, _,_-_F -I _I?_ ̧,_'

$B IR - 08.11-2227

rele_seddate 8/01/90 v

LT REPORT TR-88-1

WIDELY TUNABLE GAS LASER

FOR REMOTE SENSING

D. E. Rothe

December 1988

FINAL TECHNICAL REPORT - PHLSE II

NASA SBIR CONTRACT NO. NAS7-970

Performance Period: I August 1986 to 31 October 1988

Sponsored by:

National Aeronautics and Space Administration

Washington, DC

Monitored by:

NASA Resident Office - JPL

4800 Oak Grove Drive

Pasadena, CA 911109

Lasertechnics

5500 Wilshlre Avenue, NE

Albuquerque, NM 87113

r,

r

i"

rI '_ "-- '

T

J
"SBIR RIGHTS_ENOT

(APR 198/

Its under_fASA Contract No. NAS7-970.

of all_tems t 9 be delivered under

this_ata for Government purposes

t_fGovernment during such period

t_hat, subject to the foregoing

_ay be dlsclosed for use by support

This SBIR data is furnished with SBIR ri(

For a period of two years after acceptan(

this contract the Government agrees to u_

only, and it shall not be disclosed outs

without permission of the Contractor, ex(

use and disclosure prohibitions, such dat

contractors. After the aforesaid two-yea period the Government has a

royalty-free license to use, and to aut_ |ze others use on its behalf, this

data for Government purposes, but is r_i _d of all disclosure prohibitions

and assumes no liability for unautho_zed u_e of this data by third parties.

This Notice shall be affixed to an_reproduc_ons of this data, in whole or in

part. / _

The views and conclusions contained in this document are those of the author

and should not be interpreted as necessarily representing the official

policies, either expressed or implied, of the National Aeronautics and Space

Administration or of the U.S. Government.
REPORT DOCUMENTATION PAGE

I. REPORT NUMBER 2. GOVT ACCESSION NO.

Lasertechnics TR-88-1

TIT_E (and SwDtitle)

WIDELY TUNABLE GAS LASER

SENSING OF STRATOSPHERIC

7. A uTI..IO RI _

FOR REMOTE

CONSTITUENTS

D. E. Rothe

9. PERFORM'ING ORGANIZATION NAME AND AODRESS

LASERTECHNICS, INC.

5500 Wilshire Avenue,

Albucuerque, NM 87113

NE

11. CONTROLLING OFFICE NAME AND AOORESS

NASA Resident Office - JPL

4800 Oak Grove Drive

Pasadena, CA 91109

14, MONITORING AGENCY NAME & AOORESS (if d,N. t,on'_ Controll,ng Office)

._Ao ,,vs_,,ucPiofvs..... -

3. RECIPIENT'S CATALOG 1 NUMIIER

S. TYPE OF REPORT & PERIOD COVERED

FINAL REPORT -PHASE

1 AUG 1986 - 31 OCT 1988

6. PERFORMING ORG. REPORT NUMBER

TR-88-1

8. CONTRACT OR GRANT NUMBER(s)

SBIR CONTRACT - NASA

NAS7- 970

10. PROGRAM ELEMENT. PROJECT. TASK

AREA & WORK tJNIT NUMBERS

12. REPORT DATE 13. NO. OF PAC.,ES

December 1988 95

1S. SECURITY CLASS. (It tn,$ reQortl

UNCLASSIFIED

151. _ ECLASSl F ICATION / OOWNG RADINO

SCHEDULE

16. DISTRIBUTION STATEMENT (of ihil ,ep_U

Distribution limited to U.S. Government

Evaluation only.

See: "SBIR RIGHTS NOTICE"

Agencies for Test and

17. DISTRIBUTION STATE_IIENT (of t_l I)OItnN:l Imtl_'ed m 8tOl:_ 20. it Oitfi_en¢ from ,I@O,t)

Abstract and SUMMARY approved for public release;

Distribution unlimited

"18. SUPPLEMENTARY NOTES

'19. KEY WORDS (Cont,nue on ,ever14, Iwdo if nlclllt_y _¢1 iclenEify t)V block numGcl')

Electric Discharge Laser Prepu!se Techniaue

Rare-Gas Halide Laser Magnetic Switching

Pulse Power Technology High Efficiency

Pulse Forming Line X-Ray Preionization

Remote Sensing

2_ AOSTRACT (Co_inue _ ,e_ _ if n_eu_ry I_ _entlf¥ 0¥ _l_k numO_l

An advanced, highly efficient and reliable Rare-Gas Halide laser has

been developed. It employs: (1) novel nre_ulse techniques and impedance matching for efficient energy transfer, (2) magnetic switches

for high reliability, (3) X-ray preionization for discharge uniformity

and beam quality, (4) an integrated gas flow loop for compactness.

When operated as a XeCI laser, the unit produces 2 J per pulse with

good beam uniformity. Optical pulse duration is 100 ns. Pulse repetition rate has been tested up to 25 Hz. Efficiency is 3 per cent.

DD,; %1473

EDITION OF 1 NOV GS IS OBSOLETE

UNCLASSIFIED

SE¢IJRtTY CI.AS,_IFICATION OF THIS PAGE (When Dell EnteredJ
1,0 SUMMARY

This Report describes the development of an efficient and reliable multi-joule

high-pressure gas laser based on new state-of-the-art excimer laser

technology. Such a laser has potential applications in a space-based LIDAR

system for remote sensing of the atmosphere, as a transmitter for submarine

communications, and for a multitude of commercial uses,

The design of the laser incorporates a specially constructed low-impedance

pulse-forming network (PFN), including several magnetic switches which permit

a high-voltage prepulse to be applied to the discharge electrodes, This

prepulse technique makes it possible to impedance-match the PFN to the

discharge plasma for maximum energy transfer efficiency. Extremely uniform

preionization of the gas mixture is achieved with 70 kV X-rays produced with a

newly developed long-life X-ray generator.

When operated as an XeCI laser this device has demonstrated optical output at

2 J per pulse at 308 nm , and pulse repetition rates as high as 25 Hz.

Because of its high discharge stability, long optical pulse durations of I00

ns were easily attained. Electrical single-pulse laser efficiency of 3% has

been achieved. Good beam uniformity and excellent pulse energy stability have

been demonstrated.

Significant new knowledge and expertise has been gained during this

investigation in the construction and operation of a long-life corona plasma

e-beam cathode, in the construction and control of saturable magnetic Metglas

switches, and in the design of prepulse circuits which form an integral part

of the discharge PFN. This work has demonstrated that the high magnetic

saturation flux density of thin Metglas ribbon can be utilized to produce

high-performance switches with long hold-off periods and fast turn-on action.

Rates of rise of 1012 V/s have been achieved with the present magnetic

prepulse switch, eliminating the need for spark gaps. The three-electrode

structure of the present laser has not been found advantageous, because the

dynamic interplay between the two discharges proved difficult to understand.

A new design with two solid discharge electrodes is proposed herein.

New technology developed here will be used in a commercial follow-on excimer

laser, which has potential applications in semiconductor processing

(annealing, planarization of metal films, selective etching of films,

selective film deposition), photochemical processes, product marking

(including semiconductors and metals), and micromachining. A smaller unit

with less pulse energy but higher pulse frequency has potential applications

in the medical field (corneal machining, laser angioplasty) and for UV

lithography.

-I-
1.0

2.0

2.1

2.2

2.3

3.0

3,1

3.2

3.3

4.0

4,1

4.2

4.3

4.4

4,5

4.6

4,7

4.8

4.9

5,0

5,1

5.2

5.3

6,0

6.1

6.2

6.3

7.0

8,0

9.0

9.1

9.2

I0.0

TABLE OF CONTEBTS

SBIR Rights Notice

Documentation Page and Abstract

SUMMARY

Table of Contents

List of Figures and Tables

INTRODUCTION

Key Technical Issues

Project Objectives and Relevance to NASA Goals

Unique Features of the Discharge-Excited Multigas Laser

APPROACH FOR INCREASING LASER EFFICIENCY AND RELIABILITY

Efficiency Optimization

Minimum Stress Design (Reliability and Lifetime)

Compactness

EXClMER LASER CONSTRUCTION

Functional System Outline

Discharge Parameters

Discharge Electrodes and Feedthroughs

Pulse-Forming Network

Pulse Charging Unit

X-Ray Preionizer and Pulser

Trigger and Delay Circuits

Gas Flow and Cooling

Optical Cavity

PRELIMINARY EXPERIMENTS

Electric Field Plotting

Computer Simulation of Pulse-Forming Networks

Testing of Sample Metglas Cores

DEVELOPMENT OF INTEGRATED PREPULSE GENERATING SCHEMES

Prepulse Circuits

Magnetic Switch Design

Core Reset Techniques

TEST RESULTS

CONCLUSIONS

PROPOSED FUTURE WORK

Proposed Changes

Commercial Spin-offs and Applications

REFERENCES

i

ii

1

2

3

9

11

12

12

13

14

15

15

19

2O

24

26

26

36

36

4O

43

43

44

57

63

63

75

79

84

9O

9O

9O

93

94

-2-
LIST OF FIGURES AND TABLES

Figure 2-I.

Figure 2-2,

Ideal Pump Pulse.

Avalanche Formation Time as a Function of

Overvoltage Ratio,

Page

8

8

Figure 4-I,

Figure 4-2,

Figure 4-3.

Figure 4-4.

Figure 4-5,

Figure 4-6.

Figure 4-7,

Figure 4-8.

Figure 4-9.

Functional Outline of System Components,

Cross-Section through Discharge Chamber and

PFN Connections.

Overall View of Excimer Laser System,

Underside of Support Plate Showing Kynar

Insulated Cathode Feedthroughs,

Cathode Installed on Partition Plate.

Cathode Fin,

Secondary Cathode Installed over Primary Cathode.

Pulse Forming Line with Magnetic Switches

Installed.

Schematic of Pulse Charging Unit.

Figure 4-10, Top View of Pulse Charging Unit Showing Location

of Components.

Figure 4-11, Schematic of X-Ray Generator and Pulser,

Figure 4-12. E-Gun Cathode Design,

Figure 4-13. E-Gun Anode Design.

Figure 4-14. X-Ray Generator Pulse Characteristics.

Figure 4-15, Blower Installed in Discharge Chamber.

Figure 4-16, Magnetic Coupling Design.

Figure 4-17. Heat Exchanger Design.

Figure 4-18, Vacuum Pump and Gas Mixing Station,

16

17

18

21

22

22

23

25

27

28

31

33

34

35

38

39

41

42

Figure 5-I,

Figure 5-2,

Figure 5-3.

Figure 5-4.

Figure 5-5.

Figure 5-6.

Figure 5-7.

Electric Field Plot in Discharge Section

for Original Geometry.

Electric Field Plot in Discharge Section

with Modified Kynar Insulator.

Model of PFN. Original Version with

Magnetically Switched Positive Pulse.

Charging Current through Thyratron (Computer

Modeling).

Voltage at Screen during Charging Pulse

(Computer Modeling).

Prepulse and Discharge Voltage at Screen

(Computer Modeling).

Prepulse and Discharge Voltage at Cathode

(Computer Modeling).

45

45

46

48

49

5O

5O

-3-
Figure 5-8. Power Dissipated in Main Discharge (Computer

Modeling).

Figure 5-9. ComputerModel of X-Ray Pulser,

Figure 5-10. Calculated E-BeamAnodeVoltage Pulses for Two

Different Diode Impedances.

Figure 5-11. B-H Curve for Racetrack Core Woundfrom 15 um

Thick by 5 cm Wide 2605-C0 Metglas Ribbon,

Figure 5-12, B-H Curve for Racetrack Core Wound from 25 um

Thick by 5 cm Wide 2705-MN Metglas Ribbon,

Figure 5-13. Experimental Setup for Testing Metglas Cores.

Figure 5-14, Voltage and Current for 2705-MN Core.

Figure 5-15, Magnified Current Pulse for 2705-MN Core.

Figure 5-16. Voltage Pulse for 2605-C0 Core,

Figure 5-17. Magnified Current Pulse for 2605-C0 Core.

Page

52

54

56

59

59

60

61

61

61

61

Figure 6-I. Original Prepulse Circuit; Magnetically Switched

Positive Pulse Followed by Predischarge Switched

Negative Pulse.

Figure 6-2, Calculated Voltage Across Main Gap for LK = 5 nH

Distributed over Predischarge Circuit.

Figure 6-3. Calculated Voltage Across Main Gap for Lumped

20 nH Cathode Impedance.

Figure 6-4, Measured Screen Voltage Showing Prepulse Generated

by Circuit in Figure 6-I,

Figure 6-5, Expanded Trace of Prepulse at 50 ns/DIV Showing

Prepulse to be 25 ns Wide.

Figure 6-6, Modified Prepulse Circuit; Magnetically Switched

Negative Pulse.

Figure 6-7. Screen Voltage for Circuit in Figure 6-6

(Glow in Main Gap).

Figure 6-8. Screen Pulse at Higher Charging Voltage

(Arc in Main Gap).

Figure 6-9, Voltage-Doubling Prepulse Circuit with Voltage

Divider and Fast Switch (L2).

Figure 6-10, Calculated Cathode and Screen Voltages for

L2SAT = 50 nH.

Figure 6-11. Cathode and Screen Voltages for Spark-Gap

Switched Prepulse.

Figure 6-12. Cathode and Screen Voltages for Magnetically

Switched Prepulse.

Figure 6-13, Laser Burn Marks for Two Different Mirror

Alignments (I J Output),

Figure 6-14. Side-to-Side Intensity Profile of Laser Beam,

Figure 6-15, One of the 7-Turn Magnetic Switches Originally

Used for L2S.

Figure 6-16. Fast Magnetic Prepulser.

Figure 6-17. Measured B-H Curve for New Low-lnductance

Metglas Switch.

65

66

66

67

67

68

70

70

72

72

73

73

74

74

76

77

78

-4-
Page

Figure 6-18. Prepulse Time Jitter with Insufficient Reset.

Figure 6-19. Magnetic Core Reset Circuit Using Auxiliary

Windings on Saturable Inductors.

Figure 6-20. Magnetic Core Reset Circuit, Pulse Applied

Directly to Laser PFN.

8O

82

83

Figure 7-I.

Figure 7-2.

Figure 7-3,

Figure 7-4.

Figure 7-5.

Figure 7-6.

Cathode and Screen Pulse (200ns/DIV).

Cathode and Screen Pulse (lOOns/DIV).

Screen Voltage and Optical Pulse (New Gas Mix).

Screen Voltage and Optical Pulse (Old Gas Mix).

Beam Imprints at 2 J Output Level for Different

Resonators and Alignments.

Pulse Energy Stability at l Hz Measured with

Pyroelectric Energy Meter.

85

85

87

87

88

89

Figure 9-I. Proposed Two-Electrode PFN and Prepulse Circuit. 92

Table 2-I,

Table 5-I.

Table 5-2.

Atmospheric Species Measurable with Multigas

Pulsed Laser System,

Computer Model Parts List for Laser Discharge PFN,

Parts List for X-Ray Pulser Model.

I0

47

55

-5-
2,0 INTRODUCTION

In the past, pulsed gas lasers have not been seriously considered for spacebased applications when other alternatives were available. The reasons for

this have been two-fold: the need for switching relatively high voltages and

the need for special gas handling equipment.

The work reported here addresses primarily the first issue. This publication

describes the development of fast magnetic switches to replace the less

reliable plasma switches, such as spark gaps. It also describes progress made

in developing impedance-matched PFNs, resulting in higher laser efficiencies

and in arc-free discharges, An additional benefit is derived from the latter,

namely increased component life. The above advances greatly mitigate the

problems traditionally encountered with handling high voltages.

The second issue, involving gas handling equipment, has to be addressed for

each gas laser separately. It generally involves the use of a gas cleanup

system, which continually or intermittently scrubs the gas free of

contaminants and reaction products to maintain an optimally clean and correct

gas mixture. In addition, provisions may have to be made to completely

replace the gas after so many pulses (typically between 108 and 109 shots).

With some lasers, such as HF/DF lasers, disposal of the reaction products may

also present a problem. At this time it appears entirely feasible that

closed-cycle rare-gas halide (RGH) and CO2 lasers, with suitable gas

processing cycles, can be developed which are entirely service-free for

periods of one year. Suitable gas processors (catalytic converters, getter

type traps and cryogenic traps) are being developed elsewhere I-5, Because of

the finite resources for the present program, the gas cleanup issue has been

considered beyond the scope of this project.

2.1 KEY TECHNICAL ISSUES

The principal issues associated with the development of long-life RGH lasers

(and CO2 lasers) are the long-term production of stable, uniform, relatively

long-duration, high-power-density electric discharges in the laser gas, the

long-term maintenance of laser gas composition and purity, and the avoidance

of materials degradation. Additionally, high flow quality must be maintained

at the laser head. For the optics and wavelength control system, stable,

automated operation at the required laser frequency, and within the required

frequency bandwidth, are also challenging issues.

Traditionally, the central problems affecting the performance of all highpressure, self-sustained discharge-excited gas lasers are discharge uniformity

and stability. The need for uniform-field electrodes, uniform preionization,

and rapidly rising voltage and current pulses was recognized quite early in

the development of these so-called TE-lasers 6. Consequently, most TE-lasers

have carefully profiled electrodes, some means of preionization (usually hard

UV), and low-inductance pulsers for rapidly feeding power into the discharge.

-6-
Homogeneousdischarges can be achieved in high-pressure RGHlasers, if certain

criteria are met simultaneously. These are:

I. Uniform electric field betweendischarge electrodes prior and during

discharge formation. Field variations should be less than 1 per cent.

2. Uniform volume preionization. In a plane perpendicular to the discharge

current, the ionization level should not vary more than I0 per cent.

In the direction of current flow, the ionization level should be within

a factor of two.

3. Preionization density must be above a certain minimumvalue everywhere in

the discharge volume. This minimumelectron numberdensity is typically

around 106 cm-3, but depends on the gas mixture, gas pressure and voltage

rise 7.

4. Voltage and current must rise sufficiently fast to drive the electron

avalanche development in a uniform manner.

5. The PFN, rather than the discharge, should limit the current to inhibit

ionization runaway.

With the strongly electronegative gas mixtures used in RGHlasers, stable

discharges can generally be maintained only for a few hundred nanoseconds. In

XeCI lasers, local depletion of vibrationally excited HCl causes the discharge

to becomefilamentary at the end of a long pumppulse, causing premature

termination of the optical pulse8,9, Vibrationally excited HCl has a large

dissociative electron attachment cross-section, and normally acts as a

stabilizing agent against ionization runaway.

Recent advances in excimer laser technology I0,II,12 have been incorporated

into the present laser to improve the laser efficiency and beamuniformity.

Someof these advances are:

I. X-rays are used to provide uniform preionization in the high-pressure gas.

2. A prepulse is used to initiate the discharge. This permits adding the bulk

of the electric energy under impedance-matchedconditions.

3. A constant-voltage, constant-current transmision line drive ensures

stability and high-power excitation throughout the pumppulse.

An attachment dominated avalanche discharge acts approximately as a constantvoltage load13,14. Thus the discharge impedancedecreases inversely

proportional to an increase in current, It is necessary, therefore, to limit

the current externally, so as not to drive the discharge into ionization

and/or thermal instability for long-pulse excitation, A distributed-impedance

pulse-forming line (PFL) is ideally suited for accomplishing this.

An ideal pumppulse for exciting the laser mediumis shown in Figure 2-I. For

the gas to break downin less than 20 ns, the initial overvoltaqe _eak VpK

must be at least 4 times the steady-state discharge votage VD.IO, I_ This is

inconsistent with an impedance-matchedtransmission line, which produces an

overvoltage ratio (VpK/VD) of only two. For optimum energy efficiency, it is

necessary therefore, to apply an additional high-voltage prepulse. A strong

pulse of ionizing radiation, overlapping with the rising and falling parts of

the prepulse, assures that the avalanche discharge develops into a uniform

glow.

-7-
VO.ID

PREPULSER

VOLTAGE

PEAK

SPATIALLY INIFORPVl

"_-__ PREIONIZATION REQUIRED

HERE

,!I

j ,I

PREPULSE (LOW CURRENT)

MAIN PULSE

VD

ID

%

|

PREPULSE VOLTAGE RISE < 10 nt

MAIN PULSE CURRENT RISE < 20 ns

t

FIGURE 2-1. IDEAL PUMP PULSE

(COMPILED FROM EXPERIMENTAL DATA IN REFS. I0 & 14)

I0

i

S

Vo

]

2

,i I

1hi

DISCHARGES

Xe/HCI • 30 |

!

XesHCI • lOj

, , .1.., .... ! .... , , , l,,,J

lO 100 lps lOIJs

FIGURE 2-2. AVALANCHE FORMATION TIME AS A FUNCTION OF

OVERVOLTAGE RATIO

-8-
For a PFL with output impedance Zo to provide a constant-V, constant-I pump

pulse, the laser head inductance LHD must be low enough to make the current

risetime, LHD/Z o, less than 25% of the pump pulse duration; otherwise the

inductance effects dominate over the PFL characteristics, in which case the

PFL might as well be replaced by a single capacitor. When this is done,

however, both discharge stability (limited pulse length) and the laser

efficiency will be compromised.

During the early development phases of high-energy discharge-excited RGH

lasers / , it was believed necessary for good discharge stability that the

voltage appearing on the electrodes must rise from zero to 4V D in I0 ns or

less. More recent experiments have shown, however, that the electrode voltage

can rise more slowly up to a value of 2VD, and that reasonably stable

discharges can then be initiated with a fast-rising high-voltage

prepulselO, ll, 12 or with a strong pulse of ionizing radiation 15, This has

made it possible to eliminate the previously required fast rail gap switches

in favor of combinations of thyratrons or thyristors 16 and magnetic switches.

The present laser system is switched by a thyratron, operated in the so-called

long-life radar mode, and saturable magnetic inductors are employed to

generate the high-voltage prepulse.

Whereas it is permissible to have voltage appearing on the electrodes prior to

the breakdown pulse, the duration and amplitude of such electric prestress

must be well to the left of the curve in Figure 2-2; i.e. significantly

shorter than the avalanche formation time at low overvoltages. Otherwise

discharge instabilities will prevent formation of a uniform glow and will

result in immediate arc formation. With smoothly contoured electrodes, corona

induced streamer formation may be delayed for several microseconds at an

electrode voltage between 1.5 VD and 2 VD. If the full charging voltage

appears on the electrodes, the PFN should preferrably be charged in less than

one microsecond, however. Alternatively, a PFN should be employed which

limits the electrode voltage prior to discharge initiation to less than

1.5 VD,

2.2 PROJECT OBJECTIVES AND RELEVANCE TO NASA GOALS

The objective of this work is the development of an advanced multi-joule

TE-laser source for possible application in an orbiting LIDAR system and for

industrial applications.

The design of the laser is based on the latest advances in excimer laser

technology for optimizing the laser efficiency (3% in the UV) and for high

reliability. The device incorporates X-ray preionization, prepulse

techniques, impedance-matched pulse forming lines, magnetic switching and

careful material selection. Special techniques are introduced for producing

stable and uniform discharges.

-9-
TABLE 2-1

ATMOSPHERIC SPECIES MEASURABLE WITH MULTIGAS PULSED

LASER SYSTEM

R(3H Version DF Version CO 2 Version

Specie Method Specie Method Specie Method

0 3

502

NO 2

ClO

OH

NO

Aerosols

Winds

DIAL

DIAL

DIAL

DIAL

LIF

LIF

MIE Scat

Incoherent

UV Doppler

HCI

CHt 1

N20

SO 2

H20

CO 2

Aerosols

DIAL

DIAL

DIAL

DIAL

DIAL

DIAL

MIE Scat

03

SO 2

NH 3

N2H 4

C2H 4

HNO 3

CCI 4

CH3CI

COCI 2

CFCI 2

CF2Cl 2

Aerosols

DIAL

DIAL

DIAL

DIAL

DIAL

DIAL

DIAL

DIAL

DIAL

DIAL

DIAL

MIE Scat

In addition to having improved tunability, frequency stability and efficiency,

the optical output pulse is expected to have the following unique

characteristics:

I. Excellent beam uniformity and mode stability.

2. Optical pulse length greater than lOOns with multi-joule pulse energy in

the excimer bands.

Pulse repetition rate will be 25 Hz. Component and system life is predicted to

be in excess of 109 pulses.

NASA's interest in this project is primarily concerned with the potential use

of this type of laser for remote sensing applications. The monitoring of

atmospheric processes, as well as measuring the concentrations of chemical

species and pollutants, is an important part of a well defined multidisciplinary information gathering network for improving our understanding of

the earth as a system. This global scientific program, planned by NASA for

the 1990s, is known as the Earth Observing System (EOS), and includes a

variaty of remote sensing techniques from orbiting platforms 17. Suitable,

highly efficient laser sources for space-based LIDAR systems have to be

developed now with established technology, in order to meet this time

schedule.

-I0-
Physical and chemical processes in the stratosphere are very complex and

include radiative, chemical and dynamic interactions. Photochemistry in the

tsratosphere begins with the transport of "source molecules", such as N20,

H20, CH4, CFCI3 and CF2CI 2 from the troposphere. There they are transformed

by photolysis and chemical reactions into highly reactive radicals, such as

ClO, NO, NO2 and OH. Ozone, 03, plays a special role in atmospheric

chemistry. Because it absorbs a high percentage of the solar UV, which is

harmful to biological organisms, it acts as a protecting layer and also as a

major soyrce of upper atmospheric heating. Species that are lost from the

stratosphere by transport into the troposphere, such as HCI, SO2 and HNO 3, may

precipitate out as acid rain.

It is interesting to notice by comparison with Table 2-I that essentially all

these species can be measured by DIAL or LIF measurements with one or the

other configuration of the type of laser developed here. For some species,

such as 03 and SO2, complementary determinations can be made in the UV and the

IR for greater accuracy.

Aerosols such as H2SO4 droplets (volcanic) and dust (volcanic and meteoritic)

may be monitored at several wavelengths. Complementary Mie and Raleigh

scattering data may support each other, It is also possible to employ an XeF

(350 nm) laser to measure wind fields in the lower stratosphere by incoherent

Doppler detection 18. A UV Doppler LIDAR may be able to monitor winds in

atmospheric regions where an IR Doppler system does not encounter sufficient

return signal from aerosol scattering 19. Scattering coefficients in the nearUV are orders of magnitude higher than in the IR.

With a somewhat more sophisticated LIDAR setup, the line shapes of absorbing

or fluorescing species may be analyzed to extract temperature and pressure

data.

2.3 UNIQUE FEATURES OF THE DISCHARGE-EXCITED MULTIGAS LASER

The new excimer laser developed here can operate on all the RGH bands (XeCl,

XeF, KrF, ArF) and has several unique features not generally found in

commercially available lasers. The laser has been designed for high

efficiency, long life, high reliability, long optical pulse and good beam

uniformity.

Optical output specifications are 1 to 2 joules per pulse at pulse repetition

frequencies up to 50 Hz. Optical pulse duration is I00 ns. Beam divergence

is close to diffraction limited (2 lambda/D) with unstable resonator optics.

System weight can be kept below 500 kg.

Some of the unique design features are:

I, X-ray preionization for more uniform discharge and less gas contamination.

2. Compact preionizer design with long-life corona plasma cathode,

3, Magnetically switched prepulse to permit impedance matching and high

electrical laser efficiency (3%).

-II-
4. Constant-current, constant-voltage, voltage-matched, long-pulse PFL to

energize the gain medium.

5. Extensive use of magnetic switches (saturable core reactors) to replace

the less reliable plasma switches.

6. Magnetic assist and inverse current blocking to extend the life of the

charging thyratron.

7. Compact gas flow system which minimizes gas required to refill the laser.

8. Careful selection of discharge chamber materials for long gas life.

3.0 APPROACH FOR INCREASING LASER EFFICIENCY AND RELIABILITY

Specific lines of approach taken to produce a laser which is efficient,

reliable and compact are discussed in moree detail in this section.

3.1 EFFICIENCY OPTIMIZATION

The author has shownlO, 14 in 1981 that efficient energy transfer to the

elctric discharge is not possible with a single-pulse excitation network.

Because of the constant-voltage load characteristics of the discharge,

impedance matching is essentially achieved by voltage matching; i.e. the opencircuit output voltage from the pulser has to be twice the steady-state

discharge voltage, V D. Unfortunately, at such an overvoltage ratio of two, it

takes microseconds for the discharge to develop, ample time for instabilities

to grow to a dominating magnitude. Avalanche formation times as a function of

overvoltage ratio have been plotted in Figure 2-2.

The traditional solution has been to charge the PFN to a higher voltage, so as

to obtain an overvoltage ratio of 3 or 4, which reduces the discharge

formation time to less than I00 ns. Under these mismatched conditions,

however, the power transfer is less than 50% efficient. This is the reason

for the relatively low overall efficiency (approximately I%) of most XeCl and

KrF lasers.

Operating a laser under mismatched conditions has a further diadvantage. The

energy, which is not dissipated in the discharge during the pump pulse, will

ring back (unless "crowbarred" into a resistive 1 ohm load) and produce "postdischarge arcing" in the laser. Needless to say, the life of the electrodes,

the capacitors and the thyratron switch, as well as the gas purity, will all

be compromised by such arcing.

By applying a high-voltage prepulse (of approximately 4 VD amplitude) to the

electrodes to establish the avalanche discharge in less than 20 ns, it is

possible to feed in the bulk of the excitation energy under matched conditions

from a PFL charged to 2VD. Well executed prepulse schemes, lO,ll applied to an

XeCl laser, have demonstrated that 4% efficiency can be reached.

In order to pulse one of the discharge electrodes to 4 VD, the electrode must

be temporarily isolated from the low-impedance PFN by an isolation switch. In

early prepulse work, 9,10,20 the prepulser was a separate pulse generator which

-12-
needed additional timing and trigger pulses. In the work reported here, the

prepulser has been integrated into the discharge PFN and needs no separate

timing. Electrode isolation has been effected in the past by multichannel

spark gaps lu, glow discharges 12, saturable pulse transformers 9, and saturable

magnetic inductors 20. Whereas a magnetic switch is the ideal isolation

device, it was not clear at the onset of this work that a high-frequency, fast

magnetic switch with a high V.t product and a very low saturated inductance

could be constructed to suit this demanding application. It was for this

reason that we chose to eliminate the need for the isolation switch by

adopting a three-electrode structure as proposed in the Phase I Final

Report 21. Having to deal with two tandem discharges brought with it an extra

level of complication, however; a subject which is discussed in more detail in

Section 6.

3,2 MINIMUM STRESS DESIGN (RELIABILITY AND LIFETIME)

Early component failure in an electrical device is generally caused by

excessive electric stress; i.e. the components are operated too close to their

design limitations. Every effort has been made in the design of the present

laser to reduce such stresses to a more comfortable level. The elimination of

post-discharge arcing, by methods discussed above, has been a major factor in

extending the potential life of the system.

A saturable magnetic core, the heart of a magnetic switch, has practically no

failure mode and ,therefore, has a potentially very long life. Wherever

possible, magnetic switches have been employed for initiating or generating

fast changes in voltage or current. Conversely, they have also been utilized

for blocking voltage surges, which might damage susceptible components. The

only plasma switches in the system are rugged high-voltage ceramic thyratrons

which are operated in the "long-pulse Radar mode". Peak currents and rate of

current rise (dl/dt) have been kept well below the design specifications of

the thyratron tubes. In the pulse charging circuit, a saturable magnetic

inductor has been added to delay the thyratron from experiencing high values

of dl/dt until the thyratron has safely passed through its resistive turn-on

phase. This inductor also acts as a diode and isolates the thyratron from

inverse voltages generated by the laser discharge PFN.

To avoid early capacitor failure, the ceramic capacitors in the PFL are

stressed to only half their voltage rating. The discharge electrodes and

high-voltage feedthroughs have been shaped so as to reduce corona formation at

the "triple points", where conductors, insulators and the laser gas come

together. A semi-empirical study of the electric field distribution in the

discharge region (Section 5,1) has been of significant help in eliminating

high field areas on the insulator which separates the high-voltage cathode

from ground.

Preionizing the gas with X-rays instead of UV sparks eliminates the erosion of

spark electrodes and the accompanying contamination of the laser gas. Since

X-rays can ionize the discharge volume more uniformly, resulting in a more

uniform discharge, the erosion of the main discharge electrodes is also

reduced.

-13-
A similar low stress approach has been applied to the mechanical and optical

components of the system. For example, the pressure vessel has been designed

to safely withstand a pressure of 6 atmospheres with a safety factor of 4.

Similarly, the cross section of the optical gain volume (3.5 cm X 3.5 cm) has

been chosen large enough to keep the power density and energy density on the

laser optics well below recognized damage thresholds.

Serious consideration has been given to the corrosive environment (laser gas

may contain 0.1% HCI or F2) that the laser chamber materials are exposed to.

To keep chemical deterioration to a minimum, only stainless steel, nickel and

Kynar have been used in the construction of the discharge chamber, the

electrodes, the insulators, the heat exchangers and the gas blower. Pressure

and vacuum seals have been made with Viton O-rings to keep the cost of the

laser down. However, these can easily be replaced with superior Kalrez

(Dupont TM) O-rings if necessary.

3.3 COMPACTNESS

A cylindrical shape has been chosen for the discharge chamber for compactness

and for strength. The gas is recirculated within this circular chamber by a

tangential blower. A small cylindrical X-ray preionizer has been fitted into

the core of the chamber. By choosing this shape for the pressure vessel, the

discharge chamber weight could be kept to a minimum. A relatively thin shell

of stainless steel can easily support 5 to 6 atmospheres of pressure, as well

as full vacuum loading. The chamber is 0.4 m in diameter and 0.8 m long.

Refer to Figure 4-3 for an overall view of the system.

The PFN is contained in an oil-filled aluminum tank situated below the

discharge chamber. Overall system height is approximately 1 m, and total

weight approximately 500 kg. Separate oil-filled boxes have been used for the

pulse charging unit and for the X-ray pulser. An improved version of this

laser will have the pulse charging circuit designed into the PFN tank.

Whereas transformer oil has been used here as the insulating liquid for

economical reasons, a space qualified system would use "Fluorinert C-40"

fluorocarbon liquid instead.

We anticipate that with some redesign the output performance of the laser (at

308 nm) can be scaled up to 4 J per pulse and 50 Hz pulse repetition frequency

(PRF) without an increase in size (see Section 9.1).

-14-
4.0 EXCIMERLASERCONSTRUCTION

As form follows function, we will first discuss the functional relationships

and design parameters, and then present the details of the electricl,

mechanical and optical construction of the laser subsystems.

4.1 FUNCTIONAL SYSTEM OUTLINE

A fully operational excimer laser is itself a rather complex piece of apparatus, consisting of many functionally interrelated components (Figure 4-I).

The heart of the laser is the electric discharge passing transversely through

the gain volume, which is the source of the stimulated optical emission. The

latter is converted into the laser output beam by the optical resonator.

Electric power is taken from the supply network (240 V - 3 Phase) and is

converted into a series of excitation pulses of a desired shape (Figure 2-I)

in three stages. The electric energy flow is as follows: A gateable switched

charging supply, operating at 16 kHz, charges upon command a long-life highvoltage capacitor (Maxwell, oil-filled aluminum foil capacitor) to a voltage

which is adjustable between I0 kV and 40 kV. Normal operating voltage for the

laser is 35 kV. A triggered thyratron then transfers this energy to the PFL

in a time period between 1 and 2 microseconds. A saturable core reactor, LIS,

acts as a thyratron assist and as an inverse blocking diode to protect the

thyratron. A second (and in some cases a third) magnetic switch generates the

high-voltage propulse which initiates the electric discharge. The duration of

the excitation pulse is controlled by the "electrical length" of the PFL, here

approximately 140 ns.

Before and during application of the electric prepulse the laser gas is

preconditioned by an intense ionizing X-ray pulse. A separate thyratron

switched pulse generator, energized from a 30 kV DC supply, powers the e-gun

which generates the X-rays. A variable delay and trigger unit controls the

sequence of events. At to the switched charging supply is activated and

allows charging of the Maxwell capacitor. Forty milliseconds later the

discharge sequence is initiated by triggering the thyratron in the pulse

charging unit with trigger pulse tI. The energy transfer to the PFL is then

controlled by magnetic switch LIS. After a predetermined time, magnetic

switch L2S saturates and generates the high-voltage spike which starts the

discharge. A second trigger pulse, t2, which triggers the X-ray pulser, has

to be timed precisely relative to t I, in order for the X-ray pulse to coincide

with the prepulse.

The gas between the discharge electrodes has to be cleared out and replaced

with fresh gas between pulses to remove temperature non-uniformities which can

lead to arcing. A tangential fan blows the gas through the discharge gap in a

transverse direction and passes it through a gas-to-water heat exchanger,

where the heat added to the gas by the discharge is removed. The blower is

driven by an external electric motor via a magnetic coupling, which makes a

hermetic seal with the chamber. Figures 4-2 and 4-3 show the physica_

location of components.

-15-
to

ACTIVATE !_ SWITCHED

CHARGING

SUPPLY

INHIBIT 25 TO 35 KV

CONTROL

VARIABLE FREQ.

PULSE GENERATOR

VARIABLE

TRIGGER PULSE DELAY

GENERATOR &

DELAY UNIT VARIABLE

DELAY

VARIABLE

DELAY

THYRATRON

SWITCHED

PULSE

CHARGER

tl = to + 40 ms

PFL

t2 J _r _-ELLCTRIC U_ OPTICAL POWER

DIAGNOSTIC _ _ DISCHARGE OUT

J _ ml PREIONIZER J

I CATHODE PULSE _ -,

OSCILLOSCOPE

THYRATRON

SWITCHED

X-RAY

PULSER

t

20 TO 30 KV I

DC SUPPLY

IHEAT

EXCHANGER_--_ '' BLOWER I

f

J' MAGNETIC COUPLING I

FIGURE 4-I. FUNCTIONAL OUTLINE OF SYSTEM COMPONENTS

-16-
INTERMEDIATE

CATHODE

FIGUI_[ 4-2

C_O_- SECTION TNROUG_4

DI%CWA_G_ c_4_ _SE _

_fz

PFL

CAP AC I TO R

MAGNETIC

SWITCH

-17-
AND POWERCONTROLS SUPPLIES

i

LASER

CHAMBER

X-RAY UNIT

PFN TANK D

OPTICAL

BENCH

FIGURE 4-3. OVERALL VIEW OF EXCIMER LASER SYSTEM

-18-
4,2 DISCHARGE PARAMETERS

A typical XeCl laser mixture contains between 0.5 and I% of xenon and I/lO of

that amount of HCI in a diluent of neon. The measured steady-state discharge

field, normalized by the gas pressure, for such a gas mixture is approximately

givenlO by E/p = 0,8 kV/cm-atm. The design of the discharge electrodes and

the choice of discharge parameters is strongly influenced by this E/p-value

and by the requirement that the electric pump power density should be close to

1MW/cm3. The gas mixture and pump power density quoted are near optimum

values, based on electron kinetic and chemical kinetic studies. Under optimum

conditions approximately 14% of the applied pump power goes into the

production of XeCl* upper-lasing-state species. The gain length is determined

by considerations of optical extraction efficiency, and frequency and mode

control. Following is a list of design parameters for the present laser:

Total System Input Power

Energy Stored in PFN & Switched into Discharge

Pulse Repetition Frequency (PRF)

Pump Pulse Duration

Charging Voltage

Steady-State Discharge Voltage

PFL Capacitance

Pump Power

Discharge Impedance & PFL Output Impedance

Main Discharge Gap

Pre-Discharge Gap

Discharge Width

Gain Length

Discharge Volume

(I.0 1 incl. pre-discharge gap)

Energy Density

Power Density

Gas Pressure

Typical Gas Mixture

Steady-State Discharge Field

= 2.5 kW

U = 80 J

f = I0 to 25 Hz

t E = 140 ns

VCH = 35 kV

VD = 16 kV

C = 140 nF

P = 570 MW

RD = 0.5 ohm : VD2/P

d = 3.5 cm

d' = 1.2 cm

w : 3.5 cm

1 = 60 cm

V' = 0.74 1

UlV' = lO0 Jll

P/V' = 0.8 MW/cm 3

p = 5.5 atm

Ne:Xe:HCI:H 2 = 99.5:0.5:0.05:0.02

E/p = 0.75 kV/cm-atm

-19-
4.3 DISCHARGE ELECTRODES AND FEEDTHROUGHS

For reasons outlined before, a three electrode geometry was chosen to make it

easier to apply a high-voltage prepulse. The electrodes consist of a primary

cathode, an intermediate cathode (screen) and the anode. The predischarge gap

between cathode and screen is approximately one third of the main discharge

gap between the screen and the anode. A cross-section through the electrode

structure, including the PFN feedthroughs, is given in Figure 4-2. The

cathode and the screen are supported from a flat partition plate, consisting

of a structural stainless steel plate, with ribs to support the chamber

pressure, and a Kynar insulator. The Kynar insulator has been machined out of

a solid I0 cm X 30 cm X 120 cm block. The cathode is connected to the center

plate of a double transmission line via four oval stainless steel

feedthroughs. Access to the screen is provided by eight similar feedthroughs,

four on each side. A view of the underside of the partition plate, showing

the twelve Kynar insulated feedthroughs, is presented in Figure 4-4.

The contours of all electrodes are constructed from flat sections and

elliptical curves and closely approximate compacted Chang 22 profiles. The

cathode is an inductively ballasted electrode, consisting of a stack of

several hundred thin stainless steel fins, held together by two stainless

steel rods, running end to end (Figure 4-5). The fins (Figure 4-6) have been

chemically etched out of 0.13 mm thick sheet. The screen electrode

(intermediate cathode) has been formed from 0.6 mm thick stainless steel sheet

(Figures 4-2 and 4-7). It has a fine hole pattern chemically machined along

its center, in the discharge region, to allow UV radiation to pass from the

pre-discharge gap to the main gap and vice versa. A coarse hole pattern has

been etched into the sides of this electrode (Figure 4-7) to permit the gas

flow to pass between cathode and screen.

The anode is supported from the central support structure of the discharge

chamber which holds the X-ray generator (Figure 4-2). It has been machined

out of solid stainless steel and has a window along its center, so that X-rays

can pass into the discharge region to preionize the gas. The anode is covered

with a thin nickel plated aluminum sheet, 0.8 mm thick. The X-rays have to

pass through this sheet, as well as through two similar aluminum windows. The

additional X-ray windows are used to support the pressure differences between

the laser gas (5 atm) and the ambient atmosphere, and between the atmosphere

and the vacuum of the X-ray chamber. There is no pressure load on the anode

skin. The combined thickness of the X-ray windows is less than 3 mm, and it

is estimated that less than 35% of the X-ray flux is absorbed in these

windows, Since there exists a concern about the lifetime of the 50 micron

thin nickel coating on the anode skin, this skin is easily replaceable with

other experimental window materials, e.g. an uncoated 0.6 mm thick titanium

sheet. The anode remains at electrical ground potential. Fine-mesh nickel

wire screens, connecting the anode to the discharge chamber on both sides, act

as current return connections and as gas flow conditioners.

In operation, with the original prepulse scheme, the small gap between cathode

and screen would break down first, generating a voltage pulse on the main gap

with the aid of the closely coupled prepulse capacitors (Figure 4-5). Once

-20-
Z

0

-ri,i

t.--

__./

Cu.

Z

O_

_rF

O-I--

I__I l_l.J

I--"

__JILl

0

O_ I.I_I

D---

O--J

I

,,I

r_

LI_

-21 -
PREPULSE

CAPACITORS

KYNAR

INSULATOR

/

FIGURE 4-5,

CATHODE INSTALLED ON PARTITION PLATE

(ST. STL. SUPPORT PLATE AND KYNAR

INSULATOR)

FIGURE 4-6. CATHODE FIN (ACTUAL SIZE)

-22-
FIGURE 4-7, SECONDARYCATHODE(SCREEN) INSTALLED OVER

PRIMARY CATHODE

-23-
the main gap ignites, the discharge current has to pass through both gaps in

series. During this phase, the power density deposited in the pre-discharge

gap is very high. The inductive ballasting built into the primary cathode was

intended as an aid to keep the discharge in the small gap evenly distributed

along its length. Intense UV radiation from the "pre-discharge" was then

meant to help sustain the main discharge for extended pump periods.

Furthermore, the early phase of the predischarge was to be utilized as a

possible gain medium for a master oscillator for injection locking the power

oscillator built around the main discharge. As will be discussed in Section

6.1 this scheme has been abandoned for the time being in favor of a direct

excitation of the main gap.

4,4 PULSE FORMING NETWORK

The attainment of a smooth, stable, steady-state discharge voltage, VD, for

the duration of the power pulse is extremely important for achieving high

frequency stability and optical mode stability. The required constant-voltage

excitation pulse can best be generated with a distributed-impedance pulse

forming line (PFL). Some of the best results have in the past been obtained

with water transmission lines.7,10, II Such lines would have disadvantages for

space and airborne applications, however. For a 120 ns pulse, the PFL would

have to be 2 m long, so that it could not easily be incorporated into a

compact system. Also, special precautions would have to be taken to keep the

water from freezing. We have, therefore, chosen to work with a double transmission line PFL constructed from barium titanate ceramic capacitors. The

physical construction of the PFL is discussed hereunder, the electrical

configurations of the PFL and prepulse circuits are discussed in more detail

in Sections 5 and 6.

The ceramic disc capacitors were purchased unencapsulated, but polarized and

metallized, from the Ceramite corporation. Rounded brass corona discs were

then attached by oven-soldering with a eutectic tin-bismuth alloy (melting

point 130o C) for reasons outlined in the Phase I Report 21. Each unit was

constructed by soldering two 2.2 nF, 30 kV discs in series, with a corona disc

on each end and one in the center (Figure 4-8). These capacitor units were

then assembled into eight parallel-plate transmission lines, each two

capacitors wide and eight capacitors long. The capacitors were sandwiched

between 0.5 mm thin soft copper sheets. Low-impedance connections were made

by drawing the copper "foil" into the capacitor end caps with brass clamping

discs (Figure 4-2). The edges of the copper sheets were electrically "graded"

by encasing the sharp edges in resistive (approximately lO0 ohm-cm) siliconrubber tubing. The extruded tubing was slit along its length and cemented

around the copper edge with conductive epoxy. The eight ceramic PFL sections

were arranged in a double-transmission line configuration with an output

impedance of 0.5 ohm. The center plates were attached to the cathode

feedthroughs and the outside plates to the stainless-steel partition plate

(electrically connected to the anode). The prepulse circuits, containing

several magnetic switches were attached to the screen cathode (Figure 4-8).

-24-
-PFL

FIGURE 4-8. PULSE FORMING LINE WITH MAGNETIC SWITCHES

INSTALLED

-25-
The entire PFN has been immersed in electrical-grade insulating oil, which is

contained in a rectangular reinforced aluminum tank (Figure 4-3). The tank,

which is sealed to the bottom of the discharge chamber, is filled by first

evacuating the tank of all air, then backfilling with oil which has been

thoroughly outgassed under vacuum.

4.5 PULSE CHARGING UNIT

The pulse charging unit has been contained in a separate electrically shielded

aluminum tank, which is also filled with transformer oil. It contains a highvoltage EG&G LS-4011 thyratron, a long-life, high-PRF 160 nF Maxwell

capacitor, and a saturable magnetic inductor, which acts as a thyratron assist

and protective diode (Figure 4-9). The thyratron is a 40 kV, I0 kA, doublegrid ceramic tube, one of a new series of ruggedized thyratrons made

especially for high energy laser excitation. The saturable inductor has three

electrical turns (originally 5 turns), wound on a 2605-C0 Metglas core with a

V.t-product of approximately 8 mV-s. Also contained in the pulse charging

tank are an RC snubber circuit to help in the commutation of the thyratron, a

1 megohm bleeder resistor, two I00 ohm supply ballasts, a high-voltage diode

to isolate the core reset circuit, the thyratron grid drive circuit and the

transformers and regulators to supply the heater and reservoir voltages (6 V

AC). A co-axial high-voltage feedthrough connects the pulse charging unit to

the PFN. This feedthrough, and connector, has been constructed such that the

charging tank and the PFN tank can be disconnected and physically moved apart

without draining the oil out of either tank.

In operation, the 160 nF capacitor is command-charged from an A.L.E. 302L

high-voltage, 3 kW switching supply and an A.L.E. 302S, 3 kW slave unit,

connected in parallel, in approximately 30 ms. After being fully charged, the

energy stored in the capacitor (Maxwell) is transferred to the PFL by

triggering the thyratron. The trigger pulse is derived from a Stanford DG-535

pulse/delay generator and is amplified to 1 kV before being applied to the

thyratron grids, Several metal-oxide varistors (MOVs) protect the pulse and

delay generator from high-voltage transients. The saturated inductance of the

Metglas inductor (with three turns) is 800 nH and permits the energy to be

transferred to the PFN in approximately I,I microsecond. Before triggering

the thyratron, the core of the saturable inductor is reset by a pulse from a

core reset circuit described in Section 6.3. The arrangement of components in

the pulse charging tank is illustrated in Figure 4-10.

4.6 X-RAY PREIONIZER AND PULSER

A novel long-life, large-area X-ray generator has been developed here to

provide an intense pulse of non-contaminating ionizing radiation for

preconditioning the laser gas. The high-current-density (space-charge

limited) e-gun has been designed around a set of rugged, reliable and longlife electrodes, The electron source at the cathode is a corona plasma formed

at the interface between a conductor and a high-permittivity dielectric, The

-26-
ALE-302L

SWITCHING

SUPPLY

ALE-302S

SWITCHING

SUPPLY

SLAVEUNIT

CORE

RESET

PULSE/DELAY

GE_mmA_nDR

TRIGGER

PLEBE

AMPLIFIER

I00

i00

50 kV

DIODE

OIL

METGLAS

S_IND.

160 _ i 1 M

i00 W

T

EG&G

LS-4011

G2

200

I:8

%50V

MOV

I

t

_2.5 nF

160

I

i iTO PFL

I _IRSX]PPLY HEATER AND ]

FIGURE 4-9. SCHEMATIC OF PULSE CHARGING UNIT

-27-
!

FIGURE 4-10. TOP VIEW OF PULSE CHARGING UNIT

SHOWING LOCATION OF COMPONENTS

-28-
anode and e-beam target is liquid-cooled, emits in the more efficient backward

direction and is shaped to concentrate the X-ray flux toward the X-ray

window.

Efficient, large-area X-ray preionization of high-energy lasers requires

electron guns with space-charge limited e-beam current densities in the range

of 1 to I0 A/cm2 and short pulses of less than 1 microsecond duration. A

number of cold-cathode guns have been developed to fill this need. However,

most of these devices suffer from limited cathode life (103 to 107 pulses).

Electrically pulsed, self-sustained discharge excited, high energy gas lasers

(e.g. XeCI, KrF, XeF, C02, HgBr lasers) need uniform preionization of the

high-pressure gas mixture for discharge uniformity and stability. For optimal

laser efficiency and laser beam uniformity, the preionization level should be

between 108 and 109 electrons per cm3, Preionization uniformity should be

within +_ 20% throughout the discharge volume; hence, X-ray uniformity should

also be within 20 per cent.

Available e-guns differ primarily in the manner in which electrons are

liberated in sufficient quantity at the cathode. Hot cathodes (e.g. thoriated

tungsten) require hard vacuum, large heater power, and for thermionicall_

limited emitters, have insufficient current density (less than I00 mA/cmZ).

Cold field-emission cathodes (sharp blades, needles, carbon felt) are simple

and rugged, but exhibit very non-uniform emission characteristics after I0_

pulses (sometimes after 103 pulses). A carbon felt cathode design with a

control grid has recently been patented23. Such a cathode, however, is not

suitable for pulse repetition rates above a few hertz 24,

On!y so-called plasma cathode guns have demonstrated lifetimes in excess of

108 pulses, when operated in the short pu!se (less than one microsecond) and

high current density (greater than 1A/cm _) regime. With proper anode

cooling, plasma cathode guns can also be run at high PRF (above I00 Hz).

In 1982 a corona plasma cathode was developed by Helionetics 25 (HLX Laser,

Inc.), which had a cathode consisting of a tungsten wire coil wound around a

glass tube. The anode was a "forward emitting" gold foil target at ground

potential. When a high negative voltage pulse (-I00 kV accelerating

potential) was applied to the wire coil on the cathode, the electric field

between it and a grounded conductor inside the glass tube initiated a surface

glow on the outside of the dielectric tube. This corona plasma served as the

electron source. Because no high DC voltages had to be sustained by the

system, this type of gun could be made quite compact. This corona cathode has

a more uniform emission 24 than a carbon felt cathode 23 and has a potential

life of more than 108 pulses 25. Pulse rates as high as 200 Hz are possible

with this type of gun, although cooling becomes a problem. Whereas the HLX

gun represents a significant advance over the then existing state of the art,

it suffers from several serious drawbacks:

I. The cathode is subject to catastrophic failure due to cracking or eTectric

breakthrough of the highly stressed pyrex tube.

2. Non-uniform erosion of the dielectric tube and the thin fragile tungsten

wire tends to make the electron emission less uniform as the tube ages.

-29-
3. The device relies on the less efficient X-ray emission in the forward

direction from a high-Z foil (anode).

4. Anodecooling becomesa problem at high PRF.

The X-ray preionizer developed here has a more rugged corona plasma cathode

and has all the advantages of the HLXgun, but none of the above limitations.

The salient faetures of the Lasertechnics gun are:

I. Compactdesign. The gun is contained in a 14 cmdiameter cylinder.

2. Ruggedcathode design, The corona plasma is produced at the interface

between a high-permittivity dielectric and a thick nickel structure.

3. The cathode is not subject to catastrophic failure. The dielectric is not

used as an insulstor to support high voltages.

4. The cathode structure has enough bulk for ablation to permit the

generation of I010 corona plasmas. The life of the cathode is not limited

by the material in the cathode.

5. A central liquid-cooled anode, suitable for high PRF, acts as the electron

target, the X-rays being emitted in the more efficient "back direction".

6. The X-ray emitter is a thick, high-Z metal coating (e.g. I00 microns of

gold), which makesgood thermal contact with the cooled anode body.

7. The emitter surface is shapedso as to preferentially direct ("focus")

the X-rays toward the X-ray window.

8. The gun chamberdoes not require high vacuumlevels. Background pressure

may be as high as 5 X 10-4 tort.

9. No high DCvoltages are employed. The anode is driven to 90 kV in a

250 ns pulse, while a I0 kV pulse is applied to the cathode to generate

the corona plasma.

I0. The anode and cathode pulser circuits are switched by the samethyratron,

requiring only one timed trigger pulse.

Figure 4-11 shows a schematic of the X-ray generator and pulser. The e-gun

electrodes are enclosed in a cylindrical vacuumchamber, with the high-voltage

anode running along its center. The chamberwalls are madeof relatively

heavy (6 mmthk.) stainless steel and help to shield operating personnel from

stray X-rays. The anode pulse, used to accelerate the electrons, and the

cathode pulse, used to create the corona plsma, are generated simultaneously

by the pulser shownschematically in Figure 4-11. Approximately 5 J of

electric energy are stored in the PFN, when it is charged to 25 kV from a DC

power supply. The PFNconsists of a series of capacitors and inductors

configured to produce a 250 ns (FWHM)near-rectangular pulse, when the

thyratron switches one side of the PFNto ground. A 1:4 pulse transformer

serves to step up the voltage from 25 kV to I00 kV, which is then applied to

the anode via the high-voltage feedthrough at one and of the vacuumchamber.

The pulse transformer has been woundon a set of 32 Ferroxcube 3C8 toroidal

ferrite cores with a multifilar winding to keep the leakage inductance to a

minimum. The large numberof cores were necessary to eliminate core

saturation effects, since only three primary turns and twelve secondary turns

of high-voltage cable could be fitted through the cores (3.7 cm ID). The

cathode pulse is produced simultaneously with the anode pulse by switching a

small capacitor, containing less than one joule of energy, to ground by means

of the samethyratron. A second I:I pulse transformer inverts the polarity of

-30-
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-31 -
the cathode pulse from minus to plus. The duration and shape of this pulse is

modified by an LR cicuit in such a way as to provide a high dV/dt across the

cathode dielectric at the same time when the anode voltage is at a maximum.

The cathode (Figure 4-12) consists of a series of bare barium-titanate discs

sandwiched between a conductive bar and a 0.5 mm thick perforated nickel

sheet. The nickel sreen is in intimate contact with one surface of the highpermittivity dielectric. The edge of the screen is protected by a field

grading metal bead. The assembly has been potted in epoxy, except for the top

surface of the dielectric and the nickel screen, which remain exposed. The

metal screen is electrically grounded. When the cathode pulse of +lO kV to

+15 kV is applied to the backstrap, a high electric field is created between

the dielectric surface and the screen. This field is highest at the "triple

junctions" where metal, ceramic and vacuum meet, i.e. around the edge of the

perforations in the nickel sheet. As a consequence, a corona plasma forms in

these areas.

Whereas the corona plasma may be initiated by field emission, the surface glow

is maintained by a number of electron emission processes. Positive ion

bombardment and photoelectric emission certainly play an important part. The

corona plasma, created from material desorbed and ablated from the dielectric

surface, rapidly spreads over the nickel screen and provides a source of

positive ions, which are pulled into the nickel surface to generate sufficient

secondary electrons to maintain a space-charge limited electron beam. No

corona is formed between the backstrap and the dielectric, because the back

surfaces of the barium-titanate discs are metalized, and the backstrap is

solidly connected to these metal films.

The anode consists of a brass rod, having high thermal conductivity. It is

located near the center of the gun chamber, so that the electric field focuses

the electrons onto the anode (Figure 4-13). Copper cooling tubes have been

provided for removing heat from the anode by means of a suitable cooling

liquid (deionized water, oil, freon). The electron target area has been

plated with a 50 micron thick gold film for efficient X-ray production.

X-rays are emitted in a flat cosine distribution, resulting in a widely

diverging X-ray flux, if the target area was flat. A fair degree of

collimation has been achieved by making the target area concave, as shown in

Figure 4-13. In order that the accelerating field not be disturbed by the

concave target surface, a thin wire screen with more than 80% transmission has

been wrapped around the cylindrical anode.

Measured X-ray generator pulse shapes are reproduced in Figure 4-14.

Electrode length is 75 cm and the anode-cathode spacing is 5 cm. The anode

voltage pulse (FWHM = 250 ns) reaches a peak voltage of 90 kV. The e-beam

current has a pulse width of approximately 250 ns and reaches 300 A at the

peak. Peak power of the X-ray flux is approximately 150 kW. Values of

measured currents and voltages indicate that the e-gun is operating close to

the space-charge limited regime.

-32-
CATHODE

SCREEN

HIGH-£

DIELECTRIC

FIELD GRAD[

BEAD

METAL

BACKSTRAP

BARIUMTITANATE

DISCS

+i0 kV TO

+ 20 kV NICKEL

SCREEN

)RONA

FORMATION

TOP OF

DIELECTRIC

ENLARGED VIEW

OF METAL SCREEN

FIGURE 4-12. E-BEAM CATHODE DESIGN

-33-
COPPER

CYLI

COPPER

TUBES

WIRE

SCREEN

GOLD

TARGET

FIGURE 4-13. E-GUN ANODE DESIGN

-34-
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150 kW ,._

X-RAYS /I

POWER

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X-RAY

FIGURE 4-14. X-RAY GENERATOR PULSE CHARACTERISTICS

-35-
4.7 TRIGGER AND DELAY CIRCUITS

The system contains two thyratrons which need to be triggered at specified

times t I and t 2 relative to a reference time t O. These pulses are supplied

and controlled by a Stanford DG-532 digital delay generator with 4 output

channels (Figure 4-I).

The A.L.E. switched charging supplies have been wired so as to normally

inhibit charging. At time to, a pulse is sent to a bistable multivibrator

circuit which generates a 25 V bias to override the inhibit signal, thereby

activating the charging supplies until the bias is removed by signal t I, The

combined power of the A.L.E. 302L and 302S supplies (6 kW) permits charging

the 160 nF storage capacitor in approximately 25 ms. At time tl, generally

set for t O + 40 ms to allow operation at PRFs up to 25Hz, a trigger pulse is

sent out to fire the thyratron in the pulse charging unit. After being

amplified to 150 V, this trigger signal is stepped up to 1 kV in a 1:8 pulse

transformer to trigger both grids of the thyratron (Figure 4-9). At the same

time, the bias is removed from the inhibit circuit of the switched charging

supply, preventing further charging.

After approximately another 1.5 microseconds, a second pulse, t2, fires the

thyratron in the X-ray pulser, after having been amplified to 500 V. Pulses

t I and t2 have to be timed with an accuracy of 50 ns, so that the X-ray pulse

overlaps with the prepulse (Figure 2-I). The time interval between t I and t2

is determined by the transfer time of charge from the charging unit to the

PFL, the V.t-products of magnetic switches LIS, L2S, L3S, the degree of reset

applied to the cores of these magnetic inductors, and the inherent turn-on

delay times of the thyratrons. To position the X-ray pulse correctly in time

to coincide with the prepulse, the voltage on the laser "screen" electrode is

monitored with a GHz oscilloscope. The screen voltage and the e-beam current

pulse are displayed alternately, and the timing of t 2 is adjusted to bring the

e-beam current into coincidence with the prepulse. The timing of pulse t 3

(Figure 4-I), which is used to trigger the diagnostic oscilloscope, can be

adjusted to permit viewing of the relevant portions of the screen electrode

pulse with a I00 ns/cm sweep speed.

4.8 GAS FLOW AND COOLING

Operation of the laser at PRFs exceeding 1 pulse per second requires the gas

to be circulated within the cylindrical discharge chamber (Figures 4-I, 4-2,

4-3). The gas sweeps out the discharge volume in a transverse direction, then

passes through a set of heat exchangers to remove the heat added to the gas by

the electric discharge. The design of the components for handling and

conditioning the laser gas are dicussed in this section.

Tangential Blower

A stainless steel tangential blower, 7.6 cm in diameter and 80 cm long, has

been designed and built to suit the requirements of this particular laser.

-36-
The impeller blades and support discs have been welded together to form a

rigid structure capable of sustaining rotational speeds of 3000 RPM without

excessive distortion. The impeller, which consists of two sections, has a 9.5

mm shaft and is supported by three ball bearings, one at each end and one in

the center. The unit was bench tested with special silicon nitride ball

bearings at speeds up to 3000 RPM, at which point one of the bearings cracked.

The damage is believed to have been caused by excessive vibrational stresses.

Silicon nitride bearings were originally selected, because they can run at

high speeds without lubrication. Also the material is chemically inert to

HCl, although some concern exists about the compatibility of silicon nitride

with fluorine. Even though the bearings failed during test, we believe that

silicon nitride bearings can be made to work with the following precautions

and modifications:

I. The present application requires more robust bearings (for 12.7 mm diameter

shaft or larger), perhaps roller bearings instead of ball bearings.

2. With a three-bearing design, the bearing mounts have to be perfectly

colinear to within I/I0,000 inch (2.5 micron).

3. The impeller sections and the magnetic drive have to be perfectly

dynamically balanced.

To make the impeller in one section, supported by only two bearings, would

help a great deal. Because of time and money constraints in the present

program, a redesign of the blower and mounts was not feasible. Instead, the

silicon nitride bearings were replaced with Krytox-lubricated stainless steel

bearings, which have performed satisfactorily throughout this program. A view

of the blower assembly installed in the discharge chamber is given in Figure

4-15.

Magnetic Coupling

The magnetic coupling permits the blower to be driven with an external motor

without the use of troublesome shaft seals. The torque necessary to drive the

impeller is transmitted from a permanent-magnet driver to a permanent-magnet

rotor through a hermetically sealed non-metallic partition cylinder

(Figure 4-16).

Both the driven and the driver assemblies consist of I0 magnetic poles. The

magnets are samarium cobalt magnets with an energy product of over 20

Megagauss-Oerstedt (Recoma Magnets). The coupling has been designed to

transmit a torque of 20 N-m without slippage. At speeds between 2000 and 3000

RPM, the coupling is able to transmit several horsepower. Hence the coupling

is much stronger than required for the present application. Future lasers can

be equipped with a considerably smaller and cheaper coupling.

The magnetic coupling was originally designed with silicon nitride bearings

and an alumina partition cylinder. For reasons outlined in the "gas blower"

section above, the bearings have been replaced with Krytox-lubricated

stainless steel bearings. Similarly, a Kevlar-reinforced Kynar partition

cylinder has been subtituted for the brazed alumina cylinder, which had

fabrication problems. The thin Kynar cylinder has so far stood up very well,

both under pressure loading (5 arm difference) and under full vacuum.

-37-
BLOWER

ASSEMBLY

ANODE

SUPPORT

X-RAY

WINDOW

FIGURE 4-15. BLOWER INSTALLED IN DISCHARGE CHAMBER

(NOTE SUPPORT STRUCTURE FOR X-RAY GENERATOR)

-38-
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The magnetic coupling is driven by a variable-speed I/4 HP DC electric motor

via a single V-belt. The I/4 HP motor is able to rotate the blower assembly

in a 6 atm gas mixture up to 2000 RPM. A larger motor will be necessary to

increase the blower speed to 3000 RPM.

The magnetic drive is connected to the blower assembly by a flexible coupling.

Low-torque couplings (0.5 N-m) failed after several hours of operation. A new

high-torque bellows-type coupling has stood up well for the duration of this

program.

Heat Exchangers

Two custom designed 8 kW heat exchangers have been built and installed into

the laser discharge chamber (Figure 4-17). The units were constructed from

12.7 mm OD stainless steel tubing (I0 per unit) and 25.4 mm diameter copper

fins, which were silver brazed to the stainless tubes. After the heat

exchangers were built and leak tested (by Voss Industries), the entire units

were nickel coated by an electro-less nickel coating process.

Each heat exchanger unit has been built to the following specifications:

Heat exchange capacity:

Total surface area:

Nominal gas flow:

Pressure drop at nominal gas flow:

Gas inlet temperature:

Gas outlet temperature:

Water flow rate:

Water inlet temperature:

Water outlet temperature:

8 kW

9.7 m2

280 I/s (600 CFM)

90 P (0.32 inch H20 )

39.5o C

38.5o C

0.4 I/s (6 GPM)

15.5o C

20.6o C

Gas Handling Equipment

During initial pump-out, outgassing and leak checking, the laser chamber was

evacuated with a high-pump-rate cryopump, which was connected to the 7.6 cm

valved pump port at the side of the discharge chamber (Figure 4-3).

Routine changing of the laser gas is performed with the aid of a pump and gas

mixing station (Figure 4-18). Generally the laser gases are mixed in the

discharge chamber. Partial pressures and total pressures are measured with a

MKS baratron gauge and a 0 to I00 PSIG Bourdon gauge.

4.9 OPTICAL CAVITY

For most of the tests reported here the laser has been operated as a stable

oscillator. Power has been extracted only from the main discharge cavity.

The laser chamber has two 7.6 cm diameter, 1.3 cm thick quartz windows. The

windows are tilted 1o off the normal to the optical axis to prevent window

reflections from setting up parasitic modes. The substrates are UV-grade

-40-
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FIGURE 4-18. VACUUM PUMP AND GAS MIXING STATION

-42-
fused silica, and both faces are AR coated for 308 nm (i.e. for XeCI).

Although the window mounts are equipped with annular gas flushing slits, no

gas processing and laminar window flushing equipment has been installed.

Cavity reflectors and beam diagnosing equipment are supported from an optical

table which has been built around the discharge chamber. The optical bench

(Figure 4-3) has three adjustable legs and is supported from the concrete

floor. A more "productized" version of this laser will have the optics

supported from optical rails attached to the discharge chamber.

Only stable resonators with flat-flat and concave-flat configurations have

been tested. Various output couplers have been used with reflectivities

between 10% and 50%. A helium-neon laser has been used to align the optics.

5.0 PRELIMINARY EXPERIMENTS

A number of preliminary experiments were performed during the early part of

the design phase to help in determining optimum design features for certain

critical components. Theoretical and empirical input was required in three

critical areas: the cathode insulator design, the discharge PFN configuration, and the magnetic switch design. A semi-empirical approach was used

for the first task, a theoretical computer simulation for the second, and

experimental testing of sample cores for the third.

5.1 ELECTRIC FIELD PLOTTING

A most critical issue in the design of high-energy excimer lasers (which has

so far prevented scaling of discharge-excited RGH lasers to pulse energies

exceeding I00 J) is concerned with surface tracking along the insulator which

isolates the high-voltage electrode from ground. This insulator cannot be

made arbitrarily large, because the head inductance of the discharge circuit

must be kept as low as possible, so that the current risetime is substantially

shorter than the stable duration of the discharge. In order to avoid high

field concentrations on the insulator surfaces which may lead to tracking, a

field mapping study was undertaken to determine the electric field

distribution in the vicinity of the electric discharge. The study consisted

of an experimental determination of the equipotential surfaces and field lines

in a two-dimensional model of the electrode structure (see discharge section

in Figure 4-2). The field plotting was performed in an electrolytic tray by

measuring the voltage distribution caused by an AC current field.

In the simulation, E :V_ : i/_ , where d is the electrical conductivity of

the material. This corresponds to E =_ = D/_ in the electrostatic case.

The technique, therefore, requires dielectric materials with permittivity _ to

be simulated by semiconducting materials with conductivity _ . The free-space

permittivity £o is represented by a copper sulfate solution of conductivity

G o. Insulators, such as Kynar (Er = 7) are simulated by blocks of carbon

filled polyester resin, mixed and cured to produce a conductivity of _ =_r_o

(e.g. 7_o for Kynar).

-43-
To map out the equipotential lines, a 1 kHz AC signal of 20 V amplitude was

applied to the aluminum blocks made in the shape of the anode and screen

cathode. An AC signal was chosen to prevent electrochemical decomposition of

the copper sulfate. Equipotential lines were determined by probing for points

(with thin metal probes attached to a voltmeter) which have the same AC

voltage (or amplitude). Electric field lines were then plotted by drawing

lines which are everywhere perpendicular to the equipotential lines.

An electric field plot obtained in this fashion for the original insulator

geometry is reproduced in Figure 5-I. Note the high field region (denoted by

"H") at the tip of the Kynar barrier and the relatively large tangential field

components along its surface (promoting breakdown by surface tracking). By

curving the barrier (Figure 5-2), the field lines were made normal to the

surface, and the high field at the tip was eliminated. Another high field

region existed between the Kynar and the chamber wall. The field there was

significantly reduced by removing some of the Kynar block and by rounding the

top of the Kynar next to the wall. Eliminating these high-stress areas had

the further advantage of simplifying the machining of the Kynar. For the

final design, the head inductance was calculated to be 18 nH.

5.2 COMPUTER SIMULATION OF PULSE-FORMING NETWORKS

Extensive computer simulations of the performance of the proposed PFN resulted

in circuit improvements and simplification in the design stage of the program.

The calculations were performed with an Electronic Circuit Analysis program,

called ECA-2.30 and purchased from Tatum Labs., Ann Arbor, Michigan. The

original circuit described in the proposal was found to produce unacceptable

oscillations during the charging phase. A modified version, producing good

wave shapes, is shown schematically in Figure 5-3.

In the model, the gaseous electric discharge plasmas (thyratron and laser

discharge) were simulated by a time-variable resistance (acting as a switch,

simulating discharge development), in series with a nonlinear conductance G,

which approximated the constant-voltage load characteristics of the discharge.

For example, the conductance G2D of the main discharge was described by an

I,V-characteristic given by (see Computer Model Parts List - Table 5-I)

I = 2.7 x 10-17 V 5,

Note that the circuit uses two saturable inductors, LIS and L2S. LIS acts as

a magnetic assist for the thyratron to promote long life. When it saturates,

the pulse-forming line (PFL) is charged to the supply voltage in approximately

l microsecond. The sequence of events leading to prepulse formation and

discharge initiation are best followed by monitoring the voltage appearing on

the secondary cathode K2 as a function of time (Figures 5-5 and 5-6).

During the pulse charging phase, after LIS saturates, the voltage at K2

follows closely the sinusoidally rising negative voltage on Kl (Figure 5-5).

During this same period, the current experienced by the thyratron reaches a

peak value of lO kA (Figure 5-4). At the end of the charging period, LIS

reverts to its high-impedance state (unsaturated core) and acts as a diode,

-44-
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FIGURE 5-5. VOLTAGE AT SCREEN (NODE 7) DURING

CHARGING PULSE (COMPUTER MODELING)

-49-
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-50-
preventing the thyratron current from going negative. When L2S saturates,

capacitor Cl is rapidly charged from C4, producing a high voltage between K1

and K2 (predischarge gap), rapidly initiating the predischarge. The closely

coupled capacitors Cl, in combination with the underdamped resistance of the

highly overvolted predischarge, then results in Cl ringing through the

predischarge gap, producing a negative prepulse of nearly I00 kV across the

main discharge gap (between screen and grounded anode). This prepulse

initiates the laser discharge. A steady-state discharge voltage of

approximately 18 kV is then maintained for 150 ns. The voltage appearing on

the cathode, K1 (Figure 5-7), is very similar to the voltage pulse on the

screen, except for the L2S-induced positive voltage excursion, which is only

seen on the screen.

It should be noted that in the computer simulation the main discharge is

artificially induced by letting the discharge resistance R2D rapidly go to a

low value. In practice, the prepulse amplitude was much less than predicted,

and the prepulse was not wide enough for the main discharge to ignite

completely. These difficulties, experienced with the actual circuit, are

discussed in more detail in Section 6.1.

The calculated power dissipated in the main discharge (Figure 5-8) reaches 600

MW, close to the design value. The oscillations seen in the power curve (and

also in the discharge current) were at first believed to be due to the fact

that the PFL was constructed from discrete capacitors. However, further

investigation showed this to be untrue. The oscillations are caused by

ringing in the LC-circuit dominated by the head inductance and the stray

electrode capacitance.

Whereas only the results of computer modeling of the first PFN are presented

in this section, the ECA program has proven invaluable for modeling the later,

more successful, versions of the PFN. The B-H characteristics of the

saturable core reactors were approximated by a simple S-curve going through

the origin (i.e. saturation effects were accounted for, but hysteresis losses

were ignored). The non-linearities in the B-H curves were modeled by letting

the inductance decrease in a stepwise fashion as the current increased. This

stepwise change in L has occasionally caused non-convergences in the

integration of the set of differential and integral equations used to describe

the circuit. The ECA program has a built-in recovery program which in most

cases can successfully sidestep these non-convergences, however.

The ECA program has also been successfully applied to model the X-ray pulser

(see also Section 4.6). Figure 5-9 shows the equivalent circuit used for

calculation purposes. Table 5-2 lists the component values and initial

conditions, The e-gun diode (anode-cathode) impedance and stray capacitance

have been represented here by RD and CS, the cathode capacitance and contact

resistance (corona plasma impedance) by CC and RC. The thyratron has been

modeled by the non-linear resistance and conductance, RT and GT, in series

with the tube inductance LT. The component values of the PFN, made up of

capacitors C1 to C6 and inductors L1 to L5, have been tailored to give the

most rectangular anode pulse of 250 ns duration, The pulse transformer has

been represented by an ideal transformer with a gain of 4:1. The non-ideal

characteristics of the transformer have been taken care of by equivalent

-51 -
........................ . ........... : ........... • ........... , ........................

769,M_

609,M " _/I ..... :............ :............ "...........

V v, 599.M

,o.,iiiiiiiiiiiiiiiiiii. •

308.M i

IO9,M ....................... "".'.'."'.'". ...'.'."'.'.'.'".'.'.'.'.'.'.'.

..................................... iiiiiiiiiii i

-U]O.li ....................................................................

1.1u 1.Zu 1.3u 1,4u 1.Su 1.6u 1.Tps

FIGURE 5-8. POWER DISSIPATED IN MAIN DISCHARGE

GAP (COMPUTER MODELING)

-52-
circuit components, such as the leakage inductance LL, the primary magnetizing

inductance Lp, the winding resistance RW, the eddy current core losses Rp, and

the interwinding capacitance CEQ. Values used for LL, Lp and RW were obtained

experimentally by analysing the completed pulse transformer with an impedance

bridge. The equivalent capacitance CEQ has been calculated from the measured

primary-secondary capacitance Cps , using the expression

CEQ = (n 2 - n + I) Cps/3, where n = 4 (step-up ratio).

Figure 5-10 gives the anode voltage pulse calculated for two different diode

impedances, RD = 750 ohms and RD = 375 ohms. Modeling results have shown a

significant degradation of the anode pulse if the diode impedance becomes less

than 200 ohms. Calculated diode impedance, based on the Child-Langmuir law

and the electrode geometry, is 650 ohms.

The Child-Langmuir law states that the e-beam current density is given by

Jb : K Vb3/2 / s 2 ,

where Vb is the e-beam current denslty, s is the _D_de-cathode spacing, and

the constant K is approximately lO-b ampere-volt-J/L for typical e-gun

geometries. In practice, s is a function of time as the cathode plasma

(acting as a virtual cathode_ rapidly expands toward the anode with

velocities, Up, exceeding I0 D cm/s. Hence, to a first approximation, the

anode-cathode spacing varies with time as

s(t) = s o - Up t.

The time varying diode impedance is then given by

RD : (s o - Upt)2/AKVbl/2(t),

where A is the area of the advancing plasma front. Vb is a function of time,

because the circuit responds to the decreasing diode impedance with a

decreased output voltage (Figure 5-10). Taking this time-dependent behavior

of the e-gun characteristics into account, the actual measured anode pulse

(Figure 4-14) is in good agreement with the pulse predicted by the model.

The cathode pulse is generated simultaneously with the anode pulse by

transferring the voltage on capacitor CA to the cathode capacitance CC when

the thyratron fires. The values of Lc, R2 and CB have been adjusted, so as to

give a high value of dV/dt at the cathode at the same time the anode voltage

is at a maximum. This assures good synchronicity between the time of most

efficient corona formation and the time at which the accelerating field is at

a maximum.

-53-
(!)

MA/X/,° II0

-I Z

_,< >.

m1_ _

I'- ---1

I

® ]}I t,

I

I_ om ,_@; }{

_AAAt

II.

i

0

-k

Z

{:}

k-

_ i 1!,_0

_. u u _U

"t"

!

|1

I

0

0

o

I

L_

II

0

I'-.

go

-54-
TABLE 5-2

PARTS LIST FOR X-RAY PULSER MODEL

' XRAY PULSER

1 RT 1 0 IO.K

2 + 0.01

3 GIT 7 1 l.u

4 + O.002

5 GIIT 1 7 0,05

6 LT 8 7 lO0, n

7 C1 8 9 2.n

8 C2 8 I0 1.3n

9 C3 8 II 1.3n

I0 C4 8 12 2. n

II C5 8 13 3,3n

12 C6 8 14 4. n

13 L1 9 I0 500. n

14 L2 I0 II 300. n

15 L3 II 12 500. n

16 L4 12 13 300. n

17 L5 13 14 300. n

18 LL 15 9 2.45u

19 RW 2 15 0.03

20 CEQ 15 0 1,62n

21 RP 9 0 5.3K

22 LP 9 0 400. u

23 LX 3 0 lO0, n

24 LY 5 0 lO0,n

25 RD 4 0 750,

26 CS 4 0 50.p

27 GTI 2 0 I.

28 GT2 2 6 -I,

29 GT3 3 0 -I.

30 GT4 3 6 I,

31 GT5 4 0 0.25

32 GT6 4 6 -0.25

33 GT7 5 0 --(3.25

34 GT8 5 6 0.25

35 R2 8 17 60.

36 LC 17 18 30. u

37 CA 18 16 1,3n

38 CB 16 0 1.3n

39 LB 16 19 lO.u

41 RCH 16 0 I00.

42 CC 19 20 6.n

43 RC 20 0 25.

E'5,

I t

t t

! 1

9t

9 W

tt

9 W

99

SO.

V 30.K

V 30.K

V 30.K

V 30. K

V 30,K

V 30.K

VO.

VO.

V 30. K

VO,

VO.

-55-
128.K 128.KV

188.x _",, 188.x

58, K

/K

/. '\

88.!( !/

.....

',

• 88, K

!

/ \

/ \

/ '\ /-_ 4e.K

40.! / ",. /. "\. .

Z8.]( f 29. l

/

!

1

9, 8.

68.K

RD = 750 OHM

.. -Z8, K

299.n 490.n 509. n 896.n 1,us

X-RAY PULSER ANODE VOLTAGE

129.X IZ9. XV

186.K .......................................................... 189.K

88, K 86, K

...... ............................

58. K " 66.K

/ ',

,' \

//

49,K /- i................ ...... ,_,\ .... i..........

48.K RD

an 375 OHM

/ %,

zo.xi/ ........................... i.......... __-.._,:,............... zo.,

-28.X ............................... : ................................................ -ZO.l

O, 290,n 489,n 589, n 886,n 1,u s

FIGURE 5-10. CALCULATED E-BEAM ANODE VOLTAGE PULSES

FOR TWO DIFFERENT DIODE IMPEDANCES (RD)

-56-
5.3 TESTING OF SAMPLE METGLAS CORES

Requirements for the magnetic switches LIS and L2S Csee Figures 4-I and 5-3)

are that their V.t-products are on the order of I0 -L V-s. Only cores wound

from thin Metglas (TM Allied Signal) ribbon, less than 25 micrometers thick,

have both the high frequency response and, simultaneously, the high values of

the saturated magnetic flux density (Bm) , necessary for the present

application. Whereas high-frequency (microwave) ferrites may have equal or

better high-frequency response, their saturated magnetic induction is

generally more than an order of magnitude below that of Metglas. Hence, a

saturable inductor made from ferritic material would require a core volume ten

times as large as an equivalent Metglas inductor.

Early in the program, two sample Metglas cores, made by Allied Signal

Corporation's Metglas Division, were tested to verify their magnetic

properties. Tests were performed to establish the "V.t-product to saturation"

and the effective permeability of these cores when excited by a microsecond

high-voltage pulse. The cores had been wound into a racetrack geometry, with

an oval window measuring approximately 4 cm by 14 cm, using pre-annealed 5 cm

wide Metglas ribbon. Thin Mylar tape (6 microns thick) was wound onto the

Plexiglass mandrel together with the Metglas ribbon, to isolate and separate

the Metglas layers from each other. This was necessary to allow the magnetic

field to diffuse into the Metglas more rapidly, giving the core the necessary

high-frequency characteristics.

One core was made from 15 _m thick 2605-C0 material, the other from 25 _m

thick 2705-MN ribbon. The cross-sectional areas of these cores consisted of

approximately 6 cm2 of metal. Measured B-H curves for these cores are

reproduced in Figures 5-11 and 5-12. These curves were obtained with lowfrequency, quasi-DC excitation signals and are not necessarily representative

of the dynamic response under high-frequency service. Core responses to fast

excitation pulses (i.e. for high values of dB/dt), have been studied

extensively by Smith 26, (Allied Signal scientist) and have been published in

the "pulsed power technology" literature,

When designing a saturable core magnetic switch, the first consideration is

the hold-off period required before the current is allowed to increase. The

time to saturation is expressed by the voltage integral of the voltage pulse

applied to the winding on the saturable core, i.e. by JVdt. The core crosssectional area which is consistent with the required V.t hold-off integral is

given by

A = SVdt / NAB,

where N is the number of electrical turns wound on the core and AB is the

available excursion of the magnetic flux density,

J B = Br + Bm,

where Br is the magnetic flux density at the remanence point (see B-H loops).

-57-
The unsaturated inductance of the magnetic reactor can be calculated by the

usual equation

Lu = Msw N2 A / Im,

where Im is the magnetic path length, and #sw, as given by the magnetic

saturation wave theory26 is

_sw = 8pB m d-2 (dB/dt) -I.

Here, _ is the electrical resistivity of the core material and d is the

thickness of the Metglas ribbon. The time rate of change of the magnetic flux

can be estimated from

dB/dt = Vpeak AB //Vdt.

The saturated inductance of the magnetic reactor is calculated from the normal

air coil formulae.

Referring now back to Figures 5-11 and 5-12, note that the MN material has a

very rectangular, low loss hysteresis loop, but has a lower saturated magnetic

induction of l.l Tesla, as compared with 1.6 T for the CO material. The

transition from high permeability to low permeability at the remanence points

(knees of the B-H loop) is normally more "square" for the CO material. The

degradation at the "knees" has been caused by mechanical stresses introduced

when the Metglas ribbon was rewound onto an oval mandrel after having been

annealed in a toroidal configuration.

To test these cores under representative conditions, ten electrical turns were

wound around each core, and the completed saturable inductor was then

connected into a test circuit shown schematically in Figure 5-13. A 133 nF

capacitor, charged to voltages between 0.4 and 20 kV from a switched charging

supply, was then discharged through the magnetic inductor by means of a

triggered thyratron switch. The voltage across the inductor and the current

through the inductor were monitored by suitable probes. The circuit

inductance, other than the magnetic reactor inductance, was kept to less than

a microhenry, and the load resistance was kept to 0.3 ohm to keep the test

circuit well underdamped. The measured "air inductance" of the ten-turn coil

was 7 _H.

Figures 5-14 to 5-17 show the voltage and current traces obtained when the

capacitor was charged to 1.5 kV. The MN core saturated after 8 Ms, the CO

core after 12 _s (Figures 5-14 and 5-16), indicating that the cores were fully

reset to the -B r condition by the capacitor charging current. Because the

permeability at remanence (H = O, B = Br) is very small (near-horizontal slope

of the B-H curve), there resulted a current leakage pulse at t = 0 (Figures

5-15 and 5-17), which was more pronounced for the MN core than for the CO

core. Similar and self-consistent results were obtained at other charging

voltages. The saturated inductance of the inductor was calculated from the

width, _, of the current pulse and

(C LSAT) I/2 = T i.

-58-
il _'+_ it!_¢1. '-,I_It!:L,*Ii t

[ i _; *; 1 _4i i- _, 1

:_ : :' ',lJtilr t_Ittlllrl_:

ti!_!i!_ "kit.t'::'l*_!!t!t_;

it,! i p tit ;, ,-,[i _'_ T'

it .... _; +1....

I

1I: t::_ q,_t:. ?:l:I_l:II,_.

:I.t!'.! :: : ', '::It1 HI!=

t _ • .,', ++ '*i[ _r.... +1I tl

FIGURE 5-II, B-H CURVE FOR RACETRACK CORE WOUND FROM 15 Nm

THICK BY 5 cm WIDE 2605-C0 METGLAS RIBBON

B-H CURVE FOR RACETRACK CORE WOUND FROM 25 pm

THICK BY 5 cm WIDE 2705-MN METGLAS RIBBON

-59-
HIGH-VOLTAGE

PROBE

V

I

A.L.E.

SWITCHED

CHARGING

SUPPLY

0,4 - 25 kV

f

C = 133 nF

THYRATRON

TEST CORE

W. lO TURNS

p ....

TRIGGER___PULSE

1

R

J

I ION PHYSICS

ICURRENT

!PROBE

A

L = 0.3 Ohm

FIGURE 5-13. EXPERIMENTAL SETUP FOR TESTING METGLAS CORES

-60-
FIGURE 5-14.

VOLTAGE (500V/DIV) AND

CURRENT(50 A/DIV) FOR

2705-MN CORE. V,t = 12 mV s

I

o±

FIGURE 5-15.

MAGNIFIED CURRENT (2.5 A/DIV)

PULSE FOR 2705-MN CORE. NOTE

INITIAL LEAKAGE CURRENT.

CHARGING VOLTAGE FOR FIGURES 5-14 TO 5-17 WAS 1.5 kV.

TIME SCALE IS 2 MICROSECONDS PER DIVISION.

V

FIGURE 5-16.

VOLTAGE (500 V/DIV) PULSE

FOR 2605-C0 CORE

V.t = 18 mV s

I

o±

FIGURE 5-17.

MAGNIFIED CURRENT (2.0 A/DIV)

PULSE FOR 2605-C0 CORE. NOTE

LOWER LEAKAGE CURRENT

-61 -
The unsaturated inductance, LUS, was estimated from the rate of current rise

before saturation, after the effects of the decaying leakage pulse were

subtracted out, The current in a highly underdamped circuit (L/C >> R2) is

I = Vo (C/L)1/2 e-tR/2L sin(t L-I/2 C-I/2),

which for small times (t << (LC)I/2 << L/R) reduces to

I = Vo (C/L)1/2 sin( ) = VCH t / LUS.

The results from these tests were as follows:

I. The cores appeared to be fully reset to the opposite remanence condition

during charging of the capacitor from the pulsed charging supply

(approximately I0 A-turns peak) for all charging voltages above 350 V.

2, The measured V,t integrals were 18 x 10-3 V-s (or 1,8 mV-s/turn) for the

2605-C0 core and 12 x 10-3 V-s (or 1,2 mY-s/turn) for the 2705-MN core.

This corresponds to a delta B swing of 3.0 T for the CO material and of

2.0 T for the MN material.

3, The current leakage at t o was approximately 5% of the peak current for the

MN core and approximately 1.5% for the CO core.

4. The unsaturated inductance of the 2605-C0 core was estimated to be 3.0 mH

and that of the 2705-MN core as 2.6 mH. These numbers correspond to

relative permeabilities of 16,000 and 13,000, respectively.

5. Current pulse widths indicated that the saturated relative permeabilities

were close to unity, certainly less than 2.

6. Tests also showed that after saturation, any opposite voltage swings did

not produce any appreciable currents in the opposite direction (diode

behavior).

Whereas either core material would have proven satisfactory for the intended

application, we chose to go with the thin 2605-C0 material for the following

reasons:

I. Because of its higher available delta B, the number of electrical turns

could be reduced, resulting in a lower saturated inductance.

Alternatively, the mass of the core could be reduced for the same number of

turns.

2. The initial leakage current was less for the CO material.

3. The 15 micron thin ribbon blocks high frequency (> 2 MHz) signals more

effectively than the 25 micron ribbon, when the device is used as a

blocking diode.

-62-
6.0 DEVELOPMENT OF INTEGRATED PREPULSE GENERATING SCHEHES

The original PFN, which had been proposed in the Phase I Report 21 and which

had been thoroughly analysed with the ECA program (Section 5.2), proved to

contain several practical weaknesses. It served, however, as a useful test

bed for investigating a number of prepulse generating schemes. During this

process, important information was gained about the practical requirements for

initiating a fully developed electric discharge in an excimer laser gas

mixture. It should be kept in mind, however, that failure to make the

proposed circuit work as predicted, does not mean that the scheme is

unworkable. The modifications needed to successfully implement the proposed

excitation network were too extensive, too costly, and were realized too late

in the present program to be seriously considered. Instead, a more direct way

of generating a strong prepulse was chosen. It proved to be the most

straightforward approach to meeting the most important goals of the program,

pulse energy and laser efficiency.

The strong points and weaknesses inherent in the various prepulse techniques

investigated are discussed in this section.

6.1 PREPULSE CIRCUITS

With the original prepulse circuit described in Figure 5-3, a magnetically

switched positive pulse was generated which initiated the predischarge. A

negative backswing, caused by ringing in the predischarge circuit, then served

as the prepulse to overvolt the main discharge gap. The magnetic switches

tested with this network were constructed as follows: The magnetic thyratron

assist and inverse voltage block (LIS) was one of the Metglas cores (2605-C0)

tested in the preliminary experiments (Section 5.3). Five electrical turns

were wound on this core. Its unsaturated inductance was predicted to be close

to 1 mH and its saturated inductance approximately 2 microhenries. The

magnetic switch L2S, which was used to initiate the predischarge, consisted of

four identical 2605-C0 cores, as described in Section 5.3. Three of the cores

were trimmed until all four cores had identical V.t-products to well within

I%. Each core had an electrical winding of 7 turns of 7.6 cm wide copper

strap, wound one turn over top of the other to reduce the saturated

inductance. With the four inductors connected in parallel, the unsaturated

inductance of L2S was determined to be 125 microhenries and its saturated

inductance less than 500 nanohenries.

Early experiments showed that the saturated inductance of the magnetic assist

inductor, LIS, was closer to 5 microhenries (including the stray inductance of

the charging circuit), than to the 2 microhenries assumed in the design. As a

result, it took almost 2 Ms to charge the PFL instead of the expected

I.I microseconds. Consequently, the four magnetic Metglas reactors, L2S,

saturated before the PFL was fully charged.

Taking electrical turns off inductor LIS to speed up the charging would have

compromised the inverse voltage protection for the thyratron. Alternatively,

adding more turns onto inductors L2S would have slowed down the positive

voltage excursion. To bring the prepulse into coincidence with the peak of

-63-
the charging voltage appearing on the PFL, a set of second-stage saturable

reactors, L3S, was added as shown in Figure 6-I. This second set of saturable

inductors was wound on a set of ferrite cores and served to delay the voltage

seen by the Metglas prepulse switches L2S. These high-inductance (50 turns)

ferrite switches did not compromise the risetime of the prepulse generated,

yet permitted charging times in excess of 2 microseconds.

Preliminary tests with N2/He , with C02/N2/He, with F2/Kr/Ne, and with

HCI/Xe/Ne mixtures in the discharge chamber showed that the predischarge

between cathode and screen could be initiated with the magnetically switched

positive pulse as predicted, However, the large negative voltage spike

(produced by voltage inversion in the predischarge circuit) predicted by the

theoretical model (see Figure 5-6) could not be produced in practice. Hence,

difficulties were encountered in properly initiating the main discharge.

This discrepancy between theory and observation was traced to the model

assuming that the inductance LK (Figures 5-3 and 6-I) in the predischarge

circuit was concentrated at the cathode, whereas in reality the inductance was

distributed over the entire circuit. When the model was modified to reflect a

5 nH inductance distributed over the predischarge circuit, it generated only

very short (8 ns) and erratic negative voltage spikes, only 30% over the

charging voltage (Figure 6-2). These skinny voltage backswings were

consistent with observed behavior. Needless to say, the short voltage spikes

were insufficient to drive the avalanche discharge in the main gap through its

formation stage. Increasing the charging voltage resulted in self-breakdown

of the main discharge gap, causing the discharge to arc.

It was believed that the problem could be corrected by adding a physical

inductor (coil) between the cathode and the center plate of the PFL. With a

lumped inductance of 20 nH between cathode and PFL, the model predicted a much

stronger negative voltage prepulse, 25 ns wide and with a peak of twice the

charging voltage (Figure 6-3). Such a voltage spike should have been

sufficient to fire the main discharge.

The idea was put into practice by building a new cathode which was insulated

from the PFL, so that a physical inductor could be inserted between cathode

and PFL. A set of thin stainless steel wire connections served to make up the

20 nH inductance. When the new configuration was tested, a 25 ns wide

negative prepulse could indeed be generated (Figures 6-4 and 6-5), but its

amplitude was still only 35% over the charging voltage. As before, the main

discharge could not be initiated. At higher charging voltages, the arcs were

brighter than before, which was attributed to the increased head inductance,

estimated to be 40 nH. It was concluded from these tests, that the only way

the prepulsed double-discharge scheme could be made to work, was to place the

prepulse-generating saturable inductor between the cathode and the PFL. This

would have required rebuilding the entire electrode and feedthrough structure,

as well as the PFL.

At this point, it was decided to re-install the non-inductive cathode and to

apply a magnetically switched negative prepulse to the screen directly. To

accomplish this, the PFN was modified with the magnetic switches reconnected

in the configuration shown in Figure 6-6, The capacitors C1 between cathode

and screen were removed.

-64-
J

I

O_

CJI--

CrlZ

._j -lZ

I--C-_

__I

Z

___c_

IDd_

I

_D

(_O

I_i-

-65-
I0. KV : ................ i................ i................. i ............ '..............

0, ' " SWITCHED PULSE -- e'_/ INITIATES

CHARGING PULSE "_--______.,_. -lj

-30. g ' ....... ""

NEGATIVE BACKSWING

-4B, R I

-S@.R ................. -................................. ,.........................

-6O,R :................ •................................. -....................

l,_s 1.2u 1,4u i,6_t i,BFtm 2,

FIGURE 6-2. CALCULATED VOLTAGE ACROSS MAIN GAP FOR L K = 5 nH

DISTRIBUTED OVER PREDISCHARGE CIRCUIT

lo,RV

1

• • /-PREDI SCHARGE

/,/_i¢ INI T IATES

MAGNET ICALLY /

SWITCHED PULSE --_,,._J

, . ___ _

-3B.K

-4B, }{ ,NEGATIVE

BACKSWIP+G ....

DISCHARGE VOLTAGE

"--MAIN DISCHARGE

INITIATES

-SB,K ................................................. ................................

-60.}{ ................. ................ ..............................................

1,/_ s I,2_ts i,'lu i.6_t i.8_ts 2.

FIGURE 6-3. CALCULATED VOLTAGE ACROSS MAIN GAP FOR

LUMPED 20 nH CATHODE INDUCTANCE

-66-
oTV

(5kV/DIV)

A3 0,66 U

tl.

J

.50 lOOms

1 1 , I .__±__ 1 I I J. I I

FIGURE 6-4. MEASURED SCREEN VOLTAGE SHOWING PREPULSE

GENERATED BY CIRCUIT IN FIGURE 6-I (lOOns/DlV)

FIGURE 6-5, EXPANDED TRACE OF PREPULSE AT 50 ns/DIV

SHOWING PREPULSE TO BE 25 ns WIDE

-67-
"

7

l.m,-

.ml ,ml

>

,.._I

i,

r_

V'l

.ml

i,i

.m.l

i,i

It

c_

i.Ll

r'_

ILl

ZIZ

C.D

I---

.--i

,...I

CD

I'--

I.,I-I Z

c_

o_

ID

cD

la,,4

ILl

V'i

.,.I

CiILl

r_

ILl

i,

C)

I

l.L_

-68-
With this arrangement of PFN components, capacitor C4 remained essentially

uncharged during the slow charging pulse. When L3S saturated, C4 started to

charge, adding voltage across L2S. When L2S saturated (after its V.t-integral

was exhausted), the negative voltage on C4 was added to the PFL voltage and

applied to the screen. Typical prepulse spikes generated by this scheme are

shown in Figures 6-7 and 6-8.

With the magnetically switched negative prepulse circuit of Figure 6-6, only

weak glow discharges could be initiated under ideal conditions in XeCl and KrF

mixtures. Glow discharges were observed over a very limited voltage range.

Generally, one or more of the following conditions would be observed,

depending on charging voltage and gas pressure:

I. Voltage oscillations introduced by the saturation of L3S (see Figure 6-7)

caused the predischarge gap to break down prematurely during the slow

charging of the PFL. This invariably resulted in arc formation.

2. For conditions under which an avalanche discharge was initiated in the

main gap, the prepulse circuit lacked sufficient speed (i.e. had too

high an output impedance) to provide the necessary dl/dt for driving the

discharge impedance down to the self-sustaining 1 ohm level.

The latter observation and deduction provided the clue for identifying the

primary problem. For the weak discharges observed, the experimentally

determined values of the dischar impedance and the associated pump power

density were 20 ohm and 25 kW/cm _e, respectively. The discharge was hardly

self-sustaining and was well below lasing threshold of an RGH laser.

It has generally been assumed (see published literature) that an impedancematched discharge can be initiated by a fast high-voltage spike. With the

emphasis being on voltage, the fast current capability required for a

successful prepulser has not been given enough importance. The present

investigation has shown that the prepulser has to supply a fast current pulse

as well as a fast voltage pulse. The output impedance of the prepulse circuit

should thus be well below I0 ohm, preferrably near 1 ohm. It became clear at

this point that the prepulse generating switch had to have a lower impedance

when in the conducting mode. For a magnetic switch, its saturated inductance

should be less than 50 nH instead of 800 nH (the inductance of the four 7-turn

Metglas switches, L2S, in parallel).

A new low-impedance prepulser (Figure 6-9) was designed which applied a fast

negative pulse directly to the main discharge gap, as before, but which had

the following important improvements:

I. The saturated inductance of the prepulse generating magnetic switch was

50 nH or less.

2. To increase the prepulse amplitude, a voltage-doubling network was "piggybacked" onto the PFL. This circuit could in principle generate a peak

prepulse voltage which was three times the charging voltage. This voltage

doubler had the further advantage, that the switch inductance was not part

of its output impedance.

3. A resistive/inductive voltage divider (RK and LA) had been added to the

electrodes, which made the electric fields in the "predischsrge" gap and

in the main discharge gap approximately equal during charging.

-69-
LIS L3S

SATURATES SATURATES

MAIN GAP

BREAKS DOWN

v

(5kV/DIV)

I

I

!

!

!

!

!

p . |

i I

! ' I

.... i ! !1

$O{_s i I l 1 I 1 ] |

FIGURE 6-7. SCREEN VOLTAGE FOR CIRCUIT IN FIGURE 6-6,

(FAINT GLOW IN MAIN GAP)

oTV

i I .

1 I L. :, L L I I,,, L , i

FIGURE 6-8. SCREEN PULSE AT HIGHER CHARGING VOLTAGE

(ARC IN MAIN GAP) 5kV/DIV, 500ns/DlV

-70-
ECAmodeling of this new prepulse circuit proved that muchwider and stronger

prepulses were possible (Figure 6-10). In the calculation, the inductance of

switch L2 was taken as 50 nH, CAwas 4 nF, CBwas 8 nF, R3 was O, L3 was 4 MH,

RK was 5 ohm, and LA was 50 MH. Note that the screen voltage (K2) tracked the

cathode voltage (KI) very nicely during charging of the PFL, with the

cathode/screen voltage and the screen/anode voltage being roughly in the same

ratio as the physical electrode gaps. When L2 switched to a low impedance,

the charge on CA was inverted and a strong negative prepulse was applied to

the screen. During the rise of the prepulse, the field in the cathode/screen

gap first diminished to zero before inverting, while the main discharge field

increased monotonically to three times its original value, or to twice the

charging voltage. This made it possible for the main discharge to initiate

before (or at least simultaneously with) the discharge in the cathode/screen

region. Even though L2 was not part of the prepulse current path, its

inductance determined the width of the voltage prepulse; i.e. it controlled

the dV/dt. A fast rising prepulse therefore still required a switch with a

very low inductance.

To prove the viability of this circuit (Figure 6-9), a low-inductance voltage

doubler (Blumlein) was built and was integrated into the laser PFL. Until a

new, fast magnetic switch could be designed and built, a low-profile (Maxwell)

triggered spark gap was substituted for L2, The inductance of the spark gap

and connections was estimated to be 40 nH. Output impedance of this prepulser

was in the 2 to 3 ohm range.

With this arrangement, lasing discharges could be produced over a wide range

of charging voltages and gas pressures. With a typical XeCl mixture of 5 atm

Ne, with 0.4% Xe and 0.08% HCI, and 28 kV charging voltage, approximately

1 J/pulse of optical laser energy was extracted. Efficiencies, based on

measured voltages were in the 2.5% range. The PRF was limited to 1Hz by the

operating characteristics of the spark gap.

Cathode and screen voltages, showing the spark gap switched prepulse, are

shown in the oscilloscope record reproduced in Figure 6-11. Note that the

prepulse is between 40 and 50 ns wide. Near the peak of the prepulse, both

gaps appear to break down simultaneously, leading to a 150 ns wide discharge

pulse. The measured optical pulse was I00 ns wide.

A stable, multimode resonator with 70% output coupling was employed. The

total rear reflector was concave with a I0 m radius of curvature and the

output coupler was flat. Laser beam burn marks indicated a 2.8 cm x 3.3 cm

beam cross-section (Figure 6-13). The near-field intensity profile, obtained

by scanning a 1 mm wide slit and energy detector across the beam (side to

side) as the laser was repetitively pulsed, showed a profile halfway between

Gaussian and top-hat (Figure 6-14). With higher pump power density and with

an unstable resonator, the beam profile was expected to become more

rectangular. In the vertical direction (anode to screen) the beam intensity

profile was by necessity already much more rectangular.

Having successfully demonstrated that the fast prepulse circuit in Figure 6-9

was able to produce good lasing output, a new fast magnetic switch was

-71 -
LIS

PFL L2SA" = 50 nH

LZ_LJ

' i

CA R3 i CB _>_ _LA

LHD = I0 nH

t7 RK i K1

'3

_k._ K2j

{

150p

FIGURE 6-9. VOLTAGE-DOUBLING PREPULSE CIRCUIT WITH VOLTAGE

DIVIDER AND FAST SWITCH (L2)

i i KZ

}

i i I_I • ' L '

1,ps 1.2u 1.4u 1.6u 1.8_

Time in Seconds /I

V

-18,1{V

-28, X

-38,1{

-48, K

._ -58, X

I

-68 ]{V

2 ,,us

FIGURE 6-10. CALCULATED CATHODE (KI) AND SCREEN (K2) VOLTAGES

FOR L2SAT : 50 nH

-72-
V

(50kV/DIV)

F DISCHARGE --_

+3 o.+s + _/_+"+ ""

f,,, if

|bs

FIGURE 6-11. CATHODE AND SCREEN VOLTAGES FOR

SPARK-GAP SWITCHED PREPULSE (50ns/DIV)

oT

V

DISCHARGE

FIGURE 6-12. CATHODE AND SCREEN VOLTAGES FOR

MAGNETICALLY SWITCHED PREPULSE (50ns/DIV)

-73-
',,,. ANODE

3.5 cm

SCREEN

J

FIGURE 6-13. LASER BURN MARKS (ON THERMAL PAPER) FOR TWO

DIFFERENT MIRROR ALIGNMENTS (I JOULE OUTPUT)

]_--_

FIGURE 6-14.

I

li:i '

I

i

"" "-T-; IT ..... : .....

.... "-tit ...... i"

..L': I ..........

i

• _._ _-_. _...........

1

..... i ...........

'I'I

, _I , | ,

.-T._il.........

-_;-!;_-2-;;;iSIDE-TO-SIDE INTENSITY PROFILE OF LASER BEAM

(VERTICAL SLIT AND DETECTOR WERE SCANNED ACROSS

BEAM), REAR REFL. = _0 m RC TOTAL

OUTPUT COUPLER = 30% REFL. FLAT

-74-
designed for directly replacing the spark gap. The spark gap needed to be

replaced for increased system reliability and life, and for making it possible

to operate at higher PRF. The design of this magnetic switch is described in

the next section.

6.2 MAGNETIC SWITCH DESIGN

The original 7-turn magnetic Metglas switches, one of which is shown in

Figure 6-15, were too slow and did not even come close to the spark gap

performance. Consequently, a new single-turn magnetic switch was designed

with an estimated saturated inductance of 50 nH. The choice of material and

shape was dictated by the required switch characteristics. The core was wound

into an II cm ID by 25 cm OD toroid with 15 micron thick 2605-C0 Metglas

ribbon and 6 micron thick Mylar tape. Because there was only one electrical

turn, the core had to be relatively large and heavy to hold off 35 kV for even

300 ns. This was so in spite of the fact that the 2605-C0 material provides

the highest known delta-B swing of 3.3 T (i.e. 33,000 gauss). The toroidal

shape was chosen to minimize the degradation in the squareness of the

hysteresis loop caused by rewinding of the preannealed Metglas ribbon (see

discussion in Section 5.3). The measured B-H curve for this core is

significantly more narrow and rectangular (Figure 6-17) than the corresponding

B-H loop for the previously constructed oval cores (Figure 5-11). This

demonstrates that the geometry, as well as the annealing and winding

techniques are extremely important for the production of a fast Metglas

switch.

A pictorial drawing, showing the magnetic switch installed in the lowinductance prepulser, is given in Figure 6-16. The single electrical turn

consisted of an 8 cm diameter brass pipe going through the center of the core.

Fast switching action, comparable to the spark gap performance, was achieved

because of the low impedance construction of the core and prepulser and

because of the squareness of the hysteresis loop.

A direct comparison between the voltage prepulses produced by the spark gap

and by the fast Metglas switch can be made by comparing Figures 6-11 and 6-12.

Apart from a somewhat more rounded knee, observed with the magnetically

switched pulse, the prepulse shapes are virtually the same. The saturated

inductance of the magnetic switch is, therefore, also close to 40 nH.

Since the V.t-product of the new switch was only I.I x 10-2 V-s, it was not

capable of holding off the full charging pulse lasting close to two

microseconds. Consequently, other changes had to be made to the PFN and the

prepulser shown in Figure 6-9. These were:

I. The number of electrical turns on the magnetic assist inductor LIS was

reduced from five to two. This shortened the energy transfer time from

the storage capacitor (in the pulse charging tank) to the PFL to I.I

microseconds.

-75-
FIGURE 6-15. ONE OF THE 7-TURN MAGNETICSWITCHES

ORIGINALLY USED FOR L2S

-76-
METGLAS

25 cm

II cm

5 cm

FIGURE 6-16. FAST MAGNETIC PREPULSER

-77-
FIGURE 6-17, MEASURED B-H CURVE FOR NEW LOW-INDUCTANCE

METGLAS SWITCH

-78-
2. The fixed inductor L3 was replaced by a saturable core inductor, so that

the fast magnetic switch, L2, did not experience the full charging voltage

for the entire charging pulse. One of the original L2S inductors (see

Figure 6-15) was modified by reducing its electrical turns from 7 to 3. It

was then installed into the prepulser circuit in place of L3.

3. An non-inductive II ohm (I00 W) damping resistor, R3, was installed to

provide critical damping of the voltage perturbation caused by L3 saturating and the subsequent charging of capacitors CA and CB (Figure 6-9).

4, New values of the other components in the prepulse circuit were:

CA = I0 nF, CB = I0 nF, RK = 16 ohm, LA = 35 _H. The diode in the voltage

divider (Figure 6-9) was added to aid in the resetting of saturable-core

reactors L2 and L3,

With the original multi-turn magnetic reactors, the cores were reset

automatically when the Maxwell storage capacitor was charged from the A.L.E.

switched supply. Since this supply could generate charging pulses with only 1

A amplitude, the cores were reset with a magnetic field intensity of less than

15 ampere-turns/meter, which proved to be very marginal. With the new singleturn saturable inductors, this method of resetting the cores was no longer

satisfactory, and it became necessary to actively reset the cores with a

separate reset circuit. Active reset circuits are the subject of the next

section.

6.3 CORE RESET TECHNIQUES

Full resetting of the Metglas cores can be achieved with magnetizing fields

between 50 and I00 A-turn/m, Incomplete core reset can result in significant

time jitter in the time to saturation. This problem is illustrated in the

example given in Figure 6-18, which shows the time jitter in the prepulse as a

result of insufficient reset current for switch L2,

At first glance, resetting the magnetic cores to their opposite remanence

state appears to be a straightforward procedure. This is in fact not so, and

several months of effort were devoted during this program to perfect the pulse

resetting networks. The first thought generally is to wind an auxiliary reset

winding with many turns on each core, so that only milliamperes of current are

required to provide the necessary reset field. It must be kept in mind,

however, that any additional winding will convert the magnetic inductors into

transformers. During reset, the laser PFN acts acts as a secondary circuit

coupled to the primary circuit, modifying the reset pulse. Conversely, during

excitation of the laser PFN, the reset windings act as secondaries. Since the

laser PFN switches voltages in the tens of kilovolt, many megavolts would be

induced in the multi-turn reset windings. Even if these windings are

effectively open-circuited during pulsing of the laser, the induced voltages

would be impossible to handle without electric breakdown.

It becomes apparent, therefore, that any auxiliary reset winding should have

no more turns than the primary number of turns on the saturable inductor.

Alternatively, the reset current pulse should be applied directly to the laser

-79-
oTV

(5kV/DIV)

!

FIGURE 6-18, PREPULSE TIME JITTER WITH INSUFFICIENT RESET

(200ns/DlV)

-80-
PFN without the use of auxiliary windings, Since our fast magnetic switches

have only one or two turns, the reset current has to reach several tens of

amperes to be effective. This leaves three alternative reset schemes:

I. The cores can be reset with a steady DC current of approximately 50 A

from a low-voltage DC source (e.g, a 12 V car battery), The DC source can

be protected from the high PFN voltage pulses with high-impedance chokes.

This appears to be the simplest way to reset the cores, At low PRFs, the

scheme would be uneconomical, however, since 600 W is an appreciable power

loss when compared with a laser input power of only a few kW, At PRFs

above I00 Hz, where DC resetting would become efficient, there may not

be enough interpulse time to overcome the high V,t-products of the magnetic

inductors with only the 12 V signal, especially when more than one core

needs to be reset in a sequential manner,

2. The cores can be reset with millisecond long pulses at the 200 V level.

A circuit to accomplish this is shown in Figure 6-19. Here, the 200 V

quasi-DC signal is switched onto a set of secondary windings, added onto

the saturable cores (in approximately a I:I ratio), by a high-current

transistor (e.g. a HEXFET). The high-impedance chokes, LF, serve to

protect the HEXFET transistor from high-voltage transients induced into

the reset circuit when the PFL is pulsed, These chokes have been wound

on ferrite cores which have a very slanted B-H loop, so that these cores

have a remanence induction close to zero and do not require resetting.

They do have to be driven into saturation by the reset pulse, however,

for the reset current to reach 50 A. A 1.6 ohm damping resistor has

been inserted into the reset circuit to prevent current reversal, The

reset pulses are produced by switching the HEXFET on and off with an

optically isolated gate driver. The pulse length can be adjusted from

1 ms to 5 ms by an RC-timing network, controlling a monostable multivibrator. The reset circuit is triggered at the same time as the laser,

so that the cores are reset right after the laser pulse, This reset

circuit has worked very effectively for resetting switches L2S and L3S.

In practice, this circuit could not be used to reset LIS, because it was

difficult to add a secondary winding on this core, since it was located

in a separate tank.

3. A third way of resetting the cores is schematically shown in Figure 6-20.

This circuit has been used in most of the recent laser tests. Here, a

+6 kV pulse of approximately I0 microsecond duration is directly applied to

the laser PFN at the thyratron anode to simultaneously reset all three

cores, LIS, L2S and L3S, A 50 kV diode blocks the normal 35 kV PFN

charging voltage from getting into the reset circuit, The pulses are

produced by switching the charge on a 3 pF capacitor, charged to 1,2 kV,

through the primary of a 1:5 step-up pulse transformer with a high-voltage,

high-current thyristor (such as the AEG G6OO-RI6OOK GTO thyristor shown in

the diagram). The thyristor is protected from negative transients (fed

back from the laser PFN) by a number of inductors, shorting diodes and

metal-oxide varistors (MOVs), In practice, it was found advantageous to

apply the reset pulse at two points of the laser PFN simultaneously,

at the thyratron anode and at the laser cathode, This resulted in

more consistent resetting of all three cores and in jitter free

operation,

-81 -
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-82-
._ B _ _ _ _ _-i 7

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-B3-
7.0 TEST RESULTS

With the fast magnetic voltage-doubling prepulser and the directly applied

pulsed core reset circuit, described above, best laser performance has been

obtained with XeCI mixtures containing Xe and HCI in the ratio of I0:I and at

total pressures of 5.6 atm absolute, Typical gas mixtures would contain 2

torr HCI and 20 torr Xe in a diluent of neon. Most efficient laser operation

has been observed at 30 kV charging voltage, although the laser has operated

well over the range from 20 kV to 35 kV,

The diagnostic equipment for monitoring the laser output and the laser

operation consisted of the following instruments:

I. A I00 kV high-voltage probe (Gentec, I000:I voltage divider) for monitoring

the voltage at the cathode and at the screen.

2. Pulsed current probes (Ion Physics, 0. I V/A) for monitoring the e-beam

current and the reset current pulses.

3. Pyro-electric pulse energy meter (Gentec) for measuring optical pulse

energy.

4. Laser thermopile power meter (Scientec) for measuring average optical

output power,

5. Vacuum photodiode (ITT, 1 ns response) with S-5 cathode for measuring

optical pulse shape,

6. Various MHz, I00 MHz (storage), and 500 MHz (with electron multiplier)

oscilloscopes (Tektronix) were employed to monitor the voltage signals.

Typical voltage pulses observed on the cathode and the screen are superimposed

(two successsive shots) in Figure 7-I. Sweep speed was 200 ns/cm. The screen

voltage can be seen tracking the cathode voltage for the most part of the

charging pulse, which here takes about II00 ns to complete charge transfer.

Three hundred nanoseconds before the cathode voltage (and the PFL voltage)

reaches the peak, the screen voltage separates from the charging curve as L3S

saturates and allows the prepulse capacitors to charge up. Five hundred nanoseconds later L2S saturates and generates the prepulse appearing on the

screen. Near the peak of the prepulse, the main discharge gap braeks down,

pulling the screen voltage to ground. At this time, a large voltage is

developed across the cathode-screen gap, the small gap breaks down and brings

the screen voltage back up close to the cathode (and PFL) voltage. With both

gaps ignited, the discharge current can pass from the PFL into the discharge

plasma. For the next 150 ns (i,e. during the pump pulse), the cathode and

screen voltages again track each other. The separation between the cathode

and discharge voltages during the pump pulse reflects the physical separation

between the electrodes, This indicates that the sustaining E/p field is

approximately the same in both discharge gaps. The experimental E/p field

here works out to approximately 600 V/cm-atm, which is close to the expected

value for a lean ×eCl mixture. The details of the discharge initiation can be

followed more clearly at the faster sweep speeds. Figure 7-2 shows the

cathode and screen voltages at I00 ns/cm, Figure 6-12 at 50 ns/cm. Note that

at the relatively low charge voltages of 25 kV, the prepulse occurs a few

hundred nanoseconds after the peak of the charging pulse. At higher voltages

(30 to 32 kV), the prepulse coincides with the peak of the charging curve.

-84-
oT

V

(5kV/DIV)

........ ° ..............

il i I ............. I

! J _ .L !

FIGURE 7-I. CATHODE AND SCREEN PULSE

(200ns/DlV, 22kV CHARGE)

__j

IIII

FIGURE 7-2. CATHODE AND SCREEN PULSE

(lOOns/DlV, 25kV CHARGE)

-85-
With a new gas fill and clean optical chamber windows, optical laser pulse

energies as high as 2.1J have been measured at a charging voltage of 30 kV.

At this voltage, 64 J were stored in the PFL, 8 J in the prepulser and 4 J in

the X-ray pulser, The electrical efficiency of the laser works out to be 3.2%

based on the energy stored in the PFL, 2.7% based on the overall energy stored

in the PFL, the prepulser and the X-ray pulser, When the "predischarge

losses" are subtracted out, then the lasing efficiency of the main discharge

is 4.4%.

The measured optical pulse duration is almost exactly I00 ns FWHM (Figures 7-3

and 7-4). The laser pulse starts approximately 50 ns after both discharge

gaps have broken down. Note that there is a difference in the smoothness of

the screen voltage during the pump pulse, depending on whether the gas mixture

is fresh (Figure 7-3) or whether it has been in the laser for more than a week

(Figure 7-4). Apparently, when the gas mixture becomes contaminated with

discharge products (remember there is no gas cleanup system installed) after

105 pulses or so, the discharge becomes less stable and is subject to more

severe voltage oscillations. By that time the laser pulse energy has

generally deteriorated to approximately 1.2 J.

Only stable resonators with a concave/flat and flat/flat configuration have so

far been tested. Typical burn marks on thermally sensitive paper are shown

full scale in Figure 7-5. At the two-joule output level, the beam uniformity

appears to be considerably better and the intensity profile closer to a tophat, than at the one-joule level (compare with burn marks in Figure 6-13).

Approximately the same energy has been extracted with a 90% transmissive

output coupler as with a 70% transmissive output coupler. Optimum output

coupling is evidently around 80%. In all these tests, no attempt has yet been

made to extract energy from the predischage. The 5 cm diameter reflectors

have always been lined up with the main discharge cavity.

Note the regular filamentary structure caused by the sreen in the burn mark of

Figure 7-5 (C). This burn mark has been obtained when the laser gas mixture

was "old". It appears that a rough electrode surface promotes the formation

of cathode streamers, the first indication of discharge instability. This

observation makes a good case for smooth solid electrodes.

Without water cooling and with only a slow gas flow, the maximum PRF is 1 Hz.

At that repetition rate, the pulse energy stability is excellent (Figure 7-6),

Pulse energy variation is approximately + I%,

With water cooling and with the tangential blower rotating at 2000 RPM, the

maximum PRF appears to be 25 Hz, Beyond this pulse frequency, the pulse

energy starts to drop off substantially, At 25 Hz, pulse energy is decreased

by approximately I0%, Maximum average power measured with the Scientec power

meter has been 30 W (not a fresh gas fill). Chances are excellent that the

PRF can be increased to 50 Hz, if the blower is rotated at 3000 RPM with a new

I/3 HP or I/2 HP motor to replace the I/4 HP motor driving the magnetic

coupling now,

-86-
oT

V

(5kV/DIV)

INTENSITY

oi_ TL .....

FIGURE 7-3. SCREEN VOLTAGE AND OPTICAL PULSE

(NEW GAS MIX)

°T

V

÷

FIGURE 7-4. SCREEN VOLTAGE AND OPTICAL PULSE

(OLD GAS MIX)

-87-
(A) CONCAVE (lOm RC) TOTAL

AND 10% FLAT OUTPUT

COUPLER (O.C.)

(B) FLAT TOTAL AND

30% O.C.

(C) FLAT TOTAL AND

3O% O.C.

FIGURE 7-5. BEAM IMPRINTS AT 2 J OUTPUT LEVEL FOR

DIFFERENT RESONATORS AND ALIGNMENTS

-88-
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-89-
8,0 CONCLUSIONS

The present R&D program has significantly advanced the state-of-the-art of

high-pressure pulsed gas lasers. High laser efficiency has been achieved by

implementing novel prepulse technology with highly reliable magnetic switches.

As part of the program, high-performance Metglas switches with large V.tproducts (10-2 V-s) and with high dV/dt capabilities (1012 V/s) have been

constructed and tested, core reset circuits have been perfected for good

repeatability, and an efficient long-life X-ray generator has been developed.

When operated as an RGH laser with XeCI, the laser has produced output pulses

of more than 2 joules at 308 nm with laser efficiencies of 3 per cent.

Operation of the laser at PRFs of 25 Hz has been demonstrated. These

performance figures meet the program goals for XeCl as originally outlined in

the proposal. Performance tests for other gases (KrF, XeF and C02) remain to

be done.

It is only natural that new insights gained during this program have already

made the present configuration obsolete. We feel confident that with our new

knowledge we can build a commercial follow-on version of this laser, which is

simpler, more powerful, less costly, more reliable and more compact.

9.0 PROPOSED FUTURE WORK

During the next 12 months we propose to use the present laser as a test bed

for checking out new electrode geometries and new magnetically switched,

integrated prepulse generators and PFN configurations. Testing of the laser

with KrF and CO2 is also contemplated.

9.1 PROPOSED CHANGES

Based on our previous experience, we propose the following design modifications for an improved version of the present system.

I. The disadvantages associated with the present double-pulsed three-electrode

structure appear to outweigh its advantages. Consequently, we propose to

employ a two-electrode design with a smooth solid nickel cathode and an

anode (X-ray window) covered with a solid 0.125 mm nickel cover. The new

high-performance Metglas switches will make it possible to prepulse such a

two-electrode gap. The two-electrode geometry will improve the ruggedness

of the electrodes, it will result in more effective gas flushing of the

discharge gap, and it will result in a more uniform discharge Con account

of the elimination of the screen cathode, which provides many starting

points for streamers).

2. An integrated prepulse circuit, as shown in Figure 9-I is proposed, with

the magnetic switch located between the cathode and the PFL. In this

circuit, the magnetic inductor L2 acts more as a prepulse generating switch

-90-
than as an isolation switch. Consequently, the prepulse does not have to

be of opposite polarity to the discharqe voltage, as is usually the case

with prepulsed two-electrode designs.lO, 20 Furthermore, the voltage

appearing on the electrodes prior to the prepulse can be kept well below

the charging voltage. The 3 uH inductor shownin the schematic can

preferrably also be a saturable inductor.

3. It will be more cost effective to use high-quality TDKceramic capacitors

tested at 60 kV, than to modify bare ceramic discs as was done in the

present program. The TDKcapacitors have acceptable solid, low-inductance

terminals. Alternatively, mica capacitors could be employed.

4. The pulse charging unit should be placed in the sametank as the PFL.

benefits would be more system compactnessand faster PFL charging

capability.

The

5. Wepropose to use a single impeller (possibly nickel plated titanium) for

the tangential blower, with only two bearings. The use of silicon nitride

bearings should be re-evaluated.

6. The blower baffle and flow guide should be redesigned for more efficient

vortex formation at lower RPM.

7. We propose the use of a smaller, shorter and lower-cost magnetic coupling

constructed with readily available Nd-Co magnets. A further improvement

would be to drive the permanent magnet rotor directly with electric coils

located around the outside of the Kynar partition cylinder (i.e. no motor

and belt).

8. Redesign the X-ray generator: Locate the HV pulse transformer in an oil

filled compartment (or Fluorinert filled) attached to the X-ray tube. This

would simplify the anode feedthrough, allow for better X-ray shielding, and

eliminate the I00 kV cable between pulser and X-ray tube. Eliminate all

plastics from the X-ray tube, use hard metal seals and bakeable ceramics.

Eliminate the vacuum pump by use of molecular sieve material in a semipermanently sealed tube.

9. Make the delay between PFL charging (trigger tl) and X-ray generation

(trigger t2) automatic by letting the discharge PFN generate a pulse, that

can be amplified into the anode voltage pulse for the e-gun.

-91 -
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OUTPUT IMPEDANCE

i

CHARGE IN __..

> 2

L2UNS >- lO pH, L2SAT =< 20 nH, C l = L2SAT/Z °

ELECTRODE

(CATHODE)

VOLTAGE

_ _ <0,6 VD IV D _"-_ ._TIME

v_H_ = 4V

PK D

FIGURE 9-i. PROPOSED TWO-ELECTRODE PFN AND

PREPULSE CIRCUIT

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9.2 COMMERCIAL SPIN-OFFS AND APPLICATIONS

The scientific insights gained and the technological advances made under this

program will be applied, in the form of new engineering designs, to develop

superior laser systems and sub-assemblies for the commercial market, as well

as for research and defense. Lasertechnics plans to take the design of the

present prototype as the basis for two possible excimer laser products, which

would incorporate the modifications outlined in Section 9.1 above. A highenergy RGH laser (KrF and XeCI) with 2 to 4 joules of optical output and

modest PRF (25 to 50 Hz) will address commercial markets in the areas of

semiconductor processing, product marking and micromachining. It would also

find applications in research and defense. A smaller version with 200 mJ

output energy, but with a high PRF of 200 Hz, will address the possible

markets in UV lithography and in the medical field. A compact, selfcontained, long-life X-ray preionizer and a small, powerful magnetic coupling

are expected to become marketable products as well.

Business opportunities and applications for RGH excimer lasers are multifold,

as can be appreciated from the following partial list of potential

applications:

I. REMOTE SENSING

A. Remote Sensing of Atmospheric Species (e.g. Ozone and S02)

B. Mineral Exploration by Laser-lnduced Fluorescence

2. UV PHOTOLITHOGRAPHY

3, SEMICONDUCTOR PROCESSING

A. Annealing of Silicon

B. Planarization of Metal Films

C. Selective Etching of Films

D. Selective Photochemical Deposition of Films

4. PRODUCT MARKING AND CODING (Semiconductors and Metals)

5. MICROMACHINING (Metals and Plastics)

6, MEDICAL (Precision Removal of Biological Tissue)

A. Laser Angioplasty

3. Corneal Machining to Correct Vision

7. PHOTOCHEMISTRY

A. Laser Isotope Separation

B. Laser Catalysis (Excited State Chemistry)

8. DEFENSE

A. Submarine Communications (NAVY)

B. Detection of Toxic Agents in Atmosphere (ARMY)

C. Distant Target Illumination (SDI)

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10.0

I.

2.

1

e

.

.

.

.

.

10.

11,

12,

15.

16.

REFERENCES

R. A. Tennant, "Control of Contamination in XeCl Lasers", Laser

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M, A. Akerman, R. A. Tennant, "Discharge Pumped RGH Laser Materials

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M. S. Sorem, A. Faulkner, "Catalytic Converter for Closed-Cycle

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D. E. Rothe,_ork performed in the development of first commercial

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B. Lacour, C. Vannier, "X-Ray Photoswitching of a 1 J Excimer Laser",

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17.

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26.

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D. E. Rothe, "Widely Tunable Gas Laser for RemoteSensing of Stratospheric Constituents", Final Tech. Report, NASA/SBIRContract No.

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T. Y. Chang, "Improved Uniform Field Electrode Profiles for TEA

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C. Birnbach, J. Tanner, "X-Ra-yy Source Employing Cold Cathode Gas

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