Enhanced plasma production by ambipolar electric fields in convergent magnetic fields

Controlled unidirectional remote plasma production in a radio-frequency (rf) plasma source is demonstrated by placing magnetic nozzle throat several tens of centimeters away from the rf antenna, where a local density peak forms near the throat. The experiment indicates that the dominant electron energization mechanism responsible for the density peak in the magnetic nozzle throat is electrostatic electron acceleration driven by a spontaneously formed ambipolar electric field, yielding an energy gain of approximately 5 eV and enhanced remote ionization. These results establish plasma sources driven by spontaneous electrostatic electron acceleration.