Automatic laser beam alignment for a Thomson scattering system

A system for the automatic alignment of a pulsed ruby laser beam is under development for the Thomson scattering diagnostics of RFX, a large reversed‐field pinch machine now under construction. In this experiment the laser will be 11 m from the machine and the beam alignment at the 750×4‐mm scattering volume will be actively maintained to within 0.5 mm. The beam direction in space is measured in two reference planes fixed to the collection optics frame by means of two 80‐mm2 fast quadrant photodiodes. A double‐channel preamplifier is used for each quadrant in order to measure both the 30‐ns FWHM ruby pulse and a low‐power cw He–Ne beam propagating through the same optical path. Every time the main laser is fired, the relative direction of the two beams is determined so that the position of the He–Ne beam can be used for feedback control of the steering optics between plasma shots.