Several different approaches are continuing to produce energy in a sustainable manner. Two techniques that are widely discussed in this article are developments to improve the efficiency of solar and wind energy.

Ultimately, the source of the solar energy is probably the most sustainable of all energy generating options. The sun produces energy through the use of nuclear fusion that involves merging four hydrogen atoms into one helium atom producing a tremendous amount of energy. This process has been self-sustaining for the approximately 4.5-billion-year lifetime of the sun.

Research has been underway to emulate what occurs in the core of the sun. The objective is to develop a system that produces more energy during fusion than is required to initiate the process. This is defined as having fusion reach the point of ignition. A second consideration is that the fusion process must not only be self-sustaining but controllable.

Past research has used fuel that is a combination of the two hydrogen isotopes, deuterium and tritium (DT). To achieve fusion, the DT fuel must ignite at incredibly high temperatures facilitating a conversion to a burning plasma state that can readily produce a high degree of energy and helium.

Two fusion techniques that have been utilized to produce plasma are inertial and magnetic. Dr. Hermann Geppert-Kleinrath, physicist at Los Alamos National Laboratory in Los Alamos, N.M., says, “Our research team is using an experimental setup at the National Ignition Facility (NIF) to determine if a burning plasma can be established to carry out fusion. The technique we are using is known as indirect-drive inertial confinement fusion and starts with 192 lasers delivering up to 1.9 megajoules of frequency-tripled light into a cavity known as a hohlraum, where the walls are in radiative equilibrium with the radiant energy in the cavity. The hohlraum produces X-rays that are directed to the exposed surface of a DT fuel-containing capsule at its center. As a result, hundreds of millibars of pressure is created that forces an expansion of the the outer edge of the capsule away from the center. A partial-pressure equilibrium is formed between fuel vapor layers against the inside surface of the capsule with fuel vapor at the center of the capsule.”

Geppert-Kleinrath continues, “Inwardly directed acceleration pushes the capsule and the DT fuel inward upon itself leading to enormous pressures and temperatures. DT fuel is accelerated to 400 kilometers per second and starts fusing; this phase of the process takes only 130 picoseconds. During this step in the process, the DT fuel now obtains about 10-20 kilojoules of kinetic energy inside a small volume. Once the implosion occurs, enormous pressure of a magnitude of hundreds of gigabars is generated. The real temperature reached in the fuel is approximately three times higher than what is found in the center of the sun. In effect, our team is creating a miniature sun in the laboratory.”

The fusion reaction produces neutrons and alpha particles (helium nuclei). The self-sustaining aspect of the process is due to the alpha particles depositing their energy in the high temperature hotspot that is created. But indirect-drive inertial confinement fusion also loses energy at a very high rate. Geppert-Kleinrath says, “Energy is lost through radiation and heat conduction.”

Research has now been reported that moves forward the possibility of self-sustaining fusion.

Recent experiments
The research team conducted four recent experiments of which two that took place in February 2021 exceeded the criteria for achieving a burning plasma state in fusion. The result is the research team is now closer to achieving self-sustaining fusion than ever before.

To improve the ability of the process to produce more energy from the fuel and keep it contained in the target chamber, several improvements were made to the experimental setup. These changes includ- ed reducing the size of the fuel fill tube’s size, which was identified as a limitation by three-dimensional neutron imaging.

The energy yield achieved by the two experiments is expressed as Q_a, which is defined as the ratio of the energy obtained from self-heating of an alpha particle to the external heating input into the DT fuel. The research team estimates that Q_a for one of the experiments was in the range of 1.4-1.6 megajoules and was 1.3-2.0 megajoules for the second one.

Geppert-Kleinrath says, “We have not yet reached self-sustaining fusion, but we are on the threshold of achieving that goal. Our experiments have demonstrated that we have produced a burning plasma. Small incremental improvements beyond this stage should yield significant gains in energy yield moving us close to the stage of self-sustainment.”

An important analytical tool used by the research team is the neutron imaging setup shown in Figure 1. Geppert-Kleinrath says, “This technique produces an image of the 70-micron hotspot in the experimental setup from a distance of 30 meters away. This hotspot is equal to the thickness of a human hair. The images are produced by neutrons scattered during fusion. This analysis provides information on the composition of the capsule and the size and shape of the hot spot region.”


Figure 1. The neutron imaging setup is used to provide information on the size and shape of the hot spot region from a distance of approximately 30 meters away. Figure courtesy of Los Alamos National Laboratory.

Additional information on this research can be found in a recent article¹ or by contacting Geppert-Kleinrath at geppert@lanl.gov.

REFERENCE
1. Zylstra, A., Hurricane, O., Callahan, D., Kritcher, A., Ralph, J., Robey, H., Ross, J., Young, C., Baker, K., Casey, D., D ppner, T., Divol, L., Hohenberger, M., Pape, S., Pak, A., Patel, P., Tommasini, S., Ali, S., Amendt, A., Atherton, L., Bachmann, B., Bailey, D., Benedetti, L, Hopkins, L, Betti, R., Bhandarkar, S., Biener, J., Bionta, R., Birge, N., Bond, E., Bradley, D., Braun, T., Briggs, T., Bruhn, M., Celliers , P., Chang, B., Chapman, T., Chen, H., Choate, C., Christopherson, A., Clark, D., Crippen, J., Dewald, E., Dittrich, T., Edwards, M., Farmer, W., Field, J., Fittinghoff, D., Frenje, J., Gaffney, J. Johnson, M., Glenzer, S., Grim, G., Haan, S., Hahn, K., Hall, G., Hammel, B., Harte, J., Hartouri, E., Heebner, J., Hernandez, V., Hermann, M., Hinkel, D., Ho, D., Holder, J., Hsing, W., Huang, H., Humbird, K., Izumi, N., Jarrott, L., Jeet, J., Jones, O., Kerbel, G., Kerr, S., Khan, S., Kilkenny, J., Kim, Y., Kleinrath, H., Kleinrath, V., Kong, C., Koning, J., Kroll, J., Kruse, M., Kustowski, B., Landen, O., Langer, S., Larson, D., Lemos, N., Lindl, J., Ma, T., MacDomnald, M., MacGowan, B., Mackinnon, A., MacLaren, S., MacPhee, A., Marinak, M., Mariscal, D., Marley, E., Masse, L, Meaney, K., Meezan, N., Michel, P., Millot, M., Milovich, J., Moody, J., Moore, A., Morton, J., Murphy, T., Newman, K, Nicola, J., Nikroo, A., Nora, R., Patel, M., Pelz, L., Peterson, J., Ping, Y., Pollock, B., Ratledge, M., Rice, N., Rinderknecht, H., Rosen, M., Rubery, M., Salmonson, J., Sater, J., Schiaffino, S., Schlossberg, D., Schneider, M., Schroeder, C., Scott, H., Sepke, S., Sequoia, K., Sherlock, M., Shin, S., Smalyuk, V., Spears, B., Springer, P., Stadermann, M., Stoupin, S., Strozzi, D., Suter, L, Thomas, C., Town, R., Tubman, E., Trosseille, C., Volegov, P., Weber, C., Widmann, K., Wild, C., Wilde, C., Wonterghem, B., Woods, D., Woodworth, B., Yamaguchi, M., Yang, S. and Zimmerman, G. (2022), “Burning plasma achieved in inertial fusion,” Science, 601, pp. 542-548.