In this paper, experimental observations of spontaneously excited waves in the ion cyclotron range of frequency (ICRF) on JT-60U are described. The fluctuations in ICRF are driven by the presence of non-thermal ion distribution in magnetically confined plasmas. Two types of magnetic fluctuations are detected: one is due to high-energy D ions from neutral beam (NB) injections and the other is due to fusion products (FPs) of 3He and T ions. These fluctuations have been reported as ion cyclotron emissions (ICEs) in the burning plasma experiments on large tokamaks. This paper describes the first measurement of the spatial structures of the excited modes in the poloidal and toroidal directions. It is confirmed by using ICRF antennas as pickup loops that all modes excited spontaneously have magnetic components. The modes due to D ions have zero or a small toroidal wave number kz. On the other hand, the measurement of finite kz in the modes due to FP ions supports the excitation of the Alfvén waves, which is the possible origin of FP-ICEs. It is also observed that the excited modes due to FP ions (3He and T ions) have different characteristics: driven by different NBs and having different parameter dependences. ICE due to T ions has no harmonics and the value of ω/Ωci is smaller than that due to 3He. Both beam-driven ICEs and FP-ICEs are clearly observed and their spatial structures are obtained on JT-60U.
Electrostatic ion cyclotron waves in a low magnetized plasma
Novel internal measurements of ion cyclotron frequency range fast-ion driven modes