178 research outputs found
Intrinsic rotation with gyrokinetic models
The generation of intrinsic rotation by turbulence and neoclassical effects
in tokamaks is considered. To obtain the complex dependences observed in
experiments, it is necessary to have a model of the radial flux of momentum
that redistributes the momentum within the tokamak in the absence of a
preexisting velocity. When the lowest order gyrokinetic formulation is used, a
symmetry of the model precludes this possibility, making small effects in the
gyroradius over scale length expansion necessary. These effects that are
usually small become important for momentum transport because the symmetry of
the lowest order gyrokinetic formulation leads to the cancellation of the
lowest order momentum flux. The accuracy to which the gyrokinetic equation
needs to be obtained to retain all the physically relevant effects is
discussed
Axisymmetric plasma equilibrium in gravitational and magnetic fields
Plasma equilibria in gravitational and open-ended magnetic fields are considered for the case of topologically disconnected regions of the magnetic flux surfaces where plasma occupies just one of these regions. Special dependences of the plasma temperature and density on the magnetic flux are used which allow the solution of the Grad–Shafranov equation in a separable form permitting analytic treatment. It is found that plasma pressure tends to play the dominant role in the setting the shape of magnetic field equilibrium, while a strong gravitational force localizes the plasma density to a thin disc centered at the equatorial plane
Linearized gyro-kinetic equation
An ordering of the linearized Fokker-Planck equation is performed in which gyroradius corrections are retained to lowest order and the radial dependence appropriate for sheared magnetic fields is treated without resorting to a WKB technique. This description is shown to be necessary to obtain the proper radial dependence when the product of the poloidal wavenumber and the gyroradius is large (k rho much greater than 1). A like particle collision operator valid for arbitrary k rho also has been derived. In addition, neoclassical, drift, finite (plasma pressure/magnetic pressure), and unperturbed toroidal electric field modifications are treated. (auth
Drift-Ordered Fluid Equations for Field-Aligned Modes in Low-β Collisional Plasma with Equilibrium Pressure Pedestals
Omnigenity as generalized quasisymmetry
Any viable stellarator reactor will need to be nearly omnigenous, meaning the
radial guiding-center drift velocity averages to zero over time for all
particles. While omnigenity is easier to achieve than quasisymmetry, we show
here that several properties of quasisymmetric plasmas also apply directly or
with only minor modification to the larger class of omnigenous plasmas. For
example, concise expressions exist for the flow and current, closely resembling
those for a tokamak, and these expressions are explicit in that no magnetic
differential equations remain. A helicity (M,N) can be defined for any
omnigenous field, based on the topology by which |B| contours close on a flux
surface, generalizing the helicity associated with quasisymmetric fields. For
generalized quasi-poloidal symmetry (M=0), the bootstrap current vanishes,
which may yield desirable equilibrium and stability properties. A concise
expression is derived for the radial electric field in any omnigenous plasma
that is not quasisymmetric. The fact that tokamak-like analytical calculations
are possible in omnigenous plasmas despite their fully-3D magnetic spectrum
makes these configurations useful for gaining insight and benchmarking codes. A
construction is given to produce omnigenous B(theta, zeta) patterns with
stellarator symmetry.Comment: 37 pages, 8 figure
- …