the next step. Again, a spherical direct drive system would not rule out continuing tests with indirect drive by using approximately two-thirds of the beams.

If both the laser-indirect and laser-direct drive approaches continue to experience difficulty reaching ignition over the next 5 or so years, then it would be justified to put more resources to the MagLIF and HIF approaches. Depending on the reasons for the failure of the laser-based approach—e.g., laser plasma instabilities—it might also be appropriate to consider alternate laser driver approaches. DOE support for reactor design studies of ideas using these drivers is important, including participation by groups that are not advocates. Viable reactor designs would be required before there is a substantial ramping up of such approaches. These design studies should help guide the related decisions.

Recommendation 4-6: Although ignition was not achieved at the National Ignition Facility by the end of FY2012 as planned, efforts to achieve ignition with indirect drive should not cease. Contingent on the availability of funds and Department of Energy priorities, these efforts should continue at least until new configurations (e.g., polar direct drive) can be tested on the National Ignition Facility, which would require at least 4 years of development. However, under this scenario, a commitment should be made to undertake pretesting of polar direct drive on the National Ignition Facility and, if the pretests are successful, prepare NIF to test polar direct drive.

Even if ignition should be reached with indirect drive before polar direct drive becomes operational, the funding for direct drive will still have been well spent, for it is desirable to test polar direct drive in the hope of getting a higher gain (with the same drive energy) than may be possible with indirect drive. (A technical discussion of direct and indirect drive is given in Chapter 2.)

As discussed in Chapter 2, the energy required to achieve ignition in laser-based indirect and direct drive approaches favors direct drive. Moreover, for a fixed laser energy, the calculated gain is higher for direct drive. Nevertheless, there are important uncertainties in laser-plasma physics and implosion dynamics that must be addressed for fusion-scale targets, particularly for shock ignition. The NIF is currently a unique tool for addressing these issues, some of which could be addressed with NIF in its present configuration. Others may require modifications such as improvements in beam smoothness, or ultimately even a different illumination geometry.

Conclusion 4-5: Each target design and each driver approach has potential advantages and uncertainties to the extent that “the best driver approach” remains an open question.

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