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Title: Continuous, edge localized ion heating during non-solenoidal plasma startup and sustainment in a low aspect ratio tokamak

Abstract

Plasmas in the Pegasus spherical tokamak are initiated and grown by the non-solenoidal local helicity injection (LHI) current drive technique. The LHI system consists of three adjacent electron current sources that inject multiple helical current filaments that can reconnect with each other. Anomalously high impurity ion temperatures are observed during LHI with T i,OV ≤ 650 eV, which is in contrast to T i,OV ≤ 70 eV from Ohmic heating alone. Spatial profiles of T i,OV indicate an edge localized heating source, with T i,OV ~ 650 eV near the outboard major radius of the injectors and dropping to ~150 eV near the plasma magnetic axis. Experiments without a background tokamak plasma indicate the ion heating results from magnetic reconnection between adjacent injected current filaments. In these experiments, the HeII T i perpendicular to the magnetic field is found to scale with the reconnecting field strength, local density, and guide field, while $${{T}_{\text{i},\parallel}}$$ experiences little change, in agreement with two-fluid reconnection theory. In conclusion, this ion heating is not expected to significantly impact the LHI plasma performance in Pegasus, as it does not contribute significantly to the electron heating. However, estimates of the power transfer to the bulk ion are quite large, and thus LHI current drive provides an auxiliary ion heating mechanism to the tokamak plasma.

Authors:
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]
  1. Univ. of Wisconsin, Madison, WI (United States). Dept. of Engineering Physics
Publication Date:
Research Org.:
Univ. of Wisconsin, Madison, WI (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
OSTI Identifier:
1361014
Grant/Contract Number:
FG02-96ER54375
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 57; Journal Issue: 7; Related Information: M.G. Burke, J.L. Barr, M.W. Bongard, R.J. Fonck, E.T. Hinson, J.M. Perry, J.A. Reusch, and D.J. Schlossberg, "Public Data Set: Continuous, Edge Localized Ion Heating During Non-Solenoidal Plasma Startup and Sustainment in a Low Aspect Ratio Tokamak," DOI: 10.18138/1336395; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; tokamaks; ion heating; magnetic reconnection; non-solenoidal tokamak plasma startup; helicity injection

Citation Formats

Burke, Marcus G., Barr, Jayson L., Bongard, Michael W., Fonck, Raymond J., Hinson, Edward T., Perry, Justin M., Reusch, Joshua A., and Schlossberg, David J. Continuous, edge localized ion heating during non-solenoidal plasma startup and sustainment in a low aspect ratio tokamak. United States: N. p., 2017. Web. doi:10.1088/1741-4326/aa6f2b.
Burke, Marcus G., Barr, Jayson L., Bongard, Michael W., Fonck, Raymond J., Hinson, Edward T., Perry, Justin M., Reusch, Joshua A., & Schlossberg, David J. Continuous, edge localized ion heating during non-solenoidal plasma startup and sustainment in a low aspect ratio tokamak. United States. doi:10.1088/1741-4326/aa6f2b.
Burke, Marcus G., Barr, Jayson L., Bongard, Michael W., Fonck, Raymond J., Hinson, Edward T., Perry, Justin M., Reusch, Joshua A., and Schlossberg, David J. Tue . "Continuous, edge localized ion heating during non-solenoidal plasma startup and sustainment in a low aspect ratio tokamak". United States. doi:10.1088/1741-4326/aa6f2b. https://www.osti.gov/servlets/purl/1361014.
@article{osti_1361014,
title = {Continuous, edge localized ion heating during non-solenoidal plasma startup and sustainment in a low aspect ratio tokamak},
author = {Burke, Marcus G. and Barr, Jayson L. and Bongard, Michael W. and Fonck, Raymond J. and Hinson, Edward T. and Perry, Justin M. and Reusch, Joshua A. and Schlossberg, David J.},
abstractNote = {Plasmas in the Pegasus spherical tokamak are initiated and grown by the non-solenoidal local helicity injection (LHI) current drive technique. The LHI system consists of three adjacent electron current sources that inject multiple helical current filaments that can reconnect with each other. Anomalously high impurity ion temperatures are observed during LHI with Ti,OV ≤ 650 eV, which is in contrast to Ti,OV ≤ 70 eV from Ohmic heating alone. Spatial profiles of Ti,OV indicate an edge localized heating source, with Ti,OV ~ 650 eV near the outboard major radius of the injectors and dropping to ~150 eV near the plasma magnetic axis. Experiments without a background tokamak plasma indicate the ion heating results from magnetic reconnection between adjacent injected current filaments. In these experiments, the HeII T i perpendicular to the magnetic field is found to scale with the reconnecting field strength, local density, and guide field, while ${{T}_{\text{i},\parallel}}$ experiences little change, in agreement with two-fluid reconnection theory. In conclusion, this ion heating is not expected to significantly impact the LHI plasma performance in Pegasus, as it does not contribute significantly to the electron heating. However, estimates of the power transfer to the bulk ion are quite large, and thus LHI current drive provides an auxiliary ion heating mechanism to the tokamak plasma.},
doi = {10.1088/1741-4326/aa6f2b},
journal = {Nuclear Fusion},
number = 7,
volume = 57,
place = {United States},
year = {Tue May 16 00:00:00 EDT 2017},
month = {Tue May 16 00:00:00 EDT 2017}
}

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  • This public data set contains openly-documented, machine readable digital research data corresponding to figures published in M.G. Burke et. al., 'Continuous, Edge Localized Ion Heating During Non-Solenoidal Plasma Startup and Sustainment in a Low Aspect Ratio Tokamak,' Nucl. Fusion 57, 076010 (2017).
  • Radio Frequency auxiliary heating for Low Aspect Ratio Tokamaks can facilitate startup and access to high energy density plasmas, reduce the demand on ohmic core volt-seconds, and provide off axis electron heating for a high {beta}{sub e} plasma. Calculations for several waves both below and at high harmonics of the ion cyclotron frequency predict significant RF heating even at the lower electron temperatures and densities typical of startup plasmas. Real time adjustments of the antenna array phasing can track plasma start up evolution. {copyright} {ital 1997 American Institute of Physics.}
  • A low aspect ratio tokamak plasma was generated and sustained by injecting a series of plasmas from a magnetized coaxial gun into a flux conserver with toroidal field. The magnetized coaxial gun was supplied by an oscillating current with a d.c. component. The first few current pulses injected plasma and helicity into the flux conserver. This pulse helicity injection method worked effectively to maintain the low aspect ratio tokamak. 8 refs., 5 figs.
  • Low-aspect-ratio tokamaks with toroidal currents, {ital I}{sub {ital p}}, up to 250 kA are formed and sustained in the Helicity Injected Tokamak experiment [Nelson {ital et} {ital al}., Phys. Rev. Lett. {bold 72}, 3666 (1994)] using coaxial helicity injection. These plasmas are produced without use of a current drive transformer. Average toroidal currents are sustained at high values, {l_angle}{ital I}{sub {ital p}}{r_angle}=225 kA for 2 ms, where electron thermal energies are measured up to 80 eV with spectroscopy data suggesting burnthrough to the higher ionization states of oxygen. Currents can also be sustained for longer periods at lower values, {l_angle}{italmore » I}{sub {ital p}}{r_angle}=138 kA for 7 ms. These tokamaks are characterized by a rotating, {ital n}=1 distortion, with poloidal distortions approximately following the field line pitch, which only occur on the outer bad-curvature region. Equilibrium reconstructions show these plasmas have a tokamak {ital q} profile ({ital q}{sub 0}=5 -- 8, {ital q}{sub 95}=10 -- 12, {ital q}{sub cyl}{congruent}3.6), with a hollow toroidal current profile and up to 170 kA of closed field toroidal current in a low-aspect-ratio, {ital A}=1.68 configuration.« less
  • Coaxial helicity injection is used to form and sustain low aspect ratio tokamaks at currents of up to 250 kA in the Helicity Injected Tokamak experiment. Plasma currents can be sustained at an average of 225 kA for 2 ms, with on axis electron thermal energies up to 80 eV, or for longer times, 140 kA average for 7 ms, many resistive diffusion times. Spectroscopic measurements of the higher current discharges suggest burn-through of oxygen impurities. These plasmas have a rotating n = 1 distortion, appearing only on the outer, bad-curvature region. Equilibria reconstruction , fitting to experimental data, showsmore » tokamak q profiles achieved with hollow plasma current profiles. 11 refs., 4 figs., 1 tab.« less