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

Abstract

We present results of an experiment where, using a 200 GW CO2 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 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.Greatly increasing the electro-optical conversion efficiency from particle beams to coherent radiation has the potential to enable a new class of high peak and average power sources capable of satisfying the increasing demands of cutting-edge scientific, defense and industrial applications. These range from powering laser-based accelerators, developing defense-class high energy lasers, and improving the throughput of next generation fabrication processes for the semiconductor industry [1][2][3].The current workhorse to directly convert power from electron beams to electromagnetic radiation is the free-electron laser interaction where relativistic electron beams and electromagnetic waves exchange energy as they copropagate in an undulator magnetic field. This interaction is maximized when the electron energy, the undulator period and field amplitude satisfy the resonant condition, or equivalently the particles slip exactly one (or an integer number of) radiation wavelength every undulator period. In the classical FEL amplification scheme [4,5], the amplification process saturates at a peak power given by P sat ∼ 1.6ρP beam where ρ is the FEL pierce parameter (typically lower than 0.5 % for short wavelength radiation) and P beam is the beam power. Due to the absence of a gain medium or of a nearby metal or dielectric structure, the interaction is dissipation-free and the saturation occurs only due to the fact that the particles lose energy and fall out of the resonant interaction region.Increasing the output power beyond the FEL saturation level can be achieved by tapering the undulator parameters -that is modifying the undulator characteristics (field amplitude and/or period)-to sustain the interaction even when the particles lose a large fraction of their energy. Undulator tapering as a means to increase FEL performances has been studied since the early days of FEL technology when the FEL was proposed as a path towards a very high average power source, and typically results in few percent efficiencies. The ELF experiment in the '80s demonstrated extraction efficiencies over 30 % but for GHz frequencies and only in a waveguide-mediated interaction [6]. Recent development of the X-Ray FEL has rekindled interest in undulator tapering [7][8][9] as increase in the peak power of the X-Ray FEL resulting from 5-10 % extraction efficiencies could unlock long-term goals in x-ray science such as single molecule imaging [10,11].An even stronger tapering of the undulator parameters to maintain the resonant condition over a very larg...

Superradiant and stimulated-superradiant emission of bunched electron beams

“…In the following chapters we analyze the dynamic processes of a tightly bunched electron [99,173], that are described in Section B of Chapter VIII. Here very tight prebunching was attained using a high intensity 10.6µ CO 2 laser.…”

Some analyses on optimal energy-extraction efficiency in a free-electron laser

“…the saturation of the optical field. To heighten the FEL power, the tapered undulator was proposed [2][3][4][5][6], tapering the undulator parameters along the undulator to maintain the resonance condition and the interaction.…”

“…There are many investigations on the improvement of FEL efficiency [2][3][4][5][6][7][8]. Various strategies have been studied in theory and some of them have been demonstrated in experiments.…”

“…Normally, the FEL saturation occurs as the power reaches a certain level with the qualities of electrons deteriorated severely, namely, the parameter relations of the electron beam and the optical field have deteriorated. To increase the FEL efficiency, an appropriate way is the use of a tapered undulator [2][3][4][5][6][12][13][14]. Around the FEL saturation, the undulator parameters are tapered along the undulator to maintain the resonant condition and the energy transfer from the electrons to the radiation.…”

Very high brightness and power LCLS-II hard X-ray pulses

“…This is similar to the approach discussed in Refs. (Mak et al, 2017;Sudar et al, 2016;Duris et al, 2015), but is not limited to express the resonant phase in the form of a polynomial function, as they assume in their papers. The motivation for allowing arbitrary variation of ψ r along the undulator is due to the fact that output power depends on the trade-off between the energy loss due to the FEL interaction (dγ/dz ∝ sin ψ r ) and the fraction of electrons trapped f t .…”