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

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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

outline
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
introduction
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.
re entrant cavity 1
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.
re entrant cavity 2
Re-entrant Cavity (2)

Cavity BPM

installed in the FLASH linac

Button BPM

designed by DESY

Re-entrant cavity BPM

re entrant cavity 3
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.

re entrant cavity 4
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.
rf characteristics of the bpm
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.
rf cavity and fields 1
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

rf cavity and fields 2
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

rf measurements of the cavity bpm
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’.
cross talk of the cavity bpm
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.

signal processing 1
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.
signal processing 2
Signal Processing (2)

IF signal after Lowpass Filter on channel Δ

RF signal after Band pass Filter

Signal processing electronics installed in the hall

calibration of the electronics
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.
first beam tests on bpm system 1
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)
slide17

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
mathcad model 1
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.
slide19

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°

mathcad model 3
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.
simulation results
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

time resolution

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

future development
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

summary
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.
acknowledgements
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