Direct expansion gas cooling system for the ESO's ELT M4 adaptive mirror

The ESO's ELT M4 adaptive mirror is based on the large, contactless voice-coil adaptive mirror technology, with 5352 actuators controlling the shape of the six mirror segments. The system will dissipate up to 8 kW, which are removed by a direct gas expansion cooling system, specifically designed for the application. The key advantage of such solution is related to the fact that any possible leakage from the cooling system would not cause damages to the optics and electronics of the M4 only, but also potentially to the ones of M1, M3 and M5; with AdOptica's system, the gas spillage would cause a local evaporation with consequent temperature drop, without any further consequences. The whole cooling system, including the evaporator embedded in the M4 and the dedicated compressor unit, have been completely assembled. To facilitate the testing and calibration, we designed and built a dedicated large evaporator test unit, called Heat Load Emulator: such device allowed testing and tuning the whole cooling system over the full functional power and environmental ranges, well before the integrated M4 became available, without putting at risk the precious M4 embedded control system. In this work we present the overall cooling system, the dedicated test setup and performance tests results.