Position Measurement Method Using a Divergent Laser Beam and Arago Spot for Tracking of an Inertial Fusion Energy Target

Abstract

This paper is a complement to our previous work [Jpn. J. Appl. Phys. 46 (2007) 6000] on the accuracy of the position measurement method using an Arago spot for an inertial fusion energy target tracking system and we here report an improvement of the measurement accuracy using a divergent laser beam. By employing divergent beam illumination, we can magnify the displacement of the Arago spot compared with the actual displacement of the target without enlarging the diameter of the Arago spot significantly, allowing us to measure the small displacement of the target with high accuracy over a large measurement range. The experimental results for a 5-mm-diameter target demonstrated that a measurement accuracy of lower than 0.2 mm can be achieved when the distance between the target and a charge-coupled device camera is within the range from 2 to 10 m.

Trajectory Calculation of Horizontally Injected Laser Fusion Energy Target in Residual Gas

“…This value satisfies the criterion of the position measurement error of the flying target. Thus, an orthogonal laser irradiation on the target enables 3dimensional position measurement [4,5]. Moreover, image compression using cylindrical lens can convert a 2D Arago spot image into a 1D image [5,10].…”

“…The center of the spherical target, focal point F of the laser beam, and the Arago spot lie on a line [11]. If the spherical target is irradiated by a convergent or divergent laser beam, the shadow is magnified and the Arago spot of the spherical target can easily be observed at a large distance, thereby enabling the position measurement of the flying target from outside the reactor [4,5].…”

“…To meet the criterion of the engagement error, the error of the position measurement of the flying target as well as the calculation error must be lowered by an order of magnitude. Moreover, the position measurement units (PMU) are set along the target path [4,5]. These PMUs measure the local coordinates of the flying target in each PMU and the time of the measurement, (x, y, z, t).…”

Compression of Arago Spot Images for Rapid Position Measurement of Inertial Fusion Energy Targets

“…Hence, the center of the Arago spot agrees with the center of the projection of a spherical object. Using a divergent laser beam, an accuracy within 0.2 µm was obtained at a distance of 10 m [5].…”