An active phase compatible protection system to prevent excessive neutral beam shinethrough during JET plasmas

The JET Fast Beam Interlock System (FBIS) removes the neutral beam power with a fast response (<20 ms) under a diverse range of fault conditions. The configuration of FBIS for the tritium or active phase of operation of JET is described, in particular its expansion to include inputs from a new interlock designed to prevent excessive neutral beam shinethrough. This new system is based on visible bremsstrahlung emission from the plasma and is called the Bremsstrahlung Beam Interlock (BBI). Its main features are detailed here, including heated fibre-optics and a compensation fibre which make the interlock compatible with the active phase. In addition, the calibration of the interlock using JET's infra-red interferometer is described as well as operational experience with deuterium-tritium plasmas.