Control of internal transport barriers in magnetically confined tokamak fusion plasmas

Abstract

In the Tokamak plasma, for fusion to be possible, we have to maintain a very high temperature and density at the core at the same time keeping them low at the edge to protect the machine. Nature does not favor gradients. Gradients are source of free energy that causes instability. But we require a large gradient to get energy from plasma fusion. We therefore, apply a huge magnetic field on the order of few Tesla (1 T-10 T) that confines the plasma in the core, maintaining gradients. Due to gradients in density of charged particles (ions and electrons), there is an electric field in the plasma. Heat and particle transport takes place from core to edge mainly through anomalous transport while the E x B velocity sheer acts to reduce the transport of heat and particles. The regime at which the E x B velocity shear exceeds the maximum linear instability growth rate, as a result, the transport of particles and heat gets locally reduced is termed as the formation of a transport barrier. This regime can be identified by calculating the transport coefficients in the local region. Sometimes it can be observed in the edge where it is called an edge barrier while if it is near the core it is an internal transport barrier. There is a positive feedback loop between gradients and transport barrier formation. External heating and current drives play an important role to control such barriers. Auxiliary heating like neutral beam injection (NBI) and radio frequency (RF) heating can be used at a proper location (near the core of the plasma) to trigger or (far outside from the core) to destroy those barriers. Barrier control mechanism in the burning plasmas in international thermonuclear test reactor (ITER) parameter scenarios employing fusion power along with auxiliary heating source and pellets are studied. Continuous bombardment with pellets in the interval of a fraction of a second near the core of the burning plasma results in a stronger barrier. Frozen pellets along with auxiliary heating are found to be helpful to control the barriers in the tokamak plasmas. Active control mechanism for transport barriers using pellets and auxiliary heating in one of tokamaks in United States (DIII-D) parameter scenarios are presented in which intrinsic hysteresis is used as a novel control tool. During this process, a small background NBI power near the core assists in maintaining the profile. Finally, a self-sustained control mechanism in the presence of core heating is also explored in Japanese tokamak (JT-60SA) parameter scenarios. Centrally peaked narrow NBI power is mainly absorbed by ions with a smaller fraction by the electrons. Heat exchange between the electron and ion channels and heat conduction in the electron channel are found to be the main processes that govern this self control effect. A strong barrier which is formed in the ion channel is found to play the main role during the profile steepening while the burst after the peaked core density is found to have key role in the profile relaxation.