Quantifying Fusion Born Ion Populations in Magnetically Confined Plasmas using Ion Cyclotron Emission

Abstract

Ion cyclotron emission (ICE) offers a unique promise as a diagnostic of the fusion born alpha-particle population in magnetically confined plasmas. Pioneering observations from JET and TFTR found that ICE intensity P_{ICE} scales approximately linearly with the measured neutron flux from fusion reactions, and with the inferred concentration, n_{α}/n_{i}, of fusion born alpha particles confined within the plasma. We present fully nonlinear self-consistent kinetic simulations that reproduce this scaling for the first time. This resolves a long-standing question in the physics of fusion alpha-particle confinement and stability in magnetic confinement fusion plasmas. It confirms the magnetoacoustic cyclotron instability as the likely emission mechanism and greatly strengthens the basis for diagnostic exploitation of ICE in future burning plasmas.

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

“…First, the values of key parameters, notably the ratio of the characteristic perpend icular velocity of the energetic ions to the local Alfvén speed. This needs to be of order unity [16][17][18][19][20][21][22][23][24][25][26][27][28]. Second, the structure of the distribution of the energetic ions in velocity space, which needs to be strongly non-Maxwellian in order to excite the MCI which underlies ICE.…”

“…The value of V A differs significantly between the simulations because it is inversely proportional to the square root of the majority ion (deuteron) mass density, and hence to n e . The horizontal black line denotes f LH , below which we see excitation of the fast Alfvén wave with resonances at consecutive proton cyclotron harmonics, characteristic of the MCI [2,14,[23][24][25] which underpins ICE. Above f LH there are several weaker but significant spectrally intense regions.…”

“…Full gyro-orbit kinetics are essential for capturing cyclotron harmonic resonance effects including, as we shall see, coupling between modes driven at different harmonics by collective instability. The simulations detailed in [2,14,[23][24][25] are set up in slab geometry, hence they do not incorporate realistic toroidal geometry and the associated compressional Alfvén eigenmode structure [26][27][28][29][30]. Nevertheless this spatially localised physics approach is successful in capturing most of the observed features of ICE, including also recent results from the heliotron-stellarator LHD [31,32].…”

“…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.…”

Distinct stages of radio frequency emission at the onset of pedestal collapse in KSTAR H-mode plasmas

“…rapid 'tongue'-shaped deformation of flux surfaces reported in the Large Helical Device (LHD) [16]) or fast ions produced by charge exchange with NBI particles [17]. Furthermore, recent 1D3V particle-in-cell (PIC) simulation showed that the high concentrations of energetic particles can intensify the harmonic ICE [18], suggesting that the intensification in high-harmonic ICE described in Figure 3(c) and Figure 3(d) can be interpreted as the increase of fast ions at the outer edge region near the pedestal collapse. In addition, a similar 1D3V PIC simulation with time-varying local electron density reproduced the rapid stair-casing chirping of proton-ICEs observed during the pedestal collapse phase on the KSTAR [12].…”