Waveguide array excitation of lower hybrid fields in a tokamak plasma

1980

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A BSTRA CT.Launching of the fast wave in the lower hybrid frequency range is described. This wave is excited at the plasma edge by RF electric field's perpendicular to those required for the lower-hybrid wave. In high temperature plasmas, where the lower hybrid wave may not penetrate because of Landau damping or other effects near the edge, the fast wave might provide an alternative for heating and/or current generation in the central portion of the plasma. In addition, for high density plasmas this has the advantage that lower frequencies than those required for the lower hybrid excitation can be used. Thus wayeguides of convenient dimensions for maximum power transmission and ease of fabrication can be employed. Coupling from a waveguide array into an inhomogeneous plasma is analysed. Power reflection in the waveguides is found as a function of array design and density gradient at the edge. This reflection is fairly large (> 20%). Propagation into the plasma is then considered and the field structure and dispersion of the fast waves are found as a function of distance of penetration. Unlike the lower hybrid waves, fast waves do not form resonance cones and energy is dispersed over a large volume.

Section: Qrmentioning

confidence: 99%

Coupling to the fast wave at lower hybrid frequencies

Theilhaber

1980

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“…Using Eqs (1), (2), (8) Using these we can then obtain the electromagnetic field spectra in the plasma as excited by the dominantmode incident-field amplitude (V. 0 ) in the waveguide, and the power flow into the plasma.…”

Section: The Plasma Fieldsmentioning

confidence: 99%

“…In all cases, the finite size of all the cross-sectional dimensions of the waveguide relative to the plasma dimensions must be accounted for in the coupling problem. Previous analyses of the waveguide-plasma coupling problem have concentrated on the LHRF [1][2][3][4]. All these analyses were carried out in detail in only two dimensions: the waveguides were assumed to be perfectly conducting parallel-plates of infinite extent in the direction perpendicular to the confining magnetic field B 0 , and the excited plasma modes were thus forced to have no variation in that direction.…”

Section: Introductionmentioning

confidence: 99%

Three-dimensional theory of waveguide-plasma coupling

Theilhaber

1983

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ABSTRACT. A general, linear theory of coupling electromagnetic fields from free-space waveguide arrays to an inhomogeneous plasma in a magnetic field is presented. In contrast to previous analyses, which assumed parallel-plate waveguides, waveguides having closed cross-sections of arbitrary shape are considered; full account is taken of all the waveguide modes. Far away from the coupling region, the plasma is assumed to be absorptive and the waveguides to propagate only their dominant modes. The formulation is geared to obtain the reflection coefficients in the waveguides and the excitation of waves in the plasma. The results are applicable to RF heating of plasmas whose size is large compared to the waveguide array, as would be the case in a fusion reactor, and valid for the frequency regimes of either ion-cyclotron (harmonic), or lower-hybrid, or electron-cyclotron (harmonic) heating.

“…Reflection back into the waveguides, reduces the available power by 201. According to linear theory (13,14], from the remaining 64 Kw, about 35 to 40% is below accessibility. This power is fed to surface mode fields that remain near the plasma wall, and never penetrates in the main bulk of the plasma.…”

Section: A Inside the Plasmamentioning

confidence: 99%

A study of quasi-mode parametric excitations in lower-hybrid heating of tokamak plasmas

Villalón

1980

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A detailed linear and nonlinear analysis of quasimode parametric excitations relevant to experiments in supplementary heating of tokamak plasmas is presented. The linear analysis includes the full ion-cyclotron harmonic quasimode spectrum. The nonlinear analysis, considering depletion of the pump electric field, is applied to the recent Alcator A heating experiment. Because of the very different characteristics of a tokamak plasma near the wall (in the shadow of the limiter) and inside, t-he quasimode excitations are studied independently for the plasma edge and the main bulk of tFle plasma, and for two typical regimes in overall d&nsity, the low (peak in density, ne = 1.5 x 1 0 H cM~?) and high (7o = _ x 10 ~M3) density regimes. At the edge of the plasma and for the low density regime, it is found that higher Pz 's (nz = c kz'o) than those predicted by the linear theory are strcngly excited. Inside the plasma,. the excitation of higher wave-numbers is also signiftcant. These results' indicate that a large amount of the rf-power may not penetrate to the plasma center, but will rather be either Landau-damped on the electrons or modeconverted into thermal modes, close to the plasma edge . Moreover, for sufficiently high peaks in density it is found that all the rf-power is modeconverted before reaching the plasma center. Inside the plasma the power density of the excited sideband fields is shown to be always very small in comparasion with their excitation at the plasma edge.