[en] The errors of undulator magnet field integrals are analyzed. The formulas of first and second field integral error are given as they are induced by field error σ_B and by positioning error σ_z , respectively. The requirement for the measurement points per period also is analyzed

[en] We propose a simple method to generate a few femtosecond pulses in seeded FELs. We use a longitudinal energy-chirped electron beam passing through a dogleg where transverse dispersion will generate a horizontal energy chirp, then in the modulator, a seed laser with narrow beam radius will only modulate the center portion of the electron beam and then short pulses at high harmonics will be generated in the radiator. Using a representative realistic set of parameters, we show that 30 nm XUV pulse based on the HGHG scheme and 9 nm soft x-ray pulse based on the EEHG scheme with duration of about 8 fs (FWHM) and peak power of GW level can be generated from a 180 nm UV seed laser with beam waist of 75 μm. This new scheme can provide an optional operation mode for the existing seeded FEL facilities to meet the requirement of short-pulse FEL.

[en] An easy-to-implement scheme is proposed to produce the longitudinal electron bunch density modulation with phase-merging phenomenon. In this scheme an electron bunch is firstly transversely dispersed in a modified dogleg to generate the exact dependence of electron energy on the transverse position, then it is modulated in a normal modulator. After travelling through a modified chicane with specially designed transfer matrix elements, the density modulation with phase-merging effect is generated which contains high harmonic components of the seed laser. We present theoretical analysis and numerical simulations for seeded soft x-ray free-electron laser. The results demonstrate that this technique can significantly enhance the frequency up-conversion efficiency and allow a seeded FEL operating at very high harmonics.

[en] Short communications only

[en] A photocathode rf gun can generate trains of THz subpicosecond electron bunches by illuminating the cathode with trains of laser pulses, but it suffers from the increasing charge in the beam. The THz structure blurs and tends to disappear when the longitudinal space charge forces begin to play a significant role in the beam evolution. In this paper, we propose a scheme to restrain the space charge forces by expanding the transverse size of the laser pulses to reduce the charge density and adopting a multicell gun to increase the beam energy. Thus, quasiequally spaced ultrashort microbunches with relatively high charges can be generated according to our studies. Postacceleration can be used to freeze the longitudinal phase space dynamics. The proposed scheme is in principle able to generate intense multi-color narrow-band THz radiation and offers a promising way towards the tunable intense narrow-band THz sources

[en] The effects of undulator phase errors on the Free Electron Laser small signal gain is analyzed and discussed. The gain reduction factor due to the phase error is given analytically for low-gain regimes, it shows that degradation of the gain is similar to that of the spontaneous radiation, has a simple exponential relation with square of the rms phase error, and the linear variation part of phase error induces the position shift of maximum gain. The result also shows that the Madey's theorem still hold in the presence of phase error. The gain reduction factor due to the phase error for high-gain regimes also can be given in a simple way

[en] The 3D magnetic field is calculated and analyzed using normalized dimensions for PPM undulator. The 3D modify and the effect on the good field region of the magnet pole width are discussed for general case. The modify curve of the magnet pole width to the usually 2D approximation and the field transverse variation curve for various magnet pole widths are given

[en] The design of the first undulator UD-1 in NSRL is introduced. The design parameters and main technical requirement are given. The quality requirement on undulator magnet field are analysed considering demands of light users as well as that of the electron beam stability. It is given that the first and second integral of the magnet field should be less than 2 x 10^-4 T·m and 2 x 10^-4 T·m² respectively, the phase error should be less than 10 degree

[en] The design and computation of a undulator transformed from optical klystron are presented. The brilliance of HLS (NSRL) can be enhanced two orders by the undulator. Lots of experiments which can not be done at the present brilliance will be made possible

[en] A three-dimensional gain expression is presented and analysed for a Gaussian optical beam in small signal case. An asymptotic gain formula is given for a broader electron beam in cross section compared to the optical beam. When electron beam transverse dimension is smaller than the beam waist of the Gaussian mode, an analytic approximate formula is given and the results are in good agreement with numerical simulations