High repetition rate seeded free electron laser with an optical klystron in high-gain harmonic generation

Many high-gain free electron lasers worldwide are planning to incorporate seeding setups into their day-to-day operation. These techniques provide both longitudinal and transverse coherence and extended control of the output free electron laser radiation spectral properties. However, the output wavelength and repetition rate strongly depend on the properties of the seed laser system. With the laser peak power required for successful seeded operation, it is currently not possible to increase their repetition rate to an extent that it matches the electron bunch repetition rate of superconducting accelerators. Here, we investigate the advantages of a modification of standard seeding setups, by combining the seeding with the so-called optical klystron. With this new seeding setup, it is possible to decrease the seed laser power requirements and therefore, seed laser systems can increase their repetition rate at the same wavelength. We show simulation results in a high-gain harmonic generation (HGHG) setup for a range of harmonics (8th to 15th) and we verify the reduction of seed laser power required with an Optical Klystron HGHG scheme. Finally, we comment on the stability of the proposed setup to jitter sources and to shot-to-shot fluctuations and compare to the standard HGHG scheme.

Figure 1

In a standard HGHG setup, we need one modulator followed by a chicane, before the final FEL amplification at the amplifier. The amplifier is tuned to a harmonic of the seed laser wavelength.

Figure 2

In an OK-HGHG, we need two pairs of a modulator followed by a chicane. The two modulators are set to the same resonance, chicane 1 is responsible for increasing the bunching at the fundamental wavelength, while chicane 2 is responsible for longitudinal density modulation at a harmonic of the seed laser wavelength. The final FEL amplification of a harmonic of the seed laser wavelength is taking place at the amplifier.

Figure 3

(a) Longitudinal phase space after modulator 2 in an OK-HGHG setup (see Fig. 2). With an $R_{56,2}=51\mu \mathrm{m}$, we get $b_{10}=8\%$. (b) Longitudinal phase space after modulator in a standard HGHG setup (see Fig. 1). With an $R_{56}=67.25\mu \mathrm{m}$, we get $b_{10}=8\%$.

Figure 4

(a) Required energy spread ratio between the two schemes (${\sigma }_{\mathrm{E},\mathrm{HGHG}}^{\prime }/{\sigma }_{\mathrm{E},\mathrm{OK}\text{−}\mathrm{HGHG}}^{\prime }$) to obtain 8% bunching at different harmonics. In addition to the standard parameters (Table 1) also higher peak current and longer undulators are studied. The energy spread (${\sigma }_{\mathrm{E}}^{\prime }$) is measured upstream from the amplifier. (b) Ratio between the minimum seed laser power required for both schemes ($P_{\mathrm{seed},\mathrm{HGHG}}/P_{\text{seed},\mathrm{OK}\text{−}\mathrm{HGHG}}$) to obtain the same bunching for the cases studied in (a).

Figure 5

(a) Bunching amplitude response to seed laser power fluctuations for a standard HGHG and an OK-HGHG setup. The nominal seed laser power is 27.7 MW and 54.5 kW for the standard and the OK-HGHG, respectively. (b) Bunching amplitude response to electron beam compression factor fluctuations for a standard HGHG and an OK-HGHG setup. The nominal energy spread and peak current are shown in Table 1. They change proportionally when we vary the compression factor.

Figure 6

We compare two fully optimized simulations of a standard HGHG and an OK-HGHG setup for the 15th harmonic of a 300 nm seed laser wavelength. (a) Gain curves in amplifier. (b) Output FEL power profiles. (c) Output FEL spectra normalized to the intensity of the standard HGHG simulation.

Figure 7

SASE background in temporal domain for the five initial shot noise cases studied in this section.

Figure 8

Shot-to-shot fluctuations for a standard HGHG setup, imprinted on the output FEL at 20 nm. (a) Power profile for different initial shot noise. (b) Spectra for different initial shot noise. They are normalized to the highest spectral intensity with Seed 2.

Figure 9

Shot-to-shot fluctuations for an OK-HGHG setup, imprinted on the output FEL at 20 nm. (a) Power profile for different initial shot noise. (b) Spectra for different initial shot noise. They are normalized to the highest spectral intensity with Seed 3.

Figure 10

Shot-to-shot fluctuations of pulse energy for the standard HGHG and the OK-HGHG setup.