All Staff Memos
|Date:||November 21, 1997|
|From:||Larry Cardman and Andrew Hutton for the Nuclear Physics
Experiment Scheduling Committee
|Subject:||Accelerator Schedule: November 1997 - December 1998|
Attached is the accelerator operations schedule for the period through December 1998.
The schedule was generated by the Jefferson Lab Nuclear Physics Experiment Scheduling Committee. Committee members are: Larry Cardman and Andrew Hutton (Co-Chairmen), Roy Whitney (Secretary), Roger Carlini, Bernhard Mecking, Kees de Jager, Claus Rode, Charlie Sinclair, and Will Oren. Starting with this meeting the Committee has been augmented by the addition of Mike Seeley. Nathan Isgur also provided advice.
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 first looking at the requests for major installation work in the experimental Halls, evaluating the number and kind of people needed, and then scheduling to minimize overlap. The schedule request forms again proved quite useful in identifying the detailed requirements of each experiment. Information on other laboratory engineering priorities was also fed in 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 was again calculated using an estimated overall efficiency of hall plus accelerator operation of 50%; this value is somewhat better than last year's actual experience, but we anticipate that improved understanding of the accelerator and the elimination of many of the hall-interaction constraints on operations will improve our overall availability this year. The final schedule was then reached by a series of compromises in running order within each experiment and between halls to work around incompatibilities.
This is the first schedule with substantial polarized beam operations, and the first with polarized beam operations to multiple halls. As is well known, 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 and all 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 (within ~20 degrees) polarization to three halls.
The problem of delivering simultaneous polarization close to longitudinal in each of the three halls is over-constrained. As yet, we have no straightforward algorithm to calculate accelerator energies that provide a large longitudinal polarization component in each of the three halls, but rather arrive at these energies empirically. In cases where polarization is shown for all three halls, we have chosen accelerator energies that provide a high longitudinal component for all three halls, typically over 90% of the basic electron source polarization. It is important to note that when the polarization is not purely longitudinal, there is also a transverse polarization component in the horizontal plane. This transverse component reverses sign when the sign of the longitudinal component is reversed. This transverse component becomes quite large as the longitudinal component becomes smaller. For example, an 80% longitudinal component also implies a 60% transverse component. 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. As we study the three hall polarization problem further, we may recommend changing the accelerator energies slightly from those shown on the schedule to further optimize the polarization for everyone. The subject will be reviewed prior to the March, 1998 meeting of the Scheduling Committee, when the schedule for the second half of 1998 will be re-issued as "firm."
In an effort 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 run from period to run period and hall by hall. In the worst case the effective polarization delivered to a hall is reduced to 83% of the nominal maximum available from the cathode (due to the angle at which the polarization vector is set relative to the beam direction in the hall in a compromise that optimized delivery to all halls).
It is clear that we will 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 numbers 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.
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 drove some of the scheduling , and outline the broad program as planned.
Work continues on developing the full capability of the accelerator. During a development run in September, a beam current of 200 microamperes was delivered at the full (4 GeV) beam energy; this corresponds to the full design beam power for the accelerator and represents a considerable achievement by the accelerator operations group and a confirmation of the original design and the construction.
Polarized beams were delivered to Hall A and Hall C for experiments this year with currents as high as 75 microamperes. The accelerator, understandably, provided beam with reduced availability during this period as we learned how to use this new capability. A number of improvements will be made to the source this winter, including: improved pumping in the anode-cathode gap; improved optics in the polarized gun; replacement of the "Z" spin manipulator with a (shorter) Wien filter plus solenoid; and the installation of a three-laser drive system. These modifications should improve source lifetime and operational flexibility.
Starting in July, 1998 and continuing through December, we are committed to run with a high polarization photocathode. As noted in the last schedule release, the start date for this effort was delayed by three months to provide high current beam for Hall A and C running and to give the polarized source group more time to prepare for this demanding task. Due to the low beam current demands of the experiments running in Halls B and C during this period, we would expect a very long cathode lifetime if we were only running these two halls. We will add Hall A operations during this period only on a best-effort basis. The maximum current delivered to Hall A will be restricted to a value such that the photocathode's operational lifetime is no less than 48 hours. If this cannot be done with reasonable current delivery to Hall A, then the Hall A running will be cancelled. This is indicated on the schedule by adding an asterisk to Hall A's priority during the period in question. The hall will have priority in the normal sense if and only if photocathode lifetime stays above 48 hours; if this is not the case then Hall A operations will be cancelled and the running halls will have the priority for normal operations in the order listed.
Delivery of 5 GeV beam, tentatively scheduled to begin in the last week of August, will also be on a best effort basis. The decision to provide this high-energy beam is tentative. A final decision will be made early in August, after we have had the opportunity to evaluate the impact of high polarization and multiple-hall operations on accelerator availability. If availability without the additional stress of high beam energy operation has not been adequate, then the accelerator energy will be kept at the current nominal value of ~4 GeV.
Hall A has completed spectrometer commissioning and the first pair of "half" experiments - E89-003 (Saha) and E89-033 (Glashausser), both 16O(e,e'p) measurements. E89-033 used both polarized beam and the FPP in the hadron arm to study polarization transfer. The deuteron elastic form factor measurement of Petratos (E91-026) is now underway, and will be followed at the end of 1997 with the "dress rehearsal" for the Souder/Finn parity violation experiment (E91-010) aimed at studying the strange quark content of the sea. 1998 running will begin with the virtual Compton scattering experiment of Bertin (E93-050), followed by the first run cycle of the Souder/Finn parity experiment (contingent on a successful dress rehearsal), and then the proton electric form factor (GEp) measurement of Perdrisat (E93-027). The commissioning run in early March (basically the first operation of the hall after the winter shutdown) is the first scheduled three-hall operation. It will be done on a best-effort basis, contingent on two-hall operations having been within 10% of our operations goal of 70% averaged over a month. Continued operation in three-hall mode will remain contingent on overall availability dropping no more than 10%; running in this mode will be evaluated on a monthly basis, and the schedule revised if appropriate and necessary. If the hall commissioning cannot be run in 3-hall mode in early March, it will have to be incorporated into the beginning of the VCS run, scheduled to begin 3/12.
Both the last portion of the GEp experiment and the polarized 3He experiment of Meziani on the q2 evolution of the GDH sum rule (E94-010) will then be run in the July/December period contingent on polarized source performance, as discussed above. A readiness review must be held for E94-010. This review can be arranged through the hall leader, Kees deJager, at the convenience of the collaboration, but it must take place no later than February 1, 1998 if the collaboration wants to keep its place on the schedule.
Hall B has completed basic commissioning of both the tagging system and the CLAS detector. The first physics run is scheduled for December: the e1 group (Dytman). It will run through the end of 1997 and in February/March, 1998. We have scheduled some additional photon tagger commissioning for March, 1998. This is the second scheduled three-hall operation. It will be done on a best-effort basis, contingent on two-hall operations having been within 10% of our operations goal of 70% averaged over a month. Continued operation in three-hall mode will remain contingent on overall availability dropping no more than 10%; running in this mode will be evaluated on a monthly basis, and the schedule revised if appropriate and necessary. If the tagger commissioning cannot be run in 3-hall mode in March, it will be incorporated into the beginning of the next run group, the g1 group (Miskimen), which is scheduled for April and May. This will be followed by the g6 group (Marchand), which will run until the end of June, 1998. At that time, Hall B running will cease for the installation of the CLAS polarized target (to follow the Hall C GEn target installation) and the eg1 group (Kuhn) will run using that target for the remainder of 1998. A readiness review must be held for the eg1 group. This review can be arranged through the hall leader, Bernhard Mecking, at the convenience of the collaboration, but it must take place no later than February 1, 1998 if the collaboration wants to keep its place on the schedule.
The T20 experiment was completed in Hall C and the pion form factor measurement (E93-021, Mack) is about to begin; it will run through November. During the winter, 1997-8 shutdown, the Hall C cryotarget will be reworked to adapt it to the needs of the pion electroproduction experiment of Jackson (E91-003), which will start running in February, 1998. Then Hall C will begin the job of mounting the polarized deuterium target for the neutron form factor measurement. The start of this installation was delayed to increase the probability that the polarized source would be able to support both the high polarization beams required for E93-026 and the higher current beams required by Hall A running. E93-026 is the second major installation experiment to be run, and it will use beam for the remainder of calendar 1998. At that time the polarized deuterium target must be removed from the pivot to honor a commitment to return it to SLAC.
In Hall C we have promised to give the "major installation" experiments advance notice (of at least one year) of their anticipated start date. It is our intention in the first half of 1999 to follow the GEn experiment by a number of experiments using the basic hall equipment (HMS and SOS). The experiments to be run will be determined during the March, 1998 scheduling cycle. Then in the second half of 1999 we will mount the HNSS and execute experiment E89-009 (Hungerford); this will be the third major installation experiment. Before the March, 1999 meeting of the scheduling committee (which would place the experiment on the firm schedule) the collaboration must pass a formal readiness review. This review can be arranged through the hall leader, Roger Carlini, at the convenience of the collaboration, but it must take place no later than February 1999 if they want to keep their place on the schedule.
We tentatively plan to run the last of the Hall C "big four" major installation experiments, a second measurement of the neutron electric form factor (E93-038, Madey), in the second half of the year 2000. This late run start should permit the use of high polarization and high current beams, permitting this experiment to provide a significantly improved measurement of GEn. A final decision on this plan will be made as part of the September, 1998 scheduling cycle.
Information about the Schedule
The accompanying revised schedule is fixed for the seven-month period November 1997 - June 1998 and tentative for the following six months. Because of the complex couplings between the hall operations during polarized beam running, all halls must now run in "calendar-driven" mode. This has become a necessity. The firm schedule for the second half of 1998 will be released late March, following the meetings of the scheduling committee in that month.
Footnotes to the Schedule
We summarize here the detailed footnotes98.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. They are repeated here for clarity and information.
- March 1998 - This is the first month where more than one hall is listed in the Priority Hall column. Priority, where more than one hall is shown, will be in the order listed (left to right.)
- During March 1998, three hall operations will be scheduled to commence on a best effort basis. The third hall will be added only if beam availability for two-hall operation has been within 10% of our operations goal of 70% averaged over a month. Continued operation in a three-hall mode will remain contingent on overall availability dropping by no more than 10%. This limit potentially impacts the Hall A commissioning runs (3/1-9) and the Hall B tagger commissioning (3/14-12). It may also impact 3-hall running scheduled from mid-June through the end of 1998.
- The Souder/Finn parity experiment scheduled for April-May, 1998 in Hall A is contingent on the success of the "dress rehearsal" measurement scheduled for December, 1997. The results of the "dress rehearsal" will be reviewed on or before March 1, 1998, and a final decision on the Hall A program for April/May will be made at that time.
- 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 hall - e.g., A/B/C means that Hall A is the highest priority, Hall B has second priority, and Hall C has the lowest priority. In the period from August through December, 1998, Hall A priority is indicated with an asterisk. This means that the hall has the priority slot shown if and only if the source lifetime is greater than 48 hours; if this is not the case, beam delivery to Hall A will be terminated and the remaining two halls will have priority in the order listed.
- The Hall C SOS will be off during the M¿ller tests on 3/26 and 3/27.
- Energies listed in the schedule for the halls receiving polarized beam are the actual, delivered energies; they include the energy of the injector.
- During the period between July 3rd and July 12th, a high polarization photocathode will be installed in the gun. Polarization is expected to be at least 70% after this date. Before this date, all polarized beam operation will be with either bulk GaAs or thin unstrained GaAs cathodes.
- The electricity will be off for repairs in the Test Lab, EEL, and CEBAF Center May 23-25, 1998. Arrangements will be made to keep the Computer Center on line.
- GEn Commissioning from 6/18-6/22 is currently scheduled as 3-hall operation but will be dependent on our metric for two hall operations having been met.
- The Hall B run during the period from 8/14 to 8/24 will have to be interrupted for a two-day period to permit Perdrisat to obtain a two-pass beam. This is shown on the schedule as nominally taking place on 8/16 17; the date of the Hall B hiatus may move a few days in either direction upon mutual agreement of the run coordinators.
- The installation time for the polarized 3He target needed for the Hall A experiment of Meziani et al. is still under review. If additional time is needed the start date for the experiment (tentatively scheduled for 9/24/98) may slip. As noted above, a readiness review must be held for this experiment on or before February 1, 1998.
- During the period August - December, 1998 we are committed to run with a high polarization photocathode. Due to the low beam current demands of the experiments running in Halls B and C during this period we would expect a very long cathode lifetime if we were only running these two halls. We will add Hall A operations during this period on a best-effort basis. The maximum current delivered to Hall A will be restricted to a value such that the photocathode's operational lifetime is no less than 48 hours. If this cannot be done with reasonable current delivery to Hall A, then the Hall A running will be cancelled. If Hall A running has been cancelled, the ability of the source to deliver the necessary beam will be reviewed periodically as experience warrants, and running started in the hall as soon as we have confidence that we can do so within the constraint of >48hour operational lifetime.
- 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 periods of scheduled three-hall operation with polarization.
- When polarized beam is provided at a new energy, the first shift of polarized beam operation will be used to verify polarization in the halls.
- Delivery of 5 GeV beam, tentatively scheduled to begin in the last week of August, will also be on a best effort basis. The decision to provide this high-energy beam is tentative. A final decision will be made early in August 1998 after we have had the opportunity to evaluate the impact of high polarization and multiple-hall operations on accelerator availability. If availability without the additional stress of high beam energy operation has not been adequate, then the maximum accelerator energy will be dropped back to ~4 GeV.
Additional, General Information on Operations and Scheduling Constraints
The accompanying schedule is fixed for the seven-month period November, 1997 - June, 1998 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 last six-month period will be reviewed in March, 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 effect planned running in the other halls without the consent of the other halls. Of course, final authority for decisions about the 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 7:30 AM in the MCC.
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 accelerator to take place immediately when beam quality is not acceptable
- to insist that energy change occur as scheduled
- to obtain hall access as desired.
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 a retune of the accelerator take place within 2_ hours of the desired time (it will nominally occur at the earliest convenient break in the priority hall's schedule)
- can require access to the hall within one hour of the desired time (again, it will nominally occur at the earliest convenient break in the priority hall's schedule)
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_ 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_ hours/day.) In the event that two non-priority halls are running, the 2_ hours shall be split evenly between them in the absence of mutual agreement on a different split.
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. (Note that we anticipate that upgrades to the PSS and MPS system, together with the installation of the 3-laser drive for the polarized source, will reduce and eventually eliminate the need for this constraint.)
- can negotiate with other halls, and the accelerator and physics division for changes in scheduled energy changes (either direction)
Initial Tuneup of New Beams:
- Normally one shift is set aside for tuneup 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 tuneups shall always be done in the order in which 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.
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.
Facility Development Time
Experiments that do not get beam when scheduled, or which are unable to obtain the data required in the scheduled period due to problems with accelerator and/or experimental equipment availability will not be able to request a schedule slippage. However, the experiment may re-apply for additional beam time to complete the experiment at a later date. To facilitate this process, a period of about one week every two months or so has been put into the schedule for Facility Development. This time will be awarded by competition between experiments needing to complete data-taking, preparatory tests for future experiments, and testing new capabilities of the experimental equipment. This allows the schedule to be more rigidly adhered to (this has been a clear request from all users) but provides a degree of flexibility for unforeseen circumstances (this is, after all, an experimental program and things do not always go according to schedule). The schedule for the Facility Development periods will be determined at a meeting of the Scheduling Committee to be held one week before the start of the period. Requests for use of this time should be addressed to the Committee.
Maintenance days are shown on alternate Tuesdays, starting at 6 A.M., and will usually be followed by five shifts of accelerator restoration and machine development time. Beam will then be re-established to the experiment(s) by Thursday at 6 A.M., so that the accelerator groups will absorb the recovery time from maintenance. The machine development time will be used to prepare new experimental set-ups such as polarization, non-standard energies, three-beam operation etc. for the immediate future as well as preparatory work for higher energy operation. On a few occasions the maintenance period has been slipped a day in order to provide uninterrupted completion of a scheduled sequence of measurements within an experiment. We cannot, however, eliminate or drastically reschedule these maintenance and development periods.
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.
- Non-standard energies in one Hall will normally preclude multiple beam operation and impose shutdowns on the other Halls, unless one or more of the other Halls can also use commensurate, non-standard beam energy.
We have demonstrated an ability to run low current beams to Hall B without a higher energy "witness" beam, so the constraint listed in previous scheduling committee memos for Hall B operation has been lifted.
Polarization Constraints on Multiple Hall Operations
A technical note covering all combinations of 2-hall polarized beam running is available (TN 97-021). A tech note on 3-hall polarized running is under development; the problem is considerably more complex due to the many constraints that must be fulfilled.
Note that starting in July 1998 the schedule includes one additional day during maintenance/development periods that is listed as "cathode replacement." This is to provide the roughly 3-day period necessary for a complete cathode changeout (involving opening the gun, swapping the cathode, baking the gun to restore the vacuum quality, and activate the cathode) should such replacements be necessary to achieve good source availability during running periods. Note also that when polarized beam is provided at a new energy, the first shift of polarized beam operation will be used to verify polarization in the halls.