A possible excitation mechanism for observed superthermal ion cyclotron emission from tokamak plasmas

Abstract

Intense superthermal ion cyclotron emission (ICE) has been observed from tokamak plasmas. The power spectrum displays narrow peaks at multiple harmonics of the background ion cyclotron frequency [Cottrell and Dendy, Phys. Rev. Lett. 60, 33 (1988)] in Ohmic deuterium plasmas, and the radiation appears to be driven by the fusion ion population in the edge plasma. Theoretical investigation of this phenomenon may be rewarding, in terms of the information about the behavior of energetic ions in tokamaks that can be extracted from ICE observations. The interpretation presented here is based on the resonant excitation of fast Alfvén waves with ion Bernstein waves supported by an energetic ion species (α), in the presence of a more numerous thermal ion species (i). Because the ion cyclotron frequencies may be commensurate (lΩα=sΩi for some low integers l,s), and observations indicate that ω is comparable in magnitude to Ωi, the standard theory [Belikov and Kolesnichenko, Sov. Phys. Tech. Phys. 20, 1146 (1976)] which assumes ω≫Ωi is not immediately applicable, and is accordingly extended here to the low-frequency regime. The results show that excitation of the fast Alfvén wave at proton cyclotron harmonics can occur for fusion proton concentrations nα/ni as low as 10−7, and that multiple cyclotron harmonics can be simultaneously unstable. Furthermore, while fusion protons born at 3.0 MeV are above the energy threshold required to drive the instability, the other primary fusion products in deuterium—1.0 MeV tritons and 0.82 MeV helium-3 nuclei—fall below it, consistent with the observation that radiation at cyclotron harmonics of the latter is not detected. However, the mechanism considered here cannot easily explain the generation of radiation which is observed at deuterium cyclotron harmonics that are degenerate with proton half-harmonics.

Ion cyclotron emission from fusion-born ions in large tokamak plasmas: a brief review from JET and TFTR to ITER

“…This was strongly suggested by the original linear analytical theory approach to ICE from deuterium-tritium plasmas in JET and TFTR [1][2][3][4][5][6][7]. It is confirmed by the recent large scale kinetic simulations using PIC and hybrid codes [14,15] reviewed here.…”

“…The measured intensity of ICE spectral peaks scaled linearly with measured fusion reactivity: both between different plasmas spanning a factor of a million in fusion reactivity [3], and during single plasma pulses following the rise and fall of fusion reactivity over time [3,5]. Soon after these observations were reported, linear analytical theory together with particle orbit calculations suggested [7][8][9][10][11] that the emission mechanism is the magnetoacoustic cyclotron instability (MCI). The MCI was first identified theoretically by Belikov and Kolesnichenko [12] before it was observed in JET and TFTR tokamak plasmas, where it is driven by a subset of centrally born fusion products that lie just inside the trapped-passing boundary in velocity space, whose existence was anticipated by Stringer [13].…”

Observations of core ion cyclotron emission on ASDEX Upgrade tokamak

“…6) ). This suggests that the core ICE driver is likely to be the fusion-born protons and not the beam ions, since it is driven more readily when the fast ions are super-Alfvѐnic 14 . Indeed fusion protons were identified as the driver of ICE in the KSTAR discharges considered in Ref.…”

Nonlinear wave interactions generate high-harmonic cyclotron emission from fusion-born protons during a KSTAR ELM crash

“…This subset remains confined, because it lies on deeply passing drift orbits which carry the protons from the core to the edge and back again. Its sharply defined non-Maxellian distribution in velocity space means that this subset of the fusion-born protons can undergo the magnetoacoustic cyclotron instability (MCI) [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] in the edge plasma. The MCI drives waves on the fast Alfvén-cyclotron harmonic wave branch, both in analytical theory [16][17][18][19][20] and in first principles simulations [23][24][25], and these are likely to be the waves observed as ICE in KSTAR.…”