Fast-ion studies with three-ion ICRF scenarios in non-active JET plasmas

Significant progress with the development of three-ion ICRF scenarios in support of the ITER Research Plan has been recently achieved [1]. We report the results of JET studies where the three-ion 4He-(3He)-H scheme was applied for heating non-active H-4He plasmas, both on-axis and off-axis [2], and studying the impact of fast ions on the plasma confinement. The spatial profile of energetic 3He ions was controlled by varying the ICRF antenna phasing, resulting in significant differences in MHD behaviour and sawtooth dynamics. Our results confirm that the three-ion ICRF scenario can be applied to control the radial profile of the safety factor and sustain plasmas with an inverted q-profile at JET, complementing earlier observations of its application in D-3He plasmas [3]. Another important highlight of the JET experiments in H-4He plasmas reported here is the demonstration of the capability to measure simultaneously both He isotopes, n(4He)/ne ? 5-15% and n(3He)/ne ? 0.2%, using the high- resolution sub-divertor gas spectroscopy [4]. In this contribution, we also report on the successful application of the three-ion 9Be/22Ne/Ar-(4He)-H ICRF scheme at JET. This scenario makes use of intrinsic 9Be (Z/A ? 0.44), and/or extrinsic impurities with a similar charge-to-mass ratio, e.g., 22Ne and Ar (Z/A ? 0.45), to optimize the efficiency of ICRF absorption by a small amount of 4He ions (n(4He)/ne ? 0.5%) in hydrogen plasmas [1]. JET experimental results confirm that an additional injection of a very small amount of 22Ne impurities is beneficial for maximizing the population of MeV-range 4He ions with ICRF in the plasma, as evidenced by gamma-ray spectroscopy [5]. In the absence of a direct control of the level of 9Be impurities, an additional seeding of 22Ne or Ar impurities is a promising technique that can be also applied in future ITER plasmas.