The ISIS pulsed spallation facility at the Rutherford Appleton Laboratory has been delivering powerful beams of neutrons and muons for materials characterization studies since 1984. The negative hydrogen (H–) linac was upgraded in 2004 with the addition of a ‘pre-injector’ based around a 665 keV radio-frequency quadrupole (RFQ). A Penning-type caesium-enhanced surface-plasma ion source supplies the pre-injector with around 55 mA of H– beam current. Limitations in beam transport efficiency from both the ion source and between the RFQ and drift-tube linac (DTL) mean over 50% of beam current is lost between the ion source and synchrotron. Moreover, the Penning source lifetime is limited by cathode material sputtering inside the plasma discharge chamber. As such, facility operations must be stopped every two to three weeks to replace the ion source.
To address these issues, a project is underway to upgrade the pre-injector with the addition of a medium-energy beam transport (MEBT) line. A fast electrostatic sweep chopper is included in the MEBT and will notch the linac bunch train at the synchrotron frequency. The MEBT and chopper will increase beam transport efficiency significantly, reducing the output H– current requirements from the ion source. As such, a long-lifetime, non-caesiated, RF-driven, external-antenna H– ion source based on the successful CERN Linac4 and SNS designs is being constructed which will improve facility up-time and reliability.
This paper will highlight latest developments on the MEBT before focusing on the RF ion source. The RF ion source must deliver 35 mA of H– beam current in pulses 400 µs long at 50 Hz repetition rate, with a transverse normalised 4.RMS emittance less than 1.2 π mm mrad. The beam current and emittance are within reach of a non-caesiated H– source, whereas operating at relatively high duty cycles presents challenges in terms of thermal management. In particular, serious consideration must be paid to safe removal of a high current co-extracted electron beam. Other novel developments to be discussed include a low power electron source as a plasma igniter, a solid-state 2 MHz 100 kW RF amplifier, a 3D-printed cooling jacket, an adjustable permanent-magnet filter field and low energy beam transport (LEBT) beam tracking studies. The detailed ion source and LEBT designs are complete and first machined components are due at the end of 2019. Vacuum, high voltage and interlocks commissioning will start in Spring 2020, with first beam expected towards the end of 2020.
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