Ballooning modes in heliotron/torsatrons
- National Institute for Fusion Science, Nagoya (Japan)
The characteristics of the high-n ballooning modes and the relation between high-n and low-n modes are examined in an L = 2/M = 10 heliotron/torsatron where L and M axe the polarity of helical coils and the toroidal pitch number. The eigenvalue {omega}{sup 2} is the function with respect to {psi}, {theta}{sub k}, and {alpha} : {omega}{sup 2} = {omega}{sup 2}({psi}, {theta}{sub k}, {alpha}) where {psi} and {alpha} are the label of the flux surfaces and the magnetic field lines, respectively, and {theta}{sub k} is the radial wave number. In tokamak cases, {omega}{sup 2} = {omega}{sup 2} ({psi}, {theta}{sub k}). The local magnetic shear s is characterized by the Shafranov shift and it is shown that may disappear even in the stellarator-like global magnetic shear region. The reason results from the fact that the local compression of the poloidal magnetic field outside of the torus to keep the toroidal force balance is suppressed in the stellarator-like global magnetic shear region due to the radially increasing global rotational transform t up to over unity. It is the vacuum rotational transform that makes the base of such a t profile, so that the dependence of s on {alpha} is weak. The influence of the pressure profile on s is weak compared with one due to t. The local magnetic curvature consists of one due to toroidicity and one due to the helicity of the helical coils. The former basically has no {alpha}-dependence. The latter stems from the strength of B in the vacuum configuration, so that it has strong {alpha}-dependence. The influence of the pressure profile on the local magnetic curvature is not so significant. The influences of the pressure profile most significantly appear in the Mercier criterion because the averaged magnetic hill region always exists in the plasma periphery. The peaked pressure profiles such as CHS experimental results give slightly Mercier unstable or Mercier stable equilibria since the pressure gradient is weak in the magnetic hill region.
- OSTI ID:
- 489487
- Report Number(s):
- CONF-960354--
- Country of Publication:
- United States
- Language:
- English
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