Fusion for the Future: NIF

Fusion is only 50 years away and it will solve all of the worlds energy problems. That is the good news. The bad news is that it has been 50 years away for the last 50 years. If that situation is maddening to you then you are not alone. Leonardo Mascheroni, a retired Los Alamos National Laboratory physicist, wanted funding to build a colossal laser for producing energy from fusion and was willing to trade the United States' nuclear weapons secrets to realize his dream. Mascheroni was recently indicted on charges of treason concerning selling nuclear arms secrets and is awaiting trial sometime this year. In the meantime the United States is pressing forward with a completely separate laser fusion project called the National Ignition Facility or the NIF which uses 192 lasers fired in unison to recreate the energy source of the stars harnessed on Earth.

In this post I am going to talk about the basics of fusion and the NIF. I also have questions and answers with a physicist on the project, Siegfried Glenzer, at the end of the post. I asked him some hard questions not just about the science, but also about the politics going on around the project. Physicists would like their experiments and budgets to work in a vacuum, but alas they never are. I deeply thank doctor Glenzer for answering my questions.

What is fusion?

Fusion is the joining of two or more separate atomic nuclei into a larger nuclei. Fusion can create energy because the mass of the input and output nuclei are not necessarily equal in mass. Specifically, if the mass of the output nuclei is less than the total mass of the input nuclei then the mass difference is made up by the production of energy as Einstein taught us E=mc² (conversely if the output nuclei are more in total mass than the input nuclei then the reaction would consume energy). In particular, stars like our Sun fuse lighter elements into heavier elements up until the point the star is attempting fusion of iron which does not produce energy because iron has the largest binding energy per nucleon. Actually fusion processes in stars normally involve several intermediate nuclei or elements. The most important process for our Sun is the proton-proton chain which fuses four hydrogen nuclei, ¹₁H, to form a single helium nuclei ⁴₂He with a mass difference of ΔM. Einstein's mass energy relation shows us how much energy this process releases.

4 ⋅ ¹₁H -⁴₂He = ΔM

ΔM c² ≈ 27 MeV

The key to joining two nuclei together is overcoming the repulsive electric Coulomb force between nuclei. The positive charge on nuclei repel each other until the two nuclei actually meet and then the attractive short range strong nuclear force takes over to bind the two nuclei into new larger nuclei. The fewer the number of protons in the nuclei the easier it is to fuse. The repulsive force between nuclei may be overcome in several ways. Inside stars heat and pressure, which comes from the stars gravitational contraction, occasionally forces two nuclei close enough together for them to fuse and all together the star burns consistently for a very long time. The more massive the star the hotter and denser it is at the center so larger nuclei can be fused. The production of heavier elements by stars fusing hydrogen is essentially the origin of all elements heavier than lithium; massive stars occasionally explode, and thus we are all made of stardust. The input elements for the first fusion reactors will be the hydrogen isotopes of deuterium (H with a neutron) and tritium (H with two neutrons) because this reaction has the highest nuclear cross section and high energy yield.

Why is fusion important?

Fusion is very important; this is the kind of physics that future presidents should understand. In this post I am focusing on the basics of fusion and the prospects for the National Ignition Facility and a in a future post I will talk about another project known as ITER. I should clarify that there are effectively many different kinds of fusion machines and an important distinction is net energy positive and net energy negative machines. The ratio of fusion power to input power (often denoted Q in the field) must be positive to have a viable energy solution. There exist at this moment very many fusion machines which take more input power than they make in output power (they have a fractional Q value). Some of the current machines seem fantastic like 'table top' pyroelectric fusion devices, but the reality is that they take energy to run and have no foreseeable future in the energy game. These devices play a role as portable neutron generators in labs for various research purposes or in security as nuclear material detectors. Net energy positive machines have not yet been invented. The NIF will not produce energy, but will be a testbed for fusion technologies. The fusion technology goal is the sustainable production of energy from abundant raw elements such as hydrogen, helium, or related isotopes (Helium 3, deuterium, tritium). Fusion using these light elements is cheap, safe, and green. Fusion is cheap (however the technology development is expensive!) because the raw elements like hydrogen are abundant, further as a consequence of this virtually infinite supply (one in every 6,500 atoms on Earth is a deuterium atom) it can be considered a renewable energy. Fusion is safe because when a fusion nuclear reactor malfunctions unlike a fission nuclear reactor the reaction will snuff itself out rather than proceed uncontrollably to the point of a thermonuclear explosion. Finally, fusion is green or environmentally friendly because it produces no climate altering products.

There are so many reasons fusion is important. Fusion is the future. It is the next step in humanity's technological evolution. This video from the BBC Horizons series with physicist Brian Cox gives a cursory look at the NIF, and puts the entire endeavor into perspective (and to boot in finishes with The Kinks This Time Tomorrow which has the most appropriate lyrics ever).

How do we use fusion to make energy?

Under the correct conditions of incredibly high density, pressure, and temperature a self sustaining fusion process can occur. These conditions are of course exactly what you find at the center of a star, but on Earth these conditions are engineered via the use of confinement and heating mechanisms. The NIF will use a symphony of lasers to simultaneously heat and compress a pellet of deuterium and tritium to simulate the conditions inside of a star. A deuterium and tritium target has been chosen for this first experiment because the fusion cross section between deuterons and tritons is three orders of magnitude larger than for any other atoms. Other fusion projects like ITER will use a toroidal (or doughnut shaped) chamber known as a tokamak to confine a deuterium and tritium plasma which is then heated through magnetic field confinement or radio frequency heating kind of like a big nuclear microwave. Once the fusion process is begun radiation and fast neutrons will be emitted which will be absorbed by the walls of the machine in order to gather heat to drive a steam-turbine generator to produce electricity pretty much just like every power plant.

How does NIF work?

It all starts with a single primordial laser source with very low power which is slightly preamplified and split into 48 parts. These pulses are then amplified by a factor of 10 billion in another set of preamplifiers then they are split into 192 parts and sent to the main amplifier. Then electrical energy stored in capacitors is dumped into 7680 xenon flashtubes which operate pretty much like the flash on your camera, except they are over 6 feet tall and take 30 kilojoules of input power each. The bright incoherent full spectrum light from the flashtubes passes through Neodymium doped glass and in a stupendously inefficient process amplifies the laser beams. The lasers bounce back and fourth a few times and finally go through the amplifier and the main optics system again before heading to the target chamber. At this point the primordial laser has been amplified by a factor of 10¹⁵ (in the video below he says quadrillion which apparently doesn't even have an agreed upon meaning, I think 10¹⁵ is right). The beams travel equidistant paths into the final optics assemblies which convert the original infrared light in to UV light that enters the target chamber. Inside the chamber the light focuses onto a little cylinder called a hohlraum and then, maybe, fusion starts. This process is very complex, this video explains it way better than I can.

Finally the lasers converge in the center of the laser chamber on the hohlraum. What is a holraum and what happens next? This was one of the questions I asked Dr. Glenzer and he responds,

A hohlraum is a radiation enclosure. The laser irradiates the inside of the hohlraum wall and is converted to soft x-rays. The soft x-rays homogeneously ablate the outer layer (the ablator) of a 2.2 mm spherical fusion capsule in the center of the hohlraum. Due to Newton's third law, the dense fuel on the inside the ablator layer is accelerated towards the center producing a hot plasma surrounded by dense deuterium and tritium (1000 times solid density). The center will get very hot launching a burn wave into the dense deuterium-tritium layer: A microscopic star is born.

This is in theory, exactly what happens. The 192 lasers induce densities and temperatures sufficient for nuclear fusion by not allowing the spherical fusion capsule to explode asymmetrically (technically this is called internal confinement fusion). The rate of fusion is proportional to density squared times the temperature to the fourth power, so the more rapidly the capsule can be made to implode the better. In reality the lasers impending on the capsule will create an imploding shock which will cause instabilities to grow under high acceleration of the shell during the convergence and allow energy to escape ruining the efficiency of fusion. Compression is maximized by keeping the fuel cool hydrodynamically with several laser pulses stepped in time such that the spherical deuterium and tritium fuel capsule is qausi-isentropically compressed. When the deutrieum and tritum nuclei get close enough for the strong force to kick in fusion results in 14 MeV neutrons and 3.5 MeV alpha particles being emitted. The 3 MeV alpha paricles have a short mean free path in the dense enviroment which causes local heating and facilitates sustained fusion.

However, no one has ever measured the dynamic compression and shock breakout pressures present in the shell and the nonlinear nature of the process means is must be determined experimentally. There is no equation anyone has to say how this is going to work because the system is so complex. An example of one specific issue the physicists at the NIF face is the so called Rayleigh-Taylor instability that originates from the interface between the solid shell and the deuterium and tritiutm fuel within it. It is this instability which causes the fusion reaction to proceed asymmetrically such that the necessary temperatures and pressures are not reached because instead the capsule explodes before the implosion is complete. Overcoming physics challenges like this will lead to efficient fusion.

Politics at the NIF

The politics of the NIF are as complicated as the fusion itself. The project is wildly expensive so that alone makes it controversial and beyond fusion the NIF has a second major task that doesn't fit in their public relations campaign so well. The NIF will provide needed data on the nuclear weapons status, capability and performance in this era of nuclear weapon testing abstinence. The United States has a stockpile stewardship and management program run by the DOE which tests nuclear weapons, but it can't go around nuking like it used to as a result of the comprehensive test ban treaty established in 1996 (edit: the United States has signed but not ratified this treaty, regardless live nuke testing is frowned upon in the modern age) . The NIF should be able to experimentally simulate on small scales the conditions of pressure, temperature, and energy density close to those that occur during a nuclear explosion. I have never read anything that describes how the NIF is meeting these goals in technical terms information is tight. As for our would be spy, Mascheroni, the evidence regarding his case was placed under a limited protective order by a judge last Tuesday. Documents containing sensitive nuclear weapons information will be crucial in his case, but, like so much else at the NIF, confidential.

Hope for the NIF

The NIF has everything: science, intrigue, spies, money, and hope. The hope is that it leads the path forward to sustainable fusion. The issue currently is that there is a physics and engineering problem at the NIF and based on what is known today it seems unlikely that NIF will produce any practical amount of fusion energy because it wasn't designed to. It is a scientific experiment which will give us answers about fusion. It will light a path forward, literally, with the power of the stars.

Questions and Answers on the NIF

I leave you with some questions and answers with Dr. Glenzer.

1) What are the design challenges of the NIF?

The NIF laser is finished and operational. At this point, we can deliver more than 1.2 MJ energy and 400 TW (yes, 400 TW) on target and we have calculations that indicate a good chance at ignition at these energies if everything else works as expected.
One of our goals is to increase the laser energy and power further to 1.8 MJ and 500 TW to increase ignition margin. This requires careful placement of beam smoothing optics and proper planning of optics maintenance. We are in the process of implementing this capability while we are doing experiments in the facility every day. A very challenging task.

2) What does focusing lasers at a hohlraum with a deuterium-tritium target at the center do? And what is a hohlraum?

A hohlraum is a radiation enclosure. The laser irradiates the inside of the hohlraum wall and is converted to soft x-rays. The soft x-rays homogeneously ablate the outer layer (the ablator) of a 2.2 mm spherical fusion capsule in the center of the hohlraum. Due to Newton's third law, the dense fuel on the inside the ablator layer is accelerated towards the center producing a hot plasma surrounded by dense deuterium and tritium (1000 times solid density). The center will get very hot launching a burn wave into the dense deuterium- tritium layer: A microscopic star is born.

3) This machine creates a star on Earth?

Yes, a microscopic star will exists for a billionth of a second burning deuterium and tritium into helium nuclei.

4) In an experiment in early November of 2010 a 1.3 megajoule laser shot run produced a world record neutron yield for laser-driven fusion in internal confinement target, thus fusion is being achieved at NIF, yet this is not considered ignition. What does ignition mean and what advancements are necessary to achieve it?

Ignition means producing a burning plasma where fusion processes occur on a much higher rate than observed so far. This is about the case when the energy produced by fusion processes is of the order of 1 MJ, i.e. of the same order as the laser energy used to heat the target.
This is of fundamental interest for laboratory astrophysics and dense plasma physics. To make this process useful for energy production using fusion we are further developing targets that produce about a factor of 100 more energy than initially used by the laser.

5) This machine is designed to do vastly more than just fusion. What other fundamental physics is explored?

The rough split is 40% fusion, 40% defense, and 20% basic science. There are calls for proposals on the NIF website, and Universities around the world have responded to the first call submitting more than 40 proposals. Eight experiments were selected to be scheduled on NIF in the next few years. The proposals include the study of supernovae plasmas or states of matter of ultra-high pressures and densities never produced the laboratory before.

6) The NIF has been accused of black ops, cost overruns, political pork-barreling, and misleading the public on the reality of fusion. Does this situation put additional pressure on the scientists on the project?

I do not believe that there is additional pressure when people are asking critical questions. Fact is that there are fewer world-leading science machines left in the US than before- see LHC in CERN or the upcoming XFEL at DESY. I believe that NIF will make a big difference in science and it will be worth the investment.

7) When will we have sustainable energy producing fusion on Earth?

Good question - I believe that I will live to see it happen (I was born in 1966).

References

Glenzer, S., MacGowan, B., Michel, P., Meezan, N., Suter, L., Dixit, S., Kline, J., Kyrala, G., Bradley, D., Callahan, D., Dewald, E., Divol, L., Dzenitis, E., Edwards, M., Hamza, A., Haynam, C., Hinkel, D., Kalantar, D., Kilkenny, J., Landen, O., Lindl, J., LePape, S., Moody, J., Nikroo, A., Parham, T., Schneider, M., Town, R., Wegner, P., Widmann, K., Whitman, P., Young, B., Van Wonterghem, B., Atherton, L., & Moses, E. (2010). Symmetric Inertial Confinement Fusion Implosions at Ultra-High Laser Energies Science, 327 (5970), 1228-1231 DOI: 10.1126/science.1185634