This paper presents an overview of existing and emerging technologies on electron sources that can service various Energy Recovering Linacs under consideration. Photocathodes that can deliver average currents from 1 mA to 1 A, the pros and cons associated with these cathodes are addressed. Status of emerging technologies such as secondary emitters, cesiated dispenser cathodes, field and photon assisted field emitters and super lattice photocathodes are also reviewed.

An Energy Recovery Linac (ERL) test facility is presently under construction at BNL [1,2]. The goal of this test facility is to demonstrate CW operation with an average beam current greater than 100mA, and with greater than 99.95% efficiency of current recovery. This facility will serve as a test bed for the novel high current CW photo-cathode [3,4], the superconducting RF cavity with HOM dampers [5,6], and the lattice [7,8] and feedback systems needed to insure the specified beam parameters. It is an important stepping stone for electron cooling in RHIC [9], and essential to meet the luminosity specifications of RHICII [10]. The expertise and experience gained in this effort might also extend forward into a 10-20GeV ERL for the electron-ion collider eRHIC [11]. We report here on the use of a technique of differential current measurement to monitor the efficiency of current recovery in the test facility, and investigate the possibility of using such a monitor in the machine protection system.