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Impact of edge localized mode on photo-elastic-modulator based motional Stark effect polarimetry

机译:边缘局部化模式对基于光弹性调制器的运动斯塔克效应极化法的影响

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摘要

Intermittent increase in background light induced by edge localized mode (ELM) in high confinement mode (H-mode) plasma gives disturbances to polarization angle of incident light measured by motional Stark effect (MSE) polarimeter using photo-elastic modulator (PEM), even if the background light is not polarized. The error in the polarization angle γ pem determined by the light intensity at the frequency modulated by the PEM originates in a broadband frequency spectrum of the intermittent background light and applies to PEM based polarimeter in general. The impact of the background light induced by the ELM on the error inγ PEM is numerically investigated in terms of the polarization angle of incident light γ, the peak intensity of background light normalized by the intensity of beam emission (X_(ELM)). ELM frequency felm. ELM decay time τelm and the time constant of low-pass filter (LPF) per single stage τlpf. The error increases almost proportionally with γ between ±22.5° when inphase spectral amplitude is used inγ pem calculation. The error becomes quite large for γ approaching to the direction of PEM axis (±22.5°), when absolute spectral amplitude is used. The error increases with X_(ELM) but does not depend on τelm. When the ELM frequency is sufficiently lower than a critical ELM frequency that is given by inverse of an effective LPF time constant (τlpf times the number of LPF stages), the error has its peak, the height of which is independent of felm. soon after the ELM and vanishes between subsequent ELMs. However, when felm is higher than the critical ELM frequency, errors induced by subsequent ELMs pile up and the error increases with fELM. In case of no pileup, the maximum error becomes larger for smaller τlpf. because impact of ELM temporally concentrates in a shorter period just after the ELM. The error induced by ELMs is roughly estimated to be ~0.002° in a type-I ELMy H-mode JT-60U discharge, which is much smaller than other errors coming from calibrations (about 0.1-0.2°) and temporal fluctuation (less than about 0.05°), mainly since the background light induced by ELMs is weak compared to the beam emission (X_(ELM)) ~0.07 in JT-60U. However, MSE diagnostics experiencing large background light (X_(ELM)) should take this kind of error into accmint.
机译:边缘局限模式(ELM)在高限制模式(H-mode)等离子体中由边缘定位模式(ELM)引起的背景光的间歇性增加会干扰使用光弹性调制器(PEM)通过运动斯塔克效应(MSE)偏振计测量的入射光的偏振角,甚至如果背景光没有偏振。由在PEM调制的频率处的光强度确定的偏振角γpem的误差起源于间歇背景光的宽带频谱,并且通常适用于基于PEM的偏振计。根据入射光的偏振角,通过光束发射强度归一化的背景光的峰值强度,对ELM引起的背景光对γPEM误差的影响进行了数值研究。 ELM频率感觉。 ELM衰减时间τelm和每级低通滤波器(LPF)的时间常数τlpf。当同相谱幅度用于γpem计算时,误差几乎与γ成正比,在±22.5°之间。当使用绝对频谱幅度时,对于接近PEM轴方向(±22.5°)的γ,误差会变得很大。误差随着X_(ELM)的增加而增加,但不取决于τelm。当ELM频率充分低于有效LPF时间常数(τlpf乘以LPF级数)的倒数给出的临界ELM频率时,误差将出现其峰值,其高度与felm无关。 ELM之后不久,并在随后的ELM之间消失。但是,当felm高于临界ELM频率时,后续ELM引起的误差就会堆积起来,并且误差随fELM的增加而增加。如果没有堆积,则对于较小的τlpf,最大误差会变大。因为ELM的影响会在ELM之后的较短时间内暂时集中。在I型ELMy H模式JT-60U放电中,由ELM引起的误差大致估计为〜0.002°,比校准(约0.1-0.2°)和时间波动(小于大约0.05°),主要是因为ELM引起的背景光比JT-60U中的光束发射(X_(ELM))〜0.07弱。但是,遇到较大背景光(X_(ELM))的MSE诊断程序应考虑到这种错误。

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