An apparatus comprises an electron source chamber, an electron-beam sustained plasma (ESP) processing chamber, and a dielectric injector disposed between the electron source chamber and the ESP processing chamber. The dielectric injector comprises a first flared input region comprising a wide entry opening and a narrow exit opening. The wide entry opening opens into to the electron source chamber. The first flared input region is radially symmetric about a longitudinal axis of the dielectric injector. The dielectric injector further comprises a first parallel region comprising an input opening and an output opening. The input opening is adjacent to the narrow exit opening. The output opening is disposed opposite of the input opening. The first parallel region is cylindrical.