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The pseudospark as an electron beam source
Abstract
The pseudospark is a low-pressure, hollow-cathode gas discharge
that occurs in a special discharge geometry (pseudospark chamber) in
different kinds of gases. A modular pseudospark chamber was built to
investigate this discharge type as a source of intense electron beams.
At a breakdown voltage of 24 kV and a discharge current of 480 A, an
electron beam of 106 A and 13 ns FWHM (full width at half maximum) was
extracted through the anode hole into a drift chamber filled with
low-pressure gas. Electrical parameters of the circuit, including the
plasma channel, were evaluated by monitoring the discharge current
waveform. First results of beam profile and emittance measurements of
the produced electron beam are presented. At an axial distance of 9 cm
behind the anode, an RMS emittance of 55 mm-mrad was measured. The
results obtained make it possible to consider the pseudospark discharge
a high-brightness electron beam source
Pseudospark (PS) discharge-based electron beam sources are capable of generating high current density, high energy, high emittance, and high brightness self-focused electron beam. PS-based electron beam sources are potential candidates for a wide range of applications including high-voltage and high-power switching, X-ray source, electron and ion beam generation, free electron masers, extreme-ultraviolet radiation sources, free electron laser, and micro-thrusters, material process, and microwave devices. In this article, a focused review on various types of PS discharge-based electron beam sources, their mechanisms and applications have been presented. This content will provide an overview on the importance as well as some limitations of PS discharge-based electron beam sources.
During its conductive phase, a pseudospark discharge is able to generate a low energy electron beam with a higher combined current density and brightness compared with electron beams formed from any other known type of electron source. In this paper, a configuration is proposed to post-accelerate an electron beam extracted from a single-gap pseudospark discharge cavity in order to achieve high quality high energy intense electron beams. The major advancement is that the triggering of the pseudospark discharge, the pseudospark discharge itself, and the post-accelerating of the electron beam are all driven by a single high voltage pulse. An electron beam with a beam current of ∼20 A, beam voltage of 40 kV, and duration of ∼180 ns has been generated using this structure. The beam energy can be adjusted through adjusting the amplitude of the voltage pulse and the operating voltage of the whole structure, which can be varied from 24 to 50 kV with an efficient triggering method under fixed gas pressure below ∼10 Pa.
Summary form only given. An experimental investigation of a pseudospark-produced electron beam is being performed. The pseudospark chamber consists of modular six gap brass and insulator disks stacked between a hollow cathode and an anode. The anode has a 3.2-mm on-axis hole for the beam extraction. External capacitances are added to the self-capacitance of the pseudospark chamber to determine the beam behavior with different stored energies. The pseudospark is operated in the 5-25-kV voltage and 30-70-mtorr pressure ranges. A breakdown curve similar to Paschen's curve (for two parallel electrodes) was obtained for the pseudospark. The breakdown occurs in two phases. During the first phase, the discharge voltage drops slowly while the discharge and beam current are small. In the second main breakdown phase, the discharge voltage drops to almost zero and the discharge and beam current rise to large values. For Cext=980 pF, an electron beam with a current of 150 A and an average energy of 25 keV is produced. The RMS emittance of the beam measured at an axial distance of 9 cm from the anode is 65 mm-mrad. This corresponds to a beam brightness of 1.0×1011 A/(m2-rad2), which characterizes the pseudospark as a high brightness source
The development of thermionic electron guns is of significant importance in electron beam technology. The paper is primarily concerned with the design, fabrication and experimentation of electron beam sources developed by our research group at electron beam sources development laboratory (EBSDL). This includes the development of directly and indirectly heated thermionic electron beam sources as well as high power line source electron guns. The important features of the optimized gun designs, their emission characteristics and performance are briefly outlined and their results are presented. These guns are currently being used for melting, evaporation, welding, heat treatment and research applications.
A directly heated thermionic high power electron beam source was constructed. The circular cross-section tungsten line cathode of length 140mm with diameter 0.9mm was used. Different gun design parameters were investigated and their results are discussed in detail. A uniform external magnetic field of 50G was employed for focusing of electron beam at 180° deflection. The gun delivers uniform emission current density throughout the emission surface. The dimensions of the electron beam at worksite were comparable with the line cathode. The beam power of 57kW was successfully achieved well below the saturation limit. The spring action mechanism was especially designed to avoid any cathode deformation. The gun design facilitates the reported length and emitting surface temperature of the cathode to be further increased to obtain higher emission values. The important gun features are operational reproducibility and long time stability. The evaporation rates for stainless steel have been achieved up to 1kg/hr. An area of (280×120mm2) could be heat treated with the line source electron beam in few milliseconds. The gun is extremely useful for melting, evaporation and heat treatment.
Laser-controlled beamfront accelerator experiments achieved gradients of up to 40 MV/m and proton acceleration to 18 MeV over 40 cm with a beam energy of 900 keV. Beamfront electric field degradiation requirements beam energy to be increased as the gradient is increased. An experiment with a beam energy of 1.5 MeV achieved a 60 MV/m gradient in a 100 cm distance, matching theoretical models. Experiments with psuedospark discharges achieved electron beams of 25 KV, 1000 A/sq. cm., 10 ns with 10(11) A/(m-rad)sq brightness. Theory indicates densities greater than 10(3) A/sq. cm. and brightness greater than 10(11) A/(m- rad)sq are achievable. (jhd)
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