In most tokamaks, and in WEST in particular, the plasma is heated by radio-frequency waves in the range of 30 to 60 MHz, which are emitted by so-called Ion Cyclotron Resonance Heating (ICRH) antennas. The advantage of these antennas is that they are compact enough to be introduced close to the fusion plasma, through a port in the vacuum chamber. However, the tokamaks of the future will require a higher radio frequency power distribution with the same space constraints, and the extrapolation of these antennas to higher power densities seems to be at the limits of our current expertise.

This is why the researchers have turned their attention to another family of antennas. Known as Travelling Wave Arrays, these antennas can be used to relieve the space constraint by being deployed inside the vacuum chamber, in contrast to in-port antennas. This arrangement of antennas reduces the power density for the same power coupled to the plasma, and thus the required electric fields. The risks of electric arcing are diminished, and the reliability of the antennas is increased! Moreover, this geometry should be a priori more favorable to the coupling of radio frequency waves to the plasma, and their wide-band emission (10 MHz) does not require any adjustment (and therefore no moving parts) inside the machine.

A simplified model of a Travelling Wave Array antenna has been tested at high power under vacuum using the IRFM TITAN (Test Bed for Iter ICRH Antenna) test bench:

• 2 MW for 3 s (maximum power of the radio frequency wave generator)
• 500 kW for 60 s.

The measurements (RF, infrared, etc.) performed are in agreement with the models: the antenna is broadband (10 MHz), it does not require any adjustment during use, and the voltage inside is lower than that of conventional antennas for similar outputs. These advantages are essential for ion heating antennas used in future fusion machines!

 
This model was designed as part of a EUROfusion project led by the Laboratory for Plasma Physics of the Royal Military Academy (Belgium), and manufactured at the Institute of Plasma Physics of the Chinese Academy of Sciences (ASIPP). It was then assembled at the IRFM and installed in March in the TITAN vacuum chamber, where it was tested.