IR Free Electron Research Center

» FEL Apparatus (@FEL-TUS)

Our reseach facility or the equipped FEL apparatus is often called "FEL-TUS" (FEL at Tokyo University of Science). We have two FEL apparatus, one is "MIR-FEL" (Mid-Infrared Free Electron Laser) which has routinely oscillated for user's experiments, another is "FIR-FEL" (Far-Infrared Free Electron Laser) which is under rectification for oscillation. Both of them consist of similar parts, "RF electron gun" generating a electron beam, "accelerator" accelerating the electron beam, "undulator" (permanent magnets) which makes the electron beam undulate, and "cavity" for light amplification and oscillation. The room where the FEL apparatus is equipped is surrounded by 2-meter thickness wall to shield harmful radiation. The FEL beam for utilizing experiments are collimated by optics, and distributed to each laboratory room through a vacuum duct equipped inside an undefloor pit. A klystron system for the FEL apparatus is equipped at a room next to the FEL apparatus room, and a high-power radio-frequency wave is supplied to "RF electron gun" and "accerelator" through waveguides. Both of two FEL apparatus cannot be operated at the same time, because they require a very high electric power. We use only one FEL apparatus by switching the power-supply waveguid.

Fig.1: [FEL Apps Room & MIR-FEL Beamline]
Fig.2: [Klystron]

1-1. MIR-FEL

Fig.3: [MIR-FEL Apparatus]
Table1: [MIR-FEL]
Electron Beam SourceRF Gun
CathodeLaB6 crystal
Accelerator
Acce. Energy40 MeV (max.)
Freq.2856 MHz
UndulatorHalbach-type
Magnetic Field0.83 T (max.)
Period32 mm
Num of Period43 cycle

Our facility provides mid-IR reagion light for user's experiments by the MIR-FEL apparatus. The apparatus is a linac-type FEL, which was enabled to be downsized by adopting newly developed RF gun as a electron beam source.

The first step in FEL oscillation is to generate a bunched electron beam with an RF electron gun. We employed a OCS-type (on-axis coupled structure) RF gun which has a coupling cell between two acceleration cells to stabilize the beam loading. It was desinged to minimize the back-bombardment. The radio-frequency of 2856 MHz is provided to the gun, and the output electron beam is accelerated up to ca. 1.6MeV at its exit.

The generated electron beam is spacially/temporally compressed by passing through an alpha magnet. After this process, the electron beam is accelerated almost up to the light speed by a linear accelerator (linac), and introduced to the light generation region. The linac with 3-m CG (continuous gradient) accelerating tube accelerates the electron beam up to 40 MeV. The RF pulse from an S-band (2856 MHz) klystron is divided and supplied to the electron gun (20%) and the linac (80%).

The accelerated electron beam guided to the light generation region interacts with an undulator (periodic magnetic field), and the kinetic energy of the electron beam is transformed to syncrotron radiation (SR). Wavelength of the generated SR is related to the strength of the magnetic field. The undulator consist of many pairs of permanent magnets (SmCo) as shown in Fig. 1, resulting a modulated magnetic field with 43 cycles to be applied to the electron beam. The intensity of the magnetic field can be changed by varying the separation between pairs of magnets. The SR generated by the interction between the electron beam and the undulator is accumulated between the cavity mirrors located outside of the undulator. The accumulated SR is cohherently amplified by interactions with sequentially injected electron beams, and the amplified/coherent SR between the cavity mirrors is extracted as a laser beam through the coupling hole at the center of one of the mirror.

1-2. Characteristics of MIR-FEL Output

The MIR-FEL is a pulsed laser with the repetition rate of 5 Hz. The output consists of many micro-pulses (1-2 ps width) with temporal separation of ca. 350 ps in a 2 µs macro-pulse, as shown in Fig. 4.

Fig.4: [Output Pulse of MIR-FEL]

The tunable wavelength region of our MIR-FEL is 5-14 µm, and the maximum macro pulse energy is about 50 mJ (measured at the FEL apparatus room). Energy of the FEL beam at the exit port of each laboratory becomes down to ca. 70 % of it, since the FEL beam is introduced to laboratories using several reflection mirrors. The output energy and the oscillation wavelength of FEL varies depending on energy of the incident electron beam. Fig. 5 shows FEL output intensities at different energies of the incident electron beam. The output wavelength region is shifted to the higher energy (shorter wavelength) side at the higher energy of the incident electron beam, while it is shifted to lower at the lower energy of the electron beam. Fig. 6 shows an example of a spectrum of an output of MIR-FEL which is actually supplied to laboratries. Although the spectral resolution of the MIR-FEL output is ca. 1:75 (&Delta&lambda/&lambda = 0.1 µm/7.5 µm) in the figure, the resolution can reach up to 1:100-150 under optimized conditions.

Fig.5: [Wavelength Dependence of FEL output]

(Output energy measured at the apparatus room)
Fig.6: [Spectrum of MIR-FEL output]
Table2: [Summary of MIR-FEL Output]
Tunable Range5-14 µm
Band Width≈1.0 %
Micro-pulse Width2 ps
Micro-pulse Energy8-25 µJ
Micro-pulse Interval350 ps
Macro-pulse Width2 µs
Macro-pulse Energy‹50 mJ
Rep. Rate (Macro-pulse)5 Hz

2. FIR-FEL

Because light of the far-infrared(FIR)/terahelz(THz) region corresponds to molecular rotation energy and it also has penetrationability into materials, its applications are expected in a variaty of technology fields, for examples, material- and bio-science/technology, and imaging, etc. We have then gotten off the mark for development of an far-infrared IR-FEL apparatus based a RF electron accelerator, taking advantage of the experience of the MIR-FEL development and operation. This project is promoted as a part of the Grant-in-Aid for the Creative Scientific Researches of Japan Society of Promotion of Science (JSPS), under the title "Development of High performance Infrared Free Electron Laser and its application in Photo-science". An RF electron gun, an alpha magnet, and an acceleration tube are similar to those used for our MIR-FEL. A new light cavity (incluiding undulator, cavity chamber, cavity mirrors) has been built. The apparatus is now modified for stable oscillation of FIR light.


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