H.H. Braun, Structure WG, 13.9.2007 Permissible Trip Rates for 12 GHz structures in CLIC Type of...
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Transcript of H.H. Braun, Structure WG, 13.9.2007 Permissible Trip Rates for 12 GHz structures in CLIC Type of...
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.