Cavity Alignment Using Beam Induced Higher Order Mode Signals in the TTF LINAC Olivier Napoly, Rita Paparella CEA/DSM/DAPNIA, Gif-sur-Yvette Marc Ross, Josef Frisch, Kirsten Elaine Hacker, Roger Michael Jones, Douglas McCormick, Caolionn O'Connell, Tonee Smith SLAC, Menlo Park, California Nicoleta Baboi, Manfred Wendt DESY, Hamburg Abstract Each nine cell superconducting (SC) accelerator cavity in the TESLA Test Facility (TTF) at DESY [1] has two higher order mode (HOM) couplers that efficiently remove the HOM power [2]. They can also provide useful diagnostic signals. The most interesting modes are in the first 2 cavity dipole passbands. They are easy to identify and their amplitude depends linearly on the beam offset from the cavity axis making them excellent beam position monitors (BPM). By steering the beam through an eight-cavity cryomodule, we can use the HOM signals to estimate internal residual alignment errors and minimize wakefield related beam emittance growth. We built and tested a time-domain based waveform recorder system that captures information from each mode in these two bands on each beam pulse. In this paper we present an experimental study of the single-bunch generated HOM signals at the TTF linac including estimates of cavity alignment precision and HOM BPM resolution. Superconducting Cavity HOM modes Respond to beam position and angle Layout of the TTF Each dipole mode has 2 polarizations, Each measured with 2 couplers Each signal has a real and imaginary part Effectively 8 degrees of freedom for each cavity mode In order convert mode signals to beam position and angle need to calibrate using beam steerers and machine lattice Theoretical resolution (noise limit) 30 nanometers for 1 nanocolumb. Each SC cavity in the TTF is instrumented with 2 HOM couplers (one at each end) Calibrate position vs. mode signals (example) Assume we have a set of 100 measurements, 8 components of a single dipole mode where we sweep the beam position X. Here we show only a single direction of sweep. M A,B is the measurement of the “Bth” component of the HOM mode (2x coupler, 2x polarization, real+imaginary), for the “Ath” data record. Note: a column of “Ones” must be added to the measured data to allow for offsets. X A is the measurement of the beam position (from steerer settings) on the “Ath” data record. R B is the component of a vector (currently unknown) which when multiplied by the Measurement M gives the best estimate of the beam position Linear Regression used to find the vector R. We use the Matlab “\” operator. Resolution measurement Use LR to find the vectors “R” relating the mode components “M”of the middle cavity to the beam position “X” (from steerer settings).. E.g. MR ~= X Use LR to find the matrix “Q” relating the mode components “E” of the end cavities to the mode components of the middle cavity. E.g. EQ ~= M Use the measurements on the middle cavity to estimate Xest1=MR. Use the measurements on the end cavities to estimate the mode components of the middle cavity, which are then used to estimate Xest2= EQR. Measure the statistical difference: error = STD(Xest1 – Xest2) 4 Plots, 1 each for X, X’, Y, Y’ 8 Plots, 1 for each component Of cavity 2 mode 4 Plots (2 shown), 1 for each of X, X’, Y, Y’ Raw signal, and spectrum for data acquisition channel without bandpass filter Raw signal, and spectrum for data acquisition channel with bandpass filter. Note frequency scale zoomed to show TE111-6 and TE111-7 modes Resolution for single cavity, single mode measurement approximately 5 microns RMS. Theoretical resolution is sub- micron. Study under way to instrument all 40 cavities in the TTF with HOM based beam position monitors Work Supported by Department of Energy Contract DE-AC02-76SF00515