NIF’s 2013 data showed fusion can be feasible but geometries need adjustment.
We continue our discussion by looking at target geometry. Yes, NIF seems on a success path, but they do need to clean up various physical details. That would lead them to the point that it would be reasonable to start considering realistic IFE power plant designs.
NIF results discussed in Part 1:
- Gain: Target implosions produced about as much energy as was applied. Yes!
- Enhanced heating: fusion-produced alphas provided significant heating to the plasma, and enhanced the yield of fusion energy. Yes!
Point 2 indicates that a path to ignition surely exists in ICF studies. Ignition is the name when the fusion reaction products (the alphas) become a significant energy source to drive new fusion events. We need an ignited plasma to use the fuel. Fusion power plants can never be economical without ignition.
So… what happened to make things work?
We rely on the two presentations discussed in the previous post and on the December presentation by John Edwards, ICF Program Leader at LLNL.
- Future Modifications: There is need to adjust strike point where laser beams hits inside hohlraum cavity wall. The final core after implosion is far from a clean sphere.
- Most Important: The laser pulse characteristics were changed so that the beams ceased being effective generators of hydro instabilities (fluid-style fluctuations that can shred the collapsing core) ; but the data may yet indicate their implosions are troubled with hydro effects.
Yes, test shots function better than before, but results show that they still have issues to address before they close in on success.
Click any illustration for the full sized image.
Modifications Needed: Uniformity adjustments
In Part-1 we showed a typical target, the hohlraum radiation chamber it sits in, and a schematic of the 96 laser beams entering the chamber from both sides and striking the walls. Fig 2 is another image.
The beams enter at 4 different angles. The equatoral plane is the horizontal line through the CH (plastic) target; the polar axis is a vertical line through the center. Interior dimensions are about 10 mm ( ~ 3/8 inch) vertical, 5 ¾ mm (~ ¼ inch) horizontal.
Adjustment of the x-ray flux from the laser/wall strike points requires exquisite care to generate uniform burn-off (ablation) of the target shell with a uniform sphere as the final stagnation core. A symmetric core with a single hot spot is the right recipe for efficient fusion.
Fig 2 shows another innovation: back-filling the hohlraum with helium gas. No specific reason given, perhaps it is to help smooth beam unevenness?
Here are results from their best shots. Fast x-ray cameras took images of core emissions, as indications of fusion activity. The data images are gray shades with dark areas being high emission regions.
Fig 3 shows colorized x-ray images from one of the good shots, the papers have others. The shape from the side (equatorial plane) is quite different what is seen through the hohlraum ends (polar view). Interesting: highest emission is from two distinct regions from a single side, and the nominally spherical central core is a “square” donut. The out-of-round must be an issue in locating the beam hit-points.
Fig 4A shows a 3D reconstructed image of the core, which accurately reproduces the shape. Fig 4B is a fresh donut, to demonstrate the toroidal shape. The core doesn’t have to be symmetrical to be yummy.
Not a sphere, and more than one compact hot spot. Clearly the laser beam hit points in the hohlraum must be re-thought. We will discuss this again in Part 3.
From the Archives: The need for symmetrical illumination has been understood for many decades.
Fig 5 shows a test with the Chroma laser at KMS Fusion, Inc, in the late 1980s. We were actually testing a facility-developed x-ray framing camera using a left-over target shell and the 0.53 µm laser light. The beam was split into 2 arms and focused on the shell. Our “Equator” image looks a lot like NIF shots, but without a “Polar” view, we can only speculate that it probably had a toroidal shape.
This old recollection is to point out that ICF labs have been serving up donuts for many years. Most shots had fairly low compression with energy distributed among multiple dense kernels. KMS’ best results date from 1986 and use its radiation chamber for precise time and space symmetry adjustments. Achieved • “at least” 80× target compression • a single high density compressed core.
Most Important need: Pulse shape adjustment
The key element for the current success is that the NIF team shortened the laser pulse length (Fig 6).
Shorter shot time NIF managers trimmed the shot length from 25 to 15 ns and raised the power in the trough at the center. This is called the “high foot” (HF) mode of operation.
Their basic change was to push into the target the same energy as before, but faster … 870 GW instead of the previous 520 GW.
There had apparently been worries that the HF pulse shape might cause too much preheating of the fuel, generating back pressure that would inhibit formation of a tight stagnation core at the end of the implosion. The results show differently. The main positive seems to be that they significantly reduced previous effects from severe RT (Rayleigh-Taylor) instabilities and allowed the fuel to converge into an almost sphere with a not quite single hot spot.
We will continue the discussion of the really positive 2013 NIF results in part 3, with discussions of what might be yet be causing trouble .
Charles J. Armentrout, Ann Arbor
2014 Jun 1
Listed under Technology … Technology > ICF/IFE
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