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Title of Thesis

Sarfraz Ahmad
Institute/University/Department Details
Department of Physics/ Quaid-i-Azam University, Islamabad
Number of Pages
Keywords (Extracted from title, table of contents and abstract of thesis)
mather-type plasma focus, neutrons emission, photons emission, depleted uranium, neutron emission, x-ray emission

The work reported in this thesis is based on the systematic investigations of neutrons and X-rays emitted from a Mather type plasma focus by varying charging voltage (capacitor energy/ discharged current) and employing different pre-ionization schemes to obtain high neutrons and photons emission. Experiments are conducted with and without pre-ionization. Scintillator coupled with photomultiplier tube, GM counter along with activation foil and a pinhole camera with CR-39 detectors are used for neutron detection. While two-channel X-ray detector consisting of Quantrad Si PIN-diodes masked with Ni (l7.5µm) and Co (20 µm) filters and a multipinhole camera with suitable absorption filters are employed to study the X-ray emission.

Using a 9 µF capacitor bank charged at 16 and 20 kV (1.15 and 1.8 kJ) along with a triggertron type sparkgap, the following two experiments are conducted. In first experiment, the influence of pre-ionization around the insulator sleeve of plasma focus, by a mesh type β-source (28Ni63), on neutron emission by d(d, n)He3 is investigated by charging the capacitor bank at 16 kV (1.15 kJ) with peak discharge current of about 135 kA. The maximum axial neutron flux is enhanced by ~ 20%, whereas the increase in radial flux is ~ 35%. That lowered the neutron fluence anisotropy from 1.6 to 1.4. The neutron yield is increased from 6.5 x107 to 8.5 x107 per burst. Hence with the addition of a p-source, the neutron yield increased by about ~25%. For second experiment, the discharge circuit is modified by employing different shunt resistors of 100, 150,200,250, 300 and 350 MΩ across the spark gap. The capacitor bank, charged at 20 kV (1.8 kJ), gives peak discharge current of about 175 kA. The current continuously flows across the shunt resistor through anode to the cathode base plate and causes ionization in the gas near the junction of anode and cathode during charging of the capacitor. The maximum Cu-Kα emission of 16.0±1.0 J for 250 MΩ shunt resistor and 14.0±3.0 J per shot without pre-ionization with total X-ray yield of about 26.0 ±1.0 J for pre-ionization (250 MΩ shunt resistor) and 18.0±4.0 J without pre-ionization in 4€-geometry are estimated. The X-ray emission is enhanced by (45±5) % at the optimum pressure.

The rest of experiments are performed with 12.5 µF single capacitor charged at 17-25 kV (1.8- 3.9 kJ). This system employs a field distortion type sparkgap having 100 MΩ resistor in parallel, for potential distribution. In the first experiment, the effect of pre-ionization caused by depleted uranium (92U238) on neutron emission is investigated by varying the charging voltages from 17- 23 kV (1.8-3.3 kJ). The maximum axial signal intensity is enhanced by ~ (50±5) %, whereas the increase in radial signal intensity is ~ (45±5) % with pre-ionization as compared to without pre-ionization. The neutron signal pulse widths (FWHM) are also increased up to (20±3) %, by impact of pre-ionization. The maximum yield of 2.5x 108 is recorded at 3.5 mbar without pre-ionization. With pre-ionization, the maximum yield is about 3.8xl08. The neutron yield increases by about ~ (50±5) %. The images on CR-39 detectors demonstrate that the charge particle emission from the focus region is more intense and the pinch filament looks quite stable and large in volume as compared to without pre-ionization. The second experiment is conducted to study the effect of pre-ionization induced by a mesh type p-source (28Ni63) on the X-ray emission from a (2.3-3.9 kJ) plasma focus device with argon and hydrogen filling. The pre-ionization, besides improving the shot to shot reproducibility, enhances the characteristics as well as total X-ray emission about (25±3) % for argon filling, and about (17±2) % for hydrogen filling. Further, the pressure range of X-ray emission is broadened. In the third experiment, X-ray generation efficiency of plasma focus, in the presence of pre-ionization induced by depleted uranium (92U238) is investigated. It is found that the pre-ionization enhances Cu-Kα and total X-ray yield about 100%, broadens the X-ray emission pressure range, X-ray pulse width, and improves shot to shot reproducibility of plasma focus operation. The pinhole images of X-ray emitting zones indicate that dominant X-ray emission is from the anode tip. The last experiment is carried out by employing different shunt resistors of 200, 250, 300 and 350 MΩ across the spark gap (having already 100 MΩ for potential distribution) switch, which resulted to effective resistance for pre-ionization 66, 71, 75 and 78 MΩ. The measured values of pre-ionization current across the 66, 71, 75 and 78 MΩ effective shunt resistors are 330±5, 315±5, 301±5 and 288±5 µA respectively at 1.5 mbar Ar filling. The pre-ionization enhanced the total X-ray yield about (53±5) % at optimum pressure. The X-ray emission is found to be predominantly as a result of electrons' bombardment activity at the anode tip and hot spots along the pinch axis, which is confirmed by the images recorded by the pinhole camera. The pinch filament formed by the gas (hydrogen or argon) is not the main source of X-ray emission. Rather, the interaction of electron beam with the anode tip and the metal vapors ablated from the anode surface, that adds to the plasma give rise to X-rays, in particular with the hydrogen filling.

It is observed that the radiated energy strongly depends upon the filling pressure as well as on effectiveness of pre-ionization. There is a significant increase in the characteristics and total X-ray emission with pre-ionization caused by depleted uranium as compared to other pre-ionization schemes. The result may be explained in terms of effectiveness of depleted uranium a-source for pre-ionization. Thus pre-ionization prior to pulse discharge plays an important role in the breakdown phase of the plasma focus operation, broadens X-ray emission pressure range, enhanced X-ray emission and improves shot to shot reproducibility. The results may be interesting for using a plasma focus as an enhanced neutron and X-ray emission source for different applications.

Download Full Thesis
4328.1 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
372.54 KB
2 1 Introduction 1
1176.85 KB
  1.1 Pre-Ionization In Plasma Devices-A Review 2
  1.2 Plasma Focus Dynamics 13
  1.3 Recent Trends In Plasma Focus Research 19
  1.4 Layout Of The Thesis 27
  1.5 References 27
3 2 Experimental Setup And The Diagnostics 36
906.35 KB
  2.1 The Plasma Focus Device At Qau 36
  2.2 Basic Electrical Diagnostics 43
  2.3 Neutron Measurements 51
  2.4 X-Ray Detection And Analysis System 57
  2.5 X-Ray Photographic Film 63
  2.6 X-Ray Absorption Filters 68
  2.7 References 68
4 3 Neutron Emission With Pre-Ionization 72
707.21 KB
  3.1 Neutron Emission In The Presence Of ’-Source 72
  3.2 Neutron Emission In The Presence Of ‘-Source (Depleted Uranium ) 83
  3.3 References 96
5 4 X-Ray Emission With Pre-Ionization 99
1143.64 KB
  4.1 X-Ray Emission In The Presence Of ’-Source 103
  4.2 X-Ray Emission In The Presence Of ‘-Source (Depleted Uranium ) 118
  4.3 X-Ray Emission In The Presence Of Shunt Resistors 126
  4.4 Results And Comparison (Different Pre-Ionization Schemes ) 144
  4.5 References 147
6 5 Conclusions 150
219.42 KB
  5.1 Conclusion 150
  5.2 Suggestions For Further Work 155