摘要:

A micro‐channel using corona effect at atmospheric pressure, either in hydrogen or in air, is created prior to the triggering of the main discharge. The channel diameter has been measured to have about 10 &mgr;m diameter, with a cw intensity of the order of 0.1 mA. The main discharge is powered by a Marx generator delivering a current pulse of 240 kA during 200 ns at the end of a water line having 1 &OHgr; impedance. Four pictures of the pinch are taken at each shot delayed in time, each from the other, from 10 ns to 260 ns.The discharge is seen to remain stable against usual instabilities during more than 500 ns. When am = 1instability appears sometime, its relative amplitude seem to remain constant as a function of time, at a very low level.The expansion of the channel takes place at very high velocities. In air, the expansion is slowered at a time corresponding to the maximum intensity.The influence of the micro‐channel characteristics has been clearly checked: when it is not well‐established, the discharge becomes unstable very early, even with the use of the pointed electrodes.

ISSN:0094-243X    

DOI:10.1063/1.2949184

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

This is a review of theoretical works in the field of dense Z‐pinch physics performed by ITEP plasma group. The problems under discussion are. Onedimensional dense Z‐pinch dynamics is investigated for a wide range of parameters. A transition between classical (compressional) and dissipative regimes of Z‐pinch dynamics is followed up. Analytical expressions, determining the type of Z‐pinch dynamics and temporal evolution of hot rarefied corona of the dissipative Z‐pinches are obtained. Heterogeneous equilibrium states of dense radiative Z‐pinches, when the corona is hot and rarefied, while pinch core is very dense and “cold”, are shown to exist. The problem of instabilities of dense Z”pinch equilibrium states is investigated, taking into account relevant dissipative processes. On the basis of the results obtained, we consider the general problem of an enhanced stability of dense Z‐pinch.

ISSN:0094-243X    

DOI:10.1063/1.2949137

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The growth rates spectrum of the ideal pressure‐driven localized modes in a general screw pinch configuration is obtained in an explicit analytical form under the condition of the Suydam criterion violation. The suppression effect of the magnetic shear near the singular surface is exponential and strongly depends upon relative values of two small parameters:&mgr; = m/((krs)2+m2)and&Vegr; = (P'−P's)/P's, whereP'sis the Suydam marginally stable pressure gradient.

ISSN:0094-243X    

DOI:10.1063/1.2949152

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A study of the stability of a gas embedded Z‐pinch discharge is presented. The experiments have been carried out using pulse power generators capable of delivering dI/dt in excess of1012 A·s−1. The parameter space on the Universal Diagram of Z‐pinch stability is explored by changing initial conditions (background gas pressure, preionization scheme: laser or microdischarge initiation and with or without preheating), as well as parameter change due to the evolution of each particular discharge. For every initial condition a trajectory is obtained in the Universal Diagram. Diagnostics used are Rogowskii loop and multiframe holographic interferometry, from which, current, line density and pinch radius are obtained. Results obtained are discussed in relationship with their position in the Universal Diagram. The applicability of the diagram is confirmed. Further work to explore different areas of the parameter space is envisaged.

ISSN:0094-243X    

DOI:10.1063/1.2949164

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The hydromagnetic Kelvin‐Helmholtz instability has been extensively studied in simple plasma configurations, but more complex situations relevant for fusion research involving the combined role of effective gravity due to acceleration, velocity shear, density and magnetic field stratification has not yet been investigated for compressible plasmas. We consider an isothermal equilibrium configuration with stratified density and magnetic field in a compressible plasma. The dispersion relation can be reduced to a highly complicated algebraic equation that involves many parameters. Various cases are examined. Analytical conditions for stability are given for a stratified plasma with no density jump. When there is a density discontinuity, the conditions for the existence of unstable modes are presented. The limiting case without gravity is examined.

ISSN:0094-243X    

DOI:10.1063/1.2949175

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Rayleigh‐Taylor (RT) instability developing in a layer of matter accelerated by magnetic pressure is shown to be suppressed if the accelerated layer scoops unperturbed matter, entraining it into motion. This stabilizing mechanism is effective for multicascade systems like explosive generators of high pulsed magnetic field or multiple (nested) gas‐puff Z‐pinches, where unperturbed elements are successively involved into accelerated motion. Linear stability analysis of one‐dimensional solutions of the piston problem demonstrates that perturbation of the given wavelength &lgr; does not grow appreciably until the thickness of the accelerated layerL(t) exceeds &lgr;. Before that, if acceleration is increased rapidly enough, amplitudes of the long‐wavelength perturbations remain almost constant and an oscillatory eigenmode localized near its front surface. If acceleration is increased not too rapidly, stays constant, or is decreased, then the long‐ wavelength perturbations with&lgr; > 2L(t)are damped. For the case when acceleration of the first shell is reversed in collision, short‐wavelength perturbations are stabilized. The optimal mass ratio is estimated for a double gas‐puff Z‐pinch: The inner shell must be ∼ 3 times heavier than the outer one.

ISSN:0094-243X    

DOI:10.1063/1.2949189

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A general class of self‐similar, time dependent, non‐stationary z‐pinch equilibria is examined in detail It is shown that there are several zones of parameter space, corresponding to various types of solution. One‐dimensional simulations reveal that the self‐similar solutions act as attractors. The instaneous linear MHD growth rates for them=0andm=1instabilities are found to be almost the same for all the solutions. Thus there is no especially favourable subset of the equilibria to aim for experimentally.

ISSN:0094-243X    

DOI:10.1063/1.2949200

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We present a model of dense Z‐Pinch heating. For pinches of sufficiently small diameter and high current, direct ion heating bym = 0instabilities becomes the principal channel for power input. This process is particularly important in the present generation of dense micro‐pinches (e.g.,HDZP‐II) where instability growth times are much smaller than current risetimes, and a typical pinch diameter is several orders smaller than that of the chamber. Under these conditions,m = 0formation is not disruptive: the largeEzfield reconnects the instability cusps externally, after which the ingested magnetic flux decays into turbulent kinetic energy of the plasma. The continuous process is analogous to boiling of a heated fluid.A simple analysis shows that an equivalent resistanceRt = l4NMi &mgr;0&Vpgr;3/2 Irappears in the driving circuit, whereIis the pinch current,Nis the line density,lis the pinch length,miis the ion mass, andris the pinch radius. A corresponding heating term has been added to the ion energy equation in a0‐D, self‐similar simulation, which had been written previously to estimate fusion yields and radial expansion ofD2fiber pinches. The simulation results agree well with the experimental results fromHDZP‐II, where the assumption of only joule heating produced gross disagreement. Turbulent ion heating should be the dominant process in any simple pinch carrying meg‐ampere current and having submillimeter radius.

ISSN:0094-243X    

DOI:10.1063/1.2949211

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The MHD instability of dissipative Z‐pinches has been investigated with magnetic field diffusion, Ohmic heating, thermal conductivity and radiative losses taking into account. We pay main attention in this paper to stability of heterogeneous Z‐pinches and to investigation, how thermal conductivity influences on instability of pinches with almost constant temperature.

ISSN:0094-243X    

DOI:10.1063/1.2949225

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

For the first time, calculations of large Larmor radius (LLR) effects on the linear stability of realistic Z‐pinch equilibria have been performed. The fixed boundarym=0instability of the pure Z‐pinch (no external magnetic field) is considered (free‐boundary andm=1modes are presently under study). We use the Vlasov‐Fluid model, where ions are treated fully kinetically and electrons are modelled as a cold, massless fluid. Stability is found to be remarkably sensitive to equilibrium profiles. A flat current equilibrium is increasingly stabilized (smaller growth rate) by LLR effects as the normalized average Larmor radius &Vegr; is increased to about 0.1. Complete stabilization cannot be obtained. For larger values of &Vegr; the growth rate increases to reach above the small Larmor radius value when&Vegr;≈0.3. The Bennett equilibrium, however, is increasingly destabilized as &Vegr; increases.

ISSN:0094-243X    

DOI:10.1063/1.2949227

出版商:AIP    

年代:1994

数据来源: AIP