An optimum plasma pressure/current density profile and corresponding heating/current drive (H/CD) determination scheme is newly developed by integrating equilibrium, stability, confinement, and H/CD, self-consistently subject to maximize the fusion gain for Korean fusion demonstration reactor (K-DEMO) steady-state operation scenarios. The integrated plasma modeling package, FASTRAN/IPS, is adopted for the integrated numerical apparatus. The target pressure profile with a pedestal structure is investigated by varying its peaking, pedestal height and width as a first step. Formation of stable equilibria is evaluated by solving the Grad–Shafranov equation and checking linear MHD stability. For the case of potentially stable equilibrium, required external heating distribution is calculated by considering both power balance and external current drive alignment to reproduce the pressure profile of the stable equilibrium. Electron/ion temperature and poloidal flux evolutions are solved with the derived heating configuration to find a steady-state scenario and achieve self-consistent plasma profiles. A self-consistent target steady-state pressure and current profile parameters are proposed through designed systematic algorithm with fusion power PF  =  2070 MW, fusion gain Q  =  19.7, and normalized beta βN  =  2.8 at toroidal field BT  =  7.4 T and plasma current IP  =  15.5 MA. Feasibility of fusion power PF  =  3000 MW operation is also explored with enhanced density and temperature limit assumption.

The fast wave branch in lower hybrid resonance frequency range, especially higher than 2ωlh, has been proposed for the central or off-axis electron heating and current drive in higher density plasmas than the slow wave scheme. With a higher cutoff density for launching, efficient coupling between the antenna and plasma would be a priority issue for feasibility. The fast wave coupling characteristics with the wave frequency, gap size, and electron density profile of the Versatile Experiment Spherical Torus (VEST) device are investigated using a commercial full wave FEM solver, COMSOL. Maximum coupling between combline antenna and plasma is expected to be at ∼500 MHz with n∥∼4.5. The coupled power ranges from 90% to 60% in the gap size between 0.5 cm and 1.5 cm. The relative power fraction of the fast wave is larger than 80% at these conditions. The propagation and coupling power of the fast wave is crucially dependent on the plasma density window by launching and confluence densities. Initial experimental result with low power shows that measured coupling efficiency starts to increase as electron density in front of antenna attains the level of cutoff density for the fast wave propagation. It varies from 30% to 90% with the edge density evolution, which is consistent with the coupling simulation using the measured edge density profile. Coupling simulation verified in this study will make it possible to predict and analyze the coupling characteristics of future lower hybrid fast wave experiments.

Experimental investigation of lower hybrid fast wave (LHFW) coupling and propagation is carried out by wave measurements utilizing magnetic probes in Versatile Experiment Spherical Torus. Coupling experiments are conducted in marginal (ne≥nconfluence) and prohibited (ne≫nconfluence) regimes for fast wave coupling. In the marginal regime of Ip = 25 kA, about two folds of n∥-upshift are observed. The propagation path of the fast wave is largely modified compared to that of full wave simulation, and more fast waves are detected inside the last-closed-flux surface despite the unsatisfied accessibility condition. Since spectral broadening is measured to be narrow and linearly proportional to coupled wave power with a high density fluctuation of about 20% in the frequency range (ω0/ωlh∼15), it is thought to be crucially correlated with wave scattering rather than parametric decay instabilities. Consequently, the spectral broadening implicates that n∥-upshift can take place via a wave scattering mechanism. In the prohibited regime of Ip = 100 kA, more enhanced coupling of up to 90% is observed with similar density fluctuation to that of the marginal regime, which is explained with intensified wave scattering due to the high dielectric constant of spherical torus plasmas. It is concluded that practical windows for propagation and coupling of LHFW are widened with the aid of n∥-upshift via wave scattering even though the accessibility condition is not satisfied right in front of the antenna.

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