1. Beam Position Measurements in TTF Cavities
using Dipole Higher Order Modes
G. Devanz, O. Napoly, CEA, Gif-sur-Yvette
A. Gössel, S. Schreiber, M. Wendt, DESY, Hamburg
Frascati, 27 Maggio 2003
O. Nappoly, CEA-Saclay

2. Motivations
Beat the µm pre-alignement of cavities-modules in the Linac
δε  δycav2
Use well-measured dipole modes TE111 and TM110 to monitor the beam position w.r.t. cavity electrical axis. This is the NLC method, but we don’t know a priori the polarisation of the modes.
No cavity movers, but one steerer / module and the knowledge of the real orbit is of great help for tuning bumps.
How refined signal acquisition needs to be ?
It is a bunch train measurement
Frascati, 27 Maggio 2003
O. Nappoly, CEA-Saclay

3. TTF 9-cell cavity HOMs Modes Below cutoff  no propagation
accelerating mode
Modes Below cutoff
 no propagation
R/Q easy to compute
in one cavity. D5
Frequency
[GHz]
R/Q
[Ω/cm2]
TE111_6
1.705
11.1
TE111_7
1.730
15.6
TM110_4
1.865
6.4
TM110_5
1.875
9.0 D3 D2 D1
R. Wanzenberg
Frascati, 27 Maggio 2003
O. Nappoly, CEA-Saclay

4. HOM couplers as BPM’s
Output power P (beam loading) from a dipole mode in a cavity:
1st polarization : P1 = C1 r 2 cos2(Φ)
2nd polarization : P2 = C2 r 2 sin2(Φ)
 r 2 = P1 / C1 + P2 / C2
C1 and C2 are calculated from fi , R/Q , Qi of the HOM :
1st power measurement  r2 = x2+y2
2nd power measurement with known beam offset δy  y and |x|
Frascati, 27 Maggio 2003
O. Nappoly, CEA-Saclay

5. Data Acquisition Beam q = 3.5 nC fb = 2.25 MHz Tp = 780 ms
Module III
Module II ON
OFF
Beam
HOM 2
q = 3.5 nC
fb = 2.25 MHz
Tp = 780 ms
HOM 1
Spectrum analyser
Agilent E8563E
spectrum analyser
used as a parametric bandpass filter:
central frequency
resolution bandwidth
signals in time domain
GPIB
Att
10 dB
zero span
Frascati, 27 Maggio 2003
O. Nappoly, CEA-Saclay

6. Cable calibration Cables inside cryostat : reflection measurement
with network analyser
Cables outside cryostat :
attenuation measurement
with power-meter
Frascati, 27 Maggio 2003
O. Nappoly, CEA-Saclay

7. Check on monopole HOMs GOAL check cable attenuation Choose HOM :
check calculations
check cable attenuation
Choose HOM :
well known HOM  non propagating bands
high R/Q to get strong signal
TM011_8
TM011_9
Frascati, 27 Maggio 2003
O. Nappoly, CEA-Saclay

8. Dipole mode measurements
2 positions computed using
2 modes with the same beam
High gradient in cavities (~ 20 MV/m)
 orbit is expected
to cross ACC1
module axis
if entering at an offset
Frascati, 27 Maggio 2003
O. Nappoly, CEA-Saclay

9. Conclusion First experiment : HOM parameters not all accurate
Data acquisition not optimum
Some selected encouraging results
Program for a PhD student co-financed by Saclay-DESY (selection committee in June)
Need : 1) accurate information on TE111 and TM110 mode parameters
2) HOM signals transported out of the tunnel, with well calibrated cables, for at least one module.
Most of the measurements can be done in parasitic mode
Frascati, 27 Maggio 2003
O. Nappoly, CEA-Saclay