Experimental and theoretical investigation of the gas embedded Z-pinch

Abstract

The paper reports observations of a sub-millimetre diameter Z-pinch discharge initiated in hydrogen gas at high pressure (0.33-2 bar) and powered by a fast rising current (dI/dt ≈ 1 kA·ns−1). The conditions for heating the pinch ohmically under pressure balance are established by using a small preheat current of short duration (5 kA, 70 ns). Measurements of the radial density profiles of the electrons in the pinch and the surrounding neutral gas are presented. The peak electron temperature in the pinch is estimated from the soft X-ray emission. The observations show that during the preheat phase, when there is no equilibrium, the plasma column expands radially and drives a shock into the neutral gas. This results in an increase of the neutral gas density in a thin layer surrounding the pinch. When the main current is applied, a pinch in radial equilibrium, with a radius of about 400 μm and peak electron density on axis (2.5 × 1024 m−3 at 0.33 bar), is observed. The estimated peak electron temperature on axis (69 eV at 0.33 bar) is about twice the Bennett temperature. An increase of the line density is observed during the equilibrium. This is ascribed to a flow of neutral particles from the high density layer surrounding the pinch column. The equilibrium is destroyed by an m = 1 instability which grows from a helical perturbation on axis. The experimental results are compared with calculations based on ideal MHD stability theory, and it is shown that the existence of a centrally peaked current density profile can be inferred from the nature of the observed instabilities. An explanation is offered for the absence of the m = 0 mode, on the basis of cooling of the outer region of the plasma.

A review of the denseZ-pinch

“…The interferogram in figure 80 shows that this helical perturbation occurs first with a peak on axis, consistent with MHD theory for a centrally peaked current profile [149]. The apparent delay [53] in the onset of a measurable perturbation was consistent with a 1% initial perturbation. The associated axial magnetic field was always in the direction from cathode to anode.…”

“…In contrast, a solid fibre Z-pinch will tend to have such a high density on axis that collisions will dominate, preventing most electrons from running away. Electron beam formation on axis has been observed experimentally [115] in a plasma focus as well as a gas-puff Z-pinch [53]. Runaway electrons will particularly be generated at an m = 0 disruption (section 3.14).…”

“…by a focused pulsed laser, then, on applying a high voltage, current will flow at first along the axis of the vessel. This is the gas-embedded Z-pinch [53]. This type of Z-pinch is the only one to be dominated by the m = 1 or kink instability, which we will discuss in section 3.…”

“…It is probably this which led to the singularity in the eigenfunction at r = 0. Zhang and Ding [186,187] recently studied the effects of compressibility and axial flow profiles on stability, though they seemed unaware of the earlier work of [53]. Experiments with sheared axial flow [188] are reported in section 7.7.…”

Simulation of plasma dynamics in Gas Embedded Z-pinches

“…The density of plasma behind the shock-wave front p = 0.5.10-sg/cm3 is much lower, than the density of filling gas. This result corresponds to the results, reported in Ref [5]. At this conditions the conductivity of the central discharge zone is in several order of its magnitude higher, Than, approximately at t = 15ns, substantial mass of the plasma behind the shock-wave front separates from the shock wave.…”