Toki Conf. ITC12, Dec. 2001 STELLARATOR FUSION REACTORS –– AN OVERVIEW BEIDLER Craig D., HARMEYER Ewald, HERRNEGGER Franz, KISSLINGER Johann, IGITKHANOV Yuri, WOBIG Horst1 Max-Planck Institut für Plasmaphysik, EURATOM Association D-85740 Garching bei München, Germany. Abstract: The stellarator system offers a distinct alternative to the mainline approaches to magnetic fusion power and has several potentially major advantages. Since the first proposal of the stellarator con- cept many reactor studies have been published and these studies reflect the large variety of stella- rator configurations. The main representatives are the continuous-coil configurations and the mo- dular-coil configurations. As a continuation of the LHD experiment two reactor configurations, FFHR1 and FFHR2, have been investigated, which use continuous helical windings for providing the magnetic field. The modular coil concept has been realized in the MHH-reactor study (USA 1997) and in the Helias reactor. The Helias reactor combines the principle of plasma optimisation with a modular coil system. The paper also discusses the issues associated with the blanket and the maintenance process. Stellarator configurations with continuous coils such as LHD possess a natural helical divertor, which can be used favourably for impurity control. In advanced stella- rators with modular coils the same goal can be achieved by the island divertor. Plasma parameters in the various stellarator reactors are computed on the basis of presently known scaling laws showing that confinement is sufficiently good to provide ignition and self-sustained burn. Keywords: Stellarator, fusion reactor, modular coils, helical system, power balance 1. Introduction Table 1: Main features of a stellarator reactor The main properties of a stellarator reactor are the potential of steady-state operation and the absence of ∑ Steady-state magnetic fields. No induced current disruption; a summary of the main features is eddy currents. No enhanced fatigue of the given in Table 1. The steady-state magnetic fields structure due to pulsed thermal load. simplify superconducting magnet design, remove the ∑ Steady-state operation at high Q , Q –> ∞. need for pulsed superconducting coils, and eliminate ∑ No energy storage and low recirculating energy storage required to drive pulsed coils. Plasma power requirements. confinement during startup and shutdown is aided by ∑ Moderate plasma aspect ratio (8-12) which the presence of magnetic surfaces at all times during offers good access to the reactor core. this phase. Steady-state plasma operation after ignition is an outstanding advantage of the stellarator ∑ Start-up on existing magnetic surfaces concept. with good confinement at all instances. A stellarator can have a relatively high aspect ∑ No positioning or field shaping coils ratio and does not require expensive complicating necessary. auxiliary magnets for field shaping, position control ∑ No major disruptions that could lead to an coils and current drive. Its coil configuration permits energy dump on the first wall or on the access to the device from all sides and facilitates a divertor target plates. modular approach to blanket and shield design. ∑ Several potential methods for impurity Since stellarators and torsatrons can operate free of control and ash removal exist. Magnetic induced toroidal current and do not suffer from islands at the plasma edge can be used for major plasma disruptions, the major concern of an divertor action excessive energy dump on the first wall and plasma ∑ No toroidal current drive is required. facing components can be eliminated. 1 E-mail: [email protected] attempt at an integrated design of a stellarator 2. Stellarator reactors reactor addressing all major components of the Early stellarator reactor designs [1,2,3,4] concluded plant: the physics base, the coil system, the blanket, that the coupled problems of high coil cost and low maintenance, the power balance and, last but not system power density (i.e., low beta) were parti- least, the cost analysis. The magnetic configuration cularly severe for the classical stellarator. A particu- is basically a Heliac including elements of optimi- lar disadvantage of the classical stellarator configu- sation as have been developed in the Helias concept. ration arises from the interaction between the toroidal This explains the name: Modular Helias-like Heliac field coils and the helical windings. The torsatron (MHH). [5,6] and modular-coil configurations [7,8], however, The method to calculate the coil system after the show strong promise for alleviating the coil problem magnetic field has been specified offers the chance per se. to optimize the magnetic field first according to The MIT T-1 torsatron design [6] was the first criteria of optimum plasma performance [15] and study of a torsatron reactor. It reflects an attempt to then to compute the coil system after this procedure reduce the total power output to < 4 GWt under the has come to a satisfying result. Along this line the assumption of conservative beta limits. Helical advanced stellarator [16] has been developed. The reactor design studies are based on the LHD- concept of modular coils and the principle of optimi- concept, which has been developed at the National sation have been combined in the Wendelstein 7-X Institute of Fusion Studies (NIFS) in Toki, Japan. device [17], which will demonstrate the reactor The LHD-experiment is a torsatron with l=2 helical capability of the advanced stellarator line. HSR5/22 windings and 10 field periods [9] and the advantage and HSR4/18 are fusion reactors based on this of the LHD-concept is the natural divertor with two concept. X-points, which helically encircle the plasma. Apart The main criteria in designing stellarator reactors from this helical structure it has many features in are the space needed for a breeding blanket and common with the tokamak divertor. Two versions of shield and the conditions of self-sustained burn. The the LHD-type reactor exist: the Force-Free Helical size of blanket and shield depends on atomic physics Reactor (FFHR [10]) and the Modular Helical and is more or less the same in all fusion devices; it Reactor (MHR). The main feature of the FFHR is is on the order of 1-1.3 m. Any stellarator reactor the arrangement of the helical windings in such a must be large enough to accomodate a blanket. The way that the forces on the helical windings are confinement time must satisfy the Lawson conditon minimized. This requirement leads to an l = 3 nt > 2x1020, which implies that the confinement system with 18 field periods. E The first studies of modular stellarator reactors time must be about 1-2 seconds. Any chance to started from the well-known magnetic field configu- operate the reactor at high density relaxes the rations, which before have been realized by helical requirements on the confinement time. windings and toroidal field coils. The UWTOR-M Table 2: Helical system reactors reactor [11], designed at the University of Wiscon- sin, is one of the first devices utilising the concept of Parameters FFHR1 FFHR2 modular coils. The modular stellarator reactor Major radius [m] 20 10 (MSR) developed at the Los Alamos Laboratories is Av. radius of coils [m] 3.33 2.3 a classical l = 2, m = 6 configuration generated by Coil current [MAturns] 66.6 50 24 modular coils. The thermal output is on the order Plasma radius [m] 2 1.2 of 4000 MW. The study on the modular stellarator Plasma volume [m3] 1579 284 ASRA6C was carried out in 1987 as a joint effort of Magnetic field B(0) [T] 12 10 IPP Garching, KfK Karlsruhe and the University of Max. field on coils [T] 14 13 Wisconsin [12]. This study was based on the Average beta [%] 0.7 1.8 Wendelstein 7-AS configuration and the aim was Density n(0) [m-3] 2 1020 2.8 1020 directed towards the clarification of critical issues of Temperature T(0) [keV] 22 27 an advanced modular stellarator reactor and was not Fusion power [MW] 3000 1000 meant as a point design. Polarity l 32 The MHH is a 4-period modular stellarator reac- Field periods 18 10 tor, which has been designed in a joint effort by a Magnet. energy [GJ] 1290 147 group of fusion laboratories in the USA [13]. The approach is similar to that used in the ARIES- Another design parameter is the magnetic field tokamak reactor studies [14]: an integrated physics, strength, which should be large enough to provide engineering, reactor component and cost optimi- sufficient confinement and high magnetic pressure sation. This stellarator power plant study is the first 2 (small beta values) while on the other hand it should in a radioactive environment. With respect to be small to avoid large mechanical forces in the coils maintenance the continuous helical coil approach is and expensive superconductors. Feasibility of the the ideal one, since sufficiently large gaps between coil system and maintainability of the blanket are helical windings exist and access to the blanket and other important design criteria. Since there is no the divertor is possible nearly everywhere. consensus yet on a priority list among these criteria, stellarator reactor concepts differs appreciably with 3. Blanket in stellarator reactors respect to size, magnetic field and layout of the As in any toroidal fusion device the purpose of the blanket. blanket is to provide sufficient breeding of tritium Table 3: Modular coil reactors and to shield the superconducting coils against neutrons. The size of the blanket and its radial width Parameters MHH HSR4/18 is an absolute figure and any fusion device has to Major radius [m] 14 18 provide enough space to accomodate a blanket with Av. radius of coils [m] 4.75 5.5 about 1.3 m radial build. In contrast to tokamaks the Coil current [MAturns] 13.8 10.8 stellarator, however, requires a 3-dimensional design Number of modular coils 32 40 of the blanket, which must conform to the 3- Plasma radius [m] 1.63 2.1 dimensional shape of the plasma.