T. Akiba

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.

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.

Theory and experiments for the induction linac FEL

Abstract A free electron laser driven by an induction linac has been investigated by experiments and theory. Preliminary experimental results for a 6 MeV, 1 kA induction linac FEL are presented. In the experiment, a distributed feedback resonator is introduced to obtain a narrow spectrum. As a result, a typical double peak spectrum for a distributed feedback laser was observed. For planning future experiments, 3-dimensional simulations have been performed on the FEL energy conversion efficiency for multitransverse mode excitation. The wiggler tapering is found to be effective only for the single mode excitation, but not for the multimode excitation.

Theory and experiment for a new focusing wiggler

Abstract Presented are the theoretical analysis and an experiment on a new wiggler which has a focusing field and tapering by a movable electromagnet. The electron beam propagation experiments show that a high emittance electron beam propagates with a relatively small beam radius. The gain for this wiggler is also investigated by the 3D FEL simulation code FUELNDES.

Development of an inductive voltage accumulating system for a free electron laser

Abstract An electron injector which consists of four induction units has been designed and constructed. Its design parameters at the diode region are 2 MV voltage, 40 kA current, and 90 ns pulse width, and at present this induction accelerator is being tested with 0.8 MV, 30 kA, and 100 ns. A cathode stalk extends the length of the accelerator, adds the voltage from the cavities, and delivers the power to a high voltage anode-cathode gap. The cathode constitutes the inner electrode of an applied axial mgnetically insulated transmission line (called power delivering conductor). The design approach and first experimental results are presented.

Performance of Radial Focus Applied-B Ion Diode on a Coaxial Pulse Power Machine

A high-power magnetically insulated ion diode with radial geometry was successfully operated on a Reiden IV-H (2 MV, 200 kA) coaxial pulse power machine with across-current feed converter. The generated proton beams were focused ballistically onto the axis. Data of the beam trajectory measurements showed a large aberration of beam focusing in the diode gap region. This may be due to the electron virtual cathode distortion. The reduction of beam focusability was improved simply by correcting the shape of the anode surface. The focused beam intensity of 3×10 10 W/ cm 2 , which was measured by an ion pinhole camera, was compared with predicted values from the trajectory measurements and the beam brightness. Focus reduction factors in the diode were improved from 0.15 to 0.41.

Progress of Inertial Confinement Fusion Research by Light Ion Beam

Recent progress of inertial confinement fusion (ICF) research make possible to achieving the ignition condition of pellet implosion in near future. The small target implosion experiments have been done successfully by high power lasers as Gekko XII grass laser system.1 The scaling low of implosion fusion experiments indicates that the several MJ energy driver will realize the ICF reactor. The light ion beam generated by the pulse power technology is the promissing energy driver for ICF due to its high generation efficiency and easy to generate the large energy beam with relatively low cost.

Raman regime free electron laser experiments using a distributed feedback structure

Abstract The design and preliminary experimental results of a Raman regime free electron laser (FEL) using a superconducting helical wiggler with a distributed feedback (DFB) reflector are described here. The DFB reflector was an iris-loaded type with a period of 3.2 mm and was installed in the drift tube of the FEL interaction region. Using a 700 keV/20 A electron beam with a duration of 200 ns from a pulsed power machine, spontaneous radiation in the 7mm wavelength region was observed in the absence of the DFB reflector. After installing the DFB reflector, the spectrum of radiation exhibited growth in a particular wavelength region. This indicates that the DFB reflector showed wavelength selectivity corresponding to its period.