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Acceleration in the plasma focus and in astrophysics†

Published online by Cambridge University Press:  09 March 2009

P. F. Browne
P. F. Browne
Affiliation:
Department of Pure and Applied Physics, University of Manchester Institute of Science and Technology, Manchester M60 1QD
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Abstract

Both, in the laboratory (specifically the plasma focus) and in astrophysics, charges are accelerated in magnetically pinched current channels. The current channels are annular cylinders, defined by an electric double layer of thickness δrE and radius R due to radial polarization of the plasma. The electric field Er in the double layer and the azimuthal magnetic field Bϕ ensure drifts of charges in the axial direction, but ions do not take up the full drift velocity because their Larmor radius exceeds δrE. Magnetic pinching of the channel reduces the cross sectional area S, and an instability of drift velocity may denude a region of current channel. The inductively maintained current J then cannot be carried by free charges, even with relativistic drift velocity. A displacement current (S/4π) δEz/δt arises, Ez growing until all energy in Bϕ is transferred into Ez. During this transfer Bϕ decreases, so the pinch weakens, permitting discharge of accumulated space charge via a conduction current without reversal of J. Thereafter the cycle repeats quasi-periodically.

In astrophysics acceleration occurs in a pinched magnetic vortex tube (MVT). The difference is that now Bϕ(r, +z) = −Bϕ(r, −z) due to winding of loops of field lines. Hence J reverses across mid-pinch, which implies accumulation of space charge of one sign at mid-pinch and of opposite sign at the ends of the pinch. Now there is reversal of J when the space charge relaxes.

Type
Research Article
Information
Laser and Particle Beams , Volume 6 , Issue 3 , August 1988 , pp. 409 - 420
Copyright
Copyright © Cambridge University Press 1988

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