The influence of finite radial transport on the structure and evolution of m/n=2/1 neoclassical tearing modes on MAST

TY - JOUR

T1 - The influence of finite radial transport on the structure and evolution of m/n=2/1 neoclassical tearing modes on MAST

AU - Snape, J. A.

AU - Gibson, K. J.

AU - O'Gorman, T.

AU - Barratt, N. C.

AU - Imada, K.

AU - Wilson, H. R.

AU - Tallents, G. J.

AU - Chapman, I. T.

AU - MAST Team

PY - 2012/8/1

Y1 - 2012/8/1

N2 - Finite radial transport around magnetic islands is believed to play an important role in the threshold, spatial structure and temporal evolution of neoclassical tearing modes (NTMs). We report on novel measurements of NTMs with mode structure m/n=2/1 on the MAST spherical tokamak (ST), which have allowed a direct evaluation of the effect of transport on island behaviour for the first time on an ST. Temperature profiles obtained with the upgraded Thomson scattering (TS) system on MAST have been used to constrain the solutions of a heat transport equation for the NTM magnetic island (Fitzpatrick 1995 Phys. Plasmas 2 825), allowing the determination of the critical width for temperature flattening across an island w , an important parameter in the modified Rutherford equation (MRE) for NTM evolution. The measured value of w =0.7±0.2cm obtained for an ensemble of high β MAST discharges has been used in an analysis of the MRE for 2/1 NTM growth and saturation on MAST. Using a probabilistic method for parameter and error estimation, which takes account of the experimental uncertainty on measured equilibrium parameters, it is found that the temporal evolution of island size is well described by marginally, classically unstable NTMs with strongly destabilizing bootstrap current and stabilizing curvature terms. Finally, further analysis of a β ramp-down discharge is presented, in which the measured w value explains the observed threshold width well. © 2012 IOP Publishing Ltd.

AB - Finite radial transport around magnetic islands is believed to play an important role in the threshold, spatial structure and temporal evolution of neoclassical tearing modes (NTMs). We report on novel measurements of NTMs with mode structure m/n=2/1 on the MAST spherical tokamak (ST), which have allowed a direct evaluation of the effect of transport on island behaviour for the first time on an ST. Temperature profiles obtained with the upgraded Thomson scattering (TS) system on MAST have been used to constrain the solutions of a heat transport equation for the NTM magnetic island (Fitzpatrick 1995 Phys. Plasmas 2 825), allowing the determination of the critical width for temperature flattening across an island w , an important parameter in the modified Rutherford equation (MRE) for NTM evolution. The measured value of w =0.7±0.2cm obtained for an ensemble of high β MAST discharges has been used in an analysis of the MRE for 2/1 NTM growth and saturation on MAST. Using a probabilistic method for parameter and error estimation, which takes account of the experimental uncertainty on measured equilibrium parameters, it is found that the temporal evolution of island size is well described by marginally, classically unstable NTMs with strongly destabilizing bootstrap current and stabilizing curvature terms. Finally, further analysis of a β ramp-down discharge is presented, in which the measured w value explains the observed threshold width well. © 2012 IOP Publishing Ltd.

UR - http://www.scopus.com/inward/record.url?scp=84862731737&partnerID=8YFLogxK

U2 - 10.1088/0741-3335/54/8/085001

DO - 10.1088/0741-3335/54/8/085001

M3 - Article

VL - 54

SP - 1

EP - 11

JO - Plasma Physics and Controlled Fusion

JF - Plasma Physics and Controlled Fusion

SN - 0741-3335

IS - 8

M1 - 085001

ER -