arXiv subjects

Bruce D. Scott

Publications and source records attributed to Bruce D. Scott.

14 recordsLinked to original sources

arXiv2024

On Magnetic Compression in Gyrokinetic Field Theory

The issue of finite magnetic compressibility in low-beta magnetised plasmas is considered within the gyrokinetic description. The gauge transformation method of Littlejohn is used to obtain a Lagrangian which contains this effect additionally. The field theory version obtains a system model which guarantees exact energetic consistency. Gyrocenter drifts under this model are considered within a Chew-Goldberger-Low MHD equilibrium allowing for pressure anisotropy. The contributions to the current divergence balance, hence the dynamics, due to the difference between the curvature and grad-B drifts and to the compressibility are shown to cancel up to corrections of order beta. This recovers an earlier result with the same conclusion within linear theory of kinetic ballooning modes.

physics.plasm-ph↗

arXiv2024

Gyrokinetic Field Theory as a Gauge Transform or: gyrokinetic theory without Lie transforms

Gyrokinetic theory is a basis for treating magnetised plasma dynamics slower than particle gyrofrequencies where the scale of the background is larger than relevant gyroradii. The energy of field perturbations can be comparable to the thermal energy but smaller than the energy of the background magnetic field. Properly applied, it is a low-frequency gauge transform rather than a treatment of particle orbits, and more a representation in terms of gyrocenters rather than particles than an approximation. By making all transformations and approximations in the field/particle Lagrangian one preserves exact energetic consistency so that time symmetry ensures energy conservation and spatial axisymmetry ensures toroidal angular momentum conservation. This method draws on earlier experience with drift-kinetic models while showing the independence of gyrokinetic representation from particularities of Lie transforms or specific ordering limits, and that the essentials of low-frequency magnetohydrodynamics, including the equilibrium, are recovered. It gives a useful basis for total-f electromagnetic gyrokinetic or gyrofluid computation. Various versions of the representation based upon choice of parallel velocity space coordinate are illustrated.

physics.plasm-ph↗

arXiv2016

Ergodicity of gyrofluid edge localised ideal ballooning modes

The magnetic field structure associated with edge localised ideal ballooning mode (ELM) bursts is analysed by nonlinear gyrofluid computation. The linear growth phase is characterised by the formation of small scale magnetic islands. Ergodic magnetic field regions develop near the end of the linear phase when the instability starts to perturb the equilibrium profiles. The nonlinear blow-out gives rise to an ergodisation of the entire edge region. The time-dependent level of ergodicity is determined in terms of the mean radial displacement of a magnetic field line. The ergodicity decreases again during the nonlinear turbulent phase of the blow-out in dependence on the degrading plasma beta in the collapsing plasma pedestal profile.

physics.plasm-ph↗

arXiv2016

Gyrofluid computation of magnetic perturbation effects on turbulence and edge localized bursts

The effects of non-axisymmetric resonant magnetic perturbation fields (RMPs) on saturated drift-wave turbulence and on ballooning mode bursts in the edge pedestal of tokamak plasmas are investigated by numerical simulations with a nonlinear six-moment electromagnetic gyrofluid model including zonal profile evolution. The vacuum RMP fields are screened by plasma response currents, so that magnetic transport by perturbed parallel motion is not significantly changed. Radial transport of both particles and heat is dominated by turbulent convection even for large RMP amplitudes, where formation of stationary convective structures leads to edge profile degradation. Modelling of ideal ballooning mode unstable edge profiles for single bursts including RMP fields causes resonant mode locking and destabilization.

physics.plasm-ph↗

arXiv2014

Scalability of the plasma physics code GEM

We discuss a detailed weak scaling analysis of GEM, a 3D MPI-parallelised gyrofluid code used in theoretical plasma physics at the Max Planck Institute of Plasma Physics, IPP at Garching b. München, Germany. Within a PRACE Preparatory Access Project various versions of the code have been analysed on the HPC systems SuperMUC at LRZ and JUQUEEN at Jülich Supercomputing Centre (JSC) to improve the parallel scalability of the application. The diagnostic tool Scalasca has been used to filter out suboptimal routines. The code uses the electromagnetic gyrofluid model which is a superset of magnetohydrodynamic and drift-Alfvén microturbulance and also includes several relevant kinetic processes. GEM can be used with different geometries depending on the targeted use case, and has been proven to show good scalability when the computational domain is distributed amongst two dimensions. Such a distribution allows grids with sufficient size to describe small scale tokamak devices. In order to enable simulation of very large tokamaks (such as the next generation nuclear fusion device ITER in Cadarache, France) the third dimension has been parallelised and weak scaling has been achieved for significantly larger grids.

cs.DC↗

arXiv2011

Observation of explosive collisionless reconnection in 3D nonlinear gyrofluid simulations

The nonlinear dynamics of collisionless reconnecting modes is investigated, in the framework of a three-dimensional gyrofluid model. This is the relevant regime of high-temperature plasmas, where reconnection is made possible by electron inertia and has higher growth rates than resistive reconnection. The presence of a strong guide field is assumed, in a background slab model, with Dirichlet boundary conditions in the direction of nonuniformity. Values of ion sound gyro-radius and electron collisionless skin depth much smaller than the current layer width are considered. Strong acceleration of growth is found at the onset to nonlinearity, while at all times the energy functional is well conserved. Nonlinear growth rates more than one order of magnitude higher than linear growth rates are observed when entering into the small-$Δ'$ regime.

physics.plasm-ph↗

arXiv2010

Derivation via free energy conservation constraints of gyrofluid equations with finite-gyroradius electromagnetic nonlinearities

The derivation of electromagnetic gyrofluid equations is made systematic by using the Hermite polynomial form of the underlying delta-f gyrokinetic distribution function. The gyrokinetic free-energy functional is explicitly used to set up the model. The gyrofluid free energy follows directly. The interaction term in the gyrokinetic Lagrangian is used to obtain the gyrofluid counterpart, from which the polarisation equation follows. One closure rule is decided for taking moments over the kinetic gyroaveraging operator. These steps fix the rest of the derivation of the conservative part of the gyrofluid equations. Dissipation is then added in a form to obtain positive definite dissipation and to obtain the collisional fluid equations in their appropriate limit. Existing results are recovered, with the addition of a completely consistent model for finite gyroradius effects in the nonlinearities responsible for magnetic reconnection.

physics.plasm-ph↗

arXiv2007

Nonlinear polarisation and dissipative correspondence between low frequency fluid and gyrofluid equations

The correspondence between gyrofluid and low frequency fluid equations is examined. The lowest order conservative effects in ExB advection, parallel dynamics, and curvature match trivially. The principal concerns are polarisation fluxes, and dissipative parallel viscosity and parallel heat fluxes. The emergence of the polarisation heat flux in the fluid model and its contribution to the energy theorem is reviewed. It is shown that gyroviscosity and the polarisation fluxes are matched by the finite gyroradius corrections to advection in the long wavelength limit, provided that the differences between gyrocenter and particle representations is taken into account. The dissipative parallel viscosity is matched by the residual thermal anisotropy in the gyrofluid model in the collision dominated limit. The dissipative parallel heat flux is matched by the gyrofluid parallel heat flux variables in the collision dominated limit. Hence, the gyrofluid equations are a complete superset of the low frequency fluid equations.

physics.plasm-ph↗

arXiv2005

Flux surface shaping effects on tokamak edge turbulence and flows

Shaping of magnetic flux surfaces is found to have a strong impact on turbulence and transport in tokamak edge plasmas. A series of axisymmetric equilibria with varying elongation and triangularity, and a divertor configuration are implemented into a computational gyrofluid turbulence model. The mechanisms of shaping effects on turbulence and flows are identified. Transport is mainly reduced by local magnetic shearing and an enhancement of zonal shear flows induced by elongation and X-point shaping.

physics.plasm-ph↗

arXiv2005

Dynamical Alignment in Three Species Tokamak Edge Turbulence

Three dimensional computations of self consistent three species gyrofluid turbulence are carried out for tokamak edge conditions. Profiles as well as disturbances in dependent variables are followed, running the dynamical system to transport equilibrium. The third species density shows a significant correlation with that of the electrons, regardless of initial conditions and drive mechanisms. For decaying systems the densities evolve toward each other. Companion tests with a simple two dimensional drift wave model show this persists even if the third species is a passively advected test field. Similarity in the transport character of electrons and the trace species does not imply that the electrons themselves have a test particle transport character.

physics.plasm-ph↗

arXiv2005

GEM -- An Energy Conserving Electromagnetic Gyrofluid Model

The details of fluctuation free energy conservation in the gyrofluid model are examined. The polarisation equation relates ExB flow and eddy energy to combinations of the potential and the density and perpendicular temperature. These determine the combinations which must appear under derivatives in the moment equations so that not only thermal free energy but its combination with the ExB energy is properly conserved by parallel and perpendicular compressional effects. The resulting system exhibits the same qualitative energy transfer properties as corresponding Braginskii or Landau fluid models. One clear result is that the numerical model built on these equations is well behaved for arbitrarily large perpendicular wavenumber, allowing exploration of two scale phenomena linking dynamics at the ion and electron gyroradii. When the numerical formulation is done in the globally consistent flux tube model, the results with adiabatic electrons are consistent with the ``Cyclone Base Case'' results of gyrokinetic models.

physics.plasm-ph↗

arXiv2002

Zonal Flows and Electromagnetic Drift Wave Turbulence

Detailed computations of tokamak edge turbulence in three dimensional, globally consistent flux tube geometry show an inhibition of the standard scenario in which zonal ExB flows generated by the turbulence should lead to transport barrier formation. It is found by comparison to slab geometry and by analysis of the energetics that the zonal flow energy is depleted by toroidal coupling to the pressure through the geodesic curvature. Edge transport barriers would then depend on the physics of the neoclassical equilibrium.

physics.plasm-ph↗

arXiv2002

The Character of Transport Caused by ExB Drift Turbulence

The basic character of diffusive transport in a magnetised plasma depends on what kind of transport is modelled. ExB turbulence under drift ordering has special characteristics: it is nearly incompressible, and it cannot lead to magnetic flux diffusion if it is electrostatic. The ExB velocity is also related to the Poynting energy flux. Under quasineutral dynamics, electric fields are not caused by transport of electric charge but by the requirement that the total current is divergence free. Consequences for well constructed computational transport models are discussed in the context of a general mean field analysis, which also yields several anomalous transfer mechanisms not normally considered by current models.

physics.plasm-ph↗

arXiv2002

Drift Wave versus Interchange Turbulence in Tokamak Geometry: Linear versus Nonlinear Mode Structure

The competition between drift wave and interchange physics in general E-cross-B drift turbulence is studied with computations in three dimensional tokamak flux tube geometry. For a given set of background scales, the parameter space can be covered by the plasma beta and drift wave collisionality. At large enough plasma beta the turbulence breaks out into ideal ballooning modes and saturates only by depleting the free energy in the background pressure gradient. At high collisionality it finds a more gradual transition to resistive ballooning. At moderate beta and collisionality it retains drift wave character, qualitatively identical to simple two dimensional slab models. The underlying cause is the nonlinear vorticity advection through which the self sustained drift wave turbulence supersedes the linear instabilities, scattering them apart before they can grow, imposing its own physical character on the dynamics. This vorticity advection catalyses the gradient drive, while saturation occurs solely through turbulent mixing of pressure disturbances. This situation persists in the whole of tokamak edge parameter space. Both simplified isothermal models and complete warm ion models are treated.

physics.plasm-ph↗