Researchers at Sandia National Laboratories’ Z pulsed-power accelerator have produced a significant output of fusion neutrons, using a method fully functioning for only little more than a year. The experimental work is described in a paper to be published 24 September in Physical Review Letters online. A companion theoretical paper helps explain why the experimental method worked. The combined work demonstrates the viability of the novel approach.
Sandia senior manager Dan Sinars expects the MagLIF (Magnetized Liner Inertial Fusion) technique will be a key piece of Sandia’s submission for a July 2015 National Nuclear Security Administration review of the national Inertial Confinement Fusion Program.
Inertial confinement fusion creates nanosecond bursts of neutrons, ideal for creating data to plug into supercomputer codes that test the safety, security and effectiveness of the US nuclear stockpile. The method could be useful as a future energy source if the individual fusion pulses can be sequenced like the firing of an automobile’s cylinders.
Pulsed-power accelerators, such as the Z facility, generate large MA-scale currents, and thereby produce strong magnetic fields at small radius, in order to achieve Mbar pressures in materials. Direct magnetically-driven implosions for inertial confinement fusion (ICF) are an interesting subclass of magneto-inertial fusion (also called magnetized target fusion) approaches because the absorbed target energies and coupling efficiencies are both significantly enhanced compared with indirect radiation-driven ICF approaches.
The MagLIF (Magnetized Liner Inertial Fusion) concept has been presented as a path toward economically obtaining substantial thermonuclear fusion yields on pulsed-power accelerators via the implosion of cylindrical metal liners containing pre-magnetized and laser-preheated fuel. The first neutron-producing experiments of the concept have occurred with encouraging preliminary results.—Sefkow et al.
|MagLIF is a three step process: magnetization, laser pre-heat and compression. Graphic from a presentation (Slutz) from 2012 MagLIF Workshop on the process. Click to enlarge.|
MagLIF uses a laser to preheat hydrogen fuel, a large magnetic field to squeeze the fuel and a separate magnetic field to keep charged atomic particles from leaving the scene.
|“We are committed to shaking this [fusion] tree until either we get some good apples or a branch falls down and hits us on the head.”|
It only took the two magnetic fields and the laser, focused on a small amount of fusible deuterium (hydrogen with a neutron added to its nucleus), to produce a trillion fusion neutrons (neutrons created by the fusing of atomic nuclei). Had tritium (which carries two neutrons) been included in the fuel, scientific rule-of-thumb says that 100 times more fusion neutrons would have been released. (That is, the actual release of 1012 neutrons would be upgraded, by the more reactive nature of the fuel, to 1014 neutrons.)
Still, even with this larger output, to achieve break-even fusion—as much power out of the fuel as placed into it—100 times more neutrons (1016) would have to be produced.
The gap is sizable, but the technique is a toddler, with researchers still figuring out the simplest measures: how thick or thin key structural elements of the design should be and the relation between the three key aspects of the approach—the two magnetic fields and the laser.
The first paper, “Experimental Demonstration of Fusion-Relevant Conditions in Magnetized Liner inertial fusion,” (MagLIF) by Sandia lead authors Matt Gomez, Steve Slutz and Adam Sefkow, describes a fusion experiment remarkably simple to visualize.
|Sandia’s Z machine is the world’s most powerful and efficient laboratory radiation source. It stores up to 22 MJ of energy in capacitor banks that is compressed in both space and time into a ~100 ns, linearly rising current pulse with a peak value of up to 26 MA. This current is used to create extremely high energy density conditions.|
|Z is part of Sandia’s Pulsed Power program, which began in the 1960s.|
|Z comprises a radial series of large capacitors connected to a central vacuum chamber—10 feet in diameter and 20-feet high—by metal cables. The cables, some insulated by water and some by oil, are each as big around as a Volkswagen Beetle and 30-feet long.|
|When the accelerator fires, the powerful electrical pulses strike a target at the center of the machine. Each shot from Z carries more than 1,000 times the electricity of a lightning bolt, and is 20,000 times faster. The target is about the size of a spool of thread, and it consists of hundreds of tungsten wires, each thinner than a human hair, enclosed in a small metal container known as a hohlraum (“hollow space” in German). The hohlraum serves to maintain a uniform temperature.|
|The flow of energy through the tungsten wires dissolves them into plasma and creates a strong magnetic field that forces the exploded particles inward at extremely high velocity. The particles collide with one another along the z axis (hence the name Z machine), and the collisions produce intense radiation (2 million joules of X-ray energy) that heats the walls of the hohlraum to approximately 1.8 million degrees Celsius.|
|Time-exposure photograph of electrical flashover arcs produced over the surface of the water in the accelerator tank as a byproduct of Z operation. These flashovers are much like strokes of lightning. Source: Sandia.|
The deuterium target atoms are placed within a long thin tube called a liner. A magnetic field from two pancake-shaped (Helmholtz) coils above and below the liner creates an electromagnetic curtain that prevents charged particles, both electrons and ions, from escaping. The extraordinarily powerful magnetic field of Sandia’s Z facility then crushes the liner, forcefully shoving atoms in the container into more direct contact.
As the crushing begins, a laser beam preheats the deuterium atoms, infusing them with energy to increase their chances of fusing at the end of the implosion. (A nuclear reaction occurs when an atom’s core is combined with that of another atom, releasing large amounts of energy from a small amount of source material. That outcome is important in stockpile stewardship and, eventually, in civilian energy production.) Trapped energized particles including fusion-produced alpha particles (two neutrons, two protons) also help maintain the high temperature of the reaction.
On a future facility, trapped alpha particles would further self-heat the plasma and increase the fusion rate, a process required for break-even fusion or better.—Adam Sefkow
The actual MagLIF procedure follows this order: The Helmholtz coils are turned on for a few thousandths of a second. Within that relatively large amount of time, a 19-megaAmpere electrical pulse from Z, with its attendant huge magnetic field, fires for about 100 nanoseconds with a power curve that rises to a peak and then falls in intensity. Just after the 50-nanosecond mark—near the current pulse’s peak intensity—the laser, called Z-Beamlet, fires for several nanoseconds, warming the fuel.
According to the paper’s authors, the unusual arrangement of using magnetic forces both to collapse the tube and simultaneously insulate the fuel, keeping it hot, means researchers could slow down the process of creating fusion neutrons.
What had been a precipitous process using X-rays or lasers to collapse a small unmagnetized sphere at enormous velocities of 300 kilometers per second, can happen at about one-quarter speed at a relatively more “modest” 70 km/sec. (70 km/sec is about six times greater than that needed to put a satellite in orbit.)
The slower pace allows more time for fusible reactions to take place. The more benign implosion also means fewer unwanted materials from the collapsing liner mix into the fusion fuel, which would cool it and prevent fusion from occurring. By analogy, a child walking slowly in the ocean’s shallows stirs less mud than a vigorously running child.
This experiment showed that fusion will still occur if a plasma is heated by slow, rather than rapid, compression. With rapid compression, if you mix materials emitted from the tube’s restraining walls into the fuel, the fusion process won’t work; also, increased acceleration increases the growth of instabilities. A thicker can [tube] is less likely to be destroyed when contracted, which would dump unwanted material into the deuterium mix, and you also reduce instabilities, so you win twice.—Sandia senior scientist Mike Campbell
Besides the primary deuterium fusion neutron yields, the team’s measurements also found a smaller secondary deuterium-tritium neutron signal, about a hundredfold larger than what would have been expected without magnetization, providing a smoking gun for the existence of extreme magnetic fields.
The question remained whether it was indeed the secondary magnetic field that caused the 100-fold increase in this additional neutron pulse, or some other, still unknown cause. Fortunately, the pulse has a distinct nuclear signature arising from the interaction of tritium nuclei as they slow down and react with the primary deuterium fuel, and that interaction was detected by Sandia sensors.
The secondary magnetic field is the subject of the second, theoretical paper, “Understanding fuel magnetization and mix using secondary nuclear reactions in magneto-inertial fusion.” Using simulations, Sandia researchers Paul Schmit, Patrick Knapp, et al. confirmed the existence and effect of extreme magnetic fields. Their calculations showed that the tritium nuclei would be encouraged by these magnetic fields to move along tight helical paths. This confinement increased the probability of subsequently fusing with the main deuterium fuel.
This dramatically increases the probability of fusion That it happened validates a critical component of the MagLIF concept as a viable pathway forward for fusion. Our work has helped show that MagLIF experiments are already beginning to explore conditions that will be essential to achieving high yield and/or ignition in the future.—Paul Schmit
The foundation of Sandia’s MagLIF work is based on work led by Slutz. In a 2010 Physics of Plasmas article, Slutz showed that a tube enclosing preheated deuterium and tritium, crushed by the large magnetic fields of the 27-million-ampere Z machine and a secondary magnetic field, would yield slightly more energy than is inserted into it.
A later simulation, published January 2012 in Physical Review Letters by Slutz and Sandia researcher Roger Vesey, showed that a more powerful accelerator generating 60 million amperes or more could reach “high-gain” fusion conditions, where the fusion energy released exceeds by more than 1,000 times the energy supplied to the fuel.
An open access paper by Sefkow et al., published July 24, in Physics of Plasmas, further explicated and designed the experiments based on predictions made in Slutz’s earlier paper.
But, said Campbell, “there is still a long way to go.”
Slutz, S. A. and Herrmann, M. C. and Vesey, R. A. and Sefkow, A. B. and Sinars, D. B. and Rovang, D. C. and Peterson, K. J. and Cuneo, M. E. (2010) “Pulsed-power-driven cylindrical liner implosions of laser preheated fuel magnetized with an axial field,” Physics of Plasmas 17, 056303 doi: 10.1063/1.3333505
Stephen A. Slutz and Roger A. Vesey (2012) “High-Gain Magnetized Inertial Fusion,” Phys. Rev. Lett. 108, 025003 doi: 10.1103/PhysRevLett.108.025003
A. B. Sefkow, S. A. Slutz, J. M. Koning, M. M. Marinak, K. J. Peterson, D. B. Sinars and R. A. Vesey (2014) “Design of magnetized liner inertial fusion experiments using the Z facility,” Phys. Plasmas 21 , 072711 doi: 10.1063/1.4890298