Predictions of improved confinement in SPARC via energetic particle turbulence stabilization

The recent progress in high-temperature superconductor technologies has led to the design and construction of SPARC, a compact tokamak device expected to reach plasma breakeven with up to 25MW of external ion cyclotron resonant heating (ICRH) power. This manuscript presents local (flux-tube) and radially global gyrokinetic GENE (Jenko et al 2000 Phys. Plasmas 7 1904) simulations for a reduced-field and current H-mode SPARC scenario showing that supra-thermal particles - generated via ICRH - strongly suppress ion-scale turbulent transport by triggering a fast ion-induced anomalous transport barrier (F-ATB). The trigger mechanism is identified as a wave- particle resonant interaction between the fast particle population and plasma micro-instabilities (Di Siena et al 2021 Phys. Rev. Lett. 125 025002). By performing a series of global simulations employing different profiles for the thermal ions, we show that the fusion gain of this SPARC scenario could be substantially enhanced by up to ∼ 80% by exploiting this fast ion stabilizing mechanism. A study is also presented to further optimize the energetic particle profiles, thus possibly leading experimentally to an even more significant fusion gain.近年来高温超导技术的进展推动了SPARC紧凑型托卡马克装置的设计与建造，该装置预计可在最高25MW外部离子回旋共振加热（ion cyclotron resonant heating, ICRH）功率下实现等离子体收支平衡。本文针对低场低电流H模SPARC运行场景，开展了局部（磁通管，flux-tube）与径向全局回旋动理学（gyrokinetic）GENE模拟（Jenko等，2000，《等离子体物理》7，1904），结果表明：通过ICRH产生的超热粒子可触发快速离子诱发的反常输运垒（fast ion-induced anomalous transport barrier, F-ATB），从而显著抑制离子尺度湍流输运。其触发机制为快粒子群体与等离子体微观不稳定性之间的波粒共振相互作用（Di Siena等，2021，《物理评论快报》125，025002）。通过针对热离子采用不同分布剖面开展一系列全局模拟，我们证实：利用该快离子稳定机制可将此SPARC场景的聚变增益提升约80%。此外本文还开展了高能粒子剖面优化研究，有望在实验中实现更显著的聚变增益提升。