The Wisconsin Plasma Astrophysics Laboratory (WiPAL) is a flexible user
facility designed to study a range of astrophysically relevant plasma processes
as well as novel geometries that mimic astrophysical systems. A multi-cusp
magnetic bucket constructed from strong samarium cobalt permanent magnets now
confines a 10 m$^3$, fully ionized, magnetic-field free plasma in a spherical
geometry. Plasma parameters of $ T_{e}\approx5$ to $20$ eV and
$n_{e}\approx10^{11}$ to $5\times10^{12}$ cm$^{-3}$ provide an ideal testbed
for a range of astrophysical experiments including self-exciting dynamos,
collisionless magnetic reconnection, jet stability, stellar winds, and more.
This article describes the capabilities of WiPAL along with several
experiments, in both operating and planning stages, that illustrate the range
of possibilities for future users.Comment: 21 pages, 12 figures, 2 table
Pattern formation, observed experimentally in a radio-frequency plasma in annular geometry, and characterized by azimuthal symmetry breaking of the plasma parameters, is reported. The azimuthal modulation increases with increasing pressure in the range 1-300 Pa. These experimental observations are accurately described by a fluid model in which the transport coefficients are computed from a 0D Boltzmann kinetic equation. A linear stability analysis shows that unstable modulations develop at low and intermediate pressures, following an instability mechanism due to an energy transport effect-the instability mechanism lies in the sign of off-diagonal terms for the electron particles and energy fluxes expressed as functions of gradients of the plasma density and the electron temperature. This model is an excellent candidate to explain the occurrence of striations in radio-frequency plasmas.
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