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The LEP Machine

Explore the history, unexpected problems, and performance limitations of the LEP machine at CERN, including the need for a large radius and SC RF.

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The LEP Machine

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  1. The LEP Machine Steve Myers SL, CERN

  2. LEP Lay-Out

  3. Topics • Why is LEP so big? & Why is SC RF needed? • Principal Performance Limitations • History of LEP with Beam • Some unexpected problems • Some really unexpected events • or Bed-time stories for your children and grand-children • Concluding Remarks

  4. Why is LEP so Big? Why SC RF? Losses due to Synchrotron Radiation E0 = .511MeV for electrons and 938.256 for protons Power Dissipated in the walls of the Cu cavities Power to Beam from the SC cavities..... So to minimise power you need  to be as large as possible i.e. large radius. The radius for LEP1 was optimised for around 80GeV with Cu cavities LHC For protons since E0 is a factor of 1836 higher, the RF power is not an issue and the bending radius can be made as low as is technically possible. i.e. High fields For sc cavities the power needed is “only” proportional to the 4th power of energy. NOTE to operate LEP at 103 GeV with copper cavities would have needed 1280 cavities and 160MW of RF power!! Impossible for many reasons

  5. Principal Performance Limitations • Beam Energy • RF Volts (Gradients) • Optics to a much less extent • Luminosity • Beam current  • Beam beam limit  • Beam size at the interaction point (b*, emittance, ..) • Precision of the Energy calibration

  6. History of LEP with Beam • 1988: July 12: Octant test • 1989: • July 14, First turn (15 minutes ahead of schedule!) • August 13, First Collisions • Aug13--Aug 18: Physics pilot run • Aug 21--Sept 11: Machine Studies • Sept 20-- Nov 5 Physics • 1990--1994: Z physics • 1995: Z + 65 & 70 GeV • 1996: 80.5--86 GeV • 1997: 91--92 GeV • 1998: 94.5 GeV • 1999: 96--102 GeV • 2000: 102--104.4 GeV  Exciting period, But usually not very productive But LEP closure is one year (maybe 2) behind schedule

  7. 1989 Start-UP • The Economist August 19, 1989 “The results from California are impressive, especially as they come from a new and unique type of machine. They may provide a sure answer to the generation problem before LEP does. This explains the haste with which the finishing touches have been applied to LEP. The 27km-long device, six years in the making was transformed from inert hardware to working machine in just four weeks--- a prodigous feat, unthinkable anywhere but at CERN.............. ........Even so, it was still not as quick as Dr. Carlo Rubbia, CERN’s domineering director-general might have liked”. SLC

  8. 1989 Start-Up

  9. Summary of Performance

  10. Modes of Operation Every Year was Different

  11. Performance over the Years

  12. Performance in 2000

  13. Performance in 2000

  14. Performance in 2000

  15. Beam tube Some Unexpected problems • 97/98 shutdown • many RF antennae cables electrically damaged, some melted • Limitation on the beam current in 1998 • bunch length dependent • energy ramp modified to maximise the bunch length damaged area of cables super insulation blanket

  16. Cold Cable extrapolated From Measurements with beam Heating of RF antennae cables • antennes used for cavity control • heated by coupling to beam • 8W limit imposed • 30 antennae in the last three weeks of running in 1998

  17. Bunch Length Control during Ramp

  18. Other Problems with cables Where is the dirty rat who ate my cables?

  19. Very Unexpected Problems: Moon

  20. Very Unexpected Problems: Water

  21. Noise on the Beam Energy

  22. Very Unexpected Problem • Influence on the beam energy • the moon, sun and tides • the level of lake Geneva • the amount of rain • AND the fast train.........

  23. TGV induces current in LEP vacuum chamber

  24. RF: pumping up the voltage • Strategy to maximise physics time: • Run at an energy where we have some RF margin • Increase the RF voltage gradually • When stable at sufficient voltage, increase the energy • Drink the champagne • Repeat as many times as possible... But... keeping it there requires a huge effort! 101/100 GeV 100 GeV 98 GeV 96 GeV

  25. Distribution of cavity gradients (96 to 104 GeV) 100 GeV: Mean Nb/Cu 6.9 MV/m 96 GeV: Mean Nb/Cu 6.1 MV/m 104 GeV: Mean Nb/Cu 7.5 MV/m

  26. With plenty of volts, we’re cruising... • After 2 days at 101 GeV ... • available RF voltage 3510 MV • margin 210 MV (2 klystrons can trip)

  27. ...with a few less, it’s less easy • Still at 101 GeV... • but available RF voltage down to 3440 MV • margin 140 MV (1 klystron can trip) This fill at 100 GeV

  28. Q: 102GeV: How did we get there?? A “by lowering luminosity and breaking cavities”. Q: Can we go further?? A: “Yes, by further lowering the luminosity and breaking MORE cavities”. The RF group’s 1999 collection

  29. Oops!! LEP repeatedly trips after 10 to 30 minutes. The time between trips decreases with time unless you do not try to switch on. Problem was on the sextupole chains

  30. Some really unexpected events Could not get the beam to circulate more than 15 turns even with large bumps all around the ring. Use single turn orbit system and normalised the measurement. Single Turn Stopper QL10.L1 positrons

  31. Zoom sur Quadrupole beer bottle

  32. 10 metres to the right beer bottle Unsociable sabotage: both bottles were empty!!

  33. 1996: Heineken Beam Stopper

  34. Conclusions • LEP was a challenge and a lot of effort and fun • Physics output was exceptional (still waiting for Higgs/SUSY) • I would like to take this final (hopefully not!) opportunity to sincerely thank all the people who have worked on the LEP and the detectors for their motivation, devotion and hard work. It has been a fantastic experience for all of us

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