System Layout

The schematic diagram of the BCM system is presented in figure 2.5.

Figure 2.5: Schematic of the Hall A beam current measurement system.

1.

The Unser monitor is a Parametric Current Transformer designed for non-destructive beam current measurement and providing an absolute reference. The monitor is calibrated by passing a known current through a wire inside the beam pipe and has a nominal output of 4 mV/$\mu$A. It requires extensive magnetic shielding and temperature stabilization to reduce noise and zero drift. As the Unser monitor's output signal drifts significantly on a time scale of several minutes, it cannot be used to continuously monitor the beam current. However, this drift is measured during the calibration runs (by taking a zero current reading) and removed in calibrating the cavities. The more stable cavities are then used to determine the beam current and charge for each run. We also use the OLO2 Cavity Monitor and the Faraday Cup 2 at the Injector section to provide an absolute reference during calibration runs.

The two resonant rf cavity monitors on either side of the Unser Monitor are stainless steel cylindrical high Q ($\sim 3000$) waveguides which are tuned to the frequency of the beam (1.497 GHz) resulting in voltage levels at their outputs which are proportional to the beam current. Each of the rf output signals from the two cavities are split into two parts. One part of the signal is converted to 10 kHz signals (by the ``downconverters'') and fed into an RMS-to-DC converter board consisting of a 50 kHz bandpass filter to eliminate noise, amplified and split to two sets of outputs, which after further processing are recorded in the data stream. These two paths to the data stream (leading to the sampled and integrated data ) will now be described. (The other part of the split signal is downconverted to 1 MHz signals and represents the old system (pre Jan 99). Only the HAPPEX collaboration presently uses these signals.)

For the sampled (or EPICS or Slow) data, one of the amplifier outputs is sent to a high precision digital AC voltmeter (HP 3458A). Each second this device provides a digital output which represents the RMS average of the input signal during that second. The resulting number is proportional to the beam charge accumulated during the corresponding second (or, equivalently, the average beam current for that second). Signals from both cavity's multi-meters, as well as from the multi-meter connected to the Unser, are transported through GPIB ports to the HAC computer where they are recorded every 1 to 2 seconds via the data-logging process which is described in the calibration procedure. They are also sent through EPICS to CODA and the data stream where they are recorded at quasi-regular intervals, typically every two to five seconds.

For the integrated (or VTOF or Fast) data, the other amplifier output is sent to an RMS-to-DC converter which produces an analog DC voltage level. This level drives a Voltage-To-Frequency (VTOF) converter whose output frequency is proportional to the input DC voltage level. These signals are then fed to Fastbus scalers and are finally injected into the data stream along with the other scaler information. These scalers simply accumulate during the run, resulting in a number which is proportional to the time integrated voltage level and therefore more accurately represents the true integral of the current and hence the total beam charge. The regular RMS to DC output is linear for currents from about 5 $\mu$A to somewhere well above 200 $\mu$A. Since it is non-linear at the lower currents, we have introduced a set of amplifiers with differing gains (x3 and x10) allowing the non-linear region to be extended to lower currents at the expense of saturation at the very high currents. Hence there are 3 signals coming from each BCM (Upx1, Upx3, Upx10, Dnx1, Dnx3, Dnx10). All 6 signals are fed to scaler inputs of each spectrometer (E-arm and H-arm) . Hence we have a redundancy of 12 scaler outputs for determining the charge during a run. During calibration runs we calibrate each of these scaler outputs.