  One metric for comparing confinement properties of different magnetic fusion
energy configurations is the linear critical gradient of drift wave modes. The
critical gradient scale length determines the ratio of the core to pedestal
temperature when a plasma is limited to marginal stability in the plasma core.
The gyrokinetic turbulence code GS2 was used to calculate critical temperature
gradients for the linear, collisionless ion temperature gradient (ITG) mode in
the National Compact Stellarator Experiment (NCSX) and a prototypical shaped
tokamak, based on the profiles of a JET H-mode shot and the stronger shaping of
ARIES-AT. While a concern was that the narrow cross section of NCSX at some
toroidal locations would result in steep gradients that drive instabilities
more easily, it is found that other stabilizing effects of the stellarator
configuration offset this so that the normalized critical gradients for NCSX
are competitive with or even better than for the tokamak. For the adiabatic ITG
mode, NCSX and the tokamak had similar critical gradients, though beyond
marginal stability, NCSX had larger growth rates. However, for the kinetic ITG
mode, NCSX had a higher critical gradient and lower growth rates until a/L_T is
approximately 1.5 times a/L_{T,crit}, when it surpassed the tokamak's. A
discussion of the results presented with respect to a/L_T vs. R/L_T is
included.
