Measurements of narrow‐band (Δω/ω≤0.02) microwave emission from a tunable (7≤ω/2π≤21 GHz) Raman, free‐electron laser operating in a single TE11 waveguide mode are reported. Approximately 100 kW of rf power has been observed at an electronic efficiency of 12%, and ∼1 MW of rf power has been generated at a reduced efficiency of 8%. Frequency, gain, and rf power measurements have been carried out for various values of the guide magnetic field, below, above, and near to the resonance between the cyclotron frequency of the guide magnetic field and the frequency associated with the periodic wiggler magnetic field. The results are in very good agreement with the predictions of three‐dimensional free‐electron laser theory.

/pdf/microwave-studies-of-a-tunable-free-electron-laser-in-4t1d5dfl6c.pdf

Microwave studies of a tunable free‐electron laser in combined axial and wiggler magnetic fields

We report the results of the first gain measurements on the free electron laser being assembled on the storage ring ACO. The largest measured peak gain, averaged over the laser mode, is ? = 4.3 × 10-4 per pass. No laser induced bunch lengthening is observed within the experimental accuracy of 50 psec under a laser intensity of 1.6 kw/cm2 and under the conditions of strong anomalous bunch lengthening (??/??(I=0)) = 6.8 - 4.2.

Gain Measurement on the ACO Storage Ring Laser

The magnetic field at the entrance of a wiggler magnet in a free‐electron laser (FEL) determines the character of the electron orbits and influences the FEL gain. To obtain the desired adiabatic entrance conditions, it is necessary to investigate different methods of connecting leads to the wiggler as well as methods of gradually reducing the wiggler field. The field of a current‐driven, finite‐length bifilar helical wiggler magnet is derived and is used to investigate the magnetic field end perturbations. For a limited range of wiggler parameters, flaring the wiggler end produces an adiabatic entrance region. Alternatively, gradually terminating the wiggler current at the wiggler entrance is shown to produce an adiabatic entrance for a larger range of parameters.

/pdf/end-effects-of-a-bifilar-magnetic-wiggler-xleil37vso.pdf

End effects of a bifilar magnetic wiggler

An experimental free electron maser (FEM) facility is described which operates in the millimeter-wave regime. The experiment demonstrates that high gain and beam extraction efficiency can be obtained with low-current but high-quality electron beams. In the oscillator configuration, microwave powers of 110 kW were achieved with a 300-kV, 4-A beam, corresponding to a beam extraction efficiency of 9.2%. At low beam currents, with the system operating in the linear regime, the FEM gain curve was measured. The experiment uses a Pierce-type gun with a calculated beam emittance of 10.7-mm mrad at 300 kV and 4 A. The wiggler is of the iron permanent magnet hybrid design. The experiment is intended as a first step towards the development of continuous-wave (CW) devices that use beam energy recovery to make use of low-current, high-voltage power supplies. >

A long-pulse millimeter-wave free electron maser

A conceptual design is presented for efficient operation of a free electron maser amplifier for fusion applications. The system, which operates in the strong pump regime, is CW with output frequencies in the 100 GHz range or higher, output power levels of up to 5 MW and total efficiencies greater than 70%. The physical and technological data base for such a design is described.

Design of a high power CW free electron maser

An experiment has been performed in which microwaves are generated by the interaction of a helical magnetic wiggler field with an electron beam. This Free Electron Laser can be operated in either the stimulated Raman or stimulated Compton regime by changging the electron beam density. The high repetition rate, and long pulse length of the electron beam facilitate diagnostics and the characterization of the device. To date, the device has been operated in the superradiant and oscillator Raman scattering mode. Preliminary results show an increase of 20 db in the oscillator mode with respect to the superradiant mode. The variations in the spectrum with wiggler field amplitude and electron beam density are shown.

https://epaper.kek.jp/p79/pdf/pac1979_3830.pdf

Jesper Munch

Papers

A Free Electron Laser Experiment