An ion source includes a cathode to emit electrons, a cathode grid downstream of the cathode, a reflector electrode downstream of the cathode grid, reflector grid radially inward of the reflector electrode, and an extractor electrode downstream of the reflector electrode, the extractor electrode and cathode grid defining an ionization region therebetween. The cathode and the cathode grid have a first voltage difference such the electrons are accelerated through the cathode grid and into the ionization region on a trajectory toward the extractor electrode. The reflector grid and the extractor electrode have a second voltage difference less than the first voltage difference such that the electrons slow as they near the extractor electrode and are repelled on a trajectory toward the reflector electrode. The reflector electrode has a negative potential such that the electrons are repelled away from the reflector electrode and into the ionization region.