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Abstract
Two types of novel, non-inductive current drive concepts for starting-up and maintaining tokamak discharges, dc-helicity injection and internally-generated pressure-driven currents, have been developed on the CDX-U tokamak. To study the equilibrium and transport of these plasmas, a full set of magnetic diagnostics was installed. By applying a finite element method and a least squares error fitting technique, internal plasma current distributions are reconstructed from the measurements. Also, electron density distributions were obtained from 2 mm interferometer measurements by a similar least squares error technique utilizing magnetic flux configurations obtained by the magnetic analysis.
Neoclassical pressure-driven currents in ECH plasmas are modeled with the reconstructed magnetic structure, using the electron density distribution and the electron temperature profile measured by a Langmuir probe. In a trapped particle geometry, precessional currents contribute significantly to the net toroidal plasma current. As closed flux surfaces are formed, particle detrapping generates bootstrap current as a major contributor while decreasing the precessional current. These neoclassical currents always predict a hollow profile, while the local and global magnetic measurements indicate a centrally-peaked profile. Indeed, we find that the non-hollow current density profile, with non-classical transport using Boozer's term in Ohm's law, gives better agreement to the internal magnetic structure measured by movable magnetic probes than does the hollow profile. This confirms the existence of a non-classical current transport. This non-classical current transport should be applicable to other forms of non-inductive current drive situations, including the helicity injected case.
In the dc-helicity injection scheme, the need to increase injection current and maintain plasma equilibrium restricts possible arrangements. Several injection configurations were investigated, with the best found to be outside injection with a single divertor configuration, where the cathode is placed at the low field side of the x-point. In this geometry, plasma current as high as 10 kA and edge safety factor $q\sb{\rm a}\simeq$ 5 $-$ 10 were achieved. The formation of closed flux surfaces is shown by a two-dimensional magnetic reconstruction based on measurements. Experimental data and helicity balance show a good power efficiency and current drive efficiency independent of density.
Both pressure-driven and dc-helicity injected tokamaks show the importance of plasma equilibrium in obtaining high plasma current. Noting that initial current generation requires smaller vertical fields in both techniques, programmed vertical field operation has proven to be very important in achieving high plasma current. These non-inductive current drive techniques show great potential as efficient current drive methods (due to their relatively density-independent efficiency) for future steady-state and/or long-pulse fusion reactors.
