Transport implications of current drive by magnetic helicity injection

It is shown that in fusion plasma configurations sustained by electrode hel icity injection, the core electron temperature (in electron volts) can, at most, be 25% to 40% of the electrode voltage (in volts). This result is obt ained by assessing magnetic helicity injection as a driver of macroscopic s teady-state plasma currents in magnetic confinement devices. Coaxial helici ty injection using electrodes (CHI) and oscillating-field current drive (OF CD) are compared to inductive current drive. Magnetic helicity, K, is uniqu ely defined as the time-dependent volume integral of A.B when the surface c omponents of A are purely solenoidal. Using an Ohm's law including Hall ter ms, magnetic helicity transport modeling shows that no closed magnetic surf aces with time and volume averaged parallel currents can exist continuously within a plasma sustained only by CHI or OFCD. The 25% to 40% limitations are obtained by considering long and short electron mean-free-path models o f parallel energy transport. (C) 2001 American Institute of Physics.