Diapositive 1

1
 
HIGH RESOLUTION BPM FOR LINEAR
COLLIDERS C. Simon1, S. Chel1, M.
Luong1, O.Napoly1, J.Novo1, D. Roudier1, N.
Baboi2, D. Noelle2, N. Mildner2 and N. Rouvière3
1 CEA DSM/DAPNIA/SACM, Saclay, France,
2 DESY Hamburg, Germany, 3 CNRS IN2P3 IPN Orsay,
France Contacts
claire.simon_at_cea.fr michel.luong_at_cea.fr
Abstract
The beam-based alignment and feedback systems,
essential operations of the future colliders, use
high resolution Beam Position Monitors (BPM). In
the framework of European CARE/SRF programme, the
task of CEA/DSM/DAPNIA covers the design, the
fabrication and the beam test of a BPM in
collaboration with DESY. This system can be used
in a clean environment, at cryogenic or room
temperature. It is composed of a radiofrequency
reentrant cavity with a beam pipe diameter of 78
mm and an analog electronics having several
signal processing steps to reject the monopole
mode. The mechanics and signal processing design
is a compromise to get a high position resolution
(better than 1 µm) and the possibility to perform
bunch to bunch measurements for the X-ray Free
Electron Laser (X-FEL) at DESY and the
International Linear Collider (ILC).
BPM Design
First beam tests on BPM System

• Summer 2006, the first beam tests were carried
  out (at room temperature). The BPM system was
  calibrated to have a good measurement dynamics.

• The reentrant BPM (Fig. 1) is composed of a
  mechanical structure with four orthogonal
  feedthroughs. It is installed on the TESLA Test
  Facility Phase 2 (TTF2) at DESY.
• The cavity is fabricated with stainless steel as
  compact as possible 170 mm length, 78 mm
  aperture as illustrated in Fig. 2.

Figure 1 RF Cavity with one feedthrough
Cu-Be RF contact welded in the inner cylinder of
the cavity to ensure electrical conduction.
Figure 5 Calibration results in LINAC frame from
horizontal (left) and vertical (right) steering

• Good linearity in a range 15 mm
• Measurement dynamics /-5mm

Twelve holes of 5 mm diameter drilled at the end
of the re-entrant part for a more effective
cleaning.
Figure 2 BPM RF Cavity

• The RF measurements presented in the Table 1
  compare the computed frequencies and coupling of
  the resonant cavity (simulated with the HFSS
  code) to their measured values.

Eigen modes F (GHz) F (GHz) F (GHz) Ql Ql Ql R/Ql (O) at 5 mm R/Ql (O) at 10 mm
Calculated Measured in lab Measured in the tunnel Calculated Measured in lab. Measured in the tunnel
Monopole mode 1.250 1.254 1.255 22.95 22.74 23.8 12.9 12.9
Dipole mode 1.719 1.725 1.724 50.96 48.13 59 0.27 1.15
Figure 6 Standard deviation of the position
measurement (calibrated)

• RMS resolution 40 µm with beam jitter

System Performances
Table 1 RF characteristics of the reentrant BPM

• 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.
  The measurements of this asymmetry, called cross
  talk are shown (Fig. 3). The corresponding values
  are displayed in Table 2.

• Position resolution RMS value related to the
  minimum position difference that can be
  statistically resolved.
• To assess the system performance, a model
  (cavity signal processing) is elaborated with a
  Mathcad code based on Fourier transforms.
• The gain was adjusted to get an RF signal level
  around 0 dBm on the ? channel with
• 100 ?m beam offset.
• Noise is determined by the thermal noise and the
  noise from signal processing channel and is about
  0.4 mV.

Measured in lab Measured in the tunnel
Cross talk 41 dB 33 dB
Systems Simulated resolution (nm) Offset (µm)
hybrid with isolation 40 dB 350 -0.38
hybrid with isolation 30 dB 350 0.95
Figure 3 Representation of the cross-talk
measurement
Table 2 Cross-talk measurement
Table 3 Influence of hybrid isolation on the
position resolution and offset.
BPM Signal Processing

• The damping time is given by using the following
  formula

fd dipole mode frequency Qld loaded quality
factor for the dipole mode
with

• The signal processing uses a single stage
  downconversion to obtain ?/S (Fig. 4).
• The rejection of the monopole mode, on the ?
  channel, proceeds in three steps
• a rejection based on a hybrid coupler having an
  isolation higher than 20 dB in the range of 1 to
  2 GHz. The isolation can be adjusted around 30 or
  40 dB at the frequency of the dipole mode 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.

• Considering the system (cavity signal
  processing), the time resolution is determined,
  since the rising time to 95 of a cavity response
  corresponds to 3t.
• Figure 7 confirms the possibility bunch
  to bunch
  
  measurements.

Damping Time cavity only Time resolution cavity electronics
BPM 9.4 ns 40 ns
0.5V
Table 4 Time resolution of the reentrant BPM.
200 ns
Figure 7 RF signal measured from one pickup.
Conclusion

• High resolution reentrant cavity BPM features
• Compatibility with clean environment
• Operation at room and cryogenic temperature
• Large aperture 78 mm
• Position resolution better than 1 µm (simulated
  with 100µm of dynamics )
• Time resolution expected around 40 ns
• Winter 2006-2007, the beam tests will continue
  (at room temperature). The gain, on each channel,
  will be optimized to improve the resolution and
  confirm the simulated performances.
• This BPM appears as a good candidate for the
  XFEL project (DESY) and ILC cryomodules.

Output Signal of the band pass filter
IF Signal on ? channel
Figure 4 Signal processing electronics