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Fonck, R. J.

Publications and source records attributed to Fonck, R. J..

5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Spatial heterodyne spectroscopy for fast local magnetic field measurements of magnetized fusion plasmas

A novel spectroscopy diagnostic for measuring internal magnetic fields in high temperature magnetized plasmas has been developed. It involves spectrally resolving the Balmer- α (656 nm) neutral beam radiation split by the motional Stark effect with a spatial heterodyne spectrometer (SHS). The unique combination of high optical throughput (3.7 mm 2 sr) and spectral resolution (δλ ~ 0.1 nm) allows these measurements to be made with time resolution $\ll$1 ms. The high throughput is effectively utilized by incorporating a novel geometric Doppler broadening compensation technique in the spectrometer. The technique significantly reduces the spectral resolution penalty inherent to using large area, high-throughput optics while still collecting the large photon flux provided by such optics. In this work, fluxes of order 10 10 s –1 support the measurement of deviations of <5 mT (Δ λ Stark ~ 10 –4 nm) in the local magnetic field with 50 µs time resolution. Example high time resolution measurements of the pedestal magnetic field throughout the ELM cycle of a DIII-D tokamak plasma are presented. Local magnetic field measurements give access to the dynamics of the edge current density, which is essential to understanding stability limits, edge localized mode generation and suppression, and predicting performance of H-mode tokamaks.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Local measurements of the pedestal magnetic field profile throughout the ELM cycle on DIII-D

We report new high speed localized measurements of the pedestal magnetic field during the edge localized mode (ELM) cycle of a DIII-D High confinement mode (H-mode) discharge indicate a temporally and spatial complex redistribution of the edge current density profile, $j_{edge}$. The measurement technique extracts the magnetic field magnitude, $\textit{B}$, via the spectral separation of Stark-split neutral beam radiation in the pedestal. Single spatial channel measurements from a novel spatial heterodyne spectrometer are validated in discharges with core current profile changes. The technique measures Stark-splitting changes that imply $\textit{B}$ changes as small as 1 mT with high time resolution (50 μs). At normalized poloidal flux $ψ_n$ = 1.0, $\textit{B}$ appears saturated in the inter-ELM period and then rapidly decreases in <200 μs by ~1%, before edge recycling emission begins to increase. Radially inboard of $j_{edge}$, $\textit{B}$ increases at the ELM crash. The behavior is consistent with a rapid collapse of $j_{edge}$ at the ELM crash and subsequent pedestal recovery. In some discharges, at $ψ_n$ < 0.96, changes in $\textit{B}$ are observed throughout the ELM cycle. In others, $\textit{B}$ recovers and is relatively stable until a few ms leading up to the next crash. Measurements of $\textit{B}$ during the H-mode transition show a large increase at $ψ_n$ = 1 with little change at $ψ_n$ = 0.9, consistent with the formation of the edge bootstrap current density peak. The $ψ_n$ = 0.9 spectrum is complicated by predicted changes to the Stark component intensities with density at the L–H transition.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Effect of magnetic perturbations on turbulence-flow dynamics at the L-H transition on DIII-D

Detailed 2D turbulence measurements from the DIII-D tokamak provide an explanation for how resonant magnetic perturbations (RMPs) raise the L-H power threshold P LH [P. Gohil et al. , Nucl. Fusion 51 , 103020 (2011)] in ITER-relevant, low rotation, ITER-similar-shape plasmas with favorable ion ∇ B direction. RMPs simultaneously raise the turbulence decorrelation rate Δ ω D and reduce the flow shear rate ω shear in the stationary L-mode state preceding the L-H transition, thereby disrupting the turbulence shear suppression mechanism. RMPs also reduce the Reynolds stress drive for poloidal flow, contributing to the reduction of ω shear . On the ~100 μ s timescale of the L-H transition, RMPs reduce Reynolds-stress-driven energy transfer from turbulence to flows by an order of magnitude, challenging the energy depletion theory for the L-H trigger mechanism. In contrast, non-resonant magnetic perturbations, which do not significantly affect P LH , do not affect Δ ω D and only slightly reduce ω shear and Reynolds-stress-driven energy transfer.<

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗