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Low Vg a/ λ =0.16. High Vg a/ λ =0.19. So why?. The first cell puzzle…. - PowerPoint PPT Presentation
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June 2007, CERN
June 2007, CERN
HDS 60 (cells) copper was processed from both sides
Low Vga/λ=0.16
High Vga/λ=0.19
HDS 11 titanium
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
The first cell puzzle…
So why?
June 2007, CERN
- The breakdown initiation- The RF pulse dependence- The damage mechanism- and …
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.
will not be addressed
June 2007, CERN
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
June 2007, CERN
29 29.5 30 30.5 3140
30
20
10
0
F, GHz
S-pa
ram
eter
s, dB
Traveling wave
Configuration #1 (breakdown resonant fuse) :Resonant cavity with reduced electric surface field (HO1) is located between structure and waveguide.
Standing wave
June 2007, CERN
0 2 4 6 80.9
1
1.1
1.2
1.3
1.4
R. Zennaro (June 2007)
0 2 4 6 83
3.5
4
4.5
5
Vg=PLc/W
P/W
Stored Energy/cell
Cell #Cell #
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 #
NLC inline taper V. Dolgashev (May 2002)
Inline taper with increased stored energy
Inline taper with a “speed” bump
1-st cell
Configurations #...
June 2007, CERN
June 2007, CERN TWO FREQUENCY STRUCTURE
Transversely “0”
Transversely “pi”
Type 1 Type 2
June 2007, CERN
1
0
E x112
z09990 x0 10 20
0
0.5
11
0
0.505
0.651
E x 0( )
E_11i
z09990 x z1i z0500 z15000 10 20
0
0.5
1 1
0
E x16
z09990 x0 10 20
0
0.5
1 1
0
E x312
z09990 x0 10 20
0
0.5
11
0
E x4
12
z09990 x0 10 20
0
0.5
1
E x5
12
x0 10 20
0
0.5
1
1
0.650887
Re E z0500 tt
30 tt0 0.5 1 1.5 2 2.5 3
1
0
1
1
4.871054 10 7
0.504
0.705
E x 0( )
E_11i
z09990 x z1i z0500 z15000 10 20
0
0.5
11
8.769268 10 7
E x112
z09990 x0 10 20
0
0.5
1
E x16
x0 10 20
0
0.5
1
E x112
x0 10 20
0
0.5
1 1
1.432727 10 6
E x16
z09990 x0 10 20
0
0.5
1
0.307 0.395( )0.505
2
20.966
1
1.826586 10 6
E x312
z09990 x0 10 20
0
0.5
11
1.96676 10 6
E x412
z09990 x0 10 20
0
0.5
1
E x512
x0 10 20
0
0.5
1
1
0.704153
Re E z0500 tt
30 tt0 0.5 1 1.5 2 2.5 3
1
0
1
Type 1
Type 1
Transverse Ez (t) distributions