Range Wakefields Research Articles

Beam based characterization of the European Synchrotron Radiation Facility Extremely Brilliant Source short range wakefield model

Beam based characterization of the European Synchrotron Radiation Facility Extremely Brilliant Source short range wakefield model

Equilibrium of an arbitrary bunch train with cavity resonators and short range wake: Enhanced iterative solution with Anderson acceleration

This paper continues the work of two previous treatments of bunch lengthening by a passive harmonic cavity in an electron storage ring. Such cavities, intended to reduce the effect of Touschek scattering, are a feature of fourth generation synchrotron light sources. The charge densities in the equilibrium state are given by solutions of coupled Ha\"issinski equations, which are nonlinear integral equations. If the only wake fields are from cavity resonators, the unknowns can be the Fourier transforms of bunch densities at the resonator frequencies. The solution scheme based on this choice of unknowns proved to be deficient at the design current when multiple resonators were included. Here we return to the conventional formulation of Ha\"issinski equations in coordinate space, the unknowns being charge densities at mesh points on a fine grid. This system would be awkward to solve by the Newton method used previously, because the Jacobian matrix is very large. Here a new solution is described, which is both Jacobian-free and much simpler. It is based on an elementary fixed point iteration, accelerated by Anderson's method. The scheme is notably fast and robust, accommodating even the case of extreme over-stretching at current far beyond the design value. The Anderson method is promising for many problems in accelerator theory and beyond, since it is quite simple and can be used to attack all kinds of nonlinear and linear integral and differential equations. Results are presented for ALS-U, with updated design parameters. The model includes harmonic and main r.f. cavities, compensation of beam loading of the main cavity by adjustment of the generator voltage, and a realistic short range wake field (rather than the broad-band resonator wake invoked previously).

Longitudinal equilibrium density distribution of arbitrary filled bunches in presence of a passive harmonic cavity and the short range wakefield

We propose a novel scheme for the calculation of bunch profiles and synchronous phase shifts for arbitrary fill patterns in electron storage rings considering both the effects of the passive harmonic cavity and the short range wakefield. This scheme treats the bunch as a charge distribution rather than a point charge with a bunch form factor, and divides the voltage induced in the cavity into two groups: that induced by the previous passages and that by the present passage, where the latter is integrated with the short range wakefield from all the other vacuum components. An iteration loop for computing the bunch profiles, including a subloop using Newton iteration for calculating the synchronous phase deviations, is built and implemented into a matlab-based code. Using this code, we study the bunch lengthening and centroid transient effects for several possible fill patterns in the Hefei Advanced Light Facility storage ring under design. We find the method is very efficient, and the CPU time for the whole calculation on a regular personal computer is less than 30 seconds for each case even with more than 700 bunches. Using the case of a long gap fill as a benchmark example, both the results given by our code and the multiparticle tracking simulation with elegant are in very good agreement.

Simulation study of the long range wakefield induced multi-bunch instability in the China X-ray free electron laser

A 1.3GHz superconducting accelerator (SC) which can generate 32 bunches (0.2nC/bunch) with an interval of 2.3ns has been designed for the China free electron laser (CXFEL), the effects of the long range wakefield in the superconducting cavities are analyzed in this paper. In our simulation results, the fundamental mode of the longitudinal wakefield (beam loading) may result in an energy droop of 844eV/cavity to the bunch train, and may also disturb the bunch compression process by altering the energy chirp. The effects of the transverse wakefield are studied by injecting all bunches with an initial position error of 200µm, and also by applying a random misalignment of σΔx,y=100µm (in Gaussian distribution) to the cavities and the focusing quadrupoles, then the trajectory of the bunch train is corrected with horizontal and vertical correctors. In both cases the transverse wakefield induced bunch to bunch variation are negligible, whereas the misalignment may result in emittance degradation to all bunches.

Off-axis beam dynamics in rf-gun-based electron photoinjectors

The need to operate an rf-gun-based electron photoinjector with a beam emitted away from the cathode center can occur under various circumstances. First, in some cases the cathode can be affected by ion back-bombardment that progressively reduces the quantum efficiency (QE) in its center, making off-axis operation mandatory; second, in some cases the drive laser intensity can be sufficiently high to generate QE depletion in the cathode area illuminated by the laser, forcing off-axis operation; last, in cathodes with nonuniform QE distribution it could be convenient to operate off axis to exploit a better QE. However, operation in this mode may lead to growth of the projected transverse beam emittances due to correlations between the transverse and longitudinal degrees of freedom that are introduced within the gun and downstream rf cavities. A strategy is described to mitigate this emittance growth by allowing the beam to propagate along a carefully tuned off-axis trajectory in downstream rf cavities to remove the time-dependent rf kicks introduced in the gun. Along this trajectory, short range wakefields do not degrade the emittance, and long range wakefields degrade the emittance for very high repetition rate only.

Simulation of high order short range wakefields for particle tracking codes

We consider the effects of short range (intra bunch) wakefields in axially symmetric apertures for bunches of arbitrary shape, and show how they can be included in particle tracking codes in a computationally efficient way, including modes of any angular order. The MERLIN simulation code is given as an example. Results from these simulations strongly indicate that such higher order mode wakefield effects are not significant even for the unprecedentedly small-dimension collimation system proposed for the ILC.

Accurate modelling of charged particle beams in linear accelerators

A higher order, energy conserving discretization technique for beam dynamics simulations is presented. The method is based on the discontinuous Galerkin (DG) formulation. It utilizes locally refined, non-conforming grids which are designed for high spatial resolution along the path of charged particle beams. Apart from this formulation, the paper introduces a class of general symplectic integrators which conserve discrete energy in a modified sense. Specialized split-operator methods with optimum dispersion properties in the direction of particle motion are, additionally, derived. The application examples given in the paper are performed in a high performance computing environment. They include the self-consistent simulation of the RF electron gun developed by the Photo Injector Test Facility at DESY Zeuthen (PITZ) project and the computation of short range wake fields for ultra-relativistic electron bunches.