88 research outputs found
Phase Jump Method for Efficiency Enhancement in Free-Electron Lasers
The efficiency of a free-electron laser can be enhanced by sustaining the
growth of the radiation power beyond the initial saturation. One notable method
is undulator tapering, which involves the variation of the gap height and/or
the period along the undulator. Another method is the introduction of phase
jumps, using phase-shifting chicanes in the drift sections separating the
undulator segments. In this article, we develop a physics model of this phase
jump method, and verify it with numerical simulations. The model elucidates the
energy extraction process in the longitudinal phase space. The main ingredient
is the microbunch deceleration cycle, which enables the microbunched electron
beam to decelerate and radiate coherently beyond the initial saturation. The
ponderomotive bucket is stationary, and energy can even be extracted from
electrons outside the bucket. The model addresses the selection criteria for
the phase jump values, and the requirement on the undulator segment length. It
also describes the mechanism of the final saturation. In addition, we discuss
the similarities and differences between the phase jump method and undulator
tapering, by comparing our phase jump model to the classic
Kroll-Morton-Rosenbluth model
Emittance manipulation by first- and second-order lattice control
The lattice of the MAX-I electron storage ring has been investigated and tuned towards small momentum compaction values. By measurements of the synchrotron frequency, bunch length, horizontal beam size and beam movement, the beam has been found to behave in reasonable agreement with the predictions of the lattice model up to second order in energy deviation. Both longitudinal and horizontal emittance could be varied within a relatively large range with lattice changes and/or controlled beam energy changes
Further Studies of Undulator Tapering in X-Ray FELs
We further the studies of the model-based optimization of tapered free-electron lasers presented in a recent publication [Phys. Rev. ST Accel. Beams 18, 040702 (2015)]. Departing from the ideal case, wherein the taper profile is a smooth and continuous function, we consider the more realistic case, with individual undulator segments separated by break sections. Using the simulation code GENESIS, we apply our taper optimization method to a case, which closely resembles the FLASH2 facility in Hamburg, Germany. By comparing steady-state and time-dependent simulations, we examine how time-dependent effects alter the optimal taper scenario. From the simulation results, we also deduce that the "traditional" empirical method, whereby the intermediate radiation power is maximized after closing every undulator gap, does not necessarily produce the highest final power at the exit of the undulator line
FEL Operation Modes of the MAX IV Short Pulse Facility
The Short Pulse Facility (SPF) of the MAX IV Laboratory in Lund, Sweden features the production of ultrashort, incoherent x-ray pulses. It is driven by a 3-GeV linac and comprises two 5-metre undulator modules. While the SPF is designed for spontaneous radiation, we explore alternative operation modes in which the SPF functions as a simple free-electron laser (FEL). In this article, we characterize two of them in time-dependent numerical simulations. We perform a sensitivity study on the electron beam parameters and examine the technique of single-step tapering
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