Dissertation

Electron Bernstein wave thermal emission and mode conversion in the CDX-U spherical torus

Jones, Brent Manley

2002

Overview

In tokamaks (toroidal magnetically confined fusion plasmas), electron cyclotron emission has been used for many years to measure the electron temperature (Te) profile. A microwave radiometer detects blackbody emission, with a given frequency corresponding to a unique electron cyclotron harmonic (n fce) resonance layer within the plasma. Alternately, electromagnetic waves can be launched to heat or drive current at cyclotron resonances. These techniques cannot be used in a class of “overdense” plasmas in which the plasma frequency (fpe) is much greater than fce so that electromagnetic waves in this frequency range are evanescent. An alternate wave is considered here which enables similar techniques in overdense spherical tokamaks (ST) and other high-β plasmas. The electrostatic electron Bernstein wave (EBW) is thermally emitted at n fce resonances, and can propagate below fpe. The EBW can mode convert to electromagnetic waves at the upper-hybrid resonance (UHR) layer surrounding the plasma, at which point it can be detected using standard radiometric techniques. In this work, EBW mode conversion to X-mode waves (B-X) is studied as a means to measure Te in an overdense plasma. An in-vacuum antenna measures B-X emission in the CDX-U ST. To control and optimize the B-X conversion, a local limiter shortens the density scale length (Ln) measured with a Langmuir probe array at the UHR, which theoretically allows the mode conversion efficiency (C ) to approach unity. The emission is predominantly X-mode polarized and emitted near the n fce layer. Comparing the Te profile measured by Thomson scattering to the EBW radiation temperature (Trad) profile verifies C ∼ 100%. Large (∼50%) fluctuations in Trad are observed, though, and Trad ∼ Te is seen only at the peak of fluctuating emission. Theoretical C calculated with measured, fluctuating Ln is ∼75% correlated with Trad. Other sources of Trad variation are discussed. These results provide experimental evidence supporting B-X conversion theory and have demonstrated the feasibility of measuring Te through B-X emission. Since the inverse X-B conversion process obeys the same physics, this work also validates the prospect of performing heating/current drive in an ST via mode conversion of launched X-mode waves to the EBW.

• Publisher: ProQuest Dissertations & Theses
• Subject: Electromagnetics / Electromagnetism / Fluid dynamics / Gases / Plasma physics
• ISBN: 0493756663, 9780493756660