Dark current in TESLA linac

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Dark current in TESLA linac

• R. Brinkmann
• TESLA collaboration meeting, Saclay, April 3, 2002

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Outline of the problem

• Field emitted particles are accelerated to
  hundreds of MeV before being kicked out by
  quadrupoles
• Losses into 2K Helium can be much higher than
  indicated by measured Q value on cavity test
  stand
• Generated secondary electrons (positrons) can be
  re-accelerated ? enhancement of losses, in worst
  case avalanche effect (ongoing study by N.
  Golubeva, V. Balandin)

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Crude estimate of losses from simple simulation

• Use phase space distribution of dark current at
  exit of 9-cell cavity from Stolzenburg thesis
• Transport through downstream lattice (cavities
  and quads) until trajectory hits aperture limit
• Record energy deposition, assume all goes into 2K
  Helium, disregard generation of secondaries

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Lost particle energy and average power into 2K
along main linac (1st 50 GeV)
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Consequence need quality control beyond
measuring Q0
Assume maximum tolerable heat load at 1020 of
RF losses into cavity walls ? dark current per
cavity must be kept below 50nA at TESLA design
gradient If Q0 reduction due to dark current beam
loading, 50nA corresponds to ?(1/Q)
1/(5?1011) This is probably not visible in Q vs.
g curves!
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Preliminary results on simulation of dark current
losses including e.m. shower (Balandin/Golubeva)
Use EGS4 plus tracking code with e.m. fields in
cavities Realistic geometric setup with all
material, quads, etc. So far, at g20MV/m
additional heat load from re-accelerated
secondary charged particles seems to be small
(order of magnitude lt10 of primary loss), not
enough statistics and linac length simulated
yet Will continue to investigate limit on g from
avalanche effect
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