Soft-X-Ray Tomography Diagnostic at the Rtp Tokamak

An 80-channel soft x-ray tomography system has been constructed for diagnosing the RTP (Rijnhuizen Tokamak Project) tokamak plasma. Five pinhole cameras, each with arrays of 16 detectors are distributed more or less homogeneously around a poloidal plasma cross section. The cameras are positioned close to the plasma edge to enable viewing angles which could not be reached otherwise. The system can be used for making tomographic reconstructions of the soft x-ray emissivity with a relatively high poloidal mode number (up to both components of m = 4), which is an improvement with respect to other plasma tomography systems presented in literature. The temporal resolution of the system is 5 mus. Apart from a description of the hardware and geometrical layout of the system, the paper will focus on two important issues, namely the calibration of the detectors and the positioning of the cameras in the vacuum vessel of the tokamak. A dedicated x-ray source has been designed for the relative calibration of the detectors of each individual camera. Misalignment causes several artifacts in tomographic reconstructions obtained with multicamera systems. Therefore, two new methods are proposed which make use of the plasma itself for improving the knowledge about the positions of the cameras.