Lower Hybrid Current Drive Experiments on the Encore Tokamak

Citation

Sverdrup, Lawrence Henry (2002) Lower Hybrid Current Drive Experiments on the Encore Tokamak. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/27RZ-SM06. https://resolver.caltech.edu/CaltechTHESIS:10212009-105023039

Abstract

The work of this thesis concerns a technological aspect of a tokamak fusion power reactor. A toroidal current in tokamaks is necessary for plasma equilibrium. Ohmic heated tokamaks are inherently pulsed devices since the toroidal plasma current is essentially a single turn secondary of a transformer. A pulsed power reactor is undesirable for a number of reasons including thermal fatigue to material structures and other mechanical cycling effects. Various means to drive a continuous current have been studied. One of the more successful schemes has been to inject unidirectional lower hybrid plasma waves into a tokamak. The plasma waves Landau damp on the high velocity tail of the electron distribution, delivering wave momentum to electrons and generating plasma current.

The results of early experiments produced two plasma physics problems. First, the current drive effect disappeared above a certain plasma density that depended in some way on the particulars of the experiment. This effect became known as the 'density limit' problem. Secondly, the phase velocities of the launched lower hybrid plasma waves in most experiments turned out to be so high that essentially no electrons in the high velocity tail of the electron distribution were available to interact with the plasma waves. Despite this, large currents were indeed driven in most of the experiments. Somehow the 'spectral gap' between the launch phase velocity of the wave and the Landau damping phase velocity was being bridged.

Experiments at Caltech on the Encore tokamak failed to produce the large driven currents seen in other experiments. The reason for this and simultaneously the cause of the density limit seen in the other experiments was explained by a relatively simple and appealing theory.

Small driven currents were observed. Initially puzzling was the result that currents could be driven in the same toroidal direction regardless of the directionality of the launched lower hybrid waves. The Encore tokamak had a handedness. The cause of this handedness turned out to be a radial, horizontal, magnetic error field associated with the toroidal magnetic field which led to a horizontal spiraling of the toroidal field lines.

Thesis Files