Advanced diagnostics of laser driven plasma accelerators : electron spectormeters and optical probes



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Recent advancements in laser-driven plasma accelerators (LPAs) have enabled the acceleration of electrons to multi-GeV energy levels using powerful petawatt (PW) lasers and lower density plasmas. A notable achievement in this field is the acceleration of electrons to 7.8 GeV in a plasma with a density of 1 × 10¹⁷ cm⁻³. Addressing the challenges brought by these developments, particularly in LPA diagnostics, has been a key area of our work. For measuring high-energy electron spectra, we innovated by incorporating tungsten fiducial wires into a magnetic spectrometer. This technique enhances accuracy through precise calibration of electron trajectories. We also developed a comprehensive theory for designing, calibrating, and analyzing such spectrometers. Our findings indicate that the minimum error for these spectrometers is 3.36% when measuring electrons with energies below 10 GeV. In addition, we introduced a new concept for spectrometers that utilize the scattering properties of electrons passing through a thick plate. By analyzing the scattering distribution, we successfully reconstructed the electron spectrum. This method was tested with electrons in the 1 to 2 GeV range and compared favorably with our fiducial-equipped magnetic spectrometer. Further simulations suggested that this novel technique remains highly effective for electrons with energies exceeding 20 GeV. The technique offers significant benefits in terms of accuracy, compactness, and cost-effectiveness. To improve plasma structure measurements in low-density environments, we proposed enhancing signal detection through magneto-optical effects, including Faraday rotation and the Cotton Mouton effect. We determined that a probe wavelength of 1.8 m is optimal for measuring 1 GeV electrons and implemented an optical probe based on an Optical Parametric Amplifier (OPA). A Faraday rotation experiment was successfully conducted. We are also in the process of developing a probe simulation code using the Finite-Difference Time-Domain (FDTD) method and perturbed flow equations.



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