Experimental investigation of a plasma edge cathode scheme for high current density, long pulse electron extraction

Conventional cold cathode sources used for the extraction of long pulse electron beams are typically limited by plasma closure of the extraction gap. In the plasma edge cathode scheme, a nearly stationary plasma cathode is formed by partially intercepting a well directed plasma jet with a material edge. Electrons are extracted in a direction normal to the flow of plasma behind the obstacle. Under space charge limited extraction, the plasma boundary will experience negligible electric force since the electric field at the boundary will be reduced to near zero. As a result, if the boundary is initially stationary, it will remain so and plasma closure can be avoided. An electron current density in excess of 100 Acm^2 is theoretically possible from a plasma with density n = 4x10^13 cm^3 and kTe=1eV.

Experimental investigation of the plasma edge cathode has confirmed the underlying concept and has resulted in an achieved current density in excess of 20 A/cm^2 for 8 µs. Geometric diode gap spacings between 0.4 cm and 1.5 cm were typically used resulting in the observance of closure velocities as low as 0.08 cm/µs. Both small (4.71 cm^2) and large (42.4 cm^2) area anode configurations were used. Variation of the gap potential and spacing verified the space charge limited nature of the extracted electron current density. The density and velocity of the plasma jet are inferred from biased flux probe measurements taken at various positions in the vacuum chamber. In addition, the electron temperature both in and downstream of the extraction region was measured as a function of time with a double Langmuir probe. Finally, a multi-pinhole imaging technique was employed to measure the intrinsic divergence and microscopic brightness of the electron beam as well as to infer the effective shape of the plasma boundary.