Measurements of fast electron beams and soft X-ray emission from plasma-focus experiments

Abstract

Abstract. The paper reports results of the recent experimental studies of pulsed electron beams and soft X-rays in plasma-focus (PF) experiments carried out within a modifi ed PF-360U facility at the NCBJ, Poland. Particular attention was focused on time-resolved measurements of the fast electron beams by means of two different magnetic analyzers, which could record electrons of energy ranging from about 41 keV to about 715 keV in several (6 or 8) measuring channels. For discharges performed with the pure deuterium fi lling, many strong electron signals were recorded in all the measuring channels. Those signals were well correlated with the fi rst hard X-ray pulse detected by an external scintillation neutron-counter. In some of the analyzer channels, electron spikes (lasting about dozens of nanoseconds) and appearing in different instants after the current peculiarity (so-called current dip) were also recorded. For several discharges, fast ion beams, which were emitted along the z-axis and recorded with nuclear track detectors, were also investigated. Those measurements confi rmed a multibeam character of the ion emission. The time-integrated soft X-ray images, which were taken side-on by means of a pinhole camera and sensitive X-ray fi lms, showed the appearance of some fi lamentary structures and so-called hot spots. The application of small amounts of admixtures of different heavy noble gases, i.e. of argon (4.8% volumetric), krypton (1.6% volumetric), or xenon (0.8% volumetric), decreased intensity of the recorded electron beams, but increased intensity of the soft X-ray emission and showed more distinct and numerous hot spots. The recorded electron spikes have been explained as signals produced by quasi-mono-energetic microbeams emitted from tiny sources (probably plasma diodes), which can be formed near the observed hot spots.

Dynamics of ion beam emission in a low pressure plasma focus device

“…It was proposed by Newman et al [44] that the origin of the hard x-ray emission detected outside of the vacuum chamber to be due to the bombardment of non-thermal high energy electron beam to the anode tip. An electron beam with energies in the range of 41-715 keV, which lasts tens of nanoseconds has been reported in the PF-360U facility [45]. The interpretation of electron, ion beam and soft x-ray emission suggested the charged-particles were produced from different micro sources in the pinched plasma.…”

Development of a radiographic method for measuring the discrete spectrum of the electron beam from a plasma focus device

“…Although there are many experimental results on the measurement of the electron beam produced by PF devices, a comprehensive mechanism is not yet available to explain the acceleration of electrons in this device [6][7][8]. One of the possible explanations is a hypothesis that some electrons are accelerated inside miniature plasma diodes, which can be formed due to magnetohydrodynamic instabilities of current filaments observed in PF discharges, and particularly inside pinch columns [22][23][24].…”

“…It is well known that HXR pulses are produced mainly by fast electron beams interacting with metal electrodes, but it is not known where and when such fast electron beams are generated and accelerated [21,22]. Although there are many experimental results on the measurement of the electron beam produced by PF devices, a comprehensive mechanism is not yet available to explain the acceleration of electrons in this device [6][7][8].…”

“…In recent years, several studies have been conducted on the measurement of electron emissions in PF devices. Surala et al reported results of experimental studies on the pulsed electron beam of PF-360, the PF facility, by means of two different magnetic analyzers, which could record electrons in the vast 41-715 keV energy range [21,22]. Their results showed relatively strong electron pulses in all measuring channels from 41-715 keV.…”