The
pedestal is a region of reduced plasma turbulence that naturally
appears in the edge of tokamak plasmas when the input power is above a
threshold that is currently determined experimentally. It is widely
accepted that the electric field gradient observed in the pedestal is
responsible for shearing turbulence and reducing energy losses in the
pedestal. This electric field gradient is sustained by the pedestal
pressure gradient, i.e., the plasma tries to expand and it is pushed
inwards by a large electric field. In the usual picture of pedestal
formation, if the plasma pressure gradient increases, the electric
field and its gradient increase as well, and eventually, the turbulence
reduction due the electric field gradient dominates over the increase
in turbulence driven by the pressure gradient, leading to a sharp
transition and the appearance of the pedestal. What determines the
critical pressure gradient? One possible answer is that the critical
gradient is reached once the characteristic scale length of the
pressure is comparable to the width of the confined particle orbits in
the edge. This picture is somewhat supported by the fact that the
radial extent of particle orbits can be a large fraction of the
pedestal width. We propose to study this possibility by searching for
reduced turbulence states supported by finiteorbitwidth effects. We
will start by studying pedestals observed in current machines. It has
been possible to construct neoclassical models that incorporate finite
orbit width effects [1,2]. Based on the techniques developed in these
previous works, we will investigate the effect on stability of finite
drift orbit widths with analytical and semianalytical approaches. We
will consider both microinstabilities and MHDlike modes. The project
will build on previous experience in local gyrokinetic stability in
pedestals (see for example, [3]) and on work previously done to include
the MHD kink drive in gyrokinetics [4]. We intend to use the model to
study the formation and stability of JET and MASTU pedestals,
comparing experimental results, including DBS measurements, to
theoretical predictions. To extend the work to MASTU, in which the
drift orbit width can be comparable to the ion gyroradius, we have
developed techniques to include finite ion gyroradius effects
selfconsistently [5].



Background
Reading:
1. G. Kagan & P. J. Catto, "Arbitrary poloidal gyroradius
effects in tokamak pedestals and transport barriers," PPCF
50, 085010 (2008)
2. M.
Landreman & D. R. Ernst, "Local and global Fokker–Planck
neoclassical calculations showing flow and bootstrap current
modification in a pedestal,"
PPCF 54, 115006 (2012)
3. D. Hatch et al., "A gyrokinetic perspective on the JETILW
pedestal,"
PPCF
57, 036020 (2017)
4. I. Pusztai et al., "A currentdriven electromagnetic mode in sheared
and toroidal configurations,"
PPCF
56, 035011 (2014)
5. A. Geraldini, F. I. Parra, & F. Militello, "Gyrokinetic
treatment of a grazing angle magnetic presheath,"
PPCF
59, 025015 (2017)
4. Plasmawall interaction in magnetic confinement fusion plasmas
Supervisor: Prof
Felix
Parra Diaz
UKAEA cosupervisor: Dr Stefan Mijin
As
we build more efficient fusion devices, the amount of power released by
the fusion plasma onto the vacuum chamber walls increases rapidly. The
walls of the next generation of fusion experiments are expected to
suffer heat loads that existing materials can barely withstand.
Thankfully, there are several proposals to reduce the heat load using
concepts from magnetized plasma physics and atomic physics. Some of
these ideas (such as the superX divertor [1]) are going to be tested
in machines such as MASTUpgrade (situated at UKAEA, a few miles from
Oxford). The student working on this DPhil project will focus on one
very important aspect for the success of the several schemes mentioned
above: the interaction between the plasma and the wall. This is a
problem at the boundary between plasma physics and condensed matter
physics, and, despite being of fundamental importance for laboratory
plasmas, several key questions remain unanswered. The fact that thin
layers of nonneutral plasma form around the wall means that the
problem can be treated accurately with simplified theoretical 1D or 2D
models. The student working on this project will build on theoretical
and numerical tools developed by the Plasma Theory Group in Oxford and
UKAEA [2, 3] to study the highly nonlinear physics of the thin layers
described above. The focus of this project is on new physical regimes
that we expect to find in future reactors, and on the transitions and
bifurcations between these regimes. This project is a collaboration
with UKAEA, and the student will have the opportunity to work with the
team that will exploit the exciting new facility MASTUpgrade.



Background
Reading:
1. P.M. Valanju et al., "SuperX divertors and high power density fusion devices," Phys. Plasmas 16, 056110 (2009)
2. A. Geraldini, F.I. Parra and F. Militello, "Gyrokinetic
treatment of a grazing angle magnetic presheath,"
PPCF 59, 025015 (2017)
3. A. Geraldini, F.I. Parra and F. Militello, "Solution to a collisionless shallowangle magnetic presheath with kinetic ions,"
PPCF 60, 125002 (2018)
4. A. Geraldini, F.I. Parra and F. Militello, "Dependence on ion
temperature of shallowangle magnetic presheaths with adiabatic
electrons,"
J. Plasma Phys. 85, 795850601 (2019)
5. S. Mijin et al., "Kinetic and fluid simulations of parallel electron
transport during equilibria and transients in the scrapeoff layer,"
PPCF 62, 095004 (2020)