First observation and interpretation of spontaneous collective radiation from fusion-born ions in a stellarator plasma

Abstract

During bursty MHD events, transient ion cyclotron emission (ICE) is observed from deuterium plasmas in the Large Helical Device (LHD) heliotron-stellarator. Unusually, the frequencies of the successive ICE spectral peaks are not close to integer multiples of the local cyclotron frequency of an energetic ion population in the likely emitting region. We show that this ICE is probably driven by a subset of the fusion-born protons near their birth energy EH = 3.02MeV. This subset has a kinetic energy component parallel to the magnetic field, mH vparallel 2/2, significantly greater than its perpendicular energy mH v perp 2/2, for which vperp ∼VA, the Alfvén speed. First principles computations of the collective relaxation of this proton population, within a majority thermal deuterium plasma, are carried out using a particle-in-cell approach. This captures the full gyro-orbit kinetics of all ions which, together with an electron fluid, evolve self-consistently with the electric and magnetic fields under the Maxwell-Lorentz equations. The simulated ICE spectra are derived from the Fourier transform of the fields which are excited. We find substantial frequency shifts in the peaks of the simulated ICE spectra, which correspond closely to the measured ICE spectra. This suggests that the transient ICE in LHD is generated by the identified subset of the fusion-born protons, relaxing under the magnetoacoustic cyclotron instability. So far as is known, this is the first report of a collective radiation signal from fusion-born ions in a non-tokamak magnetically confined plasma. Disambiguation between two or more energetic ion species that could potentially generate complex observed ICE spectra is an increasing challenge, and the results and methodology developed here will assist this. Our approach is also expected to be relevant to ICE driven by ion beams with lower parallel velocities, for example in cylindrical plasma experiments.

Identification of core ion cyclotron instabilities on HL-2A tokamak

“…An approximately linear relationship between the concentration of fusion-born alpha particles, √ n α /n e , (where n α is the alpha particle density, n e is the electron density), and ICE growth rate was predicted by MCI by theories and simulations [54,56], which explained the results of JET D-T experimental result. Furthermore, experimental results in TFTR [49], AUG [22,23,25], START [27], LHD [37,39], and KSTAR [18] have been attributed to the MCI [54,56]. Recently, a new collective cyclotron resonant process between fast ions and thermalizing fusion-born alpha particles through MCI is predicted by simulation [55], which would affect the transport of helium ash in fusion plasmas.…”

“…Through comparing the detected frequency and f ci , the ICE was located around the outer edge midplane of the plasma and was believed to be excited by deeply trapped alpha particles reaching the plasma edge. Subsequently, a significant amount of research has been conducted on ICE in tokamaks, including JT-60U [12][13][14][15], KSTAR [16][17][18], TUMAN-3M [19,20], ASDEX-U [21][22][23][24][25], EAST [26], START [27], MAST [28], JET [29] NSTX/NSTX-U [30,31] and DIII-D [32][33][34][35], stellarators, including W7-AS [36] and LHD [37][38][39][40], and field reversed configuration C-2U device [41,42]. ICE is located around both the edge and the core of the plasma, and is destabilized by EP from D-T and D-D fusion reaction, neutral beam injection (NBI), and plasma heating in the ion cyclotron range of frequencies.…”

Interpretation of ion cyclotron emission from sub-Alfvénic beam-injected ions heated plasmas soon after L-H mode transition in EAST

“…Typical edge ICE observed in confined magnetic fusion devices, such as JET [4][5][6], JT60U2 [7,8], TFTR [9,10], ASDEX Upgrade [11], KSTAR [12,13], DIII-D [14,15], LHD [16], NSTX and EAST [17]. ICE from the plasma core has been reported in ASDEX Upgrade [18], DIII-D [14,19], JFT-2 M [20], JT60U2 [8], TUMAN-3 M [21], NSTX-U [22] and EAST [23] Tokamak.…”

Mechanism for Collective Energy Transfer from Neutral Beam-Injected Ions to Fusion-Born Alpha Particles on Cyclotron Timescales in a Plasma

“…ICE is widely observed from MCF plasmas. In addition to historical observations from the TFR [5] and JET [6,7] tokamaks, and from deuterium-tritium plasmas in JET [8,9] and TFTR [10], ICE has recently been reported and analyzed from the KSTAR [2,[11][12][13], JT-60U [14,15], DIII-D [16,17], ASDEX-Upgrade [18][19][20][21], TUMAN-3M [22,23], NSTX-U [24,25] and EAST [26] and JET [27] tokamaks, and from LHD [3,4,[28][29][30]. ICE is under consideration as a fast-ion diagnostic for ITER [31][32][33]; it is also observed from solar-terrestrial plasmas [34][35][36][37][38], and may be present downstream of supernova remnant shocks [39].…”

“…The thermal majority and energetic NBI ions are often of the same species. For example, the NBI ions are sub-Alfvénic deuterons at 80 to 100 keV in KSTAR deuterium plasmas [2], super-Alfvénic and sub-Alfvénic protons at 40 keV in LHD hydrogen plasmas [3], and sub-Alfvénic deuterons at 70 keV in LHD deuterium plasmas [4,30].…”