1. BEAM POSITION MONITORS USING A RE-ENTRANT CAVITY C. Simon1, S. Chel1, M. Luong1, P. Contrepois1, P. Girardot1, N. Baboi2 and N. Rouvière CEA DSM/DAPNIA/SACM, Saclay, France, 2 DESY Hamburg, Germany, 3 CNRS IN2P3 –IPN Orsay, France Contact: -
Abstract
Two designs of high resolution beam position monitor, based on a radiofrequency re-entrant cavity, are developed at CEA/Saclay. The first monitor is developed in the framework of the European CARE/SRF program. It is designed to work at cryogenic temperature, in a clean environment and get a high resolution with the possibility to perform bunch to bunch measurements. Two prototypes with a large aperture (78 mm) are installed in the Free electron LASer in Hamburg (FLASH), at DESY. The other design with an aperture of 18 mm and a large frequency separation between monopole and dipole modes, as well as a low loop exposure to the electric fields is developed for the Clic Test Facility (CTF3) probe beam CALIFES at CERN. It is operated in single bunch and multi-bunches modes. This poster presents the mechanical and signal processing designs of both systems.
Re-entrant Cavity BPM
BPM installed in the FLASH linac
Coaxial re-entrant cavities have been chosen for the beam orbit measurement because of their mechanical simplicity and excellent resolution.
Passing through the cavity, the beam excites electromagnetic fields (resonant modes), which are coupled by four feedthroughs to the outside.
Main radio-frequency modes excited by the beam in the cavity:
the monopole mode signal is proportional to beam intensity and does not depend on the beam position
the dipole mode signal is proportional to the distance of the beam from the centre axis of the monitor.
Spring 2006, the re-entrant BPM (Fig. 4) was installed in a warm part in the FLASH linac (Fig. 5) at DESY.
Cu-Be RF contact welded in the inner cylinder of the cavity to ensure electrical conduction.
Figure 5: RF Cavity installed in the FLASH linac
Twelve holes of 5 mm diameter drilled at the end of the re-entrant part for a more effective cleaning.
BPM designed for the CTF3 probe beam
A design, with a large frequency separation between monopole and dipole modes, as well as a low loop exposure to the electric fields, has been developed (Fig. 1) for the CTF3 probe beam CALIFES.
Figure 4: Drawing of the cavity BPM
124.8 mm
18 mm
35 mm
28.8 mm
RF characteristics are presented in the Table 3.
Six BPMs will be installed on the CTF3 probe beam CALIFES.
Dipole mode frequency chosen around GHz for a resonant operation with 64 bunches. The mechanical dimension Ø 28.8 mm will be adjusted to have the dipole mode frequency close to GHz.
Eigen modes
F (GHz) Ql
R/Ql (Ω)
Measured
Offset 5 mm
Offset 10 mm
Monopole mode
1.255
23.8
12.9
Dipole mode
1.724 59
0.27
1.15
Figure 6 :New design for the XFEL
cold re-entrant BPM
Table 3: RF characteristics of the re-entrant BPM
Figure 1: Re-entrant cavity designed for CALIFES
The signal processing uses a single stage downconversion to obtain Δ/Σ (Fig. 7) .
The resonant cavity was designed with the software HFSS (Ansoft). The RF measurements presented in the Table 1
Eigen modes
F (GHz) Ql
R/Ql (Ω)
Measured
in lab
Measured in lab
Offset
2 mm
10 mm
Monopole mode
3976.2
27.09
22.3
22.2
Dipole mode
5964.4
51.49
1.1 7
are an average of the frequencies and external Q measured on the six BPMs .
The cross_talk is quite high, it was measured in laboratory better than dB on each BPM.
Figure 7 : Signal processing electronics
Beam measurements with the BPM
installed in the FLASH linac
Table 1: RF characteristics of the CTF3 probe beam BPM
The position measured by the re-entrant BPM vs the calculated position was plotted for the horizontal and vertical steerings (Fig. 8).
Signal processing uses a single stage downconversion (Fig. 2).
- Isolation of the hybrids can be adjusted by phase shifters to reject the monopole mode
Figure 8. Calibration results in LINAC frame from horizontal (left) and vertical (right) steerings
Good linearity in a range  15 mm
RMS resolution : 4 µm measured on the vertical channel
8 µm measured on the horizontal channel
Figure 2 : Signal processing electronics
RMS resolution limited by the electromagnetic contamination in the experimental hall
To assess the performances of the system (Table 2), a model is elaborated with Mathcad.
- RF re-entrant cavity model is a resonant RLC circuit
- The transfer functions of different devices composing the signal processing are determined by the S parameters measured with a network analyzer.
Main features of the BPM installed in the FLASH linac:
- Measured resolution ~ 4 µm with a dynamic range  5 mm
- Time resolution 40 ns (Fig. 9)  bunch to bunch measurements (Fig. 10)
- Large aperture 78 mm
- Operated at cryogenic temperature in a clean environment
 New prototype for the XFEL
20 ns
20 mV
40 ns
Systems
Level on the Δ channel with a beam offset of 5 µm (V)
Noise level (V)
Simulated resolution (µm)
Time resolution
(ns)
CALIFES BPM (single bunch)
6*10-1
5*10-4
3.2
~ 10
CALIFES BPM (64 bunches)
~ 40
Table 2. CALIFES BPM RMS resolution
Figure 9 : Output signal from signal processing
Resolution ~ 3.2 µm with dynamic range +/- 5 mm
Operated in single and multi-bunches modes (226 bunches)
2008 6 BPMs (Fig. 3) installed in the CTF3 probe beam
Sept  first beam tests
ΔT =1µs
30 BPMs will be installed in the XFEL cryomodules.
Figure 3 : CALIFES BPM
Figure 10: RF signal measured at one pickup