We have investigated the pre-breakdown ionization processes in a pulsed capillary discharge using a capacitive probe array to measure ionization growth with time and space resolution. The experimental results indicate that pre-breakdown processes in shielded capillary discharges are characterized by the formation of a fast ionization wave. Depending on voltage polarity, the ionization wave can be associated with a mobile virtual anode with characteristic speed 10 5 m s −1 , in the case of positive polarity, or with the propagation of a high speed potential wave, of characteristic speed 10 7 m s −1 , in negative polarity case. The time and space evolution of the ionization waves is closely related with the formation of high energy electron beams, which originate due to the hollow cathode geometry of the open end capillary. A qualitative model based on the hollow cathode effect is proposed to explain the initial formation and later time evolution of the observed ionization waves.

We present experimental results on the characterization of a non-ablating fast pulsed capillary discharge, with a hollow cathode (HC) geometry, operating in argon below 1 Torr. Both the pre-breakdown and breakdown phase of the discharge are investigated with several diagnostics, which include electron beam monitoring, capacitive probe array and extreme ultraviolet (EUV) detector array. The pre-breakdown phase is found to be characterized by the emission of HC electron beams, which assist the propagation of a high speed ionization wave, with typical velocity in the 10 6 -10 7 m s −1 . Coinciding with electric breakdown a fast EUV radiation pulse is emitted. The leading edge of the radiation pulse is due to beam target emission by the HC electron beams. At the breakdown the radiation emission is mainly centered in the 5-15 nm spectral window, and is emitted from a capillary plasma which is being heated by a kiloampere level, 10 ns half-width current pulse.

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The effect of electric field penetration in the hollow-cathode region of self-initiated pulsed hollow-cathode discharge is investigated. The discharge is operated in hydrogen at pressures between 30 and 200 mTorr with applied voltages between 10 and 30 kV. Aluminum cathodes with aperture diameters and lengths from 1 to 5 and 2 to 18 mm, respectively, are used. The results show that the effect of the hollow cathode in discharge initiation is particularly strong at low pressures. In the range of parameters investigated, the time delay to breakdown and the jitter decrease when cathode aperture with larger diameters and shorter lengths are used. At high pressure the pulsed hollow-cathode effect is less important as ionization processes in the main gap become dominant.

The results of an extensive operation of a Vacuum Spark plasma using Titanium electrodes in a 120 ns 150 kA discharge are presented. The hot spots are found to form with a regular spacing in a zippering Z-pinch plasma, which forms close to the cathode and extends to approximately two thirds of the anode separation over a period of a few ns. The axis of the discharge is well defined by an initial plasma from a Nd:YAG laser focussed onto the cathode electrode surface. The statistics of the formation of the hot spots are given for the life of one anode electrode. Between one and three hotspots form and the favored positions are at 1.5 and 3.0 mm from the cathode and the strongest emission, as observed in a filtered X-ray pinhole camera, comes from the hot spot closest to the cathode. The emission spectra resolved between 50 and 350Å shows a wide range of Ti ionization which allows the temperatures of the anode blow off plasma, the Z-pinch and the hot spot plasma to be distinguished. These results are compared with filtered PIN diode signals and filtered pinhole images.