Beam position monitor tests show required accuracy

Located inside the superconducting radio frequency (SRF) cryomodules, cold beam position monitors (BPM) are critical for threading beam through the 330 superconducting cavities planned for FRIB. To demonstrate signals from the cavity will not jeopardize position measurements, tests were conducted last week. A BPM supplied by Fermi National Accelerator Laboratory was mounted to a beta = 0.041 quarter-wave resonator, cooled to 4.5 K, and excited to peak surface electric fields of 45 MV/m which produced high levels of field emission.

The measurement was sensitive enough to see signals only 6 db above thermal noise, 50 one billionths of a volt, and no cavity induced signals were observed demonstrating position errors less than 3.1 um (millionths of a meter) at 10 percent of the design beam intensity. This is thirty times smaller than the resolution required to safely operate the two million watts of beam power planned for FRIB.