Diapositive 1
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Transcript Diapositive 1
BEAM POSITION MONITORS USING A RE-ENTRANT CAVITY
C. Simon1, S. Chel1, M. Luong1, P. Contrepois1, P. Girardot1, N. Baboi2 and N. Rouvière3
1 CEA
X FEL
DSM/DAPNIA/SACM, Saclay, France, 2 DESY Hamburg, Germany, 3 CNRS IN2P3 –IPN Orsay, France
Contact: [email protected] - [email protected]
X - Ra y Fr e e - Ele ct r on La se r
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.
BPM installed in the FLASH linac
Re-entrant Cavity BPM
Coaxial re-entrant cavities have been chosen for the beam orbit measurement because of
their mechanical simplicity and excellent resolution.
Spring 2006, the re-entrant BPM (Fig. 4) was installed in
a warm part in the FLASH linac (Fig. 5) at DESY.
Passing through the cavity, the beam excites electromagnetic fields (resonant modes),
which are coupled by four feedthroughs to the outside.
Cu-Be RF contact welded in the
inner cylinder of the cavity to
ensure electrical conduction.
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.
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.
Six BPMs will be installed on the
CTF3 probe beam CALIFES.
18 mm
R/Ql (Ω)
R/Ql (Ω)
F (GHz)
Ql
Measured
Measured
Monopole
mode
1.255
23.8
12.9
12.9
Dipole mode
1.724
59
0.27
1.15
35 mm
Offset 5 mm
Offset 10 mm
Figure 6 :New design for the XFEL
cold re-entrant BPM
Table 3: RF characteristics of the re-entrant BPM
124.8 mm
Figure 1: Re-entrant cavity designed for CALIFES
The resonant cavity was designed with the software HFSS (Ansoft). The RF
measurements presented in the Table 1
Eigen
F (GHz)
Ql
R/Ql (Ω)
R/Ql (Ω)
are an average of the frequencies and
modes
external Q measured on the six BPMs .
Measured Measured
Offset
Offset
The cross_talk is quite high, it was
measured in laboratory better than
28 dB on each BPM.
RF characteristics are presented in the Table 3.
Eigen modes
28.8 mm
Dipole mode frequency chosen
around 5.997 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 5.997 GHz.
Figure 4: Drawing of the cavity BPM
in lab
in lab
2 mm
10 mm
Monopole
mode
3976.2
27.09
22.3
22.2
Dipole
mode
5964.4
51.49
1.1
7
Table 1: RF characteristics of the CTF3 probe beam BPM
Signal processing uses a single stage downconversion (Fig. 2).
The signal processing
uses
a
single
stage
downconversion to obtain
Δ/Σ (Fig. 7) .
Figure 7 : Signal processing electronics
Beam measurements with the BPM
installed in the FLASH linac
The position measured by the re-entrant BPM vs the calculated position was plotted
for the horizontal and vertical steerings (Fig. 8).
- 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
To assess the performances of the system (Table 2), a model is elaborated with Mathcad.
RMS resolution limited by the electromagnetic contamination in the experimental hall
- 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:
20 mV
Systems
Level on the Δ channel with Noise level
a beam offset of 5 µm (V)
(V)
Simulated
resolution (µm)
Time resolution
(ns)
CALIFES BPM (single bunch)
6*10-1
5*10-4
3.2
~ 10
CALIFES BPM (64 bunches)
6*10-1
5*10-4
3.2
~ 40
20 ns
- Measured resolution ~ 4 µm with a dynamic range
5 mm
- Time resolution 40 ns (Fig. 9) bunch to bunch
measurements (Fig. 10)
Table 2. CALIFES BPM RMS resolution
40 ns
- Large aperture 78 mm
Resolution ~ 3.2 µm with dynamic range +/- 5 mm
- Operated at cryogenic temperature in a clean
environment
Operated in single and multi-bunches modes (226 bunches)
- 2008 New prototype for the XFEL
Figure 9 : Output signal from signal processing
ΔT =1µs
2008 6 BPMs (Fig. 3) installed in the CTF3 probe beam
30 BPMs will be installed in the XFEL cryomodules.
Sept. 2008 first beam tests
Figure 3 : CALIFES BPM
Figure 10: RF signal measured at one pickup