Linear accelerator
The reaccelerator (ReA3) will use a state-of-the-art linear accelerator (linac) with high transmission efficiency, flexible output energy and a robust capability to deliver high beam quality to the experiments. The linac will accelerate the beam to energies up to 3 MeV/nucleon for beams with charge-to-mass ratio of 1/4 using a radio frequency quadrupole (RFQ) and 14 superconducting cavities installed in two accelerating cryomodules.
A multi-harmonic buncher will pre-bunch the beam before it is injected into the RFQ, which is operating at 80.5 MHz. The multiple harmonics (80.5, 161 and 241.5 MHz) used in the buncher help to minimize the intrinsic non-linear effects during the bunching process. ReA3 users will be provided with beams of high quality having bunch length of approximately 1 ns and energy spread of approximately 1 keV/nucleon at the experiment. The high beam quality from the EBIT charge breeder upstream should also enable beam size of approximately 1 mm at the experiment.
After the buncher, the RFQ will accelerate the beam to 0.6 MeV/nucleon while preserving the beam quality. The electromagnetic design of this device was done at MSU while the mechanical design and construction of the device is done by researchers University of Frankfurt (led by of A. Schempp's group), world experts in RFQs who have manufactured more than thirty of them through the years.
After the RFQ, the beam will be injected into a superconducting linac using quarter-wave resonators operating at a frequency of 80.5 MHz for acceleration and a superconducting solenoid for transverse focusing of the beam. The design of ReA3 is very flexible and beam energies anywhere between 0.3 to 3 MeV/u can be delivered to the experiment.
The quarter-wave resonators have been developed during the R&D phase for FRIB and successfully tested at NSCL. The laboratory has grown its rf superconducting technology expertise for more than 8 years and multiple cavities of various types (quarter-wave, half-wave, elliptical, and so on) have been designed, built and successfully tested. NSCL is also actively collaborating with other facilities around the world, such as TRIUMF (Canada) and Legnaro (Italy) where superconducting quarter-wave resonators are already in operation.
The superconducting solenoids also were developed during the FRIB R&D phase and successfully tested. The shielding of the stray magnetic field produced by the solenoid was proven to be adequate through experimental study of a test cryomodule. Superconducting cavities have been successfully operated at their full accelerating gradient without being affected by the fringe magnetic field of the solenoid. Following the results from beam simulations, dipole winding steering magnets also were included in the design of the focusing solenoids to provide active correction of the beam trajectory, which can be thrown off center due to alignment errors of the focusing elements and accelerating cavities.
Simulation and design are 80% complete; procurement and construction are underway. Commissioning of the linac is expected in 2009.
Cryomodule layout for superconducting cavities being designed for the new NSCL reaccelerated-beam capability
Prototype cryomodule: prototype layout (left) and cold mass (right).
Operation of NSCL as a national user facility is supported by the Experimental Nuclear Physics Program of the U.S. National Science Foundation.