Chirping Ion Cyclotron Emission (ICE) on NSTX-U

We report here on the discovery on NSTX-U of a qualitatively new type of non-thermal emission from plasmas in the ion-cyclotron frequency range (ICE). This new type of ICE is weakly damped, É¡_damp/??_ICE ≈ 0.14%, low order eigenmodes, excited by a fast ion population radially located near an internal transport barrier.    The ICE appears as repetitive bursts with durations of ≈ 1 ms, and the bursts exhibit frequency chirps upwards and/or downwards with maximum δf/f ≈ 1%.  We refer to this chirping ICE as ch-ICE.  The bursts are longer than the typical ICE bursts of ≈ 100 µs reported previously for NSTX and NSTX-U, for simplicity, NSTX(-U).  The chirping resembles that seen in modeling of weakly unstable fast particle driven instabilities [cf., Berk, et al., Phys. Lett. A  234, (1997) 213].  These data also provide some preliminary indications of non-linear coupling between the ch-ICE harmonics.  Despite extensive experimental and theoretical studies of ICE, theoretical understanding of ICE remains qualitative at best.  The detailed experimental observations of ICE from NSTX(-U) find a wide variation in the ICE characteristics.  The new observations of ch-ICE presented here, combined with the characteristics of ICE on NSTX(-U) previously reported [Fredrickson, et al., Phys. Plasmas 26 (2019), 032111], could guide the development of more complete theoretical models of ICE.