Title: Parameterization of FEL performance with various input conditions.
Project Description:
Motivation: Some hard X-ray experiments would benefit from very high X-ray powers. Single molecule imaging is believed to require on the order of 10^13 photons in 10 femtoseconds at a wavelength of 1 angstrom in a 100nm (minimum practical) focus, corresponding to a peak power of 2 TW. This is 10-100X higher than is available from the LCLS, or expected from LCLS_II or any of the other proposed FELs
Work G. Geloni and E. Saldin from DESY indicated that a seeded FEL could make efficient use of tapering. Later simulations by Z. Huang at SLAC indicated that by using the higher beam energy (27 GeV) available from the SLAC LINAC and with reasonable assumptions for peak current and emittance, and with a very long (700M) undulator, peak powers of 4TW at 1 angstrom could be produced. This design was based on a guess at parameters and an optimized design might produce higher power, or require a shorter undulator.
The Ming Xie formula is very useful for estimating and optimizing the performance of un-tapered FELs, but is not applicable to strongly tapered systems. Full 3-D simulations like Genesis are needed for tapered systems and their slow calculation speed makes optimization difficult.
Goal: The project goal is to develop a method to quickly optimize the design of seeded, tapered FELs.
Approach: perform a set of Genesis simulations with different beam conditions, optimizing the taper for each simulation. Then use the results to generate either an empirical formula or a fast calculation tool to approximate the performance of tapered FELs. This can then serve as a design and optimization tool, with full 3-D simulations used to verify the performance of the final design.
Milestones:
Make Genesis operational with inputs and outputs interfaced to an external program (Matlab or similar). Program a genetic optimization code based on Genesis steady state simulations to facilitate the parameter optimization. (April 2011)
Simulate the seeding and tapering experiment that will be performed on LCLS_I, to benchmark calculations. (May 2011 for simulation, experiment expected by the end of 2011)
Design an algorithm to automatically optimize the FEL taper (possibly by iteration) for a set of electron beam and undulator parameters. A good taper optimization is required to evaluate the effects of different beam conditions and undulator parameters. Available optimized undulator parameters include the start of the tapering, the ratio of undulator field over the taper length, and taper model (polynomial). Check the dependence of the optimal undulator parameters on the beam parameters. (August 2011)
Run a set of simulations (Genesis, slice) for reasonable beam conditions at SLAC to find the required undulator length and maximum peak power. Input beam conditions should include both present parameters, and reasonable extrapolations. The effect of the extrapolated parameters can serve as a guide to direct future R&D. (November 2011)
Spot check simulations with full 3-D Genesis runs and an alternate 3-D codes to check their accuracy. (December 2011)
Find a parameterization, or simple algorithm to fit the simulation results and predict the performance of tapered seeded FEL. (February 2011)
Perform tolerance studies to see if the designs are practical, or if tolerances need to be included in the optimization. (March 2011) Note, if tolerance issues are significant, this may extend the project).
Based on the calculation tool, develop a set of possible parameters for a SLAC based TW FEL.
Deliverables: An empirical formula or calculation tool to estimate the performance of seeded and strongly tapered FELs.