Irradiation tests of bolometer sensor prototypes for ITER

In ITER, the bolometer diagnostic is foreseen to provide the measurements for the total plasma radiation. Sensors need to withstand the harsh environmental conditions. Most prominent among those is the nuclear environment with neutron fluxes up 10 13 n/cm2 s to at the locations of bolometers, which result in a radiation dose of up to 0.3 dpa in Si3N4. Original metal resistor bolometer sensors based on Au-absorbers on a mica substrate with Au-meanders failed when exposed to fast neutron fluences corresponding to 0.1 dpa. Various material tests under irradiation identified ceramic materials like AlO, AlN or Si3N4 together with Pt meanders as promising alternatives. Based on these ideas a development campaign was initiated to produce a radiation hard bolometer sensor. Two approaches resulted in good mechanical stability under thermal cycling: Au absorbers supported by a Si3N4 membrane and Au absorbers on a silicon base plate which is hung up by flexure hinges. In order to assess their resistance against irradiation, tests have been conducted in the Budapest Neutron Centre. The test campaign demonstrated for the first time that bolometer sensors can withstand irradiation doses corresponding up to 0.3 dpa in Si3N4, which is higher than the value of 0.1 dpa requested by ITER project requirements, while their meander resistance measured after irradiation increased only by 20-30 O.