Beam handling


In the set-up as shown in fig. 3, the electron-optical components are outlined mechanically to better than a few tenths of a mm by the conflat flanges and rings. The drive laser can be outlined on the center of the photocathode by using a second port on the laser incoupler to observe the (very weak) circular machining grooves in the reflected laser light. The beam will still wander off the mechanical axis due to the earth magnetic field (and possibly incompletely screened fields from magnetic materials in pumps etc.). To correct for this, an x,y-deflector unit is foreseen in the lay-out of fig. 2. In addition to this correction option, it is recommended to apply compensation of the transverse component of the earth field using an orthogonal set of rectangular, U-shaped Helmholtz coils, which fit around the photogun chamber and have the open end near the sample chamber. The x,y-deflector than serves for fine-tuning only.


In order to diagnose beam transmission through the compression cavity and its position and shape at the sample, use of the detector for the diffraction pattern is a first option. However, for pump-probe experiments, the sample holder should be capable of accurately exchanging the sample for a pinhole.  Transmission of both beams through the pinhole then assures proper spatial overlap of the two beams. For adjustment of the temporal overlap of electrons and photons, one option is to exchange the sample for a TEM-grid; when irradiating this grid with the pump laser, an instantaneous, short-lived plasma is formed on the grid wires which changes the transmission of the electrons (see e.g. Dwyer, Miller, et al., Phil. Trans. Roy. Soc. A 365, 741 (2006).


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