Poloidal rotation velocities in the outer half of Alcator-C plasmas.

Abstract

Poloidal rotation velocities in the outer half of ohmically heated Alcator-C plasmas have been determined from the Doppler shift of impurity emission lines. The measurements were made using a high-resolution photon-counting detector, sensitive to wavelengths from \ensuremath{\sim}1200 to \ensuremath{\sim}2000 A\r{}, mounted in the exit plane of a 1-m Ebert-Fastie spectrometer. The following transitions were used: 2s2${p}^{1}$P${\ifmmode^\circ\else\textdegree\fi{}}_{1}$\char21{}2${p}^{2}$${}^{1}$${D}_{2}$ at 1371.292 at 1371.292 A\r{} in O v, 2s${}^{2}$${S}_{1/2}$\char21{}2${p}^{2}$P${\ifmmode^\circ\else\textdegree\fi{}}_{1/2}$ at 1242.804 A\r{} in N v (both of which exist near the limiter radius in Alcator-C), and 1s2s $^{3}S_{1}$\char21{}1s2p $^{3}\ifmmode^\circ\else\textdegree\fi{}_{2}$ at 1623.63 A\r{} in O vii (which exists at r/a\ensuremath{\sim}0.8). The measured rotation velocities are 4\char21{}5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm/s in the direction of the electron diamagnetic drift at the radius of the O v emission shell and \ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm/s at the radius of the O vii emission shell, again in the electron diamagnetic drift direction. Therefore poloidal rotation of the outer half of the Alcator-C plasma with a velocity of several times ${10}^{5}$ cm/s in the direction of the electron diamagnetic drift is firmly established by these measurements.