X-Ray Analysis for Electron Beam Enhancement in the Plasma Focus Device^*

Abstract

The plasma focus device, a form of linear pinch discharge, produces an intense x-ray and neutron (D2) burst from a magnetically heated dense plasma. Rapidly changing magnetic fields at pinch time generate large axial electric fields which accelerate electrons and ions. In the experiments reported here the x-ray production during the plasma pinch of a 96 kilojoule (at 20 kV) plasma focus device was measured.The purpose of these experiments was to evaluate the energy in accelerated electrons in the plasma focus device and to learn how to enhance these electron hursts. Well focused, megampere electron beams at a few hundred kilovolts, lasting less than 10 nanoseconds have applications in fusionable pellet heating experiments. (1) X-rays were monitored to evaluate these electron bursts using a defocusing bent crystal spectrometer, doubly diffused silicon (PIN) detectors, with Ross filters, thermoluminescent dosimeters (TLDs) with filters, and x-ray pinhole photography.Thermoluminescent dosimeters indicated maximum x-ray yields of 140 joules above 3 keV at 57.3 kilojoules stored energy (16 kV) for a conversion efficiency to x-rays of 0.2%. 40 joules are above 60 keV and 15 joules above 80 keV. The hard x-ray pulse typically rises in 3 ns and frequently has a pulse width less than 10 ns. The low energy x-ray spectrum consists almost entirely of lines from the high Z anode insert, and the high energy spectrum is characteristic of a nonthermal power law distribution with an exponent of 2.2 ± 0.8. Peak hard x-ray production is obtained at 1 torr deuterium in contrast to peak neutron production (3 x 1010) at 5 torr. The addition of argon reduces total x-ray yield and increases the relative fraction of soft x-rays.These measurements suggest that the plasma focus produces 1200 joules of electrons with an average energy of 150 keV, in 10 nanoseconds with a stored energy of 57.3 kilojoules. This is a power of 1.2 × 1011 watts and power density of 1.5 × 1013 watts cm−2.

Particle beams generated by a 6–12.5 kJ dense plasma focus

“…11. The electron spectra appear to harden with increasing IMB where the absolute value of the exponent, |x|, scales as I^3 S °\ These results are consistent with the hard-X-ray data where the absolute value of the electron beam energy power law ranged from 3.5-4.0 for 20 kJ devices [16,18] to 2.4-4.0 for a 57.3 kJ device [17], and down to 2.0 for a 375 kJ device [19]. Thus the results from the 6-12.5 kJ device reported here fit these X-ray data fairly well although these new results are less ambiguous, being the result of a direct measurement.…”

“…1) are observed by direct methods to have the same power law energy dependence at a device current at pinch time, IMB, of 560 kA. The scaling of the absolute value of the exponent of the power law was found (for electrons) to decrease with increasing IMB (hardening of the spectrum) and the values were found to be consistent with the electron energy spectra inferred from hard X-ray spectral measurements on several other devices [16][17][18][19]. It is interesting that power law spectra of similar exponents have also been observed in a double inverse pinch [41 ], cosmic rays [57], and solar flares [58],…”

“…Since much more energetic plasma foci exist such as the 1 MJ device at Frascati [50], it would be very interesting if similar measurements were performed on these devices to see if the scaling observed in the Illinois device persists at high bank energy. If so, the hardening of the energy spectra indicated by these measurements and others [16][17][18][19] may lead to new applications of the DPF as a pulsed electron beam source.…”

“…The electron beam energy spectrum of the plasma focus has generally been inferred by analysis of the hard X-ray emission spectrum [16][17][18][19][20][21] or of dendrite patterns [21][22][23] caused when the fast electrons strike a target such as plexiglas. The hard-X-ray emission energy spectra are observed [16][17][18][19]21] to have a power law dependence decreasing with photon energy, but it is not possible to uniquely determine the electron energy spectra from such X-ray measurements. Usually, a thick-target bremsstrahlung model and a power-law electron energy spectrum are assumed in order to evaluate the exponent of the power law [ 16].…”

Post-pinch generation of electron beam in a low energy Mather-type plasma focus device

“…The produced X-ray radiation by the interaction of electrons with anode tip has been used indirectly to measure electron beam energy spectrum. However, direct methods of measurement using diagnostics, such as filters (Smith et al 1985), Rogowski coils, Faraday cups (Stygar et al 1982;Smith et al 1985), self-bias Faraday cups (Neog and Mohanty 2007), magnetic electron energy analyzers (Gullickson and Barlett 1975;Stygar et al 1982;Smith et al 1985;Hirano et al 1990;Zhang et al 2005), and Cherenkov detectors (Jakubowski et al 1996(Jakubowski et al , 1997(Jakubowski et al , 1998Sadowski and Scholz 2003), have also been cited in literature.…”