John T. Donohue

Microwave production by a free-electron laser bunched beam driving a resonant cavity at 35 GHz

In the two-beam accelerator scheme, a high-current electron beam, bunched at the resonant frequency, traverses extraction cavities, where it generates power intended to drive accelerating cavities on the main beam. Here, we report on work performed to test some aspects of this scheme. We used a free-electron-laser (FEL) amplifier at 35 GHz to bunch a beam of electrons, which is then transported and focused into a resonant cavity. The results of earlier bunching experiments had demonstrated the capacity of the FEL supplied by an induction linac to generate an appropriate drive beam. A summary of this earlier work is presented along with measurements of power from the cavity. This constitutes the first observation of high-frequency power extraction using a FEL in this scheme.

Measurements of microwave power and frequency in a pulsed free electron laser amplifier

We report output power and frequency measurements of a pulsed free electron laser (FEL) operating as an amplifier at 35 GHz, without guiding field. The experiment used an induction linac, which delivers an 800-A relativistic electron beam (2.2 MeV) with a flat-top of 40 ns into the helical wiggler. The input signal furnished by a 35-GHz magnetron source is amplified to power levels of the order of 80 MW. The experimental results are in good agreement with our simulations. Frequency chirping is observed, and its behavior as a function of the basic FEL parameters is discussed.

A Compact THz Source: 100/200 GHz Operation of a Cylindrical Smith–Purcell Free-Electron Laser

We report first operation in the terahertz regime of a cylindrical grating Smith-Purcell free-electron laser. Propagation of an annular electron beam in proximity to a cylindrical grating causes strong electron bunching due to a beam-surface wave interaction. Electromagnetic radiation results from the bunching (fundamental) and, at bunch harmonics, the Smith-Purcell effect. In the experiment, over 2.5 kW was generated at 100 GHz (fundamental) and over 100 W at 200 GHz (Smith-Purcell). The results illustrate the potential of this configuration for generation of high-power terahertz radiation.

Gain enhancement in a two-frequency high-gain waveguide free-electron laser

In a waveguide monomode free-electron laser (FEL), two resonant frequencies can be amplified by the electron beam. At the CEA/CESTA facility, single-pass high-gain FEL experiments have been performed over the last five years using relativistic electron beams provided by induction linacs. Most of the work was done in the amplifier regime (at the higher frequency) with the aim of producing a 35 GHz bunched beam. However, super-radiant measurements were also made and have shown that the FEL gain at the upper frequency is higher than in the amplifier regime and may be driven by the lower frequency FEL interaction.

Characterization of a pulsed bifilar helical wiggler

An adiabatic entrance, obtained by placing resistive shunts at half-period intervals between the adjacent clockwise and counterclockwise helical current turns of the wiggler winding, is often used to assure good beam injection into the bifilar helical wiggler of a free electron laser (FEL). If the wiggler is pulsed, the time dependence of the magnetic field in the entrance may differ from that of the uniform section of the wiggler, and the magnetic field as a function of position along the axis may not be gradually increasing, in other words adiabatic. In addition, if a waveguide is used to contain the radiation, as in microwave frequency FELs, the tube itself may partially shield and retard the magnetic field in its interior. We investigate the time dependence of the magnetic field in a pulsed wiggler both experimentally and theoretically. Good agreement between measurements and our model is found. By using shunts of varying resistance, and by choosing carefully the time at which the beam enters the wigg...

Laser ablation of fused silica induced by gold nanoparticles: comparison of simulations and experiments at λ=351 nm

Simulations of laser-fused silica interactions at 0.351 μm are a key issue in predicting and quantifying laser damage in large laser systems such as LIL and LMJ. Validation of numerical simulations requires detailed knowledge of the different parameters involved in the interaction. To concentrate on a simple situation, we have made and tested a thin film system based on calibrated gold nanoparticles (0.2-0.8 μm diameter) inserted between two silica layers. The fused silica overcoat was either 2 or 10 microns thick. We have performed simulations of laser energy deposition in the engineered defect (i.e. nanoparticle) and the surrounding fused silica taking into account various laser/defect induced absorption mechanisms of SiO2 (radiative ionization, avalanche and multiphotonic ionization). We have studied crater formation produced by the absorber explosion with a 2D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. We discuss the influence of the defect depth (with respect to the surface) on the damage morphology. The simulations are compared with our experimental results.

Observation of copious emission at the fundamental frequency by a smith-purcell free-electron laser with sidewalls

Summary form only given. A three-dimensional simulation using the Particle-in-cell code “MAGIC” predicted that for a grating with sidewalls, copious emission of coherent Smith-Purcell (SP) radiation at the fundamental frequency of the evanescent surface wave would occur 1. This may be contrasted with the two-dimensional theory proposed by Andrews and Brau (AB) some years ago 2, for which emission could only occur at harmonics of that frequency. In order to validate this prediction we have performed a demonstration experiment in the microwave domain, using the same set-up as in our previous confirmation of the AB theory 3, except that the grating has sidewalls only four cm apart. We observed a forward-moving beam of SP radiation at the predicted frequency after reflection by a plane mirror, since the true emission is in the backward direction, which makes it hard to observe. Agreement with the PIC simulations is satisfactory.

Reaching high frequencies in a Smith-Purcell FEL with a multi-channel grating

Summary form only given. Three-dimensional simulations and experiments have shown that, for a grating equipped with sidewalls, copious emission of coherent Smith-Purcell (SP) radiation at the fundamental frequency of the evanescent surface wave is possible. One may reduce the wavelength by rescaling all the parameters of the grating, but this reduces the radiated power by the square of the scale factor. Both 3D MAGIC simulations and the theory of antenna arrays suggest that by laterally juxtaposing N copies of the N-fold reduced grating, the overall power reduction is only 1/N. Furthermore, the radiation is concentrated into a solid angle of size 1/N compared to that of a single reduced grating. An experiment at 32 GHz partially confirms these predictions for N = 7. Preliminary simulations indicate that with a single 20-fold reduced channel coherent Smith-Purcell radiation at 100 GHz is produced. An experiment to confirm that a 20-channel grating will produce a well-collimated radiation pattern with adequate power levels is being prepared.

Observation of coherent smith-purcell emission at 32 GHz from a multi-channelgrating with sidewalls

Summary form only given. The two-dimensional theory of Smith-Purcell free-electron laser predicts that coherent radiation can occur only at harmonics of the frequency of the evanescent wave that is resonant with the beam. Particle-in-cell simulations and a demonstration experiment at 5 GHz have shown that in a three-dimensional context, where the lamellar grating has sidewalls, coherent Smith-Purcell radiation is copiously emitted at the fundamental frequency with 10 % efficiency. Theory shows that a rescaling of all grating parameters at constant energy leads to a rescaled wavelength for the emitted radiation, accompanied by a reduction in power proportional to the square of the scaling factor. Both simulations and antenna array theory suggest that N-fold scale reductions can be partially compensated by laterally juxtaposing N copies of the reduced grating. Both the power and the solid angle into which it is emitted are reduced by 1/N. To confirm these predictions, a new experiment, with a seven-channel grating operating at 32 GHz has been performed. We present in this talk the main results in comparison with MAGIC 3D simulations. Provided that further reduction in scale can be achieved, a coherent Smith-Purcell radiation source in the Terahertz frequency range can be foreseen.

Experimental demonstration of a high power smith-purcell source using a cylindrical grating

Summary form only given. Many applications of THz radiation remain impractical or impossible due to an absence of compact sources with sufficient power. A source in which the interaction occurs between an annular electron beam and a cylindrical grating has been shown in simulations to be capable of generating very high THz power in a very compact package. The grating surface-wave produces strong beam bunching and generates significant power at the fundamental frequency and harmonics. A collaboration between Advanced Energy Systems and CEA/CESTA has been ongoing in performing proof-of-principle tests on cylindrical grating configurations producing millimeter wave radiation. Testing has been performed with a 6 mm period grating, producing power at the fundamental frequency of 15 GHz, second harmonic power at 30 GHz and although not measured, simulations show meaningful third harmonic power at 45 GHz. Comparison between simulations and the experimental results will be presented. Future plans will increase the frequency of operation to 100 GHz.