• G.M. Kazakevich
  Fermilab, Batavia, Illinois
• Y.U. Jeong, B.C. Lee, S.-H. Park
  KAERI, Daejon
• G.I. Kuznetsov
  BINP SB RAS, Novosibirsk
• V. M. Pavlov
  Strathclyde University, Glasgow

Funding: Budker Institute of Nuclear Physics RAS, Academician Lavrentyev 11, Novosibirsk, 630090, Russia; Laboratory for Quantum Optics, Korea Atomic Energy Research Institute, P. O. Box 105, Yusong, Taejon, 305-600, South Korea.

A reliable injection system of the widely tunable microtron-based terahertz Free Electron Laser (FEL) has been developed and during last few years provides stable operation of the FEL for users. The system is based on the long-life thermionic cathode assembly using 2.5 mm-in diameter monocrystalline LaB6 emitter, heated by the tungsten cylindrical filament with the power consumption less than 50 W. The cathode emits the macro-pulse current in the range of 1-1.4 A providing operation of the terahertz FEL during more than 1000 h. The cathode assembly is installed on the cover of the I-type microtron accelerating cavity in location providing an efficient injection for the acceleration with variable number of orbits. This variation widely changes the energy of the electron beam and allows on-the-fly retuning of the FEL in the range of 1-3 THz. Pulse-signal system stabilizing the emission current prevents randomized break-downs in the accelerating cavity and decreases macro-pulse power fluctuations of the FEL radiation. The fluctuations were measured to be less than 10% during long-time operation.

• Y.U. Jeong, H.J. Cha, B.C. Lee, S.-H. Park
  KAERI, Daejon
• G.M. Kazakevich
  Fermilab, Batavia, Illinois

We have developed a laboratory-scale users facility with a compact terahertz (THz) free electron laser (FEL). The FEL operates in the wavelength range of 100-1200 μm, which corresponds to 0.3-3 THz. The peak power of the FEL micropulse having 30 ps pulse duration is 1 kW and the pulse energy of the 3-μs-FEL-macropulse is approximately 0.3 mJ. The main application of the FEL is THz imaging for bio-medical researches. Transmitted THz imaging of various samples including bugs have been measured. The samples were scanned by a 2-dimensional stage at the focal point of the THz beam. The bugs were not dry because they were killed just before experiments. We could get the transmitted THz imaging of the bugs at 3 THz with the high power THz FEL. THz spectral characteristics of several materials have been studied by the FEL and a THz FTIR spectrometer. We will introduce recent results on the imaging and spectroscopy by the THz FEL.

• H.J. Cha, Y.U. Jeong, B.C. Lee, S.-H. Park
  KAERI, Daejon
• S.-H. Park
  Yonsei University, Seoul

Funding: This work was supported by Korea Research Foundation Grant (KRF-2004-042-C00053).

We are making experiments on THz(terahertz) imaging using a compact high power FEL (free-electron laser) which is operating as a users facility at KAERI. The wavelength range of output pulses is 100~1200 μm, which corresponds to 0.3~3 THz in the frequency region. We should select the optimum wavelength for the constituents of specimens to realize the imaging based on the THz FEL. A FTIR (Fourier-transform infrared) spectrometer was modified to measure the optical constants of the specimens in THz region. A polyester film of which thickness is 3.7 μm was used as a beam splitter of the spectrometer. In the case of normal incidence, the transmittance of the film was measured to be more than 90%, and the estimated loss by absorption was approximately 2% at the FEL frequency of 3 THz. Several tens of nanometer-thick-silver was coated on the polyester film to balance both transmission and reflection of THz waves in the beam splitter. We investigated FTIR spectroscopy on air, vapor and liquid water as test samples. As a preliminary step for the compact FEL-based biomedical imaging, FTIR spectroscopic experiments on the fundamental ingredients such as carbohydrates, fats, and proteins in THz region are also planned.

• S.-H. Park, Y. Cha, Y.U. Jeong, B.C. Lee, K. Lee
  KAERI, Daejon
• S.V. Miginsky
  BINP SB RAS, Novosibirsk

The KAERI SC RF linac with one 352 MHz cryomodule is routinely operating at 10 MeV. The maximum accelerating gradient achieved so far is about 7.7 MV/m and is expected to increase up to 9 MV/m, if thermal loss and/or vibration instability is sufficiently suppressed. As a next step, we plan to generate Compton X-rays using external lasers at the straight section, just after the SC linac. This beamline will be relocated to downstream next to undulator beamline for a FEL, when the recirculating beamline is built. In this presentation, we estimate the parameters of Compton X-rays at a given system and suggest the new scheme to increase the flux, or to generate fs X-ray pulses using electron beams with a few tens ps pulse duration, using an intense ultra-short laser. We discussed a coherent condition for Relativistic Nonlinear Thomson Scattered (RNTS) radiation (or Nonlinear Compton Scattered radiation).

• K. Lee, Y. Cha, Y.U. Jeong, B.C. Lee, S.-H. Park
  KAERI, Daejon

In contrast to some recent experimental results, which state that the Nonlinear Thomson Scattered (NTS) radiation is incoherent, a coherent condition under which the scattered radiation of an incident laser pulse by a bunch of electrons can be coherently superposed has been investigated. The Coherent Relativistic Nonlinear Thomson Scattered (C-RNTS) radiation makes it possible utilizing the ultra-short pulse nature of NTS radiation with a bunch of electrons, such as plasma or electron beams. A numerical simulation shows that a 25 attosecond X-ray pulse can be generated by irradiating an ultra-intense laser pulse of 4x10(19) W/cm2 on an ultra-thin solid target of 50 nm thickness, which is commercially available. The coherent condition can be easily extended to an electron beam from accelerators. Different from the solid target, much narrower electron beam is required for the generation of an attosecond pulse. Instead, this condition could be applied for the generation of intense Compton scattered X-rays with a modulated electron beam.