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Title: The effect of pre-existing islands on disruption mitigation in MHD simulations of DIII-D

Abstract

Locked-modes are the most likely cause of disruptions in ITER, so large islands are expected to be common when the ITER disruption mitigation system is deployed. MHD modeling of disruption mitigation by massive gas injection is carried out for DIII-D plasmas with stationary, pre-existing islands. Results show that the magnetic topology at the q=2 surface can affect the parallel spreading of injected impurities, and that, in particular, the break-up of large 2/1 islands into smaller 4/2 islands chains can favorably affect mitigation metrics. The direct imposition of a 4/2 mode is found to have similar results to the case in which the 4/2 harmonic grows spontaneously.

Authors:
ORCiD logo [1]
  1. Univ. of California, San Diego, CA (United States). Center for Energy Research
Publication Date:
Research Org.:
General Atomics, San Diego, CA (United States)
Sponsoring Org.:
USDOE Office of Nuclear Energy (NE); USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
OSTI Identifier:
1374817
Alternate Identifier(s):
OSTI ID: 1348025
Grant/Contract Number:
FC02-04ER54698; FG02-95ER54309; FC02-01ER25455; AC02-05CH11231
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 24; Journal Issue: 5; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Large islands; Magnetic islands; Toroidal plasma confinement; Plasma impurities; Fluid jets

Citation Formats

Izzo, V. A.. The effect of pre-existing islands on disruption mitigation in MHD simulations of DIII-D. United States: N. p., 2017. Web. doi:10.1063/1.4977462.
Izzo, V. A.. The effect of pre-existing islands on disruption mitigation in MHD simulations of DIII-D. United States. doi:10.1063/1.4977462.
Izzo, V. A.. Mon . "The effect of pre-existing islands on disruption mitigation in MHD simulations of DIII-D". United States. doi:10.1063/1.4977462. https://www.osti.gov/servlets/purl/1374817.
@article{osti_1374817,
title = {The effect of pre-existing islands on disruption mitigation in MHD simulations of DIII-D},
author = {Izzo, V. A.},
abstractNote = {Locked-modes are the most likely cause of disruptions in ITER, so large islands are expected to be common when the ITER disruption mitigation system is deployed. MHD modeling of disruption mitigation by massive gas injection is carried out for DIII-D plasmas with stationary, pre-existing islands. Results show that the magnetic topology at the q=2 surface can affect the parallel spreading of injected impurities, and that, in particular, the break-up of large 2/1 islands into smaller 4/2 islands chains can favorably affect mitigation metrics. The direct imposition of a 4/2 mode is found to have similar results to the case in which the 4/2 harmonic grows spontaneously.},
doi = {10.1063/1.4977462},
journal = {Physics of Plasmas},
number = 5,
volume = 24,
place = {United States},
year = {Mon Feb 27 00:00:00 EST 2017},
month = {Mon Feb 27 00:00:00 EST 2017}
}

Journal Article:
Free Publicly Available Full Text
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Cited by: 2works
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  • Data on the discharge behavior, thermal loads, halo currents, and runaway electrons have been obtained in disruptions on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. {bold 8}, 2A 441 (1985)]. These experiments have also evaluated techniques to mitigate the disruptions while minimizing runaway electron production. Experiments injecting cryogenic impurity {open_quotes}killer{close_quotes} pellets of neon and argon and massive amounts of helium gas have successfully reduced these disruption effects. The halo current generation, scaling, and mitigation are understood and are in good agreement with predictions of a semianalytic model. Results from {open_quotes}killer{close_quotes} pellet injection have been usedmore » to benchmark theoretical models of the pellet ablation and energy loss. Runaway electrons are often generated by the pellets and new runaway generation mechanisms, modifications of the standard Dreicer process, have been found to explain the runaways. Experiments with the massive helium gas puff have also effectively mitigated disruptions without the formation of runaway electrons that can occur with {open_quotes}killer{close_quotes} pellets. {copyright} {ital 1999 American Institute of Physics.}« less
  • Injection of massive quantities of gas is a promising technique for fast shutdown of ITER for the purpose of avoiding divertor and first wall damage from disruptions. Previous experiments using massive gas injection (MGI) to terminate discharges in the DIII-D tokamak have demonstrated rapid shutdown with reduced wall heating and halo currents (relative to natural disruptions) and with very small runaway electron (RE) generation [1]. Figure 1 shows time traces which give an overview of shutdown time scales. Typically, of order 5 x 10{sup 22} Ar neutrals are fired over a pulse of 25 ms duration into stationary (non-disrupting) discharges.more » The observed results are consistent with the following scenario: within several ms of the jet trigger, sufficient Ar neutrals are delivered to the plasma to cause the edge temperature to collapse, initiating the inward propagation of a cold front. The exit flow of the jet [Fig. 1(a)] has a {approx} 9 ms rise time; so the quantity of neutrals which initiates the edge collapse is small (<10{sup 20}). When the cold front reaches q {approx} 2 surface, global magnetohydrodynamic (MHD) modes are destabilized [2], mixing hot core plasma with edge impurities. Here, q is the safety factor. Most (>90%) of the plasma thermal energy is lost via impurity radiation during this thermal quench (TQ) phase. Conducted heat loads to the wall are low because of the cold edge temperature. After the TQ, the plasma is very cold (of order several eV), so conducted wall (halo) currents are low, even if the current channel contacts the wall. The plasma current profile broadens and begins decaying resistively. The decaying current generates a toroidal electric field which can accelerate REs; however, RE beam formation appears to be limited in MGI shutdowns. Presently, it is thought that the conducted heat flux and halo current mitigation qualities of the MGI shutdown technique will scale well to a reactor-sized tokamak. However, because of the larger RE gain from avalanching and the presence of a RE seed population due to Compton-scattered fast electrons, it is possible that a RE beam can be formed well into the CQ, after the flux surfaces initially destroyed by the TQ MHD have had time to heal. Crucial MGI issues to be studied in present devices are therefore the formation, amplification, and transport of RE and the transport of impurities into the core plasma (important because the presence of impurities can, via collisional drag, help suppress RE amplification). In the study of impurity transport, both neutral delivery (directly driven into the core by the jet pressure) and ion delivery (mixed into the core by MHD) are of interest, as both contribute to RE drag.« less
  • The contributions of the DIII-D tokamak toward the understanding and control of disruptions are reviewed. Disruptions are found to be deterministic, and the underlying causes of disruption can therefore be predicted and avoided. With sufficiently rapid detection, possible damage from disruptions can be mitigated using an understanding of disruption phenomenology and plasma physics. Regimes of high {beta} are readily available in DIII-D and provide access to relatively high energy density disruptions, despite DIII-D's moderate magnetic field and size. DIII-D, with all-graphite wall armor and wall conditioning between discharges, has proven highly resilient to the deleterious effects that disruptions can havemore » on plasma operations. Simultaneously, exploitation and adaptation of DIII-D's extensive core and edge plasma diagnostic set have allowed for unique plasma measurements during disruptions. These measurements have tied into the development of several physical models used to understand aspects of disruptions, such as magnetohydrodynamic growth at the disruption onset, radiation energy balance through the thermal quench, and halo currents during the current quench. Based on this fundamental understanding, DIII-D has developed techniques to mitigate the harmful effects of disruptions by radiative dissipation of the plasma energy and extrapolated these techniques for possible use on larger devices like ITER.« less