Abstract
Measurements are reported of electromagnetic emission close to the cyclotron frequency of energetic ions in JET plasmas heated by waves in the ion cylotron 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 system in the transmission lines of one of the ICRF antennas. The measured ion cyclotron emission 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 Alfvén eigenmodes closely spaced in frequency. In most cases the energetic ions can be clearly identified as ICRF wave-accelerated 3He minority ions, although in two pulses the emission may have been produced by energetic 4He ions, originating from third harmonic ICRF wave acceleration. It is proposed that the emission close to the 3He 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 multiple 3He 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 γ-ray emission or other collective instabilities.
Highlights
Spontaneous emission of electromagnetic radiation in the ion cyclotron range of frequencies, usually referred to as ion cyclotron emission (ICE), is a frequently-observed property of tokamak plasmas containing suprathermal ion populations [1, 2, 3, 4]
With the use of suitable filters to suppress signals corresponding to the launched ion cyclotron resonance heating (ICRH) frequencies, ICE detection is possible in this case during radio-frequency heating, thereby making it possible to detect emission excited by ICRH fast ions
We have presented measurements of ion cyclotron emission (ICE) from JET plasmas heated by a combination of ICRH and neutral beams; the measurements were obtained using a detection system in the ICRH antenna transmission lines
Summary
Spontaneous emission of electromagnetic radiation in the ion cyclotron range of frequencies, usually referred to as ion cyclotron emission (ICE), is a frequently-observed property of tokamak plasmas containing suprathermal ion populations [1, 2, 3, 4]. One-dimensional codes that co-evolve the fields and the full orbits of the ions, while employing either a kinetic or a fluid description of the electrons, have been used to model the nonlinear evolution of ICE excited by fusion α-particles in JET [13, 14], and of ICE observed during edge localised modes in the medium-sized tokamak KSTAR [15, 16]. Linear modelling of fundamental and second harmonic ICE driven by 3He fusion products in the large tokamak JT-60U has yielded estimates of the frequencies and toroidal mode numbers of the most rapidly growing modes which are in good agreement with measured values for ICE in this device [17] In this case the growth rates were obtained using a model fast ion distribution in the ICE source region (the low field side plasma edge) which was based on calculated guiding centre orbits of 3He fusion products arising from beamthermal reactions.
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