Full text loading...
Schematic diagram of the UED instrument and the grating enhanced ponderomotive scattering geometry. Pulses from the 75 MHz laser oscillator are used to generate an electrical signal used by the PLL to phase-lock the RF cavity oscillation with the femtosecond laser. The electron bunches are generated through photoemission at a bulk copper photocathode using the third harmonic (266 nm) of the Ti:Sapph laser (800 nm). The electron-laser pulse cross correlation is performed at a position 26 cm after the RF cavity by spatio-temporally overlapping sections of the electron pulse with two counter propagating laser pulses by controlling the relative arrival time with an optical delay line. The ponderomotive scattering signal, proportional to the local electron charge density, is measured as the intensity of the streaks on either side of the main beam. This is shown in the rightmost raw image data labelled a, b, and c for various schematically indicated laser-electron pulse overlap conditions.
RF cavity as a temporal lens. Impulse response functions recorded for the four RF field amplitudes indicated in the inset. Red (diamonds)= 1.518 MV/m, Green (triangles) = 1.535 MV/m, Cyan (squares) = 1.552MV/m, and Blue (circles) = 1.574 MV/m. The blue curve was taken at the optimal settings and is an average taken over 45 min (bunch charge, Q = 0.1 pC). The FWHM of the measured impulse response function (after deconvolution of the measurement impulse response ) is 334 ± 10 fs FWHM. The dashed line in the inset shows simulation results for the electron pulse duration at the measurement position performed with GPT. Note the discrepancy around the optimal compression point.
Impulse response function vs. relative delay for varying RF phase . Negative delays are equivalent to . Inset shows the fitted FWHM IRF to each impulse response function in the main figure. The dashed line in the inset shows simulation results performed with GPT.
RF synchronization jitter. Inset (A) shows the signal fluctuations measured at the peak of the IRF shown in the main panel (425 fs FWHM) over the course of 7 min. These signal fluctuations can be used to diagnose the RF synchronization jitter, , present in the synchronization system. Inset (B) shows a diffraction pattern of a polycrystalline gold sample measured at optimal compression integrated over 500 electron bunches (0.5 s) at Q = 0.1 pC.
Article metrics loading...