A miniatured built-in surface ionization oven for the compact microwave ion source

Abstract At Peking University (PKU), a compact 2.45 GHz microwave ion source was developed to obtain high-intensity Li+ beam. To improve the generation efficiency of Li+ ions, we developed a built-in lithium oven named surface ionization oven (SIO), which consists of a heating oven for lithium vapor production and a surface ionizer for the ionization of lithium neutrals. Consequently, the surface ionization and electron cyclotron resonance heating (ECRH) principles are combined within the compact microwave ion source. In this work, the structure and materials of the SIO are investigated systemically to lower thermal risk and increase ionization efficiency. The oven is heated by resistance heating. To minimize the heat loss, polished stainless steel shields are used to surround the oven. The surface ionizer is installed at the exit of the nozzle of the heating oven. It consists of six rhenium wires with the diameter of 0.25 mm inside the ionizer tube. Limited space near the microwave window, the dimension of SIO is ⊘ 13.5 mm × 40 mm. This source has been tested with only heating oven and SIO. With only heating oven, which means the absence of surface ionization, and a 200 eμA pulsed Li+ ion beam was stably extracted from this ion source. When the surface ionizer and the heating oven work together, a pulsed Li+ ion beam of 800 eμA is obtained, which increased by about 300%. The experiment proved that surface ionization could dramatically increase the discharge efficiency of lithium plasma in ECRIS.