Michael Stetter

Generation of intense pulsed electron beams by the pseudospark discharge

A low-pressure gas discharge is presented as a source of intense pulsed electron beams. The pseudospark discharge emits a short-duration pinched electron beam during the breakdown phase. At voltages of typically 20 kV, approximately 10-20% of the total discharge current appears as the electron-beam current of typically 20 ns in duration. According to the breakdown voltage in the beam, a power density on the order of 10/sup 9/ W/cm/sup 2/ is reached. Thus, this electron beam turns out to be a good tool for material processing, comparable to pulsed high-power lasers. Besides the drilling of holes into metals and insulators, an interesting application is the production of high-temperature superconducting thing YBa/sub 2/Cu/sub 3/O/sub 7-x/ films. The electron beam is used to evaporate material from a stoichiometric 1-2-3 target. Experimental results concerning the propagation behavior in neutral gas, the electron energy distribution, and the interaction with matter are reported. >

Pseudospark produced pulsed electron beam for material processing

An intense pulsed electron beam produced by a pseudospark discharge is used for material processing. The electron beam propagates in a self-focused manner in the background gas. Hardly 12 ns after the beginning of the discharge the fraction of space charge neutralization is about 96%. To sustain the neutralization effect high energy electrons (E >

Investigation of the different discharge mechanisms in pseudospark discharges

The intention of this paper is to give an overview of recent experiments explaining the development and transition of the discharge phases in a pseudospark. The reported experiments include single gap pseudospark discharges in ultra-high-vacuum systems with hydrogen as working gas, as well as multigap pseudospark discharges in argon. Temporally and spatially resolved framing photography, spectrometry, raster electron microscopy and time resolved electrical measurements are presented. The experiments comprise a current range of some hundreds of amps to 60 kA. The results are used to specify the four characteristic phases of the pseudospark: Townsend-, hollow cathode-, high current- and metal vapor arc phase. >

Investigations about triggering of coaxial multichannel pseudospark switches

A fundamental problem of pseudospark switches is erosion in the borehole area. One way to reduce erosion is to distribute the current to several discharge channels. Essential for multichannel operation is a reliable ignition of all these channels. The aim of this work was to find out the requirements for a trigger for multichannel pseudospark switches and to develop a suitable trigger device. The investigations were made with a three channel pseudospark switch. The developed trigger is a pulsed hollow cathode discharge with a 3 mA dc-preionization. A trigger voltage of 4 kV results in a current of about 6 A in the hollow cathode of the trigger-section. This hollow cathode discharge causes a trigger current into the hollow cathodes of the pseudospark chambers. The trigger current which is necessary to ignite an equally distributed discharge has to be at least 3 mA into each main switch hollow cathode. A jitter of 2 ns was achieved for the coaxial multichannel pseudospark switch. >

Spatial and time characteristics of high current, high voltage pseudospark discharges

During the early phase of the discharge (ignition), fast ionization waves are observed propagating with a velocity of 10/sup 6/ m/s from cathode to anode. During this transient phase, a first peak of an energetic electron beam develops. Simultaneously, a moderate radial expansion of the axially concentrated background plasma (produced from beam electrons) is observed, but the plasma parameter remains still smaller than the borehole diameter (equal to 3 mm). The transition into the high current phase is characterized by further continuous radial expansion of background plasma, which is interrupted by a sudden and rapid radial expansion of plasma into the last two or three gaps in front of the anode. One reasonable explanation is based upon a kind of plasma blow-up by the field of the space charge accumulated there. Part of the beam electrons, extracted from the hollow cathode and adjacent gaps, are apparently deflected or even reflected in this high local electric field. Parallel with increasing total current, the internal resistance of the system drops dramatically, synonymous with the energy of the beam electrons, too. Characteristic for the development of the hollow-cathode plasma is a stepwise expansion. The plasma itself develops a hollow structure, and the diameter of it is still larger than the borehole diameter. During the high-current phase, the diameter of this characteristic hollow structure increases rapidly to the wall, indicating the end of the first current half-wave.

Correlation of current quenching and occurrence of metal vapor in a pseudospark discharge

The quenching phenomenon, i.e., a sudden interrupt of the discharge current, was investigated in a pseudospark discharge with charging voltage of 2.5 kV, maximum current of 2 kA and discharge duration of 3 /spl mu/s. The working gas was hydrogen at a pressure of 40 Pa. Concerning electrode material and geometric parameters, molybdenun electrodes were chosen with hole diameters of 5 mm; the electrode distance was 3 mm. In this parameter range, a temporal correlation of current quenching and the occurrence of metal vapor could be detected by means of time-resolved optical spectroscopy. With each current interruption a sudden increase of emission from neutral molybdenum atoms as well as an increase of cathode spot emission, which is spatially localized on the cathode, occurs. Also oxygen ions were observed which show a similar time-dependence, however with a significant delay of the order of 200 ns. The results are discussed in the scope of the mechanism proposed for quenching, i.e., ion depletion in the plasma boundary layer, and the mechanisms occurring in the high current phase of a pseudospark discharge. >

Cathode processes of the early phases of the pseudospark discharge

The early phases of the pseudospark discharge (hollow cathode discharge-discharge within the cathode aperture-ignition of the first cathode spots) were investigated in a circuit with low rate of current rise (dI/dt<10/sup 9/ A/s) where a prolongation of the phases is achieved. A system with 3 mm cathode aperture and 3 mm gap distance was used. After ignition, a hollow cathode discharge develops with a typical forward voltage drop (FWD) of 500 to 1000 V. This discharge is stable up to currents of 600 to 800 A. With rising current a ring shaped metal vapour cloud develops at the inner surface of the cathode aperture with a thickness of 250 /spl mu/m. When the current density within the cathode aperture exceeds 10/sup 3/ A/cm/sup 2/ the FWD decreases to 200-400 V, the intensity of metal vapour ring increases, and the light emission of the hollow cathode ceases. Fast shutter camera photographs with a telescope optics (resolution of 3 pm per pixel) of the ring shaped emission showed a homogeneous and diffuse discharge. Self sputtering of cathode material seems to be the relevant cathode mechanism. This phase is terminated at currents between 1 and 2 kA (depending on anode voltage, gas pressure, and the dimension of the cathode aperture) by the ignition of a first cathode spot of type A at the edge of the cathode aperture or by quenching of the discharge. It is only at higher current that cathode spots are observed at the cathode surface facing the main gap.