Measuring   the   Rastered   Beam   Spot   Size   on   Target

Robert Michaels, rom@jlab.org, Jefferson Lab Hall A, May 2001

This file:     hallaweb.jlab.org/equipment/daq/spot.html

• PURPOSE:   To display the beam spot on target in Hall A.

Additional diagnostic BPM histograms are generated in paw file bpmana.his.

• SIMPLEST USAGE:     Start a CODA run on L-arm and admire the picture.

When a new spectrometer DAQ run starts, histograms refresh. Note: The beam must be on ( > 1 microamp) for some time in the first few thousand events. If spot++ is not running, simply type "spot++" from either adaq or adev accounts on the ADAQ Sun computers in the counting room.

• MORE DETAILS ON USAGE:     spot++   [-f codafile -w waittime -n nevents -r -v -h -p]

Type "spot++ -h" for help on the optional arguments. You can analyze a specific "codafile" for a specific number of events "nevents", etc. If you don't specify a codafile, spot++ loops forever, taking the most recent data, clearing histograms when there is a new run. Histogram file is bpmana.his in the directory where you ran spot. Several histograms exist for detailed checkout, and pedestal runs, etc. Spot++ runs only on SunOS at the moment, not Linux. You must start a new run to refresh histograms.

•  COMMON SENSE TIPS :
A few common sense tips about raster and spot size:   Do not confuse the "rastered spot size" (of order 4 mm) with the "intrinsic spot size" (typically 10 times smaller). This has been known to confuse people, though its hard to imagine why.   If your purpose is to check the spot size and dwell time to avoid destroying the target, then it makes NO sense to test at high current !! Test at low current, say 1 microamp, before going to high current on target.   You can "h/pl 502 lego" in paw using bpmana.his to clearly see the dwell time at the last BPM.   A good double check is the oscilloscope trace of the raster current, seen in the middle room (electronic rack room) of the counting house.