Neoclassical plasma viscosity and transport processes in non-axisymmetric tori

Abstract

Neoclassical transport processes are important to the understanding of plasma confinement physics in doubly periodic magnetized toroidal plasmas, especially, after the impact of the momentum confinement on the particle and energy confinement is recognized. Real doubly periodic tori in general are non-axisymmetric, with symmetric tori as a special case. An eight-moment approach to transport theory with plasma density N, plasma pressure p, mass flow velocity V and heat flow q as independent variables is adopted. Transport processes are dictated by the solutions of the momentum and heat flux balance equations. For toroidal plasma confinement devices, the first order (in the gyro-radius ordering) plasma flows are on the magnetic surface to guarantee good plasma confinement and are thus two-dimensional. Two linearly independent components of the momentum equation are required to determine the flows completely. Once this two-dimensional flow is relaxed, i.e. the momentum equation reaches a steady state, plasmas become ambipolar, and all the transport fluxes are determined through the flux-force relation. The flux-force relation is derived both from the kinetic definitions for the transport fluxes and from the manipulation of the momentum and heat flux balance equations to illustrate the nature of the transport fluxes by examining their corresponding driven forces and their roles in the momentum and heat flux balance equations. Steady-state plasma flows are determined by the components of the stress and heat stress tensors in the momentum and heat flux balance equations. This approach emphasizes the pivotal role of the momentum equation in the transport processes and is particularly useful in modelling plasma flows in experiments. The methodology for neoclassical transport theory is applied to fluctuation-driven transport fluxes in the quasilinear theory to unify these two theories. Experimental observations in tokamaks and stellarators for the physics discussed are presented.

Heavy impurity confinement in hybrid operation scenario plasmas with a rotating 1/1 continuous mode

“…4. One can notice that U 1θ,i ≈ 0 in the axisymmetric limit, as is expected from conventional neoclassical literature 23,25 , but is enhanced by an order of magnitude for the helical core in the core region. Additionally, this geometrical enhancement only appears in the flux through a factor of M 2 * /(1 + M 2 * ).…”

“…To complete the solution of the parallel flow, a prescription for the parallel flow velocity in 3D magnetic fields can be obtained through the Shaing-Callen neoclassical model 23 in the appropriate regimes. We assume that both the impurities and background ions are in the high-collisionality Pfirsch-Schlüter regime.…”

Recent ECRH/ECCD experiments aiming for higher density and temperature operations in the LHD

“…It was reported that the existence of i/2p = 1/2 near the edge of the e-ITB can realize higher central electron temperature [14]. And it is pointed out that the strong shear of the radial electric field at the boundary of the magnetic island in the helical plasma can contribute to the reduction of transport through turbulence suppression by E × B shear [24]. This mechanism can explain that higher t e_reff ≤ 0.25m is obtained with the existence of i/2p = 1/2 near the edge of the e-ITB compared to the case when i/2p = 1/2 is vanished by co ECCD superimposed on the counter NBI, as described above.…”

Simultaneous feedback control of plasma rotation and stored energy on NSTX-U using neoclassical toroidal viscosity and neutral beam injection

“…Neoclassical toroidal viscosity (NTV) is a physical effect caused by the non-ambipolar diffusion of ions and electrons. 36 The effect can be created by the application of nonaxisymmetric fields in a tokamak, and has been demonstrated to provide an actuator for plasma rotation in NSTX. 37 Depending on the spectrum of the non-axisymmetric field applied, NTV can be created over significant portions of the plasmas.…”

“…Results could strongly influence the design of future fusion reactors like the fusion nuclear science facility FNSF, [3][4][5] by helping to understand some key physics issues such as non-inductively sustaining a plasma at high normalized beta, b n . [6][7][8][9][10][11] The two main components of the upgrade are the complete replacement of the NSTX-U center stack by 36 22-footlong, 350-pound copper conductors which contain the inner-leg of the toroidal field (TF) coils, the ohmic heating (OH) solenoid, and some divertor coils, and the addition of a second neutral beam injector, oriented more tangentially compared to the old set for NSTX. This will enable the temperature inside NSTX-U to exceed the 15 Â 10 6 C core of the sun, it will double the toroidal field (TF) capability reaching about 1.0 T, and will also double the plasma current reaching about 2.0 MA.…”