The AMIT superconducting cyclotron uses cathodes made of pure tantalum to generate high-energy H- ion beams for the production of isotopes for positron emission tomography. During service, the cathodes are immersed in a pure hydrogen atmosphere and are locally impacted by back-accelerated ions from the plasma, which generates a crater-erosion profile in the cathode head region that is subjected to ion bombardment. This leads to a degradation in the performance of the electrodes, which need to be replaced periodically. This study investigates Laser Metal Deposition as a means of repairing them.

Characterisation of representative damaged parts through 3D imaging, scanning electron microscopy, and Vickers microhardness revealed that localised melting, abnormal grain growth, deformation through grain boundary sliding, and localised precipitation of tantalum nitrides occur in the region of the electrodes exposed to ion bombardment. A progressive embrittlement and increase in hardness (from ~100 HV to more than 700 HV) were also observed moving from the base to the head region of the cathodes. They were attributed to the entrapment of large amounts of hydrogen in trapping sites generated by the impact of high-energy ions.

Different repair strategies were developed considering the extent of damage of each cathode. Both high-purity tantalum wire and powder feedstocks were employed to demonstrate the ability of Laser Metal Deposition to restore the damaged electrodes to the geometry required for their application in the AMIT cyclotron and potentially improve their service life. The refurbished parts displayed a higher hardness (~315 HV and ~665 HV for wire and powder processes, respectively) than the original material, due to both dissolved oxygen and dispersed fine oxide particles providing solid-solution and precipitation strengthening effects, respectively. This may enhance the resistance of the electrodes to the impact of high-energy ions during operation in the cyclotron.