Measurement of the LCLS-II dark current using the LDMX Trigger Scintillator

The Light Dark Matter eXperiment (LDMX) is a proposed fixed-target missing momentum search for sub-GeV thermal relic dark matter. LDMX aims to probe thermal dark matter targets with 10¹⁶ electrons on target. Such an approach requires a high-repetition rate, low-current beam, with an average of one electron on target per event. These requirements are well-suited to the Linac to End Station A (LESA) facility, which will take advantage of the unused RF buckets between LCLS-II bunches to produce a well-defined low-current beam with a 26.9 ns bunch spacing. This talk will describe the results of a measurement of dark current in the Sector 30 transfer line (S30XL) of the LCLS-II beam, using a partial prototype of the LDMX trigger scintillator (TS) subsystem, as well as recent measurements taken using a full TS prototype in End Station A.