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.
PY - 2012/8
Y1 - 2012/8
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 c, an important parameter in the modified Rutherford equation (MRE) for NTM evolution. The measured value of w c=0.7±0.2cm obtained for an ensemble of high β N 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 c value explains the observed threshold width well.
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 c, an important parameter in the modified Rutherford equation (MRE) for NTM evolution. The measured value of w c=0.7±0.2cm obtained for an ensemble of high β N 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 c value explains the observed threshold width well.
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
AN - SCOPUS:84862731737
SN - 0741-3335
VL - 54
JO - Plasma Physics and Controlled Fusion
JF - Plasma Physics and Controlled Fusion
IS - 8
M1 - 085001
ER -