What is THz-CUR

The Coherent Undulator Radiation (CUR) can be generated when an electron beam having shorter pulse length than the wavelength of the undulator radiation.

In this case, radiations emitted from each electron in the electron beam can be coherently summed up.

The intensity of CUR is in proportional to square of number of electron in the electron pulse. (100 pC = ~6 x 10⁸ electron)

Then intense quasi-monochromatic radiation can be generated.

At KU-FEL, a photocathode RF gun with a bunch compression chicane is used to generate short-bunch electron beams with the kinetic energy of 4.6 MeV.

The short bunch electron beam is injected to a 70-cm undulator whose number of periods and period length are 10 and 7 cm respectively.

The maximum undulator K-value is 2.8 and the longest wavelength of the undulator radiation is 1.875 mm (160 GHz).

Because the electron accelerator can provide the electron pulse length shorter than 1 mm, CUR in THz region can be generated.

The generated THz radiation is CEP (carrier envelope phase) stable 10-cycle pulse.

There is another user facility (TELBE) utilize THz-CUR for user experiments in Germany.

The THz-CUR is considered to be used for generating THz radiation in X-ray Free Electron Lasers by using spent electron beam.

Performance of THz-CUR

The detailed properties of THz-CUR at KU-FEL facility has been reported in previous papers (1, 2, 3, 4).

The frequency of THz-CUR can be tuned from 0.16 to 0.6 THz by changing the magnetic field strength of the undulator.

The micro-pulse energy of THz-CUR was measured by a calibrated THz pyroelectric detector, THz10.

The highest peak power available is ~30 kW @0.33 THz.

The spectrum has 10% bandwidth and the spectral distribution is almost same with the ideal 10-cycle radiation.

When the charge contained in one electron pulse gets higher, the electron pulse length become longer due to space charge effect, i.e. repulsive force between electrons.

Saturation phenomena of THz-CUR in the high charge region has been observed and this phenomena limits the maximum micro-pulse energy of THz-CUR.

User Station

THz-CUR at KU-FEL facility has currently no user station since transportation of the very long wavelength radiation is difficult.

User experiments should be performed on an optical bread board very close to the undulator.

On the bread board, a house-made interferometer is available for measuring the THz spectrum.

On going development

Under the collaboration with Dr. Shigeru Kashiwagi (Tohoku University), a method to control the polarization state of THz-CUR has been developed.

Right/Left-handed polarization can be realized by using a wire-grid based optical setup.

This enables us easy, quick and high-throughput control of the polarization state of the THz-CUR. (-> check the previous report)

An application experiment of easily switchable circularly polarized intense quasi-monochromatic THz radiation will be conducted near future.

Under the collaboration with Dr. Kazuyuki Sakaue (Tokyo University), the RF gun of the THz-CUR has been replaced with a RF gun with an energy chirping cell (ECC-RF gun) for short electron pulse generation since 2020.

ECC-RF gun will enables us to generate short pulse length electron beams with higher bunch charges.

This can contribute to increase the peak intensity of THz-CUR.

THz-CUR has already been generated by electron beam provided by ECC-RF gun.

Further study will be conducted to investigate the maximum peak intensity of THz-CUR driven by ECC-RF gun.