A Study of the Beam Physics in the CLIC Drive Beam Decelerator

CLIC is a study for a Multi-TeV e+e- linear collider, in which the rf power for the main linacs is extracted from 100 ampere electron drive beams, by the use of specially designed power extraction structures. Up to 90% of the beam energy is extracted from the drive beams along one kilometer long decelerator sectors, rendering the beam transport challenging. We have identified two major challenges for robust beam transport: the significant transverse wakes in the power extraction structures, and the large energy spread induced by the power extraction process. By beam dynamics studies we have qualified power extraction structure designs, leading to the present CLIC baseline structure in which the transverse wakes are sufficiently mitigated. We have further shown that the beam energy spread induced by the deceleration implies that standard 1-to-1 correction might not ensure satisfactory drive beam transport. As alternative, we propose a decelerator orbit correction scheme based on dispersion-free steering and exploiting the structure beam loading. By simulation the proposed scheme shows excellent performance, assuming sufficient beam position monitor resolution. We have performed successful demonstrations of similar orbit correction schemes in the linac of the CLIC Test Facility 3. The results of the beam dynamics studies have lead to specifications for decelerator instrumentation and magnets, described in detail in this w ork. The first prototype of the baseline power extraction structure has been tested with beam in the Two-beam Test Stand in the CLIC Experiment Area, where a field recirculator has been installed to boost power production. We have derived formulae for rf power production and voltage, including a simple model describing power extraction with recirculation. The model has been applied to the first Two-beam Test Stand experimental result. We compare the measured rf power, phase and energy loss with reconstructed signals based on beam intensity measurements, and a good agreement between the measurements and the reconstruction is shown.