Theoretical Study on Magnetic Shielding of an Ablating Solid Hydrogen Pellet in a Plasma and Hydrogen Pellet Production and Injection Into the Apex Tokamak

The ablation rate of a solid hydrogen pellet in a plasma is the critical factor in determining the feasibility of pellet refueling of toroidal magnetically confined thermonuclear reactors. Initial experiments at Oak Ridge National Laboratory involving small pellets (diameter of order 200 (mu)m) and plasmas below the thermonuclear range have produced data in good agreement with the predictions of scaling laws developed by Parks and Turnbull. These scaling laws are based on an ablation model which visualizes the ablating pellet to be surrounded by a neutral hydrogen ablation cloud in the plasma. Other authors have pointed out that in a thermonuclear plasma the ablation cloud may be highly ionized and may exclude the magnetic field in the torus from the ablation cloud with a possible decrease in energy flux to the ablating pellet. The resulting so-called "magnetic shielding" ablation models predict pellet lifetimes significantly longer than the corresponding lifetimes of the neutral ablation cloud model. Most authors also correctly point out that the primary criterion for magnetic shielding to occur is a large magnetic Reynolds number. Unfortunately, the magnetic Reynolds number for an ablating pellet in proposed near-term magnetically confined thermonuclear devices is of order unity, leaving the validity of the different models open to question for actual reactors. In this work, the problem of magnetic diffusion into an ablation cloud and the corresponding effect on ablation rates is solved rigorously. Accurate criteria are developed for assessing the significance of magnetic shielding for various pellet and plasma configurations, and more generalized equations and scaling laws are presented which govern the ablation process in the case of partial shielding.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Nuclear Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2014

Author dc:creator

• Gilliard, Richard Phillip

• Engineering, Nuclear
• Energy

Language dc:language

eng

Identifier

(UMI)AAI8026499

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/67788