Measurement Principle and Tuning of the CLIC Crab Cavity

1. Measurement Principle andTuning of the CLIC Crab Cavity 12. Feb. 2014

2. Outline General information on the Crab Cavity and motivation for this talk Electromagnetic field pattern close to the axis [Ex, Ey, Ez, Hx, Hy, Hz] Strategy for Bead-pullmeasurements: basing them on Ey only Results of 1st Bead-pull measurements Tuning and measurement results of the Crab Cavity Summary Tuning of CLIC Crab Cavity

3. CLIC Crab Cavity • designed by Cockcroft Institute, Lancaster University, EDMS 1159170 • very good support from Graeme Burt and Praveen-Kumar Ambattu • extracted fields on axis and off-axis (CST) from Praveen-Kumar Ambattu • .sat Model from Praveen-Kumar Ambattu,(here simulated with HFSS) • symmetric structure (constant impedance) • directions: z: beamy: deflection Tuning of CLIC Crab Cavity

4. General information, Goal, Suggestions • General information: • Crab Cavity designed by Cockcroft Institute, Lancaster Universityand CERN • production coordinated by CERN (BE/RF/PM) • tuning by CERN (BE/RF/LRF) • preparation (bake out, etc.), installation and high power testing by CERN (BE/RF/PM, BE/RF/MK, BE/RF/LRF) • Goal: develop a reliable tuning method and tune the CLIC Crab in short time • => no need to re-study RF design, data provided by Cockcroft Institute, Lancaster University • => no need to measure all electromagnetic field components • => find a reliable method and apply it ! • Suggestions: • using a double bead-pull with a dielectric bead and a metallic bead + data processing to determine the electric and the magnetic field (as Ben Hall used for his PhD) • using a double bead-pull with 2 different bead shapes (e.g. a small cylinder = needle and a disc = washer) to couple to different field components complicated ! Tuning of CLIC Crab Cavity

5. Is there a simple method for Bead-pulling? • Background: • the CLIC Crab Cavity is a multi-cell cavity • most important is the correct phase advance per cell for synchronism with the beam=> automatically for good RF designs, the correct amplitude patterns settles • all cells need to be tuned = adjusted in volume to reach desired frequency / phase advance=> several bead-pull measurements need to be performed to verify effect of tuning=> ideally a bead-pull measurement after each tuning operation=> about 30 (+ 10 at different frequencies) bead-pull measurements in total (best case) • Reliability: Performing several bead-pull measurements in the same state shall give the same information (mainly phase advance per cell and amplitude profile) • => we are looking for a way to get the necessary information for tuning (= phase advance per cell, amplitude profile) with a single, reliable bead-pull measurement Tuning of CLIC Crab Cavity

6. Electromagnetic field pattern • beam is deflected in y-direction by Ey and vz*μ0*Hx=ZF0*Hx(! fields taken at different moments in time) • observations for x~0, y~0: • max(|Ey(z)|) @ location of irises • max(|Hx(z)|) in middle of cells • max(|Ez(z)|) in middle of cells • max(|Ez(z)|)<max(|Ey(z)|) Tuning of CLIC Crab Cavity

7. z- dependency: backward travelling wave • negative group velocity at operation point • wide pass band (11.9 to 12.8 GHz) Tuning of CLIC Crab Cavity

8. EM-fields, deflecting Ey Ey(x,y) nearly homogeneous close to symmetry axis (here R0.5 mm) ! Ey^2 in preparation for bead-pull pattern Tuning of CLIC Crab Cavity

9. EM-fields, deflecting Hx Hx(x,y) nearly homogeneous close to symmetry axis (here R0.5 mm)peaks of ZF0*Hx are ~20% lower than peaks of Ey Tuning of CLIC Crab Cavity

10. EM-fields, accelerating Ez Ez(x,y): ~lin. dependent on y, close to beam axis smaller than Ey and Hx(here factor 3 for R0.5 mm) Ez(0,0) not 0 in first and last cells => single feed effect Tuning of CLIC Crab Cavity

11. EM-fields, small Ex Ex negligible close to symmetry axis(factor >70 in respect to Eyfor R0.5 mm) Tuning of CLIC Crab Cavity

12. EM-fields, small Hy ZF0*Hy negligible close to symmetry axis(factor >100 in respect to Ey for R0.5 mm) Tuning of CLIC Crab Cavity

13. EM-fields, Hz different simulation (HFSS of .sat file)=> field worse adapted for 120°/cell compared to simulations from Praveen-Kumar Ambattu Hz depends lin. on x close to beam axis ZF0*Hz has similar magnitude as Ez Tuning of CLIC Crab Cavity

14. Field summary and Bead-pull measurements • Summary of fields:Ex and Hy can be neglected in vicinity of beam axis (~100 smaller)=> only Ey, Hx, Ez, Hz need to be consideredEy(x,y), Hx(x,y) ~ y0 x0 (deflecting fields)Ez(x,y)~ y1 x0, Hz(x,y) ~ y0 x1Ex(x,y), Hy(x,y) ~ 0 • Bead-pull measurement principle: monitoring change of input reflectionCharles W. Steele, IEEE Trans. on microwave theory and techniques, Vol. MTT-14, No.2 (February, 1966), p.70:S11= S11,perturbed – S11,unperturbed = _{x,y,z} {(e.*E.)^2 – (ZF0*h.*H.)^2} E2, H2: complex fields squared (phase !) at position of bead e, h: complex factors describing polarisation & magnetisation effect of the bead's material in the local EM field • Study of different factors e., h. to investigate if bead-pull measurements can be bases on a single field component => making tuning procedure simple Tuning of CLIC Crab Cavity

15. Simulation of bead-pull measurement Ey only = reference Tuning of CLIC Crab Cavity

16. Simulation of bead-pull measurement small off-set y=0.5mmEy & Ez => changes seen for dS11 at locations min(|Ey|) =max(|Ez|) Tuning of CLIC Crab Cavity

17. Simulation of bead-pull measurement off-set y=0.5mmEy & Ez but ez^2=4 => changes seen for dS11 at locations min(|Ey|) => peaks of dS11 dominated by Ey stay in the same location (amplitude & phase) Tuning of CLIC Crab Cavity

18. Simulation of bead-pull measurement off-set y=0.5mmEy & Ez but ez^2=9 => changes seen for dS11 at locations min(|Ey|) => weird phase behaviour due to "minimum passage" => peaks of dS11: phase locations identic, small changes of amplitude Tuning of CLIC Crab Cavity

19. Simulation of bead-pull measurement Ey & Hx => changes seen for dS11 at locations min(|Ey|) => peaks of dS11: amplitude and phase of peaks as for reference => difference seen for cell 1 & 12 Tuning of CLIC Crab Cavity

20. Simulation of bead-pull measurement Ey, Ez & Hx => phases of peaks at locations as reference, => amplitudes similar, only different for first and last irises Tuning of CLIC Crab Cavity

21. Strategy for bead-pull measurements: • dielectric bead => bead does not perturb magnetic field => no (or negligible) S11 • close to beam axis (x=0,y=0): • Ex negligible • peaks(|Ey|^2) >> peaks(|Ez|^2) • moreover, Ez small where |Ey| reaches maxima in regular cells • ==> Strategy for measuring and evaluating fields of operating mode in regular cells: • bead-pull with dielectric bead on axis (x=0,y=0)=> S11 • search for peaks(S11) => Ey(zn)^2 + very small error • ==> amplitude and phase advance profile of regular cells can be evaluated accurately • at the same time a sensitive method to validate the accuracy is provided bycomparing max(|Ez|) to min(|Ey|) (in the middle of cells) by looking at the complex bead-pull pattern S11 • Coupling cells: • output coupling cell is adjusted to minimise the standing wave pattern • input coupling cell is adjusted to minimise the overall input reflection S11 Tuning of CLIC Crab Cavity

22. Results of 1st RF measurements • nearly no frequency shift due to wire (for accelerating structures typically -0.50 MHz, here |df|0.04 MHz) • perturbation by bead is quite small due to relatively low field strength (typical S11 usually ~ 0.1, here ~ 0.01)=> noise is was seen on the first measurements (VNA setting IF 200 Hz)=> simple solution: decrease VNA IF bandwidth to 100 Hz for measurements during tuning – signal was clean enough, no need for making a bigger bead • due to an (un)fortunate setup-error the effect of going off-axis could be analysed Tuning of CLIC Crab Cavity

23. Results of 1st RF measurements flange was skewed (soft after brazing) => bead not on axis for the upper (first) cells Tuning of CLIC Crab Cavity

24. Results of 1st RF measurements dS11 off-axis arg(dS11) [°] on-axisd_E~-120° iris number (iris 1 between cell 1 and 2) Tuning of CLIC Crab Cavity

25. Tuning of the Crab Cavity centring V guiding the wire for bead-pull measurements nitrogen supply input (chosen and marked) tuning pins (4 per cell) temperature sensor cooling block output (marked) Tuning of CLIC Crab Cavity

26. Before tuning bead-pull @ 11.9922 GHz input reflection Tuning of CLIC Crab Cavity

27. Tuning - E. Daskalaki, A. Degiovanni, C. Marrelli, M. Navarro Tapia, R. Wegner, B. Woolley Tuning of CLIC Crab Cavity

28. Tuning of the Crab Cavity tuning pins (4 per cell) Tuning of CLIC Crab Cavity

29. After tuning bead-pull @ 11.9922 GHz input reflection Tuning of CLIC Crab Cavity

30. Summary • a simple and reliable bead-pull method has been identified to determine the phase advance and amplitude profile of the CLIC Crab Cavity with a single bead-pull measurement • the CLIC Crab Cavity could easily be tuned. A few remarks: • the cells were initially very well in shape • the tuning range per cell is about 4 to 5 times smaller than for other accelerating structures (T(D)24, T(D)26, DDS, etc.)* for the Crab Cavity the group velocity is higher (~3.3%)* the tuning pins placed ~45° off the max. magnetic field regionsbut due to the good initial shape, the tuning range was largely sufficient for tuning • we had to hammer slightly harder for tuning compared to other structures Tuning of CLIC Crab Cavity

31. Thank you for your attention Tuning of CLIC Crab Cavity