Comment on "National Ignition Facility laserperformance status"

Stephen E. Bodner689 Fearrington Post, Pittsboro, North Carolina 27312, USA

*sebodner@mailaps.org

Received 18 July 2007; accepted 24 August 2007;posted 18 January 2008 (Doc. ID 85435); published 27 March 2008

The National Ignition Facility (NIF) laser has not yet achieved two of the stated requirements needed fortesting and evaluating ignition targets. The laser focal spot size is more than a factor of 2 too large, andthe laser bandwidth is more than a factor of 2 too small. © 2008 Optical Society of America

OCIS codes: 140.0140, 140.3530.

1. Introduction

A recent article in Applied Optics [1], entitled “Na-tional Ignition Facility laser performance status,”states that “the NIF will meet its laser performancedesign criteria.” However the data from the firstoperational laser beams of the National IgnitionFacility (NIF), summarized in Table 4 of [1], arenot consistent with recently published target designspecifications. The baseline ignition target designwith 1:8MJ of laser energy has a requirement ofan elliptical focal spot size of 500 × 1000 μm; thefirst operational NIF beams had a focal spot sizeof 1160 × 1300 μm, which is a factor of 2.3 too largealong the minor axis. The baseline ignition targetdesign also has a requirement [2] of a bandwidthof 270GHz; at full energy the NIF beams had abandwidth of only 120GHz, which is 44% of thedesign value. The following sections explain thereason for and the importance of these two laserrequirements.

2. Focusing Requirements

The NIF laser fusion ignition target design consistsof a small spherical fusion target in the middle of acylindrical can, with holes at each end of the canthrough which the NIF laser beams enter, heating

the inside of the can, and producing x rays that thendrive the fusion implosion.

The spatial distribution of the focal spot of each ofthe laser beams has several restrictions. First, ofcourse, the focal spot size must be small enough toeasily fit within the entrance holes. A more restric-tive requirement is that the focal spot size must betight enough that essentially none of the laser energydirectly hits the spherical fusion target. If any signif-icant amount of laser light directly hits the sphericaltarget, it would produce a nonuniform pressurepulse, and the implosion would fail. Also, the variouslaser beams do not pass through the center of theentrance holes, which also restricts the focal spotsize. One must also be careful to prevent any signif-icant amount of laser light from illuminating theedges of these entrance holes.

When the NIF construction project was approvedin the mid 1990s, the contract included the followingphrase: “The laser spot size shall be 500microns, tobe defined as follows: The envelope of the laser pulsespatial intensity distribution shall approximate aneighth order super Gaussian at best focus. The laserbeam profile diameter at e − 1 in intensity shall notexceed 500microns, with 98% or more of the 1:8MJpulse energy contained within a circle of 600–microndiameter” [3].

After the construction project began, the target de-signers realized that they could relax their focusingrequirement. The focal profile could be an ellipse in-stead of a circle, with a spot size of 500 × 1000 μm.

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This had the advantage of reducing the peak laserintensity, thereby reducing the risk of deleteriouslaser–plasma interactions. This newer specificationis noted in the 2004 target design review article [2].Table 4 of [1] indicates that, when the initial NIF

beams were operated at their full required energy,enough to produce 1:8MJ with all the laser beams,then the focal spot size of each beam was an ellipsewith a FWHM size of 1160 × 1300 μm. This focal spotsize is a factor of 2.3 too large along the minor axis,and a factor of 1.3 too large along the major axis.Also, along the major axis, it appears that 98% ofthe energy content is in a much larger diameter ofapproximately 1600 μm; see Fig. 31 of [1].The above comparisons ignore the difference be-

tween FWHM and 1=e width, which is not criticalto this analysis.

3. Bandwidth Requirement

As discussed in [1], the front end of the NIF containstwo phase modulators. The first produces a laserbandwidth of 30GHz at 1ω. Its purpose is to sup-press stimulated Brillouin scatter in the main laseroptics. Without this phase modulator it was discov-ered that acoustic waves could break the laser glass.A second phase modulator can produce up to anadditional 150GHz of bandwidth at 1ω. This secondsource of bandwidth is needed to help control laser–plasma instabilities.When the laser beams propagate through the

plasma inside the target, they can produce a zoo oflaser–plasma instabilities. One set of these instabil-ities includes laser beam filamentation. As the laserbeam filaments in the plasma, it produces muchhigher peak laser intensities than would occur in avacuum. The higher laser intensities then enhancethe rest of the zoo of laser–plasma instabilities. Ata sufficient level, these instabilities can prevent tar-get ignition.It was discovered some years ago [4] that, if the

laser beam has spatial and temporal incoherence,then the filamentation modes could be suppressed,which then reduces the other laser–plasma instabil-ities. For the NIF, the required temporal incoherence(bandwidth) is produced at the front end of the laserwith the second phase modulator. The spatial inco-herence is then created with phase plates at theend of the laser chains. This version of induced inco-herence is called smoothing by spatial dispersion(SSD) and is described in greater detail in [1].Because the NIF target will be physically larger

than any previous laser-fusion target, one mustextrapolate from previous experiments to estimatewhat laser bandwidth will be needed for the NIFignition target. There are of course differences ofopinion within the community on how to do an extra-polation. Some experts think it likely that there willbe excessive laser–plasma instabilities with the NIFignition target, even with broadband SSD, and thatthe target has a good chance of failing. Other experts

think the SSDwill work sufficiently, and that there iseven likely to be a safety factor in the bandwidth re-quirement. The 2004 specification of the NIF targetdesigners [2] is that the laser should have the flex-ibility to produce 3Å of bandwidth at 1ω [2]. Con-verting this wavelength spread into a frequencybandwidth, the NIF requirement is 90GHz at 1ω,which broadens to 270GHz during frequency conver-sion to 3ω.

Table 4 in [1] shows that at the design energy out-put of 1:8MJ, the SSD bandwidth was only 120GHz,44% of the design requirement. Note also that the90GHz specification in the third row of Fig. 4 is in-correct. According to [2] it should be 270GHz.

4. Conclusions

The first NIF laser beamlines, when operated attheir full energy, have a bandwidth that is less thanspecified by the NIF target designers. Whether thisbandwidth limitation would lead to the failure of theNIF ignition target cannot be determined yet, be-cause as mentioned above there is uncertainty anddisagreement on how to extrapolate from previousexperiments to the larger geometry of the NIF.The increase in focal spot size is more obviously aproblem. With the larger spot size it may be impos-sible to avoid directly illuminating the sphericaltarget. Or it may require an increase in the can size,which would then reduce the energy coupling to lessthan what is needed for the fusion ignition target.

NIF operation at a reduced energy of 1:0MJ wouldsolve both of the above laser problems, as can be seenin Table 4 of [1]. At this reduced energy the focal spotis the appropriate 500 × 1000 μm, and the bandwidthis the appropriate 270GHz. It is still uncertainwhether the proposed 1:0MJ ignition target designwill be as viable as the 1:8MJ ignition target designfor which the NIF laser was originally built.

References1. C. A. Haynam, P. J. Wegner, J. M. Auerbach, M. W. Bowers,

S. N. Dixit, G. V. Erbert, G. M. Heestand, M. A. Henesian,M. R. Hermann, K. S. Jancaitis, K. R. Manes, C. D. Marshall,N. C. Mehta, J. Menapace, E. Moses, J. R. Murray, M. C. Nos-trand, C. D. Orth, R. Patterson, R. A. Sacks, M. J. Shaw, M.Spaeth, S. B. Sutton, W. H. Williams, C. C. Widmayer, R. K.White, S. T. Yang, and B. M. VanWonterghem, “National Igni-tion Facility laser performance status,” Appl. Opt. 46, 3276–3303 (2007).

2. J. D. Lindl, P. Amendt, R. L. Berger, S. G. Glendinning, S. H.Glenzer, S. W. Haan, R. L. Kauffman, O. L. Landen, and L. J.Suter, “The physics basis for ignition using indirect-drive tar-gets on the National Ignition Facility,” Phys. Plasmas 11, 339–491 (2004). See Table II-1 on p. 348, and the accompanyingtext on that page.

3. “National Ignition Facility Functional Requirements andPrimary Criteria,” Revision 1.3, Report NIF-LLNL-93-058(Lawrence Livermore National Laboratory, 1994).

4. A. J. Schmitt, “Three-dimensional filamentation of light inlaser plasmas,” Phys. Fluids B 3, 186–194 (1991).

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