1. Dark current in TESLA linac
R. Brinkmann
TESLA collaboration meeting, Saclay, April 3, 2002

2. 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)

3. 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

4. Lost particle energy and average power into 2K along main linac (1st 50 GeV)

5. Consequence: need quality control beyond measuring Q0
Assume maximum tolerable heat load at 10…20% 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!

6. 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 g=20MV/m additional heat load from re-accelerated secondary charged particles seems to be small (order of magnitude <10% of primary loss), not enough statistics and linac length simulated yet
Will continue to investigate limit on g from avalanche effect