Recent 30 GHz results @ CTF2
M. Gerbaux, S. Döbert, Jan Kovermann, R. Zennaro
Outline
•
Introduction
•
30CNSDsbCu_speed-bump

Motivation

High power RF test results

•
TM020 3.5 mm structure

Motivation

•
Effect of the speed bump on the damages
High power RF test preliminary results
Conclusion and future plans
To set the stage…
Linac
Thermionic gun
CR
F
D
F
D
D F D
D FD
D F D
D FD
F
D
F
F
D
F D
D
F
D
F
D
F
D
F
D
D F
D F
F D
D F D
CLEX
CTF2
30 GHz production (PETS line)
and test stand
30 GHz activities :
• Reminiscence of the time when CLIC operating frequency was not yet 12 GHz.
• Now used to test scientific hypothesis.
• The speed bump and TM02 structures are based on the design of the older 3.5 mm
diameter structure.
Why speed bump?
From Igor’s presentation at
the X band workshop:
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.
PINC
PINC
HDS 60 L
HDS 60 S
What we do 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.
In other words, the first cell can accept “more” energy during breakdown initiation than
the following ones.
We do not know the exact transient behavior of the breakdown and the structure
bandwidth could play important role.
Speed bump (TM03)
R=14.398 mm
0.06
Test structure in disks: 30 cell and identical mode
launcher of the conventional “3.5 mm structure”
0.05
vg/c
0.04
0.03
2nd mode speed bump
regular cell nominal value
3rd mode speed bump
3rd mode,final version
0.02
0.01
R_iris= 2.428 mm
Iris_thickness= 1mm
0
0
1
2
3
4
cell #
Goal : « protect » the structure by lowering Vg in the first cell (usually the most damaged).
Tested in both direction :
• RF fed from the input (4.1×106 pulses, 2186 breakdown) → the speed bump plays its role
• RF from the output (1.7×106 pulses, 501 breakdown) → the speed bump has no effect
→ Equivalent to 19 + 8 « SLAC hours » @ 60 Hz…
5
Breakdown rate calculation
100
Accelerating gradient
90
80
70
60
50
40
30
1606
1608
1610
1612
1614
1616
Time (h)
1618
1620
1622
1624
1626
OK
@ 50%
@ 75%
100
Pulse length
80
60
40
20
1606
1608
1610
1612
1614
1616
Time (h)
1618
1620
1622
1624
1626
6
Breakdown3.5
rate
mm
vs structures
gradient – comparison
speed bump structure
10
-2
C30-sb
Upper limits
• Vertical error bars calculated assuming an error of 1 on the breakdown
number and 500 on the number of pulses.
10
-3
• Horizontal error bars : standard deviation of the measured gradient.
• -RedOld
circles
arestructure
upper limits (no breakdown recorded).
3.5mm
• Fit doesn’t take into account these points.
Breakdown rate
No effect observed on the breakdown rate : similar results in both directions and for
10 mm structure (same design without speed bump).
the 3.5
-4
10
10
-5
-6
65
70
75
80
85
Peak gradient (MV/m)
90
95
100
C30-SB
Reversed C30-SB
All breakdowns
All breakdowns
550
550
500
500
450
450
400
400
Number of occurrences
Number of occurrences
Number of breakdowns in the two experiments
350
300
250
200
350
300
250
200
150
150
100
100
50
50
0
5
15
25
Power (MW)
35
0
5
15
25
Power (MW)
35
General view of the 30CNSDsbCu_speed-bump afer cutting
Speed bump
Iris 1
Iris 2
Iris -2
SEM inspection was performed on these irises
Iris -1
Damages
Iris 1
Iris -1
Damages
Iris 2
Iris -2
TM02 structure
Is it possible to change some global parameter without changing local field distribution?
 Only
by changing the propagating mode
TM01 regular cell “reference”
•
•
•
•
but
Same phase advance
Same P/c
Same aperture and iris shape
Same field configuration in the iris region
TM02 regular cell
• Different group velocity (4.7% vs 2%)
• Different R/Q (29 kΩ/m vs 12 kΩ/m)
Test structure in disks : 30 cells, same mode launcher as the “conventional” 2π/3, Ø 3.5 mm.
TM020 structure – preliminary results
10
10
-2
3.5 mm
C30-SB
Reversed C30-SB
TM02 upper limits only
TM02 last "real" points
3
4
27
2
-3
25
Breakdown rate
11
30
12
28
24
10
5
7
-4
1
20
14
15
17
19
13
6
10
29
10
26
8
23
9
22
-5
21
16
18
10
At the moment, equivalent to ≈26 « SLAC hours »
-6
50
55
60
65
70
75
80
Peak gradient (MV/m)
85
90
95
100
TM020 structure – preliminary results
10
Breakdown rate
10
10
10
10
-2
-3
TM02 upper limits only
TM02 last "real" points
TM02 fit
3.5 mm
C30-SB
Reversed C30-SB
-4
-5
-6
50
55
60
65
70
75
80
Peak gradient (MV/m)
85
90
95
100
Conclusion and future plans
• The 30CNSDsbCu_speed-bump worked well and the speed bump seems to
reduce the damages due to breakdowns.
• This suggests to test a « speed bump » structure at 12 GHz.
• The TM020 structure is still under test (still conditioning ?) but the results are
not very promising.
• If vg was the key parameter, the achievable gradient at a given BD rate should
be higher than for the other 3.5 mm structures.
• If it was rather surface field, the results should be more or less the same.
• The experiment confirms neither one nor the other. Is this only due to
fabrication issues or is there another key parameter?
• It underlines the difficulty to draw conclusions with a single structure !
Direct comparison of Vg
Vg
TM01: 2π/3 Vg=4.7%
d
a
TM02: 2π/3 Vg=2.0%
( as  ds  vgs )  ( a1  d1  vg1)
C30-sb
2500
Inc
Trans
Refl
In total :
• 4,101,250 pulses, mainly at 1 Hz
corresponding
18.99due
SLAC
Weird to
things
to hours at 60 Hz
calibration problems
• 2186 breakdowns
(now solved)
2000
Energy
1500
1000
500
0
0
1
2
3
4
Pulse number
5
6
Mathias Gerbaux - CTF3 Collaboration Technical meeting - 27/01/2009
7
8
x 10
4
18
C30-sb-reversed
Inc
Trans
Refl
3000
1 point every 50 pulses
In total :
2500
• 1,704,650 pulses, mainly at 1 Hz
corresponding to 7.89 SLAC hours at 60 Hz
• 501 breakdowns
Weird things due to
calibration problems
(now solved)
Energy (mJ)
2000
1500
1000
500
0
0
0.5
1
1.5
2
2.5
Pulse number
Mathias Gerbaux - CTF3 Collaboration Technical meeting - 27/01/2009
3
3.5
x 10
4
19
Reversed C30-SB
All breakdowns
All breakdowns
500
500
400
400
Number of occurrences
Number of occurrences
C30-SB
300
200
100
0
500
300
200
100
1500
2500
Energy (mJ)
3500
0
500
1500
2500
Energy (mJ)
3500