Studies of wake fields set up by relativistic electron bunches in a cylindrical dielectric-lined waveguide and application to accelerator physics.

摘要

We report on the experimental demonstration of a novel acceleration technique, proposed in 1999, which might deliver high acceleration gradients as required by future linear colliders. This technique utilizes constructive superposition of wake-fields produced in a dielectric-lined waveguide by short (psec) drive bunches which excite a broadband frequency spectrum having ∼40 eigenmodes and thereby synthesize a high-amplitude accelerating field. This experiment is compared with a related experiment by a group at the Argonne National Laboratory where the wake field consisted of ∼10 eigenmodes. We find that the axial accelerating electric field has a sharply-peaked profile with very narrow footprint as desired, and we demonstrate that fields of two bunches have been successfully superimposed. We report the development of a nondestructive technique to measure bunch rms-length in the psec range and below, by measuring the high-frequency spectrum of wake field radiation which is caused by the passage of a relativistic electron bunch through a channel surrounded by a dielectric. We demonstrate both experimentally and numerically that the generated spectrum is determined by and sensitive to the bunch rms-length, whereas it is insensitive to the axial and longitudinal charge distribution. Measurement of the millimeter-wave spectrum determines the bunch rms-length in the psec range, and this has been done using a series of calibrated mesh filters. We have developed the analysis of the factors crucial for achieving good accuracy in this measurement, and find the experimental data are fully understood by the theory. We point out that this technique also may be used for measuring fsec bunch lengths, using a prepared planar wake field microstructure. We also investigate theoretically and numerically the quantitative behavior of the dielectric wake field accelerator performance (such as the efficiency, accelerating gradient, and energy spread) vs. the dielectric wake field accelerator parameters (e.g. the inner and outer radii, the dielectric constant, the longitudinal shape of a drive/test bunch, the bunch rms-length, etc.) for the case of the cylindrical multimode monolayer dielectric wake-field accelerator.