Extension of the operating space of high-betaN fully non-inductive scenarios on TCV using neutral beam injection

The fully non-inductive sustainment of high normalized beta plasmas (betaN) is a crucial challenge for the steady-state operation of a tokamak reactor. In order to assess the difficulties facing such scenarios, steady-state regimes have been explored on the tokamak à configuration variable (TCV) using the newly available 1 MW neutral beam injection (NBI) system. The operating space is extended towards plasmas that are closer to those expected in JT-60SA and ITER, i.e. with significant NBI and electron cyclotron resonance heating and current drive (ECRH/CD), bootstrap current and fast ion (FI) fraction. BetaN values up to 1.4 and 1.7 are obtained in lower single null L-mode (H98(y,2)~0.8) and H-mode (H98(y,2)~1) plasmas, respectively, at zero time averaged loop voltage and q95~6. Fully non-inductive operation is not achieved with NBI alone, whose injection can even increase the loop voltage in the presence of EC waves. A strong contribution to the total plasma pressure of thermal and FIs from NBI is experimentally evidenced and confirmed by interpretative ASTRA and NUBEAM modeling, which further predicts that FI charge-exchange reactions are the main loss channel for NBH/CD efficiency. Internal transport barriers, which are expected to maximize the bootstrap current fraction, are not formed in either the electron or the ion channel in the plasmas explored to date, despite a significant increase in the toroidal rotation and FI fraction with NBI, which are known to reduce turbulence. First results on scenario development of high-betaN fully non-inductive H-mode plasmas are also presented.