Analysis of the Response of CVD Diamond Detectors for UV and sX-Ray Plasma Diagnostics Installed at JET

Diamond detectors are very promising candidates for plasma diagnostics in a harsh environment. In fact, they have several proprieties which make them suitable for magnetic fusion devices: radiation hardness, high thermal conductivity, high resistivity, high carrier mobility and a large bandgap (5.5 eV). The latter makes them insensitive to visible radiation and allows low noise measurements without any cooling. In 2008 two CVD (Chemical Vapour Deposition) single crystal diamond (SCD) detectors were installed at the JET tokamak as extreme UV and soft X-Ray diagnostics [1]. In this work the neutron background in these detectors was measured shielding the UV and soft X-Ray radiation by closing a local vacuum valve. The UV detector was found to be insensitive to the neutron flux, while the soft X Ray detector signal exhibited spikes during the highest neutron rate pulse (neutron rate 1016n/s, which corresponds to a flux of φn ∼105n/cm2s in the detector location). These spikes were found to be due to the (n,p) reaction within the plastic filter in front of the soft X-Ray detector. The UV SCD was also used to perform time of flight (ToF) measurements in laser ablation experiments. ToFs were found to be an order of magnitude higher than expected if only the drift velocity is considered. This discrepancy could be due to a delay between the arrival time of the impurities in the plasma and their emission in an energy range which SCD is sensitive to (Eph >5.5 eV). The delay is found to be comparable with the expected ionization times for edge plasma conditions.