10 saw 1.1
11 \begin{document}
12 The Hall C Beam Current Monitors (BCMs) are calibrated using an Unser monitor
13 located in Hall C as a reference. These
14 calibrations are used both in the EPICS system which displays the BCM1 and
15 BCM2 currents and in data analysis. This document describes how to make these calibrations
16 \begin{abstract}
17
18 \end{abstract}
19
20 \section{Background}
21 This section will contain background information on how the Unser monitor,
22 and BCM's work as well as the electronics that are used.
23
24 In summary:
25
26 There are three devices used in current measurement in the
27 Hall. Once calibrated, the two BCM's are stable and linear over a wide
28 current range. However, the BCM's, being cavity resonant devices are only
29 sensitive to CW beam and so can not be bench calibrated with known current
30 sources. Thus the BCM's are cross-calibrated with an Unser Monitor\cite{bi:unser}
31 saw 1.1 (Direct Current Transformer). The Unser Monitor has been calibrated with
32 known currents in wires, however, the ``offset'' on the calibration is
33 noisy and the time average offset drifts in unpredictable ways. By
34 measuring a series of currents over a short period of time, this drift can
35 be minimized so that calibrations can be transferred to the BCM's.
36
37 The outputs of the front end electronics for these three devices are small
38 voltages. These voltages are digitized by the use of V/F (voltage to
39 frequency) converters. Scalers are used both in the control system
40 (EPICS) and in the data acquisition to record these digitized signals.
41
42 \section{Obtaining a BCM Calibration}
43
44 Obtaining a BCM calibration consists of two steps that cross calibrate the
45 BCM's to the Unser Monitor. First the response of the Unser and the BCM's
46 is measured at a series of beam currents that cover the range of beam
47 currents to the current experiment. Second those measurements are
48 analyzed to produce calibration constants for the BCM's.
49
50
51 \subsection{Data Collection}
52 saw 1.1
53 The following is the recommended method of acquiring BCM calibration
54 data. This procedure was designed for the Spring 2003 Hall C running.
55 For other
56 experiments the procedure may be modified, depending on the range of
57 currents required by the experiment, but the procedure will be similar.
58
59 \begin{description}
60 \item[Goal] The goal is to perform a current monitor calibration over the range
61 10 to 100 $\mu$A.
62
63 \item[Estimated Duration] 1 hour
64
65 \item[When] Soon after startup, but reliable high current beam is needed.
66
67 \item[Impact on other halls] The calibration is normally invasive because, without
68 turning off the other Hall lasers, it is the only sure way to get the required
69 0.000 microA needed for the Unser zero offset measurements.
70 \end{description}
71
72 \begin{enumerate}
73 saw 1.1
74 \item The Run Coordinator needs to make pre-arrangements with the other
75 halls since we're shutting them off for one hour.
76
77 \item Put in either of the 4 cm cryotargets. The choice of target isn't
78 critical, but these are least likely to trip off the beam due to
79 excessive dose rates in the ion chambers.
80
81 \item Find out if the MCC can deliver 100 microA. If they can't, the Run
82 Coordinator needs to decide whether it's worth proceeding.
83
84 \item Make sure our data acquisition is working, and that BCM and CLOCK
85 scalers are counting. Prescale away most spectrometer triggers so the daq is
86 less likely to crash in the middle of the calibration.
87
88 \item Ask the operator to turn off non-Hall C lasers.
89
90 \item Start a "BCM Calibration Run" now before you forget.
91
92
93 \item Tell the operator your nominal current cycle will be:
94 saw 1.1
95 0, 10, 0, 20, 0, 30, 0, 40, 0, 50, 0, 60, 0, 70, 0, 80, 0, 90, 0, 100,
96
97 and should then be repeated. Each current setting should be 1.5-2 minutes
98 duration.
99
100 If the green light is flashing on the scaler crate, you're probably taking
101 data. The files are usually small enough to fit in the daq buffer, so the
102 output .log file will be nearly empty until you finally stop the run.
103
104 \item (FINAL): After the calibration run is over, please replay completely,
105 taking care to output the charge scalers \verb|via charge####.txt|.
106
107 \end{enumerate}
108
109 \subsection{Data Analysis}
110 At this point, data analysis must be done by a BCM expert (Dave Mack).
111
112 The basic principle of the analysis is as follows. The Unser monitor has
113 a well known calibration, but it has an offset that is both noisy and
114 drifts with time. In taking the calibration data, each different beam
115 saw 1.1 current is bracketed by a period of beam off so that the drifting offset
116 can be well determined for each different beam current used in the
117 cabibration. With these offsets determined, the average current for each
118 nominal current is well determined and the unser calibration can be
119 trasnferred to the BCM's. The BCM's are not linearly, particularly at
120 small currents, so the calibration fit is made using only currents over
121 the range that is required by the experiment. Typically this means that
122 zero current is excluded from the fit.
123
124 \section{Using the BCM calibration}
125
126 The result of the BCM calibration is a straight line fit, a gain and a
127 slope, that converts the V/F frequencies or total counts measured by DAQ or controls
128 scalers into current or total charge. (Usually the unit is
129 microamps/microcoulombs, but some experiments may prefer nano
130 amps/coulombs.)
131
132 \subsection{EPICS}
133
134 The EPICS variables for Hall C beam current are \verb|ibcm1| and
135 \verb|ibcm2| (Accelerator also has copies of the signals known as
136 saw 1.1 \verb|hallc:bcm1| and \verb|hallc:bcm2|. The beam currents are
137 calculated by the EPICS IOC \verb|vmec15|.
138
139 Changing the EPICS BCM calibration should be done only at the request of
140 the Run Coordinator or a BCM expert. To change the EPICS calibrations,
141 logon to \verb|cvxwrks@cdaqs1| and do the following.
142 \begin{verbatim}
|
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