WEST, like most magnetic fusion experiments, use radiofrequency waves at frequencies between 30 to 60 MHz to heat the ions of the confined plasma. This plasma heating technique, known as Ion Cyclotron Resonance Range of Frequencies (ICRF), rely on antennas located close to the plasma. ICRF systems provide several tasks on present fusion experiments: direct plasma ion heating, first wall conditioning, plasma start-up or removing of impurities from the core. Up to now, ICRF antennas are “resonant” antennas, a property that allows them to be compact enough to fit the narrow “ports”, that is, the dedicated space between two coils by which an antenna is inserted. Like a guitar or a violin, these antennas must be properly tuned to work at a specific frequency.

Next-step fusion projects like ITER or future demonstration reactors also rely on this kind of antenna. However, because the space available for these antennas is constrained and the required heating power is always larger, the power density increases and reaches the limit of the engineering know-how.

To solve this dilemma, other antenna concepts have been proposed. “Travelling Wave Array” antennas (TWA) can reduce this power density to much safer levels, but at the price of a larger surface facing the plasma. However, a TWA antenna does not require to occupy the full port space at the contrary of resonant antennas, leaving this precious space for other systems. Because these travelling wave antennas have a larger number of radiating elements, the capability of coupling the radiofrequency power to the plasma is increased. In addition, these antennas are naturally wideband, which means they can be operated at different frequencies without additional tunings, making them a more robust tool in the harsh conditions of future fusion reactors. Despite these advantages, TWA antennas have not been yet tested at the frequency range for ion heating (typically around 50 MHz in current experiments).

Hence, a TWA antenna mock-up has been designed and tested in the joint activity of EUROfusion (led by LPP/RMA, Belgium) and SIFFER (SIno-French Fusion Energy centeR), in collaboration with CEA/IRFM, as the first step toward the experimental demonstration of an ICRH TWA antenna. The main goal of this mock-up, a simplified version (flat and without active cooling)  is to demonstrate that the antenna can handle high power and to measure the voltages and thermal responses of the antenna to benchmark with numerical modelling. To this aims, a stainless-steel only mock-up manufactured by ASIPP (China) has been installed in the TITAN test-bed at CEA/IRFM and connected to one of the high RF power generators of the WEST ICRF system.

The tests have been successfully performed and up to 2 MW have been injected into the mock-up during few seconds, a value corresponding to the maximum RF power the generator could produce and not by the antenna itself. While not actively water-cooled, the mock-up has been tested during longer RF pulses with up to 500kW for 60 seconds. The temperature rise during RF pulses was monitored with the help of an infra-red camera and show thermal responses as expected by modelling. Following this successful demonstration, the next step should be to test an actively cooled TWA antenna inside a tokamak. Why not in WEST?