BOUT

1
BOUT Towards an MHD Simulation of ELMs

• B. Dudson and H.R. Wilson
• Department of Physics, University of York
• M.Umansky and X.Xu
• Lawrence Livermore National Laboratory, CA
• P.Snyder
• General Atomics, San Diego, CA

2
Outline

• BOUT motivation and philosophy
• ELM modelling the approach and objectives
• Initial benchmarking results (work in progress),
  and future aims

3
BOUT Philosophy

• BOUT is a collaborative project between
  University of York and LLNL
• The code provides a framework for developing
  plasma fluid codes
• user defined magnetic geometry (in terms of
  metrics)
• user-defined plasma model
• Flexible, user-friendly code (small compromise
  on speed)
• easy to adjust plasma physics model, and explore
  implications

4
Example of the code Ideal MHD equations
dndt -nDiv(v) V_dot_Grad(v,n)
dpdt V_dot_Grad(v,p) - gammapDiv(v)
dvdt V_dot_Grad(v,v) ((Curl(B)B) -
Grad(p))/n
dBdt Curl(vB)
5
Physics Objectives

• There are two main objectives
• Edge turbulence modelling
• Edge MHD and ELMs
• focus on the ELM modelling here

6
ELM modelling- the approach

• Two complementary approaches to tackle the ELM
  problem
• Full non-ideal MHD code, towards a model for the
  ELM crash
• A range of codes being used NIMROD, BOUT,
  JOREK, M3D, etc
• Advantage well-developed codes, some with
  2-fluid effects
• Disadvantage difficult to pull out and and
  study the impact of specific physics elements
  without a detailed knowledge of the code making
  contact with analytic theory is not easy
• Building up from simple ideal MHD model
• Basic ideal MHD model eases comparison with
  analytic theory and linear codes (eg ELITE and
  non-linear ballooning theory)
• The model can then be slowly built up,
  monitoring the impact of different physics
  effects
• BOUT is ideally suited to exploring the second
  approach
• permits the user to add and subtract physics in
  a clear way

7
Initial benchmark studies (in progress)

• The Orszag-Tang vortex provides a standard
  test of 2D ideal MHD solvers looks good,
  qualitatively
• Tests the ability to treat shocks (possibly
  important for ELMs)

Athena, Roe solver
BOUT, ideal MHD
8
Quantitative Benchmark linear ideal MHD

• We have begun to test the code against ELITE
• For initial tests, we have implemented a reduced
  ideal MHD model into BOUT
• Valid for high-n ballooning modes
• Initial case strong instability, with
  significant peeling component
• OK for intermediate n, but unable to reproduce
  higher n (yet)
• Points to a problem with the kink/peeling drive
  (sensitive to plasma-vacuum boundary)

9
Produces fingers in non-linear regime

• Mode propagates radially
• Filamentary structures are produced in the
  non-linear regime
• Cannot take too seriously while there is
  disagreement in the linear regime
• but encouraging first signs!

10
New equilibrium to minimise coupling to vacuum

• Presently exploring a more ballooning case, with
  reduced coupling to vacuum (ELITE requires some
  edge interaction)
• ELITE predicts close to marginal stability
  g/wA0.01

Equilibrium mesh
ELITE
11
The challenges of marginal stability

• Agreement has not yet been achieved (the BOUT
  runs take 12 hours, while ELITE is 3 minutes, so
  comparisons are not trivial)
• It is necessary to work close to linear marginal
  stability
• it is the experimentally relevant situation (p
  increases slowly through marginal stability
• modes that are strongly unstable linearly are
  likely to have different dynamics
• existing non-linear theories are based on
  proximity to marginal stability
• One issue with proximity to marginal stability
  is resolution of fine-scale structures near
  rational surfaces
• makes sense to use nq as the radial variable to
  improve resolution around rational surfaces (pack
  mesh there) presently exploring this
• When we go non-linear, an additional challenge
  will be the time taken to get into the non-linear
  regime
• will need to make use of scaling of mode
  structure during linear phase to speed code up
  here

12
Future plans the strategy

• Work to find a mesh and formalism that gives
  agreement with ELITE close to marginal stability
  with weak coupling to vacuum
• Extend/return to linear tests where mode couples
  to vacuum
• Extend to non-linear regime
• compare non-linear evolution with and without
  kink-component
• Extend to include non-ideal physics (care
  unphysical modes can be introduced when
  dissipation is introduceddiamagnetic effects
  will be an important first effect to include).