  This review article will offer a qualitative overview of the strongly
reversed shear profile for steady-state operation in tokamaks. For a
steady-state reactor to be commercially viable, it is necessary to have a large
bootstrap fraction. Currently, there appears great potential in an Advanced
Tokamak (AT) regime, namely the hollow current profile (strongly reversed
shear). This mode is characterized by high poloidal beta, broad current
profiles, strong internal and edge pressure gradients, and relatively good
magnetohydrodynamic (MHD) stability against Neoclassical Tearing Modes (NTMs)
and ballooning modes. The n=1 and n=2 kink modes, resistive wall modes, and
double tearing modes are of concern in the reversed shear profile, and
avoidance and/or suppression of these modes is necessary. Although there is a
relatively low net plasma current in the reversed shear, the regime appears to
have excellent energy confinement properties due to the naturally occurring
Internal Transport Barriers (ITBs) caused by the substantial bootstrap
currents, and Edge Transport Barriers (ETBs), which can form from ELM-free
H-Mode (QH-Mode), to form the Quiescent Double Barrier (QDBs). The reversed
shear can be generated by freezing the current profile, through MHD effects or
substantial heating and/or current drive during the current ramp up phase, and
is sustained by off-axis non-inductive current drive sources, such as the
Neutral Beam Current Drive (NBCD), Lower Hybrid Current Drive (LHCD), and
Helicon Current Drive (HCD). Experimental results by DIII-D, JT-60U, ASDEX
Upgrade, JET, PBX-M, COMPASS-D, and K-STAR, simulation models and codes, such
as Lower Hybrid Simulation and STELION, and theoretical reactors, such as
ARIES-RS, ARIES-AT and SSTR are referenced.
