Tokamak plasma variations under rapid compression

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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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