Orbit effects on impurity transport in a rotating plasma

In 1985, very high ion temperature (≈ 10 keV) plasmas were first produced in the TFTR by coinjecting neutral beams into low-current (≈ 800 kA), low-density plasmas. In this mode of operation, called the energetic ion mode, the plasma rotates at very high speed (≤ 10⁸ cm/s). It was found that heavy impurities injected into these plasmas diffused out very quickly. The authors have calculated the impurity ion orbits in a rotating tokamak plasma, using the equation of motion in the frame that rotates with the plasma. It has been found that heavy particles in a rotating plasma can drift away from magnetic surfaces significantly faster. This takes place because impurity ions in the plateau or Pfirsch-Schluter regime when the plasma is stationary can get into the banana regime in a rotating plasma with the same plasma parameters, provided that the rotation velocity is high enough. These orbit effects produce a large enhancement in the particle diffusion coefficient. More than one order of magnitude increase is obtained for the TFTR plasma parameters. Particle orbits near the surface of a rotating tokamak have also been analyzed. All of the results have been substantiated by numerical particle simulation.