A Mega-Ampere Spherical Tokamak Design for Beta-Limit and Confinement Studies

Recent favorable results on START have caused renewed interest in low aspect ratio tokamaks. To design an economical next-step spherical tokamak to study confinement scaling and high beta plasmas, we have developed a transport scaling and device optimization code. This code OPT, benchmarked against START, includes 10 empirical confinement scaling laws and essential tokamak physics such as stability limits. Parameters are optimized separately for each scaling law and physics goal. Using OPT we find for R/a=1.2 to 2.0 one can achieve β_N=5 and <β>=30% with just two neutral beams (P_NB<3.5 MW) for I_p≥0.75 MA, and R_o≥0.6 m. In contrast, if one insists on using the nominal device parameters, I_p=1 MA and R_o=0.8 m, with each scaling law, achieving β_N=5 requires typically P_NB⋍7.5 MW.