Overview of the TWA concept from DEMO to the high power mock-up for WEST.

The travelling wave array (TWA) concept was proposed as an RF actuator in the ion cyclotron range of frequencies (ICRF) for future fusion reactors and represents a relevant alternative to the conventional individually-fed arrays used in present-day machines. This paper presents an overview of the TWA concept, from the proposal for EU-DEMO to the successful tests of a high RF power mock-up paving the way for an experiment in a long-pulse device like WEST. The best TWA integration case considered for a fusion reactor like EU-DEMO would be part of the breeding blanket, sharing its cooling and effectively acting as a first-wall component. The antenna should be insensitive to considerable mechanical deformations arising from the high-temperature operation of the blanket. The roadmap to the proof-of-concept in a long-pulse device foresees a test at high power (up to 2 MW). We have designed and built a TWA antenna mock-up that has been successfully tested in the TITAN facility. This mock-up was an essential milestone that allowed us to demonstrate the validity of the design, confirming some key characteristics of the TWA concept. The ability to tune the antenna to be matched over a large frequency band was controlled in the design phase using trimmers and was well demonstrated during the experimental phase. The antenna frequency response showed to be resilient to thermo-mechanical deformations. The ohmic losses proved to be limited (4%). The expected operation targets were met (in TITAN) of 2 MW / 3 s and 500 kW / 60 s. In the proof-of-concept, the antenna should minimize unwanted plasma-wall interactions (PWIs). The TWA already benefits from lower fields and a very narrow power spectrum due to the larger number of straps characterizing its structure. By adjusting the layout of the antenna, we show that the power spectrum can be tuned to avoid low-k excitation. Furthermore, we show that the electric fields on the antenna limiters can be reduced. The status, challenges and opportunities of the WEST TWA proof-of-concept proposal are discussed and future work is outlined. [ABSTRACT FROM AUTHOR]