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 (tab "Short Term Schedule") 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_K#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 (tab "Short Term Schedule")

- 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 (tab "Short Term Schedule") 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.

A few points to note:

• 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.