FRIB sets record for highest uranium beam energy in linear accelerators

02 June 2021

On 2 June, FRIB accelerated a uranium-238 beam (238U37+) to a beam energy of 20 million electron-volts per nucleon (MeV/u), setting a record for the highest uranium beam energy in linear accelerators (linacs).

The beam energy of 20 MeV/u is beyond the design value of 17 MeV/u for charge-stripping optimization.

FRIB is the first operating accelerator facility to use liquid lithium to charge-strip heavy-ion beams. The charge stripper helps FRIB’s linac achieve design-goal beam energies beyond 200 MeV/u and beam power up to 400 kilowatts (kW). The heavy ions produced in the ion source have too low of an electrical charge to be accelerated efficiently. The charge stripper strips as many excess electrons as possible from heavy ions to make them electrically more positive and accelerate them efficiently. The beam passes through molten liquid lithium, where collisions between beam electrons and lithium nuclei remove nearly all of them from the beam.

The linac’s cryomodules contain superconducting radio-frequency (SRF) resonators that accelerate the beam while operating at temperatures a few degrees above absolute zero. Heavy ion beams speed up slowly starting from rest. The first three cryomodules in the linac accelerate the beam to 1 percent of FRIB’s final beam energy of 200 MeV/nucleon, the first 15 cryomodules to 10 percent, and the remaining 31 cryomodules provide the other 90 percent of FRIB’s final beam energy.

The acceleration was part of FRIB process for commissioning the beam at linac segment 1 (LS1). Commissioning involves integrated testing of individual devices and beam commissioning of devices working together. As FRIB prepares for operation, several stages of commissioning are conducted to demonstrate readiness of the different segments of the accelerator. Integrated testing examines the functionality of the system.

During the 238U37+ beam-acceleration, the SRF resonators of LS1 operated at 10 percent above the design value.