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

Motivations Beat the 300-500 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

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

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

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

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

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

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

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