Terahertz (THz) pulses with a peak power of ϳ2 kW were generated in a noncollinear phase-matched GaAs crystal at room temperature. Two 200 ns pulses from a dual-beam TEA CO 2 laser were used for difference frequency mixing in the crystal. A comb of narrow lines ͑⌬ / ϳ 10 −4 ͒ was obtained in the 0.5-3 THz range with a step of 40 GHz. By comparing the effective nonlinearity of GaSe with that of GaAs for THz generation, the electro-optic nonlinear coefficient for GaSe was measured to be d eo = 24.3± 10% pm/ V. Using simulations we show that a 1 kW THz pulse could be amplified by a factor of 2 ϫ 10 4 to a 10 MW level in a 2 m long singlepass free-electron laser.

Energy gain of trapped electrons in excess of 20 MeV has been demonstrated in an inverse-free-electron-laser (IFEL) accelerator experiment. A 14.5 MeV electron beam is copropagated with a 400 GW CO2 laser beam in a 50 cm long undulator strongly tapered in period and field amplitude. The Rayleigh range of the laser, approximately 1.8 cm, is much shorter than the undulator length yielding a diffraction-dominated interaction. Experimental results on the dependence of the acceleration on injection energy, laser focus position, and laser power are discussed. Simulations, in good agreement with the experimental data, show that most of the energy gain occurs in the first half of the undulator at a gradient of 70 MeV/m and that the structure in the measured energy spectrum arises because of higher harmonic IFEL interaction in the second half of the undulator.

A comprehensive analysis is presented that describes amplification of a seed THz pulse in a single-pass free-electron laser (FEL) driven by a photoinjector. The dynamics of the radiation pulse and the modulated electron beam are modeled using the time-dependent FEL code, GENESIS 1.3. A 10-ps (FWHM) electron beam with a peak current of 50-100 A allows amplification of a 1 kW seed pulse in the frequency range 0.5-3 THz up to 10-100 MW power in a relatively compact 2-m long planar undulator. The electron beam driving the FEL is strongly modulated, with some inhomogeneity due to the slippage effect. It is shown that THz microbunching of the electron beam is homogeneous over the entire electron pulse when saturated FEL amplification is utilized at the very entrance of an undulator. This requires seeding of a 30cm long undulator buncher with a 1-3 MW of pump power with radiation at the resonant frequency. A narrow-band seed pulse in the THz range needed for these experiments can be generated by frequency mixing of CO 2 laser lines in a GaAs nonlinear crystal. Two schemes for producing MW power pulses in seeded FELs are considered in some detail for the beam parameters achievable at the Neptune Laboratory at UCLA: the first uses a waveguide to transport radiation in the 0.5-3 THz range through a 2-m long FEL amplifier and the second employs high-gain third harmonic generation using the FEL process at 3-9 THz.

Terahertz pulses with a peak power of ϳ2 MW were generated in a noncollinear phase-matched GaAs crystal at room temperature. Efficient difference-frequency generation was achieved using 250-ps CO 2 laser pulses for which the surface damage threshold increased in comparison with standard 200-ns pulses. Tunable CO 2 lasers in combination with this technique should yield a source in the 0.1-3.0-THz range.

We have shown that a seventh-order inverse-free-electron laser (IFEL) interaction, where the radiation frequency is the seventh harmonic of the fundamental resonant frequency, can microbunch a beam of relativistic electrons inside an undulator. Using coherent transition radiation (CTR) emitted by the bunched 12.3 MeV beam as a diagnostic, strong microbunching of the beam is inferred from the observation of CTR at the first, second, and third harmonics of the seed 10 m radiation. Threedimensional IFEL simulations show that the observed harmonic ratios can be explained only if transverse spatial distribution of the steepened bunched beam is taken into account.There is a growing interest in developing new light sources in the x-ray range with improved spatial and spectral coherence properties compared to the present-day synchrotron radiation sources. Many proposals and several ongoing national projects exist worldwide to build x-ray or UV free-electron lasers (FELs) in which a high-brightness, multi-GeV electron beam has a resonant interaction with radiation in the presence of a periodic magnetic field of an undulator [1]. Because of the practical limit on the size and strength of undulator magnets, which all have periods in the cm range, the electron beam energy on the GeV scale represents one of the main constraints on the shortest reachable wavelength. To circumvent this limitation, a high-order FEL interaction has been proposed recently as a way to reduce the required beam energy [2]. In addition to FELs, inverse-free-electron laser (IFEL) acceleration [3], longitudinal current modulation of an electron beam (microbunching) [4], and creation of very short electron bursts (femtoslicing) [5] are other applications, which can potentially benefit from the high-order resonant beamradiation interaction in an undulator.High-order FEL and IFEL interactions can be understood by looking closely at the resonant condition on the axis of a planar undulator:where K u ¼ eB u =mck u is the dimensionless undulator parameter, r is the radiation wavelength, is the electron Lorentz factor, u the undulator wavelength, n the harmonic number, K u the undulator wave number, and B u the undulator magnetic field [6]. A typical IFEL/FEL operates at the fundamental frequency [n ¼ 1 in Eq. (1)] providing the highest possible efficiency of the interaction. However, since the transverse velocity component of the electron motion in the undulator becomes relativistic at K u ! 1, resonance can also occur when the seed radiation frequency is a multiple of the fundamental resonant frequency [6]. In a collinear IFEL interaction in a planar undulator, the electron beam then may exchange energy with a seed radiation of wavelength s satisfying the condition r ¼ n s , where n ¼ 3; 5; 7 . . . (the high-order interactions). Unlike an atomic resonance, the high-order IFEL resonances for n ¼ 7-11 according to simulations [7] can be very efficient reaching an interaction strength of 50% of that at the n ¼ 1 case. If this efficiency is proved experimentally,...