Experimental investigation and validation of neutral beam current drive for ITER through ITPA Joint Experiments

Joint experiments investigating the off-axis neutral beam current drive (NBCD) capability to be utilized for advanced operation scenario development in ITER were conducted in four tokamaks (ASDEX Upgrade (AUG), DIII-D, JT-60U and MAST) through the international tokamak physics activity (ITPA). The following results were obtained in the joint experiments, where the toroidal field, B t, covered 0.4-3.7 T, the plasma current, Ip, 0.5-1.2 MA, and the beam energy, Eb, 65-350 keV. A current profile broadened by off-axis NBCD was observed in MAST. In DIII-D and JT-60U, the NB driven current profile has been evaluated using motional Stark effect diagnostics and good agreement between the measured and calculated NB driven current profile was observed. In AUG (at low δ ∼ 0.2) and DIII-D, introduction of a fast-ion diffusion coefficient of Db ∼ 0.3-0.5 m2 s-1 in the calculation gave better agreement at high heating power (5 MW and 7.2 MW, respectively), suggesting anomalous transport of fast ions by turbulence. It was found through these ITPA joint experiments that NBCD related physics quantities reasonably agree with calculations (with Db = 0-0.5 m2 s-1) in all devices when there is no magnetohydrodynamic (MHD) activity except ELMs. Proximity of measured off-axis beam driven current to the corresponding calculation with Db = 0 has been discussed for ITER in terms of a theoretically predicted scaling of fast-ion diffusion that depends on Eb/Te for electrostatic turbulence or βt for electromagnetic turbulence. © 2011 IAEA, Vienna

Some Theoretical Problems of Magnetic Diagnostics in Tokamaks and Stellarators

"The main problem of magnetic diagnostics is discussed here: which plasma characteristics can be determined from magnetic measurements in tokamaks and stellarators. The reasons are elucidated why diamagnetic measurements are reliable and easily interpreted. We discuss also the capabilities of diagnostics based on the measurements of poloidal fields outside the plasma. This article is based on a lecture delivered at the Third International School on Plasma Physics and Controlled Fusion, held 15-22 June 1993 at St.Petersburg - Kizhi, Russia.

Pressure-induced Shift of the Plasma in a Helical System with Ideally Conducting Wall

"The g1obal p1asma shift is calculated analytically for a he1ica1 system with an ideal wall. The derived expression for the plasma shift, incorporating both the finite-beta p1asma expansion and the opposing reaction of the nearby ideal wall, can be used for interpreting the observable high-beta equilibrium effects in LHD and other he1ical devices.

Quasisymmetry Equations for Conventional Stellarators

"General quasisymmetry condition, which demands the independence of B^2 on one of the angular Boozer coordinates, is reduced to two equations containing only geometrical characteristics and helical field of a stellarator. The analysis is performed for conventional stellarators with a planar circular axis using standard stellarator expansion. As a basis, the invariant quasisymmetry condition is used. The quasisymmetry equations for stellarators are obtained from this condition also in an invariant form. Simplified analogs of these equations are given for the case when averaged magnetic surfaces are circular shifted, in principle, in a conventional stellarator by a proper choice of two satellite harmonics of the helical field in addition to the main harmonic. Besides, there appears a restriction on the shift of magnetic surfaces. Thus, in general, the problem is closely related with that of self-consistent description of a configuration.

Magnetic Diagnostics: General Principles and the Problem of Reconstruction of Plasma Current and Pressure Profiles in Toroidal Systems

"The restrictions of the magnetic diagnostics are discussed. Being related to the integral nature of the measurable quantities, they follow from the fundamental laws of electromagnetism. A series of particular examples demonstrating the strength of these restrictions is given and analyzed. A general rule is emphasized that the information obtained from external magnetic measurements is obviously insufficient for the reliable evaluation of plasma current and pressure profiles in tokamaks or in stellarators. The underlying reason is that outside the plasma the own field of the equilibrium plasma currents is determined by the boundary conditions on the plasma surface only.

Effect of Satellite Helical Harmonics on the Stellarator Configuration

"We discuss the problem of self-consistent analytical description of stellarators with a helical field which has, besides the main harmonic, two nearest satellites with the same period in the toroidal angle. The expression for the flux function explicitly incorporating the effect of such satellites on the shape of magnetic surfaces is obtained. It is shown that they produce the shift of magnetic surfaces. The expression for this shift is derived. Two problems are considered where the behavior of B^2 on a magnetic surface is important: Pfirsch-Schluter current at the presence of satellites and possibility of fulfillment of quasisymmetry condition (B^2 on a magnetic surface does not depend on one of the angular Boozer coordinates) at least at a single magnetic surface. It is shown that effect of satellite harmonics on the magnitude to Pfirsch-Schluter current turns out to be much smaller than it was predicted earlier on the basis of simplified model where the shift related with satellites was not taken into account. It is shown that quasisymmetry condition in a configuration with two satellites can be fulfilled only in linear approximation in helical field. The analysis is performed for conventional stellarators with planar circular axis making use of stellarator expansion.

Ideal and Conventional Feedback Systems for RWM Suppression

"Feedback suppression of resistive wall modes (RWM) is studied analytically using a model based on a standard cylindrical approximation. Two feedback systems are compared: \u27ideal\u27, creating only the field necessary for RWM suppression, and \u27conventional\u27, like that used in the DIII-D tokamak and considered as a candidate for ITER. The widespread opinion that the feedback with poloidal sensors is better than that with radial sensors is discussed. It is shown that the \u27conventional\u27 feedback with radial sensors can be effective only in a limited range, while using the input signal from internal poloidal sensors allows easy fulfilment of the stability criterion. This is a property of the \u27conventional\u27 feedback, but the \u27ideal\u27 feedback would stabilise RWM in both cases.