HIGH VOLTAGE SUPPLIES
We have used two methods for powering GEMs (usually both electrodes have to be at a voltage):
1. Direct application of the required voltages to each electrode through independent, current limited HV power supplies with a high-value protection resistor (typically 10 MOhm).
2. One power supply with an external resistive partition network for GEM and another for the upper drift (with protection resistor).
Method 1 is more convenient, since it allows independent control of the various voltages and fields (drift and transfer). We have however observed, although not always, that an otherwise harmless discharge in this case may develop in a sustained spark and result in irreversible damages for the GEM; this does not happen using method 2. We believe that this could be a consequence of different or unpredictable reaction times of the power supplies, resulting in a temporary increase of the GEM voltage. With method 2, we have subjected the same GEM to hundreds of provoked discharges without ill effects.
Note that using method 1 the lower power supply is fed reverse current (collection of electrons), probably not a legal action.
We discourage the use of method 1, unless you fully understand and control the reaction of your HV supplies on overload (for a discussion see Appendix C in our paper: Charge amplification and transfer processes in the Gas Electron Multiplier, by S. Bachmann et al, Nucl. Instrum. Methods A438, 1999, 376) GET PREPRINT.
MULTI-GEM DETECTORS POWERING
In multi-GEM detectors the simplest powering scheme is also a properly designed resistive divider. For large size GEMs, having partitions on one side, individual protection resistors connect each partition to the common node. The scheme used for powering the COMPASS Triple-GEM detectors is shown in the figure:
The scheme is designed to permit operation (at a slightly reduced gain) in case one sector develops a short. For more information, see:
C. Altunbas, M. Capéans, K. Dehmelt, J. Ehlers, J. Friedrich, I. Konorov, A. Gandi, S. Kappler, B. Ketzer, R.D. Oliveira, S. Paul, A. Placci, L. Ropelewski, F. Sauli, F. Simon, and M.v. Stenis, Construction, test and commissioning of the Triple-GEM tracking detector for COMPASS, Nucl. Instr. and Meth. A490(2002)177.