MAST The Spherical Tokamak

William Morris
1999 EurophysicsNews  
FUSION possible to its physical limits. Main areas of research in TJ-II are: studies of plasmas in low collisionality regimes aimed at understanding the nature of transport mechanisms caused by the physical properties of magnetically confined plasmas; operational limits in high pressure regimes-in particular we plan to use the flexibility of TJ-II to experimentally validate existing theories of instability development; and studies of confinement optimization and its relationship to radial
more » ... hip to radial electric fields. Encouraged as we should be by all the progress made in the last decade in magnetic confinement, one of the greatest scientific challenges confronting our community is still open: the development of models to fully explain the mechanisms underlying the transport of energy in fusion plasmas. Our hope is that the experimental flexibility of TJ-II could play an important role in finding the solution to this fundamental question. The facility is up and running and at the full disposal of our European scientific community. Fig 2 Left the calculated confining magnetic surfaces of TJ-II and right the experimental ones measured at 5% of the nominal field using the fluorescent rod method. The excellent agreement between both indicates that the positioning and geometry of all coils is within the specified construction tolerances (< 1 mm) When magnetic fusion has reached such an advanced state, it is fair to ask why another variant on the tokamak is being discussed, as well as how it is possible for a torus to be spherical. Figure 1 addresses both questions: the simplicity and compactness of such a plasma have attractions as the core of a fusion device, and the plasma is spherical in appearance. There is a firm database in support of a burning plasma device with a conventional aspect ratio (ratio of major to minor radius of the torus), as embodied in the ITER design for example. If, however, there are advantages in pursuing a somewhat different plasma geometry, for instance with a larger or smaller aspect ratio, then present confidence is limited by the restricted range of aspect ratio data in the experimental database. A major aim of MAST (Mega Amp Spherical Tokamak) is to broaden the tokamak database as well as to provide a novel environment in which to test many aspects of tokamak physics and operation, and transfer results to more conventional aspect ratio devices. Conversely, progress with the spherical tokamak (ST) concept has been extremely rapid as it is built on a wealth of experience from traditional tokamaks. The ST is largely based on theoretical ideas initially developed in the late 1970s and 1980s. One of the major costs and engineering challenges for the tokamak is the production of an adequate magnetic field to confine the plasma, so it is important to make optimal use of it. This means that the plasma pressure should be as high as possible for a given field -the fusion performance can be written in terms of the product of the density, temperature and confinement (or energy replacement) time. Theoretical calculations using magnetohydrodynamic stability codes, tested against existing tokamak data, show that the highest ratio of plasma pressure to magnetic field pressure is achieved by reducing the aspect ratio as far as possible, and raising the plasma current (and/or reducing the magnetic field). Furthermore, good plasma confinement time is found experimentally to be related to high plasma current. A natural feature of ST geometry allows plasmas with very high plasma currents to be confined at low magnetic field while maintaining
doi:10.1007/s00770-998-0233-7 fatcat:3eaxao7v3rgixgkax6ue45uici