  Measurements are reported of electromagnetic emission close to the cyclotron
frequency of energetic ions in JET plasmas heated by waves in the ion cyclotron
range of frequencies (ICRF). Hydrogen was the majority ion species in all of
these plasmas. The measurements were obtained using a sub-harmonic arc
detection (SHAD) system in the transmission lines of one of the ICRF antennas.
The measured ion cyclotron emission (ICE) spectra were strongly filtered by the
antenna system, and typically contained sub-structure, consisting of sets of
peaks with a separation of a few kHz, suggesting the excitation of
compressional Alfven eigenmodes (CAEs) closely spaced in frequency. In most
cases the energetic ions can be clearly identified as ICRF-accelerated He-3
minority ions, although in two pulses the emission may have been produced by
energetic He-4 ions, originating from third harmonic ICRF wave acceleration. It
is proposed that the emission close to the He-3 cyclotron frequency was
produced by energetic ions of this species undergoing drift orbit excursions to
the outer midplane plasma edge. Particle-in-cell and hybrid (kinetic ion, fluid
electron) simulations using plasma parameters corresponding to edge plasma
conditions in these JET pulses, and energetic particle parameters inferred from
the cyclotron resonance location, indicate strong excitation of waves at He-3
cyclotron harmonics, including the fundamental, which is identified with the
observed emission. These results underline the potential importance of ICE
measurements as a method of studying confined fast particles that are strongly
suprathermal but have insufficient energies or are not present in sufficient
numbers to excite detectable levels of gamma-ray emission or other collective
instabilities.
