Pseudospark Electron Beam Source
This page was begun on 16 January, 2012 and will be updated as the project progresses. Please use the Forum to ask questions and to provide feedback.
In the late 1950s Jens Christiansen worked on parallel plate avalanche counters for nuclear physics experiments. Near the Paschen minimum, anomalous sparks at the edge zones of these counters resulted in erratic failures. Twenty years later he reestablished the research in this almost forgotten effect. [Quote from K. Frank and J. Christiansen, IEEE Trans. Plasma Sci. 17, 748 (1989)]. In 1979, J. Christiansen and C. Shultheiss published a paper Production of high current particle beams by low pressure spark discharges, in Z. Phys. A 290, 35 (1979). In this paper, a low pressure fast discharge phenomenon between a hollow cathode and an anode is described and named as pseudospark.
A minimal pseudospark device consists of two parallel plates, each with a 3-4 mm aperture and separated by an insulator. Behind the cathode plate is a hollow cathode where the discharge initiates. The device works on the left hand side of the Paschen curve, i.e. where decreasing pressure results in a higher breakdown voltage. Additional electrodes may be added to increase the breakdown voltage. The charging circuit of a pseudospark device generally consists of a high voltage capacitor and dc power supply. The hollow cathode usually incorporates a trigger mechanism. Typical operating pressures are in the range on 100 - 500 mTorr.
There are several stages to the operation of a pseudospark device:
- Predischarge and ignition of the pseudospark
- Development of a hollow cathode discharge. This produces a brief (nanosecond scale) high energy, high brightness electron beam (up to the charging voltage) and a reverse direction ion beam.
- A high current main discharge
- Decay of the plasma
Depending on the energy available to the device, the discharge may not proceed to the high current stage. In extreme cases, the third stage can lead to a low voltage arc and significant erosion of the electrodes.
Pseudospark devices are now available as high current, fast switching alternatives to thyratrons, ignitrons and spark gaps. In addition, pseudospark devices are used as sources of electrons to pump other devices and for use in specialized thin film deposition processes.
The Patents Page has a number of patents related to pseudospark sources. Please consult them for further detail on the various uses and configurations.
My pseudospark device has gone though a number of iterations over the course of the past 15 years, much of which is detailed in the First and Second Five Years complilations. The current incarnation is depicted in the drawings at the top of this page and in the photograph below. The unit consists of 8 stainless steel electrodes separated by 0.100 plexiglas disks. The hollow cathode incorporates a flashover trigger. The source is then connected to an experiment chamber that was fabricated from a discarded liquid nitrogen foreline trap. This chamber has a view port and a pair of Helmholtz coils for electron beam deflection. The capacitor bank typically consists of one or more 2.7 nF, 40 kV doorknob capacitors.
Ongoing work consists of beam energy analysis and deposition.
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