Description
The TCV tokamak is a mid-sized device featuring versatile ECRH capabilities (2.6 MW in 2025), which have allowed fully non-inductive and steady-state discharges, with the current profile fully relaxed, to be run routinely since 1999. High confinement and a high bootstrap fraction - even reaching unity - could also be obtained in these steady-state conditions by sustaining electron internal transport barriers (eITBs) primarily with off-axis heating and/or current drive. The missing ingredient for fusion-relevant performance and, in particular, high beta_N in these long-pulse scenarios was ion heating, which is now being explored in a series of ongoing experiments.
The most promising results toward steady state were obtained with a balanced application of ECRH, all the power being used for off-axis ECCD, and NBI - yielding an empirical finding of better MHD stability with the NBI-2 beam (50-60 keV) injected counter-current than with the NBI-1 beam (20-25 keV) in co-current mode. Carefully controlling the density to within a narrow useful range for efficient current drive as well as ion heating, beta_N=2 was reached in a semi-stationary manner with Ti/Te~0.3. Both TRANSP and ASTRA simulations indicate that the safety-factor shear is reversed in the core.
As a separate investigation of particular interest to spherical-tokamak research, we have also explored the possibility of a full current ramp-up with no loop-voltage contribution from the central solenoid (CS). ECRH power was applied 15 ms after breakdown and the CS current clamped to a constant value at 30 ms, using only 10% of the available flux, and a successful ramp-up was achieved in a reproducible manner. The CS-free conditions were maintained throughout the plasma volume increase and shape formation as well as X-point creation, to the flat-top phase featuring also NBI.
It should be noted that, as all ECRH deposition is required to be off-axis to sustain the scenario, we are unable to use ECRH to heat the high-confinement central region of the plasma. There is thus reason to believe that the gradual addition of 1.9 MW ECRH power by the end of 2027 will be crucial for the culmination of this line of research.