General How-to

Beam Restoration

• Study the manual: Establishing Physics-Quality Beam. If you do not feel confident to do this by yourself, contact the Beamline Expert.

Restoring Beam after Short Downtime

• Ask for 5 nA beam current and check the 2C24A and 2H01 beam (x, y) positions against the Reference Values (at the bottom of Short Term Schedule tab) after
  • beam steering or orbit change
  • change in beam current, e.g., after a target change
  • If BPM 2C24A and 2H01 positions are off, (not within +/-0.1 mm), ask MCC to bring them to the reference values, see tab https://wiki.jlab.org/clas12-run/index.php/Run_Group_D#tab=Short_Term_Schedule Short Term Schedule]
  • The beam position at 2C21A is not important for production

Beam Tuning to Tagger Dump

• Study the manual: Establishing Physics-Quality Beam, and take into account the first steps mentioned therein.

◊ Make sure the BMT along FMT is in SAFE mode, SVT HV is OFF, (LV is ALWAYS ON), and all other CLAS12 detectors HV are OFF.

• A few points to note:

• Change the beam type to "Photon" from the "BTA Config"

• Ask MCC to energize the tagger magnet

• Move the collimator to the Blank position after the tagger is energized

• Ask MCC to mask the beam halo counters (FSDs: Upstream, Midstream, Downstream, BOM)

• Ask MCC for 5 nA beam current and make sure the Orbit Locks are OFF;

• Check the 2C21A and 2C24A beam positions (x, y) against the Reference Values (at the bottom of Short Term Schedule tab);

- If the beam positions are OFF, ask MCC to bring them to the reference values.

• In case the two BPMs (x, y) positions look good, take a harp scan (HS) at Harp 2C21 (Upstream Right Counter) and Tagger Harp (2C24; Upstream Right Counter):

- It should be noted that the beam must be ON during the scan. Once OFF, abort and restart once the beam is back.

- Compare the results with the HS References at the bottom of Short Term Schedule tab and bear in mind that the fit ranges might need to be adjusted manually to assess the fit results;

- If good, log the new scan results.

Møller run

Follow the standard procedure as described in the manual Performing Møller Runs:

• Request 20 nA , Orbit Locks should be OFF;
• The BPMs 2C21 and 2C24A may be out of range after the Møller configuration.
• Keep running until the polarization uncertainty is < 1.5%

Beam Tuning to Faraday Cup

• Study the manual: Establishing Physics-Quality Beam, and take into account the first steps mentioned therein. This is typically done after tuning to the tagger dump (and possibly taking a Moller run).

• A few points to note:
  • Change the beam type to "Electron";
  • Ask MCC to de-energize the tagger magnet, which includes degaussing the tagger magnet and turning the power supply OFF;
  • Ask the operator to use the script only, not try it by hand, as this might trip the circuit breaker of the tagger magnet power supply.
  • (If initial beam tune, target should be "Empty")
  • Make sure the Beam Blocker is OUT
  • Move the collimator to the 20 mm position
  • Ask MCC to unmask beam halos
  • Set the halo counter FSD thresholds to 1 MHz and the integration time interval to 50 milliseconds
  • Insert the Downstream Viewer, Chromax, in the beamline
  • Ask MCC for 5 nA beam current
    • If the 2C24A and 2H01 beam positions are OFF, ask MCC to bring them to the reference values (see tab Short_Term_Schedule)
    • In case the BPM (x, y) positions look good, take a harp scan at 2H01 (Midstream Left)
    • The beam must be ON during the scan. Once OFF, abort and restart once the beam is back
    • Compare the results with the harp scan references (see tab Short_Term_Schedule)
    • If good, log the new scan results.

• Once done and before resuming the production data-taking:
  • Retract the Downstream Viewer from the beamline;
  • Put the FSDs thresholds and integration time to their reference values;
  • Turn all detector HV ON, for MVT "HV ON - Full Voltage";
  • MCC might need to adjust the beam positions after going to the production beam current.