All Staff Memos
|Date:||October 19, 2001|
|From:||Larry Cardman and Andrew Hutton for the Nuclear Physics
Experiment Scheduling Committee
|Subject:||Draft Accelerator Schedule: Through December 2002|
Attached is the draft accelerator operations schedule through December 2002. Due to budget constraints, the accelerator will operate for approximately 28 weeks during Fiscal Year 2002.
The Jefferson Lab Nuclear Physics Experiment Scheduling Committee developed the schedule. Committee members are: Larry Cardman and Andrew Hutton (Co-Chairmen), Hari Areti, Roger Carlini, Leigh Harwood, Bernhard Mecking, Kees de Jager, Mike Seeley, Dennis Skopik, Steve Suhring, Will Oren, and Karen White. Franz Gross provided advice. As has been the norm, a number of meetings of this committee were necessary to resolve conflicting requirements and to ensure that sufficient resources would be available at the laboratory to properly stage and carry out each of the experiments. The schedule was derived by looking at the requests for major installation work in the experimental halls, evaluating the number and kinds of people needed, and then scheduling to minimize overlap. The schedule request forms were useful in identifying the detailed requirements of each experiment. Information on other laboratory engineering priorities was included to ensure that the required preparatory work could be completed in time. This provided a rough overview of when each hall would be down.
Each hall leader took the requests for running time submitted by the experiment spokespersons and prioritized them based on the PAC recommendations and other considerations as outlined in the scheduling committee charter. Scheduled time for Hall C was again calculated using an estimated overall efficiency of simultaneous hall and accelerator operation of 50%; this value is consistent with last year's experience for the hall. The same 50% was used for Hall B scheduling, even though the actual availability was higher; since most running in Hall B involves fractions of very long run groups, cutting the scheduling as tight as possible has no long term benefit, and we take advantage of higher running efficiency (when achieved) to complete a larger fraction of a run group's program. Scheduled time for Hall A was calculated using an estimated overall efficiency of 60%, consistent with recent experience. In a number of cases the scheduled beamtime has been adjusted to reflect significant changes in facility capabilities since the time of PAC approval of the experiment; the most obvious of these is the availability of high polarization beams with significantly higher current than was the case a few years ago. The final schedule was then reached by a series of compromises in running order within each experiment and between halls to work around incompatibilities.
To optimize polarized beam running, we have scheduled many weeks of operation at "unusual" energies that are consistent with good polarization in multiple halls. The details vary from run period to run period and hall by hall. In the worst case (5 days in July), the effective polarization delivered to a hall will be reduced to about 82% of the nominal maximum available from the cathode. This reduction is due to the angle at which the polarization vector will be set relative to the beam direction in the hall in a compromise that will optimize delivery to all halls. Details on the constraints associated with polarized beam operation are discussed in a note at the end of this memo.
The standard section at the end of this memo on "the meaning of priority on the accelerator schedule" is included for reference; there were no changes in the policy this cycle. All users with running experiments should read it carefully.
The schedule attached represents our best effort to optimize the physics output of the laboratory consistent with our resource constraints and the technical evolution of the accelerator and the experimental equipment. In the material that follows, we outline the technical considerations that influenced the scheduling, and outline the planned program.
We ran the majority of FY01 at 5.7 GeV as a compromise between the high trip rates associated with pushing the accelerator energy higher and the physics users desire for higher energy. We have been able to contain the high klystron failure rates experienced during the August 2000 6 GeV test when an average of one klystron failed per day. We have had only one klystron failure in period February 2001 through September 2001 while running primarily at 5.7 GeV. However, the RF Trip rates still makes running at energies above 5.7 GeV problematic for routine operations until more cryomodules are installed, therefore the maximum energy planned for FY02 is 5.7 GeV.
The polarized injector, with two fully operational, horizontally mounted polarized guns (one for production beam and one for a spare), continues to perform well. All beam operations, polarized or unpolarized, is conducted with high polarization cathodes. When polarized beam is not required, shorter wavelength lasers are used to take advantage of the higher quantum efficiency at these wavelengths. Typical bleedthrough from high current Halls A and C to Hall B is less than 3%. The photocathode lifetime in the new horizontal guns is excellent. In fact, the lifetime is now so long, it has become difficult to measure accurately. The current value is over 35,000 mA-hours or 300 Coulombs at high current. Though this long lifetime makes absolute statements difficult, our current experience is that the cathode deterioration can be completely removed by a simple heat treatment and reactivation. This cathode recovery can be accomplished during a normal maintenance period. This implies that a single cathode could be used essentially without limit. During the past year, over 280,000 mA-hours were delivered from the polarized guns with polarization between 75% and 80%.
The new Ti-sapphire laser needs some additional work, but has delivered ~400 mA of high polarization beam and was used for about 4 months delivering our highest combined current to date of over 140 mA to Halls A and C. We operated for ~ 3 months with a single laser rather than three. During this time, all halls received ~80% polarization and we were able to run from a single cathode location for 3 months without a reactivation or a spot move, corresponding to a lifetime of ~600 Coulombs. Single laser operation minimized polarization dilution of Halls with low current (the beams for all three halls had the same polarization at the cathode so bleedthrough did not affect the polarization). We were also able to maintain the beams to the different Halls on strictly identical trajectories, reducing beam trips due to particle loss (a common problem with three lasers caused by drifting apart of the three laser spots on the cathode). The major disadvantage of single-laser operation is that hall access or tuning operations in one hall affect all hall beams.
During the September 2001 downtime, we prepared to resume 3 laser operations following the 3 month period of single laser operations. We will once again be able to independently tune the three beams and ramp a single hall following a trip, if required. Additionally, an access to one Hall will no longer require the other two halls to stop receiving beam.
In order to support the upcoming Physics program, we have developed a new Ti-sapphire laser for the G0 experiment. This low average power laser provides the required substantially lower laser pulse rate (31 MHz versus 499 MHz) while maintaining the same overall average current as the 499MHz laser. This laser will require periodic testing and development during the months prior to the beginning of the G0 experiment.
We have not scheduled most of the shift maintenance and restore periods that took place every three or four weeks last year. Instead, we will bank the 11 shifts per month normally scheduled for machine maintenance, and draw against these preplanned accelerator down hours when a failure occurs that effectively prevents beam delivery to Physics. We will then fix the failure keeping the machine down and perform other planned maintenance activities that can be accommodated in parallel. In this manner, we will avoid the cycle of halting machine operations to perform maintenance and subsequent restoration when the machine is running well. We will also use one scheduled day following holiday shutdowns to perform necessary maintenance before the two day machine restoration. We will continue to reserve a 12 hour period every Tuesday (7am - 7pm) in order to recover RF cavities, test the G0 laser and perform other limited activities deemed critical to successful accelerator operations.
Since the last schedule was released, Hall A used the April/May shut down to remove the cryogenic target system and install the polarized 3He target system required by experiments E99-117 (Chen/Meziani/Souder) and E97-103 (Averett/Korsch). E99-117 carried out precise measurements of the neutron asymmetry An1 at large Bjorken x while experiment E97-103 did searched for higher twist effects in the neutron spin structure function gn2 (x,Q2).
The experimental schedule for the later half of 2001 had to be readjusted to allow for a delay in the delivery of the septum magnets. The waterfall target system was installed during the September/October shutdown in order to run E00-102 (Saha/Bertozzi/Weinstein/Fissum), testing the limits of the single-particle model in 16O(e,e'p). During the November 2001/January 2002 shutdown, the waterfall target system will be replaced by the cryogenic target system. A new photon calorimeter and associated charged particle sweep magnet will also be installed as well as the Focal Plane Polarimeter (FPP). This equipment is required by experiment E99-114 (Hyde-Wright/Nathan/Wojsekhowski) which will carry out exclusive Compton scattering on the proton. Experiment E01-001 (Arrington/Segel) will follow in February. E01-001 will perform a new measurement of GE/GM for the proton. The septum magnets are expected to be ready for installation during the February/May shutdown. The cryogenic target system will also be replaced by the waterfall target system during this time in preparation for E94-107 (Frullani/Garibaldi/LeRose/Markowitz/Saito). E94-107 will perform a high resolution study of 1p-shell hypernuclear spectroscopy.
The tentative experimental schedule for the later half of 2002 consist of experiment E97-110 (Garibaldi/Chen/Cates) being carried out immediately after E94-107. Experiment E97-110 is the low-Q2 (forward angle) extension of E94-010, which measured the Q2 evolution of the GDH sum rule. This will permit us to spread the "overhead" of the septa installation over two experiments. E97-110 requires the installation of the polarized 3He target in July. The September/October shut down will be used to remove the septum magnets as well as to replace the polarized 3He target by the cryogenic target system in preparation for E00-118 (Gomez/Petratos) and E00-007 (Gilman/Holt/Meziani). E00-118 will measure the forward elastic form factor (A(Q2)) of 3He. Experiment E00-007 will measure proton polarization angular distribution in deuteron photo-disintegration
Since the last schedule release, Hall B completed the first part of the g8 run (linearly polarized photons on a hydrogen target to study vector meson production in and above the resonance region, three experiments). This was followed by the completion of the g6c run (unpolarized photon beam on hydrogen, mainly search for unusual mesons) and a short test run for the Primex experiment.
After the maintenance period in September/October 2001, the e1-6 run (polarized electrons on a hydrogen target to study high-Q2 N* excitations and deep virtual vector meson production, 4 experiments) will take data at 5.7 GeV. To take advantage of the high energies available, e6 (deep inelastic electron scattering off the neutron tagged by the detection of a low-energy proton, two experiments) will follow. After the maintenance period in March/April 2002, e1 (polarized electrons on a hydrogen target, 13 experiments) will continue data collection at 4 and 5 GeV.
The tentative part of the schedule in the second half of 2002 shows the completion of the e1 run. After the maintenance period in September, the g7 run (search for medium modifications in the mass spectrum of electron-positron pairs) will occupy the remainder of the year.
Since the last schedule was released, Hall C running has included the completion of the major installation experiment E93-038 (Madey, Kowalski), a measurement of the neutron electric form factor. Then the hall was reconfigured for polarized target running and the completion the 1st of two Q2 points for E93-026 (Day), the polarized target measurement of the neutron electric form factor. In addition, the majority of the extensive cryogenic, mechanical and electrical infrastructure necessary for the G0 experiment plus the entire G0 detector assembly has been installed in the Hall.
After the completion of E93-026 in December of 2001 the GEn polarimeter and bunker will be removed and the polarized target slightly reconfigured to run E01-006, Nuclear Spin Structure Functions in the Resonance Region. E01-006 will complete in early March 2002 at which time the final dedicated G0 installation phase will begin. The G0 run will begin in Oct 2002 with several pre-commissioning injections of beam into the Hall at earlier dates. After the first G0 run a significant period of time will be dedicated to the running of shorter, standard configuration experiments in Hall C.
Note: The plan to install and commission G0 will be reviewed on January 15th, 2002. If the G0 magnet is not ready for installation at that time, the Hall will be reconfigured to the base equipment configuration and a series of small experiments will be run after completion of E01-006 in March 2002.
Information about the Schedule
The accompanying revised schedule is fixed through June 2002, and tentative for the following six months. Because of the complex couplings between the hall operations during polarized beam running, all halls must continue to run in "calendar-driven" mode. The firm schedule for the first half of 2002 (and the tentative schedule for the first half of 2003) will be released following the meetings of the next cycle of the scheduling committee.
Footnotes to the Schedule
We summarize here the detailed footnotes to the schedule. They appear in the rightmost column of the schedule listing, and are listed at the earliest date in the schedule when they are applicable; many extend for a considerable time after they first appear. The first five footnotes apply to the entire schedule. All of the footnotes are repeated here for clarity and information.
- When two or three halls are scheduled, the relative priority listed in the schedule (in the order listed from left to right) is the relative priority of the halls. For example, A/B/C means that Hall A is the highest priority, Hall B has second priority, and Hall C has the lowest priority. If one of the halls has an asterisk, it means that its priority is conditional, and the conditions are given in appropriate footnotes at the beginning of the running of the affected experiment. If the conditions are not met, then the remaining two halls will have priority in the order listed.
- Energies listed in the schedule for the halls receiving polarized beam are the actual, delivered energies; they include the energy of the injector.
- In the "Accelerator" column, a "low" under Pol(arization) indicates planned use of either a bulk or a thin (but unstrained) GaAs cathode, implying that medium (~40%) polarization can be expected. A "high" under Pol(arization) indicates planned use of a strained GaAs cathode, implying high (~75%) polarization can be anticipated.
- When polarized beam is delivered to all three halls, it is not, in general, possible to provide pure longitudinal polarization to all users. We have optimized the beam energies to provide the highest longitudinal polarization (generally over 90%) to all halls during extended periods of scheduled two- and three-hall operation with polarization. For two-hall operation we have occasionally used less than ideal linac energy settings when one or more of the halls has a scheduled pass change in order to optimize polarization delivery over the entire run. This avoids the loss of beamtime associated with a linac energy change, and it avoids energy shifts in the hall that has no interest in changing energy at the time of the transition in the other hall. See the note in the "polarization" subsection of the text on the meaning of priority in the schedule; the note is attached below.
- In all cases, the orientation of the polarization at the injector will be optimized by setting the Wien angle to a value that minimizes the differences between the hall polarizations (by minimizing the dispersion) so long as this scheme does not result in a reduction of the "sum of squares" figure of merit by more than 2% compared to the optimum figure of merit as determined by summing the squares of the polarization provided to all halls scheduled to receive polarized beam. If minimizing the dispersion results in a loss of more than 2% relative to the optimum figure of merit, we will revert to our earlier algorithm of setting the Wien filter to maximize the overall figure of merit. In all cases involving polarized beam delivery the setting of the Wien Filter shall be fixed throughout the running period unless all parties scheduled to receive polarized beam agree to a different setting.
- When polarized beam is provided at a new energy, as much time as necessary during the first shift of polarized beam operation will be used to verify polarization in the halls. This can be done by direct polarimetry in the hall(s) and/or by taking data on a reaction that is adequately sensitive to the beam polarization. By the end of the first shift of production running with polarized beam, the run coordinator(s) for any experiment(s) receiving polarized beam must report to the Program Deputy that they have measured the beam polarization and determined it to be acceptable. Otherwise, a measurement of the beam polarization will be scheduled immediately. When the polarized beam energy is being changed in only one hall (e.g. a "pass change") then that hall should measure beam polarization by the end of the first shift of production running. Further, if the change in settings of the Wien filter are substantial, all three halls should measure and report beam polarization by the end of the first shift of production running with the new setup.
- The Injector Group will install the G0 production laser during the Christmas shutdown period.
- The Hall A septa installation is now scheduled to begin on March 8, 2002. The septa construction project will be reviewed on January 15, 2002 and if there are further delays, the run schedule beginning in May, 2002 will be modified.
- Results of tests designed to confirm the likelihood of successful G0 magnet operation will be reviewed January 15, 2002. Based upon that review, a final decision will be made to either proceed with the G0 installation or configure Hall C for several small scale experiments.
Additional General Information on Operations and Scheduling Constraints
The accompanying schedule is fixed for the nine-month period October 2001 thru June 2002 and tentative for the following six months. Priorities have been assigned as "firm" for the period of the schedule that is fixed; the tentative priorities set for the July-December 2002 period will be reviewed in April, when the schedule for that period becomes fixed. As noted earlier in this memo, the operation of polarized beams in more than one hall puts severe constraints on our ability to change beam energies.
The Meaning of Priority on the Accelerator Schedule
Generally, the assignment of priority to a hall means that the identified hall will have the primary voice in decisions on beam quality and/or changes in operating conditions. We will do our best to deliver the beam conditions identified in the schedule for the priority hall. It will not, however, mean that the priority hall can demand changes in beam energy that would affect planned running in the other halls without the consent of the other halls. Of course, final authority for decisions about unplanned changes in machine operation will rest with the laboratory management.
The operation of more than one hall at Jefferson Lab substantively complicates the interaction between the experimenters and the accelerator operations group. It is in the interests of the entire physics community that the laboratory be as productive as possible. Therefore, we require that the run coordinators for all operating halls do their best to respond flexibly to the needs of experiments running in other halls. The run coordinators for all experiments either receiving beam or scheduled to receive beam that day should meet with the Program Deputy at 7:45 AM in the MCC on weekdays, 8:30 AM on weekends.
To provide some guidance and order to the process of resolving the differing requirements of the running halls, we have assigned a "priority hall" for each day beam delivery has been scheduled. We outline here the meaning of priority and its effect on accelerator operations.
The priority hall has the right to:
- require a re-tune of the accelerator to take place immediately when beam quality is not acceptable
- insist that energy changes occur as scheduled
- obtain hall access as desired
- request beam delivery interruptions for experiment-related operations such as Mott measurements of the beam polarization or pulsed operation for current monitor calibrations, temporarily blocking normal beam delivery to all halls. These interruptions shall be limited by a sum rule - the total time lost to the non-priority hall(s) due to such requests shall not exceed 2.5 hours in any 24-hour period. It is, of course, highly preferred that these measurements be scheduled at the morning meeting of the run coordinators whenever possible, and coordinated between halls whenever possible
When the priority hall has requested a re-tune, if the re-tune degrades a previously acceptable beam for one of the other, lower priority running halls, then the re-tune shall continue until the beam is acceptable to both the priority hall and the other running halls that had acceptable beam at the time the re-tune began.
Non-priority halls can:
- require that a retune of the accelerator take place within 2.5 hours of the desired time (it will nominally occur at the earliest convenient break in the priority hall's schedule)
- require access to the hall within 1 hour of the desired time (again, it will nominally occur at the earliest convenient break in the priority hall's schedule)
- request Mott measurements in the injector within 2.5 hours of the desired time (it is preferred that this be scheduled at the morning meeting of the run coordinators and coordinated between the running halls whenever possible).
The ability of non-priority halls to request retunes and accesses shall be limited by a sum rule - the total time lost to the priority hall due to such requests shall not exceed 2.5 hours in any 24-hour period. (To facilitate more extended tuning associated with complex beam delivery, with the agreement of the run coordinators for all operating halls, the sum rule may be applied over a period as long as three days, so long as the average impact is less than 2.5 hours/day.) In the event that two non-priority halls are running, the 2.5 hours shall be split evenly between them in the absence of mutual agreement on a different split.
During operations in which a single, 1500 MHz laser is being used to drive the electron source for all 3 halls, when a non-priority hall needs changes to the accelerator state (re-tuning, access, etc.), then all halls currently receiving beam need to agree on the timing of the change, and the shift leader for the priority hall should contact the crew chief to make the formal request. The upgrades to the PSS and MPS system, together with the development of the three-laser drive system eliminate the need for this constraint during 3-laser operation of the source. However, it is necessary to reinstate this constraint whenever, for reasons of source performance in service to the running experiments, a single drive laser is used. It is also necessary to reinstate the constraint on a temporary basis in situations such as a laser failure in which we are forced to operate the polarized source in a non-standard manner.)
- can negotiate with other halls, and with the Accelerator and Physics Division for changes in scheduled energy changes (either direction)
Initial Tune-up of New Beams:
- Normally one shift is set aside for tune-up whenever a new beam setup is being tuned (for unusual beam setups more time may be scheduled explicitly for tuning at the discretion of the scheduling committee). It is understood that beam tune-ups shall always be done in the order that the accelerator operations group believes will minimize the total time needed to tune all scheduled beams (i.e., the "priority hall" beam is not necessarily tuned first). In the event that obtaining the new beam setup requires more than the scheduled time, the Accelerator Program Deputy is authorized to spend up to one additional shift of tuning in an effort to deliver all scheduled beams instead of just the "priority hall" beam.
- Note that the setting of the Wien filter, which determines the polarization orientation in all halls, is NOT affected by the hall priority assignment. For two-hall operation we will always optimize the figure of merit for the two running experiments by setting the Wien filter to a value that results in identical longitudinal polarization components for the two halls. For three-hall operation we set the Wien angle to a value that minimizes the differences between the hall polarizations (by minimizing the dispersion) so long as this scheme does not result in a reduction of the "sum of squares" figure-of-merit by more than 2% compared to the optimum figure of merit as determined by summing the squares of the polarization provided to all halls scheduled to receive polarized beam. If minimizing the dispersion results in a loss of more than 2% relative to the optimum figure of merit, we will revert to our earlier algorithm of setting the Wien filter to maximize the overall figure of merit. In all cases involving polarized beam delivery the setting of the Wien Filter shall be fixed throughout the running period unless all parties scheduled to receive polarized beam agree to a different setting.
Finally, any change in the accelerator schedule that has implications for running beyond one week and/or is not agreed to by the run coordinators for all affected experiments and the accelerator program deputy must be discussed and confirmed at meetings to be held (as required) each Tuesday and Friday afternoon at 4:00 in the office of the AD for Physics
The maintenance cycle that was started last year has proven to be a considerable improvement. In the present running cycle we will continue to extend the time between full (12 shift) maintenance periods even further. During a typical 4-week cycle period the maintenance/development cycle remains the same, but the weekly, non-invasive maintenance periods will be extended from 4 to 12 hours every Tuesday (7am - 7pm). This cycle offers improved opportunity for accelerator and injector related maintenance, and should result in even higher availability for physics. The details are provided here for reference.
The schedule includes some maintenance/development periods of 11 shifts. As can be seen below, the 11-shift cycles have the same
structure as the maintenance/development periods of the previous 3-week cycle. However, the weekly, non-invasive maintenance
periods will be extended from 4 to 12 hours every Tuesday (7am - 7pm). This cycle offers improved opportunity for accelerator and
injector related maintenance, and should result in even higher availability for physics. The details are provided here for
Machine Maintenance/Development Schedule
Weeks with long maintenance periods
|Owl||Physics||Injector||Hot Checkout by Ops (no beam)||Restore Accelerator
|06:00 Rad Survey
Software priority 12:00 - 20:00
Injector check-out 18:00-20:00
Lock-up when work complete
MD on Hall lines
|12:00 Diagnosis||12:00 Restore Injector
23:00 Lock Halls
|Legend: Physics Running; Halls Locked for Accel. Use|
Weeks with no long maintenance period
|Day||Physics||RF Recovery, Injector work, software tests, MD
20:00 Start Physics
- A major block of time for Maintenance and Machine Development will be scheduled for 11 shifts roughly every 4 weeks.
- The first four hours will be spent doing MD that may require beam to the halls. At least one hall will be available for such activities, and more halls will be made available by prior agreement as necessitated by the program of activities planned.
- The halls will be open for experimental equipment maintenance from 12:00 on Monday through 23:00 on Wednesday.
- A designated period of time is provided for pre-maintenance diagnosis with beam (AES Group, Beam Physics & Instrumentation, and others) so that the maintenance time can be utilized effectively.
- Monday Swing and Tuesday Owl will be assigned to the Injector for the foreseeable future.
- There are two shifts for maintenance on Tuesday. It is anticipated that mechanical installations will start early and be finished around 6 PM. To avoid interference, the Software Group will have priority from 12:00 – 20:00, and the AES Group will have priority before 12:00 and after 20:00. Injector checkout should occur from 18:00 – 20:00.
- The Wednesday Owl, "Hot Checkout by Ops," means that:
- System Owners need to write Hot Checkout procedures to be used by Ops.
- System Owners will be required to be physically present during Hot Checkout until procedures are provided to Ops.
- These procedures will be the basis for troubleshooting and repair documentation.
- Systems found to be defective will usually be fixed on the Wednesday Day Shift although major problems may have to be addressed immediately.
- Wednesday morning is available for completing installations and for final repair of systems found to be defective during Hot Checkout.
- The accelerator will be restored starting 12:00 on Wednesday, continuing through Thursday Owl. Some additional Machine Development may occur during these shifts.
- Restoration of beam to the halls will be done on Thursday Day and Swing shifts. This will include Machine Development on the hall beamlines, which would benefit from more dedicated time.
- On the weeks when no major maintenance is scheduled, 12 hours will be assigned beginning on Tuesday morning. This will be used for recovering RF cavities to maintain a high active cavity inventory, and for tests of Beam Applications that would profit from frequent, short tests.
For holidays shown on the schedule (such as Thanksgiving) the beam will be shut down at ~noon on the last day shown as beam delivery (e.g. Wednesday noon before the Thursday Thanksgiving holiday). Beam operations for physics will be resumed at 8 AM on the first day after the holiday shown as beam operations, with the first shift devoted to beam restoration (i.e. it will be treated as initial tune-up of a new beam from the point of view of operations).
Energy Constraints on Multiple Hall Operations
The standard constraints for the different energies in the three halls during multiple hall operation are reiterated here for your information. The RF separators are able to extract one beam after each pass or, alternatively, to deliver beam to all three halls after five passes.
Therefore, it is always the case that:
- All three beams can have the same energy only on the fifth pass.
- No two halls can have the same energy, except on the fifth pass.
- Unusual beam energies in one hall will sometimes preclude multiple beam operation and impose shutdowns on the other halls, unless one or more of the other halls can also use a commensurate, unusual energy.
Polarization Constraints on Multiple-Hall Operations
There are only two beam energies (2.115 and 4.230 GeV) at which purely longitudinal spin can be delivered simultaneously to all three halls when the halls have the same energy. There are, however, many combinations of passes and linac energies at which it is possible to deliver beams with precisely longitudinal polarization to two halls simultaneously, and many combinations at which it is possible to deliver nearly longitudinal polarization to three halls. A technical note covering all combinations of 2-hall polarized beam running is available (TN 97-021). Tables of ideal energies for two-hall operation and optimal energies for three-hall operation are available at the url: http://claspc10.jlab.org/spin_rotation/
You can also determine the dependence of the polarization in all three halls on the Wien filter angle for the actual settings of the accelerator. Experimenters scheduled for periods involving multiple-hall polarized beam delivery should consider the possible impact of a transverse polarization component on their measurements, and provide the laboratory with a maximum allowable transverse component if appropriate. Because of the limitations on beam energies associated with the different combinations of linac settings and numbers of passes delivered to the different halls, we have a great deal less flexibility for changing energies in the different halls during polarized beam running. This is because there are many instances where the nominal linac energy and number of recirculations for the running halls provide reasonable polarization, but where changing the number of recirculations for one of the running halls results in nearly transverse polarization.
In an effort to optimize polarized beam running, we schedule many weeks of operation at energies that are consistent with good polarization in multiple halls. The details vary from run period to run period and hall by hall. In the worst case, the effective polarization delivered to a hall is typically reduced to no less than ~90% of the nominal maximum available from the cathode. This reduction is due to the angle at which the polarization vector will be set relative to the beam direction in the hall in a compromise that will optimize delivery to all halls. For two-hall operation we can optimize the figure of merit for both running experiments by simply setting the Wien filter to a value that results in identical longitudinal polarization components for the two halls. For three-hall operation we have previously used an algorithm that set the Wien filter to a value that maximized the overall figure of merit (the sum of the squares of the polarization provided to all halls scheduled to receive polarized beam). It has been noted that this sometimes results in situations where the delivered polarization is significantly different for the three halls. To "equalize the pain" for three-hall operation, we are adopting a refinement to this algorithm. The Wien angle for three-hall operation will now be set to minimize the differences between the hall polarizations (by minimizing the dispersion) so long as this scheme does not result in a reduction of the "sum of squares" figure of merit by more than 2% compared to the optimum figure of merit. In all cases involving polarized beam delivery the setting of the Wien Filter shall be fixed throughout the running period unless all parties scheduled to receive polarized beam agree to a different setting.