High temperature plasmas in ST40

ST40 is a high field Spherical Tokamak, built and operated by Tokamak Energy Limited, with R_geo≈0.4-0.5m, A≈1.6-1.9, κ<1.7, I_P≈0.4-0.8MA and B_T≈0.8-2.2T. Two co-injected hydrogen neutral beams deliver 0.8MW at 50kV and 0.7MW at 24kV. With deuterium the corresponding figures are 1.0MW/55kV and 0.8MW/24kV, respectively. The vessel walls were conditioned with helium GDC and boronization. Merging-compression start-up was used. The plasmas were mainly limited on the high field side, although some diverted plasmas were obtained. The primary goal for the 2021-2 operations was the attainment of the business objective, T_i(0) > 100MK (8.6keV). A central deuterium ion temperature of 9.6keV was obtained together with ??_i(0).??_??(0).??_E ≈ 6 ± 2 × 10¹⁸ m⁻³.keV.s. More detailed analysis of the high T_i data will be presented at this meeting by M. Sertoli and M. Romanelli. Such high ion temperatures have only previously been achieved in significantly larger devices and never reached in a ST. However, in order to access the high T_i regime, the electron temperature must be high enough that the collisional power transfer from ions to electrons is significantly smaller than the input power density. In the best ST40 shots, T_e(0) > 3keV was observed, which is in contrast to the T_e(0) <2.4keV in neutral beam heated STs of twice the major radius but with B_T <0.8T. The results from ST40 are consistent with the NSTX observation of an approximately linear B_T dependence of energy confinement time. A limited TF scan and comparisons between high T_i data at 1.6 and 1.92T, on axis, demonstrate the beneficial effect of high B_T. The dominant loss channel is via electrons and so the observation of high temperatures lends support to the high field ST route to economical, compact fusion reactors.