1. H.H. Braun, Structure WG, 13.9.2007 Permissible Trip Rates for 12 GHz structures in CLIC  Type of break downs  Permissible rate for type I RF breakdown  Permissible rate for type II RF breakdown  Permissible rate for type III RF breakdown  Conclusions

2. Definition of breakdown types Type I: small transverse kick, affects only collision effect Type II: medium transverse kick, affects luminosity Type III: large transverse kick, destruction of main linac components

3. T 1 pump down T 2 field recovery beam loading EAEA EBEB This requires that PETS can be switched off from one pulse to next and that field can be ramped with constant phase ! Permissible Trip Rate for type I breakdown as a Function of Linac Energy Overhead

4. If X is the probability that a structure break down in a pulse, we will have in average with N the total number of structures and M the number of structures connected to a PETS. Since N Trip is a random number it will scatter around this value with a standard deviation N Trip ½. Assuming that we want to cope with 6 standard deviations from we get the condition with R the fractional voltage reserve of the linac

5. With L=0.229 m E A =100 MV/m E B =-20.5 MV/m N=3.3 TeV/(L E A )=144106 M=2 T 1 =20 s T 2 =20 s

6. Permissible trip rate for type II RF breakdown The vertical momentum of the beam electrons have a gaussian distribution with this r.m.s. value varies along the linac in the range 5-35 keV/c Each single cell of an accelerating structure increases P Z by 954 keV/c. A change of field direction in a single cell by 0.3 0 during a breakdown event is therefore sufficient to bring the beams out of collisions during this pulse. If a field distortion of this magnitude occurs, the implication is that all machine pulses with a break-down in a single structure are lost for luminosity. If this is true the permissible breakdown probability X for a 1% luminosity loss is corresponding to a trip probability of X< 7  10 -8 This reasoning is somewhat pessimistic, since the impact of a transverse kick depends on the local  function and the (random) azimuthal direction of the kick.

7. Type III RF breakdown The geometric acceptance of the main linac is given by a transverse kick with can be sufficient to steer the beam on the structure aperture. To get 4 MV transverse kick  4 cells have to turn field by 90 0.

8. Conclusions  Permissible rate for type I RF breakdown (with 5% voltage overhead) 10 -5  Permissible rate for type II RF breakdown 10 -7 structures at low energy are more critical than those at high energy  Permissible rate for type III RF breakdown 0 structures at low energy are more critical than those at high energy TBTS experiment essential to connect breakdown probability with  P T distribution of breakdowns !

9. How important is dark current ?

11. Why bother about dark current, overfocusing of main beam quadrupoles will clean off d.c. electrons ! But at high energy end of linac quadrupoles are spaced by 10 modules, or 80 accelerating structures.

12. RF gun’s Dolgatchev data CTF II, Cu HIGGS What’s a good value for E MAX ? P-Linac people use Kilpatrick criterion for structure design with braveness factor. Braveness 2.5 is considered as pushing to the very limits.