STUDY OF THE IMPROVEMENT ON THE PROPERTY AND WORKING LIFE OF FREEMAN SOURCE

The improvement of the properties and working life of the Freeman sour ce is described. The experimental results show that the magnetic mirro r field caused by a pair of mild steel circular plates, 15 mm in diame ter and 2 mm thick, placed outside the discharge chamber can increase the ionized efficiency of the gas feed. The atom-ion ratio of nitrogen was increased from 66% to 81%, and the heating filament current was d ecreased about 30 A, from 130 A down to 100 A for a new filament of 2 mm in diameter at the same arc voltage, arc current, and external magn etic field condition. Additionally, a Mo shield cylinder 14 mm in diam eter put behind the reflect cathode surrounding the BN insulator preve nted the metallization of the BN insulators, and resulted in increasin g the stability of the arc discharge until the end of the filament wor king life of the Freeman source. The improvement occurred because it r estricted the deposition of the sputtered W and Mo atoms coming from t he hot filament and reflect cathodes onto the BN insulator, hence main taining a high resistance of 70 MOMEGA between the anode and cathode. The new design with a combination of the cathode shield cylinder and m agnetic mirror field improved the running property of the Freeman sour ce, the mass spectrum of nitrogen, and the working life of the Freeman source, to about 40 h.