Soft X-rays from Compact Plasma FocusLiu, Mahe (1996) Soft X-rays from Compact Plasma Focus. PhD thesis, Nanyang Technological University Singapore. This is the latest version of this eprint. Full text available as:
AbstractThis thesis reports investigations of soft x-ray emission from a compact plasma focus. Experiments were carried out under various operational conditions with (a) three different working gases, neon, argon or deuterium, and (b) with the central electrode operating at initial positive and negative charging voltages.A current transformer, a Rogowski current derivative coil, a resistive voltage divider, a PIN optical speed detector and magnetic probe arrays were used to study the evolution, gross dynamics, and discharge characteristics of this plasma focus device. Soft x-rays (SXR) in the spectral regime 3 - 30 Å from the neon plasma focus have been investigated in detail. An x-ray pinhole camera, a pinhole transmission grating spectrograph, a flat crystal spectrograph, a filtered 5-channel PIN soft x-ray detector, a photoelectric effect x-ray detector and an x-ray calorimeter were the diagnostics viewing the x-ray output of the pinched plasma region. The pinhole camera, the pinhole transmission grating spectrograph and the spatially resolved flat crystal spectrograph observed a non-uniform column-like x-ray source with ~ 300 micron diameter and ~ 7 mm length centred at the axis under optimum operational conditions. The x-ray spectrum with high resolution from this neon plasma source is obtained by the flat crystal spectrograph. The x-ray emission is mainly within the wavelength range from 8 Å to 14 Å. The helium alpha line (He-) at 13.447 Å and the hydrogenlike alpha line (Ly-) at 12.132 Å are observed to be the most intense features of the neon spectrum, contributing 53 ~ 67% of the total spectrum emission. The x-ray spectrum deduced from the 5-channel PIN detector shows that 44% of the total soft x-ray energy is at 13.447 Å, 20% at 12.132 Å and 36% in the wavelength range of 8 ~ 11.568 Å, which is consistent with the results obtained by the flat crystal spectrograph. The calorimeter is used to measure the total SXR yield. At optimum condition (14 kV, 4 mbar) a total SXR yield of 6 J/shot at source into 4steradians is observed from this plasma source. A radiative plasma focus model was developed. Reflected shock phase and radiative phase are added to the earlier model to simulate the x-ray emission from the plasma focus. Electron temperature of 360 eV and electron density of 1026 m-3 were computed for the uniform radiative compression. Radiation from this compression was also computed. Line radiation comprises 70% of the total x-ray emission. These results show good agreement with the experimentally measured values. The radiative plasma focus model also shows that for a good SXR yield from neon plasma a temperature range of 300 ~ 500 eV is optimum. Computations also indicate the need to reduce circuit inductance. Reduction of stray inductance from 110 nH to 10 nH will result in a 10-fold increase in proportion of stored energy converted into focused plasma energy. SXR yield will rise correspondingly. The results of this thesis form a data base for the development of a high performance, high repetition rate SXR source for lithography applications which is now in progress in our laboratory. Front Pages; Table of Contents Chapter 1: Introduction Chapter 2: Plasma Focus & X-ray Emission Chapter 3: Radiative Plasma Focus Modelling Chapter 4: Apparatus and Diagnostic Techniques Chapter 5: Experimental Results Chapter 6: Conclusion
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