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