Aperture test for internal target operation in the JLAB high-current ERL

A high current beam transmission test has been successfully completed at the JLAB FEL Facility, culminating in very low-loss transmission of a high current CW beam through a small aperture simulating the configuration of an 10¹⁸ cm^-3 internal gas target proposed for the DarkLight experiment [1]. In order to meet the technical challenges, minimal beamline modifications were made to create a machine configuration that is substantially different from those used in routine UV or IR FEL operation. A sustained 8 hour high power beam run was performed, with clean transmission through a 2 mm transverse aperture of 127 mm length. A beam size of 50 um (rms) was measured near the center of the aperture. Experimental data from a week-long test run consistently exhibited beam loss of only a few ppm on the aperture while running 4.5 mA current at 100 MeV - or nearly 0.5 MW beam power. This surpassed the users' initial expectation and demonstrated a unique capability of an ERL for this type of experimental configuration. This report presents a summary of the experiment, a brief overview of our activities, and outlines future plans.