Ribbon Beam Electron Gun Based on Discharge with Extended Hollow Cathode

Earlier we reported about positive experience of plasma electron gun design for ribbon beam generation in pressure range 1 -10 Pa. Parameters of the beam are: cross section 30 x 1 cm², total current up to 0.6 A, electron energy 2-8 keV. These figures make the gun useful for some applications such as material surface treating and plasma chemical reactions as in gas phase, so at the surface. At the same time beam current density (~ 20 mA/cm²) can´t be proposed sufficient, because reactions velocities are too low. Two ways exist for beam current density increasing. One is taking up of total beam current, and the other is reducing of beam cross section. Second way may be useful if there are limitations in power supplies. The objective of our work was investigation of possibilities of beam current density growth by second way. For this aim split aperture width in extended hollow cathode was decreased. Simultaneous narrowing of window in emission electrode leads to positive result -current density grows. However if aperture width becomes less than any value, determined by experimental conditions, ribbon beam converts to several narrow streams, which can chaotically move along hollow aperture. Further investigations showed that reason of this conversion is reconstruction of hollow cathode discharge itself due to which local plasma density maximums occur in hollow. These local maximums appear as a result of ion sheath opening in split aperture of hollow cathode. The process can take place if sheath thickness is comparable with width of split aperture. This opening has casual character because of plasma density fluctuations and mechanism of positive reverse connection between plasma density and ionization process intensity. Method of suppression is plasma density rising either by discharge current growth or by hollow surface area decreasing. Both these measures allowed to rise beam current density two times value at beam uniformity along hollow split n- ot worse than 15%.