Characterising the Pelletron beam at the University of Melbourne

The Pelletron at the University of Melbourne is a DC particle accelerator that has been used for materials analysis and ion implantation since its installation in the 1970s. Today, it is primarily used to produce proton and helium beams up to 3.5 MeV and 1.5 MeV respectively, for three beamlines with a range of different experiments. The beam stability and fundamental beam parameters such as the transverse phase space distribution and emittance have not been previously investigated in detail. We have performed systematic measurements to determine the beam current variation, finding persistent fluctuations: our study determined that internal wire scanners used for monitoring the beam profile contribute to a periodic current drop. We also found that large terminal voltage oscillations reduce the average current. In addition, the phase space has been measured using a custom-built slit-grid apparatus. We found that the Courant-Snyder (Twiss) parameters are a function of the operational characteristics of the Pelletron, with the emittance more than doubling between measurements. To parametrise the beam despite the fluctuations, we introduce a 'minimum bounding ellipse' described by a set of Courant-Snyder parameters that is representative of the beam characteristics for all the measurements: for our data, this has beta = 8.2(5) m, alpha =-5.2(3), and an emittance of 0.34(3) mmmrad. These parameters will enable future beamline design in Melbourne, and give an indication for the requirements at other Pelletron facilities. Our characterisation techniques can provide a low-cost method to probe beam parameters for improvements in the operation and efficiency of similar DC accelerators, allowing for more accurate measurements and reduced data acquisition times.