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Profile and Beam Position Monitor
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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
Property |
PM |
BPM |
data taking speed |
slow |
fast |
long-time stability |
good |
medium |
produces beam spill |
yes |
no |
gives beam width data |
yes |
no |
gives beam center data |
yes |
yes |

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Last updated by
Urs Rohrer on 6-Oct-2006
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