[en] Short communication

[en] We present a review of MHD stability issues in reversed shear configurations. This work will focus on identifying issues relevant to existing and proposed devices. The aim is to provide a framework for assessing the value of different designs. We will use available experimental data to support our conclusions where it is possible

[en] A recent analysis of plasma stability based on modifications of the extended energy principle for magnetohydrodynamic stability led to conclusions that are too optimistic. The original interpretation of this principle is indeed applicable. The present analysis demonstrates explicitly the fallacy of using the wrong functional for δW in the extended energy principle. It then shows that the original energy principle functional δW_B is also obtained for a model in which a surface mass is incorporated to provide pressure balance. This work therefore indicates, but does not prove, that the eigenfunctions that are obtained from a minimization of the extended energy principle with the proper kinetic energy norm provide a good representation of what would be achieved with an exact treatment

[en] Published in summary form only

No abstract available

[en] The present paper describes the improvements made to the conversion software for ANSYS APDL 2 FLUENT MHD Magnetic File which is able to extract the data from ANSYS APDL file and write down a file containing the magnetic field data in FLUENT magneto hydro dynamics (MHD) format. The MHD module has some features for the uniform and non uniform magnetic field but it is limited for sinusoidal or pulsed, square wave, having a fixed duty cycle of 50%. The present software, ANSYS APDL 2 FLUENT MHD Magnetic File, suffered major modifications in comparison with the last one. The most important improvement consists in a new graphical interface, which has 3D graphical interface for the input file but also for the output file. Another improvement has been made for processing time, the new version is two times faster comparing with the old one. (authors)

[en] Experiments on the maximum attainable beta values of ASDEX discharges show that the limits for this parameter lie in the range of theoretical predictions. In a previous publication we have attempted to theoretically identify the corresponding instabilities. While no significant correlation to global plasma instabilities could be found, the two-dimensional MHD equilibria calculated on the basis of ASDEX experimental parameters turned out to be close to the marginal ideal ballooning limit. In the present work we extend this investigation to resistive ballooning modes. Separatrix bounded as well as limiter controlled plasma equilibria are considered. Because of the small aspect-ratio of ASDEX (A ≅ 4) all equilibrium as well as stability calculations are performed in full toroidal geometry. After the formulation of a system of four equations describing the resistive evolution of velocity and magnetic fields in the high-m stability limit in coordinate-invariant form and its Fourier approximation in the neighbourhood of a localization field line, we solve the resulting quasi-mode equations applying methods of finite-element discretization. We find complex growth rates γ with positive real part for values of the toroidal mode number n below 100. Calculated values of Re{γ} ≤ 10^-3/τ_A s^-1 (with τ_A being the Alfven time) are small and therefore in agreement with the experimentally observed non-disruptive behaviour at the β_p- limit. Thus we believe that the characteristic signatures which govern ASDEX high-β_p discharges can be explained by resistive ballooning modes. (orig.)

[en] The stability of toroidal plasmas is often described by approximate MHD equations for a small number of scalar functions (reduced MHD equations). We propose a general representation of toroidal MHD in terms of stream-functions and potentials, which facilitates the physical interpretation of various terms and therefore is well suited for deriving different types of reduced equations. We qualitatively discuss stationary plasma states bifurcating from a given equilibrium, and we treat the particular case of a MHD-unstable cylindrical equilibrium

[en] In an earlier publication, it was shown that toroidal resistive MHD equilibria, for spatially uniform resistivity and under a standard set of assumed symmetries, do not exist. The difficulties are associated with the non-vanishing nature of curl (j x B), and disappear in the open-quotes straight cylinderclose quotes limit. Here, we inquire into whether there exist spatial dependences for the (scalar) electrical resistivity which permit axisymmetric, toroidal, zero-flow resistive steady states to exist. The question is answered in the affirmative. A differential equation derived from Ohm's law replaces the Grad-Shafranov description, and the pressure is derived from the Laplace equation obtained from taking the divergence of the equation of motion. There are some novel features to the states that result. For example, the resistivity required is not a open-quotes flux functionclose quotes -- i.e., it is not constant on a magnetic surface, thus suggesting that the temperature will not be either

[en] A discussion of a number of novel and non-standard stellarator configurations will be presented. Some of them are based of the recently considered configurations of a tokamak with inclined coils and the stellamak concept, and a few other promising configurations feature an ultra-low aspect ratio, which is very unusual for stellarators. A stellamak represents a type of a stellarator configuration, or a stellarator-tokamak hybrid configuration, which improves its properties when the tokamak features, such as plasma current, are added to the system. This is opposite to the standard stellarator configuration, where addition of the ohmic plasma current or even a small bootstrap current degrades the system, and special efforts are needed to find the configuration with the smallest bootstrap current possible. A tokamak with inclined coils gives an example of a system where the plasma current, in the direction such that current rotational transform and the vacuum rotational transform add constructively, improves the system. Another interesting configuration can be obtained by combining the standard toroidal field coils of a tokamak with an additional system of vertically inclined planar coils. Such a hybrid allows two different modes of operation, the pure stellarator and the pure tokamak modes, and successive switching between them without significant plasma loss. By keeping the plasma current in the stellarator or hybrid system we obtained a set of configurations that have a potential of combining the best properties of both, tokamaks and stellarators, to make an ohmically heated and continuously operated device. The numerical examples of the coil systems and main properties of the confining magnetic fields and plasma equilibria for each of the novel configurations considered will be presented in detail. The magnetic field tracing code, UBFIELD, the 3D MHD equilibrium code, VMEC, in its free-boundary mode, and some other standard codes are used in our calculations