1. Walter Wuensch SAREC review meeting30-31 January 2012 Direct measurement of the transverse long-range wake-field of the CLIC main linac accelerating structure G. De Michele 1,2,3, E. Adli 1,4, A. Grudiev 1, A. Latina 1, D. Schulte 1, W. Wuensch 1 1 CERN, 2 PSI, 3 EPFL, 4 The University of Oslo W. Wuensch SAREC review meeting 30-31 January 2012

2. Walter Wuensch SAREC review meeting30-31 January 2012 Motivation The main performance issues for the CLIC main linac accelerating structures are: Accelerating gradient Wakefields and higher-order-mode suppression rf-to-beam efficiency (The three issues are marvelously interconnected) The performance of the accelerating structures drives the efficiency and cost of CLIC. We dedicate significant resources to demonstrate high-gradient performance.

3. Walter Wuensch SAREC review meeting30-31 January 2012 Break down rate scaled to 100 MV/m, CLIC pulse At KEK and at SLAC High-gradient testing Status end 1011

4. Walter Wuensch SAREC review meeting30-31 January 2012 We now propose an experimental program to investigate long-range wakefields. Why? Do we expect surprises? Well no, but… Wakefields, and specifically higher-order-mode suppression, is probably being computed very accurately. But we’re currently close to beam dynamics limits and as we push the design towards lower cost we will depend on the accuracy of the simulations more and more. Also accelerating structures are complex objects, for example with lots of silicon carbide loads. The only experimental verification of a waveguide damped wakefields was in ASSET in late 1999 – we feel it is essential to check our latest design, material choice etc. Finally the wakefield measurement is a concrete exercise for our beam dynamics team and their tools. It can only be healthy for those dealing with wakefields in simulation to deal with them in a real machine. Motivation 2

5. Walter Wuensch SAREC review meeting30-31 January 2012 There is a long tradition of measuring accelerating structure wakefields in ASSET

6. XB-10, ICFA Mini-Workshop, R.M. Jones, Cockcroft Inst., 30 th Nov-3 rd Dec 2010 66 DS Q cu RDDS1 ASSET Data Conspectus of GLC/NLC Wake Function Prediction and Exp. Measurement (ASSET dots) DDS3 (inc 10MHz rms errors) DDS1 RDDS1 H60VG4SL17A/B -2 structure interleaved Refs: 1. R.M. Jones,et al, New J.Phys.11:033013,2009. 2. R.M. Jones et al., Phys.Rev.ST Accel. Beams 9:102001, 2006. 3. R.M. Jones, Phys.Rev.ST Accel. Beams, Oct.,2009. 1. GLC/NLC Exp vs Cct Model Wake

7. An Asset Test of the CLIC Accelerating Structure, PAC2000 Higher-order mode damping demonstration in ASSET 1999 150 cells/structure, 15 GHz 24 cells/structure, 12 GHz (loads not implemented yet) Then Now Double band circuit model

8. 30 January 2004 Alexej Grudiev, GdfidL for TDS wakefield calculations Full length TDS results comparison

9. Walter Wuensch SAREC review meeting30-31 January 2012 The CLIC baseline accelerating structure and its wakefield

10. Walter Wuensch SAREC review meeting30-31 January 2012 The CLIC baseline structure Average loaded accelerating gradient100 MV/m Frequency12 GHz RF phase advance per cell2π/3 rad. Average iris radius to wavelength ratio0.11 Input, Output iris radii3.15, 2.35 mm Input, Output iris thickness1.67, 1.00 mm Input, Output group velocity1.65, 0.83 % of c First and last cell Q-factor (Cu)5536, 5738 First and last cell shunt impedance81, 103 MΩ/m Number of regular cells26 Structure length including couplers230 mm (active) Bunch spacing0.5 ns Bunch population3.72×10 9 Number of bunches in the train312 Filling time, rise time67 ns, 21 ns Total pulse length243.7 ns Peak input power61.3 MW RF-to-beam efficiency28.5 % Maximum surface electric field230 MV/m Maximum pulsed surface heating temperature rise 45 K H s /E a E s /E a S c /E a 2

11. Wakefield simulations Different EM codes have been used to simulate the wake- fields in the CLIC RF prototype with dispersive materials: GdfidL, CST Particle Studio®, ACE3P Different materials (i.e. different EM properties) have been tested in order to meet the beam dynamics requirements 11 CST: superPC, 128 GB of RAM and 24 CPUs GdfidL: distributed and shared memory Transverse wake-field

12. The predicted wake-field 12 Transverse real impedance Transverse wake-field Three main dipolar bands in the Impedance spectrum Absolute value of the envelope of the transverse wake 2 nd bunch

13. What we need from measurements Roll-off speed to be confirmed up to 0.25m Wake-field threshold Identification of frequencies from wake-field measurements: high sampling of the wake-fields with the witness bunch 13 Roll-off threshold

14. Critical issue for simulation in time domain: dispersive materials GdfidL – The permittivity can be expressed with an N-th order Lorentz medium with resonant angular frequencies ω n and damping angular frequencies γ n : – This feature has been debugged in late 2011! CST 2012 version – the transient solver has undergone many improvements, in particular with respect to, e.g., broadband constant loss tangent. The broadband sensitivity analysis with respect to geometry and material property variation has also been enhanced. 14 4 th order Lorentz medium

15. Walter Wuensch SAREC review meeting30-31 January 2012 Wakefield measurements of the CLIC baseline accelerating structure in ASSET/FACET

16. Possibility of having driving and witness bunches with positrons and electrons. Adjustable bunch spacing for a timing span behind the driving bunch. Bunch length flexibility: ideally shorter than 1mm in order to resolve the 3 rd dipole band which shows up a peak around 40GHz. 16 Why FACET? Layout of the ASSET facility POSSIBLE LOCATION

17. Where in FACET 17 ASSET CLASSE LI02 sector

18. Walter Wuensch SAREC review meeting30-31 January 2012 Young people from the CLIC study (not me) are becoming familiar with the linac and the wakefield measurement

19. Experimental procedure 19 transverse wakefield drive bunch offset witness charge drive bunch charge active length of the structure energy of the witness bunch The transverse wake-field can be extrapolated from the measurements of the deflecting angle of the witness bunch drive bunch length witness bunch length Transverse wakefield

20. Experiment simulations with tracking code PLACET BPMs upstream and downstream of the structure are needed in order to determine the bunch offset in the structure, as well as dipole corrector magnets to generate this offset. only BPMs downstream of correctors are sensitive to the correction itself. That means that the lattice response is triangular. 20 Response matrix from the south extraction kicker in SRD to LI02 A. Latina G. De Michele

21. 21 Positrons: offset scan to drive wakefield The BPMs just before and after the RF prototype are LI02, 201 and LI02, 211. The correctors used are only two upstream these two BPMs. With only two correctors seems possible to shift the positron beam in the RF prototype. To be verified the maximum voltage that can be applied to the correctors.

22. Electrons: Deflected by wakefield We can compute the witness bunch deflections from betatron oscillation fits to data from the BPMs downstream of the test structure, and corrected for incoming orbit jitter using the results from similar fits to data from BPMs in the NRTL line [C. Adolphsen et al., PRL 74 13, 1995] For the fit of trajectory parameters, one needs the knowledge of BPM locations and lattice parameters, i.e. quantities which have no stochastic errors. For the fit of lattice parameters, however, one has to correlate BPM data with trajectory parameters, i.e. quantities which will in general have significant stochastic noise components. In this case, the fit must take into account the noise on the measured trajectory parameters due to finite BPM resolution [ SLAC- CN—371]. Once we have simulated all the experiment with the tracking code PLACET, our routines will be Interfaced with the SLAC Control Program (SCP). Hopefully in March 2012 first tests. 22

23. 23 BPMs resolution From the experiment proposal: An accurate data analysis is required Good knowledge of the optics is essential Orbit fit to determine the kick

24. Lattice simulations and verification in 2012 run The experiment can be reproduced numerically with all imperfections. Data acquisition as well as data analysis can be simulated. From the physical point of view the experiment can be completely simulated. We have only deal with technical problems and software integration on SCP. 24

25. Walter Wuensch SAREC review meeting30-31 January 2012 The test structure – a simplified assembly since the dipole mode Q’s are low

26. main strongback in the ASSET area FACET prototype scheme 26 six TD26 accelerating structures simple vacuum tank no power will be sent to the test structure and the double feed couplers will be terminated by the loads clamped aluminum cells available length: 76.80 inches vacuum tank  1.5m 76.80 inches beam pipe transition bellow vacuum pump vacuum pump Support and alignment system vacuum flange

27. AS CLIC accelerating structure for ASSET 6 sections, 24 cells each, compact coupler design Clamped aluminum disks To be tested at SLAC Ø180mm TD26 with Compact Couplers (x6), clamped Cell shape accuracy 20µm Aluminium disk Damping material (SiC) Threaded rod for AS clamping Clamp for SiC fixation Aluminium disks Features for Tank alignment on the girder Features for AS alignment in the Tank Reference spheres BEAM A. Solodko

28. Uses existing vacuum tank Estimated total load of the CERN installation is about 400 kg Two vacuum ion pumps (noble diode pumps, 53-55 l/s) The RF structure is aligned mechanically inside the vacuum tank The vacuum tank will be aligned to the beam axis by means of adjustable feet (drawings needed) Two tooling balls on both ends are present on top of the vacuum tank 28 Mechanical design: features

29. CLIC meetingWalter Wuensch6 May 2011 What full prototype cells look like Machining: OFHC copper diamond milled and turned disks with micron precision. +- 5 micron tolerance lines G. Riddone, S. Atieh

30. Available longitudinal space 76.80inches RF structure disks diameter: 180mm RF structure longitudinal dimensions: 6 X 250mm Beam pipe: 1inch outer diameter Vacuum flanges: 2 ¾ inches Distance between beam-pipe center to main strongback: 8.795inches Accelerating cell inner radius: 2.35mm 30 Mechanical design: dimensions

31. Walter Wuensch SAREC review meeting30-31 January 2012 Schedules

32. Coordination with FACET runs (may already be obsolete) 2012 - CLIC personnel participate in linac measurements in preparation for wakefield measurements. 2013 - After commissioning of positrons, we have two options: – Re-measure the installed DDS then install the CLIC RF structure – Install and measure only the CLIC RF structure But in any case the CLIC RF structure will be ready for September 2012. 32

33. Overall Schedule 20122013 Q1Q2Q3Q4Q1Q2Q3Q4 Changing machine parameters and BPMs data acquisition Shipping of the CERN prototype to SLAC Final assembly at SLAC Re-measurements of the DDS structures which are still in place in LI02 Installation Direct measurements of wake- fields for CLIC RF structure 33