Energy Confinement of Lower-Hybrid-Current-Driven Tokamak Plasmas

The energy content of high-density, purely lower-hybrid-current-driven plasmas has been measured under a variety of operating conditions on the Alcator-C tokamak. The energy stored in the current-carrying electron tail can be a large fraction of the total kinetic energy in an rf-driven plasma, while the energy residing in the bulk plasma is comparable to that of a similar Ohmic discharge. At densities ${\overline{n}}_e<~3\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$, the global confinement time of the rf-driven plasma is similar to that of an Ohmic plasma, but degrades relative to its Ohmic counterpart at higher densities and/or rf powers.