1. Test Accelerating Structures Designs, Objectives and Critical Issues
Riccardo Zennaro
CLIC-ACE CERN 1/16/2008

2. X band test structures 8 new designs in 2007 and 17 new structures under construction
Test of CLIC structure prototype (vg1) (4 different designs and 9 structures)
Test of different geometries (aperture and iris thickness)
Test of P/c
Comparison of damped and undamped structures
Comparison of technology: quadrant to disk
30 GHz test structures 8 new designs in 2007 and 12 new structures under construction
Test of different geometries (aperture and iris thickness)
Test of P/c
Comparison of technology: quadrant to disk
Comparison of damped and undamped structures
Test of different cleaning procedures (in collaboration with Sacley)
Test of new ideas (TM02, Speed bump)
CLIC-ACE CERN 1/16/2008

3. CLIC-ACE CERN 1/16/2008 Courtesy of G. Riddone CERN, KEK, SLAC vg1

4. The vg1 family: the design derives from the CLIC_C structure
Vg1 is short version of CLIC_C (18 regular cells instead of 24) renormalized to GHz
The group velocity in the last regular cell is ~1%c (vg1) instead of 0.65 (CLIC_C)
Easier for machining
Structure C
RF phase advance per cell: Δφ [o]
120
Average iris radius/wavelength: <a>/λ
0.12
Input/Output iris radii: a1,2 [mm]
3.87, 2.13
Input/Output iris thickness: d1,2 [mm]
2.66, 0.83
Group velocity: vg(1,2)/c [%]
2.39, 0.65
N. of reg. cells, str. length: Nc, l [mm]
24, 229
Bunch separation: Ns [rf cycles] 8
Number of bunches in a train: Nb
311
Pulse length, rise time: τp , τr [ns]
297, 30
Input power: Pin [MW]
64.6
Max. surface field: Esurfmax [MV/m]
298
Max. temperature rise: ΔTmax [K] 56
Efficiency: η [%]
23.8
Luminosity per bunch X-ing: Lb× [m-2]
1.3×1034
Bunch population: N
4.0×109
Figure of merit: ηLb× /N [a.u.]
7.7
vg1
CLIC-ACE CERN 1/16/2008
Courtesy of A. Grudiev

5. The CLIC vg1: CLIC prototype
Collaboration between CERN,KEK,SLAC
8 structures,
4 different designs
Structure name
CLIC_vg1
RF phase advance per cell: Δφ [o]
120
Average iris radius/wavelength: <a>/λ
0.128
Input/Output iris radii: a1,2 [mm]
4.06, 2.66
Input/Output iris thickness: d1,2 [mm]
2.794, 1.314
Group velocity: vg(1,2)/c [%]
2.4, 0.95
N. of cells, structure length: Nc, l [mm]
18, 179
CLIC-ACE CERN 1/16/2008

6. VG1 mode launcher
The mode launcher of theT53 has been adopted for the vg1 with some minor modification (R circuluar w.g mm instead of mm)
Acceptable E field enhancement(~45 80 MW)
Broadband coupler
CLIC-ACE CERN 1/16/2008

7. CLIC_VG1 RF evaluation before shipping to SLAC for brasing (T. Higo, K
CLIC_VG1 RF evaluation before shipping to SLAC for brasing (T. Higo, K. Yokoyama, N. Kudoh, T. Takatomi) A B
Average phase advance per cell
degrees / cell
Average phase advance per cell
degrees / cell
Courtesy of T. Higo
CLIC-ACE CERN 1/16/2008

8. Comparison of damped and undamped structures and technology (disks vs
Comparison of damped and undamped structures and technology (disks vs. quadrants)
The vg1 family (X band):
T18_vg2.4_disk
TD18_vg2.4_disk
T18_vg2.4_quad
TD18_vg2.4_quad
Direct comparison of technology (quadrant/disk)
Direct comparison of dumped/undamped structures
CLIC-ACE CERN 1/16/2008

9. HDS4_vg2.6_thick #1 (Sacley)
Comparison of damped and undamped structures, technology (disks vs. quadrants), and cleaning procedure
30 GHz
HDS: hybrid damping
NDS: No damping
Cleaning provided by high pressure water flow.
Test of bars
C30_vg_4.7_quad
C30_vg_4.7_disk (tested)
HDS4_vg2.6_thick #1 (Sacley)
HDS4_vg2.6_thick #2
NDS4_vg2.5_thick (tested)
Direct comparison of technology (quadrant/disk)
Direct comparison of dumped/undamped structures
Direct comparison of surface cleaning
CLIC-ACE CERN 1/16/2008

10. 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)
d [mm]
a [mm]
2.79
2.13
2.00
1.66
1.37
1.25
2.53
Vg: 0.7%
CLIC _vg1
output 1.0%
2.85
T53 output
1.0%
3.0
CERN-X
1.1%
Vg: 1.35%
3.87
3.89*
Vg: 2.25%
(*)
30 GHz 2π/3 ≈2.6%
T53 input
Vg: 3.3%
4.38
30 GHz 2π/3 4.7%
5.00
30 GHz
π/2 7.4%
2π/3 8.2%
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-ACE CERN 1/16/2008
(*) not very different from input vg1 (d=2.79; a=4.06)

11. The test matrix
C10_vg0.7
C10_vg1.35
C10_vg2.25
Reference structure:
C10_vg3.3 (T53 input)
C30_vg2.6
30 GHz 2p/3
30 GHz p/2
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)
CLIC-ACE CERN 1/16/2008

12. To be compared with the C30_vg4.7
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”
Same phase advance
Same P/c
Same aperture and iris shape
Same field configuration in the iris region
but
Different group velocity: 4.7% & 2%
Different R/Q: 29 kΩ/m & 12 kΩ/m
TM02 regular cell
CLIC-ACE CERN 1/16/2008
To be compared with the C30_vg4.7

13. TM02 structure
2p/3
TM02
CLIC-ACE CERN 1/16/2008

14. Why speed bump? HDS 11 Ti HDS 60 L PINC HDS 60 S PINC
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.
HDS 60 L
PINC
HDS 60 S
PINC
What do we certainly know, the breakdown ignition is a very fast process: 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.
We can tray by reducing vg in the matching cell
CLIC-ACE CERN 1/16/2008

15. To be compared with the C30_vg4.7
Speed bump (TM03)
R= mm
R_iris= mm
Iris_thickness= 1mm
To be compared with the C30_vg4.7
CLIC-ACE CERN 1/16/2008

16. Other test structures available at CERN
HDS11_small_Mo
(Molybdenum)
HDS11_small_Ti
(Titanium)
Pulse heating cavity
HDS11_small_Cu
(copper)
Strong H field enhancement (~6* MW)
CLIC-ACE CERN 1/16/2008
Direct comparison of material

17. Conclusions
The test accelerating structure program intend to evaluate the performances of the VG1 (CLIC prototype)
The test accelerating structure program intends to investigate the response of the b.d. rate to several free parameters
In particular:
geometrical parameters test matrix
P/c, Sc, field on surface test matrix
Group velocity (vg) TM02
The test accelerating structures should also give indications as to how the performances depend on the technology (quadrants/disks) and on the presence of damping waveguides
The tests will provide information not only on the b.d. rate but also on the b.d. damage distribution: speed bump
The tests could also give indications regarding the companies that made the different structures
CLIC-ACE CERN 1/16/2008