Physics of Plasmas in a Dipole Field

  • Hasegawa A.

Dipole field is the most elementary structure of a magnetic field, yet the behavior of a plasma in a dipole magnetic field is very interesting both from the point of view of a single particle motion and of a collective fluid motion. In particular when a stochastic motion and diffusion in psi-phi space is induced by RF fields at omega~mphi the plasma particles are expected to have an ambipolar diffusion toward the center of the dipole and simultaneously be heated by J and mu considerations. Here mu, J and psi are the first (the magnetic moment), the second (action of the bounce motion) and the third (magnetic flux) invariants, phi is the conjugate angle of psi and m is the azimuthal mode number of the RF. As a consequence, one can produce a confined plasma with the pressure p(r) ~r sup (-20) /3 and density n(r)~r sup (-4) using a low density and low temperature source at an outer edge of the dipole field. The plasma thus produced is marginally stable with respect to an interchange because partial f / partial psi|sub mu, J=0. Furthermore, the fact that most plasmas are "trapped" leads to almost a perfect cancellation of the driving term of the ballooning instability by the trapped particle contribution and to a stability (with respect to the ballooning) to beta>>1.

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