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Optical Design & Engineering

Interferometric adaptive optics for the National Ignition Facility

A newly constructed testbed could be employed by future fusion power plants to track and engage targets with the precision needed to release high-gain fusion energy.
2 June 2010, SPIE Newsroom. DOI: 10.1117/2.1201005.002609

Great strides have been made during the past half century in pursuit of fusion energy as a relatively clean and essentially unlimited energy source. One of two current approaches, inertial-confinement fusion (ICF), uses a large number of lasers to either directly compress a capsule containing deuterium and tritium (DT) or indirectly convert the laser photons into x-rays on the wall of a Hohlraum (an idealized cavity) and use this radiation to compress a DT-filled capsule. The latter, indirect-drive ICF is currently being pursued by the newly commissioned National Ignition Facility (NIF).

As we are closer to solving the physics issues associated with achieving fusion energy in the laboratory, we are starting to look in detail at the daunting engineering challenges that must be solved for a commercial power plant driven by fusion energy to become a reality. A main challenge is target tracking and beam correction for hundreds of laser-beam lines. An ICF power plant will focus approximately 200 laser beams onto a Hohlraum, a cylinder with a diameter of 1cm and a length of 2cm, containing a DT-filled fuel capsule. Hohlraums will be launched into the target chamber at speeds of up to 300m/s and their fuel capsules imploded at a rate of 15Hz. Each of the hundreds of lasers will be required to hit their respective positions on the Hohlraum's wall with positional accuracies of ~100μm rms. For fast-ignition ICF, the beams' positional accuracies may be as restrictive as 10μm rms. Phasing multiple beam lines together may also be required.

Figure 1. (a) Degradation in Strehl ratio due to an applied tilt error. (b) Corrected far field after an application of a piston error across half the aperture. MEMS: Micro-electromechanical systems.

One way to correct for higher-order beam aberrations, phase multiple beam lines together, and steer each of them, is to use an interferometric adaptive optics (AO) system.1 When its wavefront sensor is used with a pixilated mirror, it does not require reconstruction of the measured wavefront and can, therefore, be run at correction speeds in excess of 10kHz for systems containing more than a thousand subapertures. This is much faster than is possible with conventional AO systems.

Figure 2. Measurements examining higher-order phase errors across the aperture. (a) Unwrapped phase. (b) Corrected far-field intensity measurements.

We constructed a testbed at the Lawrence Livermore National Laboratory to assess the performance of such an interferometric AO system to simultaneously phase multiple apertures together, perform beam steering, and correct for higher-order aberrations within the beam lines before the system is implemented at the NIF for fast-ignition experiments.2 For these experiments, the laser beam will be focused at the end of a narrow fast-ignition cone of only 40μm in diameter. To meet the associated energy requirements, four beam pairs must be phased together, while the positioning of the individual beams at the end of the fast-ignition cone must be better than 10μm rms. To change the pointing of a beam, a linear phase ramp is applied across the pixilated micro-electromechanical-systems (MEMS) device. This causes the latter to become a phase grating, so that energy is taken from the m=0 order and transferred to the higher orders of the grating: see Figure 1(a), where four different levels of tilt are placed across the MEMS device. The inset shows the loss of Strehl ratio (which relates the AO-corrected peak intensity of a point source to that expected when operating at the diffraction limit) as a function of waves of tilt across the entire MEMS device. Phasing can be studied by placing microscope slides across half the device, with subsequent phase measurements and corrections for the piston error caused by the microscope slide. Figure 1(b) shows the resulting far field corrected with these dephased apertures, while the inset shows the piston phase when the microscope slide is placed across half the aperture. We achieved a corrected Strehl ratio of Sr=0:76.

We also used the testbed to correct for a higher-order aberration expected on the fast-ignition beam lines. This phase—see Figure 2(a)—produced tip/tilt-removed Strehl ratios of Sr=0:087. The corrected far field achieved Sr=0:66: see Figure 2(b). The achieved Strehl ratios met the requirements for the fast-ignition experiments and the system is now scheduled for implementation at the NIF.

Kevin Baker, Doug Homoelle, Craig Siders, Chris Barty
Lawrence Livermore National Laboratory
Livermore, CA

Kevin Baker has nearly 20 years of experience in research and development related to AO metrology and diagnostics. He has worked on AO projects for high-energy lasers, free-space communications, dark energy, and extrasolar-planet imaging.

Eddy Stappaerts
Stappaerts Consulting
Livermore, CA