The dense plasma focus (DPF) is a Z-pinch that has been studied for 50 years. Within ten years of its discovery by Fillipov and Fillipova in Russia and Mather in the USA, this dense pinch was scaled up to 2-MA currents and neutron outputs of ~ 1012/pulse. More remarkable is the fact that most of the relevant physics and scaling laws were elucidated within this first decade. The subsequent period has seen this type of pinch used as a teaching tool, developed as a portable neutron source for security applications, as a soft X-ray source for lithography, and as an energetic ion source for nanofabrication applications. This review builds upon several prior reviews. From the plasma physics standpoint, DPF physics is examined in light of fast Z-pinches to examine the similarities. More cross-fertilization between the two communities is suggested as a means to improve both types of pinches. From the applications standpoint, the many uses of DPFs are summarized to demonstrate the versatility of these pinches. Their ease of assembly and relatively low voltage operation have allowed DPFs to be disseminated worldwide as fusion testbeds (unlike their fast Z-pinch counterparts). Smaller or less economically developed nations have made valuable contributions to our understanding of the physics, as evidenced by the rich lode of publications that have advanced the field.

The Dense Plasma Focus: A Versatile Dense Pinch for Diverse Applications

The characteristics of three different low energy Mather-type plasma focus devices are investigated and compared. The effects of anode length and insulator sleeve length on neutron yield and on fluence anisotropy are studied. It is found that proper tuning of the focus tube inductance with that of the driver enhances the neutron emission from the focus region, increases the neutron pulse width and broadens the deuterium pressure range for high neutron yield. The insulator contamination which occurs due to Cu evaporated from the electrodes drastically changes the device characteristic. The possibility of using the device as a multi-pulse neutron generator is also examined.

https://iopscience.iop.org/article/10.1088/0963-0252/4/1/012/pdf

Comparative study of low energy Mather-type plasma focus devices

A new soft x‐ray source has been developed and investigated. Argon is injected into a plasma focus through a hole in the anode. The pinch emits several Joules per shot of soft x rays at λ<0.5 nm from multiple micropinches well aligned along the axis of symmetry. When viewed end‐on the pinch appears as a small source of 700 μm diam. The micropinches have the following characteristics: Range of size 40–250 μm (depending on radiation wavelength), density 1028 m−3, temperature 1.5 keV, and lifetime typically less than 1 ns. The helium‐like emission from single micropinches lasts for 250 ps only, which is much shorter than previously reported.

Observations of soft x‐ray production in the speed 2 plasma focus

The results of studies of the plasma-current sheath structure on the PF-1000 facility in the stage close to the instant of pinch formation are presented. The measurements were performed using various modifications of the calibrated magnetic probes. Studies of the influence of the probe shape and dimensions on the measurements accuracy were done. The current flowing in the converging sheath at a distance of 40 mm from the axis of the facility electrodes was measured. In the optimal operating modes, this current is equal to the total discharge current, which indicates the high efficiency of current transportation toward the axis. In such shots a compact high-quality sheath forms with shock wave in front of the magnetic piston. It is shown that the neutron yield depends on the current compressed onto the axis. This dependence agrees well with the known scaling, Yn ~ I4. The use of the total discharge current in constructing the current scaling, especially for facilities with a large stored energy, is unjustified.

https://iopscience.iop.org/article/10.1088/0741-3335/54/2/025010/pdf

Experimental study of the structure of the plasma-current sheath on the PF-1000 facility

Deuterium discharges in the SPEED2 plasma focus doped with heavy gases (e.g. neon, argon) produce two pinch modes, the micropinch mode (MPM) or the stable column mode (SCM), with a transition regime where the initial SCM is followed by the MPM. Micropinches are local radiative collapses initiated by instabilities (m = 0 type) of low-energy-density pinch plasmas. These instabilities and the successive micropinches can be suppressed by kinetic deuterons produced during dynamical compression of high-energy-density deuterium plasma sheaths. Depending on the relaxation of this fast deuteron component the pinch column can be stabilized for several tens of nanoseconds. The SCM optimized with respect to the compression ratio is a powerful linear radiation source of high density (up to 1027 m-3) and temperature (up to 1 keV).

Pinch modes produced in the SPEED2 plasma focus

Discharge initiation and sheath formation are serious problems of high current (several MA) dynamical pinches because of the enormous power input. The fast 300 kV plasma focus SPEED 2 allowed studying the discharge behaviour in a wide range of power input (10-100 GW) previously not accessible. The main results are: (i) high power input causes fast sheath formation ( 100 keV) photons with which the insulator surface is irradiated during discharge initiation and sheath formation or using insulators with coppered surface.

W Kies

Papers

Power limits for dynamical pinch discharges