Development of 2 color short-wavelength FIR laser interferometer and its application to the Large Helical Device

我们一直在研制一种新的双色远红外激光干涉仪，用于LHD上的密度测量和未来的大型聚变装置。在LHD中，已经建造了一台13ch FIR激光干涉仪，并按常规操作，几乎每一次激发都能精确测量电子密度分布，但冰球注入产生的高密度等离子体除外。当一个大尺寸的弹丸被注入到等离子体中时，会形成陡峭的密度梯度，这有时会导致条纹计数器测量到的密度轨迹上的条纹跳跃。为了克服这一困难，我们一直在开发波长在40-70μm的新的激光光源，并获得了同时振荡在57.2和47.6μm的高功率激光谱线。这两条激光谱线使我们能够构造一种新的双色激光干涉仪。为了构造一台短波长激光干涉仪，真空窗口和光束…的光学材料的选择更多的拆分器是重要的。晶体石英标准具在50μm附近有较大的吸收，是一种很好的短波长材料。在57.2μm处的吸收系数和折射率分别为0.19±0.05 cm~(-1)和2.383±0.002。然而，由于大尺寸的分束器/合束器过于昂贵，CVD-Diamond的应用仅限于特殊部件。因此，我们研究了硅在57.2和47.6μm处的光学常数，发现硅可以作为激光诊断的光学元件，在50μm左右。在此温度下，我们成功地检测到了两个色差拍信号(0.5 MHz和1.2 MHz)，具有良好的信噪比(~40分贝)。这些差拍信号经过带通滤波后送入相位比较器进行相位测量。并发现双色FIR激光器在机械振动补偿方面取得了较好的效果

We have been developing a new two color far-infrared laser interferometer for the density measurement on the LHD and for the future large fusion devices. In the LHD, a 13ch FIR laser interferometer has been constructed and routinely operated for the precise measurements of the electron density profile almost every shot except in the case of a high-density plasma produced by an ice pellet injection. When a large sized pellet is injected into the plasma steep density gradient is formed, which sometimes causes the fringe jumps on the density traces measured by fringe counters. In order to overcome this difficulty we have been developing new laser sources in the wave length region of 40 to 70 μm, and achieved high power laser lines oscillating simultaneously at 57.2 and 47.6 μm. These two color laser lines enable us to construct a new two color laser interferometer.In order to construct a short wavelength laser interferometer, the choice of optical materials for a vacuum window and a beam … More splitter is important. A crystal quartz etalon, which is often used in a FIR region, has a large absorption around 50 μm. We have confirmed that a CVD-Diamond is an excellent material for the short wavelength region. The absorption coefficient and the refractive index at 57.2 μm are 0.19±0.05 cm^<-1> and 2.383±0.002, respectively. The application of the CVD-Diamond is, however, limited to apply to the special components, since the large sized beam-splitters/combiners are too expensive. So, we have investigated the optical constants of Silicon at 57.2 and 47.6 μm, and found that Silicon can be used as optical components for laser diagnostics around 50 μm. By introducing Ge : Ga photoconductive detectors operating at liq. He temperature, we have successfully detected two color beat signals (0.5 MHz and 1.2 MHz) with excellent signal to noise ratio (〜40 dB). These beat signals were fed into phase comparators for phase measurement after passing through band-pass filters. And we found that two color FIR laser work was successful in demonstration of mechanical vibration compensation Less