Thursday, November 15, 2012
Steering high power ion beams cleanly through the FRIB driver linac and onto the rare isotope production target will be essential to achieving the FRIB scientific objectives. The beam position monitoring system will be the on-line diagnostic to measure position of the ion beam in the vacuum chamber at nearly 150 locations along the linac and the beam transport lines.
Beam position monitors (BPMs) that are planned to be utilized in the FRIB linac in the future underwent three series of tests in recent months in a collaboration among the ReA commissioning team, the FRIB diagnostics group, and the diagnostics group at Fermi National Laboratory. The first test was in June and was a preliminary test to confirm the signal levels available at the BPM output. The test revealed that noise issues were a problem for the BPMs, caused by the RF frequency and the harmonics from the RFQ amplifier that limited beam signal detection. After the tests, the amplifiers were modified by adding shielding and electromagnetic interference (EMI) filters in an attempt to solve the problem.
Those improvements were tested in August during the second test, which confirmed the noise issue had been resolved. The linearity was determined, as was the lower detection limit with respect to beam current of the system, which was above one microampere beam peak current. Also, a first test to detect beam motion with the BPMs showed good results.
The third test was run in September, which – down to 50 nanoamperes – successfully measured the beam signal’s intensity and phase relative to the master oscillator. Prior to this test, improvements to the software were made that allowed the BPMs to measure the beam signal simultaneously from both BPMs relative to the RF-clock, which enabled a first time-of-flight measurement. The beam energy was changed by changing the RF phase of the last CM2 cavity and observing the phase change between the two BPM detectors. Preliminary results show the resolution for this measurement to be better than one degree. With these highly sensitive BPM diagnostics devices, it will now be possible to develop these diagnostics for ReA pilot beams also. When fully implemented, these devices will very beneficial for future ReA linac operations.