The feasibility of a tokamak reactor with a very high aspect ratio plasma

Feasibility of a very high aspect ratio tokamak reactor is investigated. Here, the aspect ratio A is the ratio of plasma major radius RP to the minor radius aP. Over a wide range of A (=2.7∼8), the plasma size, current drive power PCD, divertor heat load WDIV are calculated for the steady-state plasma when the fusion power PFUS (=3 GW) and the maximum toroidal field BTMAX (=18 T) are fixed. Here, the inboard shield thickness δTF=1.4 m, the normalized beta βN=3, plasma elongation κ=1.4, plasma temperature T=15 keV and other parameters are similar to those of ITER. It is clarified that the acceptable current-drive power (PCD=81 MW), large fusion gain (Q=37) and small divertor heat load (WDIV∼10 MW m−2) are achieved in a very high-A plasma with RP/aP=12/1.5 m. Radial build is optimized with keeping the field ripple ratio constant. When the number of TF-coil (=20) and ripple level (=1%) are fixed, the inboard shield thickness δTF is increased by 0.8 m and outboard shield thickness by 1.4 m as A increases from 2.7 to 8. In this case, the internal diameter of TF-coil increases by about 0.4 m since the plasma minor radius aP is decreased by 0.9 m. In the future power plant using SiC structural material, the very high-A reactor is greatly superior to the low-A reactor since the large shielding region is available without degrading plasma performance. These results are dependent on the plasma elongation κ and normalized beta βN. When κ is increased from 1.4 to 1.7, the required BTMAX is reduced from 18 to 15 T. If an advanced operation (βN∼5) with reversed shear plasma is available, the required BTMAX is reduced from 18 to 14.5 T.