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PEP-II Machine Advisory Committee

PEP-II Machine Advisory Committee. Closeout Session November 17, 2007. Committee:. Unable to attend: Lou Bertolini John Corlett Katsunobu Oide Flemming Pedersen Ivan Koop. Kathy Harkay Stuart Henderson Shin-ichi Kurokawa * Pantaleo Raimondi David RIce David Rubin

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PEP-II Machine Advisory Committee

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  1. PEP-II Machine Advisory Committee Closeout Session November 17, 2007 PEP-II MAC Closeout November 17, 2007

  2. Committee: Unable to attend: Lou Bertolini John Corlett Katsunobu Oide Flemming Pedersen Ivan Koop Kathy Harkay Stuart Henderson Shin-ichi Kurokawa * Pantaleo Raimondi David RIce David Rubin Joachim Tuckmantel * Frank Zimmermann PEP-II MAC Closeout November 17, 2007

  3. 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. PEP-II MAC Closeout November 17, 2007

  4. 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 PEP-II MAC Closeout November 17, 2007

  5. Executive Summary PEP-II MAC Closeout November 17, 2007

  6. 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. PEP-II MAC Closeout November 17, 2007

  7. 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 PEP-II MAC Closeout November 17, 2007

  8. 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. PEP-II MAC Closeout November 17, 2007

  9. 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. PEP-II MAC Closeout November 17, 2007

  10. Summary of committee’s responses to its charge -- PEP-II MAC Closeout November 17, 2007

  11. 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 PEP-II MAC Closeout November 17, 2007

  12. 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. PEP-II MAC Closeout November 17, 2007

  13. 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 b* in both rings, have significant work to be done. • Uncertainties in HOM powers with combined higher currents and shorter bunch length PEP-II MAC Closeout November 17, 2007

  14. 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. PEP-II MAC Closeout November 17, 2007

  15. Performance and Upgrades PEP-II MAC Closeout November 17, 2007

  16. 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 PEP-II MAC Closeout November 17, 2007

  17. Performance - Findings -2 • Run 7 objectives – deliver 250 fb-1 in the next 10 months • 4 A LER, 2.2 A HER, lower b*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. PEP-II MAC Closeout November 17, 2007

  18. 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. PEP-II MAC Closeout November 17, 2007

  19. 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 PEP-II MAC Closeout November 17, 2007

  20. Interaction Region PEP-II MAC Closeout November 17, 2007

  21. 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. PEP-II MAC Closeout November 17, 2007

  22. 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. PEP-II MAC Closeout November 17, 2007

  23. Interaction Region Recommendations: • 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. PEP-II MAC Closeout November 17, 2007

  24. Vacuum PEP-II MAC Closeout November 17, 2007

  25. 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! PEP-II MAC Closeout November 17, 2007

  26. 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) PEP-II MAC Closeout November 17, 2007

  27. 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. PEP-II MAC Closeout November 17, 2007

  28. Vacuum Systems Recommendations • Assess short bunch operation from the HOM generation and heating standpoint early in the run period, at safely reduced beam currents. • 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. PEP-II MAC Closeout November 17, 2007

  29. RF and Feedback PEP-II MAC Closeout November 17, 2007

  30. RF Findings • With the last installed klystron in HER 4-6 the RF power potential of PEP II is now completely exploited. • 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 • 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 • 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’ PEP-II MAC Closeout November 17, 2007

  31. To avoid trips for each power dip, the klystron limit was set to the specified 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 erroneous trips. PEP-II MAC Closeout November 17, 2007

  32. 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 1005980 kW PEP-II MAC Closeout November 17, 2007

  33. Replacement of drive amplifiers Problem 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 PEP-II MAC Closeout November 17, 2007

  34. Klystron saturation Power can not reach 900 kW and goes over the top PEP-II MAC Closeout November 17, 2007

  35. Klystron linearizer ‘solves’ problem for standard operation But: power dips or drive glitches remain problem cases. Drive glitches under investigation (not solved, difficult since “rare events”) as high priority task Tuning-polynomials express cavity detuning as function of temperature for feed-forward: larger detuning requires ‘unnecessary’ RF power. Constant observation required and ‘reset/update’ to be Executed if changes take place (time consuming) Important: At highest current all stations are needed, any irrecoverable failure requires reduction in beam current(s) PEP-II MAC Closeout November 17, 2007

  36. 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). PEP-II MAC Closeout November 17, 2007

  37. 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. PEP-II MAC Closeout November 17, 2007

  38. Due to undercoupling (lower design currents), the RF voltage in the LER has to be increased (!!) to have enough RF power to accept the Run 7 beam current -> bunch shortening. (changing couplers or l/4 transformers is a major operation) PEP-II MAC Closeout November 17, 2007

  39. 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 power and and thermal load of l. kickers and loads become marginal The committee is convinced that everything was done to make Run7 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 management gives the necessary support in budget and personnel. PEP-II MAC Closeout November 17, 2007

  40. 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. PEP-II MAC Closeout November 17, 2007

  41. Beam Stability and Impedance PEP-II MAC Closeout November 17, 2007

  42. 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. PEP-II MAC Closeout November 17, 2007

  43. 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. PEP-II MAC Closeout November 17, 2007

  44. 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. PEP-II MAC Closeout November 17, 2007

  45. 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. PEP-II MAC Closeout November 17, 2007

  46. 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. PEP-II MAC Closeout November 17, 2007

  47. 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. PEP-II MAC Closeout November 17, 2007

  48. 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. PEP-II MAC Closeout November 17, 2007

  49. 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. PEP-II MAC Closeout November 17, 2007

  50. Lattice and Beam Dynamics PEP-II MAC Closeout November 17, 2007

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