Infrastructure for Chamber Operation

For the GEM-MSGC chamber operation four different high voltages are needed: drift voltage $U_{drift}$, the voltage for the upper and lower side of the GEM foil, and the cathode voltage $U_{cathode}$. In order to save space and reduce the number of power supplies in the experimental hall, one power supply provides $U_{drift}$ and $U_{cathode}$ for eight chambers. The two voltages needed for both sides of the GEM are derived from the drift voltage, the voltage difference $U_{GEM}$ can be adjusted for each chamber individually.

The front-end electronics of the Inner tracker system need a stable low voltage supply. The front-end electronics need voltages $\pm$5, +3, $\pm$2 V. The system was developed by a commercial supplier. Each half superlayer has its own low voltage power supply.

For the operation of the GEM-MSGC, a permanent exchange of the counting gas is needed. Due to aging problems specific to GEM-MSGC detectors, the gas system has to provide an extremely clean gas mixture. In order to avoid pollution of the gas, the materials used for the detector construction have been selected according to their out-gassing properties. The fact that part of the gas system is placed in the acceptance region of the experiment influenced the selection of the materials in addition. A rather critical point in the gas system is the controlling of the detector gas pressure relative to the ambient pressure. It has to be controlled with an accuracy of about 10 $\mu$bar.

The crucial point of the system is the steering system (Slow control system). It allows to tune applied voltages and to get the status information from all components of the system. Further access to the various informations (such as: actual voltages, currents, number of GEM sparks and etc ) related to the performance of the detector during data taking is possible via a slow control database.