Ion source improvement by electron injection from a ferroelectric cathode

With increasing rf power, the electron concentration in the plasma of electron cyclotron resonance (ECR) ion sources is decreasing in comparison to the ion concentration, so that the plasma is charging up positively. When reviewing the basic performance requirements of ECR sources it becomes evident that the direct injection of electrons into the ECR plasma is increasing the electron charge density and the ion current yield. Ferroelectric ceramics was used as very robust, electron emitting cathode material under the heavy-duty conditions inside the plasma chamber of an ECR ion source. The electron emission from the ferroelectric cathode is turned on by a high repetition-rate bipolar pulse of +/-1.2 to 1.6 kV amplitude to the electrodes deposited on both sides of the disk-shaped cathode. Lead-barium-zirconium-titanate (30/70/30) cathodes doped with 2 mol % Bi2O3 were installed and tested in the Ar-ion plasma of the ECR ion source CAESAR at INFN-LNS, Catania. The aim was to visibly increase the yield of the ion output current by electron injection. The plasma was heated by rf in continuous mode, while the ferroelectric cathode was pulsed at a repetition rate of 50 to 250 Hz. Pulsed electron injection at 250 Hz led to a noticeable increase of 30% of the Ar-8+ output current at a rf power level of 400 W. In addition, magnetohydrodynamic instabilities were damped during and after electron injection. These results strongly encourage further research and development on the application of ferroelectric cathodes for increasing the ion current yield from hot plasma ion sources.