Abstract: Recent advances in precise fabrication such as additive manufacturing of periodic micro- and nano-structured materials or metamaterials have led to a growing number of applications in acoustics, microwaves, photonics, mechanical engineering and biomedicine. Here we identify one possible new metamaterial application for fusion energy science and technology: Pellets of mechanical metamaterials, which extend the recent concept of hollow pellets 1 and can be used for edge-localized mode (ELM) control in H-mode plasmas. Existing LANL fabrication methods can be leveraged. 1 Z. Wang, M. A. Hoffbauer, E. M. Hollmann, Z. Sun, Y. M. Wang, N. W. Eidietis, J. Hu, R. Maingi, J. E. Menard and X. Q. Xu, Nucl. Fusion 59 (2019) 086024.
Synopsis: Mass injection is one of the promising technologies proposed for edge localized modes (ELMs) control in H-mode plasmas in tokamaks 1. Hollow pellet injection was described recently as an attractive material delivery method for ELMs control 2. Compared to a solid sphere, a hollow pellet has a similar spherical outer shell structure but without the center core. The hollow shell design is driven by two competing factors: (a.) The ELM triggering thresholds derived from experiments and simulations require that a pellet needs to be sufficiently large to trigger ELMs. In EAST tokamak, for example, a lithium pellet can’t be smaller than 500-800 microns. The optimal pellet size also depends on material properties such as ablation energy per atom, as well as the pellet injection velocity, see Figure 1. The ELM triggering thresholds for most materials including boron are yet to be characterized experimentally. The minimum pellet size is equivalent to the minimum amount mass deposition near the H-mode pedestal. (b.) Minimizing impurity contamination of the plasma core due to pellet pacing requires that only a very small amount of materials gets inside the tokamak separatrix and the H-mode pedestal. For example, low-Z atoms such as hydrogen can’t exceed 5% of the fusion ion population. High-Z atoms such as tungsten can’t exceed 2.5x10-5 of the fusion ion population. In repetitive injection scenarios for steady-state operation, the impurity contamination per pellet can be even lower. Hollow pellets can satisfy both conditions simultaneously by careful selections of the shell thicknesses and radius. An example of the minimum shell thickness as a function of injection velocity is shown in Figure1.
Here we extend the hollow pellet injection concept to include metamaterials such as metamaterial hollow shells. In addition to similar advantages to hollow pellet structure for ELM controls as summarized above, mechanical metamaterials have demonstrated superior mechanical strength to conventional isotropic materials. Such a mechanical strength would be important to acceleration of a hollow pellet of a conventional or a metamaterial structure, when an order of 1 km/s or higher final speeds may be needed, and there is one potential concern that a hollow shell may be too fragile. The superior mechanical properties of mechanical metamaterials also come with the new designs from sub-microns to nano-meter length scales, and fabrication techniques such as additive manufacturing. Below we highlight one new metamaterial application for fusion energy science and technology. Additional applications are possible, and was recently reported elsewhere 3.
Mechanical metamaterials offer a new way to optimize the charge-to-mass ratio for macroscopic objects. The velocity of a charged object through electrostatic acceleration is proportional to the product of (Q/M)1/2Vb1/2, with Q/M being the charge-to-mass ratio and Vb the voltage used for the acceleration . For conventional solid materials, Q/M decreases with the radius of a sphere as 1/R. Electrostatic acceleration is most effective only for small objects ~10 m or less in order to achieve km/s velocities without using very long accelerators (Vb is limited to ~ 10 MV/m to avoid breakdowns. In practice, the actual voltage is at least a factor of 10 less) Mechanical metamaterial allows us to use small structures (~ 10 m or less) as building blocks to construct large objects. As a result, effective mass density can reduce as a function of the object size. In Figure 2, we give two examples of terminal velocity increase. The examples assume an exponential density and size dependence, with exponents of -1/4 and -1/2 respectively. There is no difficult to extend the results to other materials and reduction in effective density.
Leveraging existing metamaterial fabrication, characterization capabilities at LANL, one example is shown in Figure 3, LANL and collaborators plan to demonstrate and validate the new applications of metamaterials to fusion energy sciences. Collaborations with materials and fabrication community are ushering in a new era of innovative interdisciplinary research, supplying new solutions to long-standing problems in plasma-material interactions encountered in the ITER and NIF class of devices, as well as motivating new designs in fusion energy science and technology.
1 Z. Wang et al 2008 AIP Conf. Proc. 1041 135; Mansfield D.K. et al 2013 Nucl. Fusion 53 113023; Baylor L.R. et al 2013 Phys. Rev. Lett. 110 245001; 2 Z. Wang, M. A. Hoffbauer, E. M. Hollmann, Z. Sun, Y. M. Wang, N. W. Eidietis, J. Hu, R. Maingi, J. E. Menard and X. Q. Xu, Nucl. Fusion 59 (2019) 086024. 3 K. Rustomji et al, Sci. Rep. 8 (2019) 5841;  Z. Wang and J. L. Kline, Appl. Phys. Lett. 83 (2003) 1662;  B. Branch, D.M. Dattelbaum, et al., J. Appl. Phys. 121 (13), 135102 (2017).
|Country or International Organization||United States|
|Affiliation||Los Alamos National Laboratory|