Outline

1. High Resolution BPM for Linear CollidersClaire Simon, Michel Luong, Stéphane Chel, Olivier Napoly, Jorge Novo, Dominique Roudier, Nicoleta Baboi, Dirk Noelle, Nils Mildner and Nelly RouvièreLüneburg 06 – November 30 2006CARE-HHH-N3-ABI workshop

2. Outline • Introduction • Mechanical design of the re-entrant cavity • Radio-Frequency Simulations • RF measurements of the RF cavity • Signal processing electronics • First measurements • Mathcad Model of the cavity and the signal processing + results • Time resolution • Future development • Summary

3. Introduction • A re-entrant beam position monitor (BPM) is developed by the CEA/Saclay in collaboration with DESY in the European framework of CARE/SRF/WP11programme. • Task of the CEA is the design, fabrication and full test of high resolution re-entrant BPM. • System can be used in a clean environment, at cryogenictemperature. • Mechanical and signal processing designs are a compromise to get: • high position resolution (better than 10 µm) • possibility to perform bunch to bunch measurements for the X-FEL at DESY and the ILC.

4. Re-entrant Cavity (1) • The concept of this cavity is from R. Bossart from CERN • This cavity has a cylindrical symmetry which allows a high precision of the machining. • Two existing re-entrant BPMs are installed in the FLASH linac at DESY • One is located at cryogenic temperature inside the cryomodule (ACC1) and has proven the cryo-compatibility. • Second is located at room temperature to validate the resolution.

5. Re-entrant Cavity (2) Cavity BPM installed in the FLASH linac Button BPM designed by DESY Re-entrant cavity BPM

6. Re-entrant Cavity (3) • The re-entrant BPM is composed of a mechanical structure with four orthogonal feedthroughs (or antennas). • It is arranged around the beam tube and forms a coaxial line which is short circuited at one end. • The cavity is fabricated with stainless steel as compact as possible: 170 mmlength, 78 mm aperture. Feedthroughs are positioned in the re-entrant part to reduce the magnetic loop coupling and separate the main RF modes (monopole and dipole) Cu-Be RF contacts welded in the inner cylinder of the cavity to ensure electrical conduction. Twelve holes of 5 mm diameter drilled at the end of the re-entrant part for a more effective cleaning.

7. Re-entrant Cavity (4) • Length of the cavity is minimized to satisfy the constraints imposed by the cryomodule. • Antennas are assembled to the cavity by a conflat gasket ( Standard CF DN16) and fulfil the conditions of Ultra High Vacuum (UHV). • Some cryogenic tests (thermal shock) were carried out on the RF feedthroughs with success. • Effective cleaning (tests performed at DESY). • In order to avoid hydrogen out gassing on site, a heat treatment at 280 °C for 15 days was applied to the BPM cavity body instead of the usual treatment (950 °C for 2h) which may have drastically reduced the RF contact elasticity. • Signal voltage of the monopole mode is proportional to beam intensity and does not depend on the beam position. • Dipole mode voltage is proportional to the distance of the beam from the centre axis of the monitor.

8. RF Characteristics of the BPM • Resonant modes • Q determined by HFSS with matched feedthroughs. • With Matlab and the HFSS calculator, we computed R/Q ratio. • R: the Shunt impedance and Q: the quality factor • and k=w/c • Difference on Q factors can be explained by the boundary conditions which are not the same during the measurements in laboratory and in the tunnel.

9. RF Cavity and Fields (1) Monopole mode (f = 1255 MHz) Simulated with HFSS E field H field E field Dipole Mode (f = 1724 MHz) Simulated with HFSS H field

10. RF Cavity and Fields (2) Monopole mode (f = 1255 MHz) Simulated with HFSS E field H field Dipole Mode (f = 1724 MHz) Simulated with HFSS E field H field

11. RF Measurements of the Cavity BPM • The dipole mode orthogonal polarizations show slightly different eigenfrequencies; the relative difference is less than 2 per 1000. • Frequencies and Q factor of modes existing in the cavity BPM were measured with network analyzer. • First and second peaks are monopole and dipole modes. • Others peaks are higher order modes which can propagate out of the cavity through the beam pipe. Cut-off frequency of the beam pipe mode TE11 is 2.25 GHz. • 1.72 GHz band pass filter, used in the signal processing, has an attenuation around -70 dB at 3 GHz and around -60 dB at 4 GHz => rejection of ‘higher order modes’.

12. Cross Talk of the Cavity BPM • Due to tolerances in machining, welding and mounting, some small distortions of the cavity symmetry are generated. A beam displacement in the ‘x’ direction gives not only a reading in that direction but also a non zero reading in the orthogonal direction ‘y’. This asymmetry is called cross talk. Monopole and dipole transmission measured by the network analyzer Transmission of 2 antennas positioned at 90° Representation of the cross-talk measurement • From those measurements, the cross-talk isolation value is estimated around 33 dB. Difference may be explained by the fact that the BPM has a tilt (11.25 degrees) with a button BPM which is very close.

13. Signal Processing(1) • The rejection of the monopole mode, on the Δ channel, proceeds in three steps: • - a rejection based on a hybrid coupler having isolation higher than 20 dB in the range of 1 to 2 GHz. The isolation can be adjusted with phase shifters and attenuators. • - a frequency domain rejection with a band pass filter centered at the dipole mode frequency. Its bandwidth of 110 MHz also provides a noise reduction. • - a synchronous detection. The 9 MHz reference signal, given by the control system, is combined with a PLL to generate a local oscillator (LO) signal at the dipole mode frequency. Phase shifters are used to adjust the LO and RF signals in phase.

14. Signal Processing (2) IF signal after Lowpass Filter on channel Δ RF signal after Band pass Filter Signal processing electronics installed in the hall

15. Calibration of the Electronics • Tuning of the phase shifters gives a high common mode rejection (30 dB at the monopole mode frequency). • Synchronous and direct detectors, as well as amplifiers and limiters for protection were adjusted to have a linearity range around +/- 10 mm. • Phase tuning for the synchronous detection was refined while visualizing the delta signal on a scope. • To get +/- 1V at the output of signal processing, the gain was adjusted to avoid saturation from ADCs. • Signal delays adjusted with cables for simultaneous acquisition with the Doocs ADC board. • Calibration for offset on the Doocs ADC board and the trigger delay adjusted.

16. First Beam Tests on BPM System (1) rf gun BPM 14ACC7 5 accelerating modules steerers H10ACC6 V10ACC6 • Magnets switched off between steerers and 14ACC7 BPM to reduce errors and simplify calculation. • Move beam with one steerer. • Average of 500 points for each steerer setting. R = transfer matrix from steerer to BPM • Calculate for each steerer setting, the relative beam position in using a transfer matrix between steerer and BPM : • Dx = R12*Dx’(angle at steerer)

17. First Beam tests on BPM system (2) • Summer 2006, the first beam tests were carried out (at room temperature) and are encouraging. • The BPM was calibrated to have a good measurement dynamics Calibration results in LINAC frame from horizontal (left) and vertical (right) steering Standard deviation of the position measurement (calibrated) • Good linearity in a range +/- 5 mm • RMS resolution <40 µm with beam jitter

18. Mathcad Model (1) • To assess the system performance, a model (cavity+signal processing) is elaborated with a Mathcad code based on Fourier transforms. • Simulation covers a span from 0 to 20 GHz. • The delivered time domain signal is determined by the RF characteristics of each mode. Each mode of the cavity is modelled as a resonant RLC circuit and single bunch response of the cavity depends on frequencyωi and external coupling Qi of the mode. • with and • where Φ(t) = heaviside function, q = bunch charge, R0 = 50 Ω, (R/Q)i = coupling to the beam and ζi = 4 if it is a monopole mode or ζi = 2 if it is a dipole mode.

19. Mathcad Model (2) Isolation of the 180°hybrid Signal from one pickup "sum" signal peak power was measured around 36 dBm and the “sum” simulated value is around 34 dBm => Mathcad model validated S parameters measurement of the hybrid 180°

20. Mathcad Model (3) • Noise determined by the thermal noise and the noise from signal processing channel • Thermal noise : • kb = Boltzmann’s constant (1.38*10-23J/K), BW (Hz) = bandwidth of the signal processing channel, and T (K) = room temperature. • Noise from the signal processing: • NF= total noise figure of the signal processing channel, G = gain of the signal processing and Pth = thermal noise. • Total noise introduced into the system by the electronics can be evaluated by the noise figure in a cascaded system : • NF = total noise factor of the signal processing, Fi and Gi respectively the noise factor and the gain of component i.

21. Simulation Results • Position resolution: RMS value related to the minimum position difference that can be statistically resolved. • Signal given by the model (cavity+signal processing) simulation with a gain adjusted to get an RF signal level around 0 dBm on the Δ channel with 100 m beam offset. • Noise level ~ 0.4 mV. • Hybrid isolation does not affect the resolution but modifies the position offset. • Offset depends also on the isolation variation inside and outside the nominal pass-band of the hybrid coupler. • Adaptation of amplifier gain for a 100 µm measurement dynamic range spoilt by a 2 factor the resolution in comparison with 10 µm range. Influence of hybrid isolation on the position resolution and offset. Results simulated of the resolution <1 µm

22. ΔT =1µs 20 mV 20 ns RF signal measured at one pickup 40 ns Time Resolution • Damping time is given by using the following formula : fd: dipole mode frequency Qld: loaded quality factor for the dipole mode With • Considering the system (cavity + signal processing), the time resolution is determined, since the rising time to 95% of a cavity response corresponds to 3τ. Time resolution for reentrant BPM Output Signal from the signal processing Possibility bunch to bunch measurements

23. Future Development • 2007, new beam tests and resolution studies : • Mixer used in the electronics will be replaced by a new one which accepts a high power RF input (around 17 dBm instead of 0 dBm). • Attenuators will be removed to change the gain of each channel and confirm the simulated performances. Resolution will be around 1 µm and measurement dynamic range will be around +/- 5 mm. • Tests in multi-bunch mode • Improvement of the mechanical design Re-entrant BPM simulation with new mixer and 10 mm beam offset

24. Summary • High resolution re-entrant cavity BPM features: • Effective in clean environment • Operation at room and cryogenic temperature • Large aperture of the beam pipe (78 mm) • Position resolution around 1 µm (simulated) with a measurement dynamic range around +/- 5 mm • Time resolution around 40 ns • Improvement of the mechanical design. • This BPM appears as a good candidate for being installed in the XFEL (DESY) and ILC cryomodules.

25. Acknowledgements • To my colleagues who work on this project • To the Andreas Peters, Hermann Schmickler and Kay Wittenburg for the invitation to this meeting • We acknowledgethe support of the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003-506395). Thank you for your attention