Fast matching of load changes in the ion cyclotron resonance frequency range

In the ion cyclotron resonance frequency (ICRF) range, it is necessary to match the antenna load to the impedance at which the generator delivers its maximum power. This is typically done by adding impedances (mechanical tuners i.e. pieces of transmission lines) at appropriate locations in parallel to the feeder line of the antenna. Large and fast load changes can result from variations in the plasma such as H-mode transitions and ELMs (edge localised modes). For optimum power delivery, those load changes have to be matched dynamically. The required variation of the impedance is performed by a change of electrical length of the tuners. A new type of tuners, fast ferrite tuners (FFT) developed by AFT, relies on the change of magnetisation of ferrites to achieve this. Since there are no mechanically moving parts, the change can be fast. Acceptance tests of a system, developed for General Atomics, using these tuners were performed successfully and first experience was gained on a proof of principle test under plasma conditions.