A multi-machine study has been carried out to investigate the impact of a strongly bounded wave propagation domain on the Lower Hybrid current drive, a condition which occurs principally in high aspect ratio tokamaks. In this regime, the condition of kinetic resonance can be far above the upper boundary of the propagation domain, and may not be achieved by the usual toroidal upshift. Therefore no tail of fast electrons can be pulled out from the thermal bulk. Nevertheless, while tokamak plasmas are in principle almost transparent to the wave in this regime so-called “unbridgeable spectral gap”, full current drive is well achieved for the two tokamaks considered in this study, TRIAM-1M (Zushi et al. Nucl Fusion 43:1600, 2003) and WEST (Bourdelle et al. Nucl Fusion 55:063017, 2015), both characterized by a very large aspect ratio $$R/a>5.5$$ . The case of the high aspect ratio tokamak HL-2A (Liu et al. Nucl Fusion 45:S239, 2005) for which the wave propagation domain has also an upper boundary, but close to the resonance condition, is considered by comparison. First principles modeling of the rf-driven current and the fast electron bremsstrahlung using the ALOHA/C3PO/LUKE/R5-X2 chain of codes shows unambiguously that the spectral gap must be already filled at the separatrix in order to reproduce quantitatively observations and some important parametric dependencies. This result is an important milestone in the physics understanding of the Lower Hybrid current drive, highlighting the existence of a powerful and likely universal alternative mechanism to bridge the spectral gap, that is not related to toroidal magnetic refraction. With an initially broad power spectrum, lobes with low parallel refractive indexes that carry most of the plasma current can be absorbed in almost single pass, restoring the full validity of the ray-tracing approximation for describing the propagation of the Lower Hybrid wave in cold plasmas.

中文翻译：

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高纵横比托卡马克的低混合电流驱动

已进行多机研究以研究强有界波传播域对下混合电流驱动的影响，这种情况主要发生在高纵横比托卡马克中。在这种情况下，动力学共振条件可以远高于传播域的上限，并且可能无法通过通常的环形升档来实现。因此，不能从热体中拉出快电子的尾部。尽管如此，虽然托卡马克等离子体原则上对这种所谓的“不可桥接光谱间隙”的波几乎是透明的，但本研究中考虑的两个托卡马克，TRIAM-1M（Zushi 等人，Nucl Fusion 43:1600, 2003) 和 WEST (Bourdelle et al. Nucl Fusion 55:063017, 2015)，两者都具有非常大的纵横比 $$R/a>5.5$$ 。高纵横比托卡马克 HL-2A (Liu et al. Nucl Fusion 45:S239, 2005) 的情况，其波传播域也有一个上限，但接近共振条件，通过比较来考虑。使用 ALOHA/C3PO/LUKE/R5-X2 代码链对 rf 驱动电流和快速电子轫致辐射进行的第一原理建模清楚地表明，必须已经在分色线上填充光谱间隙，以便定量地再现观察结果和一些重要的参数依赖。这一结果是对下混合电流驱动的物理学理解的一个重要里程碑，突出了一种强大且可能通用的替代机制的存在，以弥合光谱间隙，与环形磁折射无关。最初具有宽广的功率谱，