Design of lost alpha particle diagnostics for JET

In a future magnetic fusion reactor alpha particles will be utlilized for plasma heating. In order to achieve a high efficiency of this process, the aim has to be a good confinement of alpha particles. Therefore, direct measurement of alpha particle losses is of particular interest. Two diagnostics are being prepared for the JET Tokamak that are targeting on exactly this subject: a scintillator probe and a set of Faraday cups [A. Lioure, A. Kaye, A. Murari, J. Sanchez, T. Todd, C. Damiani, J. Pamela, JET-EFDA Contributors, The JET-Enhanced Performance Program: more heating power and diagnostic capabilities in preparation for ITER, Fusion Eng. Des. 74 (2005) 141]. These systems are capable of measuring ICRH tail ions and charged fusion products. The scintillator probe aims to allow the detection of particles with a pitch angle between 30° and 86° (5% resolution) and a gyroradius between 20 and 140 mm (15% resolution). The Faraday cup array will detect the current of fast ions at multiple poloidal locations, with a dynamic range of 1 nA/cm² to 100 μA/cm² at a temporal resolution of 1 ms. For 3.5 MeV α-particles the energy binning of the foil detector will be 15-50% of the full energy depending on the geometry of the individual collector. The experience in operating both diagnostics in a high temperature and high radiation environment will give valuable information in preparation for the design of similar diagnostics for future fusion devices. This paper covers the design and engineering of both diagnostics together with their envisaged performance.