pep-ii mac closeout november 17, 2007 1 pep-ii machine advisory committee closeout session november...

PEP-II MAC Closeout November 17, 2007 1

PEP-II Machine Advisory Committee

Closeout SessionNovember 17, 2007


Committee:Kathy HarkayStuart HendersonShin-ichi Kurokawa *Pantaleo RaimondiDavid RIceDavid RubinJoachim Tuckmantel *Frank Zimmermann

Unable to attend:

Lou Bertolini

John Corlett

Katsunobu Oide

Flemming Pedersen

Ivan Koop


Charge to the committee:

• With regard to the PEP-II facility, the Laboratory is requesting the Machine Advisory Committee (MAC) to:– a) Critique the plans for maximizing the integrated luminosity in

Run 7 which goes from December 2007 through September 2008.

– b) Appraise the luminosity improvement plans to reach the peak luminosity goal in the spring of 2008.

– c) Review the near-term focus and schedule for accelerator physics studies as they contribute to maximizing the integral for Run 7.


Closeout Outline:• Reporting assignments organized by technical

areas rather than individual talks and reports will be presented in this order:– Executive Summary– Performance and Upgrades– Lattice and Beam Dynamics– Interaction Region– Vacuum– RF & Feedback– Beam Stability and Impedance– Controls & Diagnostics


Executive Summary


Highlights:The committee continues to be impressed by the high quality work of the PEP-II staff and the effective use of resources.We have seen many advances in vacuum, RF, diagnostic, feedback and other systems during the past year.Optics analysis and correction capabilities are well developed.The operations staff is experienced and capable.


Highlights:Luminosity performance maintained/ increased

in spite of current-related events during Run6• 500 fb-1 delivered by end of Run 6 in early September• Monthly integrated luminosity record set in August• Commendable progress has been made in addressing

high current issues and installing components to accommodate the planned levels of beam current, including:– BPM replacements in LER– HOM bellows with advanced absorbers in both rings– Flex-flange RF seals in HER– IR chambers in LER Q5 and HER Q4L– RF window replaced– LER transverse horizontal kicker replaced– Beam aborts analyzed and most causes have fixes


Highlights (cont.)

• This level of performance is many factors above original design upgrade of ring components necessary.

• The end of this shutdown will leave PEP-II rings posed to go to even higher beam currents and make use of the developments in optics analysis and correction techniques to optimize delivered luminosity.


Run 7• Delivering maximum integrated luminosity will

require: – quick, efficient startup – intense focus on maintaining a high duty cycle– significant increases in instantaneous luminosity.

• Target parameters include:– Peak luminosity 1.8-2.0x1034 cm-2-sec-1

– Beam currents of 2.2 / 4.0 Amps– 250 fb-1 delivered integrated luminosity

• These objectives are challenging and will require a significant commitment of experienced accelerator physics and technical support staff.


Summary of committee’s responses to its charge --


a) Critique plans for maximizing integrated luminosity in Run 7• The run plan and timing presented represent an

aggressive but workable plan given sufficient resources and if no major problems are encountered. – 100% attention of key accelerator physics and

engineering staff– Aggressive, proactive monitoring of potential hot

spots– “SWAT team” response to problems and reliability

issues– No big surprises


b) Appraise plans to reach the peak luminosity goal in early 2008

• This is the most aggressive aspect of plans• Given this objective, the plans presented seem

reasonable to the committee; however, we note that ALL aspects of the plan must work to achieve 1.8 x 1034 luminosity.

• We note that the planned time scale for recovery of luminosity after the shutdown is much shorter than past experience supports.

• We recommend frequent reassessment of the impact of efforts to achieve the peak luminosity goal on the longer term integrated luminosity prospects.


c) Near-term focus of AP studies & schedule, and impact on integrated luminosity• Several observations presented to the

committee convince us that significant accelerator physics studies will be required to achieve maximum integrated luminosity.– Turnover in luminosity with increasing LER

current – suggestions of ion / e.c. effects– Implementation of 90 deg HER optic, lower *

in both rings, have significant work to be done.– Uncertainties in HOM powers with combined

higher currents and shorter bunch length


c) Accelerator Physics (cont.)

• It is likely that luminosity-limiting effects will be identified that will require additional dedicated MD shifts to understand and to find cures or work arounds.

• These studies are expected to be efficient and productive given the extensive knowledge of the machine acquired over the past years, and the effective array of diagnostics.

• The committee endorses the plan to have a concentration of accelerator physics studies early in the run.


Performance and Upgrades


Performance - Findings -1

• Run 6 –– 2 new RF stations working– New skew quad fix for IR2 x-y coupling– Fast dither IP feedback– Vertical vibration in IR support tube damped– HER Omega seals replaced (192 !!)– IR2 vacuum system refit almost complete


Performance - Findings -2• Run 7 objectives – deliver 250 fb-1 in the

next 10 months– 4 A LER, 2.2 A HER, lower *v to 8-8.5 mm,

bunch length

• Major points of overall plan:– Run through holidays– Do more MD at beginning of run– Find optimum condition by early Spring– Run through September with best steady-state

conditions.


Performance – Comments– Major current-limiting obstacles removed,

particularly in HER– We reasonably expect new flex flanges to reduce

beam aborts– These advances may only give opportunities to

find other, unexpected obstacles– During MD and production running regular

monitoring of temperatures and pressures will be critical.

– Increases in stored current must have coordinated supervision by experts of each subsystem.


Plans - Recommendations

• Mobilize as many experts as possible for MD activities

• Fully utilize vacuum scrubbing period for trials of new optics or tuning.

• Understand why increasing LER current did not improve luminosity – very aggressive study will be required


Interaction Region


Interaction Region

• The IR continues to perform extremely well, supporting beam currents that are 2.5x design in the LER, and 40% above design in the HER with acceptable detector backgrounds.

• This is a testament to the quality and care put into the original IR and shielding design.

• Extrapolation to anticipated run 7 performance suggests acceptable trigger rates, occupancies and dose rates

• The diagnosis and mitigation of the 10 Hz luminosity modulation arising from support tube vibration is a well-done and important piece of detective work.


Interaction Region: Comments

• The luminosity plan for run 7 requires adjusting the IR optics to reduce y* which may result in background levels higher than the scaled values

• Some experience already shows that lowered beta* increases backgrounds

• It is worth getting an early jump on these and other issues related to y* reduction.


Interaction Region

Recommendations:1. Begin machine development to deploy

lower y* interaction region optics and understand better the implications early in run 7, for example, during the scrubbing period.


Vacuum


Vacuum System

• The PEP-II vacuum system supports 40% higher beam current than designed in the HER, and 2.5 times the design beam current in the LER. This is a remarkable achievement!

• Nearly all chambers in the IR have been replaced with new designs in a very successful campaign that began in 2002 and is completed in the present maintenance period.

• All HER RF seals are being replaced in the present maintenance period to address a major operational limitation. Good work!


Vacuuum Systems comments• The run 7 plan calls for increasing the LER current

from 2.9A to 4.0A and the HER currents from 1.9A to 2.2A. The bunch length will be reduced from 11 to 9mm.

• A major risk to the plan to accumulate an additional 250fb-1 is that the vacuum system cannot accommodate the increased beam currents and shortened bunch lengths.

• The most likely limitations would result in a beam current ceiling that cannot be overcome in the remaining 10 months of operation.

• There are some failure modes that would be extremely difficult or impossible to recover from (vertex bellows, extensive fatigue effects)


Vacuum Systems Comments

• It will be important to deliberately check conditions and effects at each stage. For instance, the impact of lower bunch lengths should be studied carefully at much reduced beam currents to understand the HOM heating with bunch length before deploying short-bunch operation in the machine.


Vacuum Systems

Recommendations1. Assess short bunch operation from the

HOM generation and heating standpoint early in the run period, at safely reduced beam currents.

2. Continue to carefully monitor and regularly assess vacuum component temperatures as beam currents are increased and the bunch length is reduced; continue to monitor HOM generation via RF power balance or other methods.


RF and Feedback


RF Findings• With the last installed klystron in HER 4-6 the RF power potentialof PEP II is now completely exploited.

• 2.5 beam aborts (of total 8) per day are caused by the RF (Run 5: 2.9 , Run 6: 1.8), recovery about 30 minutes

• Most klystrons are ‘made at SLAC’, more robust than bought ones.7 spares exist, but generally old and partly limited in power output.No klystron failure in Run 6

• There is no ‘smoking gun’ causing most aborts: many distributed causes and objects. • A large number of faults could be identified and repaired ’forever’.However, power dips (power company) or arcs (‘dirt effects’)are ‘acts of god’


To avoid trips for each power dip, the klystron limit was set to thespecified 1.4 MW and the operation point was fine-tuned

Cavity RF window was replaced and coupler-box TiN coated

Monitor signals are now filtered to avoid HOM induced erroneoustrips.


RF Achievements – Run 6 (LER) (HER)

Beam Current 3.026 (1.4X) 1.96 A (2X)

RF Voltage 4.05 16.5 MV

Voltage/Cav 506 590 kV

Klystron Pwr 755.1 845.9kW

The Challenge for Run 7:

Beam Current 4.00 (1.9X) 2.2 A (2.2X)

RF Voltage 5 17.5 MV

Voltage/Cav 625 625 kV

Klystron Pwr 1005 980 kW


Replacement of drive amplifiersProblem with drive amplifier:

In presence of strong carrier, small side-bands are not amplified linearly anymore -> distortion in feedback loop

-> limits feedback gain unnecessarily low (else loop unstable)

–Bought 17 and installed 15 new pre-amplifiers

–Amplifiers show much improved response and allow

for 20 degrees of comb filter rotation

System is prepared to move to higher current realms in terms of longitudinal growth rates


Pout vs. Pin

600

650

700

750

800

850

900

950

1000

1050

1100

0 10 20 30 40 50 60 70

Pin (W)

Po

(k

W)

75.5 kV

76 kV

75 kV

Power can not reach 900 kW and goes over the top

Klystron saturation


Klystron linearizer ‘solves’ problem for standard operation

But: power dips or drive glitches remain problem cases.Drive glitches under investigation (not solved, difficultsince “rare events”) as high priority task

Tuning-polynomials express cavity detuning as functionof temperature for feed-forward: larger detuning requires‘unnecessary’ RF power.Constant observation required and ‘reset/update’ to beExecuted if changes take place (time consuming)

Important: At highest current all stations are needed, anyirrecoverable failure requires reduction in beam current(s)


The impedance of the RF system (lower order modes) and HOMs drive instabilities for high beam currents, the latter being stabilized by a wide-band longitudinal feedback and the former by a narrow band feedback using the main RF cavities as kicker (woofer).


Examination of total system behavior by simulation including non-linear system components:

The wide band feedback has still enough margin – except the heat development in the longit. kicker and its loads. Kicker timing needs tight control.

But the main RF is presently close to the limits, the highest growth rate is for m=-3.

The new pre-amplifiers are one key component allowing more phase rotation in the comb-filter branch, necessary for higher currents

The HER will keep stability with the present settings also for Run 7. The LER shows strong increase of the growth rate of m=-3 when approaching the beam current foreseen in Run 7.


Due to undercoupling (lower design currents), the RF voltagein the LER has to be increased (!!) to have enough RF powerto accept the Run 7 beam current -> bunch shortening. (changing couplers or l/4 transformers is a major operation)


Summary RF

The RF team did a great job in

• Searching the causes of present faults • Identifying possible weak points that might cause faults under the more stressed conditions of Run 7• and in parallel keeping the system running with high reliability during Run 6• However, the LOM stability in the LER, the available RF powerand and thermal load of l. kickers and loads become marginal

The committee is convinced that everything was done to makeRun7 a success and this under not easy conditions.It is hoped that the present enthusiasm remains alive even under changing conditions in the lab and that the managementgives the necessary support in budget and personnel.


Transverse Feedback• Good work on all aspects• Heating, mechanical issues addressed with

Mo plates, supports• Calculations indicate that kicker devices will

likely survive the larger HOM powers.• Front end electronics should be used for

bunch-by-bunch diagnostics where possible.• Filters between plates and RF amplifiers are

critical and should be monitored for performance and heating.


Beam Stability and Impedance


HER Flex Flange RF Omega Seals & HER DIP storms• Decided to replace all of the HER flex flange RF

omega seals with Ag-coated Inconel fingers after finding that damage to the seals was more widespread than originally thought.

• Inconel seal (non-Ag-coated) installed in July 2007 showed no damage after 1.5 months of operation, while Glidcop seals installed in Oct 2006 show same damage as original seals installed 9 years ago. It is hoped that the Inconel seal will survive in future operation.


HER Flex Flange RF Omega Seals & HER DIP storms (cont)

• Most of worst seals showed very elevated temperature readings, also correlated with vacuum spikes nearby. However, it seems that some seals that showed reasonably good temp performance found to have damage.

• 17 HER DIP storms with aborts and 25 storms without aborts occurred in Run 6. Very high correlation with HER beam on. Either disconnected offending modules or disconnected offending strings, about 10% of total number. Initiating event and mechanism not entirely understood.


Comments & Recommendations• Replacement of all 192 HER flex joint seals is highly

commendable. • Hopefully the temperature of the Inconel seals

installed in Run 6 was recorded and can serve as the baseline survival temperature to use for monitoring purposes in Run 7. There is some risk that possible damage can still occur and not be detected.

• HER DIP storms not entirely understood, but also not a major contribution to total aborts. No successful mitigation short of turning off. Misidentified DIP storms caused instead by HER flex seal failure should be eliminated.


Comments & Recommendations (cont)

• Significant weight placed on flex seal repair to resolve HER vacuum and ion instability issues. HOM effects of small gaps as modeled appear serious – further damage should be avoided in Run 7 to the extent possible.


Instabilities and HOMs

• Longitudinal CBI: Estimate adequate FB stability margin for HER to reach 2.2 A. For LER, higher gap voltage will damp longitudinal Robinson instability FB as well as lower forward power for 4 A. Wideband HOM stability adequate for LER and HER.

• Thermal limits of FB kicker components are a concern and klystron operating margins will be challenging to manage.

• Dust created by HER flex seal failure (melting) postulated to cause instability, vacuum pressure bursts in HER.


Instabilities and HOMs (2)

• HOM analysis and measurements: – Analyzing RF power balance seems to agree with HOM

measurements for LER but not HER, using same diagnostics. Experimental errors should be similar for both.

– One “missing” HER HOM source location identified through wakefield captured by LER antenna pickup, very interesting and notable achievement.

– Analysis shows redesigned HOM-damped bellow impedance appears to be adequate.


Comments & Recommendations

• Several concerns were expressed regarding klystron operating margins at full high current goals. – The CBI FB system needs close monitoring.– Operators/physicists should note that synchronous phase

adjustments for RF system interferes with longitudinal kicker timing, sensitive for HER stability.

• Ion/dust effects: – It will remain to be seen if problem goes away with flex seal

replacement. If not, measure bunch-by-bunch beam size and tune to diagnose to refine theory. Try shaking ions out. Try to understand stability with/without collisions (also dependence on gap) – this could be a factor for luminosity performance.


Comments & Recommendations(cont)

• HER estimated HOM power in Run 7 is doubled that of Run 6. Missing HER HOM source (if truly missing) is somewhat worrisome. Measure HOM power for cavity HOM loads.


Lattice and Beam Dynamics


Lattice and Beam Dynamics

• The committee is impressed with advances in understanding of ring optics, and particularly with the effective use of MIA analyses.

• LER coupling reduction using the PSKs was very successful and we feel it will open the door to further improvements in coupling.

• Initial tests of 90 degree HER optics are encouraging – work remains to fully integrate in production conditions.


Lattice and Beam Dynamics – some questions

Is PEPII already operating at the beambeam limit? With increasing current - Bigger gap transient - More ions in HER ? - More e-cloud in LER ? - Reduced beta* will increase sensitivity to gap transient (hour glass) - More background - More frequent aborts - Increasing risk of catastrophic failure With reduced bunch length - higher specific luminosity [ assuming a good operating point is found to accommodate change in synch tune] - Increased HOM


Lattice and beam dynamics

LER current limit What is the plan to understand/ overcome this limit? - e-cloud in LER - can this be measured? - HER ions - or this? - Parasitic crossings/ nonlinearity? Modeling? - Beambeam interaction - Simulation?

Probably all of the above Recommend MD/modeling/simulation to disentangle

Reduced HER beta* - Is observed background due to beam beam (bunch length) or finite aperture (peak beta)? - MD (admittedly very limited) to date suggests the latter Recommend further MD to answer this question before proceeding with 90 deg optics (or other attempts to shorten bunch)


LER couplingRecommend continued modeling/tuning to reduce residual coupling and emittance

Luminosity tuningRecommend continued development of orthogonal knobsfor tuning and employ automated feedback algorithms where appropriate. Parasitic crossing effects should be incorporated in the model. beam sizes emittance coupling beta*

Reduce beta*/ shorter bunch/ current limits 4 by vs 2 by indicate total current limit (rather than bunch current)) Recommend - reconcile simulation with observations - include parasitic crossings, ions, e-cloud.


Controls & Diagnostics


Controls & Diagnostics - comments

• Excellent work on the upgraded BCM digital bunch-by-bunch system; should improve bunch injection, bunch uniformity. Better handles gap transients.

• Fast luminosity dither feedback system upgrade is proven and was in routine operation at the end of run 6. The system helped to identify, and subsequently cancel, 9.7 Hz IP support tube vibration. This was excellent detective work.

• The IP dither system will be very important for maintaining peak performance.


Controls & Diagnostics - comments

• Bunch length measurement system appears ready for Run 7. Streak camera should be most accurate (analyze bunch-by-bunch). Use these data to calibrate BPM spectrum data accordingly to allow quick measurements.

• SLM system reports IP beam sizes that appear inconsistent with other diagnostics and the MIA model


Controls & Diagnostics - Recommendations

• Analyze bunch length bunch-by-bunch using dual-sweep streak camera data by integrating over few bunches (over small phase slew).

• Bunch-by-bunch beam size and tune diagnostics could potentially play a key role in diagnosing ion and/or electron cloud effects in Run 7. Investigate exploiting bunch-by-bunch signals already available in TFB or fast dither FB system.

• Continue efforts to resolve apparent discrepancies between the projected IP beamsizes and those obtained from the model.


In conclusion ...


During the 6 meetings of this committee we have consistently had very positive, effective, and pleasant interactions with the PEP-II and SLAC staff.We have been very impressed with the assembly of talented and committed people associated with this incredibly successful project.The contributions to both HEP and AP will be long remembered and acknowledged in future work.


The committee wishes to thank the hospitality of Jonathan, Persis, Steve, David, and John as well as the many others who have taken time and effort to put together the excellent presentations and responded enthusiastically and effectively to our numerous, and sometimes irrelevant questions.


We also thank these people who have worked behind the scenes to make this all possible and to take care of our needs while at SLAC:

– Logan Davis– Regina Matter– Robyn Mossett– Peggy Tank– Mimi Chang– Travel office

And for past meetings:– Thanh Ly– Eleanor Mitchell– Ruth Nelson– Julie Schumacher


In closing, we would like to echo comments made during this meeting regarding documentation of the impressive accelerator physics work done at B Factories during the past decade.We encourage publication of papers on the topics.We also suggest that a symposium on B factories be planned in the next year to acknowledge and document the impressive work done at PEP-II, KEK-B and CESR in this area.


Thank you and our sincere wishes for a successful Run 7.

pep-ii mac closeout november 17, 2007 1 pep-ii machine advisory committee closeout session november...

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