Dipole Wakefields Research Articles

Experimental study of transverse effects in planar dielectric wakefield accelerating structures with elliptical beams

The main obstacle to the practical implementation of the dielectric wakefield acceleration (DWA) concept is the development of the beam breakup instability due to transverse dipole wakefields generated when the beam propagates off axis in the accelerating structure. One of the methods to suppress this instability is to elliptically shape the beam and accelerate it in a planar structure. Here, we report a detailed experimental investigation of the transverse dynamics of elliptical beams in planar dielectric structure with parameters mimicking future DWA modules. Both dipole and quadrupole wakefields' effects on beam stability and projected emittance were studied. This study has highlighted the importance of counteracting quadrupole wakefields in future DWA implementations.

Local impedance measurements from closed orbit distortion due to wake fields at CESR

Abstract Short range transverse wake fields are excited by charged particles passing through a vacuum chamber. The wake fields can act back on a bunch of particles and alter its trajectory. We have measured the distortion of the closed orbit due to transverse wake fields in the Cornell Electron–positron Storage Ring. A transverse monopole wake field is induced by an asymmetric impedance associated with the insertion of a single vertical scraper. A transverse dipole wake field is generated by displacing the bunch from the symmetry axis of the chamber. The orbit is offset vertically through the gap of a pair of symmetrically inserted scrapers and also in a narrow-gap undulator chamber. A fit to the distorted orbits yields the average kick of the transverse monopole wake of the asymmetric scrapers and the kick factor of the symmetric scrapers; the scraper results are in reasonable agreement with the values from numerical simulation of the wake fields. The kick factor extracted from the closed orbit distortion of the dipole wake induced by the narrow-gap undulator chamber is significantly greater than that predicted by the numerical simulation. Possible sources of the discrepancy are discussed.

Use of a corrugated beam pipe as a passive deflector for bunch length measurements

We report the experimental demonstration of bunch length measurements using a corrugated metallic beam pipe as a passive deflector. The corrugated beam pipe has been adopted for reducing longitudinal chirping after the bunch compressors in several XFEL facilities worldwide. In the meantime, there have been attempts to measure the electron bunch's longitudinal current profile using the dipole wakefields generated in the corrugated pipe. Nevertheless, the bunch shape reconstructed from the nonlinearly deflected beam suffers from significant distortion, particularly near the head of the bunch. In this paper, we introduce an iterative process to improve the resolution of the bunch shape reconstruction. The astra and elegant simulations have been performed for pencil beam and cigar beam cases, in order to verify the effectiveness of the reconstruction process. To overcome the undesirable effects of transverse beam spreads, a measurement scheme involving both the corrugated beam pipe and the spectrometer magnet has been employed, both of which do not require a dedicated (and likely very expensive) rf system. A proof-of-principle experiment was carried out at Pohang Accelerator Laboratory (PAL) Injector Test Facility (ITF), and its results are discussed together with a comparison with the rf deflector measurement.

Analysis of Damping Requirements for Dipole Wake-Fields in RF Crab Cavities

Several current particle-antiparticle collider designs require the beams to collide at a small angle. In order to make the luminosity comparable to that of beams colliding head-on, transverse deflecting cavities known as crab cavities will be used. Additional modes, other than the required crabbing mode are excited by the beam and if these are excited resonantly then an appreciable dilution of the beam emittance will occur. We provide a single- mode analysis of the damping requirements on the dipole modes which will minimize the resulting angular kicks to the beams, by deriving an explicit solution for the frequency offset of the mode from a bunch harmonic for maximum deflection of bunches, using a novel analytical technique.