Design of an ICRF fast matching system on Alcator C-Mod

Abstract

In Ion Cyclotron Radiofrequency Heating (ICRH) for tokamaks, fast variations of the antenna loading impedance limit flexible and robust high-power operations. In this thesis, novel solutions for real-time matching and reduction of these variations are investigated and studied for implementation on the Alcator C-Mod tokamak. Load tolerant prematching networks are reviewed and a prototype configuration for E-antenna is proposed. By maintaining low voltage standing wave ratio in the network for a wide range of loading conditions, it could allow robust high power operations without the need of fast matching. However, typical conditions could create current imbalance effects ; the performance of the system could be degraded and the phasing between the antenna affected, with side-effects on the overall behavior of the antenna in plasma. Another possible option for real-time matching uses ferrite loaded transmission lines, whose electrical length could be varied over timescales as fast as a few milliseconds. A potential ferrite material is identified and experimentally characterized in a small-scale low power experiment. This leads to design guidelines for a high power ferrite phase shifter and a fast-matching network using such tuners.