Radial Electric Fields for Improved Tokamak Performance
Downum, Wesley Berry
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https://hdl.handle.net/2142/77209
Description
Title
Radial Electric Fields for Improved Tokamak Performance
Author(s)
Downum, Wesley Berry
Issue Date
1981
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Nuclear
Language
eng
Abstract
The influence of externally-imposed radial electric fields on the fusion energy output, energy multiplication, and alpha-particle ash build-up in a TFTR-sized, fusing tokamak plasma is explored. In an idealized tokamak plasma, an externally-imposed radial electric field leads to plasma rotation, but no charge current flows across the magnetic fields. However, a realistically-low neutral density profile generates a non-zero cross-field conductivity and the species dependence of this conductivity allows the electric field to selectively alter radial particle transport.
"The plasma burn time until reaching a given ash fraction (the ""time-to-buildup"") is used as a measure of plasma performance. Because the fuel species has a higher cross-field conductivity than the ash, imposition of an inward-directed field enhances fuel confinement more than ash confinement. An outward-directed field, on the other hand, drastically reduces the fuel density, but has little effect on the ash density. Therefore, the time-to-buildup is increased by an inward-directed field, but decreased by an outward-directed field."
To evaluate the spacial dependence of radial electric field effects on plasma performance, three charge-deposition profiles (shallow, moderately deep, and deep into the plasma) are taken to generate the field. Electric-field-induced transport is then self-consistently modeled. That is, the charge deposited inside the plasma is balanced by the current flow which results. The introduction of about one amp of charge current near the plasma edge allows nearly steady-state (t > 100 sec) operation with energy multiplication, Q, greater than unity (vs. Q (--->) 0 within tens of seconds without an imposed electric field). Deep penetration of the plasma by the charge source leads to even greater improvements in Q (Q > 4 at t > 100 sec).
Possible mechanisms for the injection of charge into the plasma are also discussed. Since straightforward schemes (e.g., direct injection of electrons across the magnetic field) require excessive amounts of power, new and original techniques are needed. Two such schemes (Fat Banana Carrier Injection and Ripple-Trap Carrier Injection) are explored and found to be promising methods of low-energy-cost charge injection.
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