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Cathode HV Discharge Mitigation Design

Document #:
Document type:
LBNF/DUNE document
Submitted by:
Sergio Rescia
Updated by:
Anne Heavey
Document Created:
31 May 2016, 14:47
Contents Revised:
31 May 2016, 14:57
Metadata Revised:
13 Aug 2020, 17:03
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The cathode plane of a Liquid Argon Time projection Chamber (TPC) is an equipotential plane held at a large negative voltage (about -180,000V) to create the drift field for electrons created in the active volume. It presents a capacitance toward any other electrode of the TPC. The largest capacitance components are toward the membrane of the cryostat, held at ground potential or toward the ground plane near the liquid-gas interface which limits the electric field in the gas region. Depending on its location, the energy stored in the cathode plane electrodes may exceed 100J.
No definitive understanding of the cause of the breakdown of the liquid argon has so far emerged. A discharge could be caused by a local higher value of the electric field, perhaps due to a construction defect (“point effect”) and it may be triggered by a thermally generated bubble, or a microbubble caused by a cosmic ray or a radioactive decay. At any rate, a sudden and uncontrolled discharge, though not sufficient to damage the cryostat, can create considerable damage to the electrode structure or to the readout electronics. A cathode discharge (or a discharge of any electrode of the field cage) creates a voltage difference across adjacent electrodes much larger then the nominal operating voltage, which can damage the voltage divider resistors. The Dune designs uses varistors to limit the voltage difference across the divider resistor. A sudden discharge though couples capacitively to the wire planes, and it can damage the CMOS preamplifier in the front-end. A robust design for the Dune cathode plane needs to control the discharge, which can be achieved using two methods:
1. Segmenting the cathode, so the entire structure does not discharge at once, and
2. Using resistive elements to introduce long time constant to slow down the discharge.
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