Focused beam control in super-ASHURA KrF laser system By I. MATSUSHIMA, I. OKUDA, Y. MATSUMOTO, T. TOMIE, E. MIURA, H. YASHIRO, E. TAKAHASHI, S. KATO, and Y. OWADANO Electrotechnical Laboratory, 1-1-4, Umezono, Tsukuba, 3058568, Japan ~ Received 14 January 1999; Accepted 14 January 1999! Beam smoothing techniques and focused beam profile control techniques for high efficiency KrF laser fusion are reported. We have developed 1D and 2D broadband random phase ~ BRP! irradiation techniques. Our next idea is the combination of a laser oscillator with a wide beam divergence angle and phase plates. The wide divergence angle laser beam eliminates the speckle patterns caused by the phase plates. Well characterized focal spot patterns have been observed with the front-end pulse of the Super-ASHURA KrF laser system. Amplification experiments are on going. 1. Introduction Uniform target compression is required to achieve a high gain in laser fusion. Various kinds of techniques have been proposed to improve laser irradiation uniformity. Kinoform phase plates ~ KPPs!~ Dixit et al. 1996! and 2D smoothing by spectral dispersion ~2D SSD!~Skupsky et al. 1989! are recently of interest to glass laser system developers. For KrF lasers, the broad bandwidth is useful for improving the laser irradiation uniformity. For example, the induced spatial incoherence optical smoothing ~ ISI ! technique ~ Lehecka et al. 1995! could successfully make a uniform irradiation profile ~ Pawley et al. 1997!. The authors have also proposed the broadband random phase ~ BRP! irradiation technique ~ Matsushima et al. 1991! as a beam smoothing method for the Super-ASHURA KrF laser system ~Okuda et al. 1997!. This BRP technique has been extended to 2D smoothing ~ Matsushima et al. 1996!. In this paper, our next idea is proposed. It is the combination of a laser oscillator with a wide beam divergence angle and phase plates. The wide divergence laser beam eliminates the speckle patterns caused by the phase plates. The effectiveness of this concept has been experimentally observed with the front-end pulse of the Super-ASHURA KrF laser system. Amplification experiments for the full energy shots and further improvements for the beam profile control are on going. 2. Beam smoothing concepts Although binary phase plates such as random phase plates ~ RPPs!~ Kato et al. 1984! or Fresnel phase zone plates ~ PZPs!~ Ross et al. 1995! are simple and useful methods for focused beam profile control, the focused intensity distributions include highly modulated speckles from the interference between different phase plate elements. To smooth the speckle pattern, we have proposed the BRP irradiation technique. As shown in figure 1a, the speckle pattern in the RPP irradiation is smoothed by a spectral dispersion effect using a broadband KrF laser and a spectral dispersing optic that is inserted in front of the RPP. The high spatial frequency components in the speckle pattern caused by RPP irradiation are removed by incoherent over- Laser and Particle Beams ~1999!, vol. 17, no. 4, pp. 693–699 693 Printed in the United States of America © 1999 Cambridge University Press 0263-0346099 $12.50