CLIC Breakdown Wor kshop 5/20/2008 1 RF Design Options for Quenching Breakdowns Igor Syratchev, Riccardo Zennaro
CLIC Breakdown Workshop 5/20/2008
1
RF Design Options for Quenching Breakdowns
Igor Syratchev, Riccardo Zennaro
CLIC Breakdown Workshop 5/20/2008
2
HDS 60 L
PINC
HDS 60 S
PINC
Very often we do observe, that after accelerating structure processing the most of the surface modifications take place in a few first cells. Also the number of cells involved is correlated with the group velocity, the less the Vg the fewer cells modified.
As one of the conventional explanation one could expect the statistical distribution of the events in a chain model. However with adopted processing strategy (trip rate ~10-3) the event probability and normalized to that damage distribution is calculated to be very flat.
0 20 40 600
0.5
1
1.5
Cell no
Nor
mal
ized
tota
l dam
age
HDS 60
HDS 11 titanium
So why?
CLIC Breakdown Workshop 5/20/2008
3
What do we certainly know, the breakdown ignition is a very fast
process: 0.1 -10 ns. If so, one can propose the main difference
between the “first” and “second” cell is accessible bandwidth.
And the lower group velocity the more the difference.
The first cell, if breakdown occurs is loaded by the input
coupler/waveguide and is very specific in terms of bandwidth.
Other words, the first cell can accept “more” energy during
breakdown initiation then consequent ones.
Worse to mention that we do not know the exact transient
behavior of the breakdown and the structure bandwidth could play
important role.
CLIC Breakdown Workshop 5/20/2008
4
RF current source Iejwt
Structure: 2pi/3 aperture 3.5 mm (Vg=4.5%)
Ib, kA/mm2
Pout
Pin = 50MW
28.5 29 29.5 30 30.5 310.01
0.1
1
F, GHz
Breakdown ‘naive’ modeling in HFSS
Radiation spectra(breakdown in cell#1 )
To the output coupler
To the input coupler
Missing energy plot
CLIC Breakdown Workshop 5/20/2008
5
29 29.5 30 30.5 3140
30
20
10
0
F, GHz
S-p
ara
mete
rs,
dB
Traveling wave
Configuration #1 (breakdown resonant fuse) :Resonant cavity with reduced electric surface field (HO1) is located between structure and waveguide.
Standing wave
CLIC Breakdown Workshop 5/20/2008
6
NLC inline taper
V. Dolgashev (May 2002)
0 2 4 6 80
1
2
3
4
Vg=PLc/W
0 2 4 6 80.2
0.4
0.6
0.8
1
1.2
Stored Energy/cell
Cell #Cell #
Inline taper with a “speed” bump
1-st cell
CLIC Breakdown Workshop 5/20/2008
7
Why speed bump?
Can we reduce the bandwidth and provide a tapering without increasing the overall length?
We can tray by reducing vg in the matching cell
CLIC Breakdown Workshop 5/20/2008
8
Speed bump (TM03)
R=14.398 mm
R_iris= 2.428 mm
Iris_thickness= 1mm
Bandwidth
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
29.5 29.6 29.7 29.8 29.9 30 30.1 30.2 30.3 30.4 30.5
f (GHz)
S12
(dB
) Speed bump TM03
Speed bump TM02
Original
0
0.01
0.02
0.03
0.04
0.05
0.06
0 1 2 3 4 5
cell #
vg
/c
2nd mode speed bump
regular cell nominal value
3rd mode speed bump
3rd mode,final version
To be compared with the C30_vg4.7
CLIC Breakdown Workshop 5/20/2008
9
Same phase advanceSame P/cSame aperture and iris shapeSame field configuration in the iris region
but
Different group velocity: 4.7% & 2%
Different R/Q: 29 kΩ/m & 12 kΩ/m
TM02 structureIs it possible to change some global parameter without changing local field distribution?
Only by changing the propagating mode
TM02 regular cell
TM01 regular cell “reference”
To be compared with the C30_vg4.7
CLIC Breakdown Workshop 5/20/2008
10
TM02 structure
Predicted gradients for the test structures calculated for different parameters (normalized to the experimental results of T53 for b.d.r.=10-6 and pulse length=100 ns)
0
20
40
60
80
100
120
140
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
vg (%)
E (
MV
/m)
P/c
Sc
E on surface
1/sqrt(vg)
Measured
2p/3TM02
CLIC Breakdown Workshop 5/20/2008
11
Conclusions
Experimental results shows a correlation between
RF damage and group velocity
This correlation could be explained in terms of
accessible bandwidth
The speed bump structure should inhibit the large
breakdown damage in the first cells
The TM02 structure should provide indications on
the correlation between the gradient at a given
b.d. rate and the group velocity
CLIC Breakdown Workshop 5/20/2008
12
Extra slides….just in case
CLIC Breakdown Workshop 5/20/2008
13
d [mm]a [mm]
2.79 2.13 2.00 1.66 1.37 1.25
2.53 Vg: 0.7% CLIC _vg1output 1.0%
2.85 T53 output1.0%
3.0 CERN-X1.1%
Vg: 1.35%
3.873.89*
Vg: 2.25%(*)
30 GHz 2π/3 ≈2.6%
T53 inputVg: 3.3%
4.38 30 GHz 2π/3 4.7%
5.00 30 GHz π/2 7.4%
30 GHz2π/3 8.2%
The test matrix (all structures in disks)
In red: 11.4 GHz new structures
In blue: 30 GHz new structures (scaled values for a and d)
(*) not very different from input vg1 (d=2.79; a=4.06)
Direct comparison of variation of a and P/c
Direct comparison of variation of d
Direct comparison of variation of P/c
Test for a relatively large group velocity
CLIC Breakdown Workshop 5/20/2008
14
0
20
40
60
80
100
120
140
160
180
200
220
240
0 1 2 3 4 5 6 7 8 9 10
vg (%)
E (
MV
/m)
(P/c)
(Sc)
E on surface
1/sqrt(vg)
Reference structure:
C10_vg3.3 (T53 input)
C10_vg0.7 C10_vg1.35
C10_vg2.25
C30_vg2.6
C30_vg8.2
Predicted gradients for the test structures calculated for different parameters (normalized to the experimental results of T53 for b.d.r.=10-6 and pulse length=100
ns)
The test matrix