FRIB will need a production target different than what is presently used at NSCL. The beam power is 2-3 orders of magnitude higher than the Coupled Cyclotron Facility. Static targets, as now used in the A1900, will not work and even single rotating targets would not meet the requirements of FRIB. Extensive R&D led to a production target concept based on multi-slice rotating targets made of graphite that are radiation cooled. FRIB’s targets will be about 30 cm in diameter and rotate with 5000 rpm. With FRIB’s 400 kW beam power up to 10 kW will be absorbed in each target slice, leading to temperatures of about 1900oC. The target is required to survive these conditions for about two weeks.
A 50 kW multi-slice target prototype has been built that has all the features of the anticipated final target system for FRIB. It has five target slices that rotate inside a copper heat exchanger. The target prototype design was based on extensive earlier successful studies with single slice targets bombarded with a low-energy electron beam. However, in order to make a meaningful test of the multi-slice target a high-energy high-power electron beam was needed that is available only a few places worldwide. For the tests of the FRIB prototype target, the Budker Institute for Nuclear Physics (BINP) in Novosibirsk, Russia was chosen.
Members of the Target Systems Group (Wolfgang Mittig, Mikhail Avilov, and Frederique Pellemoine) successfully collaborated with BINP on the 50 kW prototype tests at Novosibirsk with a 1 MeV electron beam in the period from 23 September to 15 October. A beam power of up to 40 kW was used without problems for the target, leading to a power deposition of 10 kW in a slice, which is the maximum expected at FRIB for rare isotope production with the heavier primary beam at 400 kW beam power.
In addition to validating the design concept for the multi-slice rotating graphite target for FRIB as a complete device, the tests also provided valuable information on further design improvements of the heat exchanger, and the targets themselves.