Dependence of alpha-particle-driven Alfven eigenmode linear stability on device magnetic field strength and consequences for next-generation tokamaks

Recently-proposed tokamak concepts use magnetic fields up to 12 T, far higher than in conven- tional devices, to reduce size and cost. Theoretical and computational study of trends in plasma behavior with increasing field strength is critical to such proposed devices. This paper considers trends in Alfven eigenmode (AE) stability. Energetic particles, including alphas from D-T fusion, can destabilize AEs, possibly causing loss of alpha heat and damage to the device. AEs are sensitive to device magnetic field via the field dependence of resonances, alpha particle beta, and alpha orbit width. We describe the origin and effect of these dependences analytically and by using recently- developed numerical techniques (Rodrigues et al. 2015 Nucl. Fusion 55 083003). The work suggests high-field machines where fusion-born alphas are sub-Alfvenic or nearly sub-Alfvenic may partially cut off AE resonances, reducing growth rates of AEs and the energy of alphas interacting with them. High-field burning plasma regimes have non-negligible alpha particle beta and AE drive, but faster slowing down time, provided by high electron density, and higher field strength reduces this drive relative to low-field machines with similar power densities. The toroidal mode number of the most unstable modes will tend to be higher in high magnetic field devices. The work suggests that high magnetic field devices have unique, and potentially advantageous, AE instability properties at both low and high densities.

近年来提出的托卡马克（tokamak）装置概念采用最高可达12特斯拉的磁场，其强度远高于传统装置，以此缩减装置尺寸并降低建造成本。针对磁场强度提升过程中等离子体（plasma）行为的变化趋势开展理论与计算研究，对这类新型装置至关重要。本文聚焦阿尔文本征模（Alfven eigenmode, AE）的稳定性变化趋势。包括氘氚（D-T）聚变产生的α粒子在内的高能粒子，可能会使阿尔文本征模失稳，进而引发α粒子热损失并对装置造成损坏。阿尔文本征模对装置磁场的敏感性，源于共振、α粒子β值以及α粒子轨道宽度的磁场依赖性。本文通过解析方法与新近开发的数值技术，阐述了上述依赖性的起源与影响（Rodrigues等，2015，《Nucl. Fusion》55 083003）。本研究表明，聚变产生的α粒子处于亚阿尔文速度或近似亚阿尔文速度的高磁场装置，可部分阻断阿尔文本征模的共振，降低其增长率以及与之相互作用的α粒子的能量。高磁场燃烧等离子体（burning plasma）工况下，α粒子β值与阿尔文本征模驱动项均不可忽略，但高电子密度可缩短α粒子的慢化时间；相较于功率密度相近的低磁场装置，更高的磁场强度会削弱该驱动项。高磁场装置中，最不稳定的模的环向模数通常会更高。本研究表明，高磁场装置在高低密度工况下均具备独特且可能具有优势的阿尔文本征模不稳定性特性。