J. Plasma Fusion Res. SERIES, Vol.5 (2002) 149-155 Stellarator Fusion Reactors - an Overview BEIDLER Craig D., HARMEYER Ewald, HERRNEGGER Franz, KISSLINGER Johann, IGITKHANOV Yuri and WOBIG Horstr Max-Planck Institut ftir Plasmaphysik, EUMTOM Association D-85740 Garching bei Miinchen, Germany (Received: 11 December 2OOl I Accepted: 4 June 2OO2) 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 concept many reactor studies have been published and these studies reflect the large variety of stellarator configurations. The main representatives are the continuous-coil configurations and the modular-coil configurations. As a continuation of the LHD experiment two reactor configurations, FFHRI 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 stellarators 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 o1'presently known scaling laws showing that confinement is sufficiently good to provide ignition and self--sustained burn. Keywords: stellarator, fusion reactor, modular coil, helical system, power balance 1. Introduction current drive. Its coil configuration permits access to the The main properties of a stellarator reactor are the device from all sides and facilitates a modular approach potential of steady-state operation and the absence of to blanket and shield design. Since stellarators and current disruption; a summary of the main features is torsatrons can operate free of induced toroidal current given in Table 1. The steady-state magnetic fields and do not suffer from major plasma disruptions, the simplify superconducting magnet design, remove the major concern of an excessive energy dump on the first need for pulsed superconducting coils, and eliminate wall and plasma facing components can be eliminated. energy storage required to drive pulsed coils. Plasma confinement during startup and shutdown is aided by the 2. Stellarator Reactors presence of magnetic surfaces at all times during this Early stellarator reactor designs 11,2,3,4) concluded phase. Steady-state plasma operation after ignition is an that the coupled problems of high coil cost and low outstanding advantage of the stellarator concept. system power density (i.e., low beta) were particularly A stellarator can have a relatively high aspect ratio severe for the classical stellarator. A particular and does not require expensive complicating auxiliary disadvantage of the classical stellarator configuration magnets for field shaping, position control coils and @20O2 by The Japan Society of Plasma t C o rre s p ondin.g autho r' s e -mail : w obi g @ ipp. mp g. de Science and Nuclear Fusion Research t49 Beidler C.D. et al., Stellmator Fusion Reactors - an Overview Table 1 Main features of a stellarator reactor Table 2 Helical system reactors o Steady-state magnetic fields. No induced eddy Parameters FFHRl FFHR2 currents. No enhanced fatigue of the structure Major radius [m] 20 10 pulsed due to thermal load. Av. radius of coils 3.33 2.3 . *. [m] Steady-state operation at high A, A-> Coil current [MAturns] 66.6 50 r No power energy storage and low recirculating Plasma radius [m] 2 1.2 requirements. Plasma volume [mt] 1 579 284 r Moderate plasma aspect ratio (8-12) which offers Magnetic field B(0) tTl 12 10 good access to the reactor core. Max. field on coils tTl 14 13 r Start-up on existing magnetic surfaces good with Average beta l%1 0.7 1.8 at instances. confinement all Density n(0) [m-'] 2x1O2o 2.8x1020 . No positioning or field shaping coils necessary. Temperature f(0) [keV] 22 27 No major disruptions that could lead to an energy Fusion power IMW] 3000 1000 dump on the first wall or on the divertor target Polarity / 3 2 plates. Field periods 18 10 . potential Several methods for impurity control Magnet. energy IGJI 1290 147 and ash removal exist. Magnetic islands at the plasma edge can be used for divertor action r No toroidal current drive is required. coils. The thermal output is on the order of 4000 MW. The study on the modular stellarator ASRA6C was arises from the interaction between the toroidal field carried out in 1987 as a joint effort of IPP Garching, coils and the helical windings. The torsatron [5,6] and KfK Karlsruhe and the University of Wisconsin [14]. modular-coil configurations [7,8], however, show strong This study was based on the Wendelstein 7-AS promise for alleviating the coil problem per se. configuration and the aim was directed towards the The MIT T-l torsatron design [6] was the first clarification of critical issues of an advanced modular study of a torsatron reactor. It reflects an attempt to stellarator reactor and was not meant as a point design. reduce the total power output to < 4 GWt under the The MHH is a 4-period modular stellarator reactor, assumption of conservative beta limits. Helical reactor which has been designed in ajoint effort by a group of design studies are based on the LHD-concept, which has fusion laboratories in the USA [5]. The approach is been developed at the National Institute of Fusion similar to that used in the ARIES-tokamak reactor Studies (NIFS) in Toki, Japan. The LHD-experiment is studies [16]: an integrated physics, engineering, reactor a torsatron with / = 2 helical windings and 10 field component and cost optimisation. This stellarator power periods [9] and the advantage of the LHD-concept is the plant study is the first attempt at an integrated design of natural divertor with two X-points, which helically a stellarator reactor addressing all major components of encircle the plasma. Apart from this helical structure it the plant: the physics base, the coil system, the blanket, has many features in common with the tokamak maintenance, the power balance and, last but not least, divertor. Two versions of the LHD-type reactor exist: the cost analysis. The magnetic configuration is the Force-Free Helical Reactor (FFHR [10,] l]) and the basically a Heliac including elements of optimisation as Modular Helical Reactor (MHR t12l). The main feature have been developed in the Helias concept. This of the FFHR is the arrangement of the helical windings explains the name: Modular Helias-like Heliac (MHH, in such a way that the forces on the helical windings are see Table 3). minimized. This requirement leads to an I = 3 system The method to calculate the coil system after the with l8 field periods (see Table 2). magnetic field has been specified offers the chance to The first studies of modular stellarator reactors optimize the magnetic field first according to criteria of started from the well-known magnetic field optimum plasma performance [17] and then to compute configurations, which before have been realized by the coil system after this procedure has come to a helical windings and toroidal field coils. The UWTOR- satisfying result. Along this line the advanced stellarator M reactor [13], designed at the University of Wisconsin, [18] has been developed. The concept of modular coils is one of the first devices utilising the concept of and the principle of optimisation have been combined in modular coils. The modular stellarator reactor (MSR) the Wendelstein 7-X device U9l, which will developed at the Los Alamos Laboratories is a classical demonstrate the reactor capability of the advanced I = 2, m = 6 configuration generated by 24 modular stellarator line. HSR5/22 and HSR4/18 are fusion r50 Beidler C.D. et al., Stellarutor Fusion Reactors - an Overvrew Table 3 Modular coil reactors superconductor, which avoids heat treatment after the winding procedure. Modular coils offer the chance to Parameters MHH HSR4/18 be fabricated separately and be tested before installing Major radius [m] 14 18 these into the reactor. Heat treatment is possible also Av. radius of coils [m] 4.75 5.5 Coil current [MAturns] 13.8 10.8 after finishing the winding process. Spacing beween Number of modular coils 32 40 modular coils should be small to avoid ripple trapping Av. plasma radius [m] 1.63 2.1 of localized particles, however this conflicts with the Plasma volume 735 1567 [mt] requirements for maintenance. Maintenance through Magnetic field B(0) ITI 4.94 5 Max. field on coils ITI 14.5 10.3 portholes needs sufficiently large gaps between adjacent Average beta I%l 5 3.7 coils. In the Helias reactor HSR5/22 or HSR4/18, l0 DensiV <n> [m-t] 1.3x1020 2x1O2o coils per period turned out to be a compromise between Temperature <I> [keV] 10 15 physics requirements and the need for sufficient access. Fusion power tMWl 1730 3000 Field periods 4 4 In MHH the alternative method has been choosen: a Magnet. energy IGJI 80 100 whole sector of the coil system together with the blanket will be moved horizontally and replacement of blanket This list displays only the recent stellarator reactors. (MHH data from ref. [15]). and divertor components occurs through the horizontal gaps. The critical issue of this procedure is the seperation and reconnection of the sector, which must be reactors based on this concept. done by remote control in a radioactive environment.