|Turbulent transport limits the confinement of tokamak plasmas, i.e., plasmas immersed in axisymmetric—and thus 2D—magnetic fields. Consequently much effort has gone into understanding the properties of turbulence and transport in tokamaks. However, perfect axisymmetry of the confining magnetic field is marred by design constraints and by large-scale plasma instabilities. Furthermore, non-axisymmetric fields are potentially preferable for fusion reactors because they do not require a current through the plasma for confinement and thus allow for steady-state operation. There is thus considerable interest in stellarators, which use 3D magnetic fields to confine the plasma. Despite the importance of the problem, there has been little work done on turbulence in 3D magnetic fields. The aim of this project is to understand how breaking axisymmetry affects plasma turbulence. In particular, can 3D effects on turbulence explain some of the mysterious transport phenomena observed in tokamaks, and how much turbulent transport can we expect in optimised stellarators? The student would address these questions with a combination of analytical theory and high performance computing.||
sufficient power is injected into magnetically confined fusion plasmas,
a region of reduced turbulence develops near the walls that contain the
plasma. In this region, known as pedestal, the plasma gradients grow to
be large because they are not reduced by the turbulent fluctuations
that limit gradients elsewhere in the plasma. The pressure jump across
the pedestal improves the performance of fusion plasmas significantly.
Unfortunately, even though pedestals are robust experimental
observations, there is no quantitative model that can explain their
appearance and evolution. In this project, the student will develop an
analytical kinetic model to determine what effect stabilises the plasma
in the presence of large gradients that drive wild instabilities
elsewhere in the fusion device. Examples of the effects to be
considered are the interaction of the hot pedestal plasma with the much
colder plasma near the wall or the fact that typical particles can
sample a significant fraction of the pedestal width before they collide
or are affected by turbulent fluctuations.
1. J. W. Connor, “A review of models for ELMs,” Plasma Phys. Control. Fusion 40, 191 (1998)
student will investigate, using relativistic fluid theory and
Vlasov-Maxwell simulations, the local heating of a dense plasma by two
crossing electron beams generated during multi-PW laser-plasma
interactions with a pre-compressed, inertial fusion target. Heating
occurs as an instability of the electron beams that drives Langmuir
waves, which couple non-linearly into damped ion-acoustic waves and
into the background electrons. Initial simulations show a factor-of-2.8
increase in electron kinetic energy with a coupling efficiency of 18%
. By considering the collisionless energy deposition of these
electron beams, we are able to demonstrate, via sophisticated
radiation-hydrodynamic simulations, that this results in significantly
increased energy yield from low convergence ratio implosions of
deuterium-tritium filled “wetted foam" capsules, as recently
demonstrated on the National Ignition Facility . This approach
promises to augment the heating of the central hot spot in these
targets, and is attractive as a complementary approach that of fast
ignition inertial fusion. The student will:
• Simulate (Vlasov or possibly particle-in-cell) parameter scan of the energy cascade. The question is how dependent are we upon the electron energy, thermal spread, divergence, beam-to-background density ratio.
• Simulate the energy cascade process in an inhomogeneous plasma.
• Simulate energy cascade using finite beams.
• Help design experiments verifying the energy cascade process.
The student will also use machine learning to study the optimisation of the energy deposition process.