comparative study of dynamics and x-ray emission of several ...

COMPARATIVE STUDY OF DYNAMICS AND X-RAY EMISSION
OF SEVERAL PLASMA FOCUS DEVICES
SHAN BING
SCHOOL OF SCIENCE
NANYANG TECHNOLOGICAL UNIVERSITY
2000

COMPARATIVE STUDY OF DYNAMICS AND X-RAY EMISSION
OF SEVERAL PLASMA FOCUS DEVICES
SHAN BING
SUPERVISED
BY
PROFESSOR LEE SING
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
PHYSICS DIVISION, SCHOOL OF SCIENCE
NANYANG TECHNOLOGICAL UNIVERSITY
2000

Acknowledgments
I wish to express my deepest gratitude and thanks to my supervisor
Professor Lee Sing (School of Science, National Institute of Education, Nanyang
Technological University, Head of Physics Division) for his kind directions,
inspiring guidance, and invaluable discussions throughout the course. Without
his patience and encouragement, this work would never be fulfilled. I am very
grateful to have such a fulfilling and enriching experience of working under
him.
I am also very grateful to Dr. Liu Mahe, Dr. Paul Lee Choon Keat and Dr.
Rajdeep Singh Rawat, for their invaluable suggestions and discussions ranging
from basic concepts to complicated PF modeling. They also joined some
experiments with NX2 and always give me generous help when I have
difficulties.
I would also wish to express thanks and appreciation to Dr. Takeshi
Yanagidaira of Gunma University. He provided me with the parameters and the
newest results of the gas-puffed Gunma plasma focus. The discussions over
internet with him did me a great help in modeling the Gunma machine.
I would also like to express thanks to Mr. Alin-Constantin Patran for
providing and operating UNU/ICTP PFF with me. I am very impressed by his
zealous attitude and skilled working ability.
Thanks are also due to Mr. Ashutosh Srivastava, who collaborated with
me going through with all the experiments with NX2.
I am also very thankful to all the various people of the plasma research
group for their help and friendship, in particular: Prof. Vladimir A Gribkov, Dr.
Muhammed Shahid Rafique, Miss Cecelia M.N. Selvam, Dr. Adrain Serban, Dr.
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S. V. Springham and Dr. Vladmir Kudryashov. It has been a pleasant
experience to work with them.
Special thanks are given to Prof. Hou Xun (Member of the Chinese
Academy of Sciences, Chairman of the Academic Council of Xi'an Institute of
Optics and Precision Mechanics, Chinese Academy of Sciences) and my father,
Prof. Shan Lun (Academician of the Chinese Academy of Engineering, Delegate
of the National People's Congress, Chairman of the Academic Council of
Northwest Institute of Soil and Water Conservation, Chinese Academy of
Science) for their constant confidence on me that encourage and inspire me to
work hard in the scientific research.
In addition, many other people in NIE have given me hands in a variety
of aspects ranging from daily life to sophisticated research. Their names,
though not mentioned herewith, are remembered in the innermost recesses of
my heart.
Finally, I am deeply indebted to my wife Yang Hongsa and my lovely
daughter Shan Shan for all their support and understanding. In the whole
course, they give me a sweet home which I cannot find words to express my
appreciation.
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Abstract
This thesis describes the theoretical and experimental investigations of the
x-ray radiation properties and the plasma dynamics in several plasma focus
devices. The purpose of the research is to optimize the x-ray radiation behavior
of the plasma focus and to seek the conditions for generating shorter
wavelength x-rays with argon gas.
The characteristics of neon and argon plasmas are investigated in detail
based on the corona model. The results show that much higher temperature
and energy are required to generate x-ray radiative argon plasma. This explains
why it is much easier to obtain soft x-rays from the neon plasma focus.
An improved plasma focus model is proposed with emphasis on detailed
analysis of energy transfer processes and thermodynamic processes. Agreement
between the experiment results and the computations for neon and argon is
noted.
The model is extended to describe, for comparison purpose, the gas-puff
plasma focus. Modifications are made based on the measured gas pressure
profile and assumed gas composition. Applying the modified model to the gas-
puff focus of Gunma University reveals that the gas-puff scheme is more
efficient to heat the plasma and to generate x-rays. And it also results in a more
stable plasma column. By comparing with published results, agreement is
obtained in the major points regarding plasma dynamics, plasma column
stability and appearance, plasma temperature, and x-ray radiation properties.
Experiments are mainly carried out with the repetitive NX2 plasma focus,
under various charging voltage and gas pressure with a series of anode lengths.
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A Rogowski current derivative coil, a resistive voltage divider, and a PCD x-ray
detector are installed to measure the current derivative, tube voltage and x-ray
signal. All these detectors have been carefully calibrated to give quantitative
information. A fast four-channel data acquisition system has been set up to
record the fast data flow from the high-repetitive rate machine.
X-ray radiation properties from the plasma focus with neon have been
investigated in detail. The optimized working region is located by scanning the
working conditions. The x-ray yield in this region is ~ 12 Joule/shot with the
peak power in the GW order. The correlation between x-ray yield with various
operating parameters such as repetition rate and reproducibility is studied.
The dynamics of the plasma focus is studied with the recorded current
and voltage signal. The correlations of discharge current, focusing time, tube
inductance, axial speed and detail features of x-ray radiation are investigated.
Efforts of generating argon x-rays in NX2 are made under various
experimental conditions. However, in argon the NX2 device does not focus in
the expected high voltage, low pressure working region. The experiments are
then carried out in the UNU/ICTP PFF (United Nations University/International
Center for Theoretical Physics Plasma Focus Facility) to study the mechanism
for argon x-rays. The characteristic Ar lines are recognized from the measured
signal. The neon experiments are also carried out with this machine for
comparison. The results show the optimized working pressure for neon x-ray is
much higher than for argon. This confirms the theoretical conclusions that
mush higher energy and temperature are required to generate x-ray from argon
plasma.
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Author’s Publications during 1999 – 2000 (Related to this thesis):
[1] B. Shan, T. Yanagidaira, and K. Shimoda, “A gated, four-frame pinhole camera with
quantitative image acqusition”, Jpn. J. Appl. Phys., 38(1999)1558-1563
[2] B. Shan, T. Yanagidaira, K. Shimoda, and K. Hirano, “Quantitative measurement of
x-ray images with a gated microchannel plate system in a z-pinch plasma
experiment”, Rev. Sci. Instrum., 70(1999)1688-1693
[3] T. Yanagidaira, T. Yamamoto, B. Shan, and K. Hirano, “Spectroscopic investigation
of z-pinch with a spatial and temporal resolution”, Jpn. J. Phys. Soc., 68(1999)852-
856
[4] R.S. Rawat, W.M. Chew, P. Lee, T. White, B. Shan, S.P. Moo and S. Lee, “Roomtemperature
growth of titanium nitride thin films on 304-stainless steel substrates
using plasma focus device”, Regional Meeting on Plasma Research in 21 st Century,
Bangkok, Thailand, 7-12 May 2000, to be published
[5] B. Shan, M.H. Liu, R. S. Rawat, and S. Lee, “X-ray Emission Properties of Neon and
Argon Plasmas in Conventional and Gas-puff Plasma Focus Devices”, Regional
Meeting on Plasma Research in 21 st Century, Bangkok, Thailand, 7-12 May 2000, to
be published
[6] M.H. Liu, B. Shan and S. Lee, “Optimum plasma temperatures for neon and argon
SXR output in plasma focus”, submitted
[7] B. Shan, P. Lee, M.H. Liu, and S. Lee, “Improving the Energy Efficiency of Plasma
Focus Devices”, submitted
[8] B. Shan, M.H. Liu, P. Lee, and S. Lee, “Simulations With Conventional and Gas-puff
Plasma Focus Devices”, submitted
v

CHAPTER 1
Introduction
Table of Contents
1.1. Research Background 1
1.1.1. X-rays and plasma x-rays 1
1.1.2. X-rays from plasma focus 2
1.1.3. Plasma focus x-rays for lithography 3
1.1.4. Scope of research 4
1.2. Plasma Focus Device: Structure & Dynamics 7
1.2.1. Structure and working principle 7
1.2.2. Breakdown phase 10
1.2.3. Axial rundown phase 11
1.2.4. Radial phase 13
1.3. X-ray Radiation from Plasma Focus 19
1.3.1. Classification of x-ray sources 19
1.3.2. Development of the plasma focus x-ray source 19
1.3.3. Characteristics of plasma focus x-ray source 21
CHAPTER 2
CHARACTERISTICS OF NEON AND ARGON PLASMAS
2.1. Simplified Plasma Equilibrium Model for Plasma Focus 26
2.1.1. General consideration of plasma equilibrium model 26
2.1.2. Corona model 28
2.1.3. Calculate ionization states 29
2.2. Calculations for Argon and Neon Plasmas Using Corona Model 31
2.2.1. Equilibrium properties 31
2.2.2. Estimate the working parameters of neon and argon plasmas in
plasma focus 34
2.3. X-ray Emission Properties of Argon and Neon Plasmas 37
2.3.1. General x-ray emission processes from plasmas 37
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2.3.2. X-ray power density from plasmas 40
2.3.3. X-ray emission intensities from neon plasma 42
2.3.4. X-ray emission intensities from argon plasma 46
2.3.5. Optimized conditions for x-ray radiative neon and argon plasmas 47
CHAPTER 3
DYNAMIC PROCESSES AND MODEL OF PLASMA FOCUS
3.1. Introduction 48
3.1.1. Simplified evolution processes in plasma focus 48
3.1.2. Symbols and notations used in the modeling 52
3.2. Electrical Properties and Circuit Equations 54
3.2.1. Equivalent circuit equations 54
3.2.2. Axial phase 56
3.2.3. Radial phase 57
3.2.4. Plasma resistance 58
3.3. Shock Wave and Shock Wave Equations 59
3.3.1. Equations for 1D shock wave 59
3.3.2. Parameters for 1D shock wave 61
3.3.3. 1D shock waves with changing piston pressure or changing ambient
gas pressure 62
3.3.4. Shock wave for 2D cylindrical configuration 64
3.4. Plasma as Thermodynamic Gases 67
3.4.1. Thermodynamic properties of plasma gas 67
3.4.2. Effective specific heat ratio of plasma gas 69
3.4.3. Thermodynamic equation for plasmas 71
3.5. Energies and Temperature of the Plasma in Plasma Focus 72
3.5.1. Mechanisms of energy transfer into plasma and plasma tube 72
3.5.2. Driving parameter S 74
3.5.3. Energy transfer process 76
3.6. Equations and Expressions of the Plasma and Plasma Focus 78
3.6.1. Axial phase 78
3.6.2. Radial inward shock phase 79
3.6.3. Reflected shock equation 82
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3.6.4. Slow compression equation 83
3.6.5. Dependence of the plasma dynamics on gas properties 83
3.7. Simulation With NX2 Plasma Focus Machine 85
3.7.1. Numerical calculating method 85
3.7.2. Results for NX2 plasma focus machine 86
3.8. Modifications for Gas Puff Plasma Focus Machine 92
3.8.1. Pressure distribution in gas puff plasma focus machine 92
3.8.2. Modification of equations for gas puff plasma focus 95
3.8.3. Simulation results for Gunma machine 96
CHAPTER 4
NX2 PLASMA FOCUS MACHINE AND DIAGNOSTIC TOOLS
4.1. Technical Consideration in System Construction 100
4.2. System Arrangement 104
4.2.1. Layout 104
4.2.2. Vacuum chamber and DPF head 105
4.2.3. Electrodes and insulators 107
4.2.4. Cooling system 108
4.3. Electrical Structure 109
4.3.1. Layout 109
4.3.2. Energy bank and charger 111
4.3.3. Switches 120
4.4. Diagnostics of NX2 Machine 115
4.4.1. Measurement of discharge current 115
4.4.2. Measurement of tube voltage 116
4.4.3. Measurement of x-ray signal from plasma 117
4.4.4. Calibration of the x-ray detector system used in NX2 119
4.4.5. Data acquisition system 123
4.4.6. Data processing 127
4.5. Measurement of Circuit Parameters by Short Circuit Test 129
4.5.1. Principle 129
4.5.2. Circuit parameters of NX2 machine 131
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CHAPTER 5
EXPERIMENTAL RESULTS AND DISCUSSION
5.1. General 134
5.2. Optimizing for X-ray Output with Neon Gas 136
5.2.1. Optimizing X-ray output by changing anode length, working voltage,
gas pressure and anode material 136
5.2.2. Reproducibility of the X-ray Output 141
5.2.3. Dependence on repetition rate 143
5.3. Plasma Focus Dynamics and Parameter Correlation 145
5.3.1. General characteristics of discharge current 145
5.3.2. Focusing time 148
5.3.3. Tube inductance and axial speed 152
5.3.4. Energy & power into plasma focus tube 155
5.3.5. Detailed features of the x-ray radiation pulse 156
5.3.6. Plasma dynamics for different gases 160
5.4. Argon x-ray from NX2 162
5.5. Experiment with UNU/ICTP PFF 167
5.5.1. System arrangement 167
5.5.2. Results from argon 171
5.5.3. Results from neon 176
CHAPTER 6
CONCLUSIONS AND SUGGESTED FUTURE WORKS
6.1. Conclusions 179
6.2. Suggested Future Works 184
APPENDIX
A. Parameters for related plasma focus machines 187
B. Characteristic data for working gases 189
C. Symbols and equations of plasma focus model 193
REFERENCES 199
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List of Figures
1-1 A test lithography result by a plasma focus x-ray source 4
1-2 Mather and Filippov-type Plasma Focus devices 7
1-3 Gas puff plasma focus device (Gunma machine) 9
1-4 Plasma compression process captured by a gated MCP camera 15
2-1 Neon Ion fractions at different temperature 31
2-2 Argon Ion fractions at different temperature 32
2-3 Effective ionization number of argon, neon and hydrogen 33
2-4 Thermal & ionization energies for generating plasmas 35
2-5 Bound-bound, free-bound and free-free transitions 38
2-6 Emission intensities of neon Lyα, Lyβ, Heα, and Heβ lines 44
2-7 Continua emission intensities of Ne 10+ , Ne 9+ and Ne 8+ ions 45
2-8 Emission intensities of argon Lyα and Heα lines 46
2-9 X-ray (Lyα + Heα ) intensities of argon and neon plasmas 47
3-1 Simplified processes in plasma focus 49
3-2 Equivalent circuit structure of the plasma focus 54
3-3 Typical discharge current traces in NX2 55
3-4 Equivalent plasma configuration in axial phase 56
3-5 Equivalent radial phase configuration 57
3-6 Shock wave induced by a fast piston 60
3-7 Relationship between shock speed and shock temperature 61
3-8 Equivalent geometry for the cylindrical compression 64
3-9 Calculated specific heat ratio of neon and argon gases 71
3-10 Energy transfer process in plasma focus 77
3-11 Simulation results of NX2 87
3-12 Simulated evolution in radial inward shock phase of NX2 88
3-13 Simulated plasma temperature vs. gas pressure for neon in NX2 90
3-14 Simulated plasma temperature vs. gas pressure for argon in NX2 91
3-15 Measured gas pressure profile with the gas-puffed Gunma Machine 92
3-16 Gas pressure profile and constitution for simulation 94
3-17 Simulation results of Gunma machine 98
3-18 Evolution in radial inward shock phase of Gunma machine 99
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4-1 Block diagram of NX2 plasma focus 102
4-2 Overall view of the NX2 plasma focus machine 104
4-3 Front view of the main body of NX2 105
4-4 Vacuum chamber and plasma focus head in NX2 106
4-5 Electrical structure of NX2 Plasma Focus Machine 110
4-6 Block diagram of HVPS 802 power supply 111
4-7 Output power & current of the HVPS 802 power supply 112
4-8 Structure of the rail gap switch 113
4-9 Rogowski coil & its equivalent circuit 115
4-10 Schematic of the resistive voltage probe 117
4-11 Metal-insulator-metal PCD x-ray detector 118
4-12 Spectrum weight for x-ray calibration 121
4-13 Fast measurement system in NX2 124
4-14 Typical results by the NX2 measurement system 125
4-15 Processed results of the data in Fig. 4-14 128
4-16 Get circuit parameters by short circuit test 132
5-1 X-ray energy, peak power and focusing depth versus charging voltage and
gas pressure with 5.0 cm stainless steel anode 137
5-2 X-ray energy, peak power and focusing depth versus charging voltage and
gas pressure with 5.0 cm copper anode 138
5-3 X-ray energy, peak power and focusing depth versus charging voltage and
gas pressure with 4.3 cm copper anode 139
5-4 Correlation between x-ray peak power and x-ray yield 140
5-5 Histogram of X-ray intensity distribution 141
5-6 X-ray reproducibility of 5.0cm stainless steel anode 142
5-7 Working properties' dependence on repetition rate 143
5-8 Discharge current at different charging voltage 146
5-9 Current traces at different gas pressure 146
5-10 Simulated current traces for NX2 plasma focus 147
5-11 Focusing time of the 4.3 cm Cu anode with neon gas 150
5-12 Correlation between x-ray yield and focusing time of 5.0 cm stainless steel
anode 151
5-13 Correlation between focusing time and x-ray Yield of 4.3 cm Cu anode 151
5-14 Tube inductance and axial speed 153
5-15 Details of the tube inductance and axial speed 154
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5-16 Energy & power into PF tube 155
5-17 Details feature of the X-ray radiation and dI/dt signal 157
5-18 Detailed neon x-ray pulses from different working pressures 158
5-19 dI/dt traces for neon and deuterium at same mass density 160
5-20 Four channel signals from NX2 with argon gas 163
5-21 Compare detailed features of argon and neon x-ray pulses 164
5-22 Overall dI/dt traces of neon and argon with same mass density 165
5-23 Simulated radial speed and dI/dt of neon and argon 166
5-24 Technical parameters & x-ray Sensitivities of BPX65 diode 169
5-25 Five-channel PIN x-ray detector 169
5-26 Sensitivities of the five-channel pin-diode detectors 170
5-27 Sensitivity ratio of channel 2 to channel 3 171
5-28 Discharge current traces at different pressures of argon 172
5-29 Argon x-ray signals from UNU/ICTP PFF 172
5-30 X-ray signals from 0.75 mbar argon 173
5-31 X-ray signals from 0.25 mbar argon 174
5-32 X-ray signals from 2.0 and 2.5 mbar argon 175
5-33 Neon x-ray signals from UNU/ICTP PFF 177
5-34 X-ray signals from 1.5 mbar neon 178
A-1 Typical discharge current from Gunma machine (Ne puff) 188
A-2 Typical discharge current from Gunma machine (Ar puff) 188
B-1 X-ray emission spectrum from neon plasma 192
B-2 X-ray emission spectrum from argon plasma 192
xii

List of Tables
2-1 Optimization conditions for x-ray radiative neon and argon plasmas 47
3-1 Symbols and notations in the model 52
3-2 Thermodynamic properties of the plasma gas in plasma slug/column 68
4-1 Specifications for the model 40200 rail-gap switch 114
4-2 Typical specifications for PCDs 118
4-3 Symbols used for calibrate the response of the PCD detector 119
4-4 Calibration factors for argon & neon x-ray in the NX2 PCD configuration 122
4-5 Signal time of the four channels in the measurement system 123
4-6 Format of the data file of the NX2 data acquisition system 126
4-7 Main Electrical parameters of NX2 133
5-1 Experiments on NX2 plasma focus machine 135
5-2 Technical parameters of UNU/ICTP PFF 168
A-1 Technical parameters of related plasma focus machines 187
B-1 Basic parameters of working gases 189
B-2 X-ray emission spectrum of highly ionized neon plasma 190
B-3 X-ray emission spectrum of highly ionized argon plasma 190
C-1 Symbols and notations in the expressions and the equations of the PF
model 193
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