BPM  Beam Position Monitor

Description from the Design Manual:

5.9.4 Beam Position Monitors

5.9.4.1 Scope

The Beam Position Monitor System will provide closed orbit measurements for the Ring.

Stripline detectors will be located in 44 of the 52 quadrupoles.

5.9.4.2 Design Requirements

Because of the large number of detectors, the beam position monitors (BPMs) must

present low impedance to the beam to prevent instabilities. Calculations indicate that at 1 x 10¹⁰ protons of circulating beam, circuit noise will provide a resolution of 1- mm RMS, improving to 50 microns at 2x10¹⁴ protons. Position measurements will be made on a turn-by-turn basis with data from multiple cycles stored locally for recall if a beam fault should occur. Data for

computation of the closed orbits will be available from the front-end processor on each cycle if

desired.

5.9.4.3 Design Description

The dual plane stripline BPMs will be located adjacent to the quadrupole, nestled under

the coil ends. It will be keyed to the quadrupole for positive location. The design will make use

of techniques developed in the RHIC units. The electrodes will be 25 cm long and subtend an arc

of 70 degrees.

The injected beam will have the basic character of the signal in the HEBT for the first few turns. As the particles diffuse they will eventually take on the character of a single bunch rotating around the Ring with a period of 945ns and a duration of 645ns. To monitor this beam, two distinctly different types of processing electronics will be required. For the early turns, electronics similar to that of the HEBT must be employed. This is an RF sensitive front end system provided by LANL. A number of BPMs using this approach will be required to monitor the first few turns. After the particles have had time to diffuse, the RF character will be reduced and a base-band sensitive front end system will be required. It is estimated that the time required to establish a base-band signal is about 3 turns. The bulk of the electronics provided for the Ring will be of the base-band design. A dual mode design is under investigation.

The base-band analog electronics will consist of an impedance matching front-end

diplexing 0.1-5 MHz filter, followed by an amplification stage. Another low pass filter

precedes the 14-bit ADCs that sample the signal at about 68 mega-samples/second (MSPS) providing an anti-aliasing capability. The over-sampled data is then stored in on-board shared memory. The data will be digitally processed at the front end to provide the position information. The local memory will be of sufficient size to store several cycles of position history data (useful following a beam inhibit/dump). Upon request, averaged, single revolution or the full memory may be obtained.

The wide dynamic range in signal will be covered in 8 ranges. Analog switches

will direct the signal through amplifiers providing 0dB to 70dB of gain (in 10dB steps).

Triggering should be straightforward since the rf is of constant frequency. Delay modules

with separate channels for each BPM and nano-second setability will trigger the ADC transfers.

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