Soft X-ray Pulse High Analysis (PHA) is a diagnostic that measures the X-ray spectral plasma emission from high temperature plasmas, from which we can derive Te, Zeff and detect the presence of heavy impurities and/or non-maxwellian electron velocity distributions, using simple numerical analysis. The PHA at TCV uses a new generation compact Silicon Drift Detector (SDD) in combination with modern Digital Signal Processing (DSP). The small diode anode capacitance, which is almost independent of the active area, results in high energy resolution with short shaping time-constants (0.25-0.5µ s) making it suitable for high flux, fast count rate applications. The leakage current level is low enough to permit operation at room temperature with spectroscopic resolution (FWHM<200eV at 5.9keV). With the addition of moderate cooling by a single stage Peltier element, the energy resolution is improved (FWHM<170eV at 5.9keV) whilst maintaining a high quantum efficiency (>75% from 1->10keV), ~10 times higher count rate than a conventional diode without showing saturation effects. The space and orientation requirements for the cooling system are considerably simpler than a cryogenic dewar, allowing flexible geometrical arrangements and positioning around the plasma device.
The DSP directly digitises the signal from the diode pre-amplifier, with all further pulse treatment performed within a single electronic module with computer control over amplifier and spectrometer controls including gains, peaking times and pileup inspection criteria, etc. Data is collected into a spectrum of up to 8k channels with up to 1024 time slices available in the module's local memory, which can be passed to the host without stopping the collection process. The throughput can be further improved with additional modules, which will enable real-time signal processing that could provide a robust signal for plasma temperature and/or impurity content feedback control.
This paper will describe the performance of the electronic analysis, which permits spectra to be obtained in a period close to the TCV ion confinement time (a few ms). The high efficiency and limited etendue requirements would make this diagnostic very suitable for routine observation of a burning plasma, where strong neutron shielding is required.