FRIB commissions liquid lithium charge stripper in linear accelerator

30 June 2021

FRIB has successfully commissioned its liquid lithium charge stripper installed in FRIB’s linear accelerator (linac). 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 mega electron volt/nucleon (MeV/u) and beam power up to 400 kilowatts (kW). The beam particles have too many electrons 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 though molten liquid lithium, where collisions between beam electrons and lithium nuclei remove nearly all of them from the beam.

From 8-13 April, FRIB conducted the world’s first beam-charge-stripping test using liquid lithium film. The charge stripper functioned properly on the first attempt. FRIB ran 124Xe26+ and 36Ar10+ beams through the charge stripper. Previously, lithium was not used as a charge stripper because of the technical difficulties of handling liquid lithium. FRIB has now demonstrated an effective engineering design for liquid lithium charge strippers.

FRIB also produced the maximum peak beam intensity that it could attain with the current accelerator configuration to test the thermal response of the film to a high-power beam. FRIB staff ran an argon-36 beam with the peak current of 12 particle microamperes (pμA) with 5.4-percent duty cycle. The equivalent beam power on target would be > 74 kilowatts (kW) if the duty cycle was 100 percent (average power at the lithium stripper was 400 W). Both film and beam were stable.

Subsequently, FRIB used the liquid lithium charge stripper to strip a uranium beam at 20 mega electron volt/nucleon (MeV/u) energy to an average charge state of 78+. 

The liquid lithium charge stripper is used instead of a conventional carbon-foil stripper because the expected beam intensity is too high for any solid strippers to withstand.

The project

In November 2010, the concept of making liquid lithium film was demonstrated at Argonne National Laboratory. A liquid lithium film was established for a short period of time and characterized. 

In April 2013, after FRIB restored the LEDA proton ion source from Los Alamos National Laboratory, the liquid lithium film was tested with the proton beam of 65 kilo electron volts (keV) at Argonne National Laboratory. The test equivalently produced 200 percent of the maximum power density deposition expected at FRIB. It also demonstrated the liquid lithium film’s excellent heat-removal performance.

In August 2018, FRIB successfully circulated liquid lithium and established a lithium film in its charge stripper. Following the first circulation in August, in September, the charge stripper completed 50 hours of continuous attended operations with round-the-clock monitoring by trained operators. The charge stripper operated safely, reliably, and stably.

In March 2019, FRIB successfully circulated liquid lithium in its charge stripper for more than seven days, relying only on remote monitoring. This new milestone of unattended operations demonstrated that the system can function without direct supervision by experts.

In June 2019, after the successful unattended continuous operation of the charge stripper, FRIB staff installed an electron-gun (E-gun) system into the stripper module. The E-gun system evaluates the thickness of the lithium film and measures the stability of the film.

In September 2019, FRIB’s electron-beam diagnostics system measured the thickness of the liquid-lithium film in the charge stripper for the first time at MSU.

In April 2021, the system was moved to the FRIB accelerator tunnel and successfully produced a stable and splash-free liquid lithium film. During the first commissioning run from 8 to 13 April, FRIB conducted charge-stripping experiments with the xenon and argon beams.

In June 2021, FRIB conducted the second commissioning run with the liquid lithium charge stripper to strip the uranium beam.