We outline the fundamental coherent radiation emission processes from a bunched charged particles beam. In contrast to spontaneous emission of radiation from a random electron beam that is proportional to the number of particles, a pre-bunched electron beam can emit spontaneously coherent radiation proportional to the number of particles -squared, through the process of (spontaneous) superradiance (SP-SR) (in the sense of Dicke's). The coherent SP-SR emission of a bunched electron beam can be even further enhanced by a process of stimulated-superradiance (ST-SR) in the presence of a seed injected radiation field. In this review, these coherent radiation emission processes for both single bunch and periodically bunched beams are considered in a model of radiation mode expansion.We also analyze here the SP-SR and ST-SR processes in the nonlinear regime, in the context of enhanced undulator radiation from a uniform undulator (wiggler) and in the case of wiggler Tapering-Enhanced Stimulated Superradiant Amplification (TESSA).The processes of SP-SR and TESSA take place also in tapered wiggler seed-injected FELs. In such FELs, operating in the X-Ray regime, these processes are convoluted with other effects. However these fundamental emission concepts are useful guidelines in efficiency enhancement strategy of wiggler tapering. Based on this model we review previous works on coherent radiation sources based on SP-SR (coherent undulator radiation, synchrotron radiation, Smith-Purcell radiation etc.), primarily in the THz regime and on-going works on tapered wiggler efficiency-enhancement concepts in various frequency regimes.

Photonic structures operating in the terahertz (THz) spectral region enable the essential characteristics of confinement, modal control, and electric field shielding for very high gradient accelerators based on wakefields in dielectrics. We report here an experimental investigation of THz wakefield modes in a three-dimensional photonic woodpile structure. Selective control in exciting or suppressing of wakefield modes with a nonzero transverse wave vector is demonstrated by using drive beams of varying transverse ellipticity. Additionally, we show that the wakefield spectrum is insensitive to the offset position of strongly elliptical beams. These results are consistent with analytic theory and three-dimensional simulations and illustrate a key advantage of wakefield systems with Cartesian symmetry: the suppression of transverse wakes by elliptical beams.

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...

In this paper we analyze the high gain, high efficiency tapered free-electron laser amplifier with a prebunched electron beam. Simple scaling laws are derived for the peak output power and extraction efficiency and verified using 1D simulations. These studies provide useful analytical expressions which highlight the benefits resulting from fine control of the initial conditions of the system, namely the initial electron beam bunching and input seed radiation. When time-dependent effects are included, the sideband instability is known to limit the radiation amplification due to particle detrapping. We discuss two different approaches to mitigate the sideband growth via 1-D time dependent simulations. We find that a more aggressive taper enabled by strong prebunching and a modulation of the resonance condition are both effective methods for suppressing the sideband instability growth rate.

The generation of X-rays and γ-rays based on synchrotron radiation from free electrons, emitted in magnet arrays such as undulators, forms the basis of much of modern X-ray science. This approach has the drawback of requiring very high energy, up to the multi-GeV-scale, electron beams, to obtain the required photon energy. Due to the limit in accelerating gradients in conventional particle accelerators, reaching high energy typically demands use of instruments exceeding 100’s of meters in length. Compact, less costly, monochromatic X-ray sources based on very high field acceleration and very short period undulators, however, may enable diverse, paradigm-changing X-ray applications ranging from novel X-ray therapy techniques to active interrogation of sensitive materials, by making them accessible in energy reach, cost and size. Such compactness and enhanced energy reach may be obtained by an all-optical approach, which employs a laser-driven high gradient accelerator based on inverse free electron laser (IFEL), followed by a collision point for inverse Compton scattering (ICS), a scheme where a laser is used to provide undulator fields. We present an experimental proof-of-principle of this approach, where a TW-class CO2 laser pulse is split in two, with half used to accelerate a high quality electron beam up to 84 MeV through the IFEL interaction, and the other half acts as an electromagnetic undulator to generate up to 13 keV X-rays via ICS. These results demonstrate the feasibility of this scheme, which can be joined with other techniques such as laser recirculation to yield very compact photon sources, with both high peak and average brilliance, and with energies extending from the keV to MeV scale. Further, use of the IFEL acceleration with the ICS interaction produces a train of high intensity X-ray pulses, thus enabling a unique tool synchronized with a laser pulse for ultra-fast strobe, pump-probe experimental scenarios.