Performance of a Pulsed Ion Beam with a Renewable Cryogenically Cooled Ion Source

Summary form only given. A magnetically insulated diode configuration is commonly used as a source of high-power ion beams. A desirable ion source operated within such a diode should feature high-purity of selectable ions, and localize ion formation within the anode-cathode (A-K) gap to maximize the potential for high-brightness operation. We have developed a pulsed ion source based on the use of refrigeration of the anode, followed by the freezing of gas upon the anode surface. The operation of such a 'cryo-diode" has been previously demonstrated , and like the earlier version, the source is operated within a magnetically insulated ion diode in extractor geometry. We have replaced the use of liquid He dewar cooling in the previous versions. by the use of a lower cost and more convenient closed-cycle renewable He cooling source, namely a cryo-pump connected directly to the anode. The ultimate operating temperature of the anode surface then is determined by the cooling capacity of the cryo-pump, as well as the thermal insulation of the anode with respect to the surrounding environment. This allows for both repetitive and long-term operation of such an ion source. Two versions of this cryo-diode have been operated, the first with a 7.5-watt cryo-pump, and the second a 60-watt version. In the first case, the anode surface temperature reached 60K, and Xe was introduced from the cathode side and frozen onto the anode. We observed the formation of a pure Xe+1 ion beam, as determined from time-of-flight (TOF) measurements. A 30 A/cm² peak current density was observed at 33 cm propagation distance (in vacuum). The beam showed prompt turn-on, but was of shorter duration than the power pulse, suggesting source-limited behavior. In the second 60 watt series, anode surface temperature was reduced to 32K, allowing for freezing out of CH₄, Kr, Xe, and Ar. The beams produced in this series were of full power pulse duration (100 ns). The beam from CH₄ was the most powerful, reaching 1.2 J/cm² on axis of mostly C+l, with 700 A total beam indicated at 45 cm distance within 6 cm radius. Beam turn-on delay appears to be 20 ns or less, but peak current densities listed are only attained after repeated pulsings from a given ice loading, up to 24 in the case of Xe ice. The beam puritv was observed to remain constant despite noticeable discoloration of the Xe ice.