Inverse free electron laser accelerator for advanced light sources

We discuss the inverse free electron laser (IFEL) scheme as a compact high gradient accelerator solution for driving advanced light sources such as a soft x-ray free electron laser amplifier or an inverse Compton scattering based gamma-ray source. In particular, we present a series of new developments aimed at improving the design of future IFEL accelerators. These include a new procedure to optimize the choice of the undulator tapering, a new concept for prebunching which greatly improves the fraction of trapped particles and the final energy spread, and a self-consistent study of beam loading effects which leads to an energy-efficient high laser-to-beam power conversion.

Figure 1

Scheme for a diffraction dominated inverse free electron laser accelerator.

Figure 2

Left: Available gradient vs normalized Rayleigh range for two different tapering schemes. Right: Resonant energy as a function of undulator length for three different ratios $z_r/L_u$. The maximum final energy is achieved for a Rayleigh range of $L_u/5$.

Figure 3

(a) Resonant energy along the undulator for different cases. (b) Variation of undulator period ${\lambda }_w$ along the undulator. (c) Variation of magnetic field amplitude $B$ along the undulator.

Figure 4

Longitudinal phase space at the end of the undulator for the constant resonant phase and for the varying resonant phase undulator.

Figure 5

Ponderomotive pendulum like potential. Harmonic oscillator potential and approximation with first 4 harmonics.

Figure 6

Scheme of a nearly ideal harmonic prebuncher which uses nonlinear conversion crystals to generate up to the fourth harmonic of an infrared laser driver.

Figure 7

Longitudinal phase space and longitudinal distribution of a typical and a harmonic prebuncher.

Figure 8

Histogram of bunching factor results for 1000 different simulations where amplitude and phase for each laser lines vary randomly from their design values by 10% and 0.5 radians, respectively.

Figure 9

IFEL output longitudinal phase space and longitudinal distribution for a typical (above) and a harmonic (below) prebuncher.

Figure 10

Output IFEL parameters as functions of input beam current.

Figure 11

Final resonant energy versus design beam current for a compensated undulator.

Figure 12

Output IFEL parameters as functions of input beam current for a compensated tapered undulator designed for 20 kA.

Figure 13

(a) Mean beam energy vs position along undulator for a 0 kA and 20 kA beam. The 20 kA beam slips from resonance by the 9th undulator period. (b) Laser profiles at period 8 for the 0 kA and 20 kA beams (electron beam distribution shown in gray). (c) Laser intensity profile for the 0 kA beam at the 8th period. (d) Laser intensity profile for the 20 kA beam at the 8th period.

Figure 14

Power absorbed by accelerated electrons and remaining laser power vs rms transverse beam size for the case of the uncompensated undulator and a 18 kA beam.