Current

The Beam Current Monitor (BCM) is designed for stable, low noise, non-intercepting beam current measurements. It consists of an Unser monitor, two rf 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 m upstream of the target.

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 not used anymore.

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 uA to somewhere well above 200 uA. 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.

There are 6 scaler channels (17-22) which are responsible for beam information in both helicity gated and non-helicity gated scalers. Channels 17-19 handle information form upstream BCM gained by factors 1, 3 and 10 respectively. Channels 20-22 is the same for downstream BCM. Each scaler event is appears in data with event number 140. Scalers provide the number of counts. To get the current value one should know the conversion between number of counts in scalers and actual current value. That conversion include the offset (VTOF-module has some constant frequency even when beam current is 0) and gain-factor. Both of these coefficients can be received by calibrating scaler data with epics read out of BCM. To got the offset one have to read out number of counts in scaler when epics shows that beam current is zero. To got the gain-factor we have to compare scaler read-out value with epics data on different beam current values.

To got the total charge accumlated during run, one have to use 105 KHz clock. There are 2 variables in data-file used for that purpose one is clock - which is 105 kHz clock and another one is dclock - which is duration of one scaler event in 105 kHz clock cycles. Charge accumulated during one scaler event can be calculated by following equation:


Multiplying that by number of scaler events one can get total charge accumulated during run.

Following pictures reproduce linearity of beam information from u1, u3, u10 one to each other and to the triggers rate.

U3vsU1	U10vsU1
T1vsU3	T2vsU3	T3vsU3

T3(coincidence) trigger rate proportional to the product of T1(Neutron Arm) and T2(BigBite) triggers rate therefore T3 trigger rate has quadratic dependence on current.