Left: collimated electron trajectories entering a uniform magnetic field are bent and overlap when the bending angle reaches 90°. Center: convergent (divergent) electron trajectories overlap earlier (later) relative to a collimated beam. Right: entering a dipole at an angle will also shift the focus.
Computer-aided design image of laser wakefield experiments. Not shown are the ICT and optics for the side view probe beam used for shadowgraphy and interferometry analysis of the gas jet. The magnets are shown in blue. (a) The PM spectrometer with PM material shown in blue, (b) view of the EM magnet exit aperture from below.
Electron trajectories overlaying a magnetic field map of the combined magnetic spectrometer with a 0.91 T PM and larger 1.4 T electromagnet. Trajectories are shown for energies of 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 75, 100, 125, 150, 200, 250, 300, 350, and 400 MeV for an object distance of 75 cm and divergence of 5 mrad rms. Red lines indicate locations of detectors with the dashed red line indicating the optional detector placement for high energy electron detection using only the PM spectrometer. marks on the inset indicate the image location. The plot on the right shows the spot size in the two transverse planes as a function of central energy. The black line indicates the spot size growth over a free space drift distance equivalent to the trajectory length in the spectrometer.
The left plot shows the dispersive strength of the PM alone and the spectrometer together. When combined with the focused spot size including aberrations [Fig. 3(b)], the resolution limit (right plot) can be determined. The inherent magnet resolution is shown for two electron beam divergences (blue: 2 mrad, red/circles: 5 mrad) with (solid) and without the EM spectrometer (dashed). Also shown are the detector resolutions as currently installed. Black-plus marks indicates the CCD/Lanex resolution and black-stars marks for the fiber spectrometer.
Angles and dimensions of the two magnet pole faces. All dimensions are in meters and degrees.
The scintillating fiber array schematic and photograph of the array installed at the low energy exit plane of the PM.
Example electron energy spectra from LWFA. Shown are the spectra obtained by the fiber scintillators (red) and phosphor screen (blue) for the same laser shot. The total charge in the peak is 1.1 pC.
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