The W7-X Stellarator Project

The W7-X Stellarator Project F. Wagner Max-Planck-Institut für Plasmaphysik, Garching EURATOMAssociation Design work in preparation for Europe’s next major step in stellarators as an alter native for magnetically confined plasma fusion has demonstrated that advanced devices can be optimized. Europe is following two strategic paths on its way to demonstrating a commercially viable fusion reactor for electric power generation. One path envisages energy production at the GW level in the shortest possible timescale using a self-sustaining, energy pro ducing plasma (a so-called burning plasma). To achieve this important goal, the four major fusion programmes (Europe, USA, Japan and Russia) have merged their knowledge and will jointly build and operate the International Thermonuclear Experimen tal Reactor (ITER). The device will demon strate the feasibility of large-scale fusion energy production and will address ques tions related to the physics and fusion tech nology of the burning plasma state, notably ignition and control, plasma stability in the presence of a large population of highenergy particles, α-particle losses, power exhaust, and tritium breeding. ITER will be a tokamak since this configuration is at the most advanced stage of development and provides the most credible basis for extra polating to the size and requirements of a burning plasma. Although the ITER line has the best prospects for demonstrating and exploring a burning plasma it nevertheless admits to drawbacks with respect to an economically viable fusion reactor. Being founded on existing data, ITER represents the highcurrent tokamak line which is unlikely to lead to a reactor operating in the steadystate. Moreover, its plasma is easily dis rupted (i.e., prone to sudden current quenches) and performance peaks in the high-current corner of operational space. So ITER will be forced to walk along a brink: any loss of equilibrium will incur a cur rent quench within a few milliseconds ac companied by mechanical and electrical Friedrich Wagner is responsible for the experimental aspects of work on the W7-AS stellarator at the MaxPlanck-lnstitut fur Plasmaphysik (IPP), BoltzmannStrasse 2, D-85748 Garching. He is a member of the IPP Directorate and has been an Honorary Professor at the TU Munich since 1993. Professor Wagner graduated in 1970 from TU Munich where he was awarded a Ph.D. in low-temperature physics in 1972. He joined the IPP in 1975 after spending two years at Ohio State University, USA, becoming the project head of ASDEX in 1986 and a Fellow of the MaxPlanck Gesellschaft in 1988. hazards. The device will be designed to withstand disruptions, but their frequency has to be limited. Concept Improvement Tokamaks The deficiencies of the ITER line — diffi culty of steady-state operation, the stresses accompanying disruption of the plasma, and the large size of a high-current device — are being tackled in Europe’s second path, which is called concept improvement. Existing concepts satisfy the physics requi rements and provide some of the techno logy for the most urgent next step, namely realization of a burning plasma. But im provements are needed and in the case of tokamaks, they should lead to the so-called Advanced Tokamak offering superior con finement and stability in smaller, low-current devices. Rigourous optimization of the mag netic and kinetic ingredients in turbulent transport and of magneto-hydrodynamic stability is essential. Unfortunately, many critical aspects can only be assessed using empirical data. A self-sustaining bootstrap current is of utmost importance in tokamaks. Such devices are based on a strong toroidal current flowing within the plasma which provides a pololdal field component super imposed on the toroidal one to establish a rotational transform, where magnetic field lines twist and wind helically to embrace the torus. The current is called a bootstrap current because it flows without external excitation, being generated by pressure dif ferences within the plasma and not, as is usually the case, by induction as in a trans former. The strategy for optimizing advan ced tokamaks is centred on a large boot strap current. If the fraction of the total current provided by the bootstrap current is large it might be possible to design a steady-state, low-current tokamak that re quires a moderate external current drive (such a current is created by Injecting either particles or high-frequency waves). Fig. 1 — The outermost flux surface of the W7-X stellarator with a band of field lines and some of the modular coils. The W7-X’s five toroidal field periods will maintain a plasma with obvious poloidal field components and a strong indentation such that the plasma is bean shaped at the corners of the pentagon. The rotational transform of W7-X is chosen to be about 1 as this value allows the safe realization of maximal magnetic shear. The major radius of the device will be 5.5 m at an aspect ratio of 10. As stellarators are made for steady-state operation, W7-X will have superconducting coils. The maximum field is 3 T and is matched to the present electron cyclotron heating frequency of 140 GHz. Europhys. News 26 (1995) 3 Stellarators Stellarators represent Europe’s second approach for confining toroidal plasmas. They aim to do this using magnetic fields generated by currents lying outside the plasma region. They share with the tokamak the basic concept of nested magnetic sur faces for achieving confinement, but a net toroidal plasma current is not required. Such devices are especially attractive since they provide intrinsic steady-state operation and are inherently disruption-free. However, the classical stellarator based on Lyman Spitzer’s original design published in 1958 also requires concept improvement to qua lify as a commercially viable reactor. The basic element of the stellarator coil system is a helical coil which embraces the toroidal plasma. The suitability of helical systems for reactors has been questioned from the technical point of view since they are difficult to fabricate and are operated close to the technical limits: a modular design based on a set of single coils is therefore essential. The modular coils inevi tably have to be non-planar because they also provide the poloidal field component. Such an approach would facilitate optimiza tion, thereby removing one of the major drawbacks of the classical stellarator. Stellarators (cover illustration) form part of the concept improvement programme and several major issues are being tackled. The plasma cross-section in a classical stellarator has a larger aspect ratio than for tokamaks, and the twist angle of rotational transform in the plasma core Is generally smaller. As a result, the equilibrium of the nested flux surfaces suffers from a large outward shift of the core region (the Shafranov shift) and operation is limited to low values of the ratio β of the plasma to mag netic field energy. Indeed, stellarators (...truncated)