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Title: Perturbative momentum transport in MAST L-mode plasmas

Abstract

Non-axisymmetric magnetic fields are used to perturbatively probe momentum transport physics in MAST L-mode plasmas. The low beta L-mode target was chosen to complement previous experiments conducted in high beta NSTX H-mode plasmas (beta N = 3.5-4.6) where an inward momentum pinch was measured. In those cases quasi-linear gyrokinetic simulations of unstable ballooning micro-instabilities predict weak or outward momentum convection, in contrast to the measurements. The weak pinch was predicted to be due to both electromagnetic effects at high beta and low aspect ratio minimizing the symmetry-breaking of the instabilities responsible for momentum transport. In an attempt to lessen these electromagnetic effects at low aspect ratio, perturbative experiments were run in MAST L-mode discharges at lower beta (beta N = 2). The perturbative transport analysis used the time-dependent response following the termination of applied 3D fields that briefly brake the plasma rotation ( similar to the NSTX H-mode experiments). Assuming time-invariant diffusive (chi(phi))and convective (V-phi) transport coefficients, an inward pinch is inferred with magnitudes, (RV phi/chi(phi)) = (-1)-(-9), similar to those found in NSTX H-modes and in conventional tokamaks. However, if experimental uncertainties due to non-stationary conditions during and after the applied 3D field are considered, a weak pinch ormore » even outward convection is inferred, ( RV phi/chi(phi)) = (-1)-(+5). Linear gyrokinetic simulations indicate that for these lower beta L-modes, the predicted momentum pinch is predicted to be relatively small, ( RV phi/chi(phi))(sim) approximate to -1. While this falls within the experimentally inferred range, the uncertainties are practically too large to quantitatively validate the predictions. Challenges and implications for this particular experimental technique are discussed, as well as additional possible physical mechanisms that may be important in understanding momentum transport in these low aspect ratio plasmas.« less

Authors:
ORCiD logo [1];  [2];  [2];  [3];  [1]; ORCiD logo [1];  [4]
  1. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  2. Culham Science Centre, Abingdon (United Kingdom). Culham Centre for Fusion Energy (CCFE), EURATOM/UKAEA Fusion Association
  3. VTT, Espoo (Finland)
  4. General Atomics, San Diego, CA (United States)
Publication Date:
Research Org.:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES)
OSTI Identifier:
1353400
Report Number(s):
PPPL-5359
Journal ID: ISSN 0029-5515
Grant/Contract Number:  
AC02-09CH11466
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 57; Journal Issue: 5; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Momentum transport; Gyrokinetics; Spherical tokamaks; Tokamaks; Turbulence

Citation Formats

Guttenfelder, W., Field, A. R., Lupelli, I., Tala, T., Kaye, S. M., Ren, Y., and Solomon, W. M. Perturbative momentum transport in MAST L-mode plasmas. United States: N. p., 2017. Web. doi:10.1088/1741-4326/aa6501.
Guttenfelder, W., Field, A. R., Lupelli, I., Tala, T., Kaye, S. M., Ren, Y., & Solomon, W. M. Perturbative momentum transport in MAST L-mode plasmas. United States. https://doi.org/10.1088/1741-4326/aa6501
Guttenfelder, W., Field, A. R., Lupelli, I., Tala, T., Kaye, S. M., Ren, Y., and Solomon, W. M. 2017. "Perturbative momentum transport in MAST L-mode plasmas". United States. https://doi.org/10.1088/1741-4326/aa6501. https://www.osti.gov/servlets/purl/1353400.
@article{osti_1353400,
title = {Perturbative momentum transport in MAST L-mode plasmas},
author = {Guttenfelder, W. and Field, A. R. and Lupelli, I. and Tala, T. and Kaye, S. M. and Ren, Y. and Solomon, W. M.},
abstractNote = {Non-axisymmetric magnetic fields are used to perturbatively probe momentum transport physics in MAST L-mode plasmas. The low beta L-mode target was chosen to complement previous experiments conducted in high beta NSTX H-mode plasmas (beta N = 3.5-4.6) where an inward momentum pinch was measured. In those cases quasi-linear gyrokinetic simulations of unstable ballooning micro-instabilities predict weak or outward momentum convection, in contrast to the measurements. The weak pinch was predicted to be due to both electromagnetic effects at high beta and low aspect ratio minimizing the symmetry-breaking of the instabilities responsible for momentum transport. In an attempt to lessen these electromagnetic effects at low aspect ratio, perturbative experiments were run in MAST L-mode discharges at lower beta (beta N = 2). The perturbative transport analysis used the time-dependent response following the termination of applied 3D fields that briefly brake the plasma rotation ( similar to the NSTX H-mode experiments). Assuming time-invariant diffusive (chi(phi))and convective (V-phi) transport coefficients, an inward pinch is inferred with magnitudes, (RV phi/chi(phi)) = (-1)-(-9), similar to those found in NSTX H-modes and in conventional tokamaks. However, if experimental uncertainties due to non-stationary conditions during and after the applied 3D field are considered, a weak pinch or even outward convection is inferred, ( RV phi/chi(phi)) = (-1)-(+5). Linear gyrokinetic simulations indicate that for these lower beta L-modes, the predicted momentum pinch is predicted to be relatively small, ( RV phi/chi(phi))(sim) approximate to -1. While this falls within the experimentally inferred range, the uncertainties are practically too large to quantitatively validate the predictions. Challenges and implications for this particular experimental technique are discussed, as well as additional possible physical mechanisms that may be important in understanding momentum transport in these low aspect ratio plasmas.},
doi = {10.1088/1741-4326/aa6501},
url = {https://www.osti.gov/biblio/1353400}, journal = {Nuclear Fusion},
issn = {0029-5515},
number = 5,
volume = 57,
place = {United States},
year = {Tue Mar 28 00:00:00 EDT 2017},
month = {Tue Mar 28 00:00:00 EDT 2017}
}

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