THE MULTICHANNEL MOTIONAL STARK-EFFECT DIAGNOSTIC ON TFTR

Although the q profile plays a key role in theories of instabilities and plasma equilibrium, it has been quite difficult to measure until the recent development of the motional Stark effect (MSE) diagnostic. A multichannel motional Stark effect polarimeter system has recently been installed on the Tokamak Fusion Test Reactor (TFTR). The diagnostic can measure the magnetic field pitch angle [gamma(p) = tan-1(B(p)/B(T))] at ten radial locations. The doppler shifted D(alpha) radiation from a TFTR heating beam is viewed near tangential to the toroidal magnetic field via a re-entrant front surface reflecting mirror. The field of view covers from inboard of the magnetic axis to near the outboard edge of the plasma with a radial spatial resolution of 3-5 cm. A high throughput f/2 optics system results in an uncertainty for gamma(p) of approximately 0.1-degrees-0.2-degrees with a time resolution of approximately 5-10 ms. Initial pitch angle profiles from TFTR have been obtained. The MSE data is consistent with the estimated magnetic axis position from external magnetic measurements and the q = 1 radius is in good agreement with the inversion radius from the electron cyclotron emission temperature measurements.