Stellarators have significant operational advantages over tokamaks as ignited steady-state reactors because stellarators have no dangerous disruptions and no need for continuous current &"ive or power recirculated to the plasma, both easing the first wall, blanket, and shield design; less severe constraints on the plasma parameters and profiles; and better access for maintenance. This study shows that a reactor based on the torsatron configuration (a stellarator variant) could also have up to double the mass utilization efficiency (MUE) and a significantly lower cost of electricity (COE) than a conventional tokamak reactor (ARIES-I) for a range of assumptions. Torsatron reactors can have much smaller coil systems than tokamak reactors because the coils are closer to the plasma and they have a smaller cross section (higher average current density because of the lower magnetic field). The reactor optimization approach and the costing and component models are those used in the current stage of the ARIES-I tokamak reactor study. Typical reactor parameters for a 1-GW(e) Compact Torsatron reactor example are major radius RO = 6.6-8.8 m, on-axis magnetic field B0 = 4.8-7.5 T, Bmax (on coils) = 16 T, MUE = 140-210 kW(e)/tonne, and COE (in constant 1990 dollars) = 67-79 mill/kW(e)h. The results are relatively sensitive to assumptions on the level of confinement improvement and the blanket thickness under the inboard half of the helical windings but relatively insensitive to other assumptions.
Loss of alpha particles in compact torsairon reactors is studied. For 6,9,and 12 field period reactors, the direct loss is a relatively weak function of radius and energy and varies from 2= S3 % for M = 6 to ?z 18 % for M = 12. Loss of alpha particles through scattering into the loss region ts calculated using the Fokker-Plank equation for fast ions and found to contribute an additional alphaparticle energy loss of cz 15 %. The consequences of these relatively large losses for torsatron reactor design are discussed.The relationship between the direct particle losses and the magnetic field structure is also studied. Orbit losses from a variety of stellarator configurations are calculated and a figure-of-merit that characterizes the orbit confinement of a magnetic configuration is deduced from these calculations. This figure-of-merit is used to show how the direct losses might be reduced at low aspect-ratio. Effects of finite beta on the direct particle losses are also addressed, and are shown to significantly increase the direct losses in some configurations.The compact torsatron sequence 1 is a family of low aspect-ratio 1 = 2 torsatron configurations optimized for high-beta operation. These configurations make interesting reactor candidates because they have the capablity for high-beta operation in the second stability regime, possess natural divertors, and have relatively open coil geometry for access to the plasma. Compact torsatrons are, however, optimized for MHD properties, not orbit confinement. A large fraction (== 5) of particles in these devices are trapped in the relatively large helical ripple in the magnetic field strength B. Toroidal effects at low aspect ratio cause orbits of these trapped particles to deviate significantly from flux surfaces leading to increased transport and in some cases to loss of the particles from the confinement region. Radial electric fields which develop to maintain quasineutrality of the plasma can reduce the deviation of the trapped particles from the flux surface through E x B orbit rotation, but only for those particles which have kinetic energies less than their potential energies
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