An achromat for the ANU 14UD linac

A compact magnetic achromat has been designed and constructed to deliver the horizontal output beam of the ANU 14UD Pelletron tandem accelerator to a superconducting booster accelerator to be located in part of the experimental hall of the laboratory. The achromat provides 90° deflection of the ion beam and is fully achromatic with respect to energy spread in the beam. Due to space constraints in the laboratory, it has been necessary to locate the beam chopping device and bunching cryostat upstream of the 90° bend, thereby requiring that the beam trajectory following the bend be independent of beam energy. The optical performance of the achromat has been investigated in first order using the matrix transfer beam calculation code TRANSPORT, and in high order using the particle tracking code RAYTRACE. In first order, the achromat is shown to have precise achromatism and to be isochronous with the exception of small and predictable time waist shifts. High order calculations lead to an expectation of less than 6% worsening of the transverse beam emittance and less than 9 ps timing degradation for 170 MeV ⁵⁹Ni¹³⁺, an isotope of interest in accelerator mass spectrometry. The effect on the transmission of this isotope through the subsequent acceleration stages and beam-optical elements is negligible.