Mitigation of Beam Instabilities in the Echo-Enabled Harmonic Generation Beamline for FLASH2020+

With the FLASH2020+ upgrade, one of the beamlines of the free-electron laser FLASH at DESY will be based on the Echo-Enabled Harmonic Generation (EEHG) seeding scheme and provide high-repetition-rate, coherent radiation down to 4 nm. To reach this wavelength, it is necessary to imprint intricate structures on the longitudinal phase space of the electron bunch at a very high harmonic of the seed laser wavelength, making the scheme potentially vulnerable to beam instabilities. Part of the beamline is a strong chicane, which is necessary to create the dispersion required by EEHG. Resulting effects such as Coherent Synchrotron Radiation (CSR) can be very detrimental for the bunching process and have to be taken into account already in the design of the beamline to ensure optimum FEL performance. We investigate and propose possible mitigation solutions to such instabilities in the FLASH2020+ parameter range. INTRODUCTION In the course of the FLASH2020+ upgrade [1] of the superconducting free-electron laser (FEL) user facility FLASH [2–4] in Hamburg, Germany, it is foreseen that one of the beamlines will be based on the Echo-Enabled Harmonic Generation (EEHG) seeding scheme [5]. EEHG provides a defined electron beam density distribution, which makes the startup process in the FEL not dependent on the stochastic nature of the shot noise and thus allows for shot-toshot reproducibility of fourier limited pulses [6,7]. However, beam instabilities arising within the EEHG section could be a limiting factor to achieve the necessary bunching [8]. Intrabeam Scattering (IBS) and Incoherent Synchrotron Radiation (ISR) have been studied based on analytical formulas [9] for the FLASH2020+ parameter space and result in no significant reduction of the bunching. Coherent Synchrotron Radiation (CSR) describes the phenomenon that electrons traveling through a dipole magnet can emit coherent radiation at wavelengths comparable to the bunch length. Due to the curved trajectory, the radiation can take a shortcut, which leads to a tail-head interaction. This finally results in an energy modulation along the bunch [10]. In the following, the effect of this energy modulation on the bunching is studied with the general-purpose accelerator simulation code ELEGANT [11] for EEHG at 4 nm. Two mitigation solutions, taking into account the duration of the seed laser and different EEHG working points, are examined. ∗ fabian.pannek@desy.de THEORY In the EEHG seeding scheme, the longitudinal phase space distribution of the electron bunch is manipulated in a beamline which consists of two undulators, so-called modulators, and two chicanes, as shown in Fig. 1. M 1 Q C 1 Q M 2 Q C 2 Q 3.3 m 6.6 m 3.3 m 3.3 m Figure 1: EEHG beamline used in the simulations with Modulators (M), Quadrupoles (Q) and Chicanes (C). In each modulator the electron bunch is modulated in energy by interacting with a seed laser, decribed by the energy modulation amplitudes A1,2 = ΔE1,2/σE, that is the energy modulation ΔE produced by the seed laser expressed as a multiple of the beam energy spread σE. The first chicane is used to create multiple energy bands in the longitudinal phase space and therefore requires a large dispersion R(1) 56 . The second dispersive section, described by R(2) 56 , compresses the energy modulated bands, creating a density modulation at the wavelength λE = λ1/aE. For two seed lasers operating at the same wavelength λ1 = λ2, the harmonic number is given by aE = n + m, where n and m are non-zero integers of opposite signs. The degree of bunching is decribed by the bunching factor |bn,m|. Its maximum value is approached for A1 ⪆ 3 and n = −1 and it scales approximately as |b−1,m| ≈ 0.39 ⋅ m−1/3 for m > 4 [12]. For optimized bunching at a specific harmonic, the ratio of the dispersions has to be close to R(1) 56 /R (2) 56 ≈ aE/|n|. Since the required strength of the second chicane is inversely proportinal to the energy modulation imposed in the second modulator, R(2) 56 ∝ 1/A2, a large A2 decreases the required dispersion of both chicanes. This is, however, accompanied by an increase in energy spread, resulting in a decreased FEL performance. CSR STUDY For this study, a σz = 100 μm Gaussian electron beam with an energy of E = 1.35 GeV, an energy spread of σE = 150 keV, a peak current of Ip = 500 A and a normalized emittance of εn = 0.6 mm mrad is used. Both seed lasers are set to λ1,2 = 300 nm. The chicane and modulator parameters used for the simulations are shown in Table 1. Each modulator and chicane is followed by quadrupoles to 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-FRXA06 FRXA06 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I 4514 MC2: Photon Sources and Electron Accelerators A04 Circular Accelerators ensure proper matching. The energy modulation amplitudes are set to A1 = 3 and A2 = 5. The corresponding bunching factor |b| can be calculated analytically and is shown in Fig. 2 for different chicane configurations. Since maximum bunching can be achieved for n = −1, simulations with the FEL code GENESIS1.3, v4 [13,14] have been carried out for this case to optimize the dispersive strengths for power gain and spectral properties in the radiator beamline. For the upper bunching peak in Fig. 2 optimum values were found to be R(1) 56 = 7.05 mm and R (2) 56 = 81.25 μm. Table 1: Simulation Beamline Parameters Chicanes 1 2 Modulators length (m) 6.124 2.824 λu (mm) 82.6 Ldipole (m) 0.42 0.31 Periods 30 Ldrift (m) 2.00 0.57 K 9.97 75 80 85 90 95 100 105 110 115 R (2) 56 (μm) 2 3 4 5 6 7 8 9 10 R (1 ) 56 (m m ) n = −1