Ion Cyclotron Range of Frequency Power Challenges and Solutions

The use of power in the Ion Cyclotron Range of Frequency (ICRF) to heat the plasma has encountered challenges: the sensitivity to the plasma edge density profile, with the difficulty to couple power to the plasma and the enhanced plasma-antenna interaction with, among others, the resulting impurity production. These challenges have essentially been solved. Problems related to the sensitivity to the plasma edge density profiles, were encountered when strong ELMs changed massively the antenna coupling and affected the operation of the generators, and when low edge density lead to insufficient coupling and high voltages. The first problem has long been solved with the use of 3-dB couplers. The second problem was addressed in recent experiments, where local gas puffing helped to tailor the density profile and increase the coupling. A systematic study confirmed that the coupling can be calculated if the local density profile is known. The development of a theoretical approach to model the local density profile that was benchmarked against the recently acquired ability to measure locally the density profile is developing into a predictive capability to calculate and optimize the coupling also for future machines. The hypothesis concerning the enhanced plasma-antenna interactions is that they are mainly due to RF sheaths at the antenna and that those sheaths are a consequence of induced current driven at inappropriate locations. New 3-strap antennas in ASDEX Upgrade were designed to reduce those unwanted currents. This approach leads indeed to a strong reduction of the impurity production. Whereas, with the original W-coated 2-strap antennas, the increase with ICRF of the W impurity concentration in the edge plasma was about twice the increase as when the B-coated 2-strap antennas were in operation, with the new 3-strap antennas (still W-coated) the W concentration is not higher than when using the B-coated 2-strap antennas. Direct measurements of the impurity production at the limiters of the W-coated antennas show a reduction by a factor of 2 between the 3-strap and the 2-strap antennas. Theoretical approaches to model in detail the formation of the sheaths are being developed and are being checked against measurements on ASDEX Upgrade. IShTAR, a dedicated test stand to measure the electric fields will further allow to benchmark the codes. These newly developed and benchmarked code capabilities will benefit the design of antennas for future machines.