Achieving high fusion reactivity in high poloidal beta discharges in TFTR

摘要

High poloidal beta discharges have been produced in TFTR that achieved high fusion reactivities at low plasma currents. By rapidly decreasing the plasma current just prior to high-power neutral beam injection, relatively peaked current profiles were created having high l(sub i) > 2, high Troyon-normalized beta, (beta)N > 3, and high poloidal beta. (beta)(sub p) (ge) 0.7 R/a. The global energy confinement time after the current ramp was comparable to supershots, and the combination of improved MHD stability and good confinement produced a new high (epsilon)(beta)(sub p) high Q(sub DD) operating mode for TFTR. Without steady-state current profile control, as the pulse lengths of high (beta)p discharges were extended, l(sub i) decreased, and the improved stability produced immediately after by the current ramp deteriorated. In four second, high (epsilon)(beta)(sub p) discharges, the current profile broadened under the influence of bootstrap and beam-drive currents. When the calculated voltage throughout the plasma nearly vanished, MHD instabilities were observed with (beta)(sub N) as low as 1.4. Ideal MHD stability calculations showed this lower beta limit to be consistent with theoretical expectations.