Energetic-particle-driven instabilities and induced fast-ion transport in a reversed field pinch

Abstract

Multiple bursty energetic-particle (EP) driven modes with fishbone-like structure are observed during 1 MW tangential neutral-beam injection in a reversed field pinch (RFP) device. The distinguishing features of the RFP, including large magnetic shear (tending to add stability) and weak toroidal magnetic field (leading to stronger drive), provide a complementary environment to tokamak and stellarator configurations for exploring basic understanding of EP instabilities. Detailed measurements of the EP mode characteristics and temporal-spatial dynamics reveal their influence on fast ion transport. Density fluctuations exhibit a dynamically evolving, inboardoutboard asymmetric spatial structure that peaks in the core where fast ions reside. The measured mode frequencies are close to the computed shear Alfv en frequency, a feature consistent with continuum modes destabilized by strong drive. The frequency pattern of the dominant mode depends on the fast-ion species. Multiple frequencies occur with deuterium fast ions compared to single frequency for hydrogen fast ions. Furthermore, as the safety factor (q) decreases, the toroidal mode number of the dominant EP mode transits from n ¼ 5 to n ¼ 6 while retaining the same poloidal mode number m ¼ 1. The transition occurs when the m ¼ 1, n ¼ 5 wave-particle resonance condition cannot be satisfied as the fast-ion safety factor (q fi ) decreases. The fast-ion temporal dynamics, measured by a neutral particle analyzer, resemble a classical predator-prey relaxation oscillation. It contains a slow-growth phase arising from the beam fueling followed by a rapid drop when the EP modes peak, indicating that the fluctuation-induced transport maintains a stiff fast-ion density profile. The inferred transport rate is strongly enhanced with the onset of multiple EP modes. V C 2014 AIP Publishing LLC. [http://dx.

Fast ion transport in the quasi-single helical reversed-field pinch

“…26 Indeed, the bursting nature of the destabilized modes follows predator-prey relaxation oscillations characteristic of localized fast ion transport. 22 The absence of bursts at a high core-resonant mode amplitude shown in Fig. 1(a) suggests a lack of drive from the fast ion distribution, hinting at fast ion losses.…”

“…An 11-chord Far infrared (FIR) interferometry system on MST provides spatial measurements of the Alfv en instabilities through correlations between electron density perturbations associated with the wave induced magnetic fluctuations. 21,22 The spectral coherence between the density and magnetic fluctuations is given by where F represents the associated Fourier transform and h i denotes an ensemble average over the Alfv enic bursts. Figure 2 presents the spectral coherence for hydrogen and deuterium injected fast ions in three topologies: the MH state, a transitional state, and the QSH state.…”

Upgrading a high-throughput spectrometer for high-frequency (<400 kHz) measurements

“…The Ion-Doppler Spectrometer II (IDSII) used for Charge Exchange Recombination Spectroscopy (CHERS) 1 was originally designed to measure ion dynamics associated with tearing modes with frequencies up to 50 kHz. We could also address the physics of plasma fluctuations at higher frequencies, e.g., micro-instabilities 2,3 and Alfvénic fluctuations associated with energetic particle modes, 4 by increasing the bandwidth of the spectrometer electronics to 400 kHz. We developed a model that generates synthetic digitized signals to facilitate the uncertainty estimation in the spectroscopic measurements and the bandwidth optimization.…”

An upgraded interferometer-polarimeter system for broadband fluctuation measurements

“…1,2 Neutral beam heated plasmas and improved confinement plasmas provide a range of phenomena (drift waves, energetic particle modes, Alfvénic modes) at higher frequencies which have been diagnosed primarily by density fluctuation measurements. 3 While the strongest of the observed fast-ion instabilities have been diagnosed through polarimetry, 4 many of these high-frequency instabilities have been too weak to resolve magnetic fluctuations. Additionally, broadband magnetic turbulence may play a role in stochastic transport of particles and energy.…”