Angular distribution of nonlinear harmonic generation in helical undulators: A comparison between experiments and theory

Abstract

Using harmonic emission from circularly polarized undulator is a procedure that is normally employed on synchrotron beamlines in order to extend the covered spectral range. A similar capability is likewise beneficial for next generation free-electron lasers. In this paper, we perform a first quantitative experimental analysis of the angular distribution of free-electron laser harmonic emission from helical undulators. Experimental results are compared to those obtained by means of a theoretical model based on the paraxial solution of Maxwell's equations.PACS numbers:

Theoretical Analysis of the Radiation Properties of Some Major X‐Ray Free Electron Lasers

“…The angular distribution of the radiation from elliptic undulators was earlier studied in ref. [93] there the comparison between the experiments and theoretical results for the angular distribution of nonlinear harmonic emission, based on the paraxial solution of Maxwell's equations, was performed. The effect of the undulator field harmonics on the UR consists in partial suppression or enhancement of the radiated high harmonics.…”

“…Experiments on nonlinear harmonic generation in a FEL with direct seeding [34] demonstrated greatly improved tem-poral coherence of the radiation and reduced saturation length, even with very low-intensity seed. A coherent harmonic generation (CHG) [35,36] in FELs relies upon the bunching created in an undulator, called the modulator, for generating coherent emission in a subsequent undulator, called the radiator, tuned at one of the harmonics of the modulator. The implementation of CHG requires the use of an external coherent source, acting as a "seed," which can be provided by one of the harmonics of a Ti:sapphire laser.…”

“…Compliant with the FEL theory (see refs. [14][15][16][17][18][19][20]23,[32][33][34][35][36][37][38][39][40][49][50][51]), and based on the numerical simulations in refs. [26,[63][64][65] and on the comparison with a number of FEL experiments at various installations, the following formula was worked out for the description of the induced harmonic power evolution by two terms: [104] Q n (z) ≅ Pn,0 e n z∕L g…”

“…The relevant scheme of the HGHG is due to Yu [39] and it has been broadly discussed in refs. [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] The principle was realized in the UV FEL [114] with the amplification of the third harmonic; other studies followed, for example, ref. [115].…”

Coherent control schemes for the photoionization of neon and helium in the Extreme Ultraviolet spectral region

“…While the relative power between the fundamental and the harmonic field can be adjusted by taking advantage of the different growth rates characterizing the two fields, there is very little margin to control the relative phase between the two. Moreover, the method can only be efficiently applied for the third harmonic and for linearly polarized FEL emission, since in the case of even harmonics or for circularly polarized undulators, the harmonic radiation is emitted off-axis 47 – 51 , and phase coherence is not conserved during beam transport. This is because the off-axis radiation follows a different, and slightly fluctuating, path with respect to the on-axis radiation.…”