  The architecture of the novel MPGD-based photon detectors of COMPASS RICH-1
consists in a large-size hybrid MPGD multilayer layout combining two layers of
Thick-GEMs and a bulk resistive MICROMEGAS. Concerning biasing voltage, the
Thick-GEMs are segmented in order to reduce the energy released in case of
occasional discharges, while the MICROMEGAS anode is segmented in pads
individually biased at positive voltage, while the micromesh is grounded. In
total, there are ten different electrode types and more than 20000 electrodes
supplied by more than 100 HV channels. Commercial power supply units are used.
The original elements of the power supply system are the architecture of the
voltage distribution net, the compensation, by voltage adjustment, of the
effects of pressure and temperature variation affecting the detector gain and a
sophisticated control software, which allows to protect the detectors against
errors by the operator, to monitor and log voltages and current at 1 Hz rate
and to automatically react to detector misbehaviors. The HV system and its
performance are described in detail as well as the electrical stability of the
detector during the operation at COMPASS.
