POWERING GEM DETECTORS
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.
