The LEP Machine

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The LEP Machine

• Steve Myers SL, CERN

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LEP Lay-Out
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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

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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
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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

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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
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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, CERNs domineering director-general
  might have liked.

SLC
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1989 Start-Up
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(No Transcript)
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Summary of Performance
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Modes of Operation
Every Year was Different
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Performance over the Years
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Performance in 2000
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Performance in 2000
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Performance in 2000
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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
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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

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Bunch Length Control during Ramp
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Other Problems with cables
Where is the dirty rat who ate my cables?
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Very Unexpected Problems Moon
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Very Unexpected Problems Water
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Noise on the Beam Energy
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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.........

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TGV induces current in LEP vacuum chamber
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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
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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
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With plenty of volts, were cruising...

• After 2 days at 101 GeV ...
• available RF voltage 3510 MV
• margin 210 MV (2 klystrons can trip)

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...with a few less, its 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
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The RF groups 1999 collection

• 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.

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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
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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
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Zoom sur Quadrupole
beer bottle
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10 metres to the right
beer bottle
Unsociable sabotage both bottles were empty!!
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1996 Heineken Beam Stopper
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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