Franck Peauger, Riccardo Zennaro Status of Wakefield Monitors developments for CLIC accelerating structures 25 Sept. 2009 Franck Peauger, Riccardo Zennaro Alexandre Samoshkin

Outline Context and requirements Time domain simulations of wakefields in AS Design of WFM RF transition Integration in the Two-Beam Test Stand 180° Hybrid coupler Test set-up in CTF3

Context Wakefield Monitors are Beam Position Monitors integrated to the drive beam Accelerating Structures (AS) It allows beam-based alignment of AS to remove wakefield effects and emittance growth Emittance growth is very well improved by aligning the AS to an RMS accuracy of 5 µm to the beam

Wakefield kicks from misaligned AS can be cancelled by another AS Context Girder Movers AS with WFM Electron bunch D. Schulte Wakefield kicks from misaligned AS can be cancelled by another AS One WFM per structure and mean offset of the 8 AS computed

WFM dev. plan & requirements Step 1 (2009 - 2010): build one WFM prototype and integrate it into a CERN structure and test on TBTS with CALIFES probe beam Step 2 (2010 - 2011): build 2 or 3 structures fully instrumented and test on TBTS + CALIFES

WFM additionnal requirements Since there is no place available in the linac, the damping waveguides of the middle cell are used to measure the beam position inside the structure we cannot define the geometry, frequency mode, Q factor … as we do for BPM the WFM works necessarily with strong damped modes (Q ~ 10) !! The WFM design consists in studying the HOM modes and their sensitivity to a beam offset, choosing an operating mode, designing an RF transition to couple this mode and processing the signal We must attenuate the 12 GHz high power accelerating signal to -150 dB !! The RF transition must not reflect any signal to the cell (typically -20 dB from 10 to 35 GHz) The middle cell equipped with the WFM must keep its strong damping functionality (SiC loads) The WFM must be cheap, easy to integrate mechanically in the Two Beam Module and avoid if possible any major additional machining of the cell

Wakefield simulation with beam offset GDFIDL Simulations: Five cells meshed (no symmetry) with a mesh step of 0.1 mm volume limited to +/- 15 mm in the X and Y directions PML set at the waveguide extremities (Xmin, Xmax, Ymin, Ymax, Zmin, Zmax) Beam: 1 bunch of 0.6 nC, σz=3 mm with offset Simulation stopped at 6.66 ns. Rectangular ports at the end of the damped waveguides of the middle cell. The two first modes are selected in GdfidL : ydamphaut xdamphaut xdampbas ydampbas 1st port mode: Fc = 13.3 GHz 2st port mode: Fc = 21.4 GHz Total of 90e6 meshs Time computation of 9 to 13 hours with 36 hosts on LXCLIC cluster E field E field TM like mode for the beam TE like mode for the beam

18 GHz = First dipole-band mode TM like modes with beam offset of 1 mm Y+ Port signal amplitude (voltage) X- & X+ Beam dx=1mm offset X- X+ Y+ & Y- Time (s) Y- Port signal amplitude (voltage) Port signal amplitude (voltage) 11.94 GHz 18 GHz = First dipole-band mode Time (s) F (Hz)

monopole / dipole mode configuration Monopole mode Dipole mode E Field E Field Opposite ports signals are in phase Opposite ports signal have opposite phase When we substract the opposite port signals, the monopole mode is cancelled and the dipole mode amplitude is increased

TM modes after 180° perfect recombination Recombined port signal amplitude (voltage) Y+ Beam dx=1mm offset DX=X+-X- X- X+ DY=Y+-Y- Y- Time (s) Recombined port signal amplitude Recombined port signal amplitude (voltage) 18.19 GHz 14.81 GHz 11.95 GHz F (GHz) Time (s)

Variation of beam offset amplitude Recombined port signal amplitude DX (voltage) dx = 1 mm dx = 0.8 mm dx = 0.6 mm dx = -1 mm Time (s)

The 18.2 GHz mode works quite well as a cavity BPM mode ! Linearity Max. amplitude of the delta signal (voltage) Offset dx (mm) The 18.2 GHz mode works quite well as a cavity BPM mode ! But we need to design an RF transition that couple this mode, attenuate 12 GHz mode, avoid reflection, allows strond damping, etc…

RF transition design Long waveguide with cut-off above 12 GHz + 90° E-bend Coax coupler designed to have -10 dB transmission 55 mm Load location 15 mm Cell radius of CLIC accelerating structure (D = 140 mm) 70 mm =

Geometry of WFM RF transition Coaxial waveguide (K type) Damped waveguide F1 Antenna F2 p Load R2 R1 d b L2 a Cell L1 Cell axis

RF transition transfer function T(f) -11 dB 1 2 3 S11 T (f) (S parameters) S13 -145 dB S12 12 GHz 18 GHz

~ 1 – 3 GHz band pass filter around 18.1 GHz TM modes after RF Transition and 180° perfect recombination with dx=1mm beam offset Recombined port signal amplitude (voltage) Y+ U0 U1 X- DX=X+-X- X+ DY=Y+-Y- Y- Time (s) Recombined port signal amplitude 18.14 GHz 13.88 GHz Given by HFSS Given by Gdfidl ~ 1 – 3 GHz band pass filter around 18.1 GHz F (GHz)

Resolution Dipole mode 18 GHz: Accelerating mode 12 GHz: The power and voltage are linked by: Dipole mode 18 GHz: Simulation results (at the coax pick-up, after the RF transition) dx=1mm, q=0.6nC → U1 ≈ 1 V Extrapolation for the commissioning case (voltage varies linearly with offset and charge) dx=5µm, q=0.06nC, (s=70µm) → U1 = 500 µV Extrapolation for the nominal case dx=5µm, q=0.6nC, (s=60µm) → U1 = 5 mV Accelerating mode 12 GHz: For 60 MW input power, there is 3 nW at the coax pick-up, after the RF transition → equivalent to UHP = 56 µV + Thermal noise and noise from signal processing to be evaluated

Integration in the Two Beam Test Stand Design a support and ensure good electrical contact between the WFM and the structure Must make a hole in U support for the “Y- waveguide”

Integration in the Two Beam Test Stand Or We would like to reserve one available flange (150 mm diameter) for a special CF flange with four feedthroughs

180° Hybrid coupler Tapered Coupled Line Hybrid Magic Tees Hybrid couplers are the special case of a four-port directional coupler that is designed for a 3dB (equal) power split and a 180 degree phase shift between two output ports Ring Hybrid Junction Tapered Coupled Line Hybrid Magic Tees When using as a combiner, input signals are applied at port 2 and 3, the sum of the inputs will be formed at port 1 while the difference at port 4. In theory: S12 = S13 = -3dB and S11 = S14 = -40 dB

180° Hybrid coupler Two tapered-line directional couplers cascaded Excellent phase and amplitude matching Realized with a three-layer stripline configuration Etched on opposite sides of a thin coupler circuit board, sandwiched between a pair of equal thickness Duroid boards

Layout of WFM prototype test with Hybrid : WFM – transition, qty 4, output connector = K female, UHV compatible : semi rigide cable, Type?, qty 4, length ≈ 400 mm , input connector = K male, output connector = K female, UHV compatible : CF flange feedthrough, qty 1, 4 connectors, input connector = K male (vacuum side), output connector = K female, UHV compatible : Flexible cable, qty 4, length ≈ 30 m , input connector = K male, output connector = K female, : 180° Hybrid coupler, input connector = K male, output connector = K female TBTS rack Scope Diode detector Bandpass filter ≈ 30 m Klystron gallery Front Back Accelerating structure TBTS Accelerating structure tank CLEX – beam tunnel

Conclusion (1/2) The WFM Context, specifications and development program well defined with the objective to demonstrate the WFM concept before end of 2010 We have developed a methodology to run Gdfidl from CEA Saclay on the CERN Cluster, and couple the results to HFSS simulations The TM like mode at 18.2 GHz has been identified and well studied in time domain. It will be used as classical cavity BPM dipole mode with 180° recombination We proposed a simple design of the WFM RF transition which should meet the long list of requirements We would like to investigate TE like modes around 23 GHz. Additional simulations are required But we also would like to freeze the design of the WFM prototype soon in order to start the mechanical study and procurement of components We should not forget to work on the electronic for signal processing

Thank you for your attention Conclusion (2/2) Important topic for CLIC Very interesting R&D program Good continuation after CALIFES for CEA Saclay Thank you for your attention

Extra - slides

Wakefield simulation with one symetry GDFIDL Simulations: Five cells meshed with one symetry (half of the structure is meshed) Perfect magnetic boundary condition on XZ plane mesh step of 0.05 mm PML set at the waveguide extremities (Xmin, Xmax, Ymax, Zmin, Zmax) Beam: 1 bunch of 0.6 nC, σz=1 mm, offset Δx = 1 mm Simulation stopped at 6.66 ns. Rectangular ports at the end of the damped waveguides of the middle cell. The first modes is selected in GdfidL so that longitudinal (TM) modes can be recorded Total of 156.8e6 meshs 14 hours with 36 hosts machines lxclic

TM modes responses Beam 1mm offset Y+ X- X+ 11.83 GHz 15.15 GHz

TM modes after 180° perfect recombination Y+ DX=X+-X- X- X+ 11.98 GHz 15.19 GHz 18.47 GHz

RF cable under vacuum

CF Flange feedthroughs CF Flange: Dext152mm x2 Dimensions (inches) A=0.63 B= 0.92 C=2.75 (69.85mm) D= 1.38 E=0.87 Flange Size=2.75"CF Number of feedthrough = 4 Grounded 50-Ohm Ref. = IFDCG042013

Two Beam Module integration Vac. Manifold: 30 x 30 mmm²