|experiment schedule memo|
Attached is the accelerator operations schedule for the period through June 2002. The overall operations continue to be constrained to a level of approximately 30 weeks/year as a result of the laboratory’s operating budget level for FY2002.
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, Bernhard Mecking, Kees de Jager, Mike Seeley, Charlie Sinclair, Dennis Skopik and Will Oren. 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.
Tests of 6 GeV operation were carried out in August 2000. While the accelerator briefly reached 6 GeV, 107 µA, the RF trip rate was extremely high and would not have been acceptable for physics running. In addition, there were many klystron failures (one per day of the test) due to running them at a higher voltage to obtain more power. A series of action items have been established, and improvements are being carried out in the May downtime. Another 6 GeV test has been scheduled in September 2001. Until then, the accelerator will be operated for physics at up to 5.7 GeV, a compromise between reaching new physics and achieving acceptable availability.
The polarized injector now has two fully operational, horizontally mounted polarized guns. All beam operation, polarized or unpolarized, is now 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. This has been very successfully demonstrated during the current running period: “unpolarized” beam up to 130 mA has been delivered to Hall A while high polarization beam was delivered to Hall B at 5 nA. Typical values for the “feed-through” from Hall A to Hall B are about 50 pA. The photocathode lifetime in the new horizontal guns is excellent. The current value is over 35,000 mA-hours. 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. For the first time, measured polarizations of 80% were measured and it is hoped to maintain this level.
During January, a test run was conducted with a new Ti:sapphire laser, which delivered ~400 mA of high polarization beam. The final version of this laser will allow this current to be more than doubled. This laser was installed during the August shutdown giving us the ability to deliver high polarization beam at full current to one hall. A second laser of this type will be prepared, to allow high polarization operation to both Halls A and C. Based on the very successful operation of the new guns, and the installation of this new laser, we have removed the capability of delivering beam from the thermionic gun.
We expect to be operating for at least part of the upcoming period with a single laser rather than three. This will minimize polarization dilution of Halls with low current (all Halls will have their polarization vector aligned so bleedthrough will not affect the polarization). We also expect to be 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 down side is that there will no longer be independent tuning of the three beams and if one hall requires the beam to be ramped following an RF trip, then all halls may be similarly delayed. Additionally, access to a hall effects beam delivery to the other running halls. Users with runs during single laser operation should review the relevant section of the memo on “the meaning of priority” later in this memo. These operating modes will be closely studied to optimize the physics results for all Halls.
The three-week maintenance cycle that was started last year was a considerable improvement and we tried extending the time between full (12 shift) maintenance periods even further – to roughly once per month. This was sufficiently successful that we will continue this cycle. During the 4-week cycle periods the (now monthly) 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 is required to maintain the complement of active RF cavities at a high level to reduce the unwanted RF trips. The details are provided below in the section on Maintenance/ Development for reference.
Since the last schedule was released, Hall A completed successfully four experiments: E97-111, E99-007, E94-104 and E98-108. E97-111 (Templon/Mitchell), was aimed at a systematic probe of short-range correlations via 4He(e,e'p)3He. E99-007 (Brash/Jones/Perdrisat/Punjabi), extended the GpE / GpM ratio measurement via polarization transfer to Q2 = 5.6 (GeV/c)2 using a considerable part of the focal-plane lead-glass calorimeter as an electron calorimeter on the floor. After completing these two experiments, two aerogel Cerenkov detectors were installed in the focal plane and the lead-glass calorimeter reassembled. Experiments E94-104 (Gao/Holt) and E98-108 (Baker/Chang/Frullani/Iodice/Markowitz) then started in Jnauary of 2001. E94-104 will study the fundamental process photo pion production in 2H and 4He. E98-108 will perform kaon electro-production measurements including an L/T separation up to Q2 = 3 (GeV/c)2.
After completion of E98-108, Hall A will use 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 will carry out precise measurements of the neutron asymmetry An1 at large Bjørken x while experiment E97-103 will search for higher twist effects in the neutron spin structure function gn2 (x,Q2).
The tentative experimental schedule for the later half of 2001 had to be readjusted to allow for a delay in the delivery of the septum magnets. In September of 2001 the waterfall target will be installed in order to run E00-102 (Saha/Bertozzi/Weinstein/Fissum), testing the limits of the single-particle model in 16O(e,e'p). The septum magnets are then expected to be ready for installation in late November. Starting in February E94-107 (Frullani/Garibaldi/LeRose/Markowitz/Saito) will then perform a high resolution study of 1p-shell hypernuclear spectroscopy. After installation of the polarized 3He target in May, that experiment will be followed by E97-110 (Garibaldi/Chen/Cates), the low-Q2 (forward angle) extension of E94-010, measuring the Q2 evolution of the GDH sum rule. This will permit us to spread the "overhead" of the septa installation over two experiments.
Since the last schedule release, Hall B completed the second eg1 run (polarized electrons on polarized hydrogen and deuterium targets, three experiments).
After the accelerator shut down in April/May the g8 run group (linearly polarized photons on hydrogen target) will take data for three experiments to study vector meson production in and above the resonance region. This will be followed by the completion of the g6 run (photon beam on hydrogen, mainly meson production) and a short test run for the Primex experiment. Following the September down time, the e1-6 run (polarized electrons on hydrogen target to study high-Q2 N* excitations and deep virtual vector meson production, 4 experiments) will take data at 5.7 GeV.
The tentative part of the schedule in the first half of 2002 shows a fourth e1 data run (polarized electrons on hydrogen target at 4 and 5 GeV, 13 experiments) followed by e6 (6 GeV polarized electrons on deuterium target, 2 experiments) to study tagged deep-inelastic scattering. The next run to be scheduled is e2 (polarized electrons on a variety of nuclear targets).
Since the last schedule was released, Hall C running has included the completion of two major installation experiments E89-009 (Tang/Hungerford), an investigation of the feasibility of performing hypernuclear physics experiments in which a nucleon in the nucleus is replaced by its strange counterpart, the Lambda hyperon and E93-038 (Madey, Kowalski), a measurement of the neutron electric form factor. A third standard configuration experiment E99-118 (Keppel/Bruell/Dunne), a measurement of the nuclear dependence of longitudinal to transverse cross section ratio at low Q2 has also been comducted.
During the spring/summer of 2001 the Hall will be reconfigured for the execution of the remaining portion of E93-026 (Day/Mitchell), the polarized target measurement of the neutron electric form factor. The G0 experiment (E00-006) will have a dedicated one month installation period starting in September after which E93-006 will resume running for the remainder of calendar year 2001. Dedicated G0 installation begins in Feb. 2002 after the removal of E93-006, with an engineering run scheduled for June 2002. This installation date is subject to the conditions outlined in footnote 10 of the schedule. After the first G0 run a significant period of time will be dedicated to the running of shorter, standard configuration experiments in Hall C.
Information about the Schedule
The accompanying revised schedule is fixed through December 2001, 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 second half of 2002) will be released in October, 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.
Additional General Information on Operations and Scheduling Constraints
The accompanying schedule is fixed for the nine-month period October 2000 thru June 2001 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 January-June 2001 period will be reviewed in September, 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:
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:
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.)
Initial Tune-up of New Beams:
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 four-week 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 – to roughly once per month. During the 4-week cycle periods 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.
4-week Maintenance/Development Cycle
The new, 4-week cycle of the schedule includes maintenance/development periods of 11 shifts every four weeks. 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 reference
Machine Maintenance/Development Schedule – 3 Week Cycle
Weeks with long maintenance periods
Weeks with no long maintenance period
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:
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.