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Measurements are reported of electromagnetic emission close to the cyclotronfrequency of energetic ions in JET plasmas heated by waves in the ion cyclotronrange of frequencies (ICRF). Hydrogen was the majority ion species in all ofthese plasmas. The measurements were obtained using a sub-harmonic arcdetection (SHAD) system in the transmission lines of one of the ICRF antennas.The measured ion cyclotron emission (ICE) spectra were strongly filtered by theantenna system, and typically contained sub-structure, consisting of sets ofpeaks with a separation of a few kHz, suggesting the excitation ofcompressional Alfven eigenmodes (CAEs) closely spaced in frequency. In mostcases the energetic ions can be clearly identified as ICRF-accelerated He-3minority ions, although in two pulses the emission may have been produced byenergetic He-4 ions, originating from third harmonic ICRF wave acceleration. Itis proposed that the emission close to the He-3 cyclotron frequency wasproduced by energetic ions of this species undergoing drift orbit excursions tothe outer midplane plasma edge. Particle-in-cell and hybrid (kinetic ion, fluidelectron) simulations using plasma parameters corresponding to edge plasmaconditions in these JET pulses, and energetic particle parameters inferred fromthe cyclotron resonance location, indicate strong excitation of waves at He-3cyclotron harmonics, including the fundamental, which is identified with theobserved emission. These results underline the potential importance of ICEmeasurements as a method of studying confined fast particles that are stronglysuprathermal but have insufficient energies or are not present in sufficientnumbers to excite detectable levels of gamma-ray emission or other collectiveinstabilities.

nuclear fusion

Observations and modelling of ion cyclotron emission observed in JET  plasmas using a sub-harmonic arc detection system during ion cyclotron  resonance heating