Squeezed superbananas and improved superbanana transport in stellarators

It is well known that the size of the superbananas in stellarators scales with the minor radius. Thus, in some stellarator configurations superbananas may not be confined. To improve stellarator confinement, a theory is developed to include the effects of the magnetic well and the radial gradient of the radial electric field to squeeze superbananas. It is found that the size of the superbananas is reduced by a factor of S-1/2, where S is the superbanana squeezing factor that depends on the magnetic well depth and the radial gradient of the radial electric field. The corresponding fraction of the superbanana is increased by a factor of S-1/2. The diffusive loss is thus reduced by a factor of S-3/2. (C) 2002 American Institute of Physics.