King J D, Makowski M A, Holcomb C T, Allen S L, Hill D N, La Haye R J, Turco F, Petty C C, Van Zeeland M A, Rhodes T L, Meyer W H, Geer R, Morse E C
Lawrence Livermore National Laboratory, Livermore, California 94550-9234, USA.
Rev Sci Instrum. 2011 Mar;82(3):033515. doi: 10.1063/1.3568827.
Accurate measurement of internal magnetic field direction using motional Stark effect (MSE) polarimetry in the edge pedestal is desired for nearly all tokamak scenario work. A newly installed 500 kHz 32-channel digitizer on the MSE diagnostic of DIII-D allows full spectral information of the polarimeter signal to be recovered for the first time. Fourier analysis of this data has revealed magnetohydrodynamic (MHD) fluctuations in the plasma edge pedestal at ρ ≥ 0.92. By correlating edge localized mode fluctuations seen on lock-in amplifier outputs with MSE spectrograms, it has been shown that edge pedestal tearing mode fluctuations cause interference with MSE second harmonic instrument frequencies. This interference results in unrecoverable errors in the real-time polarization angle measurement that are more than an order of magnitude larger than typical polarimeter uncertainties. These errors can cause as much as a 38% difference in local q. By using a redundant measure of the linear polarization found at the fourth harmonic photo-elastic modulator (PEM) frequency, MHD interference can be avoided. However, because of poorer signal-to-noise the fourth harmonic signal computed polarization angle shows no improvement over the MHD polluted second harmonics. MHD interference could be avoided in future edge pedestal tokamak polarimeters by utilizing PEMs with higher fundamental frequencies and a greater separation between their frequencies.
对于几乎所有托卡马克实验工况研究而言，都需要在边缘台基区利用运动斯塔克效应（MSE）偏振测量法精确测量内部磁场方向。DIII-D装置的MSE诊断系统上新安装的一台500 kHz 32通道数字化仪，首次实现了对偏振仪信号完整光谱信息的恢复。对该数据进行傅里叶分析后，揭示了在ρ≥0.92的等离子体边缘台基区存在磁流体动力学（MHD）波动。通过将锁定放大器输出端观测到的边缘局域模波动与MSE频谱图进行关联，结果表明边缘台基撕裂模波动会对MSE二次谐波仪器频率产生干扰。这种干扰会导致实时偏振角测量中出现不可恢复的误差，其大小比典型的偏振仪不确定度大一个数量级以上。这些误差会使局部q值产生高达38%的差异。通过利用在四阶光弹调制器（PEM）频率处测得的线性偏振的冗余测量值，可以避免MHD干扰。然而，由于信噪比更低，四阶谐波信号计算得到的偏振角相比受MHD污染的二阶谐波并无改善。未来，通过使用具有更高基频且频率间隔更大的PEM，有望在边缘台基托卡马克偏振仪中避免MHD干扰。
