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Flexible MHD tools for 3D boundaries and resistive wall coupling in fusion devices
By Chris Hansen*, Jeff Levesque, John Schmitt and Thomas Jarboe
*- University of Washington, [email protected]
Topics- D (A, C)
Presentation- Yes
Purpose: This whitepaper proposes the further development of simulation tools that
enable macroscopic modeling of full fusion devices through geometric flexibility and
multiphysics capabilities for coupled models of relevant physical regions (plasma,
resistive wall, etc.).
Motivation and background: Macroscopic MHD modeling of fusion devices is a
crucial tool in understanding observed plasma phenomenon in existing experiments and
predicting performance for future designs. However, the majority of existing codes used
in fusion modeling are tailored to model a single plasma region within an axisymmetric
geometry. While this provides desirable numerical properties when modeling the
Tokamak and other nominally axisymmetric devices, it limits study of physical
phenomenon related to or requiring 3D boundaries. For example, devices that inherently
contain complex 3D boundaries, such as the Helicity Injected Torus with Steady Injection
(HIT-SI)1 and stellarator devices, cannot be fully represented within an axisymmetric
boundary code. Even in nominally axisymmetric devices, such as the Tokamak, the 3D
structure of nearby conducting walls may have a significant impact on feedback response,
RWM torque and the resulting locking behavior2. 3D boundary effects are especially
important in small devices, such as the Lithium Tokamak eXperiment (LTX)3 and the
High Beta Tokamak – Extended Pulse (HBT-EP)4, where the impact of toroidal
asymmetries in the first wall can have first order effects on plasma equilibrium and mode
structure. Wall asymmetries are currently investigated using linear theory, where the
impact of 3D conductors on growth and vacuum field structure of a single toroidal mode
number is considered5. Extending study to the full, coupled wall-plasma magnetic
interactions requires multiphysics frameworks that support general 3D geometry and
allow the model description of the physical system to change from region to region. This
type of framework also enables simulations of a broader range of devices with plasma
regions that possess complex boundary shapes.
Opportunity: Significant research opportunities exist in the development and application
flexible simulation tools. Foundational research within the fusion community has
produced tools with some of the desired capabilities for these studies, which been applied
successfully to university scale fusion devices6,7. To leverage the existing capabilities we
propose further development of these tools to support application to resistive wall mode
studies and other coupled multiphysics problems. As stated above university scale
experiments are well suited for initial investigation of these phenomena, utilizing the
existing simulation capabilities within this community most efficiently. In order to
support broader development of flexible multiphysics tools for fusion modeling, research
into the use of modeling frameworks from the broader computational physics community
should also be considered along with integration of leading numerical techniques for
multiphysics simulations from other fields into fusion community tools. This research can
benefit significantly from connection between the fusion modeling community and the
broader ASCR community.
Impact: Resistive wall modes and 3D plasma boundaries are an important area of current
research for the advanced Tokamak (high beta, ELM/disruption-free operation).
Improved theoretical understanding of the governing dynamics of these phenomena in
existing devices will significantly aide in the development of advanced operating
scenarios required for economical reactor scenarios. In small devices, the ability to model
accurate machine boundaries can significantly improve the understanding of eddy current
and other geometric effects that can play an important role in plasma dynamics. This
understanding can in turn improve the interpretation of experimental results directly
relevant to larger devices (where wall/geometry effects may be secondary to the studied
physics).
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Figure 1- Unstable mode structure in the presence of a non-axisymmetric first wall similar to HBTEP as computed by the Plasma Science and Innovation center TETrahedral mesh code (PSI-TET)
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1!T. R. Jarboe, et al. “Spheromak Formation by Steady Inductive Helicity Injection” Phys. Rev.
Lett. 97, 11 (2006).!
2!M. Baruzzo, et al. “3D effects on RWM physics in RFX-mod”, Nuclear Fusion 51, 8 (2011)!
3
J. C. Schmitt, et al. “Magnetic diagnostics for equilibrium reconstructions with eddy currents on
the lithium tokamak experiment”, Review of Scientific Instruments 85, 11E817 (2014)
4!J.P. Levesque, et al. “Multimode observations and 3D magnetic control of the boundary of a
tokamak plasma”, Nuclear Fusion 53, 7 (2013)!
5!J. Bialek, et al. “Modeling of active control of external magnetohydrodynamic instabilities”,
Physics of Plasmas 8, 2170 (2001)!
6!C. Hansen, et al. “Simulation of injector dynamics during steady inductive helicity injection
current drive in the HIT-SI experiment”, Physics of Plasmas 22, 042505 (2015)
7!S. Knecht, et al. “Effects of a Conducting Wall on Z-Pinch Stability”, Plasma Science, IEEE
Transactions on (2014)