Topic/Type: 2.4 Gyro-kinetic and gyro-fluid methods, Oral

Optimisation of the GS2 gyrokinetic software

W. Arter1, M. A. Barnes1, C. M. Roach1, N. J. Hammer2, R. Hatzky2

1 EURATOM/UKAEA Fusion Association
2 High Level Support Team (HLST), Max-Planck-Institut f\"ur Plasmaphysik

GS2 is a time-dependent 5-D gyrokinetic fluid code extensively
validated for fusion work and widely used on HPC facilities for the
simulation of plasma turbulence, in particular for predicting tokamak transport.
Inefficiencies identified in a previous study
by Belli [1] will be addressed by a programme of work designed ultimately
to deliver improvements in GS2 accuracy and speed of execution.

Belli showed that two related issues, choice of (1) discretisation and (2)
solver for the Boltzmann
equation, needed to be addressed. These issues are common to a wide set of
compute-intensive transport
codes, including software for neutron transport [2] and radiative transport [3].
However, the plasma problem is
more general since aside from its nonlinearity, in the language of radiation
transport it is highly anisotropic, whereas most transport problems have no
or weak anisotropy and algorithms tend to exploit this property to produce
so-called multi-group energy discretisations.

Discontinuous Galerkin (DG) schemes are relatively new. However, DG
has been successfully implemented in the commercially available
Attila\textregistered [4] software for solving the (linear) Boltzmann equation.
DG has the important property for HPC that each element has data which is
minimally coupled to other elements.
Results for DG applied to a 1-D model
advection problem are compared with those from a number of popular, high
order schemes, and demonstrate the good accuracy of the method. Results for a 2-D
model Boltzmann test problem will also be presented.

Work partly funded by UK EPSRC and EURATOM

[1] E.A. Belli. \textit{Studies of Numerical Algorithms for Gyrokinetics and
the Effects of Shaping on Plasma Turbulence}.
PhD thesis, Princeton University, 2006.

[2] M.L. Adams and E.W.~Larsen.
\textit{Progress in Nuclear Energy}, 40(1):3--159, 2002.

[3] E.~Meinkohn, G.~Kanschat, R.~Rannacher, and R.~Wehrse.
In J\"ager, W. and Rannacher, R. and
Warnatz, J., editors, \textit{Reactive Flows, Diffusion and Transport}, pages
485--526. Springer, 2007.

[4] Transpire Inc.,