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The LCLSII superconducting linac accelerates 250kW of electrons to 4 GeV in bunches spaced 1.08usec (929kHz) apart and extracts them to either of two undulators or passes them to a beam dump. The LCLSII RF gun operates at 186MHz. Each of the 200 bunches in between those destined for the undulators will carry electrons from the gun HV “dark current” or can be seeded with a very low charge by an independent source laser operating at low power at the gun frequency. DASEL (Dark Sector Experiments at LCLSII) is a proposal to seed and then extract  the central ~60% of these low charge bunches and to transport them to the existing End Station A beam line for use in an experimental program designed to search for evidence of dark matter in the MeV-GeV mass range. DASEL will extract these bunches after they pass the second of the two LCLSII kickers and before the beam is bent down to the 250kW dump. This proposal is to develop the four main elements of the DASEL: the source laser, the kicker-septum, the ~250m beamline and a spoiler-collimator system that can shape the beam phase space to the requirements of the experimental program. The time structure and energy of the electron beam will enable the planned experiments to provide definitive results on low-mass thermal relic dark matter.

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