Design study for the cascaded HGHG experiment based on the SDUV-FEL

Abstract

Cascading stages of high-gain harmonic generation (HGHG) free electron laser (FEL) is a promising way to produce fully coherent X-ray radiation. As a test facility for modern FEL R&D, the Shanghai deep ultraviolet FEL (SDUV-FEL) is now under upgrading for the cascading two stages of HGHG experiment. Since the energy of the electron beam is as low as about 185 MeV after upgrade, the total harmonic number of this two stages HGHG is only 2×2, and the wavelength of the final radiation is 196.5 nm which is the 4th harmonic of the 786 nm seed laser. With help of three-dimensional simulation codes, design studies on the FEL physics for the cascaded HGHG experiment are present based on the parameters of the upgraded SDUV-FEL facility. It is found from the simulation results that the part of the electron beam which has been used in the first stage can still generate powerful radiation in the radiator of the second stage, and this radiation will be difficult to be separated from the radiation generated by the fresh part of the electron beam. To overcome this problem, a novel method based on the energy spectrum of the electron beam is proposed in this paper to demonstrate the "fresh bunch" technique. Free electron lasers (FELs) hold great promise to produce coherent short wavelength radiation with high brightness and ultra-fast time structures which will enable scientists in physics, chemistry, biology and medicine to study nature down to the molecular and atomic level at a time-scale that fits this resolution. Because of its unique performance, FEL is complement to the 3rd generation light source like the Shanghai Synchrotron Radiation Facility (SSRF) [1]. Selfamplified spontaneous emission (SASE) [2,3] and seeded harmonic generation schemes [4][5][6][7] are two leading candidates for approaching deep ultraviolet (DUV) to X-ray region. The SASE process produces short wavelength radiation with high peak power and an excellent spatial mode. As the SASE FEL starts from electron beam shot noise, the output radiation typically has poor temporal coherence and large power fluctuations. An alternative way for generation of fully coherent short wavelength radiation is using the high-gain harmonic generation (HGHG) scheme [4,5]. HGHG consists of two undulators separated by a chicane. The electron beam is first energy modulated by a seed laser in the first short undulator (modulator) and then sent through a chicane (dispersion section) which converts the energy modulation into a density modulation. The density modulated beam is then sent through the second long undulator (radiator) to generate powerful radiation at the high harmonic of the seed frequency. The output property of the HGHG is a direct map of the seed laser's attributes which can have a high degree of temporal coherence and much smaller energy fluctuations than SASE. These theoretical predictions have been demonstrated in the HGHG experiments [8][9][10]. However, significant bunching at higher harmonics would degrade the quality of the electron beam

Coherent photon beam based diagnostics for a seeded extreme ultraviolet free-electron laser

“…The SDUV-FEL is a multipurpose test facility for novel FEL principle studies, where echo-enabled harmonic generation FEL [26], crossed-planar undulators [46] and two-stage HGHG [47][48][49] have been successfully demonstrated. During the experiment of coherent photon beam diagnostics, the beam energy of SDUV-FEL was set to be 148 MeV.…”

Demonstration of a widely-tunable and fully-coherent high-gain harmonic-generation free-electron laser

“…Here we show the first results from a two-stage cascaded HGHG experiment, which is conducted at the SDUV-FEL with a new stage added [45], as shown in Fig. 5.…”

Two-beam based two-stage EEHG-FEL for coherent hard X-ray generation

“…In order to improve the frequency multiplication efficiency, several techniques with more complicated setup have been developed, mainly including the cascaded HGHG [22] and the echo-enabled harmonic generation (EEHG) [23,24]. In cascaded HGHG, the FEL generated by intermediate radiator can be used as the seed laser for the following stage with the help of the ''fresh bunch'' technique, which has been demonstrated with two stages setup both at Shanghai deep ultraviolet FEL test facility (SDUV-FEL) [25,26] and Trieste FERMI [27] recently. However, in order to further extend the output wavelength to sub-nanometer region, one may need to cascade three or even more stages of HGHG.…”