1,720,970 research outputs found
Magnetic Diagnostics: General Principles and the Problem of Reconstruction of Plasma Current and Pressure Profiles in Toroidal Systems
No full text"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."research repor
Suppression of Pfirsch-schluter Current by Vertical Magnetic Field in Stellarators
No full text"This article is devoted to the problem whether it is possible to make stellarator configuration insensitive to plasma pressure using external vertical magnetic field as a control parameter. Conventional stellarators with a planar circular axis are analyzed. It is shown that for these systems the condition of Pfirsch-Schluter current suppression can be formulated as a two-dimensional equation with all values expressed through the vacuum magnetic field only. The major advantage of this formulation is that it allows to solve the problem without solving equilibrium equations. It is used as a basis to show that complete suppression of Pfirsch-Schluter current by means of the vertical field is possible, in principle, either in shear-free systems or in stellarators with l geq 3. In l=2 stellarators with a shear it is possible to get significant reduction of the current and to suppress pressure-induced plasma column shift. This can be called an integral independence on beta in contrast to the true local independence in other cases. In all cases large inward shift of plasma column is necessary. It is out of operational range in all existing devices except Heliotron E. In recent experiments in Heliotron E the state ""integrally"" independent on beta was almost achieved. The present study was stimulated by this result."research repor
Quasisymmetry Equations for Conventional Stellarators
No full text"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."research repor
Pressure-induced Shift of the Plasma in a Helical System with Ideally Conducting Wall
No full text"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."research repor
Anisotropic Pressure Effects on Plasma Equilibrium in Toroidal Systems
No full textdepartmental bulletin pape
Control of Pfirsch-Schluter Current by External Poloidal Magnetic Field in Conventional Stellarators
No full text"The problem is analyzed whether it is possible to create net-current-free equilibrium configurations without Pfirsch-Schluter current (j zeta = 0) in conventional stellarators with a planar circular axis. Recently such analysis was presented for the case, when only vertical magnetic field was a control parameter. Here external poloidal field is considered as arbitrary, which must be found from the condition j zeta=0. It is shown that such suppression of Pfirsch-Schluter current by means of external poloidal field is possible, theoretically, in shear-free systems and in stellarators with l geq 3 only. It can be said also by other words that in ordinary l=2 stellarators with a shear plasma equilibrium with j zeta = 0 is impossible at any choice of external poloidal field. In these systems, nevertheless, dipole component of Pfirsch-Schluter current can be suppressed or strongly reduced by joint effect of vertical and quadrupole fields. It is shown that in this case phase inversion of Pfirsch-Schluter current current could be obtained at realistic conditions. These results explain, in particular, experimental observations of configurations almost insensitive to plasma pressure in Heliotron E."research repor
Ideal and Conventional Feedback Systems for RWM Suppression
No full text"Feedback suppression of resistive wall modes (RWM) is studied analytically using a model based on a standard cylindrical approximation. Two feedback systems are compared: 'ideal', creating only the field necessary for RWM suppression, and 'conventional', 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 'conventional' 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 'conventional' feedback, but the 'ideal' feedback would stabilise RWM in both cases."research repor
Some Theoretical Problems of Magnetic Diagnostics in Tokamaks and Stellarators
No full text"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."research repor
Effect of Satellite Helical Harmonics on the Stellarator Configuration
No full text"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."research repor
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