G. Decker

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.

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.

Pinch formation and reaction proton spectra of speed 1 focus discharges

Abstract SPEED 1 fusion source characteristics differ from those of conventional plasma focus discharges in this energy range (20 kJ): (i) intensity maximum of end-on proton spectra is pressure dependent; (ii) fusion reactions start prior to maximum compression; (iii) isotropic emission corresponds to rather stable pinches during fusion activity. Conclusions on possible mechanisms are drawn.

Spectral Investigations of Micropinches in the Speed 2 Plasma Focus

Soft X‐radiation mainly emitted by ‘micropinches’ is generated in the driver SPEED 2 by injection of heavy rare gases (e.g. argon) into a discharge of pure deuterium. It has been proved by a fast diagnostic in the VUV range based on a microchannelplate, that the short‐living micropinches with sub‐mm‐size are developing where neckings in the plasma column can lead to local radiative collapses as predicted by the collapse model.Spectra of the whole pinch column in the wavelength range close to the ArXVII ‐resonance line (λ = 0.3948 nm) were used to improve temperature and density determination. The results of computer simulated fits using a Monte‐ Carlo‐method for the photon transport including effects of optical density for evaluation of plasma parameters (electron density 1028 m−3 … 1029 m−3, temperature 1 keV … 1.5 keV) agree well with estimates based on the corresponding model. Spatially resolved spectra reveal that about 90 % of the line radiation stems from micropinches.The energy output in small chan...

Micropinch Formation in the SPEED 2 Plasma Focus

Micropinch formation is described as local collapses initiated in necked regions of a pinch plasma column by enhanced plasma radiation and particle outflow [1]. While the final stage of this formation is well known from soft X‐ray measurements (λ < 2 nm) [2] the local and temporal conditions of the initial phase has not yet been clarified.Therefore a fast (ns) four frame pinhole camera system has been applied to the SPEED 2 plasma focus in order to investigate the micropinch formation process in a wavelength range from VUV to the soft X‐ray (0.4 nm).The experimental setup consists of a configuration of four pinholes each imaging the pinch plasma. These pictures are taken with a special microchannel plate which is divided into four independent sectors. Each sector can be independently gated by a short triggerpulse (FWHM 5 ns) leading to an exposure time of a few nanoseconds. Therefore four differently time delayed pictures are taken during a single discharge.Different filters allow to determine the spectra...