Completing the viability demonstration of direct-drive inertial fusion energy target engagement

A significant challenge for the successful implosion of direct-drive inertial fusion energy (IFE) targets is the repeated alignment of multiple laser beams on moving targets with accuracy on the order of 20 mum. Adding to the difficulty, targets will be traveling up to 100 m/s through a chamber environment that may disturb their trajectories. In the High Average Power Laser (HAPL) program, we have developed a target tracking and engagement system capable of meeting the goals for an inertial fusion power plant. The system consists of separate axial and transverse target detection techniques, and a final correction technique using a short-pulse laser to interrogate the target's position 1-2 ms before chamber center. Steering mirrors are then directed to engage the target at chamber center. Over the past few years, we have constructed and improved upon an integrated tabletop demonstration operating at reduced speeds and path lengths. In August 2007, initial engagement of moving targets in with a simulated driver beam in air was 150 mum rms. Since then, we have taken an encompassing look at all error sources that contribute to the overall engagement error. By focusing on those individual component errors that have the most influence and improving their accuracy, we have substantially reduced the overall engagement error. In August 2008, we had achieved engagement of 42 mum rms in air by using this approach, and in March 2009, 34 mum rms in vacuum. The final elements, which we believe are necessary to meet our goal, necessitate engaging lightweight targets in a prototypic vacuum environment with an understanding of the scalability of demonstration-scale errors to full-scale errors. In this paper, we present the latest improvements from the identification and reduction of errors and the resulting engagement data demonstrating near-completion of the viability demonstration of direct-drive target engagement to 20 mum.