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A polarization modulation technique for far-infrared polarimetry in large plasmas

机译：大型等离子体中用于远红外极化的极化调制技术

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In future ITER tokamak experiments the poloidal magnetic field and the plasma electron density profiles will be simultaneously measured using a far-infrared polarimeter. This paper presents the bench testing of a new polarimetric technique in which these quantities can be obtained by phase measurements only. Its application will result in a simpler instrument, more robust and less affected by fringe jumps and vibration problems than the conventional interferometer-polarimeter systems in use today. In this experiment a polarization modulation of the input beam is generated by recombining two coherent linearly polarized components, produced by a CO2 pumped FIR laser (lambda = 118.8 mum), one of which is frequency shifted by a rotating grating. The beam is then sent through a half-wave plate rotated mechanically at 125 Hz and a fixed quarter-wave plate. When such a beam traverses the plasma, a time dependent phase shift is observed whose Fourier components in phase and in quadrature with the rotating wave-plate are related to the Faraday rotation angle and to the Cotton-Mouton effect, respectively. For this experiment the effect of the plasma has been simulated by a combination of a half-wave and a quarter-wave plates. Simple but effective phase measuring electronics have been developed to recover the two polarimetric parameters from the phase of the detected signal. The experiment has shown that variations in the azimuth and ellipticity angles of the polarization ellipse of less than 1degrees can be measured with a time resolution of the order of 10 ms.
机译：在未来的ITER托卡马克实验中，将使用远红外旋光仪同时测量极向磁场和等离子体电子密度分布。本文介绍了一种新的偏振技术的台架试验，其中只能通过相位测量获得这些量。与当今使用的常规干涉仪-偏振仪系统相比，其应用将使仪器更简单，更坚固，受到条纹跳跃和振动问题的影响更小。在该实验中，通过将两个相干的线性偏振分量重新组合来产生输入光束的偏振调制，这两个线性偏振分量是由CO2泵浦的FIR激光器（λ= 118.8 mum）产生的，其中一个通过旋转光栅进行频移。然后，光束通过机械旋转速度为125 Hz的半波片和固定的四分之一波片发送。当这样的光束穿过等离子体时，观察到随时间变化的相移，其与旋转波片同相和正交的傅立叶分量分别与法拉第旋转角和Cotton-Mouton效应有关。对于该实验，已经通过结合半波片和四分之一波片来模拟等离子体的效果。已经开发了简单但有效的相位测量电子设备，以从检测到的信号的相位中恢复两个极化参数。实验表明，可以用10毫秒量级的时间分辨率测量小于1度的偏振椭圆的方位角和椭圆度角的变化。

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