1 - Nuclear Sciences and Applications - IAEA

672 SIMONEN et al.
1.6 2.0
FIG. 3. The dependence of driven plasma current on central electron temperature. The solid line is the
total driven current predicted by ONETWO. The dashed line is without the bootstrap contribution.
We estimate the L/R time constant to be 2 sec. Since the plasma
current is essentially constant for 1.5 sec, we conclude that the current is
largely sustained by the neutral beams. Transport code [13,14] studies of
these discharges predict 0.3 to 0.4 MA of beam-driven current. In addition
to the above experiments with constant ohmic heating coil current
we have also carried out experiments with: (1) feedback control for constant
plasma current, (2) feedback control for zero loop voltage, (3) ohmic
heating coil opened at zero current, and (4) counter-injected neutral beam
current. These experiments are also consistent with transport code modeling.
The beam-driven current is proportional to the electron temperature
Te, as seen in Fig. 3. The variation in the low electron temperature data
resulted from the plasma X—point contacting unconditioned portions of
the wall. The ohmic power supplies were disconnected and the loop voltage
was less than 50 mV. The closed circles were taken during the period
of constant plasma current and the open circles during the slow decay of
the current. Also shown in Fig. 3 are ONETWO transport code modeling
of the beam-driven and bootstrap currents. The calculated [13,14] and
measured total currents agree. In spite of the high poloidal beta, the calculated
bootstrap contribution is small for the low electron temperatures
(i/« > 1) and the broad radial density profiles of these discharges.
3. HIGH BETA POLOIDAL CHARACTERISTICS
While the toroidal beta is only 0.5%, the poloidal betas [15] are
remarkably high. Several discharges with MHD 0P > 3.4 and diamagnetic
/3p > 3.0 have been produced without disruptions or coherent n ^ 0
modes. The f3p — 3.5 discharge shown in Figs. 1 and 2 has an inverse
aspect ratio e = 0.31, so that e(3p = 1.1. While these values of (3P and e/3p
are among the highest achieved in a tokamak, they are not remarkable
in themselves. What is remarkable is the absence of the large amplitude