Ion cyclotron resonance heating (ICRH) is one of several methods of heating the plasma in a fusion reactor by injecting powerful radio frequency (RF) waves. An ICRH system consists of amplifiers, transmission lines and antennas that transmit the waves to the plasma. The antenna/plasma system can be regarded as single joint system. Transmitting the RF waves works well only if the impedances – the resistance to electromagnetic waves – of the amplifier and the antenna/plasma system match. The impedance of the antenna/plasma system, however, depends on the density at the plasma edge. In H-mode plasmas, which are intended for ITER, events like ELMs cause rapid changes in the plasma edge conditions and thus in the impedance. As a result, the waves are reflected back to the RF amplifier, which switches off (‘trips’) briefly as a result thus preventing overloading. The heating does not function steadily and power is lost in the reflections. It was essential to develop a “load resilient” antenna for ITER which can compensate impedance variations and supply continuous power.

A typical ICRH antenna consists of one or more “straps” (metallic blades) that radiate the waves into the plasma. Each strap is connected individually to an amplifier. In recent years, the problem of varying impedances has been combated by combining the reflections generated in two straps. So-called 3dB hybrid couplers at ASDEX Upgrade direct the reflected power to a so-called dummy load, preventing the RF amplifier from tripping. At Tore Supra and TEXTOR, conjugate T-junctions confine the reflected power within the two straps and these compensate each other at the Tjunction because the impedances of the two branches are carefully adjusted using variable capacitors.

Both techniques have been used with JET’s four “old” A2 antennas to demonstrate their viability. The straps of two antennas were combined with 3dB hybrid couplers, the straps of the other two antennas were combined using conjugate T-junctions.

Another challenge at ITER is the limited amount of space: Large amounts of power must pass through a small aperture, thus calling for high power densities at high operating voltages. The problem lies in the possible arcs caused by the high voltages needed. Their occurrence depends on the geometry of the antenna. The new ITER-like antenna, ILA, was designed in a collaborative effort between LPP/ERM-KMS Brussels, CCFE (formerly UKAEA-Fusion, Culham), Oak Ridge National Laboratory and CEA Cadarache. It is made up of eight straps combined in conjugate Tjunctions to form four pairs. The ILA is very compact with a total surface of 0.9 m2 and is designed to deliver a maximum of 7.2 MW which corresponds to a power density of 8 MW/m².

The ILA and the four modified A2 antennas have been commissioned and extensively tested at JET in the 2008/09 experimental campaigns. A total of 8.3 MW ICRH power could be delivered into ELMy H-Mode plasmas and the load resiliency of the systems could be demonstrated. Due to the continuous supply of power, the modified A2 antennas delivered 2.9 MW and 3.6 MW in case of the d3b and the conjugate T coupling, respectively. The A2 antennas can now inject a maximum of 3 – 4 MW power per pair into the plasma, depending on the plasma conditions. The maximum power density reached at the ILA, 6.2 MW/m2 is a vast improvement over the limit of 1.8 MW/m2 of the “old” A2 JET antennas. With 42 kV, the maximum voltage on the straps of ILA was close to the ITER design value and exceeded, by far, the previously achievable 30 kV. These results greatly increase the confidence in the ICRH antenna design for ITER.

Thanks to Jef Ongena, JET, for his input.

For more information about the JET ICRH improvements, please contact Jef Ongena or Marie-Line Mayoral at JET:

More about ICRH and other heating methods can be found in JET’s Focus on: