Application of target tracking method using an Arago spot and divergent beam for in-chamber measurement

Abstract

A position measurement method using the Arago spot and a divergent beam is discussed for the in-chamber measurements of an inertial fusion energy (IFE) target. By employing a divergent beam, the actual displacement of the target is measured by the magnified displacement of the Arago spot, and the shadow of the target is also magnified, whereas the diameter of the Arago spot is not enlarged significantly. This fact enables us to measure the small displacement of the target with high measurement accuracy over a large measurement range. The Arago spot can be identified by the maximum intensity in the diffraction pattern due to the magnified target shadow. A measurement accuracy of 0.2 μm is achieved with a measurement range of 2 to 10 m for a 5-mm-diameter target.