Measuring Δ′ from electron temperature fluctuations in the Tokamak Fusion Test Reactor

A method is developed for determining directly from experimental data the classical tearing mode stability parameter ${\Delta }^{\prime }.$ Specifically, an analytical fit function is derived for the electron temperature fluctuations ${\mathrm{(T̃}}_e)$ in the vicinity of a magnetic island. Values of ${\Delta }^{\prime }$ determined from the fit function parameters for $\text{m/n=2/1,}$ $\text{3/2}$ and $\text{4/3}$ modes ($m$ and $n$ are poloidal and toroidal mode numbers) are obtained using the high resolution ${\mathrm{T̃}}_e$ profile data from major radius shift (“jog”) experiments on the Tokamak Fusion Test Reactor (TFTR) [D. Meade et al., Proceedings of the International Conference on Plasma Physics and Controlled Nuclear Fusion. Washington, District of Columbia, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, pp. 9–24]. It is found that the $\text{n⩾2}$ modes have ${\Delta }^{\prime }\text{<0.}$