J.L. Rullier

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.

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...

Electron trajectories in a helical wiggler with a uniform axial guide field: a comparison of numerical results with results obtained using an approximate but analytical method

Abstract A comparison is made between two methods of calculating electron trajectories in a magnetic field consisting of a helical wiggler and a uniform axial guide field. The code SOLITUDE, developed by Gouard, was used to calculate with high precision the trajectories of electrons whose initial positions and velocities were chosen at random from a Gaussian distribution. After propagation through an adiabatic entry, the positions and velocities at the point where the wiggler field becomes steady were used as input to an analytical calculation developed by Donohue and Rullier. Although the calculation is analytic, the method involves truncating the true Hamiltonian, and is therefore not exact. Despite this, generally good agreement is found between the trajectories generated by the two methods, in both the normal and reversed-field configurations. The analytical approach provides physical insight into certain unusual aspects of some of the numerically generated trajectories. Several illustrative examples are compared in detail.

PRODUCTION OF HIGH POWER MICROWAVES FOR PARTICLE ACCELERATION WITH AN FEL BUNCHED ELECTRON BEAM

Abstract Among the studies in the framework of high gradient linear electron–positron collider research, the Two-Beam Accelerator (TBA) is a very promising concept, and two projects are in progress, the Compact Linear Collider project at CERN (W. Schnell, Report no. CERN SL/92-51 and CLIC note 184; K. Hubner, CERN/PS 92-43, CLIC note 176; S. Van der Meer, CERN/PS 89-50, CLIC note 97.) and the Relativistic Klystron-TBA project at LBNL (Technical Review Committee, International Linear Collider Technical Review Committee Report 1995, SLAC-R-95-471, 1995). In a TBA an extremely intense low-energy electron beam, called the drive beam, is bunched at the desired operating frequency, and upon passing through resonant cavities generates radio-frequency power for accelerating the main beam. Among the different approaches to the production of a suitable drive beam, the use of an FEL has been proposed and is under active study at CEA/CESTA.