High resolution RF cavity BPM design for Linear Collider

1. High resolution RF cavity BPM design for Linear Collider Andrei Lunin 8th DITANET Topical Workshop on Beam Position Monitors

2. Outline • Introduction • Operating parameters of the cavity BPM for CLIC project • Strategy of the Cavity BPM design • Cavity BPM spectrum calculation • Monopole mode coupling - mechanical tolerances analysis - multi-bunch regime • Dipole modes cross coupling • Cold RF measurements • Analog Downconverter R&D • Conclusions

3. Cavity BPM for CLIC project Waveguides Beam Pipe Cavity The beam position monitor (BPM) have to have both, high spatial and high time resolution ! CouplingSlot WG-CoaxialTransitions

4. Cavity BPM for CLIC. Operating Principles Magnetic coupling with waveguide The off-axis beam passing the cavity induces two orthogonal dipole TM110 modes with amplitudes proportional to the off-axis shift. A resonant cavity behaves like a damped oscillator with the EM- field decaying exponentially in time: where τ= 2Q/ω0The maximum loaded Q-factor is given by: For tmax = 50ns, Qmax ~ 300

5. Cavity BPM Design. Waveguide Dimentions. 14 2 4.84 R11.23 2 R11.22 5 R3.9975 R4 0.5 5.2 20 R0.2 R1.8 5.6 • The width of waveguide (14 mm) was chosen such, that its cut-off frequency is located between TM010 and TM110 cavity modes. • The monopole signal is exponentially decaying along the waveguide, therefore, it is better to minimize the height (2 mm) • The length (20 mm) was chosen in order to eliminate a waveguide resonance.

6. loop • Cavity BPM Design 2. Cavity spectrum calculations: - Frequency - R/Q, Q - TM11 output voltage General idea: - low Q-factors - monopole modes decoupling BPM parameters: - Cavitylength - Waveguide dimensions - Coupling slot - Coaxial transition 4. Cross coupling: - waveguide tuning - 2 ports vs 4 ports 3. Parasitic signals: - monopole modes - quadruple modes loop • 5. Tolerances calculation: • coupling slots • waveguide to cavity • - cavity to pipe

7. Cavity BPM Design. Waveguide Matching. The waveguide is matched to the output coaxial by a resonance antenna coupling 4.84 5 c_dr R0.4

8. Cavity BPM Design. Spectrum Calculation. Mode TM11 Mode TM01 Mode TM02 Mode TM21

9. Cavity BPM Design. Waveguide Resonances. Mode WG_TM21 Mode WG_TM11

10. e- r • Cavity BPM Design. Output Signal Calculation. HFSS EigenMode Calculation HFSS Data: W - Stored Energy Pcoax- Exited RF Power Ez - E-field along bunch path gsym - Symmetry coefficient Scale Factor: Output Power: (II) Bunch trajectories (I) Matched Impedance, Pcoax Estimated Sensitivity (q0 = 1nQ): V/nQ/mm

11. ~∆αx Hz ∆x Hφ • Cavity BPM Design. Monopole Mode Coupling. 1. Slot rotation causes the non zero projection of TM01 azimuth magneticfieldcomponent (Hφ) in the cavity to a longitudinal one (Hz) of TE10 mode in the waveguide. Small slot shift is equivalent to rotation with angle: αx ~ arctan(Δx/Rslot). Therefore both slot rotation and shift cause strong magnetic coupling of monopole mode to waveguide. Strong Magnetic Coupling Slot Rotation Slot Shift ∆α Hz Hφ 2. Slot tilt causes the non zero projection of TM01 azimuth magnetic (Hφ) and longitudinal electric (Ez) filelds components in the cavity to a transverse (Hx) and vertical (Ey) components of TE10 mode in the waveguide. Because both Hx and Ey are close to zero near the waveguide wall tilt error causes the weak electric and weak magnetic coupling of monopole mode to waveguide. Weak Electric Coupling Weak Magnetic Coupling Slot Tilt + Hx ∆θ Ey Ey Hφ

12. ∆x ∆α • Cavity BPM Design. Monopole Mode Coupling. Slot Rotation Slot Shift a) Waveguide Tilt If we accept machining tolerances of ~10 μm, the equivalent slot rotation computes 2Δx/Lslot ~ 0.16 degree, which corresponds to ~50 mV output voltage. Therefore, the total TM010 mode leakage caused by all machining errors on the coupling slot could be roughly estimated to be less than 100 mV for each coaxial output. ∆θ

13. +0.5 degree rotation -0.5 degree rotation • Cavity BPM Design. Monopole Mode Phase Flip. 180 degree phase flip

14. B • Cavity BPM Design. Frequency Discrimination. Spectral density B – Filter Passband f, [Hz] Monopole mode rejection (red) B, [Hz]

15. Cavity BPM Design. Multi-bunch Regime. Single Bunch Signals : TM01, TM11, TM21 Multi-bunch regime (2 GHz) TM11 signal TM01 signal Rejection: TM21 signal Rejection: Time, [s]

16. Cavity BPM Design. Spectrum of Output Signal. 1 - Stainless steel resonator material 2 – RMS value; normalized to 1 nC charge 3 - Signals are from a single coaxial output at the eigenmode frequency. Multipole modes are normalized to 1 mm off-axis shift 4 – For TM210 only

17. Cavity BPM Design. Predicted BPM resolution. 1 – Stainless steel material was used. 2 – RMS value of the sum signal of two opposite coaxial ports at the 14 GHz operating frequency after all filters applied; signals are normalized to 1 nC charge

18. Port 1 Port 2 • Cavity BPM Design. Dipole Modes Crosscoupling. a) b) c) Vertical Waveguide coupling with slots Vertical Waveguide coupling, no slots Horizontal Waveguide coupling, no slots Crosscoupling The waveguide to coaxial transition brakes coupling symmetry and hence the orthogonality of the dipole modes !

19. Cavity BPM Design. Dipole Modes Crosscoupling. Port 1 a) Port 2 b) Vertical Waveguide coupling with slots (case a)has the lesser TM11 mode cross coupling due to geometry errors. Nevertheless, the case b) was chosen due to manufacturing simplicity. c)

20. Cavity BPM Design. Dipole Modes Crosscoupling. The cross coupling between the two polarizations of the TM110mode limits a dynamic range of the beam position measurement. The actual effect of cross coupling depends on amplitude and phase of reflected signals from the read-out electronics front-end, e.g. LLRF parts like hybrids or band-pass filters. For our estimation we assumed a worst case scenario, i.e. the reflected signals are in-phase and the SWR of the LLRF components is about -20 dB. Limitations of BPM resolution due to TM110 modes cross coupling 1 - In-phase signals reflection (worse case) is taken into account. 2 – The reflection from LLRF part is assumed less than -20 dB.

21. Cavity BPM Design. Mechanical Drawings.

22. Cavity BPM Design. Cold Measurements. The first prototype of the BPM was manufactured by CERN and sent to RHUL for low power RF measurements. All parts have been assembled together using special clamps and leveling brackets. For monopole and dipole modes excitation we used a coaxial antenna inserted through the upper end of a beam pipe.

23. Cavity BPM Design. Cold Measurements. *– Results depend on the antenna penetration

24. Cavity BPM Design. Cold Measurements.

25. Analog Downconverter R&D • Fermilab has several analog downconverter R&D activities: • 714 MHz -> 15.1 MHz downconverter for ATF damping ring • >90 dB usable dynamic range (for each attenuator/gain setting)! • Low noise amplifier (LNA) with switchable gain • 28 dB step attenuator • Image rejection (SSB) mixer • Remote control (CAN-bus) of attenuator & gain, read-back of voltages, LO-level, temperatures, etc. • PCB boards for RF and CAN-bus controls • 4…10 GHz -> 70 MHz donwconverter for cavity HOM coupler signals • Connectorized experimental setup (no PCB yet) • Beam studies in February 2012 (DESY FLASH 3.9 GHz HOM studies) • CLIC BPM analog downconverter proposal • Based on ATF/HOM concepts, e.g. SSB-mixer, att. & LAN, CAN-bus controls • 15 GHz -> 70 MHz • IF FD/TD optimized BPF (quasi Tchebycheff) defines waveform • On-board PLL-locked (to external RF) local oscillator (LO)

26. HOM BPM Single Channel Downconverter

27. Analog Downconverter R&D • The CLIC cavity BPM delivers a pulse-like beam signal with high frequency (15 GHz) contents. The delivered signal levels of the dipole mode cavity are ranging from nV to mV. The downconverter is an analog signal conditioning system to adapt the cavity BPM signals to the digitizer, providing two functions: • Frequency translation: • using an image rejection or single sideband (SSB) mixer is preferable • the digitizer, operating in the first Nyquistpassband • digitizer sampling rate is in the 200...250 MS/s range • The proposed IF frequency is 70 MHz • 2. Variable signal gain with minimum distortion: • adaption the large input signal level range to the typical ± 1 volt input level range of the digitizer • lowest noise and highest linearity (wide dynamic range) are key elements for choosing the electronics components • the IF section needs to provide an anti-aliasing low-pass filter at the downconverter output

28. Analog Downmixer (prototype) The downconverter needs to be located physically close to the BPM, in the tunnel, because of high insertion losses of signal cables at 15 GHz. This calls for remote control of attenuator and gain settings, as well as read-back of some parameters, e.g. supply voltages, LO signal level, temperatures, etc. We developed a CAN-bus control system for our donwconverters, at the VME crate level it is managed by a PMC CAN-bus card, located at the crate controller CPU.

29. Cavity BPM Design. Summary. • We designed a high resolution cavity BPM for CLIC project. • The BPM can operate in single and multi-bunch regimes with a submicron resolution at acceptable mechanical tolerances. • The first cold RF measurements show a promising results and a good coinciding with numerical simulations. Still there are areas of improvements on the coupling scheme and the BPM mechanical design. • The BPM parts are ready for brazing and further experiments at CTF3 beam facility are planned. • Fermilab continues various R&D activities on a high precision analog signal processing.