Wire Initiation Critical for Radiation Symmetry inZ-Pinch–Driven Dynamic Hohlraums

Abstract

Axial symmetry in x-ray radiation of wire-array z pinches is important for the creation of dynamic hohlraums used to compress inertial-confinement-fusion capsules. We present the first evidence that this symmetry is directly correlated with the magnitude of the negative radial electric field along the wire surface. This field (in turn) is inferred to control the initial energy deposition into the wire cores, as well as any current shorting to the return conductor.

Overview of the Dynamic Hohlraum X-Ray Source at Sandia National Laboratories

“…Importantly, the paper discuss the mystery behind why increasing the mass of the outer array diminishes axial x-ray power [29], why the addition of the inner array only marginally increases the axial x-ray power [29], and why a significant difference exists in the axial radiation exiting the top REH relative to that exiting the bottom [18]. These observations, which are all in contrast to the predictions of MHD simulations, are shown in this article to be related to the discrete nature of the wires, and in the last case, to the large radial electric field on the surface of the outer array of wires before the plasma-shell forms [30].…”

“…This departure from axial up-down radiation symmetry is discussed in Section VIII, and will be shown to depend on the negative radial electric field E r at the surface of the outer wire array ( Fig. 2A), which decreases as the diameter decreases [30].…”

Overview of the dynamic-hohlraum x-ray source at Sandia National Laboratories.

“…As with the departure from axial radiation symmetry observed with the diameter variation (Fig. 3A), Section VIIIB will show that the asymmetry is strongly correlated with E r , which decreases as N increases [30]. The increase in power with increasing wire number (and decreasing interwire spacing), in contrast, is related to the transition from the implosion of individual wire plasmas to that of a plasma shell where the wire coronas have merged and to the increased azimuthal symmetry of the merging coronas [19,20].…”

New paradigm : controlling initiation phase is necessary for optimization of wire-array z-pinch performance.

“…Later breakdown enables higher temperature of the wire core by prolonged Joule heating. Hence, an axial variation of the radial electric field maps to an axial variation of the wire temperature at breakdown-the so-called polarity effect [6,12].…”