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Theses and Dissertations 1. Thesis and Dissertation Collection, all items 


1979-09 


Induced evaporation of metal from an 
aluminum surface by a normal pulse 
neodymium laser 


Johnson, Christopher Brinton 


Monterey, California. Naval Postgraduate School 
http://ndl.handle.net/10945/18759 


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INDUCED EVAPORATION OF METAL 
FROM AN ALUMINUM SURFACE 
BY A NORMAL PULSE NEODYMIUM LEVEL 


Christopher Brinton Johnson 








NAVAL POSTGRADUATE SCHOOL 


Monterey, California 





THESIS 


INDUCED EVAPORATION OF METAL 
FROM AN ALUMINUM SURFACE 
BY A NORMAL PULSE NEODYMIUM LASER 


by 


Christopher Brinton Johnson 


September 1979 


Thesis Advisor: F. Schwirzke 





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. TITLE (and Subtitte) 
Induced Evaporation of Metal From an 
Aluminum Surface by a Normal Pulse 

Neodymium Laser 
















F AUTHOR(e) ; : - | &. Cn TRACT OR GRANT NUMBER(8) 


Christopher Brinton Johnson 


- PERFORMING ORGANIZATION NAME AND ADORESS 10. PROGRAM ELEMENT, PROJECT, TasK 


Naval Postgraduate School 


AREA &@ WORK UNIT NUMBERS 


Monterey, California 93940 





12. REPORT DATE 


September 1979 


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Laser Induced Evaporation 
Neodymium Laser 





20. ABSTRACT (Continue an reveree side if necessary and identify sy biock number) 


Laser induced evaporation of material from the surface of an 
aluminum target in a vacuum was studied. Based on a literature 
examination, material removal using a normal pulse laser was judged 
to be more efficient than for a Q-switched laser. The experiment 
vas conducted using a neodymium glass laser modified for normal 3 
pulse operation. The energy density was varied from 8.5xl02 J/cm 
here no breakdown occurred to 5x10 T/om* where the threshold for 


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breakdown was exceeded. The normal pulse duration was 600 us. 
Analysis of the ejected material was achieved by using a 
Hughes Ionization Gauge placed in the path of the ejected 
Material. Oscilloscope traces of the ionization gauge output 
Show that the gauge "Sees" what is flying past it. There 

1S good correlation between laser radiation, plasma radiation 
and ionization gauge fluctuations. The ionization gauge 

gave distinguishable signals for ions, electrons, and 

neutral particles ejected from the target surface. Signal 
sequence was dependent on the particle velocity. By measuring 
the elapsed time after ejection from the surface and the 
target to collector distance, the first arriving neutral 
particle velocity was determined to be 5.2x.104 cm/s. 


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Induced Evaporation of Metal 
from an Aluminum Surface 
by a Normal Pulse Neodymium Laser 


by 


Christopher Brinton Johnson 
Maso~, United States Army 
B.A., Washington State University, 1968 
M.S. University Of “Southern Calafornia, 1975 


Submitted in partial fulfillment of the 
requirements for the degree of 


MASTER OF SCIENCE IN PHYSICS 


Erom une 


NAVAL POSTGRADUATE SCHOOL 
September 1979 





ABSTRACT 


Laser induced evaporation of material from the surface 
of an aluminum target in a vacuum was studied. Based on a 
literature examination, material removal uSing a normal 
pulse laser was judged to be more efficient than for a 
Q-switched laser. The experiment was conducted using a 
neodymium glass laser modified for normal pulSe operation. 


: J/om* where no 


The energy density was varied from 8.5xl0 
breakdown occurred to 5x10° 3/em- where the threshold for 
breakdown was exceeded. The normal pulse duration was 

600 us. Analysis of the ejected material was achieved by 
using a Hughes Ionization Gauge placed in the path of the 
ejected material. Oscilloscope traces of the ionization 
gauge output show that the gauge "Sees" what is flying past 
it. There is good correlation between laser radiation, 
plasma radiation and ionization gauge fluctuations. The 
1onization gauge gave distinguishable signals for ions, 
electrons, and neutral particles ejected from the target 
surface. Signal sequence was dependent on the particle 
velocity. By measuring the elapsed time after ejection 
from the surface and the target to collector distance, the 
first arriving neutral particle velocity was determined to 


be oe cm/s. 





TABLE OF CONTENTS 


ive INTRODUCTION ----- 2-9-2 ------ -- - - - - - - -  - - - - - - - - - -- 
jGdee THEORY 2-22-92 oo a nr a a rn - - - - - - 
A. REFLECTIVITY --------------------------------- 

B. HEATING OF THE MATERIAL ---------------------- 

C. MATERIAL REMOVAL --------99------------------- 

III. EXPERIMENTAL DESIGN 7-992-292-9999 eo 
A. EQUIPMENT ------------------------------------ 

1. Laser System ----------------------------- 

2. Target Chamber --------------------------- 

3. Ionization Gauge ------------------------- 

4, Instrumentation -------------------------- 

B. PROCEDURE ------------------------------------ 

1. Ion Gauge MeaSurement Study -------------- 

2. Material Mass Study ---------------------- 

IV. RESULTS AND DISCUSSION --------------------------- 
A. PLASMA ---------------------- = = - - - = - - - - - - - = 

B. DESORBED GASES ------------ 2-2-2222 

C. NEUTRAL PARTICLES ---------------------------- 

D. MASS OF MATERIAL REMOVED --------------------- 

ie CONCLUSIONS eer re ee ee re ee ee Se 
BIBLIOGRAPHY -------------+------------+--— ------ --+--- +--+ 
INITIAL DISTRIBUTION LIST -------- oe ee ere 


70 


76 





ACKNOWLEDGMENT 


I wish to thank Robert Sanders, Technician, for his 
valuable assistance in calibrating and repairing the equip- 
ment necessary for this experiment. I would like to extend 
my Sincere gratitude to Professor Schwirzke for his help and 
guidance throughout the experiment. His comments, sugges- 
tions, and academic assistance, during the writing of the 
thesis, were of great benefit. This work was supported by 


the Defense Nuclear Agency, MIPR Number 79-512. 





I. INTRODUCTION 


A wire explosion 1S a complex phenomenon resulting from 
very rapid electrical heating of a piece of metal which has 
a small cross sectional area. The wire 1s heated and under- 
goes phase change. There is a bright flash of light and a 
loud report (in air). Thus, the term "exploding wire" has 
been coined to describe this phenomenon. More appropriately, 
the term "exploding conductor" will be used to denote the 
explosion of a single wire, a multiple wire array, a metal 
foil, or a gas puff by a high current pulse. This technique 
1s a relatively simple way of generating a plasma, which 
can be used to study the dense plasma itself or for further 
experimental work such as spectroscopy. 

The idea of exploding conductors is not a new one [3]. 
As early as 1773, Edward Nairne conducted experiments 
exploding a .15 mils diameter iron wire in proving that 
current in all parts of a series circuit is the same [45]. 
Eighty years later, Michael Faraday reported on wire explo- 
sions used to produce a metal film or mirror. The use of 
exploding wires was of little importance until the work of 
John A. Anderson in the 1920's. At Mt. Wilson Observatory, 
he showed that the temperatures involved approached those 
of the sun, in excess of 3000°C [48]. In the years that 
followed, research on exploding conductors was principally 


conducted by Nabaoka, Kleen, Wrana, Eiselt, Conn, Kvartskhava 





and Lebedev [3]. Since 1950, exploding conductors have 
become a matter of great scientific interest. Perhaps the 
largest number of researchers to use exploding conductors 
have capitalized on the extremely short, intense light out- 
put. These intense pulses of light have been used in spec- 
troscopy and high speed photography, as well as in radiation 
chemistry and laser technology as an excitation source for 
coherent radiation [50]. 

Other uses of exploding conductors are the production 
and study of aerosols, production of thin metal films [27], 
Sllvering of mirrors, and chemical synthesis [1]. In the 
past 10-12 years, the use of exploding conductors as bridge- 
type electrical detonators has found widespread application 
both as a method for multi-point detonation of explosives 
[46] and as a method to generate shock waves in different 
materials [9]. Another area of intense research, and one 
in which this paper is related to, is the use of exploding 
conductors in simulating nuclear weapons effects. In recent 
years, progress has been made in the development of exploding 
conductors as x-ray (photon) sources for use in Simulating 
nuclear weapons effects on various satellite "black boxes". 

The emission of electromagnetic radiation is a result 
Of suddenly electrically heating thin wires. Four confer- 
ences held between 1958 and 1967 dealt exclusively with 
this subject [17,18,19,20]. Toward this end, several varia- 
tions of the exploding conductor approach to high energy 


photon generation have been explored in the past few years. 





Single and multiple wire arrays [8,47], cylindrical foils 
[60], and gas puffs have all been tried to date with limited 
success. The energy of the emitted x-rays has been too low 
to be used in nuclear explosion simulations. The problem 
centers around insufficient energy being coupled to the 
plasma to allow for radiation in the x-ray region. 

Whatever the application or shapes of the conductor, 
the electrical circuit and the components are similar and 
perform similar functions, the only difference being in 
Size and characteristics. Each circuit contains an energy 
storage device, a switch or triggering device, and an explo- 
Slve conductor. The energy storage device may be a bank 
of large capacitors or special multiterawatt generators, 
such as Pithon at Physics International, Gamble II at the 
Naval Research Laboratory, or Blackjack IV at Maxwell 
Laboratories, Inc., [63]. These devices are capable of 
producing 100 KJ pulses of 50-75 nanosecond duration. This 
high rate of energy release requires special switching 
devices and transmission lines. Basically, a Marx-capacitor 
bank (200~300 KJ) is used to charge a single, large, water 
capacitor. This capacitor discharges into a water dielec- 
tric coaxial transmission line that tapers to a 1-22 output 
resistance at the point where the pulse is extracted into 
a vacuum diode. Thin wires, ribbons or foils are stretched 
between the electrode gap in the diode [41]. In the case 


Of a wire (13,29,52,53,68] or a foil [66], the initial part 





of the electrical discharge causes the conductor to rapidly 
vaporize uniformly along its entire length. The plasma so 
formed is then compressed ina Z-pinch. As a result, the 
plasma consists of "pinched" and "flared" structures char- 
acteristic of the m = 0 sausage instability to which Z- 
pinches are susceptible. It is from these pinched and flared 
areas that radiation is emitted [22]. Gas puff experiments 
conducted at Physics International are similar to the 
exploding conductor ones just described except that a gas 
puff is substituted for the wires. In addition, the gas 
puff is ionized with microwaves prior to the arrival of the 
electrical pulse to insure conductivity of the gas. 

Most of the methods giving rise to electromagnetic 
radiation emitted by plasmas are atomic in nature, that is, 
they are due to transitions involving bound or free states 
of atoms or ions. There are three basic methods of energy 
radiation: bremsstrahlung, recombination radiation, and 
discrete radiation. Bremsstrahlung or free-free radiation 
occurs when a free electron collides with an ion or neutral 
particle and makes a transition to another free state of 
lower energy with the emission of a photon. The energy of 
the photon may amount to any fraction of the initial kinetic 
energy of the electron, KT therefore, the bremsstrahlung 
has a continuum of frequencies determined by KT: At 
4 


T = 10 


Fs °K, the bremsstrahlung lies in the visible and 


infrared regions, while at T = LO °K most of the radiation 


lies in the x-ray region. Recombination occurs when a 


10 





free electron 1S captured by an n-times 10nized atom and 
makes a transition to a bound state of the (n-1) times 
ionized atom. The surplus energy 1S emitted as recombina- 
tion or free-bound radiation. The energy 1s equal to the 
sum of the kinetic energy of the free electron, KT, and 
its binding energy. Since free electrons have a continuous 
energy spectrum, the photons emitted form a continuous 
energy spectrum. Discrete radiation or bound-bound radiation 
occurs when a bound electron of an atom or ion 1s excited 
to a higher energy state by particle collisions. When the 
electron returns to a lower state, it yields energy at various 
discrete frequencies. Whenever a plasma contains ions that 
have not been completely stripped of orbital electrons, this 
radiation will appear. Generally, it takes greater plasma 
energy to completely strip atoms or molecules with higher 
atomic numbers. As the electron temperature is increased 
more tightly bound electrons are removed. This will be 
associated with an increase in the excitation energy of the 
ions. As a result, the line spectrum will shift from the 
visible to the x-ray region. Although the radiation emitted 
from exploding conductors is a combination of the above 
processes, it is dominated at high temperatures by bremsstrah- 
lung and discrete radiation in the x-ray region [30,34]. 

The spectral yields and hardness of the emitted radiation 
from Expileding conductors depends on the ionic state and the 


electron temperature which in turn are influenced by a 


ed 


combination of competing physical processes. Principal 
among these are the available generator power, the magneto- 
hydrodynamic fluid motion of the wire plasma, the coupling 
of the wire plasma to the driving electrical circuit and the 
atomic physics and energetics of the plasma. The z-number, 
geometry and mass of the wire material also play an impor- 
tant role in the above process. A good deal of the under- 
standing of how these competing processes affect the dynamic 
and radiative behavior of the wire plasma is obtained from 
experimental data. In addition, computer codes such as 
WHYRAC [51] have been developed to calculate the detailed 
time history of the wire implosion. The codes provide in- 
Sight into the coupling of plasma to the generators and into 
the energetics of the implosion. Although many of the 
processes are fairly well understood, their relationship 

and affect on one another is not. 

It as theorized that one way to obtain x rays with’ the 
desired characteristics may be to use materials with differ- 
ent, higher z values or even combinations of materials. 

Many materials can not be fabricated into thin wires or 

do not have a high enough vapor pressure to allow the forma- 
tion of a vapor. In response, investigators have therefore 
proposed that the vapor puffs be generated by using a laser. 
One of the vacuum diodes would be fabricated out of the 
target material. The target electrode, irradiated by the 


laser, would experience a steady temperature rise. If the 


12 





laser beam is intense enough, the surface of the target 
would melt and begin to vaporize. The vapor expands, 
exposing fresh target layers to heating and vaporization. 
Depending on the laser intensity, the vapor may remain 
un-ionized or a plasma may be generated. The high voltage, 
high current pulse from the Marx-generator would then be 
timed to pass through the cloud of plasma, neutral particles 
and molten metal. This procedure advances the possibility 
of using virtually any material or combination of materials, 
in addition to being able to vary the mass and temperature 
of the vapor cloud by varing the power and intensity of the 
laser pulse. 

As indicated earlier, one important parameter on which 
the dynamics of the z-pinch and x-ray emission depends is 
the mass of the pinched column. This thesis investigates 
the mass of material which is blown off a surface as a func- 
tion of the laser parameters and a method by which it can 
be measured. Aluminum was selected as the initial target 
material because of the extensive work done with aluminum 
exploding wires, thus facilitating a comparison of the two 


methods in the future. 


is 





Ti. THEORY 


The effect caused by the absorption of high power laser 
radiation on the surface of an opaque solid slab includes 
surface heating, melting and vaporization. If the laser 
intensity is low enough the vapor is un-ionized and remains 
transparent to the incoming laser radiation. At higher 
intensities, the vapor becomes ionized generating a plasma. 
Throughout this thesis, I will use the term "vapor" to de- 
note an ejected cloud of material which contains both neu- 
tral particles and plasma. In conjunction with the radiation 
impinging on the opaque surface, a cycle of events occurs. 
This is depicted in Figure 1. Prior to considering these 
events individually, a few comments on lasers in general 
are appropriate. 

First, lasers can be divided into two categories, pulsed 
and continuous. Only pulsed lasers were considered in this 
study. Pulsed lasers, in turn, may be subdivided into 
normal pulse and Q-switched depending on pulSe operation. 

A normal pulse laser is one which is pumped by a flashlamp 
and the radiation allowed to emerge when the threshold con- 
ditions for laSing are reached. Normal pulse lasers emit 
in pulses lasting tens of milliseconds with peak powers of 
He to 10> watts. The term Q-switched laser denotes lasers 
employing an element of variable loss within the cavity and 


emitting peak powers greater than 108 watts and lasting 


14 





LASER BEAM 
—_—— a. |) i on a 
| 

THE MATERIAL SURFACE ~ WSS 





‘a TAR ABSORPTION BY 
\ eae : os THE MATERIAL SURFACE 


( paprarion Vwq — — SURFACE 
V\. boss y .- HEATING 





APORI ZATION 
OF MATERIAL 


HIGH 


PRESSURE 
VAPOR 











Figure 1. Events diagram for laser-target interaction. Dotted boxes 
are considered negligible or do not contribute to the study 
at hand and will not be considered. For simplicity, 
feedback mechanisms between processes are not included. 


15 





several tens of nanoseconds. With Q-switched lasers, flux 
densities in excess of no watts/cm* are easily attained. 

In this regime, the vaporization temperature of any metal 
will be reached in less than one nanosecond. At this point, 
the input energy begins to supply the latent heat of vapori- 
zation to a thin layer of material at the surface and break- 
down (avalanche lonization) will occur. During breakdown, 
the few naturally occurring thermal electrons from the target 
Surface are heated by the laser beam. They will gain energy 
until they are capable of ionizing neutral particles by 
collision. Each collision will produce ions and more elec- 
trons which will also be heated by the laser beam to the point 
where they too can ionize neutral particles. The process 
thus continues exponentially producing electrons and ions 

and absorbing more and more of the laser radiation. The 
"breakdown threshold" is the laser intensity required to 
generate a high density plasma at the target surface. Experi- 
ments have shown that breakdown over a laser-irradiated sur- 
face exhibits a sharp threshold in laser intensity. Several 
models have been developed that predict this breakdown thres- 
hold [40,54]. Below threshold, insufficient energy is avail- 
able to propogate the avalanche ionization and the vapor 
remains relatively transparent. When the breakdown threshold 
1s reached, a high absorption plasma cloud is formed on the 
target surface. For example, consider Q-switched neodymium- 


glass laser radiation (1.06 um) incident upon an aluminum 


16 


target. <A typical pulse could have a flux of oe to 107° 


watts/em- and a duration of 10-100 nanoseconds. Figure 2, 
taken from Ref. 4 models the relationship between the thres- 
hold intensity required for breakdown and the laser pulse 
duration. The prediction is that breakdown will occur in 
this case. Because of breakdown, the energy in the beam is 
devoted to heating a small amount of vaporized material to 

a high temperature, while the heat transfer from the hot 
vapor to the bulk of the target material is limited due to 
the short pulse duration. A normal pulse laser has a series 
of spikes, many of which are capable of producing breakdown. 
The difference is that the individual spikes do not possess 
enough intensity to perpetuate the ionization process. The 
vapor is heated, breakdown occurs and absoprtion of energy 
increases, but the laser spike intensity falls off rapidly 
and breakdown ceases. This process is repeated many times 
resulting in more energy being coupled to the target and thus 
more material being ejected. Consequently, a given amount of 
energy delivered at very high power is less effective in 
causing vaporization than the same amount of energy delivered 
in a longer, lower power pulse. It is for this reason that 


only normal pulse lasers were considered in this thesis. 


A. REFLECTIVITY 
When one considers the coupling of laser radiation to 
the surface of a target, one first needs to know how much 


energy is absorbed into the material. The Drude-Lorentz free 


ey, 





10° 


¢ 


O 
nN 


O 


PULSE DURATION 


Oo 
th 


Figure 2. 


1o7-¢ oO"! O 


| 10 2 
INTENSITY GW/CM? 


Pulse duration time vs breakdown threshold intensity for an 
aluminum target irradiated by a 1.06 um laser pulse. 

Alternatively, this graph gives the breakdown time vs peak 
value of incident laser beam intensity. Taken from Ref. 4. 


18 





electron theory provides an acceptable model for metals 
interacting with infrared wavelength photons [66]. According 
to the model, electromagnetic radiation interacts only with 
free electrons ina metal. The absorbed photon energy raises 
electrons to higher energy states in the conduction band. 
The excited electrons, in turn, collide with lattice phonons 
and other electrons as they give up their energy. The electron- 
phonon collision frequency 1S proportional to the phonon popu- 
lation in the metal. The phonon population, on the other hand, 
determines the temperature of the metal. Thus, the rise in 
temperature strongly affects the electron-phonon collision 
frequency which in turn affects the reflectivity of the metal. 
Based on the Drude-Lorentz theory several authors have developed 
theoretical expressions that predict the absorbtivity of 
metals [49,66]. | 

The reflectivity of a "real metal" surface is largely an 
empirical matter. Bonch-Bruevich et al. [10] investigated 
the reflectivity at 1.06 um of aluminum, copper, dural, steel, 
and silver as the metals were irradiated with a laser beam. 
The investigators surrounded the sample with a sphere to 
monitor the reflected radiation, Figure 3. Pronounced changes 
both during each individual spike and over the pulse as a 
whole were reported. Figure 4 shows the generalized reflec- 
tivity behavior of a metal during an individual spike. The 
region of rapid change of reflectivity a-b is due to initial heating 
of the metal. The segment b-c has a constant reflectivity which the 
authors attributed to the constant temperature during the time 


when the melting wave iS propagating into the metal. Zavecz [71], 


19 





PROMO CELL 
DEE CT OR 







NO GLASS 
LASER 


TARGET 


PHOTOMETRIC SPHERE 


Figure 3. Schematic of the apparatus used by Bonch-Bruevich, 
et al. [10] for measuring the change in reflecting 
power of a metal under the action of laser 
radiation. 


20 





\ 
\ 
8-4 \ 
> \ 
-— \ 
~ \ 
= ¢ 
} 
O 
4 
4 
Te D 
LJ 
OF 
2 
0 2 4 6 8 | 2 
TIME — ps 
Figure 4. Change of reflecting power of silver at 1.06 um 


during a single spike. The form of the laser spike 
with energy of 7.5 KJ/om? is also depicted [10]. 


ou 





disagreeing, feels that this low reflectivity plateau is 

a function of the instantaneous pulse intensity and the 
Specimen's equilibrium temperature. Finally, the surface 
ee begins to rise again, thus causing a further drop 
in reflectivity, segment c-d. The increase in reflectivity 
to the right of point dis due to the decay of the spike 

flux density and the subsequent drop in temperature. Similar 
results were published by Chun and Rose [21] as reflected in 
Figure 5. In addition, Chun and Rose examined the dependence 
Of absorptance on the depth (internal volume) of the laser 
crater. These results, depicted in Figure 6, indicate that 
absorptance changes as the depth of the crater changes. This 
means that even if the initial reflectivity is high, much of 
the energy in a focused normal pulse laser will be absorbed 
by a metal surface. Although only experimental results for 
incident radiation at 1.06 um has been described, experimental 
work at other wavelengths has been done [57]. In all cases 
the experimental observations show that under high-intensity 


radiation, metal absorptivity increases non linearly. 


B. HEATING OF THE MATERIAL 
Possibly one of the most important effects of intense 
laser irradiation is the conversion of optical energy in the 
beam into thermal energy in the material. The radiation mean 
free path for visible and infrared wavelengths in a solid 
4 


material such as a metal, is typically of the order of 10 ~ om 


or less, so that the deposition of laser energy can be considered 


22 





<1) 


REFLECTIVITY 


ho 


Figure 5. 


CU 


ae 
MO 


6 8 |O 


4 
TIME- lOO ys 


Time dependent reflectance measurement at 
1.06 um from Ref. 21. A single laser pulse 
with a power density of 10/ W/cm* was used. 


20 





8 e 
O 9 oF 
é ; <a 
A = 
af 
G 
4 a) 
=| 
| 
n 3 QO AL 
Or x NI 
© O cu 
Y) 
m+ ©O MO 
< 
J 
| 2 3 4 5 6 7 8 910 


ARBITRARY SCALE (m/p) 


Figure 6. Dependence of absorbtance at iLO6ssimt On Cracer 
volume from Ref. 21. m/p is the ratio of 
mass removed to material density. 


24 





to be a surface phenomenon insofar as the transport of energy 
in a solid is concerned. The Drude-Lorentz free electron 
theory as previously discussed is the basis of thermal 
response of the material. The time required (relaxation time) 
for electrons, excited by irradiation, to transfer their 
energy to the lattice by means of electron-phonon collisions 


a2o sec [36,72,74]. When compared 


amounts to approximately 10 
to a laser pulse, the relaxation time is short, allowing one 
to assume that the heat transfer to the solid is instantaneous 
because local equilibrium is rapidly established. Therefore, 
One is justified in assuming that temperature is a valid 
concept and so the normal equations of heat flow may be 
applied. The following development has been principally 
adapted from Harrach [30,32], but other authors have similar 
developments [7,49]. 

Suppose that at time t = 0 laser radiation is directed 
upon an Opaque, solid slab of metal. A portion of the radia- 
tion, iS absorbed; the rest being reflected as described in 
the previous section. A temperature distribution T(r,t) 
develops throughout the material. When a temperature gradient 
exists in a body, there is an energy transfer from the high- 
temperature region to the low-temperature region. The heat 
conduction equation (without phase changes) is [15] 

caer ie) 


Ved (r,t) + PC IE = A(r,t) (1) 


25 





where T(r,t) 1s the temperature, J(r,t) 1s the thermal energy 
crossing unit area per unit time, pc is the heat capacity per 
unit volume, and A(r,t) is the net heat energy per unit 

volume time generated. Fourier's law relates the temperature 


to the heat flux 
i @ ae 3) ee — 8 OI ae we (2) 


To simplify the calculations, it is assumed, as in Refs. 4 and 59, 
that the problem is planar, heat 1s introduced only from the 
surface and flows into the material in the x-direction. The 
thermophysical constants are independent of temperature, 

radiation and convection from the surface is negligible, and 

the liquid phase of the material can be ignored. The heat 

flow can now be modeled using the Fourier heat-conduction 

equation [54] 


2 
9“T(x,t) , 1 a 


Ox 


ee ce} 


where k is the thermal diffusion constant equal to pKC and 
K is the thermal conductivity constant. The rate of heat 


production can be written as 


A = oCé (4) 


where @ is the surface absorptivity, I is the flux density 


of the incident radiation, 6 is the penetration skin depth of 


26 





radiation into the target material, op is the density of the 
target material, and C is the specific heat. Now by substi- 
tution of (4) into (3), the model equation becomes 

oti e) at (x,t) 


+ See SKE (5) 


_ eeeedse) 
2 at PCO 


ox 


In order to solve this differential equation, the various 
parameters must be known or at least estimated. With the 
exception of §, each parameter has an accepted value which 
can be used. The value for § 1s not so easy to determine. 
It 1s expected to be small, 107° cm [74]. It can be assumed 
that the laser radiation is absorbed in an infinitesimally 
thin surface layer (§ > 0). The source term may now be 
deleted from (5). The term is not lost however, since it 


will be incorporated into the surface boundary conditions. 


Equation (5) now becomes 


3°T (x, t) 
2 


Ox 


ats, c) 
Oe 


- K + 0 (6) 
The specific boundary conditions depend on whether the sur- 


face of the target 1s being vaporized or not. For the case 


at hand, no vaporization, the surface boundary condition is 


GPx se . Ete) 


re = aC for x = 0 


= 


27 





or 


Ort) (7) 


A second boundary condition arises when the target is con- 


Sidered to be a semi-infinite solid. 


Pim, obit 
X00 ox 


And finally, the third condition is 


Dexe0) = 0 foe, 


In general, equation (6) does not yield an exact solution. 
One method of solution, utilized by Harrach, is the “heat 
balance integral method" [30]. This method, which is based 
On the assumption that "the partial differential equation is 
required to be satisfied only in an average sense throughout 
the solid rather than at each point x," is outlined below. 
The integral method reduces the nonlinear boundary value 
problem to an ordinary initial value problem where solutions 
can be expressed in closed analytical form. 

Integrating eqution (6) over the spatial interval of the 


target, Ko SxS & gives 


28 





= ax (10) 


In general taking the derivative of an integral yields 


3 


g 
d = oT zs oe 
o f Tax = f ae ax + T(x=2,t) Fo 
Xs Xs 
Cid} 
ax _ 


Using equation (10) and the results of equation (11), noting 


ad2/dt = 0 and assuming k is constant 
ss ox 
dT fi d S 
KD =| ) = => f T dx + T(x_,t) = iz) 
ox =) Sar ac 2 S ae 


If the target surface is stationary then dx _/dt = 0. Goodman 
[26] points out that the key step in the careful choice of 
the solution form, T(x,t). The idea is to choose T(x,t) such 
that it can be integrated explicitly. This gives an ordinary 
First order differential equation when substituted into 
equation (12). Now to apply this technique to the problem 
defined by equation (6) and the boundary conditions (7), 


(8), and (9) 


kalI (t) (13) 


29 





Consider the solution form 


Texte) 


T(0,t) [L-x/&(t)]* exp[-x/e(t)], (14) 


BOT “0 <<. Ett) 


and 


EAM pC) ee Or aha ne 


where T(0,t) is the front surface temperature and £&(t) 1s a 


time dependent thermal penetration depth. Integrating 


equation (14) over x from 0 to 2 and incorporating equation 


(13) yields the ordinary differential equation 
SeIT(0,t)e(t)] = [kal (t)]/[1-2e7*]K (15) 
Substituting equation (14) into equation (7) gives 
E(t) = BRUCE) ale) (56) 


Solving equations (15) and (16) simultaneously yields 


t 
TOE ee =) (ee ey 2 (ere) (Tie )aet) 
3(1-e2°")K 0 


ay) 


30 





and 


alte jac’ 
al({t) 


1/2 


Several special cases can now be considered by stipu- 
lating the laser profile I(t) and the dependence of the 
absorptivity on temperature, a = a(t). 

GASE a: 

Let the absorptivity coefficient be constant, aoe and 
the laser pulse be a step function. The solution to equation 


(17) is now found to be 





T(O,t) = ——-( ae uve (19) 
3(1-2e 7) 
and from equation (18) 
E(t) = (3KT/(1-2e71)1/4 (20) 
For comparison, the exact solution as outlined by Carslaw 
and Jaeger [61] is 
2al 
T(x,t) = 2 (RE) 1/2¢ 4, exp (-x?/4kt) 
4(rkt) 
(21) 


ms serfclx/(4nt)*/*]} 


In the above equation erfc denots the complementary function 


31 





erfc(A) = Jj - erc(,) 


A 2 
= 2/n f exp(-u~) du 
0 


The agreement between the approximation, equation (19) and 
the exact solution, equation (21) seems to be quite good. 
For T(0,t), equation (21) is less than 1% higher. 
Now the solution just described applies up to a time t 
At T(O,t.) = T the vaporization time is 
2 


t= (3(1- 267) /kK}{KT /a lI} (22) 


for = /3kr. > 1. 
O v= 


CASE 2: 

The laser pulse is a step function and the absorption 
coefficient increases linearly with the surface temperature 
up to the vaporization temperature ue according to the 


relation 
a = [a+ (a - ao) ](T(0,t)/TL] 


The solution of the heat flow equation (14) at x = Q 1S 


then given by 


T(O0,t) = (a-a_)T /fa_-q (23) 


32 





with 
EA) = 3KT (a-d.)/lala -a)to] (24) 


and t. is as before. 


C. MATERIAL REMOVAL 

Material removal begins with vaporization. As vaporiza- 
tion proceeds, the absorption of the laser beam increases 
rapidly as the depth of the crater increases. At first the 
evaporation rate and boundary temperature are equal to zero 
and all of the laser radiation is used in heating the opaque 
solid as pointed out in the previous section. If the external 
flux 1s intense enough, the surface will be brought to the 
vaporization temperature ue at time te For later times, 
= > toe the problem is still defined by equation (6); but 
one of heat transfer in the presence of a moving phase boun- 
dary requiring new boundary conditions. 

For the first boundary condition, the surface boundary 
energy balance relation, equation (7) must be changed to 
include a term to account for the energy expended on melting 
and vaporization. It must also account for the fact that 
the surface is no longer stationary. 


—~k oT (x,t) = eG) = <1 —— (25) 


OX = 
x=x_ (t) 


33 





where x. (t) 1s the instantaneous position of the surface. 
For example, x, = 0 whent<t,. L is the effective latent 


heat of sublimation described by L = L The 


+ : 
melt Lsapor 
second boundary condition 1s as before, that 1s the target 


is assumed to be a semi-infinite solid 


lim ——— = 0 (26) 


The final condition is based on the temperature distribution 


in the material at to. 


Deane) -= Lis (27) 


where the function f(x) is determined by the prevaporization 
solution. 
With the boundary conditions just described, equation 


(6) becomes 





: k 
i Wide) = “Seler(t) = ob 


x 
S 


If dx_/dt = Q then equation (28) reduces to the prevaporiza- 
tion form, equation (13), as expected. The trial solution 


for the vaporization case may be chosen to be 


x-x _ (t) ea) 


ash) eee ~ Gives (ey! “SES ter 


(29) 


34 





for 0 < x- x(t) < g(t) - x(t) and T(x,t) = 0 for 
x— x. (t) > E(t) ~ x(t). The surface temperature 1S fixed 
at T as the position of the surface x. (t) moves into the 


solid. The surface recession velocity can be represented 


by 
a(T )I(T_) CT 
: = Vv Vio yt, _ ae _ Vv Bee 
x, (t) = or 2 ie es a ie 5 (3 + 7 ey ee, 


(30) 
When the material is exposed to large constant flux and 
begins vaporizing after time toe the rate of material removed 
will approach a steady state as the corresponding surface 


recession velocity reaches a steady state. 


Xs = a(T J I(T) /o (Lb + CT.) ene 
The steady state penetration depth, D(t) can now be written 


as 


Des = Ko L/KT = L/CT 
Figure 7 from Ref. 30 depicts the normalized thermal penetra- 
tion depth D(t), surface position x(t), and surface recession 
velocity x, (t) for aluminum. Next the crater depth de at 

a specific time t. can be determined from the integral 


equation 


35 





AND SURFACE POSITION 


PENETRATION DEPTH, 


NORMALIZED RECESSION VELOCITY, 





Figure 7. 


4 6 8 10 l2 I4 I6 
NORMALIZED TIME (t/t )-1 
Time dependence of normalized values of surface 
recession velocity x(t), thermal penetration 


depth D(t), and surface position x _(t), for 
aluminum from Ref. 30. 2 


36 





20 = f x(t) dt (32) 


assuming that the target is effectively infinitely thick. 

The previous discussion indicates how the temperature of 
a local spot on a target can be raised to the point where 
vaporization begins. At the same time, thermal diffusion 
begins as the local spot temperature rises. As a result, 
for most metals, the material is removed as a consequence 
of three processeS: vaporization, the development of pressure 
in the cavity by vapor expansion and the creation of molten 
metal. Generally, vaporization begins at the surface. The 
initial velocity of a vapor jet flowing into a vacuum is 
equal to the local sound velocity. As the crater depth 
increases, the vapor velocity increases to a supersonic flow 
[4], which provides a mechanism for the washing of liquid 
metal from the walls of the crater. The fraction of material 
removed in the liquid state increases with pulse duration [21] 
because of the increase in temperature of the crater's inner 
walls with time. 

Theoretical results are generally difficult to compare 
to experimental ones because of several factors. First, the 
amount of molten metal washed out of the crater and the 
initial fraction of absorbed energy is not well known. Next, 
a change in absorbed energy with temperature is generally 


unknown. And finally, estimates of the area on which the 


37 





beam was focused can involve considerable error. The dis- 
tribution of illumination over a spot can be very nonuniform. 

In addition, experimental results on the amount of material 
vaporized by a normal pulse laser exhibit considerable varia- 
tion even for reported "similar conditions". The literature 
provides a number of sources where values of the amount of 
metal ejected by a normal pulse laser are given [59,58,1ll, 
Ze,472, 2: 

Figure 8 depicts the ejected mass as a function of beam 
energy for a 700 us duration pulse from a normal pulse neo- 
dymium glass laser [ll]. The beam's total energy output was 
1-15 joules and its spot size was oes om? In each case, 
material removal was accompanied by breakdown. The group 
of low melting metals experience larger amounts of material 
removed. Figure 8 shows that the amount of material etedeed 
increases rapidly (exponentially) with increased energy. 

Chun and Rose conducted similar experiments a few years 

later [21]. Figure 9 shows the dependence of material removal 
on pulse duration at an average laser power of 30 KW. To 
obtain the data in Figure 9, they used different laser pulse 
durations but constant average laser power. The mass of 

the ejected material was determined by measuring the amount 

of material collected on the walls of a glass box surrounding 
the specimen. 

In conclusion, one can say that the effect of normal 


pulse laser radiation on the surface of an opaque solid is a 


ite: 





MASS EJEC TED— Grams 


10- 
SN 
PB in, 
10 B; 
10> a 
Ni 
lo- W 
lO 
0 4 8 12 16 
BEAM ENERGY-—Joules 
Figure 8. Experimental mass removal for various metals 


using a_700 us Nd-glass laser focused to 
1072 cm? (Ref. 11). 





MO 


oO) 


CU 


Ul 


AL 


> 


ON 


MASS REMOVED— mg 


e) 200 400 800 1600 


PULSE DURATION-wUS 


Figure 9. Time dependent material removal measurements. 
These measurements are the average weight of 
Material removed from three specimens for 7 5 
laser pulses having an average power of 10’ w/cm 
but different time durations (Ref. 21). 


40 





pronounced one. I have considered an ordinary thermodynamic 
approach for the calculation of the target temperature dis- 
tribution, time to vaporization, and crater depth. Experi- 
mental results On material removal for lasers providing 
fluxes up to 10° -10’ watts have been outlined. Repre- 
sentative values for mass removed and crater depth are on 
the order of a few milligrams and a few millimeters, 


respectively. 


4l 





III. EXPERIMENTAL DESIGN 


A. EQUIPMENT 

The arrangement of the equipment utilized during this 
experiment is depicted in Figure 10. 

ie Laser Svseem 

A neodymium glass laser emitting a wavelength of 1.06 

micrometers was used for this experiment. The pockel's cell 
of the Korad K-1500 laser was replaced by a simple mirror to 
allow for normal pulse operation. Figure lla shows a typical 
oscilloscope display of the laser pulse. Figure llb is an 
expanded view of the same pulse to show the individual 
Spikes. A detailed description of the laser system can be 
found in Appendix A of Ref. 55. 

Z. Larger Chamber 

The target chamber was made from a 6 inch cube of 

unbaked aluminum. Its internal volume is 12.9 + .03 liters. 
The vacuum system uSing a mechanical forepump and an oil 
diffusion pump, was capable of providing chamber pressures 
On the order of 6° Torr. The target and ionization gauge 
were mounted as shown in Figure 12. The targets were made 
Of 6062 aluminum, 50 mm in diameter and 7 mm thick. The 
surfaces were machined "smooth" on a metal lathe. A smaller 
target made from sheet aluminum (12mm x 20mm x .5mm) was also 
used. They were mounted on a holder described by Hwang 


[35]. 


42 





CW Alignment Laser 


é 


/ 


Mirror xo 
Polarizer ey, 


/ 


Oscillator 


Amplifier 


Laser Metric 
Detector / 
/ 
‘. /_ ——{ | Energy Meter Photodiode 
\ 7 


\ 7 Beam Splitters 


/ 


/ 
VY) Focusing Lens 





Nnizatio 
Gauge 


Target 


Figure 10. Block diagram of the general experimental 
equipment layout. 


43 





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‘ * ‘ : . ~ : ue z 


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Tiana > 


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op Ags «- sae 


oe 8 ef 
Walh2 ee OA 


RAR A een RD, 


bn Cah 





Figure lla. Oscilloscope display of the laser pulse at 
100 us/div. 


» 
4 
¥ 
7 
> 





Figure llb. Oscilloscope display of the laser pulse showing 
the individual spikes. Time is 2 us/div. 





LENS 





Figure 12. Top view of the target chamber. There is a 
port in the top of the chamber where the 
detector for the plasma radiation was 
located. C denotes the positive ion 
collector, G denotes the accelerating grid 
and F the Filament. 


45 





3. lonization Gauge 


A Hughes ionization gauge tube type 6578 was used as 
a device tO measure the material ejected from the surface of 
the target. Gauge #1 was placed inside the target chamber 
in the path of the excited material. Gauge #2 was located 
below the target in the manifold to the chamber. Figure 13 
Pictures the gauge #1 plus mounting hardware. The gauge was 
designed with a very fine collector wire suspended ina 
cylindrical grid. A filament placed outside the grid is 
heated by passing a current (6 amps) through it. The grid 
cylinder, which has a positive potential applied to it, 
accelerates the electrons from the filament. A simple elec- 
tric circuit for an ionization gauge is diagramed in Figure 14. 
There is a high probability that collisions between electrons 
and molecules will result in the formation of positive ions. 
Since the collector has a negative applied voltage, it collects 
ions within the grid area. The variations of collector cur- 
rent can then be measured and equated to the number of par- 
ticles in the chamber [23]. If a plasma passes through the 
10n gauge, the collector will sense the positive ions resulting 
in an appropriate current increase. This is similar in opera- 
tion to an electric probe used for plasma diagnostics [33]. 

4. Instrumentation 

The laser output signal was sensed using a Lasermetric 
Series 3117, High Speed Detector. To protect the detector, 
50, 5, and 1 percent transmission filters (at 1.06 um) were 


placed in line with the radiation. The signal from the 


46 





o> 


J, 
ed 
oe 


"Wagon 





Figure 13. Hughes Ionization Gauge, gauge l, with 
mounting hardware. 


47 





C 





-30V 


Figure 14. 


0 +150 V 


An example of a sample electrical circuit 
for an ionization gauge. C denotes the 
positive ion collector, G denotes the 
accelerating grid, and F the filament. 


48 





detector's high speed (< 150 picosecond) PIN diode was dis- 
played on a Tektronix 7904 Oscilloscope. Total energy in 
the laser pulse was determined using a Laser Precision RK- 
3200 Series Pyroelectric Energy Meter. A 10 percent trans- 
mission filter (at 1.06 um) was placed in front of the detec- 
tor face, to insure that the level of intensity reaching the 
detector did not exceed the damage threshold of the detector. 
This instrument, which gave direct digital display of the 
total energy in a pulse, was calibrated using a Hadron Model 
118 Thermopile. The calibration accuracy was no better than 
20 percent which is to be expected [59]. 

The plasma radiation was detected using a Hewlitt 
Packard pin diode detector with response time < ] nanosecond. 
To eliminate any signal from 1.06 um radiation, a filter with 
optical density of 4.5 at 1.06 um was placed over the face 
of the detector. The detector's signal was displayed ona 
Tektronix 7704 Oscilloscope. 

The two 10n gauges in the system, one in the chamber 
and one in the elbow beneath the chamber, were each connected 
to a Veeco Vacuum Gauge Control Panel. An oscilloscope was, 
in turn, connected to the recorder connection on each panel. 
The amplifier output was -2 volts with respect to the ground 
for a full scale pressure reading on all scales. A set of 
Granville-Phillips control panels was also used in lieu of 
the Veeco models. These control panels provided a -100 
mMillivolt output for a full scale pressure reading on all 


scales. 


49 





Due to the singularity and duration of the pulses, 
all oscilloscope displays had to be recorded by photographing 
with the appropriate Tektronix camera. Polaroid 3000 speed 
black and white film was used exclusively. 

Target weight measurements were made with a Cahn 
Electrobalance, Model DTL. This scale was capable of 
measurements as small as .01 milligrams with a reported 


accuracy to the nearest .01 milligram. 


B. PROCEDURE 


1. Ion Gauge Measurement Study 

A target was affixed to the target holder and the 
chamber evacuated to about H00° TOrr,; Prior €O takang data, 
all oscilloscopes and ionization gauges were adjusted and 
calibrated. 

For all experiments the laser was set to provide an 
output of 15-18 Joules. The beam intensity (focus) was ad- 
jJusted with a simple lens focusing system. A complete sequence 
of shots was taken varying the influence on the target, while 
measurements of the plasma and laser outputs and ionization 
gauge outputs were made. Some shots were repeated with 
gauge filament both off and on to distinguish the effect of 
the plasma on the collector current and to show the effect of 
the 10nizing filament electrons. 

2. Material Mass Study 
The system was prepared and operated as before except 


that special small targets were used. The procedure began 


50 





by weighing a "standard" on the Electrobalance and recording 
the weight. The target sample was then weighed. After 5 

or 6 laser shots on the target with the beam focused, it 

was rewelghed. The "standard" was also reweighed to determine 
if the balance had "drifted". The difference in weight 
readings for the sample as well as the "standard" were 


recorded. 


a1 





IV. RESULTS AND DISCUSSION 


One objective of this research was to find a method by 
which one could measure the amount of material ejected from 
a target irradiated by a laser beam. Initially, a Granville- 
Phillips ionization gauge control panel was used to monitor 
the ionization gauge signals, but was found to be unacceptable. 
The control panel's electronic circuits saturated when the 
laser was fired, yielding outputs that were not characteristic 
of the chamber pressure fluctuations. The data described 
was collected using Veeco control panels. Through the use 
of an 1lonization gauge installed in the target chamber, 
gauge 1, I was able to sense the plasma, neutral particles, 
and desorbed gases evaporated from the target surface. The 
reaction of the lonization gauge to each will be discussed 


below. 


A. PLASMA 

When the 600 us pulse was focused on an aluminum target, 
material was evaporated and a crater formed on the surface. 
The crater was oblong having dimensions of 2mm by l1mm by 
0.5mm deep for pulses with energy 15-18 J. Assuming a coni- 
cal shape for the crater, the calculated mass removed is 
0.71 mg; however, not all of this material 1s vaporized. 
The crater was ringed with metal ejected from the crater. 


This metal was deposited on outer edges as a result of the 


a2 





vapor flow scraping molten metal off of the inner cavity 
walls. A detailed display of the laser output and target 
plasma radiation from the same pulse is depicted in Figure 15. 
The beam was focused resulting in an energy density approxi- 
mately 1.9 x10> 3/om*. Light emission from the plasma was 
measured with a photo detector located on the top of the 
target chamber. A neodymium absorption filter was used to 
eliminate any direct radiation from the laser. Figure 15 
indicates that, for this case, all of the laser spikes result 
in a plasma being generated. The laser spike is relatively 
symmetrical, but the plasma emission lineshape rises sharply 
as breakdown threshold is reached and then trails off exponen- 
tially as the plasma expands, cools, and recombines. There 

is also a good correlation between plasma radiation and the 
1Onization gauge output. Figure 16 depicts these two traces 
for a single laser pulse which was defocused more than the 
pulse in Figure 15. Figure l6a shows breakdown spikes at 


ty = 0.38 ms and another at t. = 0.66 ms with approximately 


2 
the same amplitude. The corresponding ionization gauge output 


can be seen in Figure 16b. The two plasma spikes at t, are 


al 
manifested as one ionization gauge deflection. Both curves 
Show an exponential decay. Of particular interest is the 

0.8 us amd 24 us time for the 1/fe decay of the plasma radia- 
tion and ionization gauge, respectively. When the gauge's 
filament was turned off, the signal displayed on the oscillo- 


scope was the same. This is to be expected because the 


negatively biased collector acts like an ion collecting 


a3 





~@@oe¢ 


PAG AAA 
ARE EE 


a. Oscilloscope trace of laser output at 1.06 um. 


i 





a es BN he SNR 

. ee, f > ot 
Ne cats Fr a 
4 Ete 4 Fe. sn 
bers 
>. 9, » » e ae, 
: ! 
- . 
'§ Pe eae ie 
an ; * a fas bees oo} 
> 


- u's , 
‘ | x 
Mi awos ows erred Persea ty af) 
i . 
f Pe " 
rn ‘ s - 





b. Oscilloscope trace of plasma radiation with 1.06 um 
narrowband absorption filter in place. 


Figure 15. Oscilloscope traces of laser output and plasma 
radiation taken from the same pulse. Trigger 
was delayed 0.22 ms. The pulse energy was 15 
Joules. The beam was focused to approximately 
8 x 1073 cm?. Time reference for both traces 
was 5 us/div. 


54 








a. Oscilloscope trace of plasma light. The wide base 
Signal is due to laser flash lamp radiation. The 
spikes above the background noise are light emitted 
from a breakdown plasma. 


r é 
Bhp tts ee idees & 
* 


b 4 
gerbe he Goku 


Bade 
ree 


a 
ON at 
~ We 
° 
* 
on 
. 4 
* 
aS 
S$ 
a, Sr! 
* 
eaodee 
Nes 
oe 
a» Ch 
An 
ear 
eka 





b. Oscilloscope trace of chamber ionization gauge output with 
filament on and control panel set on the 10 Torr range. 


Figure 16. Oscilloscope traces of plasma radiation and 


ionization gauge output. The time for both 
traces was 0.1 ms/div. 


55 





electric probe. The plasma contains many ions which, in 

turn, provide a signal for the ionization gauge. Figure 16 
also shows that there was no detectable time lag between 
plasma radiation and the corresponding ionization gauge 
Signal. Actually, the plasma, traveling at about 2 x oh cm/s 
[14], took approximately 5 us to cross the 10.4 cm distance 
from the target to the collector, but this time was not per- 


ceivable on the 0.1ms scale in Figure 16. 


B. DESORBED GASES 

To demonstrate that the ionization gauge deflections 
measured "fly by" material, the laser beam was defocused to 
approximately 1.8 x 1G =< om. At this point, there was 
insufficient energy density, J/om*, to ionize, melt, or 
vaporize the target surface. Only gases were desorbed off 
of the surface. A visual inspection of the target showed 
a 0.4 cm diameter circle where the target had been heated. 
Figure 17 shows traces from the two different ionization 
gauges mounted in the system. The upper trace is from the 
chamber gauge, gauge 1, while the lower one corresponds to 
the gauge located below the chamber, gauge 2. The top trace 
shows a definite fluctuation, while the lower trace is 
relatively flat though its gain was 5 times greater. When 
the laser is fired several times in the same spot, the first 
three or four pulses caused a much higher positive pressure 
indication than subsequent ones. This effect has been pre- 


viously reported [55,62]. It is due mainly to the desorption 


56 











Figure 17. 


Figure 18. 


r=, 
TS . 
a 
af) 
cc 
OC | 
—_ 
pr 
S 
© 
" 
w 
om) 





Oscilloscope display of the ionization gauge 
Te eer with the control panel range set at 

10°° Torr range. The upper and lower displays 
are from gauge 1 and 2 respectively. The beam 
was defocued to 1.8 x 1072 cm¢ (no plasma 
generation). Time was 0.2 ms/div. Gain was 

1 V/div for the upper trace and 0.2 V/div. 

for the lower. 


4 ™~ VA ry AS oe, pala SAA AA AE We 
* ~~ = ‘im » ee Je > 
r os ’ ‘3 
7, 


~~ 





Oscilloscope trace of #1 lonization gauge. A 

700 G horseshoe magnet was placed parallel to 

the target surface to deflect charged particles 
form the target surface. The beam with energy 5 
of 15 J was focused to approximately 1.8 x 1072 cm*. 
The reference for time was 0.2 ms/div. The, 

control panel was set to operate on the 10 

Torr range. 


57 





of impurities such as nitrogen, oxygen, hydrogen, water, and 
carbon dioxide found on the target surface. 

A baffling result was the initial signal in the negative 
direction with the onset of lasing. If a magnet is placed 
adjacent to the target to deflect charges particles, the nega- 
tive signal disappears and only a positive signal is seen, 
Figure 18. If the gauge's filament is turned off, the trace 
is flat for the same operaing conditions. No voltage drop 
on the +150 volt accelerating grid was observed. One explana- 
tion for the negative signal may be thermal or photo electron 
emission from the target surface. The gauge's filament has a 
negative space charge around it. The additional, probably 
more enercecic, electrons from the target surface would 
increase the space charge and act as a shield returning (repel) 
thermal electrons back to the filament surface. The net 
effect is a reduction of the number of electrons accelerated 
by the grid. This, in turn, means less ionization takes 
place and therefore a drop in collector current. The positive 
portion of the trace, starting at the dips minimum, Figure 17, 
was due to the pressure increase by gases desorbed from the 
target surface. The conclusion that can be drawn is that the 
chamber ionization gauge was actually sensing what was flying 


past it. 


C. NEUTRAL PARTICLES 
Neutral particles present a special problem of measure- 


ment. It was expected that electrons from the ionization 


58 





gauge's filament would ionize a fraction of the neutral 
aluminum particles allowing for their collection. This, in 
fact, happened. A comparison of Figure 19 and 20 shows that 
the neutral particles (and desorbed gases) were not collected 
if the filament was off, resulting in a steep decline and 
a flat trace after the plasma passed, Figure 20. 

It was observed that when breakdown at the target surface 
occurred, plasma and neutral particles were ejected. The 


plasma velocity is on the order of (kT /M,)*7?, approximately 


Zo. 19° =) 2x 10/ cms, depending on the laser pulse parameters 
[14]. The plasma was therefore much faster than the neutral 
particles which leave the surface at the thermal velocity of 
20x iE cms [59]. In Figure 19, this is demonstrated. The 
plasma passed the chamber ionization gauge first, immediately 
followed by the neutrals. The velocity of the neerel Darti- 
cles can be determined from the times depicted in Figure 2l. 
The laser beam was focused to approximately 0.018 cm? and a 
700 G horseshoe magnet placed next to the target to suppress 
thermal and photo electrons as previously discussed. A posi- 


tive rise in the oscilloscope trace began at t, = 0.42 ms, 


2 
0.22 ms after lasing began, ty: The first arriving neutral 
particles traveled the 10.4 cm from the target to the collector 
ait So.2 Xx 107 cm/s. The most probable thermal velocity of 

an aluminum particle leaving the target surface at the vapori- 
zation temperature of 2330 °K can be calculated from (2Kt/m) 2/2. 


he is 3,6 x 10° cm/s. The velocity of particles evaporated 


Sie) 





Figure 19. 


Figure 20. 








ry 
Pe et 
4 


a ae 


rs 
* 
J 
ra 
. 
° 
AREA RAAn ASE URL ORe RHO Kye 


« 
wf 
af 
be oie 


: 
: r os ny kf » * 
By Bre reorient aS yo ar we “* fe 


iio 


‘Neutral's 
psignal 


a 


C e 
SA ne a epee SS sh a plo aval olen ae a 
5 * ne YP ERICK, 


Pa ; . 
Py een Weert creme Tene tert Sen igs tele ens ee Pan damdey hp iay eed 
BY ey rf 


2 . x 
" 


7 


2 nt 


OILERS Se 
. 


eri ere rss Ty 
x 


- 
. 


a 
i 
ice 
ye 
> « 
rs 
ia» a 
s 
- 
agen! 
ie 


Oscilloscope display of the #1 ionization 
gauge output with filament on and with the 
control panel set on the 10° ~ Torr range. v 
Beam was focused down to 2 x 1072 cm2. 

Time is 0.5 ms/div. 


nn 


Neutral 's Bees 


Reg f 
~ 
Ps 
> 
2 


i a 


Ionization gauge oscilloscope trace for the 
same conditions as those in Figure 17 except 
that the filament current was turned off. 


60 





Figure 21. 





Oscilloscope trace of #1 ionization gauge. 

A 700 G horseshoe magnet was placed parallel 

to the target surface to deflect charged 
particles from the target surface. The beam 
with energy of 15 J was focused to approximately 
1.8 x 10°* cm? The reference for time was 

0.2 ms/div. The control panel was set to 
Operate on the 10-2 Torr range. wv 


61 





from the surface can probably be characterized by a Max- 
wellian distribution. The faster experimental velocity 
value can be attributed to particles with velocities in the 
tail-Oof the distribution. 

Figure 22 depicts the results of a six shot sequence 
fired at one spot. Again, a magnet was used to deflect 
photon and thermal electrons away from the filament. The 
spot size of 1.8 x 10° cm” and time scale of 0.22 ms/div 
were kept constant as the laser energy was increased from 
moo tO 36.6 Joules (16.8, 19.6, 20.5, 22.9, 24«6, and $6.6 
Joules). The traces have increased amplitude with increased 
energy. The relative height of that portion of the curve 
eee puted to neutral particle emission from the target was 
very dependent on laser flux. For all cases, positive deflec- 
tions due to neutral particles occurred at the same time, 

t, = 0.42 ms. The final shot shows an initial spike, caused 
by a small plasma cloud, followed by a sharp rise due to 
neutral particles. One can conclude that as energy was 
increased, the initial arrival time of neutral particles at 
the collector remains constant. Further, the rise time 
decreased and the amplitude of the pressure signal increased 
as the beam energy density was increased. 

As an example, Figure 23a depicts the ionization gauge 
Signals as material, ejected from the target surface, passed 
by it. In this case, the experiment was conducted without 


the target magnet. Figure 23b is the corresponding trace 


of the plasma radiation. Both traces were from the same 


E2 








Fagure 22. 


. a Ale tat . ‘* 


M'icutral's 


Oscilloscope trace of chamber ionization 
gauge. A 700 G magnet was placed parallel to 
the target. The laser beam was increased with 
each shot as the spot size, 1.8 x 1072 cm2, 
and the time, 0.2 ms/div. were kept constant. 
The gauge control panel was set on the 107° 
Torr range. 


63 








Oscilloscope display of lonization gauge output with 
control panel set on 107° Torr range. The lower trace 
is from gauge #2 with the filament off. The small 


ripple was due to plasma radiation. The upper trace 
is from the chamber gauge. 


¥ aes baat oh bela btn she) a eto 
. a x 


. 
2 A aE 

. oH. ae 
= beth at oe = 
5 t 
-% 


“ 


. r * 
Kanth So awbebtetey 





b. Oscilloscope trace of plasma radiation. 


Figure 23. Oscilloscope displays of ionization gauge 
outputs and plasma radiation for the same 
shot. The beam which was _defocused to 
approximately 2 x 1072 cm? had energy of 


18 J. The time reference for both displays 
was 0.2 ms/div. 


64 





laser pulse and had 


= 7 A 
- 2S 


aa 


ith enercyv o 


Wo 


pulse, 


pox 10 


plasma 


. 


23b shows a2 


1so 


~_ 


qe traces a 


ion cau 


iS 


tebe mlvas: 


plasma 


the 


When 


a. 


Dike, 


of the plasma s 


Girection was 


heater 


Brom tne laser 


—_— 


which have a slowe 


5 a 
— 


— 


eons i1 ce 


- 


Q.2 ms/div. 


= 


LEU, 


Cont 


ment b-c shows 


4) 


par 


y 


s 


baZge 


Vado) 


SEOCS, 


-_ 


° 
o_o 


= 
> > mm 
we er Oe oe ae 


seqment c-c. 


curve, 


"4 


Te) 
oO 








a. Oscilloscope display of #1 ionization gauge output. 
Control panel range was set on 10-5 Torr range. 





b. Oscilloscope trace of plasma radiation. 


Figure 24. Oscilloscope displays of ionization gauge 
Output and plasma radiation for the same 
shot. The beam was focused from that in 


Figure 22. Energy was approximately 18 J. 
Time reference was 0.2 ms/div. for both 
displays. 


66 





D. MASS OF MATERIAL REMOVED 

In order to equate the chamber ionization gauge signal 
to a specific number of particles ejected or ultimately the 
mass of the material removed, one must first be able to 
measure the mass of the ejected material. Small aluminum 
targets were weighed and places in the target chamber. The 
laser, with energy about 18 J, was fired at five locations 
On each target. Beam spot size was approximately 0.016 om? 
The target craters were approximately 0.5 mm deep and had the 
usual metal splashed around the periphery of the crater. 
From the volume of the crater, the estimated mass removed 
was 0.7 mg. However, upon trying to reweigh the target, 
it was found that only a few hundredths of a milligram of 
material seem to have been removed. Compared to previous 
experimental work, this amount seems to be extremely low 
[11,21]. One explanation may be the difficulty encountered 
in weighting small amounts. The Electrobalance used to weigh 
the targets tended to drift. Fluctuations varied from hun- 
dredths to tenths of milligrams for the same sample. A 
standard weight was used to eliminate some of the drift, 
but only the long term effects could be eliminated. As a 
consequence, the mass of material removed was not accurately 


determined. 


67 





¥. “GONCEUSIONS 


Laser induced evaporation of material from a target 
surface has been investigated using a normal pulse laser. 

The technique of using an lionization gauge to sense the 
ejected materials does work. The gauge essentially "sees" 
what is flying past it. When breakdown on the target surface 
occurs, a crater is formed. The breakdown is accompanied by 
generation of a plasma and neutral particles. Breakdown 

can be avoided by defocusing the laser to a larger spot on 
the target surface. In this case, only gas desorption from 
the surface occurs and much smaller signals are observed 
With the ionization gauge. 

A good correlation between incident radiation, plasma 
radiation, and the ionization gauge fluctuations was observed. 
For laser energy densities sufficient to cause many break- 
downs on the target surface, the ionization gauge reacts with 
large positive signals to the plasma which arrives first, 
and then to the slower velocity neutral particles. From 
time-of-flight measurements the first arriving neutral parti- 
cle velocity was determined to be 5.2 x 10 cm/s which com- 
pares favorably with published data. For less intense laser 
radiation, surface breakdown did not occur or occurred only 
weakly a few times during a normal laser pulse. For this 
case, the ionization gauge initially gave a negative signal 


in conjunction with the onset of lasing (excluding positive 


68 





°¢ 


plasma caused fluctuations). This can be attributed to ther- 
mal and photo electrons ejected from the target surface. 

Once the slower velocity neutral particles (and desorbed 
gases) arrive at the gauge, the signal increases and becomes 
positive. The signal amplitude is commensurate with the 
laser energy density. 

Mass removal measurements were inconclusive. Experimental 
measurements of crater volume indicated that 017 mg of 
aluminum were removed; however, some of this material was 
deposited in a ring around the crater opening. Further 
experimental work needs to be done on weighing the mass 
ejected from the target surface. Once an acceptable technique 
is developed, then correlation experiments between the weight 
of the mass removed and ionization gauge fluctuations should 


be conducted to "calibrate" the ionization gauge output. 


69 





IE)? 


13 BF 


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Vo 





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76 








Thesis 186733 
J5745 Johnson 
Cou Induced evaporation of 

metal from an aluminum 

surface by a normal 

pulse neodymium laser. 


thesJ5745 
Induced evaporation of metal from an alu 


UTE 


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