Dense plasma in Z-pinches and the plasma focus

Abstract

Many of the earliest experiments in controlled thermonuclear fusion research were Z -pinches. However these pinches were found to be highly unstable to the m = 0, the m — 1 (kink), and the Rayleigh-Taylor instability. The addition of an axial magnetic field and the removal of end losses by proceeding to a toroidal geometry has led to the class of discharges known as tokamaks and the reversed field pinch. But, at fusion temperatures and with practical values of applied magnetic field this restricts the plasma density to 10 20 to 10 21 m- 3 , thereby requiring a containment time of several seconds and a plasma radius of about 1 m. Meanwhile studies of the plasma focus, which after its three-dimensional compression closely resembles a Z -pinch, have shown that a plasma of density 10 25 m- 3 and temperature 1 keV can be achieved in a narrow filament of radius 1 mm. It has enhanced stability properties which might be attributable to the effects of finite ion Larmor radius. Its neutron yield in deuterium can be as high as 10 12 per discharge, with a favourable empirical scaling law, but the thermonuclear origin of the neutrons is doubtful because of the evidence of centre-of-mass motion and the formation of electron and ion beams. The development of high voltage, high current pulse technology has permitted the reconsideration of the Z -pinch to attain dense fusion plasmas which might be stabilized by scaling the ion Larmor radius to be comparable with the pinch radius. Experiments at Imperial College show that the plasma remains stationary for about twenty Alfven radial transit times, limited only by the period of the current waveform. Theory indicates that a dense compact Z -pinch can satisfy Lawson conditions with a power input dependent on the enhanced stability time, or, if stable, with ohmic heating balancing axial heat losses. Preliminary results on a laser-initiated Z -pinch are also presented.

A review of the denseZ-pinch

“…Turning to the principal pulse of neutrons, there are important features; (i) the neutrons occur at the time of an m = 0 instability, a point confirmed experimentally by Bernard et al [238], (ii) the neutrons have an anisotropic distribution consistent with originating from an ion beam in the z-direction: This was also confirmed by Bernard who employed a deuterated Figure 32. The rate of emission of neutrons with (shaded) and without a deuterated polyethylene target 25 mm from the anode in a plasma focus [238] or [84]. Reprinted with permission from The Royal Society.…”

Breakthrough in 4π ion emission mechanism understanding in plasma focus devices

“…In such line of studies, the emission of ions in conic geometry within a small solid angle is confirmed by measuring some ion characteristics only at certain locations in a PFD space, usually along the X or Y axes, above the vacuum chamber. Some researchers believe that a PFD generates ions during the progress of sausage instability towards the top of a PFD chamber in a conical fashion within a small solid angle with ion energies ranging from tens of keV to few MeV151617. As a result, for all plasma-material interaction studies by means of different scale PFDs from PF-1000141819 to miniature PFDs20, the specimens are usually placed along the z-axis of the PFD chamber to be exposed to ions by applying several pinch shots21.…”

Correlation between plasma pinch intensity, current sheath symmetry, and HXR yield by APF plasma focus device

“…The plasma column is magnetically confined, and the temperature of the plasma further increases by ohmic heating. In addition, the magnetically confined column is hydromagnetically unstable to sausage and kink instability modes [3]. Towards the end of this phase the magnetic field starts diffusing into the plasma column.…”