X-ray free electron lasers (XFELs) are capable of producing x-ray beams with intense peak brightness, full transverse coherence, and femtosecond-scale pulse duration through Self-Amplified Spontaneous Emission (SASE). However, the SASE FELs suffer from noisy spikes in time and spectrum due to radiation originating from electron beam shot noise. To overcome these limitations and realize bright, fully coherent FEL sources, self-seeding is a promising solution. In this study, we utilized the forward Bragg-diffraction (FBD) monochromator at PAL-XFEL to generate almost fully coherent hard X-ray self-seeded (HXRSS) free-electron laser (FEL) pulses with an unprecedented peak-brightness and a narrow spectrum. Our HXRSS FEL demonstrated outstanding performance across a photon energy range spanning from 3.5keV to 14.6keV. These findings provide valuable insights for the development of advanced X-ray sources and their applications.
In order to meet the demands of experimental applications such as resonant inelastic X-ray scattering, nuclear resonance scattering, and X-ray Raman spectroscopy, we have developed x-ray energy scanning method utilizing a double crystal monochromator (DCM). This approach offers improved spectral purity and a fully calibrated energy scale. In this study, we will present recent experimental findings on the characteristics of hard X-ray self-seeded FEL at PAL-XFEL. These results have important implications for the advancement of X-ray spectroscopy and related research fields.
Femtosecond MeV electron beam generated by laser-plasma accelerators (LPA) is a promising source for ultrafast electron diffraction (UED) application. Compared to conventional UED instruments which limit temporal resolution to a few tens of fs, plasma electron accelerator-based UED is possible to make sub-10 fs temporal resolution because of no intrinsic time jitter between pump-probe pulses and ultrashort electron bunch length. Some groups have shown that a few MeV electron beam can be produced by using a few mJ laser pulse as it has shorter pulse duration (single- or few-cycle). In this regime, the laser pulse is tightly focused onto gas target, and thus electrons in relatively high density plasma (1020 cm-3) are self-injected and accelerated. However, the electron beam quality like energy spread and emittance should be still improved for applications. Here, we introduce plan of two laser pulses-based plasma electron acceleration research for UED application at Pohang Accelerator Laboratory (PAL). A laser pulse is separated to two pulses that one is used to drive plasma wakefield and the other one is delivered to induce electron injection in a plasma bubble. Since the driving pulse intensity is retained under threshold of self-injection to suppress electron injection, the electron injection occurs in a localized region the injection pulse is focused, resulting in the high quality electron generation. In addition, researches conducting for better electron beam quality are presented in this presentation.
Comparing a conventional undulator, the concept of plasma undulator is hard to realize because of a small-scaled cavity structure and a longitudinal acceleration field in the plasma cavity. So, the electron beam with betatron motion should stay in the longitudinal center of the plasma cavity. An other issue is the plasma cavity should propagate keeping a stable cavity formation and speed for entire plasma target. Moreover, high photon energy like UV and soft x-ray is much challenge since it requires longer plasma length and higher speed plasma cavity. To satisfy such conditions we investigate the electron beam-driven plasma cavity using PIC simulations. The life-time of the plasma cavity depends on how long the driving electron beam survives. We test several plasma profile, electron beam, and plasma lens parameters by testing matching conditions of the electron beam. In the presentation we also introduce new numerical technique in PIC to eliminate the numerical Cherenkov radiation causing unwanted increases of emittance. Finally we discuss a possibility of FEL from electron’s betraon motions.
Laser plasma wakefield accelerations (LWFA) are the most promising candidates for future compact accelerators and also can be used for next-generation free-electron lasers (FELs). However, due to the insufficient electron beam quality, such as a few percent of energy spread, stability, and reproducibility, the electron beam from the LWFA has difficulty to be directly used for FELs with a range from soft X-ray to hard X-ray. To overcome this limitation of the beam quality from the laser wakefield acceleration using various injection techniques, one of the most reliable way is to use the electron beam with short duration, lower energy spread, and emittance from the RF photocathode.
This external injection technique is planning with conventional S-band RF photocathode gun and final energy of 70 MeV, few tens fs duration, and lower emittance at Pohang Accelerator Laboratory Injector Test Facility (PAL-ITF). In this presentation, we show a simulation result on the characterization of the electron beam from LWFA using external injection for soft X-ray free-electron lasers.
Conference Committee Involvement (1)
Coherent and Incoherent Radiation Sources based on Relativistic Plasma Waves V
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