Improved operating space of the ICRF system in ASDEX upgrade

Following the improvements of compatibility of ICRF system with the tungsten (W) wall in ASDEX Upgrade (AUG) by using the boron-coated limiters on the 2-strap ICRF antennas and the 3-strap antennas with the W-limiters, the ICRF operating space was further extended in 3 areas: power, phase control between the antennas and frequency range. The available ICRF power in AUG was increased by using an additional RF generator to feed the central straps of the 3-strap antennas which require a 1.5:1 or 2:1 power ratio between the central strap and the two outer straps (combined) to minimize the ICRF-specific W sources. This increases the total experimentally achievable launched ICRF power in the optimized configuration to ~ 5.7 MW. A new phase control system enabled phase-locked operation of all 4 AUG ICRF antennas. This allows a better control of the launched k|| spectrum which on its turn is modified when both neighboring antennas are active, as well as of the structure of the global RF field distribution within the AUG vessel. Measurements by the RF magnetic field (B-dot) probes show that the RF field distribution can indeed be significantly affected by the variation of the phasing between the ICRF antenna pairs. However, its effect on the core plasma and on the residual ICRF specific plasma-wall interactions is small, for the cases so far limited to the H-modes at medium plasma densities. The available frequency range was extended and now covers four discrete frequencies (f): 30.0, 36.5, 41.8 and 55.1 MHz. In addition to the standard hydrogen (H) minority in deuterium (D) on-axis heating scheme at the magnetic field (Bt) of 2.0, 2.5, 2.8 and 3.1 T, the RF frequency range allows the use of the 3-ion D-(³He)-H and ⁴He-(³He)-H heating schemes at Bt=2.5-3.1 T (f=30 MHz – off-axis and on-axis), on-axis 3ωc D heating at Bt=1.9 T (f=41.8 MHz) and at Bt=2.5 T (f=55.1 MHz) and 2ωc H heating at Bt=1.9T (f=55.1 MHz).