J. Gardelle

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.

Strong coupling operation of a FEL amplifier with an axial magnetic field

This paper presents the results of a FEL experiment at 35 GHz, using a highly relativistic electron beam (T = 1.75 MeV). The electron pulse length is 30 ns FWHM with a peak current of 400 A. The FEL is designed to operate in the high-gain Compton regime, with a negative coupling parameter (Φ < 0) leading to a stong growth rate. More than 50 MW of RF power in the TE11 mode (43 dB gain) has been obtained with good reproducibility. The experimental results are in good agreement with predictions made using the 3D stationary simulation code SOLITUDE.

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...

The Arletron: A New Buncher for High-Repetition-Rate HPM Operation

In this paper, I present simulations of a new buncher that can be used in a compact high-repetition-rate high-power microwave device. The bunching mechanism involves the growth of the II-mode of two coupled cavities traversed by a medium-power annular electron beam (200 keV, 1 kA). The cavities are simple pillboxes, and frequency tuning can be obtained by using plungers to vary the internal radius of the cavities. Since the magnetic field needed to ensure beam transport is small enough to be provided by permanent magnets, no external source is required.

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.

High current fast Marx generator and vircator experiments

This paper describes a compact pulsed power generator. This system stores 10 kJ and is capable of delivering up to 40 GW to an electron diode. It has been optimized to drive high power microwaves devices that require high current. In order to limit the parasitic inductance and to reduce the rise time of the current to less than 100 ns, two Marx generators have been associated in parallel. Some typical results of vircator operation in S band are also presented.

Optimizing the emission, propagation, and focusing of an intense electron beam

Intense electron beams can be used to study the dynamical response of materials under shocks in order to adjust the models developed for hydrodynamics simulations. We present in this paper a characterization of beams produced in a field emission diode coupled to the generator RKA at CEA/CESTA. Cherenkov emission, produced by the beam interacting in a fused silica disk, was observed by fast optical cameras to estimate beam homogeneity. GEANT4 simulations were performed to estimate the transfer function of the silica target and to optimize the anode foil. First, we chose the best cathode material available among the most common materials used in field emission systems. In addition, we found that by optimization of the anode thickness, we could improve the spatial homogeneity of the beam which is of prime importance for computing the interaction of the beam with materials. Next, we changed the beam fluence by increasing the beam current and by reducing the beam radius. Finally, we studied the propagation and...