S. Kuruma

Helical distributed feedback free-electron laser

Results of the implementation of a distributed feedback (DFB) reflector in a free‐electron laser (FEL) are described. The reflector was positioned in the interaction region of the FEL waveguide in order to produce narrow bandwidth features in the output spectrum. Both the DFB and wiggler were of helical configuration. The energy, current, and pulse width of the electron beam were 1.5 MeV, 100 A, and 50 ns. By employing the DFB structure the intensity of the two specific frequencies may be increased by factors of 3 and 5. These frequencies are in good agreement with those predicted by theory. Temporally radiation waveforms exhibit a double peak, revealing a sensitivity of the oscillation condition to beam energy.

Focusing permanent magnet undulator

Abstract A plane polarized undulator does not have the ability to focus an electron beam in the undulating plane (X-Z plane) because the magnetic field decreases away from the X-axis. To focus in the bending plane, a quadrupole field must be added to the undulating field. This can be achieved using curved magnets or trapezoid-shaped magnets. In this article, we analyze a few magnet configurations which produce X-Z focusing. It has been found that semicircular magnetics or trapezoid magnets having 1.2° angle are suitable as focusing permanent magnet undulator.

Experiment and theory on CO2 laser powered wiggler and induction linac FEL

Abstract As an electromagnetic wiggler experiment, CO 2 laser light (100 J/100 ns) is injected into a high current electron beam (1 kA, 0.5 MeV). Scattered visible light (λ = 8000-5000 A) is observed. From the spontaneous emission power the gain for the CO 2 laser powered FEL is estimated. The induction linac for the FEL which is developed at our institute and the FEL driven by the induction linac are also reported.

Multi-order harmonic lasing with a modified wiggler

Abstract A novel modified wiggler for higher harmonic lasing, modified wiggler, has been developed at ILT/ILE. Its magnetic field is expressed as the sum of the harmonic magnetic field components. A trial modified wiggler produced a considerable harmonic field component. A modification ratio, B 3 B 1 , of more than 10% was achieved. Using this wiggler, we observed the enhanced 3rd harmonic spontaneous emission. Further, the development of a modified wiggler for harmonic lasing will be presented.

Electrostatic microwiggler FEL

Abstract We propose here a new type of microwiggler in which a transverse electric field is applied to a relativistic electron beam. Not only the electrostatic wiggler works as a magnetic wiggler, but also the periodically arranged electrodes interact with the electron beam as a Smith-Purcell radiator. Inside the wiggler region, these radiations are multiply diffracted by the periodic structure, to constitute standing waves, to modulate and to bunch the beam.

Distributed feedback and gas-loaded FELs driven by induction linac SHVS

Abstract The induction linac SHVS at ILE, Osaka University, delivers a high current electron beam of 1.5–4.5 MeV energy to drive millimeter and submillimeter free electron lasers. In this article, we review the previous distributed feedback (DFB) FEL experiments and present a theoretical analysis of the output radiation spectrum. In the experiment, both the DFB and the wiggler were of a helical configuration. By employing the DFB structure, the output spectral intensity increases by factors of 3 ∼ 5 for radiation which satisfies the Bragg reflection condition. Future FEL experiments at ILE are also discussed, in which effects of the DFB and gas loading are investigated. These schemes are expected to make output spectra shorter and narrower and increase the gain. Furthermore, the numerical analysis of these FELs is presented in order to clarify the above-mentioned effects.

Temporal evolutions of frequency resolved radiations from a Raman regime free‐electron laser

In pulse power driven free‐electron laser experiments for a diode voltage 300–600 kV, a maximum electron beam current 730 A, with a 110‐ns width of beam voltage pulse and a helical wiggler with a pitch of 2 cm, the temporal evolution of radiation spectra is observed. The frequency variations are explained theoretically by using instantaneous values of beam voltages and currents. In the calculation we take account of the variation in the beam cross section of the beam voltage drop and the wiggler field. The spatial growth rate of the radiation power was also measured. The observed growth rate is 1 dB/cm and the saturation level is 250 kW with the spector peak at 58 GHz.

Magnetic field analysis of a hybrid helical microwiggler

Abstract A hybrid helical microwiggler has been designed analyzing its magnetic field by a computational code. It consists of fan-shaped permanent magnets and ferromagnets which are constructed in four layers each period. It is so small that the outer diameter is about 4 cm and periodical length 1 cm. The magnetic field is 3.73 kG for the gap spacing of 5 mm.

Simulation code development for the start-up of an FEL oscillator

Abstract A one dimensional multifrequency simulation code has been developed. Using this code, we analyze the spontaneous emission from an electron beam in an FEL. The start-up time of an FEL oscillation is investigated by the simulation code, appropriate by taking into account spontaneous emission. For the parameters of the ILE/ILT experiment (electron beam energy E b =6 MeV, current I b =1 A, radius r b =3 mm, wiggler period λ w =6 cm, amplitude B w =0.32 T, length L c =1.8 m, cavity length L c =3.6 m) the radiation wavelength is approximately 500 μm and the start-up time is estimated to be about 3.7 μs. Hence, in this case, it is necessary that the electron beam macropulse length be longer than 3.7 μs for FEL oscillation.

Coherent spontaneous emission from a bunched electron beam

Abstract A one-dimensional multi-frequency simulation code has been developed. Using this code, we analyze the spontaneous emission from a bunched electron beam in a FEL. With this simulation code the radiation spectral intensity and temporal profile have been evaluated for various pulse shapes of bunched electron beams. It is found that, in the case of a square electron pulse, the spontaneous emission at the edge is much stronger than the emission from numerical shot noise, and the radiation power is higher than that of a Gaussian electron pulse case.