Beam accelerated through first ReA3 cryomodule

Beam commissioning of the ReA3 reaccelerator continued last week through the first cryomodule. On Friday, April 14, an ion beam was accelerated using the first cryomodule of the ReA3 reaccelerator. This is the first ion beam acceleration through an SRF cavity at MSU.

ReA3 is designed to accelerate rare isotope beams produced by NSCL’s Coupled Cyclotron Facility and will be part of FRIB. Once completed, ReA3 will serve the nuclear science community by delivering a wide variety of exotic isotopes for nuclear science experiments at variable energies with high beam quality.

A He+ beam from the pilot ion source was accelerated by the room temperature Radio Frequency Quadrupole (RFQ) to 600 keV/u and then further accelerated through the first SRF cavity in the rebuncher to about 640 keV/u. To determine the absolute energy gain and energy spread of the beam, a time of flight diagnostic will be necessary and is under development.

For these beam tests the rebuncher SRF cavity designed for an optimal beta of 0.041 was operated in acceleration mode. The kinetic energy of the accelerated beam particles after the cryomodule was measured with a foil silicon detector. Figure 1 shows the energy spectrum from the accelerated beam using only the RFQ (SRF cavity is grounded, red trace) together with the spectrum obtained when the SRF cavity is energized to a gradient of about 8 MV/m (blue trace). Also shown in the spectrum is the peak from the calibration alpha source, which is permanently mounted off axis in the diagnostic box. The calibration source provides alpha particles at a total energy of 5.5 MeV and is used to calibrate the detector. 

In addition, the beam energy was measured relative to the cavity phase with respect to the beam provided by the RFQ. As expected, the energy gain is proportional to a cosine-like function with the maximum energy gain at 0 phase (Figure 2, 0 phase corresponds to 260° in the graph). By measuring the energy gain as a function of the cavity phase, the phasing between the RFQ and the SRF LINAC can be calibrated.

The commissioning will continue through the next cryomodule using the same procedure to determine the relative phase for each cavity.