Bromberg, L.

Cohn, D.R.

Williams, J.E.

DSpace@MIT

PFC/JA-82-13THE LOW BETA PULSED TOKAMAK AS AMODULAR COMMERCIAL REACTOR WITH VERY LONG PULSESD.R. Cohn, J.E.C. Williams and L. BrombergPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139July 1982THE LOW BETA PULSED TOKAMAK AS AMODULAR COMMERCIAL REACTOR WITH VERY LONG PULSESD.R. Cohn, J.E.C. Williams and L. BrombergPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139AbstractWe describe a tokamak design concept which could provide solutions to some of the possible problemsof using a pulsed tokamak as a commercial power reactor, namely effects of pulsed operation, complexity andmaintainability. The design concept is a device which provides very long pulses (several hours - 1 day) withthe voltsecond capability of the ohmic heating transformer. It operates at low beta with a near circular plasmaand a simple equilibrium field system. The superconducting toroidal field coils are individually removable andthe tokamak is completely modularized. Illustrative parameters are a major radius of 15 m, an aspect ratio of 6,magnetic field on axis of 7.2 T, an average toroidal beta of 0.017, and a pulse length of 14 hours.1IntroductionConcerns about the use of tokamaks as commercial power reactors include effects of pulsed operation,complexity and maintainability. In this paper we consider a design concept which could provide solutions tothese problems while at the same time using relatively conservative assumptions about physics perfonnance.The concept is a tokamak which is completely modularized and could provide very long pulses (several hours -1 day) with the voltsecond capability of the ohmic heating transformer.Design FeaturesCapability to provide very long pulses is obtained by using a large major radius design. The volt-secondcapability of the ohmic heating transformer increases rapidly with major radius and a pulse length on the orderof several hours - one day should be possible in a tokamak with a major radius which is about twice that ofpresent commercial power reactor designs. Operation with very long pulses will greatly reduce thermal fatigueof the first wall and allow for more gradual plasma startup and shutdown.A potential problem with a large major radius design is that very large values of reactor thermal power areproduced at wall loadings which are high enough for acceptably low cost/kW. The thermal power level can bereduced by use of a moderately high aspect ratio and a near circular plasma in order to minimize first wall area.The use of a relatively large aspect ratio facilitates the use of a toroidal field (TF) magnet design in whichthe superconducting TF coils are individually removable and the entire tokamak system is modularized. Thisconcept has been used in a recent design study of a tokamak reactor with helical coils.' Each TF coil would bein a separate dewar and one -or two TF coils could be removed while the other TF coils remain in place and arekept at liquid helium temperature. An important feature of this concept is the separation of the TF coils in thethroat of the tokamak; this separation is facilitated by a relatively high aspect ratio design. Contact between thelow temperature region of the magnet and the casing, which is at room temperature, is made through struts ofG-10 insulation which has an acceptably low heat leak.In order to facilitate maintenance the tokamak would be modularized into pieces which contain one ortwo TF coils plus blanket and shield and first wall. If it is necessary to place EF coils inside the TF coils, theywould be joined by pressure contacts so as to faciliate removal of the modules. Removal of a module would beaccomplished by a relatively small number of operations.2Although the allowed value of toroidal beta is predicted to go as # ,:: 1/A where A is the aspectratio, the ratio of the magnetic field at the plasma axis to the magnetic field at the toroidal field (TF) coilincreases with increasing A. As a result the ratio of plasma pressure to the magnetic field pressure at the insideedge of the TF coil decreases rather slowly with increased aspect ratio.The use of a near circular plasma in the design significantly simplifies the equilibrium field (EF) magnetdesign. It may not be necessary to locate EF coils inside the TF coils. The reduction in plasma elongation alsofacilitates the use of more circular shaped TF coils with reduced size and reduced requirements on external EFcoils.A pump divertor is used for impurity control in order to maintain the simplicity of the design.Illustrative Design -ParametersAn engineering scoping study of the concept has not been performed. However, a set of rough illustrativeparameters can be obtained by extrapolation from the HFCTR power reactor design 2 and from recent designwork on high field tokamaks with DD-DT fuel mixtures and reduced tritium breeding requirtements. Theseillustrative parameters are given in Table 1.The illustrative design should provide a pulse length of 14 hours. The 15 m major radius provides sufficientspace for an OH drive capability of 2800 voltseconds. The field in the OH transformer is swung from +9T to -9T. The time between burn pulses might be minimized by driving the transformer, the EF coils and the plasmacurrent in opposite directions during each successive pulse. In this way the time between pulses would belimited by the startup and shutdown time of the plasma rather than by the time to recock the OH transformer.With a modest valu.e of average toroidal beta, 0.017, the magnetic field of 7.2 T on axis provides a total wallloading of 2.3 MW/m 2. 4300 MW of thermal power is produced.According to the INTOR scaling law of the electron energy confinement time the value of nTe inthe plasma is more than sufficient to obtain ignition. The margin of ignition (MI) where MI =(nrE)INTOR/(Nre)IGNITION is 3.5.The stored energy in the TF magnet in the illustrative design, which is D shaped, is 150 GJ. A significantreduction in the stored energy should be possible by use of a circular TF coil.3SummarySignificant improvements in the engineering features of a pulsed tokamak power reactor might be ob-tained with the concept described here. In addition, the physics requirement for increased beta from plasmaelongation is removed. A range of variations about the illustrative design are possible and should be studiedto determine the potential usefulness of the concept. Engineering studies of the magnet system and inodulariza-tion concept should be performed and the use of a divertor as an alternative to a pump limiter should beassessed.4References1. R.E. Potok et al.,"Physical and Engineering Constraints for Tokamak Reactors with Helical Coils," MITPlasma Fusion Center Report, RR-82-10, (1982).2. D.R. Cohn et aL., "High Field Compact Tokamak Reactor (HFCTR)" MIT Plasma Fusion Center ReportPFC/RR-79-2(1979); J. Schultz, F.S. Malick, D.R. Cohn and J.E.C. Williams, "Advantages of High ForceDensity Magnets and Total Modularization," MIT Plasma Fusion Center Report PFC/RR-80-3(1979).5Table 1Illustrative ParametersMajor radius:Minor radius:Plasma elongation:Aspect ratio:Magnetic field on axis:Magnetic field at the TF coil:Toroidal beta:Ml [= (nrE)1NTOR/(nrE)JGN]:Plasma current:(for q = 3)Average electron temperature:(parabolic profile)Pulse length:Total wall loading:thermal power:(21 MeV/fusion reaction)Number of TF magnet coils:.TF magnet stored energy:Tensile stress in TF magnet structure:Tensile stress in OH transformer structure:Voltsecond capability of OH transformer:15 m2.5 m1.267.2T9.9T0.0173.57.2 MA1514 hours2.3 MW/m 24300 MW20150 GJ200 MPA225 MPA2800 voltsec6

Low beta pulsed tokamak as a modular commercial reactor with very long pulses

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Low beta pulsed tokamak as a modular commercial reactor with very long pulses

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