I= TAILORING A LOW ENERGY PLASMA FOCUS AS A HIGH EFFICIENCY X-RAY SOURCE FOR RADIOGRAPHY
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Title of Thesis
TAILORING A LOW ENERGY PLASMA FOCUS AS A HIGH EFFICIENCY X-RAY SOURCE FOR RADIOGRAPHY

Author(s)
Safdar Hussain
Institute/University/Department Details
Department of Physics/ Quaid-i-Azam University, Islamabad
Session
2005
Subject
Physics
Number of Pages
155
Keywords (Extracted from title, table of contents and abstract of thesis)
plasma focus, x-ray source, radiography, mather type plasma focus, z inserts, field distortion

Abstract
The work adopted in this thesis is mainly based on the systematic investigations of X-rays emitted from a Mather type plasma focus by making suitable changes in experimental parameters like charging voltage, capacitor energy, discharged current, anode shape and insert material at the anode tip and filling gas to obtain high energy/ intensity photons for radiographic applications. Experiments are conducted with Pb and W insert at the anode tip. A three-channel X-ray detector consisting of Quantrad Si PIN-diodes with differential filtering and a multipinhole camera with suitable absorption filters are employed to study the X-ray emission.

In the first experiment thorough studies on X-ray emission from a plasma focus with different Pb inserted anode shapes in hydrogen gas are presented. A 9 uF capacitor bank, charged at 20 kV and giving peak discharge current of about 175 kA is used to power the device. The maximum values of energy integrated X-ray flux are found to be 1.36 ± 0.06, 2.70 :t 0.11 and 2.17 ± 0.09 J/sr for cylindrical anode with a 75° cut at the tip, tapered anode with a 75° cut at the tip and tapered anode without any cut respectively. The maximum X-ray emission in 4 €-geometry is estimated to be 17.09 :t 0.75. 27.91 ± 1.28 and 27.25 ± 1.13 J with cylindrical anode with a 75° cut at the tip, tapered anode with a 75° cut at the tip and tapered anode without any cut, respectively, and corresponding wall plug efficiencies for X-ray generation are 0.95%, 1.55% and 1.51 %. It is observed that the X-ray yield strongly depends upon the anode shape as well as filling pressure. The X-rays are emitted mainly from the anode tip through thick-target bremsstrahlung mechanism, by impact of energetic electrons from the pinch region. The device is successfully operated for radiographic imaging of different objects mounted outside the vacuum chamber.

For further enhancement of X-ray generation efficiency of plasma focus, the next experiment is carried out with a 12.5 uF capacitor charged at 15 - 27 kV (1.4 - 4.6 kJ), giving peak discharge current of about (160 - 265) kA. In 13 - 25 and 5 - 20 ke V energy ranges, the maximum X-ray yield of 8.6 ± 0.4 and 18.6 ± 0.9 J per shot in 4 € sr are obtained for 23 kV charging voltage and 225 kA discharge current, respectively. For total discharge energy of 3.3 kJ, one may estimate the average efficiency of the system in these energy ranges, which is about 0.26% and 0.56% respectively. The maximum X-ray yield at an optimum hydrogen operating pressure was estimated to be 46.4 ± 2.5 J/shot in to 47 € sr for 23 kV charging voltage and 225 kA current, which was 1.40% of the stored electrical energy. The X-ray emission from the device increases almost linearly up to 21 kV charging, saturates at 23 kV, and then deteriorates at higher voltages. The saturation and deterioration of the X-radiation emission is found due to enhanced addition of Pb vapours sputtered from the anode tip, which significantly changes the current sheath dynamics.

The third experiment is conducted with W insert at the anode tip, using 12.5 ulF capacitor charged at 19 - 29 kV (2.3 - 5.2 kd), giving peak discharge current of about (188 - 290) kA. In 5 - 20 and 13 - 25 keV energy ranges, the maximum X-ray yield of 25 : ± 1.3 and 9.5 ± 0.5 J per shot in 47 € -geometry are obtained respectively at 29 kV charging voltage and corresponding X-ray generation efficiency of the system in these energy ranges, is 0.48% and 0.18%. In this experiment, X-ray emission yield of 63.7:1: 3.4 J is recorded for 29 kV charging and 290 kA peak discharge current, that corresponds to wall plug efficiency of 1.20%. Pinhole images of the X-ray emitting sites reveal that soft X-rays are emitted from the focus region, whereas hard X-rays are emanated from the anode tip. The results are interesting for using a low energy plasma focus as a high efficiency X-ray source for flash radiography and other applications.

Download Full Thesis
2591.65 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
257.19 KB
2 1 Introduction 1
612.76 KB
  1.1 Present X-Ray Sources For Radiography 2
  1.2 Plasma Focus - Development And Dynamics 18
  1.3 Outline Of The Thesis 28
  1.4 References 28
3 2 Experimental - The Device 33
430.08 KB
  2.1 Plasma Focus System - A Brief Description 33
  2.2 Field Distortion-Type Spark Gap For Plasma Focus 41
  2.3 References 45
  3 46
  3.1 Basic Electrical Diagnostic Methods 46
  3.2 X-Ray Detection And Analysis System 55
  3.3 References 72
4 4 Tailoring Of Enhanced X-Ray Yield With High - Z Inserts 74
904.48 KB
  4.1 Lead Insert - 20 Kv Charging 77
  4.2 Radiation Saturation And Deterioration 99
  4.3 X-Ray Emission With Tungsten Insert 113
  4.4 Scope Of Plasma Focus As X-Radiation Source For Radiography 120
  4.5 References 128
5 5 Conclusions And Possibilities For Future Experiments 132
94.72 KB
  5.1 Conclusions 132
  5.2 Possibilities For Future Experiments 135