High Efficiency Energy Extraction from a Relativistic Electron Beam in a Strongly Tapered Undulator

We present results of an experiment where, using a 200 GW ${\mathrm{CO}}_2$ laser seed, a 65 MeV electron beam was decelerated down to 35 MeV in a 54-cm-long strongly tapered helical magnetic undulator, extracting over 30% of the initial electron beam energy to coherent radiation. These results, supported by simulations of the radiation field evolution, demonstrate unparalleled electro-optical conversion efficiencies for a relativistic beam in an undulator field and represent an important step in the development of high peak and average power coherent radiation sources.

Figure 1

Undulator period and magnetic field amplitude (left) and resonant energy (right) vs the coordinate $z$ along the undulator axis.

Figure 2

Nocibur experiment beam line layout.

Figure 3

(a) Electron beam spectrum with no laser seed. (b),(c) Deceleration spectra from Nocibur spectrometer for two consecutive shots having slightly different input laser seed energy, compared with GPT simulation $(1/N)(dN/dE)$ vs $E$ (bottom).

Figure 4

(a) Hall probe measurements of the prebuncher field varying chicane gap. (b) Fraction of fully decelerated particles as a function of the injection phase controlled by varying the chicane gap compared with GPT simulations with seed energy 0.55 J (yellow), 0.45 J (red), and 0.35 J (blue). (c) Longitudinal phase space for ${\mathrm{\Psi }}_r=\pi /4$ IFEL ponderomotive potential with phase space curves for trapped and untrapped particles. The prebunched beam longitudinal phase space is also shown color coding the particles within the separatrix. (d) GPT simulation of the $e$-beam spectrum at Nocibur exit showing full deceleration for the prebunched particles.

Figure 5

(a) Evolution of the radiation energy gain and total energy in electron beam along the undulator from Genesis simulation. (b) Transverse shape of Nocibur generated radiation at undulator exit.

Figure 6

Particle trajectories along the undulator from the GPT simulations. The ($\theta$, $\gamma$) longitudinal phase space at the undulator exit from Genesis simulation is displayed to show the remarkable agreement in all details of the energy spectrum. The ponderomotive potential bucket height is represented by dashed lines. The estimates for the positions of the detrapped energy peaks are also shown (points).