A quasi-isodynamic stellarator with poloidally closed contours of the magnetic field strength B (Mikhailov 2002 Nucl. Fusion 42 L23) has been obtained by an integrated physics optimization comprising MHD and neoclassical theory. For a configuration with six periods and aspect ratio approximately 12, a main result is the attainability of an essentially MHD-stable high-β (β ≈ 0.085) plasma with low neoclassical transport, approximately vanishing bootstrap current in the long-mean-free-path regime and excellent α-particle confinement.

International audienceThis work presents the physics design for a simple quasi-axially symmetric stellarator. A plasma configuration described by a modest number of Fourier coefficients was found to establish this symmetry with good accuracy. The low rotational transform results in a relatively simple coil set exhibiting low curvatures and comfortable clearance between adjacent coils. As another consequence, the maximum achievable plasma pressure will be limited to about 0.5%. An experiment along the lines proposed would allow an exploration of the confinement properties of a quasi-axially symmetric configuration

Key physics issues in the design of a high-β quasi-axisymmetric stellarator configuration are discussed. The goal of the design study is a compact stellarator configuration with aspect ratio comparable to that of tokamaks and good transport and stability properties. Quasiaxisymmetry has been used to provide good drift trajectories. Ballooning stabilization has been accomplished by strong axisymmetric shaping, yielding a stellarator configuration whose core is in the second stability regime for ballooning modes. A combination of externally generated shear and non-axisymmetric corrugation of the plasma boundary provides stability to external kink modes even in the absence of a conducting wall. The resulting configuration is also found to be robustly stable to vertical modes, increasing the freedom to perform axisymmetric shaping. Stability to neoclassical tearing modes is conferred by a monotonically increasing ι profile. A gyrokinetic δf code has been used to confirm the adequacy of the neoclassical confinement. Neutral beam losses have been evaluated with Monte Carlo codes.

New experimental observations of the plasma potential using the heavy ion beam probe diagnostic are presented together with a theoretical description of the formation of the electric field E r in the T-10 circular tokamak (B 0 = 1.5-2.5 T, R = 1.5 m, a = 0.3 m). Ohmically heated (OH) deuterium plasmas with main plasma parameters ne = (0.6-4.7) × 10 19 m −3 , T e (0) < 1.3 keV, T i (0) < 0.6 keV are characterized by a negative potential ϕ(ρ) with maximum negative values of ϕ(6 cm) = −1400 V with respect to the wall. The potential profile monotonically increases towards the plasma edge. A density rise due to gas puff is accompanied by a plasma potential that becomes increasingly negative. When the density approaches values in the range ne = (2.5-3.5) × 10 19 m −3 , the value of the plasma potential saturates, while the energy confinement time still increases up to a saturation value that is obtained at a slightly higher density. With auxiliary heating by electron cyclotron resonance heating (ECRH) up to 1.2 MW, T e (0) increases (up to 3 keV) and the absolute value of the plasma potential decreases. In some cases the plasma potential changes its sign and becomes positive at the edge. The radial profile of E r and its dependence on n e and T i are qualitatively explained by a neoclassical model in the core, and a turbulent dynamic model (Braginskij magnetohydrodynamic equations) in the edge.

5291 Integrated physics optimization of a quasi isody namic (qi) stellarator with poloidally closed contours of the magnetic field strength has led to a promising high β (〈β〉 ~ 0.1) stellarator configuration, [1]. In this context it is worthwhile to investigate whether the high β plasma state can be reached from a low β plasma state without major configurational changes. The demonstration of this useful property is the pur pose of this brief communication.To this end, the following procedure is adopted. The high β configuration is completely specified by its boundary shape and its pressure profile; there is no net toroidal current on its magnetic surfaces since the bootstrap current has been optimized to be negligible (for the relation between quasi isodynamicity and bootstrap current see, e.g., the Appendix). To find a corresponding low β equilibrium, a small β has been selected (〈β〉 ~ 0.025) and a re optimization has been undertaken to establish similar plasma properties as in the high β case. The result is that such properties are obtained at a boundary shape close to that of the high β case which proves the assertion.The main objective of the re optimization has been to establish a low β equilibrium which avoids low order rational values of rotational transform (i.e., in the situation considered here, ι = 6/6 and ι = 6/7) and, simultaneously, avoidance of these values for the same boundary considered as a vacuum field flux surface.In the following, the result of this re optimization is described in some detail. Figure 1 shows a compari son of the two plasma boundaries. The main differ ences are slightly stronger indentation and slightly less triangularity of the high β case. The rotational trans form profiles are seen in Fig. 2 and show that plasma boundaries quite close to each other keep the rota tional transform in the interval [6/7, 6/6] in the whole 1 The article was translated by the authors. range of β values considered, 0 < 〈β〉 < 0.09. The phys ical situation underlying this property can be seen in Fig. 3. Although the aspect ratio of the configuration is ≈12, the toroidal effect seen in the strength of B, namely B 1, 0 /B 0, 0 , and in the Jacobian , namely / , expressed in magnetic coordinates corresponds to A > 30 and A > 40, respectively.So, it remains to be shown that the MHD stability and the neoclassical properties of the low β case are favorable, too. As to the MHD stability, local balloon ing stability holds at half the normalized toroidal flux, which has to be compared to the slightly unstable situ ation in the high β case (see Fig. 11 of [1]). As for the neoclassical properties, the structural characteristics of the magnetic field strength of the high β case are well preserved. The contours of B on magnetic surfaces are poloidally closed (see Fig. 4). The surfaces of con stant B = const (see Fig. 5) do no longer enclose a true local minimum of B (see Fig. 4 of [1]) but except for a small fraction of very deeply reflected particles, most reflected particles see concave surfaces of B = const...