Grid-controlled electron emission from a hollow-anode electron source

The operation of a hollow-anode (HA) electron source with a biased output grid and an incorporated ferroelectric plasma source (FPS) in a diode powered by a 200 kV, 400 ns pulse is described. It was found that the FPS allows reliable ignition and sustaining of the HA discharge with current amplitude ≤1.2 kA and pulse duration ≤2×10^-5 s at N₂ gas pressure of (1-3)times;10^-4 Torr. Parameters of the hollow-anode plasma were studied by different electrical, optical and spectroscopic diagnostics. It was found that the plasma density is ∼2times;10 ¹⁴ cm^-3 and ∼4times;10¹² cm^-3 at the vicinity of the ferroelectric surface and at the output anode grid, respectively. Three different electrical schemes for the hollow-anode grid bias were tested and compared. The use of an autobias grid allows electron beam extraction with current I_h ≤ 1.2 kA in an emission-limited mode and insignificant plasma pre-filling of the accelerating gap. The use of an externally biased hollow-anode grid (either with a positive or negative potential) showed the possibility to control the plasma emission properties (I_h ≤2 kA) without changing the amplitude of the discharge current. Drastic changes in the parameters of the plasma were obtained during the accelerating pulse. The applied accelerating pulse causes a significant increase of the plasma potential (φ_pl) inside the hollow anode. For a discharge current amplitude of 0.3-1 kA, φ_pl increases relative to the hollow-anode electrode as 6.6-3.1 kV, relative to the ferroelectric plasma source as 0.2-0.4 kV, and relative to the hollow-anode grid as 2.9-1.2 kV. A model which explains the electron emission from the positively charged plasma is proposed. The model suggests screening of the hollow-anode grid potential by the space charge of ions oscillating through the hollow-anode output grid. These oscillations occur inside the potential well formed between the hollow-anode plasma boundary and a virtual anode formed inside the accelerating gap. Generation and characterization of a high-current electron beam with current amplitude of ∼1.2 kA was achieved under an accelerating pulse amplitude ≤300 kV and ∼400 ns pulse duration.