Direct Wakefield measurement of CLIC accelerating structure in

Direct Wakefield measurement of CLIC accelerating structure in FACET Hao Zha, Andrea Latina, Alexej Grudiev (CERN) 18/06/2015 High Gradient work shop 2015 in Tsinghua University 1

Transverse long-range Wakefield in CLIC-G structure Structure name CLIG-G TD 26 cc Work frequency 11. 994 GHz Cell 26 regular cells+ 2 couplers Length (active) 230 mm Iris aperture 2. 35 mm 3. 15 mm transverse long-range wakefield calculation using Gdfidl code: Peak value : 250 V/p. C/m/mm At position of second bunch (0. 15 m): 5~6 V/p. C/m/mm Beam dynamic requirement: < 6. 6 V/p. C/m/mm Beam dynamic requirement Very strong damp : 40 ~ 50 times Position of second bunch 2

Experiment history and participant • • • 2010. 07. 15: Letter of intent to SAREC for a program of measurements for the CLIC study at the FACET facility 2011. 04. 05: Recommendation from SAREC for submitting a proposal for the experiment at FACET 2011. 10. 14: Proposal is submitted to SAREC 2012. 01. 31: Proposal is presented at SAREC review 2012. 04. 18: Proposal is accepted by SAREC and experiment has got a number: E-208 • 2013. 02. 13: Delay of positrons at FACET. No positrons in 2013. • • • 2014. 01. 18: Good news on positrons. Positrons commissioned in 2014. 06. __: Structure prototype has been shipped to SLAC 2014. 11. 27 -2014. 12. 03: Measurements has been done successfully!!! • •

Direct wakefield measurement in FACET • Prototype structure are made of aluminium disks and Si. C loads (clamped together by bolts). • 6 full structures, active length = 1. 38 m • FACET provides 3 n. C, 1. 19 Ge. V electron and positron. • RMS bunch length is near 0. 7 mm. • Maximum orbit deflection of e- due to peak transverse wake kick (1 mm e+ offset): 5 mm, BPM Damping resolution: 50 um AS AS AS Aluminium disk material (Si. C) e-, NRTL e+, Driven bunch e-, Witness bunch Dipole Dump Dipole e+ Downstream BPMs Kick CLIC-G TD 26 cc e+, SRTL e. Transverse offset deflected orbit AS 4

Procedure of measurement Driven bunch offset typically 1 mm Structure length: 1. 38 m Positron charge: ~3 n. C Given positron offset respond matrix of positron Bump positron Kick electron Witness beam orbit Response of BPM to the kick Measure electron orbit respond matrix of electron Calculated wake 5

• correctors Nearest Upstream BPM Simulated kick Downstream BPMs Tested structure Nearest Downstream BPM “response orbit” 6

Measurement in FACET • Before we measured deflect orbit: - Measure the response matrix of e+: in order to bump orbit of driven bunch with given offset. Measure the response matrix of e-: in order to calculate the absolutely value of wake kick. Dispersion free correction: decrease e- orbit jitter due to energy jitter. • We measure: - 252 points in time-domain wake potential (by changing e+/e- timing); Charge transmit of both bunches were monitored in real time; Each point with 5~7 e+ offset. Each offset we take data of 100 shots. 7

Measurement in FACET Almost overlap e+/e- spacing = 2 mm e+/e- spacing = 15 cm CLIC bunch separation Decay 40~50 times 8

Wakefield analysis Deflected Orbit [mm] Response orbit R 12[mm] of 1 Ke. V kick One point Shot 1 …… Shot 100 Offset 1 kick 11 …… Kick 1, 100 Offset 2 kick 21 …… Kick 2, 100 ⁞ ⁞ …⁞ … ⁞ Offset 7 kick 71 …… Kick 7, 100 ⁞ 9

Slow beam orbit drift • Averaging on 100 shots removes fast jitter. • Slow random drift of e- orbit is observed (2 ~ 5 Ke. V equivalent) • This limits the minimum resolution (2σ) of results by 0. 5 V/p. C/m/mm. Kick of e- without e+, should = 0 SVD (singular value decomposition) 10

Slow drift correction • Use SVD to identify drift mode (or drift source) • Use linear algebra to remove drift modes from orbit. • Can remove 2 major modes (1 betatron oscillation + 1 dispersion) Orbit of e- without e+, should have no kick Orbit decomposition Kick Signal + = Drift From orbit to kick (vector product or projection) + = Calculated kick Real Kick Drift correction = Calculated kick Real Kick Error due to projection of drift + No projection No error 11

After drift correction • This drift correction is used for when wakefield kick is zero or very weak (<= 0. 1 V/p. C/m/mm resolution is achieved). • Drift correction will not be used for strong wakefield kick because of: - Resolution (2σ) ≈ 0. 6 V/p. C/m/mm Orbit drift is much smaller than deflected orbit, signal noise ratio is already very high. It will change the calculated wake by a certain ratio, means for strong kick the error will increase. Resolution (2σ) ≈ 0. 4 V/p. C/m/mm Resolution (2σ) ≈ 0. 1 V/p. C/m/mm 12

Final results • Peak value is 12% lower than simulations. • Wake potential at second bunch separation = 5 V/p. C/m/mm < 6. 6 V/p. C/m/mm 13

Timing shift • There is a timing shift in the measurements. • Should coming from the phase calibration (confirmed from SLAC people) Frequency shift ~300 MHz Unphysical-like mode Timing (or spacing) mismatch 14

Timing correction 15

Systematic error? • • • Measured results are ~12% lower than simulations Error of positron charge? Error of longitude bunch length? Error of BPM response to kick ? Should be unidentified systematic error; W ⊥ should be enlarged by 1/0. 88 -1=14% in the worst case: W ⊥(one bunch seperation) = 5*1. 14 = 5. 7 < 6. 6 V/p. C/m/mm (requirement) Measurements Simulations Frequency Amplitude Q-factor [GHz] [V/(p. C m mm)] 17. 1 180 6. 3 17. 0 205 6. 8 26. 9 53 11. 5 26. 8 59 13. 7 40. 0 27 23. 1 39. 7 31 16 39. 8

Conclusion • We successfully measured the absolutely long-range transverse wakefield potential in CLIC-G TD 26 cc. • The results show expedited attenuation of HOMs, and meet the BD requirement. • After applying lots of tools (DFS, SVD, etc. ), we manage to get very accurate results. Minimum resolution (2σ) < 0. 1 V/p. C/m/mm. • Artificially adjust the timing shift will have more physical results, and the shift like to be a real effect. • Excellent agreement between measured results and simulated one were demonstrated, increasing our trust in the simulation code. • Unidentified systematic error is observed, but results still meet the requirement. 17

Acknowledgement • We thank Paul Scherrer Institute (PSI) financially support the prototype manufacturing through FORCE 2011 funding. • We would give great thank to Giovanni De Michele for the preparation of the prototype and experiments. • We thank Anastasiya Solodko by the hard work of engineer design and fabrication of the prototype. • We thank SLAC people: Christine Clarke, Jerry Yocky, Nate Lipkowitz and all the operators’ great help during the measurements. 18