The TESLA Post-linac Collimation System

1 Introduction In high-energy physics colliders, the 'halo' particles surrounding the core of the beams would cause intolerable background in the physics detectors if not removed. Collima-tion systems consisting of mechanical 'spoilers' on the order of a radiation length are placed close to the beam and used to 'scrape' the halo particles off: the particles scattered by the spoilers are then collected in ∼ 20 radiation length 'absorbers', placed at suitable locations downstream of the spoilers. The absorbers are normally well within the optical shadow of the spoilers, so that the latter protects the former from a direct hit from the beam. Collimation of the beam halo in the next generation of linear colliders is a complex and difficult problem. The first collimation system designs for a linear collider were proposed by the NLC group at SLAC [1], and have formed the basis of much of the work reported here. The high-energy of the halo particles (close to the beam energy, 250 GeV) requires multiple-stage collimation; the extremely small beam emittances — and corresponding beam sizes — pose significant safety issues for the design of the mechanical spoilers: relatively large β-functions are generally required to increase the beam-sizes at the spoiler, to both enlarge the spoiler gap, and reducing the peak particle density on the spoiler if it is hit by the beam. The TESLA concept [2, 3] has several advantages which relieve the constraints on collimation system: the large bunch spacing (337 ns) allows a head-on collision scheme using large aperture superconducting final doublets, which result in a relatively large required collimation depth (13σ x × 80σ y , see section 2); the long bunch train (2820 bunches) and large bunch spacing allows the majority of the beam to be safely extracted in the event of some machine error or failure; the amount of halo expected from the superconducting linac is extremely small (∼ 10 4 particles per bunch, see section 3). Since publication of the TESLA Conceptual Design Report (CDR) [2], the post-linac collimation system has undergone a major design change. The change reflects a shift in philosophy with respect to the machine protection issue: the system presented in the CDR [4] was based on the philosophy that all the spoilers should withstand a direct hit from a few design bunches of the bunch train. The system was characterised by large (18 km) β-functions, and the tolerances on …