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This paper describes a number of deleterious interactions between radio-frequency ( rf ) waves and the boundary plasma in fusion experiments. These effects can lead to parasitic power dissipation, reduced heating efficiency, formation of hot spots at material boundaries, sputtering and self-sputtering, and arcing in the antenna structure. Minimizing these interactions is important to the success of rf heating, especially in future experiments with long-pulse or steady-state operation, higher power density, and high-Z divertor and walls. These interactions will be discussed with experimental examples. Finally, the present state of modeling and future plans will be summarized.

The two-dimensional fluid turbulence code SOLT is employed to study the role of midplane turbulence on the scrape-off-layer (SOL) heat flux width of tokamak plasmas. The physics simulated includes curvature-driven-interchange modes, sheath losses, and perpendicular turbulent diffusive and convective (blob) transport. Midplane SOL profiles of density, temperature and parallel heat flux are obtained from the simulation and compared with experimental results from the National Spherical Torus Experiment (NSTX) to study the scaling of the heat flux width with power and plasma current. It is concluded that midplane turbulence is the main contributor to the SOL width for the low power ELM-free H-mode discharges studied, while additional physics is required to explain the plasma current scaling of the SOL width observed experimentally in higher power discharges. Additional simulations predict a transition to a convectively-dominated SOL at critical values of power and connection length.

Reciprocating Langmuir probes are used to investigate the structure of electrostatic potentials, E x B drifts, and fluctuations in the edge (ρ < 1), scrape-off layer (SOL) and divertor of single null diverted discharges in the DIII-D tokamak. These measurements demonstrate that the X-point geometry suppresses potential fluctuations in the drift wave range of frequencies (20 kHz ≤f≤ 300 kHz) as predicted by theory [N. Mattor and R.N. Cohen, Phys. Plasmas 2 (1995) 4042], and suppresses quasi-coherent modes in the edge of H-modes. Consequently, edge turbulence and ballooning mode stability calculations, such as those used in L-H transition and H mode pedestal theories, must incorporate realistic X-point magnetic geometry to quantitatively reproduce experimental results. In the divertor plasma, root-mean-square potential fluctuations o fl . are found to be larger in H mode than in L mode, and in detached versus attached divertor plasmas. These measurements have been used to benchmark turbulence simulations with two unique codes that incorporate realistic X-point magnetic geometry: the 3-D boundary turbulence code BOUT and the BAL shooting code with high-n ballooning formalism.

We simulate boundary plasma turbulence using a 3D turbulence code BOUT and a linearized electromagnetic instability shooting code BAL. The code BOUT solves fluid equations for plasma vorticity, density, ion temperature and parallel momentum (along the magnetic field), electron temperature, and parallel momentum. A realistic DIII-D X-point magnetic geometry is used. The focus is on the possible local linear instability drives and turbulence suppression mechanisms involved in the L–H transition and on the consistency of the computed turbulence with observed temperature and density profiles. Comparison is made with data from the DIII-D tokamak where probe measurements provide turbulence statistics in the boundary plasma and transport modeling.

The present work is motivated by the importance of r.f. sheaths in determining the antenna-plasma interaction and the sensitivity of the sheaths to the complicated three-dimensional structure of modern ion cyclotron range of frequency (ICRF) antennas. To analyze r.f. sheaths on the plasma facing regions of the launcher, we first calculate the contact points of the tokamak magnetic field lines on the surface of the antenna Faraday screen and nearby limiters for realistic three-dimensional magnetic flux surface and antenna geometries. Next, the r.f. voltage that can drive sheaths at the contact points is determined and used to assess the resulting sheath power dissipation, r.f.-driven sputtering, and r.f.-induced convective cells (which produce edge profile modification). The calculations are embodied in a computer code, ansat (Antenna Sheath Analysis Tool), and sample ansat runs are shown to highlight the physics and geometry-dependent characteristics of the r.f. sheaths and their relationship to the antenna design. One use of ansat is therefore as a design tool, to assess the strengths and weaknesses of a given design with respect to critical voltage handling and edge plasma interaction issues. Additionally, examples are presented where ansat has been useful in the analysis and interpretation of ICRF experiments