Ultrasoft x-ray telescopes for fluctuation imaging in fusion plasmas

We present the design for a novel spectroscopic instrument for two-dimensional imaging of low-amplitude, fast fluctuations from magnetically confined fusion devices. The instrument is similar in principle to an ultrasoft x-ray (USXR) telescope recently developed for astrophysics application and uses a large (10-20-cm-diam), spherical multilayer mirror to select and concentrate a bright USXR impurity line (e.g., C VI Ly α at 33.7 Å, or C VI H α at 182 Å), onto a low-noise fast, two-dimensional detector. A first version of the telescope will use a fast phosphor to convert the USXR image into visible light, which is then guided using fiber optics to a fast/gated charge-coupled-device camera outside the vacuum. Detailed signal-to-noise analysis indicates that, due to the high energy and brightness of the USXR line emission and large reflectivity of the present mirrors, such an instrument would allow imaging of subpercent amplitude fluctuations with ≤ 10 μs time resolution and sub-cm spatial resolution. At the same time, ray-tracing calculations show that a quite large plasma region (up to a few tens of cm extent) can be imaged. Another version of the telescope would use a two-dimensional array of extreme ultraviolet diodes at the focal plane, in which case fluctuations up to ≈0.1% could be imaged.