Alpha Particle Physics Studies on JET with ICRH-accelerated 4He Beam Ions
JET experiments with 4He plasmas have provided a unique opportunity for alpha particle physics studies. Dedicated experiments have been performed adding ICRH power to 4He neutral beam injection in order to produce high-energy ICRH-accelerated 4He ions to simulate fusion-born alpha particles. The third-harmonic 4He scenario, with the w 3w(4He) resonance in the plasma centre, was chosen in order to avoid complications, such as cold-plasma cutoff for wave propagation and central hydrogen damping which occur for waves tuned to the fundamental and second-harmonic 4He resonance, respectively. Up to 7.5MW of ICRH power was applied at a frequency of 51MHz and a magnetic field of 2.2T. Clear evidence for a 4He tail and its effects on the plasma was obtained using low-power (1.5MW) 4He beams with the highest available energy (120keV) and with the most central power deposition. The use of the high-energy 4He beams maximised the ICRH damping strength on 4He and the ICRH power per 4He beam ion was large enough to produce a significant 4He tail. The 4He ions with energies above 2MeV were detected using gamma-ray emission from the nuclear reaction 9Be(a,ng)12C between ICRH-accelerated 4He beam ions and 9Be which is one of the impurity species in JET plasmas. This reaction has been proposed earlier as a diagnostic for fusion-born alpha particles . The direct measurement of high-energy alpha particles (although not fusion-born), together with other alpha particle observations in these experiments, are discussed in detail. These include the first excitation of Alfvén eigenmodes by alpha particles on JET, heating of the background plasma, triggering of H-mode as well as indications for sawtooth stablisation by alpha particles.  V. Kiptily, Fusion Techn. 18, 583 (1990).