• Book : 431()
• Pub. Date : 2025
• Page : pp.113727
• Keyword :

• Book : 431()
• Pub. Date : 2025
• Page : pp.113726
• Keyword :

• Book : 604()
• Pub. Date : 2025
• Page : pp.155470
• Keyword :

• Book : 431()
• Pub. Date : 2025
• Page : pp.113720
• Keyword :

• Book : 227()
• Pub. Date : 2025
• Page : pp.112363
• Keyword :

• Book : 178()
• Pub. Date : 2025
• Page : pp.105520
• Keyword :

• Book : 178()
• Pub. Date : 2025
• Page : pp.105492
• Keyword :

• Book : 227()
• Pub. Date : 2025
• Page : pp.112385
• Keyword :

Abstract

Thanks to the tail of fast electrons generated by the lower hybrid (LH) waves, the plasma conductivity changes significantly with respect to the ohmic heating status (Fisch 1985 Phys. Fluids28 245). In this paper, the synergy effects of lower hybrid current drive (LHCD) in the presence of a DC electric field (namely, the loop voltage Vloop ≠ 0) on the EAST tokamak are studied numerically using the ray tracing and Fokker-Planck tools of C3PO/LUKE. In addition to the normal case of a positive loop voltage with an asymmetric power spectrum (namely, co-current LHCD), the synergy effects of negative loop voltage with an asymmetric power spectrum (namely, counter LHCD), and positive/negative loop voltage with a symmetric power spectrum are also investigated, which are previously rarely reported. It is found that the total plasma conductivity can be increased by a factor of 1.68-2.18 (depending on the value of Vloop) when a 1.1 MW LH power at 2.45 GHz with a symmetric power spectrum is injected. Furthermore, unlike the asymmetric power spectrum, the direction of the sum of the pure LH current (Ilh0) and the synergetic current (Iad) resulting from the variation in plasma conductivity is always in line with the ohmic current (Ioh) for a symmetric power spectrum, which is favorable for AC tokamak operation. These simulated results are helpful for LHCD applications, especially in current ramp-up and AC operation.

• Book : 65(1)
• Pub. Date : 2025
• Page : pp.016016
• Keyword :

Abstract

The neoclassical ambipolarity condition governing the radial electric field in stellarators can have several solutions, and sudden transitions (in radius) between these can then take place. The radial position and structure of such a transition cannot be determined from local transport theory, and instead a non-rigorous model based on a diffusion equation for the electric field is usually employed for this purpose (Turkin et al 2011 Phys. Plasmas18 022505). We compare global (full plasma volume) drift-kinetic simulations of neoclassical transport in the Wendelstein 7-X stellarator with this model and find significant discrepancies. The position r0 of the transition is not predicted correctly by the diffusion model, but the radial structure of the transition layer is in reasonable agreement if the diffusion coefficient is chosen appropriately. In particular, it should depend on the plasma temperature in the same way as the plateau-regime coefficient of neoclassical transport theory or the gyro-Bohm diffusion coefficient. In the small-gyroradius limit, the prediction of r0 by the diffusion model simplifies to the so-called Maxwell construction (Shaing 1984 Phys. Fluids27 1567-9; Shaing 1984 Phys. Fluids27 1924-6). However, this property also emerges from a wide range of other mathematical models in the appropriate limit. The basic assumption underlying these models is that the diffusion, or generalisations thereof, is independent of the radial electric field, which is however unlikely to be the case in practice. Presumably this fact explains the discrepancy between the diffusion model and the drift-kinetic simulations. Finally, it is found that global simulations replicate the phenomenon of spontaneous root transitions driven by variations in the electron-to-ion temperature ratio, as predicted by local theory in the small-gyroradius limit.

• Book : 65(1)
• Pub. Date : 2025
• Page : pp.016019
• Keyword :