Energetic ion experiments with three-ion ICRF scenarios in non-active plasmas at JET

Significant progress with the development of three-ion ICRF scenarios [1,2] in support of the ITER Research Plan has been recently achieved [3]. We report the results of energetic ion experiments at JET, where the three-ion ⁴He-(³He)-H scheme was applied for heating non-active H-⁴He plasmas, both on-axis and off-axis [4]. 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 [5]. 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)/n_e ≈ 5-15% and n(³He)/n_e ≈ 0.2%, using the highresolution sub-divertor gas spectroscopy [6].

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 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 in hydrogen plasmas [3]. 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 (n(⁴He)/n_e ≈ 0.5%), as evidenced by gamma-ray spectroscopy [7]. 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.