Harp scans

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In the process of editing and updating this page. Please contact Juliette Mammei crowder@jlab.org with questions or suggestions.

PREX Main<< HOW TOs << Expert Tools



See also: Bullseye Scans

Taking a harp scan

  • Safety information
    • The harps should not be run with more than 5 microA CW beam
    • This is an invasive measurement!
  • IHA1H04A and IHA1H04B are the harps on the diagnostic girder (near the target)
  • 1H00 is upstream of the Moller polarimeter
  • The other seven harps in the Hall A are labeled 1C*
  • 9E01 is near the start of arc 9, not relevant for PREX/CREX

Analyzing a harp scan

  • Harp Fitter Tool in JMenu




Drawing of the Hall A beamline

This is out of date! Requested a new song sheet from Joyce. Please check this link for the drawing of Hall A beamline done by E. Forman in 2014. It shows the locations of the different beamline components, their names and the labels. (NOTE that the beamline near the target needs to be updated to reflect the addition of new parity BCMs which was done in Fall 2015.)


COMMON SENSE TIPS for Rastered Beam Spot Size

  1. If you ask for 2x2 mm2 raster, you probably will NOT get that. Why? Because of quadrupoles between the raster and the target; note, the control software assumes a 23-m drift with no magnetic fields. What to do? With target out, ask for 2x2 mm2 (for example) and observe whatever you observe as the real spot size. Then multiply the requested size by the ratio of what you want to what you observe, and then ask MCC for that. This should give you the desired spot size.
  2. Do not confuse the "rastered spot size" (of order 4 mm) with the "intrinsic spot size" (typically 10 to 100 times smaller).
  3. If your purpose is to check the spot size and dwell time to avoid destroying the target, then test at low current, say 1-2 μA, before going to high current on target or test with no target in the beam. Note that the beam position lock does not work well below 1 μA.
  4. A good double check is the oscilloscope trace of the raster current, seen in the middle room (electronic rack room) of the counting house.
  5. If you see a funny looking plot with a lot of scatter? Possibly the beam was off, or sometimes off. Make sure the beam is on (> 2 μA).

Initial BPM/Raster checks/Beam centering [3-4 hours]

  1. First MCC centers the beam on the beam dump using the ion chambers.
  2. Establish CW beam.
  3. Note that we will NOT be going through the Compton chicane.
  4. With no more than 5 μA of stable beam find the center of the target using the steps below:
    1. For each step, make sure to record both the rates from the right and left HRS and the BPM positions at BPMA and BPME.
    2. Move the target to the BeO target. Find the unrastered beam on the BeO target.
    3. Then have MCC restore beam at 5 μA with a nominal 2x2 mm2 rastered beam size at the target.
    4. Move the target to the Empty 1 position.
    5. Before asking for beam, check the right and left HRS (T1 trigger) rates. Record these rates in the HALOG.
    6. Run rastered beam with the target in the Empty 1 position at 5 μA, while checking the right and left HRS (T1 trigger) rates. Record the rates and the beam positions.
    7. Move the target to the Carbon hole position.
      • Check the rates in the right and left HRS (T1 trigger). Record these rates.
      • If you can see a difference in rates between the foil and no foil, then continue. Otherwise contact the run coordinator.
    8. Use the spot++ tool on both L-HRS and R-HRS (see Raster calibration, spot++)
    9. Verify that the raster is approximately centered on the Carbon hole target. If it is not, ask MCC to move the beam position in x and y until the hole is roughly centered on the raster image.
      Do not increase the raster size. If you cannot find the hole with the raster ON, please contact the run coordinator (Contact information is on the white board).
    10. After the raster is centered on the hole, have MCC turn off the raster. Since the hole in the Carbon foil is 2 mm in diameter, and the beam spot is much smaller than this, the rates should drop to the rates with the Empty target.
    11. Now move to the "Raster Target". Ask MCC for 5 μA of beam with a 2x2 mm2 raster. If the beam is centered, the rates should match the rates of the empty target.
    12. If you notice excess rate, then slowly move the beam position until the rates are the same as those with the empty target.
    13. After beam centering, update on the white board the nominal beam positions and the "MCC" raster size to get 2x2 mm2 raster on target.
    14. Make sure you make a detailed log entry with the results of the position scan.
  5. Superharp Scan with Raster off to check if they are working and provide signals:
    1. Have MCC scan the two superharps (IHA1H04A and IHA1H04B) near the target with raster off.
    2. Request MCC to make an ELOG entry with the Harp results.
    3. Make sure the three wires are clearly visible and have reasonable resolution for each harp: ~200-500 μm.
    4. If the harp scans are successful, then proceed with the bullseye scan below.
    5. Make a record of the results of the harp scans in the HAlog along with the ELOG entry numbers.

BCM Calibrations [2 hours] (DAY or SWING shift preferable)

Accelerator has a written procedure to follow if you wish to cross check the current at the injector with the current in the Hall.

  • The Hall A BCM and scalers need to be cross-calibrated to the Faraday cup and a BCM in the accelerator (OLO2) and the Hall A Unser. (If night shift then do NOT bother cross calibrating with the Faraday cup. You can just stay on the line with MCC and request the appropriate beam currents following the rules below.)
  • Before beginning the procedure, make sure the Hall A BCM logger and the HRS DAQ are started, which will record the signals from all relevant BCM used during the calibration. [Here ] is how to start the logger, choose the BCM logger script (solution 2).
  • Also start a HRS run to record the scalers during the calibration procedure, if necessary use the clock to take reasonably high data rate (above about 50 Hz at least)
  • It is useful to have someone over in MCC to help coordinate the calibration with them.
  • Take data on carbon target with I = 0, 3, 0, 5, 0, 7, 0, 10, 0, 15, 0, 20, 0, 25, 0, 30, 0, 40, 0, 50, 0, 60, 0, 70, 0, 80, 0 μA (or as high a current as available). Each beam period (with or without current) should last 90 s. If you get a trip during a beam up period, take 90s of beam off and restart that beam up period.

    ARC Energy Measurement (DAY or SWING shift ONLY)

    1. The procedure will be conducted by Douglas Higinbotham.
    2. With the ARC in achromatic optics, the energy measurement can be done at any time though with more uncertainty as the effects of the ARC quads need to be taken into account.
    3. If you wish to do a full dispersive optics measurement, it need to be coordinated well in advance by the run coordinator as it is invasive to the other halls. Yves from accelerator should be in MCC if the procedure has been done in while. It is presently unclear if it is even possible to do a full dispersive optics measurement with 11 GeV beam due to the energy spread of the beam as well as sync. radiation.


    More info (not instructions!)

    • Retrieved from "https://prex.jlab.org/wiki/index.php?title=Harp_scans&oldid=6945"