A Profile Monitor (PM) consists mainly of a tungsten rod or wire driven across the beam by a motor. The position is monitored by a linear potentiometer and the current produced by the collected electric charge is amplified with a logarithmic amplifier. A Beam Position Monitor (BPM) is a high frequency (HF) device. An electronic device switches (with a frequency of about 1 kHz) between the two 100 MHz HF pickup coils (left/right or up/down) and a superposition radio wave receiver and demodulator produces a position dependent DC signal (design sensitivity = 100 mV/mm). 2 Beam Position Monitors are mounted together with 2 Profile Monitors into the same vacuum box. For a simplified scheme or diagram of the two monitor types see Fig. 1 (14 kB). If you need more detailed information about beam diagnostic elements please contact the PSI Diagnostic group.
Note: Because a Beam Position Monitor is a HF device, it can only be calibrated accurately (slope and offset) with the help of the proton beam and by comparing the measured output voltages with the profile centers computed (first moments) with the data originating from the corresponding profile monitor and taken at the same time. Fortunately a profile monitor can be calibrated without the proton beam, because all relevant parts are mechanical (the usage of a precision linear potentiometer is essential). Fig. 2 (5 kB) shows a typical 2D-scatter plot of beam center data collected with a profile monitor / beam position monitor tandem (MCP20/MCS20). The vertical axis represents the measured Beam Position Monitor's output signal in volts and the horizontal axis represents the computed profile monitor centers in mm. The dotted line is the least square fit through all the data points. The plotted data were taken with a periodic program running on a PDP-11/40 and capable of measuring and storing the beam positions of up to 10 monitor tandems every 10 minutes. Data collected and displayed in such a way are showing if the BPMs behave as expected and are long-time stable or not. Usually the slope and the offset for each monitor are more or less different. In table 1 some of the main differences of the two monitor types are listed.
Table 1: Comparison of Profile Monitor (PM) and Beam Position Monitor (BPM) properties
Urs Rohrer on 6-Oct-2006