The Beam Current Monitor (BCM) is designed for stable, low noise, non-intercepting beam current measurement. It consists of Unser monitor, two cavities, the electronics and a data acquisition system. The cavities and the Unser monitor, are enclosed in a box to improve magnetic shielding and temperature stabilization. The box is located 25 meters in front of the target. It is the first beam line instrument in Hall A, and you can recognize it as grey object on the stands, about 2 meters from the wall of the hall.
The purpose of the Unser monitor is to calibrate the cavities, which are used only for relative current measurement. Since the Unser, which is a parametric current transformer, has large error at low current, the calibration of the cavities should be done at currents higher than 100 A (or as high as possible). At 100 A the error of Unser is 0.4%, at 50 A it is 0.8 %, at 25 A it is 2.5 % at 10 A it is 8% and at 1 A it is about 42 %. However, the cavities have high linearity, and by performing their calibration at high current it is possible to obtain accurate measurements also at low current. At 1 A, the accuracy can be as good as 1.5 %.
The schematic diagram of the BCM system is presented in figure2.2.2.1 One can see that the signal from Unser, after going through the front and the back PCT (Parametric Current Transformer) electronics is measured by a HP 3458 A multimeter. There is also a current source connected to the Unser, used for its calibration.
The high frequency, 1.457 GHz signals from cavities are fed to DC 200 down converters, where they are transformed to 1 MHz signals that are later filtered and amplified by the RF unit and finally are measured by the HP digital multimeters.
Signals from both cavities multimeters, as well as from the multimeter connected to the Unser are transported through GPIB port (IP 488) to a computer, and then through epics to coda and data stream. In this moment the amount of charge is calculated by off-line analysis by integrating current during the time, where "go" and "stop" times are obtained from data stream. However on-line analysis will be established. In a final design GPIB signal will go to VME crate and will be put in coincidence with "go" and "stop" time signals, which will make analysis of data easier and more accurate.
The DC 200 down converters and the Unser front end electronics are located in Hall A. The temperature controller, the Unser back end electronics and its calibration current source, cavity's RF unit and all multimeters, VME crate and computers are located in Hall A control room.