Run Plan (DVCS3)
For the 12 GeV DVCS experiment, go to [DVCS3 wiki ].
For this Winter 2016 we are only scheduled for 10 week. Therefore we estimate that we will only be able to take the data points highlighted in green.
For the Fall of 2014 only, assume:
- longitudinally polarized beam
- max 20 uA but for short times when it can go higher
- 7.3 GeV-4 pass and 5.5GeV-3Pass. No five passes in the Fall of 2014.
- 1 Commissioning/ Calibration
- 1.1 Proposed timeline for commisionning
- 1.2 Beam instruments
- 1.3 Luminosity
- 1.4 HRS
- 1.5 Calorimeter
- 1.6 Coincidence setting
- 2 Main DVCS kinematics
Proposed timeline for commisionning
Our first priority for the commisionning is to establish the HRS-Calo coincidence without damaging the calo (ie getting crazy radiation on it). To do this we need to set he experiment in some kind of realistic state.
DVCS trigger (event type 9) should be used at all time.
Steps with ** are must, steps without * are should, steps without stars are could.
Step with a # can be done later (ie they might be important but are not necessary to establish the HRS_calo coincidence)
- Period with no HRS and no cryo-target.
- ** Establish beam in hall (MCC)
- ** BPM calibration with harp and spot++
- ** BCM calibration
- ** Establish raster functionality
- #** check that the helicity comes to the hall and is correctly decoded in the main analyzer
- #** Polarization measurement (including a mini-spin dance)
- #* ARC measurement
- # calibrate the injector table to minimize the charge asymmetry
- Period with HRS and cryo target
- ** beam centering using the hole target
- ** determine the max current one can put on target when HV are on the DVCS calo (using long cryo-target)
- #* use compton detector to make sure beam is as clean as it can (going to Compton chicane)
- ** initial check out of HRS detectors (DVCS trigger)
- ** Compare DIS rate measured with the DVS trigger to the one measured with the standard HRS trigger
- ** start setting up the calo-LHRS coincidence (at least up to a point when the calibration in energy of the calo makes sense)
- ** calibrate the calo in energy (elastic calibration, proton in the LHRS)
- ** finish setting up the calo-LHRS coincidence for the main DVCS trigger (LHRS+Calo) this is our first priority
- #** check out reconstruction of the foil-target (DVS kinematic)
- #** measure an elastic setting (on elastic setting, electrons in the LHRS, dp/p~0) this is our second priority
- #** determine efficiency of HRS trigger (with DVCS trigger)
- # checkout "target boiling" (DVCS only anticipate to use beam up to 25 uA or so).
- #** study deadtime of the data acquisition with multiple simultaneous triggers (LHRS + Calo, DVCS-DIS)
- #** compare DIS rate with and without the collimator
- # only if things look bad with elastic and foil target, sieve measurements
BPM/Raster checks/Beam centering
- First MCC centers the beam on the beam dump using ion chambers.
- Ideally the beam should be able to clear up the Compton chicane. But this is not a requirement.
- Send unrastered beam to the four corners of a 2*2 mm square around assumed (0,0) position (as defined by RAD reading in beam dump). At each position compare harps against BPMs reading.
- Check size raster using the spot++ tool. Pending target boiling studies result, we will use a 2*2 mm2 raster. or 3*3mm2??
- Insert carbon hollow target to center the beam through the hollow carbon target. Setup the raster to 2*2 mm, use spot++ to check if the beam is centered around the hole. Move the beam if necessary. What is the size of the hole?
- Check that the BCM are temperature controlled.
- Use the procedure that is under .....
Note: The calibration against the Faraday cup in the injector relies on no beam loss in the machine. To insure that the (absence of) beam loss can be measured, about 100 uA should be going through the machine. We also need to be the only hall receiving beam.
- Unser calibration??
Goal : ΔP/P~2%
We have to be mindful of the issue of the intrinsic polarization and the issue of its direction. We want the beam polarized in the longitudinal direction to 99% of it's value.
- Make sure that the linac imbalance, the Wien angle and the photo-cathode QE are recorded in the end and start run logbook entries and the archiver.
- Measure the polarization with the Moeller. Do a mini-spin dance with the Wien. Can we do a min-spin dance with the Linac imbalance?
- Measure the polarization in the Mott polarimeter (not at 2% level yet but they are making measurement this fall to go there).
According to D. Hinginbotham spreadsheet, for the Fall 2014 run, we probably want to restrict the maximum imbalance between the lilacs to 0.5%, this assume an injector at 120 MeV and the nominal lilacs energy at 900 MeV.
Estimated fraction of longitudinal polarization for Fall 2014
Goal: Δ E/E ~ 0.5 %
Goal: hopefully less than 0.5% boiling at 20 uA.
No need to use the calorimeter, trigger on DIS events. Any DVCS setting will work.
- Make sure the info (fan speed, temp, ...) for both loops are in the end/start run epics scripts
- At maximal beam current, measure rate on hydrogen as a function of the fan speed and the raster size.
- At the best combination of fan speed and raster size, drop down the beam current in steps to measure the absolute density reduction.
- Repeat the beam current scan using the Carbon target. The Carbon target is 450 mg/cm^2 while the Hydrogen on is 1062 mg/cm2, so in the similar DIS setting we expect about 1.7 times more rate in Carbon than in Hydrogen.
We probably need to do this for both target loop (standard and Silviu's one).
The plan is to have multiple scalers that would let us check the deadtime. One of them is inside the DVCS trigger module, the next one is a set of scalers out side the module itself.
- First use the simplest configuration (only the main DVCS trigger, other tigers disabled)
- Check that the redundant deadtime measurements for the trigger are consistent (internal trigger module scalers, EDTM, external busy gated scalers)
- Measure rates as a function of decreasing luminosity. Check that deadtime correction return stable rates whatever the current. We may want to use two carbon thicknesses instead of playing with the current...)
- At constant current, study the rate of DIS and DVCS events for various values of the DIS pre scale. DIS events are triggered by the DVCS trigger module. Check that DIS and DVCS rates are stable witch ever pre scale.
We want to maintain the charge asymmetry below 100 ppm per run. To be done preferably after BCM &BPM calibrations.
- Check that the HWP state, the RHWP angle and the PITA and the IA voltage and the charge asymmetry are recorded in the run-start and end-of-run logbook entries.
- Check that we can measure a charge asymmetry using the BCM scalers. Check that a change in RHWP angle (~45deg) or PITA voltage (~1V) results in a change in measured charge asymmetry. About 15 min for each measurement. Request HWP out.
- Set the PITA voltage in the middle of its range. Measure the charge asymmetry each 45deg of the RHWP from 0 to 180 deg. Estimate the best angle to null the charge asymmetry.
- Measure the charge asymmetry for 4 or so voltage of the PITA. Estimate the best PITA voltage to further null the charge asymmetry.
- Measure the charge asymmetry for the new nominal setting you determined. Repeat measurement with HWP in. Re-measure the charge asymmetry, hopefully it is about the same result that what you had with HWP out.
We could use the IA too (not sure what is better to use).
Initial detector check out
- S2, S0 amplitude check
- Gas cerenkov HV tune
- Pion rejector HV tune
Measure efficiencies S2m, Cerenkov and wire chamber
Goal: to measure the efficiency of the s2 and the cerenkov in the HRS. At any given DVCS settings, measure 1 Million events for each of the following triggers. Use the DVCS trigger module in forced validation mode (when possible) and the standard Hall A daq (MLU)
- s0 and s2,
- s0 and cerenkov, and
- s2 and pion rejectors (not possible with DVCS trigger module).
The resolution of the calorimeter (about 3.5% in energy and 2mrad in angles) dominates the resolution of our experiment. So our optics needs are quite limited. In a first attempt we believe that it will be sufficient for our purpose to check that the target foils are reconstructed with about sigma=1.2mm/sin(spectro angle).
We might also want to measure an elastic point detecting electrons in the HRS and no detection in the calorimeter. Assuming a 15 cm LH2 target, a spectrometer acceptance of 6 msr and dipole form factor to compute the central cross-section.
|Ebeam (GeV)||k' (GeV)||θe (deg)||Q2 (GeV2)||dσ/dΩ 10-7 (GeV-2)||counts per μA per second||minutes for 5k events at 20μA|
Set the threshold of the trigger to the energy of the smallest DVCS photon minus about 0.5 GeV, repeat the measurement with half that offset (0.25 GeV).
Check HRS timing for charged particle
Take any DIS run (without requiring cluster on the DVCS calo), and look at the measured HRS TDC. Potentially add delay to the individual trigger signals if the overlap time between the different trigger is not adequate.
Check timing between the calo and the LHRS
Take any DIS run (without requiring cluster on the DVCS calo). Check position of photons signals in the ARS, estimate the arrival time of the signals within the trigger module ADC window.
Main DVCS kinematics
 Proposed DVCS3 settings for Fall 2014 and Spring 2015]
|Kin||Pass||Year||kBeam||xB||Q2||W2||kHRS||θe||θq||θq Calo-5-6||θq Col1-2||DCalo||IBeam|