Project Overview

Tokamak designs must satisfy two conflicting constraints: on the one hand, they must confine a very hot and dense core plasma to produce fusion energy; on the other hand, the walls can only survive a sufficiently cold and low-power-density edge plasma. Moreover, one needs to exhaust the helium ash produced by the fusion reactions through this edge plasma. The exhaust of helium ash requires directing the plasma to an enclosed region, known as the divertor, where the helium can be pumped out. The plasma is guided towards the divertor walls by magnetic field lines (divertor legs) that start close to the hot core plasma. The plasma slowly diffuses into these divertor legs and then rushes along them towards the divertor walls. As a result, the small wall surface enclosed by the divertor receives all the heat flux generated by the hot core plasma -- unless the power is radiated before reaching the divertor. It is crucial that the heat flux on the divertor plates is spread as much as possible to avoid damaging the wall, but this requirement is in contradiction with the need to confine the plasma to magnetic field lines to keep the core plasma hot and dense. The reconciliation of these conflicting requirements has been identified by the FESAC report ``Powering the Future'' and the NASEM report "Bringing Fusion to the US grid" as a major challenge for Fusion Pilot Plants (FPPs): the Core-Edge Integration (CEI) challenge.

The objective of this project is to investigate the effectiveness of several actuators to integrate core and edge requirements using advanced computational tools. We will study three actuators.

• Magnetic field geometry
  By changing the currents and positions of external magnets, we can change the shape of the magnetic field lines. The geometry of these lines has a striking effect on the plasma performance. We will focus on two aspects of magnetic field geometry: the location of the X-point (the origin of the divertor legs), which determines whether improved confinement regimes such as the I-mode are accessible; and negative triangularity, which has recently been shown to have excellent confinement without many of the shortcoming of more traditional plasma shapes (e.g. it has no dangerous explosive instabilities known as ELMs).

• Impurity injection
  Impurities (ions different from deuterium and tritium, the fuel for fusion) can be injected into the plasma to increase radiation and change its confinement properties. This technique has proven particularly important for machines with walls made of tungsten, one of the materials preferred for FPPs. Tungsten is durable and does not absorb deuterium and tritium, but it affects plasma confinement negatively for reasons still poorly understood. In several experiments, the deleterious effect of tungsten has been reversed with impurity injection, and we need to understand why.

• Low recycling walls
  Lithium is another FPP wall material candidate because retains cold neutrals (an effect known as low recycling), maintaining the edge plasma very hot and the core plasma even hotter. Low recycling experiments have demonstrated impressive confinement, and a liquid lithium coating can protect the wall from large heat fluxes.

We will develop computational tools to evaluate the effectiveness of these CEI actuators for FPPs. Our models will include turbulent and collisional transport, impurities, radiation, and plasma-neutrals and plasma-wall interactions, to list some of the important physics. We will need to use the most powerful computers on Earth, and for this reason, we need the support of the SciDAC Institutes. As part of this project, computational scientists in the SciDAC Institutes will develop and implement new data management, coupling and meshing techniques, new algorithms, and new statistical procedures to check our models against experimental data.

With the computational tools that we develop, we will evaluate and design divertors, impurity injectors and low recycling coatings for FPPs. Our results and computational tools will allow the fusion community to take the qualitative step between the use of these CEI actuators in current machines and their use in the much larger, denser and hotter plasmas of FPPs.

Organization

Principal Investigator:
Felix I. Parra, Princeton Plasma Physics Laboratory

Participating Institutions:

• Princeton Plasma Physics Laboratory (PPPL):
  F. I. Parra, J. Dominski, R. Hager, A. Hakim, G. Hammett, I. Kaganovich, S. Ku, N. Mandell, S. Shiraiwa, B. Sturdevant, G. Wilkie

• General Atomics (GA):
  E. Belli, T. Bernard, J. Candy

• Oak Ridge National Laboratory (ORNL):
  S. Klasky, D. Pugmire, E. Suchyta

• Rensselaer Polytechnic Institute (RPI):
  M. Shephard, O. Sahni, C. Zhang

• Jubilee Development:
  A. Scheinberg

• Southern Methodist University (SMU):
  D. Reynolds

• University of Southern California (USC):
  R. Ghanem

• University of Texas at Austin (UT Austin):
  D. Hatch, M. Kotschenreuther