Effects of a Current Hole on the Fast Ion Distribution in Tokamaks

K.Schoepf1), V.Yavorskij1,2), V.Goloborod’ko1,2), P.Neururer1)

1)Institute for Theoretical Physics, University of Innsbruck, Austria, Association EURATOM-OEAW, A-6020 Innsbruck, Austria
2)Kiev Institute for Nuclear Research, 252028 Kyiv, Ukraine

In the present study we investigate theoretically the consequences of a hollow toroidal current [1-4] for fast ion confinement in tokamaks. We employ a simplified model of the equilibrium magnetic configuration in a current hole tokamak, which is based on an analytical approximation of the poloidal flux function shape [5]. In the constants-of-motion space we calculate the influence of slowing down on the distribution function of energetic particles (charged fusion products, NBI ions) in current hole tokamaks. The distribution function obtained is in satisfactory agreement with those based on numerical current hole equilibria [6].
It is demonstrated that hollow current profiles result in a significant reduction and flattening of the distribution function of fast ions as well as of the profiles of power deposition to electrons and ions in the plasma core. The slowing-down of fusion alphas in the presence of a current hole results in a non-monotonic radial and pitch-angle distribution of partially thermalised alphas in the central area of the plasma (r/a<0.6-0.7). This peculiarity of the charged fusion product distribution function may be important for alpha driven instabilities in hollow current tokamaks. Both the DT alpha distribution function as well as the corresponding power deposition profiles calculated here are compared graphically with previous numerical Fokker-Planck simulations [6,7] accounting for slowing down and pitch angle scattering. Evidently, for the plasma core the analytical analysis developed provides results in good agreement with quantitative numerical Fokker-Planck predictions.

References

[1]N.C. Hawkes et al., Phys. Rev. Lett., 87(2001)115001
[2]T. Fujita et al., Phys. Rev. Lett. 87(2001)245001
[3]F.M. Levington et al., Phys. Rev. Lett. 75(1995)4417
[4]R. Goldston, Phys. Plasmas 3(1996)1794
[5]V.Yavorskij et al., Confinement of Fusion Alpha Particles in Hollow Current Tokamaks, submitted to Pl. Phys. & Contr. Fus.
[6]V. Yavorskij et al., P-1.98 at 30th EPS Conf. on Contr. Fusion and Plasma Physics, St. Petersburg, July 2003; http://eps2003.ioffe.ru/public/pdfs/P-1.98-pre.pdf
[7]V.Yavorskij et al., Nucl. Fusion 43(Oct 2003)

This study has been partially carried out within the Association EURATOM-OEAW.