1. An Overview of the CLIC RF Structure Development Program ~
A Tale of Two Structures
W. Wuensch
CLIC ACE

2. Outline Introduction to the PETS and accelerating structures
Specialties of structure development
People, organization and resources
Collaborations
Organization of this meeting

3. CLIC module Main Beam ~1 A Accelerating structure, Alexej
+100 MV/m, 64 MW, 229 mm
Drive Beam 100 A
rf distribution
PETS, Igor
-6.5 MV/m, 136 MW, 210 mm
Module in more detail Germana

4. Fundamental mode properties
PETS
CLIC_G
Aperture [mm] 23
Phase advance
π/2
2π/3
Vg/c
0.45
R’/Q [kΩ/m]
2.2
15-16
Length [mm]
210
229
Pmax [MW]
136 64
Esurfmax [MV/m] 56
245
Pin/CtPp1/3[MW/mm ns1/3]* 13 18
ΔT (ºK) * 2
Allowable breakdown rate (Daniel’s talk)
O(10-7 to10-6)
O(10-7)
moding
overmoded
single
* pulse length 240 ns
High-power constraints, Alexej

5. Full structure properties
PETS
Accelerating structure
linear field increase
Power flow
Direction may be a crucial issue for damage potential (Igor) along with degree of tapering.
Pulse shape
(Frequency domain calculation almost ready to verify energy spread compensation, Raquel Fandos)

6. Higher order mode properties
The drive beam has single bucket spacing and the main beam has six bucket spacing.
Damping mechanism
PETS: Heavy damping through eight symmetry slots
Accelerating structure: Heavy damping with detuning. Heavy damping from four cutoff waveguides in CLIC_G. Option of addition of damping slots if pulsed surface heating too strong a constraint.
cell
first
middle
last Q1 10
7.7
6.3
A1 [V/pC/mm/m]
117
140
156
f1 [GHz]
16.74
17.21
17.67
PETS
Accelerating structure

7. Higher order mode properties continued
Wakefields computed using GDFDL and HFSS. Benchmark from old ASSET experiment. Main uncertainty now is from load material but this can be resolved later.

8. Accelerating structures
Power couplers
E max (135 MW)=41.7 MV/m
H max (135 MW)=0.07 MA/m
PETS couplers were extremely difficult to figure out due to over-moding. Now this robust choke–based coupler solution must be verified with low and high power tests.
Accelerating structures
Mode launchers are used in current generation of test structures for simplified interpretation of results.
More compact couplers will be added once structures are working OK. Electric or magnetic coupling will depend on damping.

9. Tolerances Accelerating structure
PETS: 10 μm precision so not a critical issue.
Accelerating structure: Shape and transverse-alignment tolerances 5 μm. Good progress towards this goal on test pieces, disks and quadrants. Going as low as 1 to 2 μm for the bookshelf. This is not obvious… Review of requirements by Riccardo and achieved progress by Mauro.
Accelerating structure
± 2.5 μm band
PETS
2 μm shape error with ‘banana’ removed

10. Accelerating structures
Technology
PETS: Milled octants in copper, forget about disks. Copper can be heat treated if necessary due to low pulsed surface heating.
Accelerating structure: Baseline test-structure assembly is disks in copper that have diamond turned irises with milled damping waveguides, then brazed/bonded (NLC/JLC and very old CLIC). BUT soft copper may not be tolerable so CuZr alloy milled quadrants may be required. Also this might be the better mass production solution anyway… And no heating cycle after machining best for maintaining tolerances. And the PETS are milled. More from Mauro.
PETS
Accelerating structures

11. PETS on/off
Key question: After a breakdown does the power to the PETS or accelerating structures need be turned down to get the structure to run stably again?
If yes only for the accelerating structures we can implement a waveguide variable attenuator.
If yes for the PETS then we need a way to suppress power production even with the beam going through the PETS. This is hard. If we need to ramp the PETS back on again, this is probably very hard. Igor’s talk.
We will only know the behavior of the PETS once we have tested it, and even then it won’t be certain immediately so we have initiated a PETS on/off study in the mean time.

12. Specialties of structure development
Structure and linac design: Beam dynamics, wakefields, bookshelves, tilts, HOM suppression, tolerances and alignment, couplers, high-gradient-high-power performance and overall machine optimization.
High-gradient-high-power RF testing: Power sources, infrastructure (components, electronics, etc.), test structure fabrication and testing itself.
Technology: Fabrication, achieving tolerances, materials, material states, surface preparation, vacuum.
High-power physics and mechanics and specialized experiments: Understanding breakdown and its dependencies. Lifetime - pulsed surface heating. RF constraints.
High-gradient-high-power RF and beam testing: Breakdown kick, beam loading and breakdown rate, PETS operation, acceleration and deceleration. Two-beam test stand and TBL.
CLIC module integration: Cooling, Vacuum and instrumentation like wakefield monitor.
Cost:

13. Schematic view of the structure development program
dc spark testing
High-power RF testing
High-power RF constraints
Technology
Linac design
Accelerating structure design
PETS design
Beam-based testing
Pulsed surface heating tests
Beam-based testing

14. People, organization and resources
The CERN manpower working on structures comes from a number of sections, groups and departments. Roughly ¾ are in AB and ¼ are in TS . In AB most are from the RF group, in TS most are from the MME group.
A significant effort comes even from outside of CERN
Total number involved
Total combined
staff full-time on structures 4
staff part-time on structures* 9
3.5
Fellows, Doctoral students, and associates 7
Visitors at CERN 5
Guessing at outside CERN
4-5
Total
23.5
* not including services we pay for

15. People, organization and resources
Most of the progress comes from the efforts of highly talented and highly motivated individuals who make their own direct contacts with each other.
To increase the coherence of the radiated output from these individual sources, we have a weekly ‘team meeting’ (chaired by me). Here we have a broad exchange of information and a forum to describe work in progress and launch new ideas among colleagues. Presentations stored in a special directory.
Over the past year a series of specialized meetings have been put into place and chaired by Germana (more information in talk) for work that requires precise coordination including:
Test structure production - weekly
Mechanical design – weekly
Two-beam test stand – weekly
CLIC module working group – biweekly
Structure tolerances – approximately monthly
Phone meeting with SLAC and KEK - approximately monthly
Minutes of all these meetings are available on EDMS with actions clearly defined.

16. People, organization and resources
Sergio holds a bi-weekly ‘Breakdown Studies’ meeting for detailed technical discussion among those, especially fellows and PhD students, working on the dc spark, rf breakdown and pulsed surface heating experiments.
Then there is the ‘Structure Working Group’. Here larger subjects like testing plans, proposed new nominal CLIC and test structures, optimization results etc. are presented, discussed, possibly modified and approved. The makeup is the structure team plus other relevant members of the CLIC study.
There is also the CTF3 operational meeting which is also used to discuss test results during 30 GHz testing.

17. People, organization and resources
Last year we had a materials budget of 1.2 MCHF and we committed about 1.9 MCHF which just so happens to be this year’s budget.

18. Collaborators with specific linear collider expertise
SLAC: High-power testing, pulsed-surface-heating testing, test structure fabrication and a significant exchange of ideas.
KEK: Test structure fabrication and high-precision machining. High-power testing is planned.
Cockcroft Institute (Roger Jones): Structure and beam dynamics simulations are planned.
PSI (Micha Dehler): Interest in structure design and breakdown theory.
(not Fermilab or DESY)

19. Active collaborators
Helsinki Institute of Physics: Precision mechanical engineering. Breakdown simulation.
Uppsala University: Two-beam test stand beam-lines, instrumentation and experiments. Breakdown experiments.
CIEMAT: Test-PETS and RF components
US High-gradient collaboration
Gycom/Nizhniy Novgorod: High-power RF components
Pakistan: Two-beam test stand hardware.
Dubna: Pulsed surface heating experiment
DAPNIA, SACLAY: Superconducting-level cleaning now. Next hopefully test accelerating structure fabrication and wakefield monitor.

20. Organization of this meeting
New collaborators
Frascati: Accelerating structure RF design.
Ukraine: Plasma physics applied to RF breakdown and exotic materials
Organization of this meeting
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