W. Kies

Neutron emission from a fast plasma focus of 400 Joules

In dynamic pinches, short-duration high-temperature and high-density plasmas are produced, which can emit x rays and intense neutron pulses ~when deuterium is used in the discharge!. A plasma focus ~PF! is a particular pinch discharge in which a high pulsed voltage is applied to a low pressure gas between coaxial cylindrical electrodes. The central electrode is the anode partially covered with a coaxial insulator. The discharge starts over the insulator surface, and afterwards the current sheath is magnetically accelerated along the coaxial electrodes. After the current sheath runs over the ends of the electrodes the plasma is compressed in a small cylindrical column ~focus!. In most of the devices these three stages last a few microseconds. The pinch compression should be coincident with peak current ~really with the magnetic flow ! in order to achieve the best efficiency. The pinch generates beams of ions and electrons, and ultrashort x-ray pulses. Using deuterium gas, plasma focus devices produce fusion D‐D reactions, generating fastneutrons pulses (;2.5 MeV) and protons ~leaving behind 3 He and 3 H). The neutrons burst usually lasts about tens to hundreds of nanoseconds. The emitted neutrons can be applied to perform radiographs and substance analysis, taking advantage of the penetration and activation properties of this neutral radiation. The plasma focus is a pulsed neutron source especially suited for applications because it reduces the danger of contamination of conventional isotopic radioactive sources. A passive radioactive source of fast neutrons with similar energy ~for instance 252 Cf with similar mean energy or Am/Be with a harder spectrum! emits continuously, causing inconveniences in handling and storing. In turn, plasma-focus generators do not have activation problems for storage and handling. During the last 30 years, substantial effort and resources

Demonstration of neutron production in a table-top pinch plasma focus device operating at only tens of joules

Neutron emission from a deuterium plasma pinch generated in a very small plasma focus (6 mm anode diameter) operating at only tens of joules is presented. A maximum current of 50 kA is achieved 140 ns after the beginning of the discharge, when the device is charged at 50 J (160 nF capacitor bank, 38 nH, 20–30 kV, 32–72 J). Although the stored energy is very low, the estimated energy density in the plasma and the energy per particle in the plasma are of the same order as in higher energy devices. The dependence of the neutron yield on the filling pressure of deuterium was obtained for discharges with 50 and 67 J stored in the capacitor bank. Neutrons were measured by means of a system based on a 3He proportional counter in current mode. The average neutron yield for 50 J discharges at 6 mbar was (1.2 ± 0.5) × 104 neutrons per shot, and (3.6 ± 1.6) × 104 for 67 J discharges at 9 mbar. The maximum energy of the neutrons was (2.7 ± 1.8) MeV. Possible applications related to substance detection and others are discussed.

Research on pinch plasma focus devices of hundred of kilojoules to tens of joules

At present the Plasma Physics and Plasma Technology Group of the Comision Chilena de Energia Nuclear (CCHEN) has the experimental facilities in order to study fast dense transient discharges in a wide range of energy and current, namely: I) energy from hundred of kilojoules to tens of joules, II) current from megaamperes to tens of kiloamperes. Also several diagnostics have been implemented. An overview of the work being carried out on dense pinch plasma focus discharges at the Comision Chilena de Energia Nuclear is presented. The plasma energy density and scaling laws for the neutron yield are discussed. Possible applications of the radiation emitted are also discussed.

Pinch evidence in a fast and small plasma focus of only tens of joules

The pinch evidence in a deuterium-filled plasma focus (PF) of only tens of joules is presented. The system operates at a very low energy in the tens of joules range (160 nF capacitor bank, 38 nH, 20–35 kV, 32–98 J, ~150 ns current rise time), maintaining the same energy density as in large devices. The typical dip in the current derivative signal and the typical peak in the voltage signal observed in PF devices with energies of 1–1000 kJ, which are associated with pinch compression, were observed in a deuterium-filled PF operating at 50 and 67 J. The time to pinch and time to the peak current versus deuterium filled pressure were also obtained.

Pinch modes produced in the SPEED2 plasma focus

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).

Terawatt fiber pinch experiments

Pinch formation in fiber pinch experiments has been investigated in the lower terawatt regime. The main results are: (1) there are upper limits of breakdown voltage (∼700 kV) and current rise rate (∼20 kA/ns) beyond which leak discharges develop within the vacuum feed of the pulseline KALIF; (2) there is a lower limit of fiber radius (∼10 μm) below which pinch disruptions take place at a pinch current of ≳300 kA; (3) the hot (Te≤1 keV) inhomogeneous pinch plasma develops typically 10 ns after local collapses (micropinches) at a pinch current ≳400 kA and lives for more than 50 ns; (4) neutron emission (yield of CD2 fibers ∼1010) appears mostly isotropic; (5) all fiber pinches show global expansion with velocities reaching from typically 10 μm/ns (initial expansion) to ≳100 μm/ns; and (6) the power requirements for the fiber ablation process are contradictory to those for the final pinch phase.

High performance 300 kV driver speed 2 for MA pinch discharges

Drivers for dynamical pinch discharges (z-pinch, plasma focus) have to meet requirements (impedance, voltage) that are not easily satisfied if discharge currents in the MA range are needed. These requirements are discussed and the fast 300 kV driver SPEED 2, a capacitor bank (187 kJ) of unusually high impedance (60 mω) and current efficiency (I0W0 = 27 AJ) is introduced. Results are given from a first application of SPEED 2 to a plasma focus.

Micropinch actuation in the SPEED 2 plasma focus

The occurrence of hot (kilo-electron-volts), dense () and short-lived (nanoseconds) substructures in pinch plasmas (micropinches) is usually very erratic in space and time. On doping the deuterium discharge of the fast plasma focus SPEED 2 (180 kV, 1.5 MA, 400 ns) with argon, typically ten micropinches appear in an orderly manner along the axis of symmetry forming a straight chain of successive flashes within 30 ns that starts near the anode and ends after 2 cm. Four frame pictures taken in the VUV ( nm) at different times during pinch formation and careful comparison with stronger filtered x-ray pinhole pictures ( nm) show that micropinches are actuated in necked pinch regions that preferentially undergo radiative collapse via bremsstrahlung and line radiation from argon. However, not every necking leads to micropinch formation so that necking is a necessary but not sufficient condition for micropinch actuation.

Pinches and micropinches in the SPEED 2 plasma focus

Dynamical pinches (z-pinch, plasma focus) produce two different modes of pinch plasmas, unstable columns with successive short-lived micropinches or stable narrow columns with lifetimes around 50 ns. These two modes depend on the energy density of the plasma sheath and the atomic number of the working gas. Low energy density and/or heavy gases result in the micropinch mode; high energy density and/or light gases result in the stable column mode (SCM). Micropinches are actuated in pinch plasmas by local radiative collapses, the necessary condition of which is that the pinch column shows neckings (m = 0 type) at these places. In the SPEED 2 plasma focus with fixed energy input (70 kJ, 180 kV, 1.5 MA) neckings and micropinches appear only if the atomic number whereas with Z<18 macroscopically stable pinch columns are formed. Attempts to trigger micropinches on predetermined axial positions by inserting solid particles failed because of the short interaction time of the dynamic pinch ( ns) with the target. Only surface plasma around the solid core is created and deep compression only takes place in front of the tip of solid fibres.

Spectral selective plasma imaging in the wavelength range 2.4 - 4.5 nm in SPEED 2 device

X-ray images of the compression phase of a deuterium-embedded argon pinch plasma generated with the SPEED 2 driver in a plasma focus configuration were obtained. To record spectral selective images, an x-ray microscope based on cylindrically bent multilayer mirrors (MLMs) mounted in a parallel configuration was developed. The plasma images were taken at different wavelengths in the interval from 2.4 to 4.5 nm with a spectral resolution of and a spatial resolution of approximately 80 m within a 30 mm diameter field of view. A strong wavelength dependence of the shape and the size of the emitting regions of the plasma was found. Images of bright spots with a size of about 0.5 mm identified as being early stages of micropinches were obtained.