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

我们一直在开发一种新的双色远红外激光干涉仪，用于LHD和未来大型聚变装置的密度测量。在LHD中，已经构建了一个13通道远红外激光干涉仪，并进行常规操作，以便几乎每次发射都精确测量电子密度分布，除了冰球注入产生的高密度等离子体的情况外。当向等离子体中注入大尺寸弹丸时，会形成陡峭的密度梯度，有时会引起条纹计数器测量的密度迹线上的条纹跳跃。为了克服这一困难，我们研制了40 ~ 70 μm波长范围的新光源，并获得了57.2和47.6 μm波长同时振荡的高功率激光线。为了构造短波长激光干涉仪，对真空窗口和光束的光学材料的选择进行了讨论 关于我们 分离器很重要。常用于远红外区的石英标准具在50 μm附近有很大的吸收。我们已经证实CVD金刚石是短波长区域的优良材料。在57.2 μm处的吸收系数和折射率分别为0.19±0.05 cm-1<-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金刚石的应用仅限于应用于特殊部件。因此，我们研究了硅在57.2和47.6 μm波段的光学常数，发现硅可以作为50 μm波段激光诊断的光学元件。介绍了工作在液态下的Ge：Ga光电导探测器。在室温下，我们成功地检测到两个彩色差拍信号（0.5MHz和1.2MHz），具有良好的信噪比（± 40 dB）。这些差拍信号通过带通滤波器后送入相位比较器进行相位测量。并且我们发现双色远红外激光器的工作在演示机械振动补偿方面是成功的

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