The magnetoacoustic cyclotron instability of an extended shell distribution of energetic ions

Yun

et al. 2017

Sci Rep

Solitary perturbations (SPs) localized both poloidally and radially are detected within ~100 μs before the partial collapse of the high pressure gradient boundary region (called pedestal) of magnetized toroidal plasma in the KSTAR tokamak device. The SP develops with a low toroidal mode number (typically unity) in the pedestal ingrained with quasi-stable edge-localized mode (QSM) which commonly appears during the inter-collapse period. The SPs have smaller mode pitch and different (often opposite) rotation velocity compared to the QSMs. Similar solitary perturbations are also frequently observed before the onset of complete pedestal collapse, suggesting a strong connection between the SP generation and the pedestal collapse.

Section: Resultsmentioning

confidence: 99%

Solitary perturbations in the steep boundary of magnetized toroidal plasma

Yun

et al. 2017

Sci Rep

“…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.…”

Section: Discussionmentioning

confidence: 99%

“…In addition, the velocity distribution of the energetic ion population must be sufficiently non-Maxwellian that sufficient free energy is available to be unlocked via resonant processes. The first process, identified by Belikov and Kolesnichenko in 1976 [16][17][18], involves wave-wave resonance between additional cyclotron harmonic modes supported by the energetic ion population, and the fast Alfvén wave. The second process [19,20] involves wave-particle cyclotron resonance between gyrating energetic ions and the fast Alfvén wave.…”

Section: Introductionmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

et al. 2018

The radio frequency detection system on the KSTAR tokamak has exceptionally high spectral and temporal resolution. This enables measurement of previously undetected fast plasma phenomena in the ion cyclotron range of frequencies. Here we report and analyse a novel spectrally structured ion cyclotron emission (ICE) feature in the range 500 MHz to 900 MHz, which exhibits chirping on sub-microsecond timescales. Its spectral peaks correspond to harmonics l of the proton cyclotron frequency f cp at the outer midplane edge, where l = 20-36. This frequency range exceeds estimates of the local lower hybrid frequency f LH in the KSTAR deuterium plasma. The new feature is time-shifted with respect to a brighter lower-frequency chirping ICE feature in the range 200 MHz (8f cp ) to 500 MHz (20f cp ), which is probably driven (Chapman et al 2017 Nucl. Fusion 57 124004) by 3 MeV fusion-born protons undergoing collective relaxation by the magnetoacoustic cyclotron instability (MCI). Here we show that the new, fainter, higher-frequency chirping ICE feature is driven by nonlinear wave coupling between different neighbouring spectral peaks in the lower-frequency ICE feature. This follows from bispectral analysis of the measured KSTAR fields, and of the field amplitudes output from particle-in-cell (PIC) simulations of the KSTAR edge plasma containing fusionborn protons. This reinforces the identification of the MCI as the plasma physics process underlying proton harmonic ICE from KSTAR, while providing a novel instance of nonlinear wave coupling on very fast timescales.

Plasma Phys. Control. Fusion

“…It is found that the MCI unfolds so fast that this PIC simulation duration is sufficient to take one through the linear phase of the instability, but not far into the nonlinear phase of the MCI, which is more fully explored in the hybrid simulations. The fluid treatment of the electrons in the hybrid model also maps to the analytical treatment [5][6][7][8][9][10][11][12]20] of the MCI, which derives from multi-species dielectric tensor elements whose electron component is calculated in the cold plasma approximation with the addition of a Landau resonance term.…”

Section: Modelling Ion Cyclotron Emissionmentioning

confidence: 99%

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

Dendy¹,

McClements²

2015

Ion cyclotron emission (ICE) was the first collective radiative instability, driven by confined fusion-born ions, observed from deuterium-tritium plasmas in JET and TFTR. ICE comprises strongly suprathermal emission, which has spectral peaks at multiple ion cyclotron harmonic frequencies as evaluated at the outer mid-plane edge of tokamak plasmas. The measured intensity of ICE spectral peaks scaled linearly with measured fusion reactivity in JET. In other large tokamak plasmas, ICE is currently used as an indicator of fast ions physics. The excitation mechanism for ICE is the magnetoacoustic cyclotron instability (MCI); in the case of JET and TFTR, the MCI is driven by a set of centrally born fusion products, lying just inside the trapped-passing boundary in velocity space, whose drift orbits make large radial excursions to the outer mid-plane edge. Diagnostic exploitation of ICE in future experiments therefore rests in part on deep understanding of the MCI, and recent advances in computational plasma physics have led to substantial recent progress, reviewed here. Particle-in-cell simulations of the MCI, with fully kinetic ions and electrons, were reported in 2013, using plasma parameters for JET ICE observations. The hybrid approximation for plasma simulations, where ions are treated as particles and electrons as a neutralising massless fluid, was then applied and reported in 2014. These simulations extend previous studies deep into the nonlinear regime of the MCI, and corroborate predictions by linear analytical theory, thereby strengthening further the link to ICE measurements. ICE is a potential diagnostic for confined alpha-particles in ITER, where measurements of ICE could yield information on energetic ion behaviour supplementing that obtainable from other diagnostics. In addition, it may be possible to use ICE to study fast ion redistribution and loss due to MHD activity in ITER.