Beam monitoring of relativistic charged particle beams is of great interest for various applications including a high-order harmonic generation-seeded free-electron-laser. In particular, in phenomena with low repeatability/reproducibility such as laser-plasma acceleration experiments, it is important to measure spatio-temporal density profile of accelerated charged particle beams in a single shot. In this research, single-shot ultrafast spatio-temporal density profile measurement of relativistic electron beams in radiofrequency accelerators is conducted via obtaining spatio-temporal electric-field profile around the beams with combination of electro-optic sampling and echelon-based single-shot method. Here, we introduced an analytical model derived by special relativistic electromagnetism to deduce longitudinal and transverse beam sizes by measured spatio-temporal electric-field profiles.
Electro-optic (EO) sampling is a powerful non-destructive technique for measuring terahertz (THz) electric field with high temporal resolution. EO sampling is a popular method for monitoring the electron beams in accelerators because the measurement of the electric field pulse width directly corresponds to the width of the electron beam. Previous researches mainly focused on measuring the temporal profile. In this work, we also measure the spatial (radial) profile that is perpendicular to the propagation (longitudinal/temporal direction). The measurement of the electric field profiles in both time and space paves the way to access the spatio-temporal electron beam profile, which makes this method promising for a high-order harmonic generation-seeded free-electron laser (FEL). In this research, we investigated the electric field strength profile and the pulse broadening in the radial direction based on the spatio-temporal electric field around a picosecond relativistic electron beam with an energy of 35 MeV. Special relativity predicts that the electric field contracts in the propagation direction and becomes like a disk with a uniform thickness. With this postulate, the Gauss’ theorem in cylindrical coordinates can be applied to the experimental results to deduce the electron beam size from the electric field profile.
The development of second-generation short-pulse laser-driven radiation sources requires a mature understanding of the relativistic laser-plasma processes as e.g. plasma oscillations, heating and transport of relativistic electrons as well as the development of plasma instabilities. These dynamic effects occurring on femtosecond and nanometer scales are very difficult to access experimentally.
In a first experiment in 2014 at the Matter of Extreme Conditions facility at LCLS we demonstrated that Small Angle X-ray Scattering (SAXS) of femtosecond x-ray free electron laser pulses is able to make these fundamental processes accessible on the relevant time and length scales in direct in-situ pump-probe experiments [Kluge et al., Phys. Rev. X 8, 031068 (2018)]. Here we report on a recent follow-up experiment with significantly higher pump intensity reaching the relativistic intensity domain, improved targetry, XFEL shaping and particle diagnostics. We give an overview of the new capabilities in combining a full suite of particle and radiation diagnostics including ion-, electron-, bremsstrahlung- and K-alpha-spectrometer, proton beam profile imager and SAX scattering. Especially probing at resonant x-ray energies can give new insight into the ultra-fast ionization processes, plasma opacity and equation-of-state in non-equilibrium plasmas.
Respresenting the collaborations of the latest two MEC SAXS experiments we will give an overview of the experimental setup and the technical implementation of radiation and particle diagnostics as well as imaging methods. We will exemplify the capabilities on the specific example of probing the correlation of thin layers under high-intensity laser irradiation and its consequences for modelling the heating of buried layers and rear surface expansion.
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