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

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

Trip Recovery

-50

0

50

100

150

-10 0 10 20 30 40 50

Time (s)

E s

truc

ture

(M

V/m

)

T1 pump down T2 field recoverybeamloading

EA

EB

This requires that PETS can be switched off from one pulse to next andthat field can be ramped with constant phase !

Permissible Trip Rate for type I breakdown as a Function of Linac Energy Overhead

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 NTrip is a random number it will scatter around this value with a standard deviation NTrip

½ . Assuming that we want to cope with 6 standard deviations from <NTrip> we get the condition

with R the fractional voltage reserve of the linac

REPTrip TTXMNN 21

9618

21 TTMN

RNRNX

REP

With

L=0.229 m EA=100 MV/m EB=-20.5 MV/m N=3.3 TeV/(L EA)=144106 M=2 T1=20 s T2=20 s

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 PZ by 954 keV/c.A change of field direction in a single cell by 0.30 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< 710-8

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.

Y

YZY PP

2

CMSE

GL

NX

100100

1

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 900.

2RA

ZT

PRP

A

Z

T

P

Pwith

@1500GeV 54m@9GeV 4.2m

mm13.2

R

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

PT distribution of breakdowns !

0 5 10 15 20 25 300

20

40

60

80

100

120

140

160

Frequency (GHz)

Ac

ce

lera

tin

g G

rad

ien

t (M

V/m

)

Dark current capture limit

dark current captureCLIC oldCLIC newNLCILC

2

e

mcGcapture

How important is dark current ?

Why bother about dark current, overfocusing of main beam quadrupoleswill clean off d.c. electrons !

But at high energy end of linac quadrupoles are spaced by 10 modules,or 80 accelerating structures.

mA 0.24

gain)power beammain of (1%W 180,80

)1(

)1(

quadrupolenext

toed transportiscurrent dark captured all that Assuming

1

dark

dark

darkstructpulsrep

darkdark

darkstructpulsrep

N

kdarkstructpulsrepdark

I

PN

NNIVTf

PI

NNIVTf

IVkTfP

RF gun’s

Dolgatchev data

CTF II, Cu HIGGS

What’s a good value for EMAX ?P-Linac people use Kilpatrick criterion for structure design with braveness factor.Braveness 2.5 is considered as pushing to the very limits.