IPP Annual Report 2007 - Max-Planck-Institut für Plasmaphysik ...

Generally, ITG turbulence is more difficult computationally
because it generates large scale “zonal” flows (“zonal”
refers to the flux surface average component). These flows
represent the perturbed equilibrium and are involved in geodesic
acoustic oscillation “modes” (GAMs). They serve to
moderate the turbulence and represent a long-term background
phenomenon to which the turbulence is coupled.
Hence, runs must be longer and noise and saturation issues
increase the difficulty. In 2007 the massively parallel scaling
properties of the code were improved and it now runs reliably
on at least 10,000 processors. A novel diagnostic to
quantify the noise issues was developed; recent cases are
run to saturation with the noise in the range of 5 percent.
Kinetic electrons have been included. A run on 10 k processors
on the Edinburgh Hector platform found well behaved
long term saturation and converged scale separation
(between turbulence and profile relaxation). In 2008 the
Alfvén dynamics will be tested and full scale ITER cases
will be run.
Global electromagnetic computations of ITG/Alfvén turbulence
were run using GEM for various tokamak sizes.
For domain sizes of at least 200 ion gyroradii complete
scale separation was found: the turbulence no longer has
any effect on the profile of poloidal (specifically, E×B)
rotation. The rotation profile is determined by neoclassical
effects (balances between parallel forces/divergences and
magnetic drifts). For JET and ITER scale (400 or 800 gyroradii)
even the details of the profile shape change only on
transport time scales. In the course of this work the GEM
code was improved so as to scale to 512 processors on the
IBM Regatta architecture at well over 80 percent efficiency
Theoretical Plasma Physics
84
in the hard scaling. Similar performance up to 2048 processors
on the new Blue GENE/P was achieved.
Gyrofluid/Gyrokinetic Studies of Edge Turbulence
The study of self consistent geometry up to now requires
analytical models for the metric. Changes in the q-profile
and the Shafranov shift due to the background currents and
pressure profile are followed. This model was extended to
include proper separatrix geometry, which will be incorporated
into the GEM code.
Several series of parameter scalings were run using both
GEM and dFEFI in fluxtube mode (radially local gyrofluid
and phase space gyrokinetic treatments of the same problem
set) to test hypotheses involving temperature scaling of
sheared equilibrium E×B rotation and turbulence. A weak
dependence of shear suppression on temperature and flow
shear was found, but too weak to overcome the larger ρ ∗ at
higher temperatures.
The ideal MHD instability scenario for ELM events was
investigated using GEM. In contrast to other studies using
MHD models, GEM also treats the generic drift wave turbulence
down to and below the ion gyroradius scale. No threshold
was found because the turbulence remains driven by
temperature gradients (both species) when the ideal ballooning
instability is absent. The transition to MHD dominance
is gradual. Converged cases are found only when the spectrum
reaches all the way down to the ion gyroradius. The time
scale and energy content of the blowout are commensurate
with experimental observations, but the scenario remains a
hypothesis because clearly self consistent representations
of the H-mode state are still lacking.
Figure 1: Electron density contours before, during, and after an ideal ballooning mode blowout. The original instability has toroidal mode numbers 7-9, visible at
the beginning of the event. The blowout eventually saturates upon its own self-generated turbulence, whose scale range reaches down to the ion gyroradius.
Work carried out in collaboration with A Kendl, Uni Innsbruck.