Notes from Meeting Dec. 13, 2003

Septum update: Status

• Both septa had new bore tubes installed to remove the heat leak to the bore seen on the right magnet. This heat leak is gone; however, it appears that there is a remaining heat leak between the shield and the coil in both magnets. The shields will be cooled with Helium to reduce this problem. Work arounds were found for additional cooling problems seen in each septum, as described below.
• Left Septum has a leak in the insulating vacuum. The vacuum can be maintained by pumping on the insulating vacuum and on the cryo loop cooling the Nitrogen Shield. The septum can be used by cooling the shield with He (thus ruining the vacuum in the insulating chamber) and then evacuating the shield cryo line and pumping the vacuum back down (process takes approximately 2 hrs). The magnet can then function without cooling the shield for between 10-20 hours, depending on who you talk to, before the shield must be re-cooled. Septum has trained to 215 A with 3 quenches. BWXT tech will come next week to try to resolve lingering slow controls problems.
• Right Septum was cooling very slowly. Speculation centered on some kind of blockage, possibly due to a specific check-valve malfunctioning. After changes were made to improve cooling of the shield, the septum reached near 5 K and the coils were superconducting. Minor slow controls issues (unrelated to those on Left magnet) prevented a test at high current. It is not clear whether the ability to cool the septum was due to the configuration changes or a disappearance/resolution of the problems in the primary cooling loop.
• It is not clear whether one or both magnets may require repair before HAPPEX can run. A detailed schedule for such repair has not been made; it _may_ be possible to perform the repair during the 8 week installation period between hypernuclear and HAPPEX commissioning.
• Although they were surveyed into position, there is an apparent mis-alignment of the septum magnets. The back aperture of the right magnet is 4 mm farther from the beamline than the left. It isn't clear what the effect on HAPPEX will be.

Septum Heating Discussion

• Estimates from tests during GDH suggest that the septum yoke may approach 10 K when run at the HAPPEX luminosity, if total cooling capacity is unchanged.
• Results of a Monte Carlo from the Radcon group were reviewed. The radial distribution show a significant heat load even 20-30 cm from the beamline. It was concluded that the uncertainty in extracting greater details reduce the value of further simulation, so no further effort to simulate the heating process will be made at this time.
• There appears to be significant benefits available from shielding on the outside of the septum acceptance. Kent will consult Al Gavalya regarding the possibility of using thicker collimators inside the sieve box, and in adding additional shielding to some portion of the face of the septum box.

Septum Aperture/Background Discussion

• All tungsten is not alike; often tungsten is a composite with an iron fraction ranging from .1% to few percent. What tungsten do we have?
• It appears that with the old collimators, the acceptance aperture of the HRS is effectively defined by the stainless steel bore tube of the septum. Paul Souder fingers this as the most likely source of asymmetric background. A study of this is needed.
• John Lerose has done some ray-tracing studies of the aperture defined by the bore tube. Kent will get a summary of this work from him.
• Effective aperture may be dependent on target length. The choice between 15 cm and 20 cm cell will, in part, depend on this effect.
• Eugene Chudakov will develop a GEANT simulation of the target/septum configuration to study the possible backgrounds due to interactions with the septum bore. The goal is to have first results from this study in February.

Optics / Q^2

• Tim Holmstrom will work on optics commissioning / optimization for HAPPEX. He will participate in the optics commissioning before hypernuclear in preparation for this work.
• KK points out that we should have one true septum expert among the grad students or postdocs heavily invested in HAPPEX. We hope to designate someone to become the expert on this system, possibly during HAPPEX commissioning.
• Rob Feuerbach has agreed to take on the Q^2 calibration for HAPPEX. The most precise and reliable method will be Nilange's recoil method. Data taken during optics calibration for HAPPEX will prove very useful practice, and possible initial results, for this analysis.
• The measurement of Q^2 profile from an extended target is a non-trivial task; high rates may introduce efficiency distortions in high-rate areas. The measurement procedure and analysis approach needs to be specified for this task.
• The focal-plane profile scanner mechanical stability is being tested at UMass, and a setup for testing PMT linearity is also being made. The scanner detector element can be swapped, so either quartz Cerenkov for integrating or the scintillator for counting can be used. The scanner should be ready for install during the UMass spring break at (mid-March). There are still significant questions regarding readout of this detector which need to be answered. An expert on this system who can answer the readout questions, develop a measurement procedure and analysis plan, and who will be in residence during the run is needed.

Schedule

• Schedule now posted on jlab website.
  • HAPPEX installs Mar 3
  • Commission Apr 16
  • Run 4He Apr 19-26
  • Runs H Apr 28-June 20
• 4He target cannot run at full power after Apr 16 if in conflict with Hall C HMS cooldown. It _may_ be possible to run at reduced power.
• Kees reports that the end of the HAPPEX run (June 20) is fixed. Any days lost, due to accelerator problems or septum repair, for example, would not be extended on this run period (but would, of course, be part of future run periods).
• It was suggested a detailed commissioning plan be available for discussion at next collaboration meeting (Feb).

4He Radiation Budget

• 4He target is 2.8 g/cm2 (compare to 1.5 g/cm2 for LH2 target). Radiation load at site boundary for full experiment is on order of 80% of annual JLab limit. This is 5 times what is usually allowed for an experiment. It is difficult to shield the neutron load from the target, and a significant component of this increased load is reported to come from absorption of scattered rays downstream of the target.
• There is no problem in the brief (1 week) run planned for this run period. The estimate can be checked with measurements made at site boundary, in the hope that the simulated estimate may overstate the problem.

G0

• G0 is concerned with position differences which are rather large compared to goals. They are further very concerned to have not yet established a reliable feedback method for position differences.
• Concern was expressed that lab and accelerator management may need guidance in interpreting the difficulty in meeting beam requirements for HAPPEX-II, given the difficulty in meeting G0 requirements. The HAPPEX requirements are more strict, however, G0 faces additional problems in terms of beam size and space charge effects which should not impact HAPPEX.
• G0 major problems appear to be caused by space charge effects on beam transport. It was clear from the discussion that the specific effects they observed were not understood by those in attendance. Kent will consult with G0 collaborators to get a more clear understanding of issues they face. There are two questions: do we have any expertise or information which may be of assistance, and what can we learn from their data which will improve our preparations for HAPPEX?

Source/ITS studies update

Injector

• HC corrector magnets

  • 4 air-core magnets will be placed in the injector for use as a synchronous helicity-correlated feedback system. The control hardware is being developed at this time.
  • Kent will verify that the procedure for removal of real-time helicity from the control electronics is complete and verifiable.
  • We are not convinced that a feedback tool is necessary for reducing position differences, but even so there are potential uses, including beam studies.

• Ground loops

  • There is an effort now being made by accelerator and G0 to identify common grounding in the injector and anticipate and eliminate possible ground loops from the source to other electronics.
  • Kent and KK will study the source/injector electronics configuration in January with a goal of understanding the present configuration. It is expected that any serious work identifying and removing ground loops will need to wait until run time, when the existence of electronics pickup (and the effectiveness of a solution) can be verified on a configuration which is unlikely to change.

• Wien Filter

  A question had been asked: can the Wien filter be used for slow helicity reversal? The Wien dynamic range is only about 220 degrees, so unless the initial configuration is near the edge of the Wien range, probably not. In addition, changes to the Wien require significant reconfiguration of the injector, which may reduce the effectiveness of cancellation for this slow helicity reversal technique.

Transverse Asymmetry

• It is desired to measure the transverse asymmetry of electron scattering in the HAPPEX-II kinematics. A significant transverse asymmetry (~20 ppm) could possibly create a source of systematic error at the 40 ppb level. G0 expects to measure this asymmetry at the few ppm level for scattering off of Hydrogen. It will be necessary to make this measurement for Helium, and it may be necessary to repeat the G0 results on hydrogen.
• This asymmetry can only be measured by the Hall A apparatus with a vertical beam polarization. Such a polarization is possible, however, it will require hardware modifications in the source as well as require all halls to receive this vertical polarization. Kent will coordinate with accelerator to be sure that they know our requirement for this.

DAQ

Beamline

• Dither magnets are now instrumented, and it is hoped to test them during hypernuclear. The ability to disable fast position feedback during dithering will also be tested.
• A proper functioning of the beam monitors is critical to establish during hypernuclear. Double differences and BPM regressions may prove useful in verifying that repair of the systems is not required during the HAPPEX installation period.
• A commissioning plan, including pedestal calibration procedures, routine pedestal verification procedures, and BCM linearity tests, should be designed.
• All small-angle lumi vessels are installed, however, no more lumi detectors have been built. Riad is out of the country and awaiting a visa to return.
• Cabling is not complete for the lumi monitors. It is hoped to have this in place during hypernuclear run so monitors can be easily inserted into Hall when they are available.
• Air purge (to protect PMTs from Helium) is not yet designed.

Analysis Projects

• Septum Bore tube scattering, simulation project (Eugene)
• Q^2 calibration (Rob Feuerbach)
• Q^2 distribution
• Regression (Bryan?)
• Dithering
• Lumi (is it separate?)
• Linearity/ beam monitoring systematics

List of tests to be done during Hypernuclear

• Septum Heating
• Lumi commissioning
• Beamline monitors commissioning
• New Cavity Monitor commissioning (late Jan?)
• Q^2 calibration, recoil method (parasitic to optics commissioning.)