Macron Formed Liner as a Practical Method for Enabling Magneto-Inertial Fusion
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- Kirtley, D. & Slough, J. J Fusion Energ (2010) 29: 561. doi:10.1007/s10894-010-9314-y
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To take advantage of the smaller scale, higher density regime of MIF an efficient and repeatable method for achieving the compressional heating required to reach fusion gain conditions is needed. The macro-particle (macron) formed liner compression of the field reversed configuration (FRC) provides such a method. The approach to be described employs an assemblage of small, gram scale, macrons to form a more massive liner that both radially and axially compresses and heats the FRC plasmoid to fusion conditions. The large liner energy (several MJ) required to compress the FRC is carried in the kinetic energy of the full array of macrons. The much smaller energy required for each individual macron is obtained by accelerating the macron to ~3 km/s which can be accomplished remotely using conventional inductive techniques. 3D numerical calculations demonstrate that macron convergence can form a coherent liner provided minimum velocity and timing accuracy is met. Experimental results have demonstrated that a cylindrical or spherical macron can be accelerated to velocity within 2 m/s and timing less than 1 microsecond. Initial testing of a 6-stage launcher yielded 280 m/s at a final coupling efficiency of greater than 40%.
KeywordsMagneto-inertial fusionMagnetic liner compressionFRC
Introduction and Methodology
To take advantage of the smaller scale, higher density regime of magneto-inertial fusion (MIF) an efficient method for achieving the compressional heating required to reach fusion gain conditions must be found. The method needs to be simple and capable of repetitive operation. The macroparticle (macron) formed liner compression of the FRC is such a method, and uniquely addresses the major challenges facing magneto-inertial fusion. Specifically, it embodies a compression scheme that can very efficiently and repetitively generate the kinetic liner energy required to reach fusion gain. It provides for both the target plasma and liner energy to be generated remote from the reactor vessel. This is critical as the reactor environment is likely to be incompatible with the specialized pulse power equipment employed in conventional liner approaches. The timescale for forming and accelerating both the FRC and liner can be much longer than the time that the energy is thermalized in the implosion. This avoids the need for the very high voltages required to produce multi-megampere compression currents. In fact the switching requirement for the macron formed liner (MFL) is well within the range of currently available solid state devices. The time and cost for testing the concept is negligible when compared to alternative approaches. The process required for target FRC formation is already routinely produced at the MSNW laboratory .
The metallic liner is also produced by merging as well, however, in this case by a cluster of small masses (macrons) that are injected into the reactor by an array of launchers, also situated outside the reactor vessel. In this way the energy required to compress the FRC can be delivered on a timescale that is much slower than the FRC compression, and produced by a source that is highly efficient as well as broadly distributed spatially. Most importantly, the liner formation system can be located outside the reactor in a manner compatible with long-term repetitive operation.
The macron launchers are initiated in such a manner that a large array of small metallic masses arrive at the central section of the reactor (see Fig. 2) and converge to form a contiguous liner at smaller radius (rL ~ 0.1 m). The FRC is then introduced within the liner by merging two translated FRCs.
The shape of the macron will be considered later, but for this discussion its radial size and mass are the relevant parameters. The scale and number of macron launchers required for the liner can be readily estimated by what was achieved in the MTF liner experiments performed at AFRL . A cylindrical aluminum shell liner of roughly 300 g mass was imploded using the 4.5 MJ Shiva Star capacitor bank. The 5 cm radius liner was compressed with an axial current of 12 MA at 84 kV and produced a final liner kinetic energy of ~1 MJ in 22 μs . Employing the macron launcher it should be possible to exceed this liner energy by a factor of three for a similar stored energy. For example, the target parameters for each launcher are a macron mass of 4 g accelerated to a velocity of 3 km/s (3 mm/μs). The kinetic energy of each macron would thus be ~20 kJ. For a 1 cm radius macron, the 30 macron launchers depicted in Fig. 1 would converge to make contact at a radius of roughly 10 cm and be well merged at 5 cm. Five such rings arranged axially would initially form a liner of length comparable to the AFRL liner, and have roughly twice the mass (0.6 kg). The liner kinetic energy, however, would total 3 MJ. And it would compress from 5 cm radius to the minimum (r < 2 mm) in less time (~15 μs). In addition to a higher kinetic energy, the macron liner can be made to converge axially thereby increasing the compressional energy even further. This is an significant advantage as the FRC equilibrium length contracts axially (l − r2/5) as it is compressed. With a fixed liner length, the efficiency of compression is greatly diminished by this effect. Without electrical contacts, the experimental apparatus and vacuum system can also be greatly simplified. Difficulties with the post implosion vacuum integrity are also much easier to avoid. A unique benefit of macron assembled liners is the ability to impart a specified amount of rotation. The rotating inner surface will be stabilized to instabilities during the final compression stages. The amount of rotation to be added to the liner must balance stability, shear, and overall energy efficiency.
It should be noted that the small aperture in the chamber wall required for macron introduction is quite small (~3 cm2), so that even with several hundred such holes, the portion of reactor wall area exposed would be less than a small fraction of a percent of the total area (<0.1% of a 3 m diameter chamber). The time from launch to the arrival of detritus from the liner implosion will be several milliseconds providing sufficient time to aperture the small ports closed. The neutron fluence to the launcher structure is also significantly reduced by being completely outside the blanket with only a small acceptance angle for exposure due to the long connection tube length through the blanket. The distributed launcher array greatly reduces the demand on the power delivery systems. With a constant acceleration of the macron to 3 km/s over a distance of 3 m, the liner energy input can occur over a time span as long as 2 ms—two orders of magnitude slower than the conventional Z pinch method. As will be seen, the energy input from each macron launcher is low enough and slow enough that it can be provided with commercial solid state electronics.
There are three key issues that must be solved for the macron formed liner concept to be validated. First, the timing accuracy and velocity spread must be able to deliver the required liner behavior. Secondly, the dynamical behavior of the liner during compression must be understood, specifically how it is influenced by non-uniformities or rotation. Finally, the optimal projectile geometry, trajectory, and number must be fully characterized. The answer to these three questions from the initial results of the 3D modeling indicates that a suitable liner can be formed. The modeling results are discussed in more detail in section “Modeling Results”. The most critical issue is to demonstrate the ability to produce the desired macron kinetic behavior in a consistent and repeatable manner. It is clear from the considerations above; the launcher must be simple, reliable and robust. It is believed that the prototype design possesses all these features. Initial testing of the launcher is described in section “Experimental Results”.
The 3-D colliding macron liner was simulated using ANSYS Multiphysics which has the capability of calculating the behavior of the colliding aluminum macrons well into the non-linear plastic deformation regime. The initial calculations were performed for macrons comprised of hollow aluminum spheres, 6.7 g in mass which were launched radially inward at 3 km/s. The spheres were observed to collide, deform, and finally form a complete, uniform liner. The entire process occurred with little diminution in the radial velocity. For the target velocity of 3 km/s the macron merging and compression is sub-sonic. The simulation was 3 dimensional and both stabilizing rotation (initiated with small non-radial velocity component) and 3-D compression (using 2–3 arrays of macron liners) were modeled. Finally, a complete treatment of the transient structural physics of the macron collision and integration was performed, including full plastic deformation stress–strain relationships, detailed surface roughness, and interaction studies.
Several aspects of the macron liner dynamics still need to be resolved. A significant issue concerns the behavior of the inner surface of the merged macron liner during field compression and stagnation. With macrons there is the possibility of a more irregular inner surface which could be prone to spiking. There are two effects that should ameliorate this tendency. As has been mentioned, it is trivial to add a large rotational motion to the inner wall by a coherent non-radial displacement of the macron initial trajectory. As can be seen in Fig. 4 this causes a rapid winding up and smearing of the inner boundary. The rotational motion is also stabilizing to the Rayleigh–Taylor modes that are the primary cause of the spiking. With a compressed megagauss magnetic field, there will also be large wall currents that would lead to rapid melting of small protuberances on the inner wall.
It should be possible to resolve the full macron liner behavior during magnetic field compression and stagnation with the same ANSYS Multiphysics code used in the initial simulations. To achieve this result, future code work will incorporate an axial magnetic field along with modifications to include material phase change and a temperature dependent resistivity. It will also require a finer mesh than was used in these calculations. With these improvements it should be possible to establish the dynamic stability and compressive efficiency of the MFL.