An ion source includes a cathode emitting primary electrons, a cathode grid downstream of the cathode, a reflector electrode downstream of the cathode grid, a reflector grid radially inward of the reflector electrode, and an extractor electrode downstream of the reflector electrode. The cathode and the cathode grid have a voltage difference such that the electric field accelerates the primary electrons on a trajectory toward the extractor electrode. The reflector grid and the extractor electrode have a voltage difference such that the electric field repels the primary electrons on a trajectory away from the extractor electrode and toward the reflector electrode. The cathode and reflector electrode have a voltage difference such that some primary electrons strike the reflector electrode, creating secondary electrons. The reflector grid has a positive potential such that the electric field attracts the primary and secondary electrons into the ionization region where they interact with ionizable gas.