DESIGN STUDY FOR THE PEHG EXPERIMENT AT SDUV-FEL*

In this paper, design studies for the proof-of-principle experiment of the recently proposed phase-merging enhanced harmonic generation (PEHG) mechanism are presented. A dogleg and a new designed transverse gradient undulator should be added in the undulator system of SDUV-FEL to perform the phase-merging effect. With the help of 3D simulation codes, we show the possible performance of PEHG with the realistic parameters of SDUV-FEL. INTRODUCTION High-gain seeded FEL schemes have been developed for producing stable and fully temporal coherence laser pulse from deep UV down to the x-ray regime. The most famous frequency up-conversion scheme is so called the high-gain harmonic generation (HGHG) [1], which uses an external laser pulse to interact with the electron bunch for the generation of coherent micro-bunching. The property of HGHG output is a direct map of the seed laser’s attributes, which ensures high degree of temporal coherence and small pulse energy fluctuations with respect to self-amplified spontaneous emission (SASE) [2]. However, significant bunching at higher harmonics usually needs to strengthen the energy modulation in HGHG, which will result in a degradation of the amplification process of FEL. Thus the requirement of FEL amplification on the beam energy spread prevents the possibility of reaching short wavelength in a single stage HGHG. Recently, a novel seeded FEL scheme termed phasemerging enhanced harmonic generation (PEHG) [3, 4], has been proposed for significantly improving the frequency up-conversion efficiency of harmonic generation FELs. Generally, a transversely dispersed electron beam and a transverse gradient undulator (TGU) [5] are needed in PEHG for performing the phasemerging effect purpose: when the transversely dispersed electrons passage through the TGU, around the zerocrossing of the energy modulation, electrons with the same energy will merge into a same longitudinal phase. Several ways have been proposed [3, 4, 6] for performing the phase-merging effect as shown in Fig. 1, where doglegs are added before modulators for transversely dispersing the electron bunch. Fig. 1(a) shows the initial proposed PEHG scheme, where a short TGU is used for the energy modulation and to precisely manipulate the electrons in the horizontal dimension. It is found later that these two functions of TGU can be separately performed by employing a modified design, as shown in Fig. 1 (b). In this scheme, a normal modulator is used for the energy modulation, and the TGU is responsible only for transverse manipulation of the electrons, a design that will be much more flexible for practical operation. Fig. 1(c) shows a much simpler scheme that adopts a normal modulator and a wave-front tilted seed laser pulse to realize the phase-merging effect. Analytical and numerical investigations indicate that all these three schemes have the potential of generating ultrahigh harmonic bunching factor with a relatively small energy modulation. To demonstrate these theoretical predictions, a proof-of-principle experiment for PEHG has been planned at Shanghai deep ultraviolet freeelectron laser facility (SDUV-FEL) [7, 8]. In this paper, we present the design studies for this experiment. LAYOUT AND MAIN PARAMETERS The SDUV-FEL is an integrated multi-purpose test facility for seeded FEL principles, capable of testing various seeded FEL working modes. A new beam line will be added after the linac of SDUV-FEL for the PEHG and Thomson scattering experiments in this year. The layout of the PEHG experiment is shown in Fig. 2. A dogleg is adopted for switching the electron bunch and introducing a large transverse dispersion into the electron beam. After that, a conventional modulator and a new designed TGU, as shown in Fig. 3, are employed for energy modulation and manipulating the electron beam. The energy modulation will be converted to density modulation by the dispersion section (DS). Then the coherently bunched beam is sent through the radiator for high harmonic radiation. Figure 1: Modulation schemes for PEHG. ___________________________________________ *Work supported by National Natural Science Foundation of China (11475250, 11175240, 11322550 and 11205234) #denghaixiao@sinap.ac. cn Proceedings of FEL2014, Basel, Switzerland MOP070 Seeded FELs ISBN 978-3-95450-133-5 219 C op yr ig ht © 20 14 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s One can find that the undulator beam line in Fig. 2 is suitable for testing all the three modulation schemes in Fig. 1. Here we only present the design and simulation results for the second scheme (Fig. 1(b)), which is the most flexible scheme for PEHG. To roughly estimate the optimal parameters of the PEHG experiment, here we adopt the 4-dimensional linear beam transport matrix in the x-z plane, i.e., ( , ', , ) x x z is used in the following. The 4 4 beam transport matrix for the dogleg is 1 0 0 1 0 0 0 1 0 0 0 1 D