Hollow cathode effects in the pre-breakdown phase of a pulsed capillary discharge

Abstract

Experimental observations of the hollow cathode effect (HCE) in an open end pulsed capillary discharge (PCD) are presented. In the HCE axial electron beams emitted from a pre-breakdown plasma produced spontaneously in the hollow cathode region (HCR) assist ionization growth in the interelectrode volume. The PCD operates in argon at 0.6-1.4 Torr, ∼10 kV applied voltage. Time resolved spectroscopic measurements, with 15 ns time resolution, are used in conjunction with photomultiplier observations of light emission from the capillary ends, and Faraday cup measurements of axial electron beams, to characterize the pre-and post-breakdown processes in the HCR of the discharge. The HCR emission is found to be dominated by Ar II lines. Comparison between measured and synthetic spectra indicates that the pre-breakdown HCR plasma is characteristic of a collisional low pressure, low density plasma, whereas the post-breakdown HCR plasma, tens of nanoseconds after breakdown, is due to plasma ejection from the capillary volume. Experimental evidence of a zippering effect in post-breakdown capillary plasma heating, due to an initial axial pressure gradient, as predicted by computer simulations, has been found.

Fast plasma discharge capillary design as a high power throughput soft x-ray emission source

“…3 here͒. The precursor e-beams are also in evidence in another recent work 36 where both a Faraday cup and scintillator were used to measure the precursor and main e-beam signal at breakdown. As the overall energy density of the plasma was at least one order of magnitude lower than this work, only the optical emission was analyzed.…”

“…For fast repetition rate operation and with a longer ϳ1 s energy capacitor storage a longer capillary length must be used to allow operation to 25 kV, as we have presented here. Subsequently the original implementation was rendered suitable for a fast repetition rate by direct transformer charging using an IGBT based driver, although the charging time was substantially increased to 2 s. 36 It was found that operation at a frequency above 20 Hz in argon and nitrogen greatly increased the reproducibility of the discharge. This work uses a configuration of less than 0.1 J stored energy and observes optical and e-beam signals to establish experimentally the role of the hollow cathode generated e-beam on subsequent plasma evolution.…”

Observations of soft X-ray emission and plasma dynamics of a compact capillary discharge operated in xenon

“…8,27 These electrons beams play a very important role in the emission process, they shift the average ionization stage to higher values, making possible to obtain emission from highly ionized states that are only possible to obtain with Maxwellian plasmas at much higher temperatures. For example, emission in argon discharges has been found from ionization states as high as Ar XI.…”

Hollow Cathode Electron Beam Formation and Effects on X-Ray Emission in Capillary Discharges

“…Prebreakdown electron beams are well-centered ON-axis and induce the formation of a fast ionization wave, which propagates axially through the capillary length [8]. The characteristic energy of the electron beams emitted during the high-speed ionization wave propagation can be equal to, but often substantially higher than the energy expected from the applied potential [21], [22]. For the second electron beam, typical currents of around tens of milliamps have been measured [23].…”