Dieter H. H. Hoffmann

Scientific and technological progress of pseudospark devices

This paper presents an overview on the state-of-the-art of research and development with pseudospark devices. There is an ongoing interest worldwide in this novel low pressure gas discharge device. This is proven by the several papers recently published. Careful studies of breakdown characteristics with two-electrode pseudospark devices show that the simple relation of the old Paschen law is modified for this geometry. Especially for operating the pseudospark reliably at low gas pressure, it is necessary to superimpose external magnetic fields to initiate the discharge. At low pressure intense beam formation is enhanced but in parallel is hampered by less efficient space-charge-neutralization. Based on the original pseudospark geometry several modified beam configurations were developed like the channel spark and the preionization-controlled open-ended hollow cathode system. In pulsed electrical circuits for discharge currents below 10 kA, distinct discharge phenomena appear which have to be suppressed for any application. One of these is transient impedance transition, correlated with steps in forward voltage drop. By geometry and choice of electrode material the irregular transitions in impedance can be controlled over a wide parameter range. Another annoying effect is quenching obvious by sudden and irregular interruption of the discharge current. Quenching is observed as a random effect, which is influenced by a manifold of parameters. Results from the experiment indicate that quenching is strongly dependent on the number density of gas atoms in the discharge volume. Since silicon carbide (SiC) as part of the switch electrode downsizes the quenching current to negligible values (<1 kA) optical spectroscopy was used to investigate the influence of this semiconducting material on the temporal development of the discharge, by looking for emission lines of the released silicon and/or carbon atoms. The technological aspects of pseudospark devices are naturally to achieve higher lifetime and improved overall reliability. Multichannel configurations and two-gap systems are under development to reduce erosion rate and to increase hold-off capability, respectively. Under clean conditions a hold-off voltage of 65 kV was realized by a two-gap system.

The high current plasma lens: investigations of fine focusing of high energy heavy ion beams

Abstract Fine focusing and transport of heavy ion beams in plasma lenses and z-discharge channels is a significant issue for inertial confinement fusion. Therefore, some of the corresponding questions are addressed in an experimental programme at GSI, using a wall-stabilized discharge as an active plasma lens for fine focusing experiments of heavy ion beams. Very accurate and reproducible focusing of ion beams (Ne, C, Ar, Au) at energies between 10 and 300 MeV a.m.u.−1 is now achieved. Emittance measurements and the determination of the emittance growth that results from scattering in thin foils of the heavy ion beam have been possible with the plasma lens. The minimum achieved spot diameter was 390 μm in the vertical plane at a target distance of 53 mm and 150 kA plasma current, with a plasma column of length 100 mm and diameter 14 mm. The corresponding beam emittance was 40 π mm mrad (r.m.s.).

Heavy-ion-beam-induced motion in rare-gas cryo targets

Abstract At Gesellschaft fur Schwerionenforschung Darmstadt the first hydrodynamic motion of a heavy-ion beam volume heated target has been demonstrated. Therefore krypton and xenon cryogenic crystal were irradiated by an intense 300 MeV u −1 Ne 10+ beam. The beam was focused onto the crystals with a minimum beam waist of 1.12 mm × 1.42 mm full width at half maximum. A specific energy of 60 J g −1 has been deposited, leading to hydrodynamic motion and the creation of shock waves in the target and finally to its total destruction. The escape velocity of the gas streaming out of the heated volume was estimated to be 200 m s −1 . In addition, spectra in the vacuum UV region could be obtained showing the 172 nm xenon emission band.

The borehole phase of the pseudospark discharge-a transition between hollow cathode and high current phase

The borehole phase is one of the five phases in the development of a pseudospark discharge. In chronological order, the borehole phase follows the low current predischarge and the hollow cathode phase with currents up to some 100 A. This discharge phase makes the transition between the hollow cathode phase and the high current phase which is connected to the appearance of cathode spots. The transition is fast and is characterised by a sudden decrease of the switch impedance. One problem in understanding the borehole phase is the cause of the high current density of more than 10/sup 4/ A/cm/sup 2/ and the mechanism responsible for the emission of such a high density of electrons. Self sustained self sputtering of cathode material, thermionic field emission and the emission caused by impact of discharge gas ions are discussed as possible processes. Different optical and spectroscopic measurements show that secondary emission by gas ion bombardment is the main reason for the high current density. During the borehole phase, only neutral atoms and single ionised ions from the cathode material can be detected, which seem not to have enough energy to extract electrons from the surface. However, bulk ions (i.e. hydrogen) have enough energy to generate secondary emission of electrons.

Experimental study to accumulate, accelerate and focus a massive plasma beam onto a target

Abstract Experimental investigations are presented, where a device similar to a plasma thruster using j × B forces has been used, to accumulate and accelerate a plasma with a mass of about 0.2 mg to beam velocities up to 110 km/s. A pulse line with a capacity of 108 μF was used to provide discharge currents up to 450 kA for a pulse length of 6.5 μs. The discharge vessel consists of a set of coaxial electrodes with a diameter of 5 cm at a length of 50 cm. The total energy efficiency of the plasma acceleration from this process is estimated to be better than 10%. An estimate of the mass of the plasma beam was achieved by measuring the momentum transfer from plasma beam to flyer plates. Using conus-shaped electrodes at the exit of the plasma accelerator a focusing of the plasma to a diameter below 1 cm was achieved.

Intense electron beams produced in pseudospark and PCOHC for beam–plasma interaction experiments

Abstract High-voltage hollow-cathode glow discharges are used to generate intense pulsed electron beams with remarkable parameters. A comparison of the electron beam characteristics and the development of beam-induced plasma in the drift space is made for two of these configurations: preionization controlled open ended hollow cathode (PCOHC) and pseudospark, both in a multigap geometry. The dependency of electron beam parameters on external capacity, gas pressure and breakdown voltage is discussed.

Evidence for recombination XUV lasing at 52.0 nm and 49.8 nm in a fast, compact Z-pinch discharge

Evidence of gain in a recombining Z-pinch plasma has been obtained at 52 and 49.8 nm. The amplified XUV radiation originated from the 4 f -3 d and the 4 d -3 p transitions of Lilike oxygen, OVI. The plasmas were generated in a small diameter Z-pinch discharge with moderate currents of 40 kA. The gain coefficient was determined by variation of the pinch tube length, leading to a gain-length product of 2.5 (4 f -3 d ) and of 2.2 (4 d -3 p ) for a length of 9 cm.

Electron beam based space charge measurement of intense ion beams

In order to determine the transverse profile of an intense, strongly focused ion beam, like those at the HHT experimental area at GSI or at the future facility FAIR, a non-invasive methode has been successfully developed and tested. This methode is based on the electrons which pass perpendicular through the ion beam and get deflected because of its the electromagnetic field and was designed as a possible alternative to gas scintillation [1].

Study of the slow ion beam penetrating the low density plasma target

Energy loss of the slow ion beam impacting on a low density hydrogen gas target was measured. A good agreement between the experimental and theoretical results is found. The result of the ion beam passing through a fully ionized hydrogen plasma target is shown too.

Optical diagnostics of high current pseudospark discharges

The pseudospark is a high voltage, low pressure transient discharge of high discharge current and current rise. Several optical diagnostics were used to study the discharge development and to study the different plasma parameters. Laser induced fluorescence, laser absorption spectroscopy, laser tomography measurements on pseudospark switches and fast shutter and streak measurements on the electron beam pseudospark devices are summarized.