Session GP1 - Poster Session IV.
POSTER session, Tuesday afternoon, October 30
Exhibit Hall B,
The TCS device has been equipped with an intensified CCD spectrometer to determine ion toroidal rotation via the Doppler effect. C-III emission at 229.7 nm is viewed at six chords, allowing determination of a rotation profile. In the presence of electrons synchronous with the RMF, the ions experience a frictional drag force and will spin up in a short time \tau=(m_i/m_e)\nu _ei^-1, or ~100 \mus for TCS. This is not observed in these low-density experiments; rather, the ions spin up to a small fraction of the electron velocity (3% at ømega _RMF=1.0 MHz, 15% at ømega _RMF=0.5 MHz) and the current is maintained in steady state, even with ømega _ci/ømega _RMF=0.65. The primary braking mechanism may be charge-exchange with a background of cold, non-rotating neutrals. The inferred charge-exchange rate requires a neutral density equal or greater to the plasma density, which is higher than suggested by power balance. Even at these low spin-up fractions, the ions are rotating faster than their thermal velocity, causing a rotational instability. Tomography shows that a non-destructive rotating n=1 distortion develops at low ion velocities, while at higher velocities the n=2 mode is dominant.