Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

ent drive alone cannot account for the observed broad current profile. similarly, simulations for
iteR indicate that the day-one mix of heating and current drive capability is insufficient to robustly
maintain the requisite flat shear for an extended duration. The mechanism leading to the
low shear with q(0) ≥ 1 in present-day devices is correlated with the appearance of benign mhd
instabilities, for example, the appearance of fishbone activity in asdeX-Upgrade and a low-level
3/2 neoclassical tearing mode in diii-d. These instabilities prevent the central safety factor from
dropping below unity, which results in stabilization of the sawtooth instability normally found in
h-mode discharges. more research is required to determine if this “natural” current profile broadening
can be expected in iteR.
experimental results also suggest that performance in the hybrid regime is adversely affected by
operating characteristics expected to be encountered in iteR, such as low rotation and t e /t i =1.
improvements in the ability to simulate these changes using theory-based transport models is
needed to confirm whether improved performance can be realized in iteR.
Steady-state Operation: steady-state operation of iteR is highly desirable, not only to carry
out the nuclear testing part of the iteR mission, but also to demonstrate the tokamak’s potential
to form the core of a practical fusion reactor. in general, steady-state operation of a tokamak is
based on replacing the current normally generated by induction with current generated by other
means. although toroidal current can and will be produced in iteR by neutral beam injection and
the various forms of radiofrequency current drive, the efficiency of these systems is too low to
generate a substantial fraction of the total current, i.e., the power required to replace the normal
inductive current would be comparable to the total power derived from the fusion reactions. Thus,
the approach to steady state for iteR, and indeed for a tokamak reactor, is to maximally exploit
the “bootstrap current,” which is naturally produced in a toroidal plasma by the plasma’s temperature
and density gradients. achieving this “self-organized” state requires operation at relatively
high beta, which means that steady-state tokamak operating regimes represent a delicate balance
between generating sufficient bootstrap current and respecting mhd stability limits.
Good progress in achieving practical, near-steady-state tokamak operating regimes has been
made in the worldwide tokamak research effort, in particular, in the diii-d tokamak in the Us.
as in the case of hybrid modes, the magnetic shear appears to play a key role in forming highbootstrap-fraction
discharges. in diii-d, fully noninductive discharges have been obtained with
a bootstrap fraction of ~ 60% in regimes with flat shear and q min ~ 1.5. another example of nearsteady-state
operation in low-shear regimes is provided by the so-called high poloidal beta regimes
obtained in Jt-60U. in these discharges, the central safety factor remains in the range 1 <
q(0)