Perturbative momentum transport in MAST L-mode plasmas

W. Guttenfelder, A.R. Field, I. Lupelli, T. Tala, S.M. Kaye, Y. Ren, W.M. Solomon

    Research output: Contribution to journalArticleScientificpeer-review

    3 Citations (Scopus)

    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 (β 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 (β 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 (χ ) and convective (V ) transport coefficients, an inward pinch is inferred with magnitudes, (RV ) = (-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 ) = (-1)-(+5). Linear gyrokinetic simulations indicate that for these lower beta L-modes, the predicted momentum pinch is predicted to be relatively small, (RV ) sim ≈ -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.

    Original languageEnglish
    Article number056022
    JournalNuclear Fusion
    Volume57
    Issue number5
    DOIs
    Publication statusPublished - 28 Mar 2017
    MoE publication typeA1 Journal article-refereed

    Fingerprint

    momentum
    low aspect ratio
    convection
    electromagnetism
    brakes
    complement
    broken symmetry
    simulation
    transport properties
    physics
    probes
    predictions
    magnetic fields

    Keywords

    • gyrokinetics
    • momentum transport
    • spherical tokamaks

    Cite this

    Guttenfelder, W., Field, A. R., Lupelli, I., Tala, T., Kaye, S. M., Ren, Y., & Solomon, W. M. (2017). Perturbative momentum transport in MAST L-mode plasmas. Nuclear Fusion, 57(5), [056022]. https://doi.org/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. In: Nuclear Fusion. 2017 ; Vol. 57, No. 5.
    @article{d8046071ac8b49828e8747d94cf50466,
    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 (β 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 (β 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 (χ ℓ) and convective (V ℓ) transport coefficients, an inward pinch is inferred with magnitudes, (RV ℓ/χ ℓ) = (-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 ℓ/χ ℓ) = (-1)-(+5). Linear gyrokinetic simulations indicate that for these lower beta L-modes, the predicted momentum pinch is predicted to be relatively small, (RV ℓ/χ ℓ) sim ≈ -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.",
    keywords = "gyrokinetics, momentum transport, spherical tokamaks",
    author = "W. Guttenfelder and A.R. Field and I. Lupelli and T. Tala and S.M. Kaye and Y. Ren and W.M. Solomon",
    year = "2017",
    month = "3",
    day = "28",
    doi = "10.1088/1741-4326/aa6501",
    language = "English",
    volume = "57",
    journal = "Nuclear Fusion",
    issn = "0029-5515",
    publisher = "Institute of Physics IOP",
    number = "5",

    }

    Guttenfelder, W, Field, AR, Lupelli, I, Tala, T, Kaye, SM, Ren, Y & Solomon, WM 2017, 'Perturbative momentum transport in MAST L-mode plasmas', Nuclear Fusion, vol. 57, no. 5, 056022. https://doi.org/10.1088/1741-4326/aa6501

    Perturbative momentum transport in MAST L-mode plasmas. / Guttenfelder, W.; Field, A.R.; Lupelli, I.; Tala, T.; Kaye, S.M.; Ren, Y.; Solomon, W.M.

    In: Nuclear Fusion, Vol. 57, No. 5, 056022, 28.03.2017.

    Research output: Contribution to journalArticleScientificpeer-review

    TY - JOUR

    T1 - Perturbative momentum transport in MAST L-mode plasmas

    AU - Guttenfelder, W.

    AU - Field, A.R.

    AU - Lupelli, I.

    AU - Tala, T.

    AU - Kaye, S.M.

    AU - Ren, Y.

    AU - Solomon, W.M.

    PY - 2017/3/28

    Y1 - 2017/3/28

    N2 - 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 (β 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 (β 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 (χ ℓ) and convective (V ℓ) transport coefficients, an inward pinch is inferred with magnitudes, (RV ℓ/χ ℓ) = (-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 ℓ/χ ℓ) = (-1)-(+5). Linear gyrokinetic simulations indicate that for these lower beta L-modes, the predicted momentum pinch is predicted to be relatively small, (RV ℓ/χ ℓ) sim ≈ -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.

    AB - 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 (β 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 (β 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 (χ ℓ) and convective (V ℓ) transport coefficients, an inward pinch is inferred with magnitudes, (RV ℓ/χ ℓ) = (-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 ℓ/χ ℓ) = (-1)-(+5). Linear gyrokinetic simulations indicate that for these lower beta L-modes, the predicted momentum pinch is predicted to be relatively small, (RV ℓ/χ ℓ) sim ≈ -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.

    KW - gyrokinetics

    KW - momentum transport

    KW - spherical tokamaks

    UR - http://www.scopus.com/inward/record.url?scp=85017413651&partnerID=8YFLogxK

    U2 - 10.1088/1741-4326/aa6501

    DO - 10.1088/1741-4326/aa6501

    M3 - Article

    VL - 57

    JO - Nuclear Fusion

    JF - Nuclear Fusion

    SN - 0029-5515

    IS - 5

    M1 - 056022

    ER -

    Guttenfelder W, Field AR, Lupelli I, Tala T, Kaye SM, Ren Y et al. Perturbative momentum transport in MAST L-mode plasmas. Nuclear Fusion. 2017 Mar 28;57(5). 056022. https://doi.org/10.1088/1741-4326/aa6501