Schematic diagram and photograph of MIR-FEL

Electron Linac and FEL generation

At KU-FEL facility, an electron linac is used to generate MIR-FEL.

Electron beam with the macro-pulse duration of 7 micro-second and the kinetic energy of 8.3 MeV is generated by a 4.5-cell thermionic RF gun.

The electron beam is filtered by a dog-leg section composed of two dipole magnets, three quadrupole magnets and one slit.

Then the electron beam is further accelerated by a traveling-wave type accelerator tube. The maximum kinetic energy of electron beam is 40 MeV.

Those acceleration structures (RF gun and accelerator tube) are powered by independent micro-wave amplifiers (Klystron) whose micro-wave frequencies are 2856 MHz.

The longitudinal and transverse shapes of electron beam are manipulated by dipole and quadrupole magnets after the accelerator tube.

The electron beam is finally injected to a 1.8-m hybrid undulator.

In the undulator, at first, spontaneous radiations are generated from the electron beam.

The spontaneous radiations are stored in an optical cavity and amplified by the successively injected electron beams.

The amplification is continuously repeated for 100 to 200 times. Then the intensity of stored light in the cavity get more than 1 million times stronger than the spontaneous radiation. And then the laser oscillation and power saturation can be achieved.

Normally, we use thermionic electron emission from a LaB6 thermal cathode in the RF gun for user operations.

The cathode also can be used as a photocathode when the cathode temperature is adjusted below thermionic emission threshold and UV laser pulses are injected from outside.

The maximum electron charges in one bunch for thermionic cathode and photocathode operation are 55 and 190 pC, respectively.

The micro-pulse repetition rate for thermionic cathode and photocathode operation are 2856 and 29.75 MHz, respectively.

The thermionic cathode operation can provide higher macro-pulse energy (= average power) than the photocathode operation.

Contrary, the photocathode operation can provide higher micro-pulse energy (= peak power) than the thermionic cathode operation.

The photocathode operation is also available for user experiment if user need higher peak power and less heat effect for their experiment.


MIR-FEL at KU-FEL covers the wavelength from 3.4 to 26 micro-meter. The highest macro-pulse energy available at the user station #1 is 40 mJ at  9 micro-meter.

The MIR-FEL can provide the highest macro-pulse energy and the widest tunable wavelength range in Japan.

The FEL bandwidth can be tuned by slightly changing the optical cavity length.

The highest FEL power can be obtained when the FEL bandwidth is widest (~4% in FWHM).

The FEL bandwidth can be reduced to ~1% in FWHM with reduced FEL power (~1/3 of the highest power condition).

We achieved the highest conversion efficiency (5.5%) of FEL oscillator driven by a normal conducting accelerator. (-> please check this open access paper)

We achieved the world highest conversion efficiency (9.4%) of FEL oscillator under the photocathode operation. (-> please check this open access paper)

Macro-pulse Energy V.S. Wavelength

Typical spectrum

 User Stations

MIR-FEL at KU-FEL has three user stations. 

For more detail of user stations, please see the user stations page.