Download - Definition of breakdown types Type I: small transverse kick, affects only collision effect
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.