Summary of Working Group 7, Part II: Linac protection and collimation of megawatt micron sized 250--500 GeV electron beams

摘要

The average beam powers and beam size anticipated for next generation linear colliders make them awesome tools of destruction. Systems for protection will be crucial. A scheme for linac structure protection by sacrificial collimators is presented in Section 3. No matter what precautionary measures are taken, the tails of the beam will be populated by hard coulomb collisions along the linac. To remove these halos before reaching the final focus system optics, where particle showers can blind the detector, it will be necessary to collimate these beams. Section 5 discusses the equations governing the parameters of a conventional collimation system. Wakefields determine gap sizes and lattice functions. Materials properties dictate minimum beam sizes at collimators so they can withstand occasionally mis-steered beams. Spoiler scattering and edge scattering effects mandate that the final doublet phase be collimated twice, and depending on the results of further tracking studies, it may be necessary to collimate each phase two times. Section 6 describes a nonlinear collimation system that can collimate beams to smaller apertures than the conventional system. The tolerances for such systems resemble final focus tolerances. Section T addresses the problem of repopulation of the tails after the collimation system. The main conclusions are that it appears possible to collimate the beams for these machines with conventional passively protected collimation systems. However the length of present designs, which collimate energy and both transverse planes and meet the requirements of complete tail scraping, exceed one kilometer per linac. A collimation system may also be desirable at the low energy end of the linac to minimize collimation of high energy particles.