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.

Abstract

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 DSM/DAPNIA/SACM, Saclay, France, 2 DESY Hamburg, Germany, 3 CNRS IN2P3 –IPN Orsay, France

Contact: claire.simon@cea.fr - michel.luong@cea.fr

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) 6*10-1 5*10-4 3.2 ~ 40

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.

Table 2. CALIFES BPM RMS resolution

Re-entrant Cavity BPM 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.

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.

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.

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) R/Ql (Ω)

Measured in lab

Measured in lab

Offset

2 mm

Offset 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

Figure 2 : Signal processing electronics

Eigen modes F (GHz) Ql R/Ql (Ω) R/Ql (Ω)

Measured Measured Offset 5 mm Offset 10 mm

Monopole mode

1.255 23.8 12.9 12.9

Dipole mode 1.724 59 0.27 1.15

Spring 2006, the re-entrant BPM (Fig. 4) was installed in a warm part in the FLASH linac (Fig. 5) at DESY.

Figure 4: Drawing of the cavity BPM

Cu-Be RF contact welded in the inner cylinder of the cavity to ensure electrical conduction.

Table 3: RF characteristics of the re-entrant BPM

Figure 5: RF Cavity installed in the FLASH linac

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).

Figure 8. Calibration results in LINAC frame from horizontal (left) and vertical (right) steerings

20 ns

20 mV

40 ns

BPM installed in the FLASH linac

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 28 dB on each BPM.

   

Figure 1: Re-entrant cavity designed for CALIFES

Signal processing uses a single stage downconversion (Fig. 2).

- Isolation of the hybrids can be adjusted by phase shifters to reject the monopole mode

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. 2008 first beam tests

Figure 7 : Signal processing electronics

The signal processing uses a single stage downconversion to obtain Δ/Σ (Fig. 7) .

Good linearity in a range 15 mm

RMS resolution : 4 µm measured on the vertical channel

8 µm measured on the horizontal channel

RF characteristics are presented in the Table 3.

30 BPMs will be installed in the XFEL cryomodules.

Figure 9 : Output signal from signal processing

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

- 2008 New prototype for the XFEL

124.8 mm

18 mm

35 mm

28.8 mm

Figure 10: RF signal measured at one pickup

ΔT =1µs

Figure 6 :New design for the XFEL cold re-entrant BPM

RMS resolution limited by the electromagnetic contamination in the experimental hall

Figure 3 : CALIFES BPM

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

XFELX-Ray Free-Electron Laser