HIGH RESOLUTION BPM FOR LINEAR COLLIDERS

C. Simon 1 , S. Chel 1 , M. Luong 1 , O.Napoly

1 , J.Novo

1 , D. Roudier 1 , N. Baboi 2 , D. Noelle 2 , N. Mildner 2 and N. Rouvière 3 1 CEA DSM/DAPNIA/SACM, Saclay, France, 2 DESY Hamburg, Germany, 3 CNRS IN2P3 –IPN Orsay, France Contacts: [email protected] – [email protected]

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

 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.

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) Q l R/Q l (Ω) at 5 mm R/Q l (Ω) at 10 mm Monopole mode Dipole mode Calculated

1.250

1.719

Measured in lab

1.254

Measured in the tunnel

1.255

Calculated

22.95

1.725

1.724

50.96

Measured in lab.

22.74

48.13

Measured in the tunnel

23.8

59 12.9

0.27

12.9

1.15 BPM BPM Port 2 Port 1 Network analyzer Hybrid   Port 2 50 Ω Network analyzer Port 1

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.

Measured in lab Measured in the tunnel Cross talk

41 dB 33 dB

Figure 3:

Representation of the cross-talk measurement

Table 2:

Cross-talk measurement

BPM Signal Processing

 The signal processing uses a

single stage downconversion

to obtain

Δ/Σ

(Fig. 4).

The rejection of the monopole mode, on the Δ 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.

Output Signal of the band pass filter Figure 4 :

Signal processing electronics

IF Signal on Δ channel

First beam tests on BPM System

 Summer 2006, the first beam tests were carried out (at room temperature).

T he BPM system was calibrated to have a good measurement dynamics.

Figure 5:

Calibration results in LINAC frame from horizontal (left) and vertical (right) steering  Good linearity in a range ~15 mm  Measurement dynamics : +/-5mm

Figure 6:

Standard deviation of the position measurement (calibrated)  RMS resolution ~40 µm with beam jitter

System Performances



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 Δ 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 .

Systems Simulated resolution (nm) Offset (µm)

hybrid with isolation 40 dB hybrid with isolation 30 dB 350 350 -0.38

0.95

Table 3:

Influence of hybrid isolation on the position resolution and offset.

 The damping time is given by using the following formula : with

BW



f Q d ld

f d : dipole mode frequency    * 1

BW

Q ld : loaded quality factor for the dipole mode  Considering the system (cavity + signal processing), the time resolution is determined, since the rising time to 95% of a cavity response corresponds to 3τ.

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

200 ns

Table 4:

Time resolution of the reentrant BPM.

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.