To accommodate long focusing distance, the beam is directed to an extension chamber at the south end of the room. Two options are available – an f/20 (3m) parabola located in the extension, or an f/40 parabola reflected from a periscope located in the extension. The f/40 is commonly used because it produces a focus more suitable for laser wakefield acceleration (LWFA) of electrons in a gas target, but careful design is needed to ensure the fluence remains below the damage threshold of the folding mirrors. A typical electron diagnostic layout is shown, where the electrons are generated by LWFA.
