RHIC Wall Current Monitors (WCM)

The purpose of a Wall Current Monitor is to measure longitudinal beam shape. A Wall Current Monitor detector, very simply stated, is a ceramic break in the beam pipe which has many parallel resistors spanning the break, all surrounded by an enclosure which has ferrite dampening material to extend the bandwidth of the flat signal response from 3kHz out to 6GHz. The resistors are equally spaced around the gap so the monitor is insensitive to the beam position. The transfer impedance is 1 Ohm. The ion beam which passes through the beam pipe generates an equal current in the beam pipe wall called an image current. When this image current passes through the resistors spanning the ceramic break, a voltage signal is generated which is transported via high quality cable to a nearby service building for analysis. There are 3 Wall Current Monitors in RHIC, two are located in sector 2, one in each ring, these are used for beam diagnostics. Another is located in sector 4 which is used as an input to the radio frequency acceleration system.

RHIC Wall Current Monitor System Design PAC paper, pdf, 28k bytes

Longitudinal Emittance & Wall Current Monitor Design paper, Bob Shafer, BIW 1989, pdf, ~5M bytes

RHIC Wall Current Monitor used for the RF system block diagram, pdf.

RHIC Wall Current Monitor used for the RF system photo, jpg, ~200kbytes.

RHIC WCM Ring Device Photo, jpg format about 250k. In the photo the Wall Current Monitor is the gray colored cylidrical device located on the right side of the stand between the two identical copper colored cylinders which are microwave absorber gaps. These microwave absorber gaps consist of a 12" ceramic cylinders loaded with microwave absorbing material. They are used to attenuate high frequency image currents generated elsewhere when the beam passes non-uniform beam pipe apertures such as vacuum bellows, steps, and other discontinuities. These signals propagate in the beam pipe with a different velocity than the beam. Without the microwave absorbers, the Wall Current Monitor would respond to these high frequency "noise" currents. The wideband WCM signals are brought out of RHIC on 7/8" Heliax cables shown feeding into the cable tray above.

RHIC Wall Current Monitor Application Waveforms while running polarized protons, 2.6e12 total intensity in 56 bunches, energy at store 205GeV. Top half of screen is data (spectrum & raw waveforms of individual bunches) from the Blue ring, bottom half from the Yellow ring. jpg file 76k.

Questions or Comments? Contact Dave Gassner.