Controlling transverse multibunch instabilities in linacs of high-energy linear colliders

Abstract

In this paper, we study multibunch beam breakup, with emphasis on theoretical methods applicable to the design of a linear collider with a center-of-mass energy near 1 TeV. One way to significantly improve the luminosity and energy transfer efficiency of such a collider is to accelerate a train of bunches rather than just a single bunch each time the linac accelerating structure is filled with a pulse of rf energy. For the required bunch charges and intensities, the transverse instability due to the wake fields produced in the accelerating structure is very severe unless measures are taken to control it. Therefore, we examine the effects of several methods of reducing this instability: (1) use of damped acceleration cavities, (2) placing the bunches near nodes of the transverse wake fields produced by preceding bunches, (3) introducing a spread (over different cells of the accelerating structure) of the individual mode frequencies in the transverse wake field, and (4) varying the strength of the transverse focusing from bunch to bunch, in such a way as to partially cancel the effects of the wake fields from preceding bunches. We present examples illustrating the effectiveness of these cures, using realistic linear-collider design parameters.