A Study of Coupler-trapped Modes in X-band Linacs for the Glc/nlc

Each of the X-band accelerating structures for the GLC/NLC (Global Linear Collider/Next Linear Collider) consists of 55 cells which accelerate a train of charged bunches. The cells are carefully designed to ensure that the transverse wakefield left behind each bunch does not disrupt the trailing bunches, but some modes are trapped in the region of the coupler cells. These modes can give rise to severe emittance dilution if care is not taken to avoid a region of resonant growth in the emittance. Here, we present results on simulations, cold test experimental measurements and beam dynamics simulations arising as a consequence of modes trapped in the coupler. The region in which trapped modes have little influence on the beam is delineated. Paper presented at the 22nd International Linear Accelerator Conference (LINAC 2004), Lübeck, Germany, 16 -20 Aug 2004 This work is supported by Department of Energy grant number DE-AC02-76SF00515 A STUDY OF COUPLER-TRAPPED MODES IN X-BAND LINACS FOR THE GLC/NLC R.M. Jones, V.A. Dolgashev, Z. Li, J.W. Wang; SLAC Abstract Each of the X-band accelerating structures for the GLC/NLC (Global Linear Collider/Next Linear Collider) consists of 55 cells which accelerate a train of charged bunches. The cells are carefully designed to ensure that the transverse wakefield left behind each bunch does not disrupt the trailing bunches, but some modes are trapped in the region of the coupler cells. These modes can give rise to severe emittance dilution if care is not taken to avoid a region of resonant growth in the emittance. Here, we present results on simulations, cold test experimental measurements and beam dynamics simulations arising as a consequence of modes trapped in the coupler. The region in which trapped modes have little influence on the beam is delineated.Each of the X-band accelerating structures for the GLC/NLC (Global Linear Collider/Next Linear Collider) consists of 55 cells which accelerate a train of charged bunches. The cells are carefully designed to ensure that the transverse wakefield left behind each bunch does not disrupt the trailing bunches, but some modes are trapped in the region of the coupler cells. These modes can give rise to severe emittance dilution if care is not taken to avoid a region of resonant growth in the emittance. Here, we present results on simulations, cold test experimental measurements and beam dynamics simulations arising as a consequence of modes trapped in the coupler. The region in which trapped modes have little influence on the beam is delineated.