Theoretical Study on Magnetic Shielding of an Ablating Solid Hydrogen Pellet in a Plasma and Hydrogen Pellet Production and Injection Into the Apex Tokamak
Gilliard, Richard Phillip
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Permalink
https://hdl.handle.net/2142/67788
Description
Title
Theoretical Study on Magnetic Shielding of an Ablating Solid Hydrogen Pellet in a Plasma and Hydrogen Pellet Production and Injection Into the Apex Tokamak
Author(s)
Gilliard, Richard Phillip
Issue Date
1980
Department of Study
Nuclear Engineering
Discipline
Nuclear Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Nuclear
Energy
Language
eng
Abstract
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.
In addition a solid pellet injector is developed which produces relatively large solid pellets (diameters in the range of 400 to 900 (mu)m) and accelerates them to speeds higher than 10('4) cm/sec. In addition, the device is capable of continuous pellet production at an adjustable rate ranging from roughly a hundred to several thousand per second. The results of injection into the APEX tokamak at Argonne National Laboratory are presented, and the validity of the newly developed scaling laws is evaluated in light of existing experimental data.
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