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Detection and Mitigation of Propagating Electrical Discharges Within the Gas Electron Multiplier Detectors of the CMS Muon System for the CERN HL-LHC

par Starling, Elizabeth Rose
Président du jury Clerbaux, Barbara
Promoteur De Lentdecker, Gilles
Publication Non publié, 2020-12-14

Thèse de doctorat

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Résumé :

In preparation for the High-Luminosity Large Hadron Collider (HL-LHC) at CERN, the Compact Muon Solenoid (CMS) Detector is undergoing a series of upgrades to its existing infrastructure, and is adding in several completely new subdetector systems. The first of these new systems, called GE1/1, is a series of 144 gas electron multiplier (GEM) detectors, arranged as 36 two-detector "superchambers" in each of the muon endcaps of CMS. These detectors are a subtype of micropattern gas detectors, and consist of three layers of "GEM foils", thin sheets of polyimide coated with 5 um of copper on each side and chemically etched with holes of 50 - 70 um diameter at a pitch of 140 um. These layers are stacked on top of a printed circuit board (PCB) readout and sealed within a gastight volume that is filled with Ar:CO2 70:30, and a high voltage is applied to the foils to create electric fields within the GEM detectors. When a muon enters the detector and ionizes the gas within, the ionized electrons encounter these fields and multiply in Townsend avalanches at each successive foil layer, until they are read out at the readout PCB at a gain of ~10^4. In early 2017, a demonstrator system known as the "slice test" was installed into the negative endcap. Consisting of 10 GEM detectors, the two-year-long slice test served as both a proof of concept for the GE1/1 system and an invaluable learning experience that would permanently impact not only the GE1/1 project, but future GEM systems GE2/1 and ME0 as well. During the slice test, it was observed that readout channels were being lost in the course of operation to such a degree that the operational lifetime of the system was in serious jeopardy. These losses were attributed to damage to the front-end readout ASIC (VFAT) inputs, caused propagating electrical discharges within the detectors, and a dedicated campaign to study the discharges was launched. The results of this study will be presented in this dissertation. A campaign to mitigate these discharges and their resulting damage was launched. In order to protect the sensitive VFAT from damage, several external protection circuits were proposed and thoroughly tested. The results of these tests, which are presented herein, determined that a series of resistors totaling 470 Ohms would be installed on the VFAT hybrid. When coupled with an additional 200 kOhm resistor on the HV filter, this reduced the probability of damage following a discharge from 93% to 3% As GE2/1 and ME0 are not due to be installed for another few years, more complex discharge-prevention measures can be put into place. As such, the following measures have been examined, and results will be discussed herein: A new, larger VFAT hybrid is being manufactured, whose larger surface area can accommodate more robust protection circuits than those considered and used for GE1/1. As well, double-segmented GEM foils, in which both the top and bottom of each foil is segmented into < 100 cm^2 sectors that are separated by resistors, were examined for use in the detectors. These double-segmented foils were found to introduce a cross-talk signal in the detectors that results in false signals being treated as true signals, which causes a saturation of the GEM bandwidth and results in unwanted dead time. These cross-talk signals, as well as the compromises made to reduce the cross-talk while maintaining robust discharge prevention, will be discussed.