The Z-pinch, perhaps the oldest subject in plasma physics, has achieved a remarkable renaissance in recent years, following a few decades of neglect due to its basically unstable MHD character. Using wire arrays, a significant transition at high wire number led to a great improvement in both compression and uniformity of the Z-pinch. Resulting from this the Z-accelerator at Sandia at 20 MA in 100 ns has produced a powerful, short pulse, soft x-ray source >230 TW for 4.5 ns) at a high efficiency of ~15%. This has applications to inertial confinement fusion. Several hohlraum designs have been tested. The vacuum hohlraum has demonstrated the control of symmetry of irradiation on a capsule, while the dynamic hohlraum at a higher radiation temperature of 230 eV has compressed a capsule from 2 mm to 0.8 mm diameter with a neutron yield >3 × 1011 thermal DD neutrons, a record for any capsule implosion. World record ion temperatures of >200 keV have recently been measured in a stainless-steel plasma designed for Kα emission at stagnation, due, it was predicted, to ion-viscous heating associated with the dissipation of fast-growing short wavelength nonlinear MHD instabilities. Direct fusion experiments using deuterium gas-puffs have yielded 3.9 × 1013 neutrons with only 5% asymmetry, suggesting for the first time a mainly thermal source. The physics of wire-array implosions is a dominant theme. It is concerned with the transformation of wires to liquid-vapour expanding cores; then the generation of a surrounding plasma corona which carries most of the current, with inward flowing low magnetic Reynolds number jets correlated with axial instabilities on each wire; later an almost constant velocity, snowplough-like implosion occurs during which gaps appear in the cores, leading to stagnation on the axis, and the production of the main soft-x-ray pulse. These studies have been pursued also with smaller facilities in other laboratories around the world. At Imperial College, conical and radial wire arrays have led to highly collimated tungsten plasma jets with a Mach number of >20, allowing laboratory astrophysics experiments to be undertaken. These highlights will be underpinned in this review with the basic physics of Z-pinches including stability, kinetic effects, and finally its applications.

A review of the dense Z-pinch

A hybrid fluid-particle (Monte Carlo) model to describe the initiation of pseudospark discharges has been developed. In this model, time-dependent fluid equations for the electrons and positive ions are solved self-consistently with Poisson's equation for the electric field in a two-dimensional, cylindrically symmetrical geometry. The Monte Carlo simulation is used to determine the ionization source term in the fluid equations. This model has been used to study the evolution of a discharge in helium at 0.5 torr, with an applied voltage of 2 kV and in a typical pseudospark geometry. From the numerical results, the authors have identified a sequence of physical events that lead to the rapid rise in current associated with the onset of the pseudospark discharge mode. They find that there is a maximum in the electron multiplication at the time which corresponds to the onset of the hollow cathode effect, and although the multiplication later decreases, it is always greater than needed for a steady-state discharge. >

Pseudospark discharges via computer simulation

The pseudospark, an axially symmetric gas discharge at low pressures of typically 10 to 100 Pa and high voltages (of some hundred volts up to several hundreds of kilovolts), has become of interest in fast high-power switching, in producing well-pinched high intensity electron or ion beams, and as an X-ray source. The essential features of this type of discharge are described. The results of fast time-resolved spectroscopic investigations and breakdown delay statistics of the pseudospark are discussed. >

The fundamentals of the pseudospark and its applications

A surface dielectric barrier discharge (DBD) in atmospheric pressure air was excited either by low frequency (0.3–2 kHz) high-voltage ac or by short, high-voltage pulses at repetition rates from 50 to 600 pulses s−1. The short-pulse excited discharge was more diffuse and did not have the pronounced bright multiple cathode spots observed in the ac excited discharge. The discharge voltage, current and average power deposited into the discharge were calculated for both types of excitation. As a measure of plasma-chemical efficiency, the ozone number density was measured by UV absorption as a function of average deposited power. The density of ozone produced by ac excitation did not increase so rapidly as that produced by short-pulse excitation as a function of average power, with a maximum measured density of ~3 × 1015 cm−3 at 25 W. The maximum ozone production achieved by short-pulse excitation was ~8.5 × 1015 cm−3 at 20 W, which was four times greater than that achieved by ac excitation at the same power level.

https://iopscience.iop.org/article/10.1088/0022-3727/39/20/016/pdf

Comparison of high-voltage ac and pulsed operation of a surface dielectric barrier discharge

We present the first micromachined rotational electret power generator, linearized theoretical model of electret power generation, and novel method to produce uniformly charged electret. We also improved our previously developed (1996, 1999) thin film Teflon AF 1601-S electret technology with respect to dielectric thickness, charge uniformity, and processability. In demonstration, our prototype power generator successfully generated > 25/spl mu/W with electret thickness of 9/spl mu/m, effective charge density of -2.8/spl times/10/sup -4/C/m/sup 2/, and rotational speed of 4170RPM.

/pdf/micro-electret-power-generator-3m7sgzspvi.pdf

Micro electret power generator

A review is presented of recent developments in a new group of high-power hollow-electrode switches, including the pseudospark and the backlighted thyratron (BLT). Experiments demonstrate that for several key high-power switching performance factors, the pseudospark and BLT switches are superior to either high-pressure spark gap switches or thyratrons or, in some cases, both. High performance has been demonstrated in peak current (>100 kA), current rate of rise (>10/sup 12/ A/s), switching precision, trigger efficiency, current reversal (100%), and recovery time. Several electrical and optical trigger methods have been demonstrated and are described. >

/pdf/high-power-pseudospark-and-blt-switches-y7mw5yw4gr.pdf

High-power pseudospark and BLT switches

The paper describes the sealed-off pseudospark switches that have recently been developed for various pulsed power applications. The switches with the trigger unit based on a flashover are intended for a use in fast electric circuits at a pulse current of the order of 100 kA in microsecond range of the pulse duration. The switches with the trigger unit based on an auxiliary glow discharge have been designed for fast current circuits with a moderate range of current (of the order of 10 kA) and high pulse repetition rate.

Sealed-off pseudospark switches for pulsed power applications (current status and prospects)

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. >

Generation of intense pulsed electron beams by the pseudospark discharge

Deals with an interpretation of the current sustaining mechanisms at different stages of the high-current low pressure pulsed gas discharge with hollow cathode (pseudospark discharge). The analysis of experimental data for the stage of delay time to breakdown and the subsequent stages is presented. The principal features of glow stages are interpreted in the framework of the concept of nonmonotonic potential distribution in the near cathode regions. It is shown that in spite of extremely high current density the main fraction of the discharge current at the cathode surface is carried by the ion component. In the superdense glow stage the ion current is provided due to numerous microexplosions of the cathode surface, ionization of the metal vapor inside the cathode voltage drop region, and returning the metal vapor back to the cathode in a form of ions. The glow mode in this case is determined by a uniform distribution of the explosion centers over the cathode.

K. Frank

Papers

The fundamentals of the pseudospark and its applications

High-power pseudospark and BLT switches

Sealed-off pseudospark switches for pulsed power applications (current status and prospects)

Generation of intense pulsed electron beams by the pseudospark discharge

Discharge formation processes and glow-to-arc transition in pseudospark switch