Abstract
A numerical algorithm for simulating electron beam shot noise in free electron lasers (FELs) is presented. Shot noise is a source of spontaneous emission that may be amplified in the self-amplified spontaneous emission regime of operation. This regime is of great importance to XUV and x-ray FELs where the spontaneous emission is currently the only effective source available for amplification. The algorithm uses a quasiuniform phase-space distribution of appropriately charge weighted macroparticles. The statistical properties of the macroparticles are derived directly from the temporal Poisson statistical properties of the real electron distribution. Unlike previous algorithms, ours does not rely upon any averaging over a resonant radiation period time scale and so more correctly describes the underlying physics. The algorithm also allows shot noise to be modeled self-consistently in unaveraged FEL models which are able to describe subwavelength phenomena such as coherent spontaneous emission (CSE). The algorithm is used in the unaveraged 1D FEL numerical simulation code FemFel and demonstrates spontaneous emission due to shot noise and CSE in both rectangular and Gaussian electron pulse current profiles. The preliminary results show good qualitative and quantitative agreement with theory.
Highlights
The free electron laser (FEL) is an important source of high power tunable radiation that spans a wide range of the electromagnetic spectrum from the microwave to VUV and potentially beyond
The right-hand side of the wave equation is written in terms of the real electron distribution, where N is the total number of electrons in the electron pulse and at the beginning of the interaction region, z 0, so that z1
We demonstrate the use of the macroparticle model above by considering the bunching in a uniform current electron beam of 1 kA with a uniformly distributed relative energy spread of half width " =" =2 at a radiation wavelength of 10 nm and with a FEL parameter of 103
Summary
The free electron laser (FEL) is an important source of high power tunable radiation that spans a wide range of the electromagnetic spectrum from the microwave to VUV and potentially beyond. FELs such as that at the TESLA facility in Germany [6], the proposed LCLS facility at SLAC [7] in the U.S, and the U.K. 4GLS project [8], it would seem timely and prudent to derive a particle loading algorithm from first principles that may be traced directly back to the physics of the individual electrons. Such a model would allow the introduction of shot noise into FEL models that have not been averaged over a radiation period in a consistent way. Macroparticle model directly from a Poisson statistical electron distribution and demonstrates its use in a one dimensional finite element code FEMFEL currently under development
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