Setup of the experiment. The laser pulse is focused by an OAP mirror into a supersonic helium gas jet where electrons are accelerated to between 50 and 70 MeV. The transverse electron beam profile is monitored on a removable scintillating screen. (a) A 1 cm thick, 1 cm inside diameter lead aperture (b) protects the undulator magnets from off-axis electrons. The spectrometer is shielded from direct laser and transition radiation exposure by a thick aluminum foil (c) placed at the entrance of the undulator. The electrons traverse the undulator, produce synchrotron radiation, and are then dispersed in the magnetic electron spectrometer. Undulator radiation is collected by a lens (d) and analyzed by an optical spectrometer.
The relativistic plasma channel, self-illuminated at and used to characterize the length and position of the plasma channel.
The transverse electron beam profile detected on the scintillating Lanex screen at the entrance of the undulator. Electron divergences in the range 2–10 mrad are observed. The shot displayed has a rms divergence of 2 mrad.
(a) The electron spectra and (b) the corresponding optical undulator radiation spectra for three shots. The blue (solid) and green (dotted) lines have been multiplied by 5 and 10, respectively for clarity. The black (solid) lines show the predicted spectra.
(a) The electron spectra data points (red solid line). The simulated instrument response function of the electron spectrometer, (blue dashed line). (b) The undulator radiation spectra data points (black solid line). A best-fit curve to the data points, (red dashed line). The simulated instrument response function of the magnetic undulator, (green dotted line). The deconvoluted undulator spectrum, (blue dash-dotted line). Note: The upper and lower axes in these plots give the respective wavelength and energy [Eq. (2)].
(a) Shows the spatial distribution of Lorentz factor of electrons at dephasing length (triangles for no beam loading, red (lower line) and green (upper line) for beam loading with 18 and 10 pC of charge, respectively), and (b) spectrum. Here (c) shows the energy compression as a function of propagation distance; triangle for no beam loading, square for beam loading when electron bunch charge is 18 pC (circle and triangle for beam loading when the bunch charge is 10 and 18 pC, respectively).
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