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. Based on
the techniques developed in previous works [1,2], we will investigate
the effect on stability of finite drift orbit widths with analytical
and semianalytical approaches. The project would build on previous
experience in local gyrokinetic stability in pedestals (e.g., [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 et al., "Radially global delta f computation of neoclassical phenomena in a tokamak pedestal," PPCF 56, 045005 (2014) 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) 
