Ion extraction from a saddle antenna RF surface plasma source

Existing RF Surface Plasma Sources (SPS) for accelerators have specific efficiencies for H ⁺ and H ^- ion generation around 3 to 5mA/cm ² per kW, where about 50 kW of RF power is typically needed for 50mA beam current production. The Saddle Antenna (SA) SPS described here was developed to improve H ^- ion production efficiency and SPS reliability and availability. At low RF power, the efficiency of positive ion generation in the plasma has been improved to 200mA/cm ² per kW of RF power at 13.56 MHz. Initial cesiation of the SPS was performed by heating cesium chromate cartridges by discharge as was done in the very first versions of the SPS. A small oven to decompose cesium compounds and alloys was developed and tested. After cesiation, the current of negative ions to the collector was increased from 1mA to 10mA with RF power ∼1.5 kW in the plasma (6mm diameter emission aperture) and up to 30mA with ∼4 kW RF power in the plasma and 250 Gauss longitudinalmagnetic field. The ratio of electron current to negative ion current was improved from 30 to 2. Stable generation of H ^- beam without intensity degradation was demonstrated in the AlN discharge chamber for a long time at high discharge power in an RF SPS with an external antenna. Continuous wave (CW) operation of the SA SPS has been tested on the small test stand. The general design of the CW SA SPS is based on the pulsed version. Somemodifications weremade to improve the cooling and cesiation stability. The extracted collector current can be increased significantly by optimizing the longitudinalmagnetic field in the discharge chamber. CW operation with negative ion extraction was tested with RF power up to 1.8 kW from the generator (∼1.2 kW in the plasma) with production up to Ic=7mA. Long term operation was tested with 1.2 kW from the RF generator (∼0.8 kW in the plasma) with production of Ic=5mA, Iex ∼15mA (Uex=8 kV, Uc=14 kV).