Exerimental Studies Using Compact Torus Injector
Fusion energy has been a persistent pursuit for decades due to its abundance in fuel supplies and low greenhouse gas emission. Tokamak is the most promising concept as a reactor in which strong magnetic fields are used to confine the hot and dense plasma in a toroidal chamber. Despite the steady progress made, several key issues remain, such as (a) plasma instabilities affecting the confinement, (b) possible plasma disruption causing damage to the reactor, (c) interaction between the plasma and the tokamak chamber wall which increases the impurity in the plasma and fuel recycling, and (d) directly fueling of the reactor core among several other issues. Compact torus injection (CTI) has been identified as the only promising technology for direct core fueling of a reactor-grade tokamak reactor. Beside fueling effects of the compact torus injector, CTI has been utilized in several experimental studies in my Ph.D. research program including (a) for momentum transfer into the STOR-M discharges to create/modify toroidal plasma rotation which is beneficial for improvement of plasma confinement and avoidance of disruptions, (b) for studies of plasma-material interaction between the CT plasma and selected samples, and (c) for demonstration of fuelling when the fuel recycling is greatly reduced after Lithium coating of the STOR-M chamber. Several methods such as neutral beam injection (NBI) and resonant magnetic perturbation (RMP) have been used to modify the toroidal flow velocity of the tokamak plasma. However, the suitability of those technologies for the reactor grade tokamak is still uncertain. The high velocity CT carries not only the particles, but also momentum, on the order of ten-folds of plasma toroidal flow momentum of STOR-M tokamak. Experiments with tangential compact torus injection (CTI) into the Saskatchewan Torus-Modified (STOR-M) tokamak have confirmed that CTI is capable of modifying plasma toroidal flow in the tokamak, adding a completely new technology for creating plasma flow needed in the future fusion reactors. Interaction of the hot and dense plasma with tokamak wall, with a energy up to 100 MW/m2 with a pulse duration up to 1 ms, is still one of the unsolved problems for a tokamak reactor. Divertors are used to intercept the large power flux and their configurations are constantly improved. Different wall materials have been proposed and tested as the tokamak first wall and divertor materials and in tokamaks. Different techniques such as NBI, high power lasers have been utilized to study wall materials. Compact torus is able to deposit a very high power to the surface, on the order of 7.5 GW/m2. In this Ph.D. work, University of Saskatchewan compact torus injector (USCTI) has been used to inject the high heat and particle flux on the surface of stainless samples with three different types of coatings including stainless steel plate coated by tungsten (type 1), and other two types of samples with an additional layer of coating, either nickel (type 2) or chromium (type 3) on the type 1 samples, all provided by General Fusion Inc., to study the effects of extremely high heat load on sample surfaces. Tokamak wall conditioning plays an important role in the plasma confinement. Bombardment with plasma particles on the tokamak wall introduces wall material and foreign impurities absorbed on the wall to the plasma causing increased radiative energy losses. Coating the tokamak first wall with low Z material has been considered as an effective method to deal with this issue. Lithium coating of the tokamak inner wall has been used to improve the tokamak wall conditioning. In this Ph.D. work, a thin lithium film was coated on the STOR-M chamber wall. The experimental results have shown the reduction of impurity contents in the tokamak plasma and reduced recycling of gas from the wall signified by the reduced plasma density by a factor of 50%. USCTI has then been used to restore the plasma density, an example of the capability for fuelling and density control by CT injection.
compact torus, fusion, plasma toroidal flow
Doctor of Philosophy (Ph.D.)
Physics and Engineering Physics