Enhanced Electron Emission and Its Reduction by Electron and Ion Impact

Broad area electrodes show enhanced electron emission at fairly low dc and rf fields E ~ 107 V/m, thus preventing the attainment of higher electric field strengths E. The enhanced electron emission is initiated by hot electrons, which gained their energy fdxeE in dielectrics (¿dust¿). Due to their excess energy, hot electrons have an enhanced emission probability, which is enhanced further by resonant tunneling via adsorbed H2O. The emitted electrons gain energy fdxeE in the vacuum and then impinging on surfaces, giving rise to secondary electrons and atoms. These excited electrons and atoms are partly emitted into the vacuum and gain additional energy in the electric field producing more secondaries and so on, preventing the attainment of high field strength between the electrodes. The enhanced electron emission can be reduced by a thin layer of unsaturated, polymerized hydrocarbons coating the electrodes, which capture electrons and transfer their energy to the phonons. This layer of unsaturated, polymerized hydrocarbons is formed from adsorbed hydrocarbons by electron, photon or ion impact, which desorb hydrogen from C-H bonds. This explains on one hand the higher field strength reached after conditioning and on the other hand the plasma development causing discharges. Ion impact conditions mainly by sputtering H2O away and ¿switchingoff¿ the dielectric particles, reducing the enhanced emission. Both conditioning mechanisms reduce the enhanced secondary and field emission by several orders of magnitude.