PHYSICS PROGRESS TOWARDS COMPACT TOKAMAK REACTORS WITH NORMAL CONDUCTING TOROIDAL FIELD COILS

Very low aspect ratio (R /a ~ 1.2) tokamaks utilizing a demountable, normal conducting center leg to carry the toroidal field current are estimated to lead to compact (R0 + a ~ 5.4 m) and economically competitive fusion power plants, given certain key plasma properties. Calculations are carried out to determine if these properties can be obtained for such low R la, using tokamak physics models recently improved by data from START. High toroidal field utilization (/p//lfc ~ 1-2) and strong plasma "shaping" S ( s l^q^/aB^ 100-200 MA-nT1 -T"1) can be obtained using only two pairs of outboard poloidal field coils. The fraction of the scrape-off layer (SOL) connected to the inboard side of such plasmas diminishes as RJa is reduced, while the remainder is diverted. Order-unity toroidal average betas ( = 2/^0/fi^ - 0.5-1) which are ballooning stable are obtained having a second-regime core of low magnetic shear and high q0 (5 > q0 > 3), and a first-regime periphery of high magnetic shear and high edge q^ > 11). Large self-driven currents (I^f/Ip < 0.7), well aligned with a hollow plasma current in these equilibria, are calculated for e/3j < 0.9. For marginal MHD stability of themore » outboard SOL, it is shown that die minimum pressure e-folding length Xp scales approximately according to ( 7j/i?)(Z,c/,B)2, /i and 7j being the edge ion density and temperature, respectively, and Lc being the connected field line length. A large outboard SOL pressure e-folding thickness can therefore exist for low RQ and B^ configurations of very low RJa (Xp ~ AS0L/3 ~ 10 cm in the case of reactors). This reduces the divertor heat flux expected in compact fusion systems of high power density. Progress in these topics will help ensure that very low R la tokamaks are effective vehicles for fusion power as well as blanket testing.« less