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The ISIS spallation neutron and muon facility is undergoing an upgrade to the negative hydrogen (H−) linac preinjector 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 the beam transport efficiency significantly, reducing the output H− current requirements from the ion source. As such, a long-lifetime, noncesiated, RF-driven, external-antenna H− ion source based on the successful CERN Linac4 and SNS designs is constructed, which will improve facility uptime and reliability. This paper will highlight the 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 a 50 Hz repetition rate, with transverse normalized 4 rms emittances less than 1.2 π mm mrad. The beam current and emittance are within the reach of a noncesiated H− source, whereas operating at relatively high duty cycles presents challenges in terms of thermal management, which this paper will address. [ABSTRACT FROM AUTHOR]

The ISIS Penning ion source can routinely produce a 55 mA beam of negative hydrogen ions in 250 µs pulses at 50 Hz repetition rate. Extending the beam pulse length to 2 ms requires eliminating the 15%–30% droop of the current observed in long pulse operation and benefits from the suppression of discharge breakdown oscillations otherwise forcing to prolong the pulse length to 2.2 ms or longer. The droop can be compensated by ramping the discharge current during the pulse, whereas the discharge oscillations are suppressed by modifying the ancillary circuit of the pulsed arc power supply. [ABSTRACT FROM AUTHOR]