Abstract
Changes in plasmas undergoing large, rapid compressions are examined numerically over the following range of aspect ratios A: 3≳A≳1.5 for major radius compressions of circular, elliptical, and D-shaped cross sections; and 3≲A≲6 for minor radius compressions of circular and D-shaped cross sections. The numerical approach combines the computation of fixed boundary magnetohydrodynamic equilibria with single-fluid, flux-surface-averaged energy balance, particle balance, and magnetic flux diffusion equations. It is found that the dependences of plasma current Ip and volume-averaged poloidal beta β̄p on the compression ratio C differ significantly in major radius compressions from those proposed by Furth and Yoshikawa. The present interpretation is that compression to small A dramatically increases the plasma current, which lowers β̄p and makes the plasma more paramagnetic. Despite large values of volume-averaged toroidal beta β̄t (≳30% with safety factor q≈1 at the magnetic axis, q≈3 at the plasma edge), this tends to concentrate more toroidal flux near the magnetic axis, which means that a reduced minor radius is required to preserve the continuity of the toroidal flux function F at the plasma edge. Minor radius compressions to large aspect ratio agree well with the Furth–Yoshikawa scaling laws.
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