[en] A slow wave structure known as a multijunction grill is often used in lower hybrid current drive and plasma heating experiments in tokamaks. The distance between the junction of subsidiary waveguides in the main line and the grill mouth is one of the most important grill parameters determining the spectrum of radiated waves, the total reflection coefficient, power distribution among waveguides and the actual phase shift of individual waves. For maximum lower hybrid current drive efficiency, the grill must also be optimized with respect to this parameter. The optimization procedure is shown by using the results of a numerical investigation of the four-waveguide multijunction grill operated in small tokamak CASTOR. (J.U.). 5 figs., 9 refs

[en] The design of a multijunction grill which is supposed to be used in current drive experiments, calls for the optimization over three most important parameters: the phase shift ΔΦ among the adjacent waveguides, the length of the multijunction grill L_g (from the junction to the mouth) and the surface density n₀. This was demonstrated by the results of a numerical investigation of the four-waveguide grill designed for a small tokamak. It was shown that current drive efficiency (or the directivity) can reach 50% for practically arbitrary ΔΦ (≠0deg, 180deg), if L_g is dosen adequately. Enhanced current drive efficiency at ΔΦ≠90deg follows from an unevenly distributed power among separate waveguides and from the self-adaptive adjustment of the phases of the incident waves. The risk of power overload of the waveguides grows with decreasing n₀. A brief description of the author's variant of the theory of wave diffraction on the junction is given in the Appendix. (author). 13 figs., 17 refs

[en] The propagation was studied of high-frequency electromagnetic waves in a plane-stratified weakly inhomogeneous plasma. The density gradient was assumed to be perpendicular to the external magnetic field and the wave vector was not considered to be generally parallel to the plane given by the two former vectors. The analysis shows that an ordinary wave may penetrate the plasma resonance region if the orientation of the vacuum wave vector is appropriately chosen. Analytical expressions for the reflexion and transmission coefficients were obtained and their dependence on the direction cosines of the wave vector of the incident wave was studied. It is also briefly shown that after the transmission through plasma resonance the ordinary wave was transformed into an extraordinary wave and the latter was reflected back to the region of hybrid resonance. In this region the extraordinary wave was fully transformed into the Bernstein modes. (author)

[en] An approximate analytic theory of the general multijunction grill is developed. Omitting the evanescent modes in the subsidiary waveguides both at the junction and at the grill mouth and neglecting multiple wave reflection, simple formulae are derived for the reflection coefficient, the amplitudes of the incident and reflected waves and the spectral power density. These quantities are expressed through the basic grill parameters (the electric length of the structure and phase shift between adjacent waveguides) and two sets of reflection coefficients describing wave reflections in the subsidiary waveguides at the junction and at the plasma. Approximate expressions for these coefficients are also given. The results are compared with a numerical solution of two specific examples; they were shown to be useful for the optimization and design of multijunction grills.For the JET structure it is shown that, in the case of a dense plasma,many results can be obtained from the simple formulae for a two-waveguide multijunction grill. (author) 12 figs., 12 refs

[en] A proposal is given of a new launcher of lower hybrid waves for the current drive in future big thermonuclear facilities operating in the 10 GHz frequency range. The principle has been combined of the quasi-optical grill with the concepts of the hyperguide and the multiinjection grill. As an example, a six rod structure model was optimized mounted in a oversized waveguide and irradiated by the oblique plane wave emerging in the form of a higher mode from an auxiliary oversized waveguide. The rods of the optimum structure have the elongated form of the cross-section with the resonant length in the direction of wave propagation equal to a multiple of the half-wavelength of the fundamental mode of the hyperguide. This row of rods forms a multiinjection grill with zero phase shift between waveguides. The second row of rods supporting the constructive superposition of the incident and doubly reflected waves enhances the efficiency of the structure. The optimum structure has a power spectrum with narrow peaks (the main N_||=-2.15 and the parasitic N_||=3.15), low power reflection (R_tot=15%), high coupled power directivity (δ_CP=70%), reasonable N_||-weighted directivity (|δ_CD^w|=35%) and the peaking factor on the electric field equal to 3. Based on the optimization it is possible to design parameters of a big structure with tens of rods. The number of the construction elements of the structure can be reduced 20 times compared with the standard multijunction array. (author) 14 figs., 22 refs

[en] Two four-waveguide grills of an operating frequency of 2.45 Gcps currently under construction are planned for new experiments on lower-hybrid current drive in the T-7 tokamak. One should transmit the main power, the other only the part containing strongly slowed-down waves. The waveguide widths are 14 mm for one grill and 7 mm for the other. In the paper the power spectra and the total reflectivities of the grills, as well as the reflected power distributions between input waveguides are computed for various phase shifts between incident waves and two values of surface plasma density. As the effect of the wall corrugation is found to be relatively small, most computations are performed for the grills imbedded in a smooth wall. The much stronger effect of surface plasma density is discussed. (J.U.)

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[en] In connection with the problem of plasma heating by high-frequency waves a numerical code was developed which makes it possible to study the incidence of an electromagnetic wave on an inhomogeneous plasma slab. Using a one-dimensional model, non-magnetized plasma is described by means of two-fluid equations with finite electron pressure and with the adiabatic condition for all processes. It is shown that at normal incidence of a wave on a cold plasma, the wave is reflected from the region of plasma resonance. A standing wave arises which generates an electrostatic standing wave of a double frequency. At the same time the density gradient sharply rises in this region. In a warm plasma the incident wave again creates a standing wave but nonlinear perturbations propagate from the region of plasma resonance at ion acoustic velocity to the whole plasma volume. In this case the density gradient does not change very much. In the region of plasma resonance ion acoustic waves are also generated. (author)