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[71.03] A Laboratory Plasma Experiment for Studying Magnetic Dynamics of Accretion Disks and Jets

S. C. Hsu, P. M. Bellan (Caltech)

We describe a laboratory plasma experiment and initial experimental results which give insight into the magnetohydrodynamics (MHD) of accretion disks and jet formation. We utilize a magnetized plasma gun with concentric electrodes to simulate the topology of a star-disk system. The inner electrode, which respresents the star, is a 20.3~cm disk. The outer electrode, which represents the accreting disk, is an annulus with outer diameter of 50.8~cm. Our plasmas satisfy the MHD criteria (S \gg 1, \rho_{\rm i} \ll L, and V_{\rm A} \ll c, where S is the Lundquist number, \rho_{\rm i} the ion gyro-radius, L the plasma size, and V_{\rm A} the Alfvén speed) and are allowed to evolve and expand freely with minimal interaction with the vacuum chamber walls which are far away. Using a high-speed multiple-frame CCD camera, we have obtained high quality images of the formation and evolution of three distinct plasma regimes, each corresponding to a possible accretion disk/jet phenomenon. The regimes depend on \alpha_{\rm gun}=\mu₀ I_{\rm gun}/\psi_{\rm gun} (where I_{\rm gun} is the gun current and \psi_{\rm gun} is the poloidal magnetic flux linking the gun electrodes). For low \alpha_{\rm gun}, a plasma column forms along the geometric axis of the gun and persists for many Alfvén times (\tau_{\rm A} ~ 1~\mus), as required for a stable astrophysical jet. For intermediate \alpha_{\rm gun}, the column develops a coherent helical perturbation when \alpha_{\rm gun} L \gtrsim 4 \pi, where L is the column length. This condition is equivalent to the Kruskal-Shafranov limit, indicating that the observed instability is an ideal current-driven instability which strongly affects global column structure. For high \alpha_{\rm gun}, the plasma detaches from the plasma gun and propagates along the geometric axis at a significant fraction of the estimated Alfvén speed (V_{\rm A} ~50~km/s). The detachment process may be related to disk winds and flares and other quasi-periodic behavior associated with accretion disks. The experimental results support a unified picture of accretion disk and jet MHD based on magnetic helicity injection and plasma relaxation. The processes are remarkably similar to the dynamics of spheromak formation in magnetic fusion research.

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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.