Dettrick, Sean Alexander
Description
This thesis is concerned with neoclassical transport in the H-1NF heliac,
and contains an examination of drift-orbit geometries, a description of a
neoclassical Monte Carlo transport code, and a description of a method to
use that code to self-consistently calculate ambipolar radial electric fields.
We set out to study the contributions to neoclassical transport in H-1NF,
by first describing the topology and the abundance of collisionless, trapped
particle orbits in the presence of radial...[Show more] electric fields. We give an overview of
the trapped orbit geometries in H-1NF, and develop a method to numerically
classify the trapped particle orbits. On average, the trapped particle fraction
in H-1NF is 40%, with approximately 5%, 15%, and 20% of the orbits in the
deeply trapped, helically trapped, and toroidally trapped states, respectively.
A condensed version of this component of the thesis has been submitted to
Nuclear Fusion.
The orbit studies provide a background for the development of a neoclassical
Monte Carlo transport code, MCMuPPeT (for Monte Carlo, Multi
Processing Plasma Transport). Using the code, we compare several Monte
Carlo transport diagnostics, taken from the literature. Confinement times
and diffusion coefficients are calculated for plasma conditions which will be
achievable in H-1NF after the National Facility upgrade.
Since the electric field can dominate in the determination of the transport,
we develop an iterative method to self-consistently calculate the ambipolar
radial electric field, using the Monte Carlo code. The method is applied to the Argon plasma conditions observed in H-1NF, in the experimentally
observed Improved Conhnement Mode (ICM). To help interpret the results,
the ambipolar electric fields were calculated in the same conditions using a
well-known analytic model which was geometrically-fitted to H-1NF for our
purposes. Qualitative agreement was found between both of the neoclassical
models and the experimental results; the electric fields predicted in the
ICM conditions are typically twice as large as those predicted in the conditions
before the transition. The two models were also used to look for the
neoclassically predicted transition from negative to positive radial electric
field. Positive radial electric fields were observed, at long mean free path,
in Hydrogen plasma conditions which will be achievable in H-1NF after the
National Facility upgrade.
We have also developed methods to optimise the Monte Carlo code for
both parallel and vector computing environments. Two Message Passing
algorithms that we use to parallelise the MC code are presented in the appendix.
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