Bolometric diagnostics in JET

Radiation losses from the JET plasma are measured (temporally and spatially resolved) with three pin‐hole cameras which observe one poloidal plasma cross section from underneath (one vertical camera with 14 bolometers) and from the side (two horizontal cameras with 10 bolometers each). In addition, eight collimated single bolometers monitor toroidal symmetry of plasma emissivity. An improved metal resistor bolometer capable of surviving at least 5000 D–T pulses in JET is composed of two identical foils (4‐μm‐thick gold absorber, 7.5‐μm‐thick Kapton substrate, and 0.1‐μm‐thick gold resistor), mounted one behind another in one housing. The rear foil serves as a reference bolometer in a bridge circuit. With an integration time constant of 20 ms a detection limit of 70 μW/cm2 was measured in JET. Examples of measuring results in JET are presented: strong poloidal asymmetries of the radiation density as precursor signals of a density limit disruption and the formation of a poloidally symmetric, cold plasma man...