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A Laboratory Plasma Experiment For Studying Magnetic Dynamics Of Accretion Discs And Jets

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White Paper: A Laboratory Plasma Experiment For Studying Magnetic Dynamics Of Accretion Discs And Jets

ABSTRACT
This work describes a laboratory plasma experiment and initial results which should give insight into the magnetic dynamics of accretion discs and jets. A high-speed multiple-frame CCD camera reveals images of the formation and helical instability of a collimated plasma, similar to MHD models of disc jets, and also plasma detachment associated with spheromak formation, which may have relevance to disc winds and flares. The plasmas are produced by a planar magnetized coaxial gun. The resulting magnetic topology is dependent on the details of magnetic helicity injection, namely the force-free state eigenvalue ∂gun imposed by the coaxial gun.

INTRODUCTION
The accretion disc occupies a leading role in astrophysics, figuring prominently in young stellar objects (YSO), binary star systems, and active galactic nuclei (AGN). An unsolved mystery is the origin of highly collimated bipolar jets and episodic ares associated with accretion discs. It was proposed some time ago that magnetic field dynamics can supply the necessary jet formation and collimation mechanisms (Blandford 1976; Lovelace 1976). Magnetohydrodynamic (MHD) simulations have shown that jets are a natural consequence of a rotating disc in the presence of a magnetic field (e.g. Shibata & Uchida 1985). Jet structure, such as \knots," and also episodic behavior are observed in the simulations (Ouyed & Pudritz 1997; Goodson, Bohm & Winglee 1999; Nakamura, Uchida & Hirose 2001). The details of these models are unlikely to be tested by observations anytime in the near future. Therefore, data from laboratory experiments could be very useful for this purpose. It should be noted that astrophysical jets have been compared theoretically with plasma guns (Contopoulos 1995), and that plasma experimentalists have interpreted coaxial gun plasma ows in the context of astrophysical jet morphology (Caress 1996).

This work describes a new plasma gun based laboratory experiment and initial results which should give insight into the magnetic dynamics of accretion discs and jets. This experiment is the first to use a plasma gun which explicitly simulates the geometry and topology of a magneticallylinked star-disc system by using a co-planar disc-annulus electrode setup. The experiment reveals (1) the formation and helical structure of a magnetically-driven collimated plasma, similar to proposed models of astrophysical jets, and (2) plasma detachment which may be relevant for disc winds and ares and field-line opening in disc coronae. The resulting magnetic topology depends on the details of magnetic helicity injection, namely the force-free state eigenvalue imposed at the boundary. The plasmas in this work satisfy the MHD criteria globally (S >> 1, pi <>L, and VA <>c, where S is the Lundquist number, pi the ion gyro-radius, L the plasma scale length, and VA the Alfven speed). It should be noted that S ~ 103 in the experiment, similar to that in MHD numerical simulations.

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White Paper: A Laboratory Plasma Experiment For Studying Magnetic Dynamics Of Accretion Discs And Jets