OBSERVATION OF SUPRATHERMAL ELECTRONS DURING MAGNETIC RECONNECTION AT THE SAWTOOTH INSTABILITY IN DIII-D TOKAMAK. PDF Download
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Languages : en
Pages : 11
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OAK A271 OBSERVATION OF SUPRATHERMAL ELECTRONS DURING MAGNETIC RECONNECTION AT THE SAWTOOTH INSTABILITY IN DIII-D TOKAMAK. Intense bursts of x-ray and electron cyclotron emission are observed during sawtooth instabilities in high-temperature plasmas in the DIII-D tokamak. The bursts are initiated around the X-point of the m = 1, n = 1 magnetic island at the beginning of the sawtooth crash and are displaced to larger radii later during the temperature collapse. Reconstruction of the magnetic configuration using motional Stark effect (MSE) data and numerical simulations indicates that the bursts can be connected with suprathermal electrons (E{sub r} (almost equal to) 30-40 keV) generated during reconnection of the magnetic field around the q = 1 surface.
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Category :
Languages : en
Pages : 11
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Book Description
OAK A271 OBSERVATION OF SUPRATHERMAL ELECTRONS DURING MAGNETIC RECONNECTION AT THE SAWTOOTH INSTABILITY IN DIII-D TOKAMAK. Intense bursts of x-ray and electron cyclotron emission are observed during sawtooth instabilities in high-temperature plasmas in the DIII-D tokamak. The bursts are initiated around the X-point of the m = 1, n = 1 magnetic island at the beginning of the sawtooth crash and are displaced to larger radii later during the temperature collapse. Reconstruction of the magnetic configuration using motional Stark effect (MSE) data and numerical simulations indicates that the bursts can be connected with suprathermal electrons (E{sub r} (almost equal to) 30-40 keV) generated during reconnection of the magnetic field around the q = 1 surface.
Author: R. V. Budny
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ISBN:
Category :
Languages : en
Pages : 21
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Author: F. M. Levinton
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ISBN:
Category : Magnetohydrodynamic instabilities
Languages : en
Pages : 15
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Author: John Anthony Goetz
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Category :
Languages : en
Pages : 278
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Author: R. V. Budny
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ISBN:
Category :
Languages : en
Pages : 21
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Author: F. M. Levinton
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ISBN:
Category : Magnetohydrodynamic instabilities
Languages : en
Pages : 15
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Author: Christopher Michael Muscatello
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ISBN: 9781267260550
Category :
Languages : en
Pages : 150
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Superthermal ions in tokamak plasmas play a critical role in heating and current drive, and their confinement within the core of the plasma is crucial for obtaining ignition and sustaining burn in future reactors. At the DIII-D tokamak, a suite of fast-ion measurements is available to diagnose various properties of the superthermal population. This thesis work involves a contribution to DIII-D's fast-ion diagnostic collection: the 2nd generation fast-ion deuterium alpha (2G FIDA) detector. FIDA works on the principle of measuring the light that is emitted from neutralized fast ions that undergo charge exchange events with injected neutral atoms. 2G FIDA complements the other FIDA installations on DIII-D with its unique velocity-space sampling volume. Output from a synthetic diagnostic code (FIDAsim) that predicts FIDA emission levels is compared with measurements from 2G FIDA. We find that, while the predicted and measured shapes of the FIDA spectra agree well, the absolute magnitude of the spectral amplitudes are inconsistent. Results from various FIDAsim trials are presented adjusting several parameters, and it is hypothesized that mischaracterization of the diagnostic neutral beams is a major source of error. Instabilities in tokamaks can cause fast-ion transport. The sawtooth instability is particularly important because the crash phase has been observed to cause reductions up to 50% in the central fast-ion density. Passing ions of all energies are redistributed, but only low energy trapped ions suffer redistribution. The observations are consistent with transport by flux-attachment. Comparisons with theory suggest that the intensity of sawtooth-induced transport depends on the magnitude of toroidal drift. Instabilities characterized by toroidal and poloidal mode numbers and real frequency can coherently interact with energetic particles through mode-particle resonances. During a sawtooth crash, even fast ions whose energies are above the threshold for flux-attachment can experience transport if their orbits satisfy the bounce-precessional resonance condition. On DIII-D, a spatially localized population of beam ions accelerated above the injection energy by ion-cyclotron radio frequency (ICRF) heating is diminished at a sawtooth crash. Furthermore, fast-ion losses concurrent with sawtooth crashes are observed. Calculations show that mode-particle resonances could be responsible. Transport of energetic particles by resonant interactions pertains to many types of instabilities; other examples besides sawteeth will also be presented. Analysis shows that large amplitude modes cause significant resonant transport of fast particles. Even small amplitude modes can resonantly drive transport if multiple harmonics exist.
Author: Alexander Nevil Tronchin-James
Publisher:
ISBN: 9781124679686
Category :
Languages : en
Pages : 165
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Energy production is a continuing problem in the modern world, and nuclear fusion in tokamak reactors may be a viable solution. One remaining problem for tokamak is the generation of runaway electrons (RE) during shutdown of these reactors, the focus of this thesis. Energy and runaway electrons are both briefly reviewed, with emphasis on prior theses, prior theoretical developments, and prior experimental studies which establish context in the pre-existing body of knowledge. New experimental techniques tailored for studying RE are described. These techniques include plasma shaping optimized for RE generation via argon killer-pellet shutdown, which increased the probability of RE plateau in a shutdown from 30% to over 80%. A newly developed hard x-ray sensing scintillator array is described in detail, and this new diagnostic is used along with pre-existing diagnostics to explore the temporal and spatial character of hard x-ray emission resulting from RE. X-ray emission associated with RE impact at divertor strike points was observed after thermal quench (TQ) but before current quench (CQ). Instabilities of the RE current were observed during the plateau and at termination. Experiments probing RE in-situ by injecting polystyrene diagnostic pellets are also discussed. Pellets were observed disintegrating before reaching the last closed flux surface (LCFS), suggesting that substantial RE transport beyond the LCFS occurs, which is consistent with observed activation of the low field side midplane limiters. Inference of loop voltages during the pre-current quench (CQ) phase using inverse techniques and a discussion of limitations of this technique are also presented. Loop voltages exceeding 1kV are inferred peaking well before the beginning of CQ, and are capable of accelerating RE to energies of over 10MeV at the time of the first x-ray emission from RE impact with the wall. During the later CQ phase, this inferred voltage matches a simpler estimate for the loop voltage -LdI/dt.
Author:
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ISBN:
Category :
Languages : en
Pages : 141
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The sawtooth crash is a hazardous, disruptive phenomenon that is observed in tokamaks whenever the safety factor at the magnetic axis is below unity. Recently, Tokamak Test Fusion Reactor (TFTR) experimental data has revealed interesting features of the dynamical pressure evolution during the crash phase. Motivated by the experimental results, this dissertation focuses on theoretical modeling of the finite pressure effects on the nonlinear stage of the sawtooth crash. The crash phase has been studied numerically employed a toroidal magnetohydrodynamic (MHD) initial value code deduced from the FAR code. For the first time, by starting from a concentric equilibrium, it has been shown that the evolution through an m/n = 1/1 magnetic island induces secondary high-n ballooning instabilities. The magnetic island evolution gives rise to convection of the pressure inside the inversion radius and builds up a steep pressure gradient across the island separatrix, or current sheet, and thereby triggers ballooning instabilities below the threshold for the axisymmetric equilibrium. Due to the onset of secondary ballooning modes, concomitant fine scale vortices and magnetic stochasticity are generated. These effects produce strong flows across the current sheet, and thereby significant modify the m = 1 driven magnetic reconnection process. The resultant interaction of the high-n ballooning modes with the magnetic reconnection process is discussed.
Author: A. Takahashi
Publisher:
ISBN:
Category : Magnetohydrodynamic instabilities
Languages : en
Pages : 29
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