For Wendelstein 7-X

MODULAR STELLARATOR REACTORS AND PLANS FOR WENDELSTEIN 7-X G. GRIEGER, C. BEIDLER, E. HARMEYER, W. LOTZ, J. KIJ3LINGER, P. MERKEL, J. NUHRENBERG, F. RAU, E. STRUMBERGER, H. WOBIG, Max Planck Institute for Plasma Physics, IPP-EURATOM Association, BoltzmannstraJJe 2, D-8046 Garching, Germany, (49) 89-3299 1781 ABSTRACT of the beneficial ones of the tokamak) the comparison is best made by concentrating on Modular stellarator reactors of the Advanced the differences between the two systems. This Stellarator type appear to have the potential to way the comparison becomes relative and is offer desirable reactor properties: i) a single thus much more conclusive than a point to NbTi coil system is sufficient; ii) steady-state point comparison between two independently operation is inherent and rests upon refuelling developed and designed reactors. and exhaust only (no current drive system is The stellarator is a concept for confining needed); iii) there is no possibility for a major toroidal plasmas with magnetic fields genera- current disruption because there is no net to- ted by currents exclusively outside the plasma roidal current; iv) reactor volume, mass and region. The plasma contributes only by reac- magnetic field energy are comparable to those tive, pressure-driven (in the case of the Ad- of tokamak reactors. The Wendelstein 7-X ex- vanced Stellarator essentially diamagnetic) periment will provide an integrated concept currents. The stellarator shares with the to- test which is needed for producing convincing kamak the basic concept of nested magnetic predictions on the properties of this reactor surfaces for achieving confinement. However, a approach. net toroidal plasma current, as needed in tokamaks, is not required in stellarators. Thus, I. INTRODUCTION stellarators without this current can be opera- ted in steady state, without disruptions, wi- With the start of construction of experi- thout request for an external current drive mental fusion reactors, like ITER, getting clo- system, and without need for more than one ser, it is now becoming urgent to check whe- single coil system for generating the confining ther after ITER the presently followed toka- magnetic field. These properties indeed are of mak approach will automatically lead to the major importance for fusion reactors. After optimum fusion reactor concept, or whether ignition, a stellarator reactor would work con- there are other, perhaps rather congenial ap- tinuously on refuelling and exhaust alone. Since proaches, which might offer decisively more a net toroidal current provides free magnetic desirable solutions. This comparison has to be energy to drive instabilities in particular of made for full-scale reactor conditions and has disruptive nature, this reservoir is minimum in to include all questions of relevance to reactor stellarators since they do not have this current. performance like unit size, power and particle The stellarator is the toroidal confinement exhaust, pulse length, power density, power system with the smallest free energy. loadings, plasma stability, technical require- In Section II, a short description of those ments, maintenance approach, costing, etc. properties of Advanced Stellarators will be gi- In this paper it will be argued that the ven which are essential for the intended com- Advanced Stellarator has indeed the potential parison of their reactor properties with those to offer a set of more desirable reactor pro- of tokamaks. Section III will define a reactor perties than the tokamak system. Since the based on the Advanced Stellarator concept. stellarator differs only in some prospects from Section IV will then provide an analysis of a tokamak (whereas in most prospects is basi- possible advantages and disadvantages of cally very similar to it and shares with it most stellarator as compared to tokamak reactors. FUSION TECHNOLOGY VOL. 21 MAY 1992 1767 Grieger et al. PLANS FOR WENDELSTEIN 7-X Section V will analyse some properties of Ad- reactor conditions. In essence these are: vanced Stellarators which - before drawing • High quality of vacuum-field magne- final conclusions - have to be checked for their tic surfaces to yield good transport compatibility with the expected reactor per- properties, i.e. avoidance of major formance. Section VI will deal with WENDEL- resonances and small thickness of is- STEIN 7-X, which is planned to provide an in- lands. tegrated concept test to yield convincing pre- • Good finite-^ equilibrium properties dictions on the properties of ignited plasmas to achieve a stiff configuration with in Advanced Stellarators. Section VII will then rising JJ at fixed coil currents. provide a short summaiy and conclusions. • Good MHD stability properties to achieve <£>> « 5%. n. SHORT DESCRIPTION OF • Small neoclassical transport under BASIC PROPERTIES OP reactor conditions. ADVANCED STELLARATORS • Small bootstrap current to minimize the ability of the plasma to affect the The basic stellarator concept was confinement configuration. Invented by Lyman Spitzer at Princeton in the • Good collisionless a-particle contain- early 1950's1. IPP's interest in this concept ment at operational values of J5. started only a few years after this date, and • Good modular coil feasibility for coils from then on both, stellarators at IPP and the providing a sufficiently large distance IPP development of the Helical Advanced between coils and plasma. Stellarator (HELIAS) concept2 have played a The achieved stiffness of the magnetic major role in achieving the goals that have configuration results from the fact that it was been essential for the viability of this toroidal possible to reduce the internal plasma cur- confinement approach. This approach has rents to values which are approaching the been described elsewhere3. Here only those pure diamagnetic current (< j||2/jl2 > « 1/2). It points will be briefly summarized which are is important to note that mutual compatibility needed as a basis for the ensuing assessment. and simultaneous achievement of all of the The HELIAS concept is a result of a stella- above criteria have been proven. The nature of rator optimization with respect to all criteria the optimization result can be characterized as which are considered indispensible for good follows: Fig. 1. Plasma and coils of Wendelstein 7-X. 1768 FUSION TECHNOLOGY VOL. 21 MAY 1992 Grieger et al. PLANS FOR WENDELSTEIN 7-X The simultaneous achievement of the TABLE I above set of criteria essentially determines the Parameters of a Helias Reactor (HSR) structure of the magnetic field strength distri- bution of the configuration, and its geometrical shape is then a consequence of Average major radius [m] 19.5 this structure. Average coil radius [m] 3.9 The resulting configuration, together with Number of coils 50 the coils producing it, is displayed in Fig. 1. It Number of field periods 5 is clearly visible that the optimization makes extensive use of non-axisymmetry. In fact, Induction on axis [T] 5.0 most of the favourable properties are not Max. induction on coils [T] 10.7 compatible with axisymmetry. The configura- Tot. magnetic energy [GJ] 70 tion is of five-fold rotational symmetry. For Rot. transform on axis 0.84 fixed constraints, four-fold symmetry is not Rot. transform on boundary 0.97 sufficient for achieving the goals quantitatively and six-fold symmetry is leading to en- Average plasma radius [m] 1.6 3 gineering difficulties in an unnecessary Plasma volume [m ] 103 fashion. As will become evident later, the optimi- Surface of first wall [m^] 2.1x103 zation according to the above criteria directly Volume of blanket [m3] 780 yields very desirable properties also for other (d=0.4 m, coverage 85%) properties which are essential for reactor Mass of blanket (p=4) [t] 3.1x103 engineering and maintenance, and also 3 promises reduction of anomalous plasma Volume of shield (d=0.6m) [m ] 1.59x103 transport. Mass of shield (p=5) [t] 7.95x103 3 m. A STELLARATOR REACTOR BASED ON Volume of reactor core [m ] ~104 THE HELIAS CONCEPT (Coils, cryostat, structure, etc.) Total weight [t] 4 A stellarator reactor has to observe a num- «2.4xl0 ber of constraints if the properties offered by Winding pack the Advanced Stellarator concept are to be Radial height [m] 0.6 optimally exploited: The magnetic field is limi- Lateral coil width [m] 0.54 ted to 5T in order to keep the NbTi Max. toroidal elongation [m] 2.0 technology available for the superconducting Volume [m3] 9.0 magnet. The configuration and the number of 2 modular coils will be the same as for W7-X Current density MA/[m ] 31.5 because the W7-X configuration (see later) has Total current [MA] 10.4 been designed such that by proper scaling-up Average force [MN/[m3] 86 the desired reactor properties would be density achieved. There will also be no provision for Max. net force [MN] 115 current drive because the configuration essentially eliminates the bootstrap current 3 and a residual one is tolerable. For given Total coil volume [m ] 450 plasma pressure, the plasma temperature is Total coil mass [t] 2.8x103 selected for about maximum fusion power Mass of structure [t] »8.4X103 output. This allows high plasma density and the exploitation of the stellarator-typical Virial stress [MPa] 150 increase of confinement time with increasing Superconductor NbTi density together with the absence of a Temperature [K] 1.8 disruptive density limit4. The magnetic configuration is scaled up linearly in dimensions from R=5.5m for W7-X to R=20m. A minor radius of 1.6m has been selected. burn, and 10% for a burning plasma. The These conditions lead to the data listed in data listed in Tab. II show that - for the Table I. The dimensions of blanket and shield reactor - <J5>, n, T are within the intended are rough estimates resulting from the ASRA limits. The fusion power output is in the 5 right range.

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