First and second harmonic ECRH experience at gyrotron frequencies at LLNL Page: 4 of 20
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Although both sources affected the hot electron populations, the second
harmonic was clearly dominant in our experiments.
The transmission systems used to deliver gyrotron power to the
experiment with extraordinary mode polarization are shown 1n Fig. 2, for
thermal barrier heating, and 1n Fig. 3 for the potential peak. The
gyrotron produces its output power mainly in the TEq2 mode 1n oversize
waveguide. Both systems retain the overmoded waveguide for low loss
transmission. For the thermal barrier a TEQ2 to TEQ1 mode converter and a
focussing twist reflector were used to produce a linearly polarized and
focussed microwave beam (500 V-cnf1 peak field) for maximum heating
efficency.^
For fundamental heating both highly focussed Vlasov antenna
illumination (1 kV-cnf* peak field) and a polarizing slot radiator were
used to illuminate the highly elliptical (10:1) resonance surface. The
slot radiator produced an over-size beam with rf electric field up to
150 V-cm-1. Only 30-40* was directly incident on the plasma cross-section.
The remaining power filled the surrounding cavity and could be absorbed at
higher harmonic (2. ^ 2) resonances by the hot electrons. Measured cavity
field varied from 20 to 80 V-cm-1.
Thermal barriers with potential depression 1n excess of 500 V have
been produced as shown in F1g. 4. Potentials were Inferred by measuring
the change in energy of loss-cone injected neutrals which were Ionized in
the thermal barrier and escaped out the ends of the device. The potential
dip 6$ was driven by second harmonic heating, as shown in the figure, and
disappeared when ECRH was shut off. Such potential wells would provide
very long confinement times for high Z Ions. However, to produce such deep
wells requires a large fraction of mirror trapped electrons.
The locations of heating, magnetic field lines, and mod-B magnetic
surfaces in an end cell are shown in Fig. 5. The field has a quadrupcle
m1n-B with a radial well depth of about 2%. The axial mirror ratio is 4.
In TMX-U it was desirable to produce a mean hot electron temperature
of 50 keV, with a trapped electron fraction exceeding 0.8. To achieve
control of the electron energy, the scheme illustrated in Fig. 6 was tried.
Using a microwave beam with the power profile spatially limited to the
vicinity of the cold electron resonance (w a 2wce), electron heating should
become weak for electrons whose relativlstlcally shifted resonance moves to
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Stallard, B. First and second harmonic ECRH experience at gyrotron frequencies at LLNL, article, November 1, 1987; [Livermore,] California. (https://digital.library.unt.edu/ark:/67531/metadc1087422/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.