Fueling Studies on the Lithium Tokamak Experiment

Abstract

Lithium plasma facing components reduce the flux of "recycled" particles en-

tering the plasma edge from the plasma facing components. This results in

increased external fueling requirements and provides the opportunity to control

the magnitude and distribution of the incoming particle flux. It has been pre-

dicted that the plasma density profile will then be determined by the deposition

profile of the external fueling, rather than dominated by the recycled particle

flux.

A series of experiments on the Lithium Tokamak Experiment demonstrate that

lithium wall coatings facilitate control of the neutral and plasma particle in-

ventories. With fresh lithium coatings and careful gas injection programming,

over 90% of the injected particle inventory can be absorbed in the lithium wall

during a discharge. Furthermore, dramatic changes in the fueling requirements

and plasma parameters were observed when lithium coatings were applied. This

is largely due to the elimination of water as an impurity on the plasma facing

components.

A Molecular Cluster Injector (MCI) was developed for the fueling of LTX plas-

mas. The MCI uses a supersonic nozzle, cooled to liquid nitrogen temperatures,

to create the conditions necessary for molecular cluster formation. It has been

predicted that molecular clusters will penetrate deeper into plasmas than gas-

phase molecules via a reduced ionization cross-section and by improving the

collimation of the neutral jet. Using an electron beam diagnostic, the densities

of the cryogenic MCI are measured to be an order of magnitude higher than

in the room-temperature jets formed with the same valve pressure. This indi-

cates increased collimation relative to what would be expected from ideal gas

dynamics alone.

A systematic study of the fueling efficiencies achieved with the LTX fueling

systems is presented. The fueling efficiency of the Supersonic Gas Injector (SGI)

is demonstrated to be strongly dependent on the distance between the nozzle

and plasma edge. The fueling efficiency of the gas puffer was improved by the

addition of a guide tube to keep the gas flow collimated and directed towards

the plasma edge. Contrary to results reported on other devices, the MCI yields

fueling efficiencies that are at best equal to the SGI. This suggests that any

enhanced penetration from cluster formation is nullified by the reduced velocity

of the cryogenic gas. Furthermore, at very high neutral fluxes, the fueling

efficiency of the MCI dropped off dramatically, suggesting that the build-up of

pressure in the scrape-off-layer is producing a self-shielding effect. All of these

results are consistent with the idea that the best gas-based fueling source is one

that brings highly directed neutrals to the plasma edge, without being scattered

or ionized in the scrape-off-layer.

The density profiles of LTX plasmas are measured during external fueling with

the gas puffer, SGI, and cryogenic MCI. While the density profile amplitude

increases with higher fueling rates, the shape of the profile remains constant.

This suggests that particle transport is such that differences in the neutral

deposition cannot be discerned. The density profiles are consistently hollow,

suggesting a region of reduced particle transport in the core of LTX plasmas.