1 - Nuclear Sciences and Applications - IAEA

662 SCOTT et al.
The TFTR "supershot" regime [13,14], characterized by high neutron emission,
high ion temperatures, and a broad electron temperature profile, is
generally optimized by the use of near-balanced injection. It was recognized
early [2] that classical effects associated with large toroidal rotation
velocities could impair the performance of purely co-injected discharges:
the relative energy between the incident beam neutrals and the plasma
is reduced in the rotating plasma frame, leading to poorer fusion reactivity,
poorer beam penetration, and broader beam power absorption profiles;
beam power invested in pushing the plasma near the center is returned as
heat to the thermal ions only as viscous dissipation at larger minor radii.
Whether these effects are sufficient to account for the difference between coinjection
and balanced supershot injection, or alternatively whether there is
an underlying difference in the energy transport, is a question of considerable
interest. To address this issue, the beam power scan was supplemented
by a number of balanced discharges at the same current with P\, = 14 MW
(Table 1). Profile measurements of v^,{R) and Ti(R) were obtained for coinjected
discharges of slightly lower power, 11.6 MW. As expected, neutron
emission improved by a. factor of nearly two with balanced injection, but in
many respects the balanced discharges were only marginally within the supershot
regime: the density peakedness ne(0)/ rose to 2.3 (compared
to 3.0 on the best supershots), and the global energy confinement time derived
from diamagnetic measurements was 130 ms (compared to > 160 ms
in the best supershots). Figure 5 shows the calculated ion and electron
thermal diffusivities for the balanced and co-rotating plasmas. There is
no significant difference between the two cases, suggesting that the rotation
speed of 1.5 x 10 5 m/s at r = a/2, corresponding to mach 0.25 at this
radius (mach number = v^j\JTi/m = 0.6 at the plasma, center) has little
effect on heat transport.
4. Plasma Rotation and Impurity Transport
Impurity transport in discharges with 11-14 MW of co-, counter-, or balanced
beam injection was studied by impurity transport code analysis of
line emissions of germanium injected using the laser-blowoff technique. The
time evolution of extreme ultraviolet lines emitted by the core ionization
state Ge +29 is shown in Fig. 6. The time to peak emission and subsequent
decay time were shortest for counter-injection (Trjse ==-36 msec,
r(jecay ~ 50 msec), somewhat longer for co-injection (50 msec; 70 msec)
and considerably longer for balanced-injection (72 msec; 115 msec), indicating
that impurity confinement is longest during balanced injection. This