JET soft x-ray diode array diagnostic

The arrays of silicon diodes recently installed on the Joint European Torus (JET) measure the plasma soft x-ray emission with high temporal and spatial resolution, allowing magnetohydrodynamic (MHD) effects to be investigated. The diagnostic consists of two ‘‘pinhole’’ cameras which view the same poloidal cross section in orthogonal directions. A data-acquisition system based upon three cascaded digital filters can convert the 200-kHz detector signal analog-to-digital converter (ADC) output to lower sampling rates of 10, 1, and 0.1 kHz. Initially, studies have concentrated on the sawtooth collapse. By means of a trigger system data on the sawtooth crash have regularly been recorded at a sampling frequency of 200 kHz. Using tomographic reconstruction techniques, the soft x-ray emissivity cross section during the crash has been reconstructed. First results during ion cyclotron radio frequency (ICRF) heating show no precursor activity prior to the sawtooth collapse, while during the crash, the hot central plasma is displaced off axis by about 30 cm on a time scale of 50–200 μs, after which a hollow soft x-ray profile forms.