[en] Antenna coupling to the shear Alfven wave by both direct excitation and fast wave resonant mode conversion is modelled analytically for a plasma with a one dimensional linear density gradient. We demonstrate the existence of a shear Alfven mode excited directly by the antenna. For localised antennas, this mode propagates as a guided beam along the steady magnetic field lines intersecting the antenna. Shear Alfven wave excitation by resonant mode conversion of a fast wave near the Alfven resonance layer is also demonstrated and we prove that energy is conserved in this process. We compare the efficiency of these two mechanisms of shear Alfven wave excitation and present a simple analytical formula giving the ratio of the coupled powers. Finally, we discuss the interpretation of some experimental results. 45 refs., 7 figs

No abstract available

[en] Experimental results are presented which show that shear Alfven and ion-ion hybrid waves in a hydrogen-deuterium plasma are strongly guided by magnetic fields, even at frequencies close to the cyclotron frequencies. Rays were generated by a small dipole loop antenna located at the edge of a Tokamak plasma, and detected with a poloidal array of magnetic probes. The rays were observed to follow a trajectory along helical magnetic field lines intersecting the antenna. An additional ray path through the hydrogen cyclotron layer was also observed. These results are of relevance to the problem of edge heating in ICRF experiments. (author)

[en] Experimental observations are presented of the magnetically guided Alfven wave excited directly by a small dipole loop antenna located in the scrape-off layer of a tokamak plasma. This wave is excited most efficiently by antenna current elements aligned along the magnetic field and measurements indicate that, at all frequencies below the ion cyclotron frequency, the wave propagates for several transits around the machine with a high degree of localisation about magnetic field lines intersecting the antenna. Along the field, the wave has both a slowly varying amplitude and phase with predominantly radial electric and azimuthal magnetic field components. These experiments demonstrate that the Alfven wave can propagate as a magnetically guided TEM mode in plasmas which are highly inhomogeneous. A simplified mathematical description of the wave is also presented. 20 refs., 8 figs

[en] Short communication

[en] Antenna coupling to the shear Alfven wave by both direct excitation and fast-wave resonant-mode conversion is modelled analytically for a plasma with a one-dimensional linear density gradient. We demonstrate the existence of a shear Alfven mode excited directly by the antenna. For localized antennae, this mode propagates as a guided beam along the steady magnetic field lines intersecting the antenna. Shear Alfven wave excitation by resonant-mode conversion of a fast wave near the Alfven resonance layer is also demonstrated and we prove that energy is conserved in this process. We compare the efficiency of these two mechanisms of shear Alfven wave excitation and present a simple analytical formula giving the ratio of the coupled powers. Finally, we discuss the interpretation of some experimental results. (author)

[en] Short communication

[en] Despite the wide acceptance of electrostatic screens in ICRH for the protection of the plasma from the near fields of rf antennas, it has always been considered that low voltages at low frequency have made such screens unnecessary in Alfven wave heating (AWH). Despite this, AWH performs rather poorly as a heating method; the results being confused by a density rise up to 300 % of the target density. It is known that the density increase arises neither from impurity injection nor from a change in recycling. In addition, an extensive range of phenomena have been observed in the plasma scrape-off layer (SOL). During AWH, the SOL density is observed to decrease, the SOL floating potential is perturbed in a way that reflects the Alfven wave spectrum, the antennas charge negatively and draw a large current from the plasma and harmonics have been observed on the edge wave fields. The cause and correlation of these effects with each other and their impression on the bulk plasma response was not known. Experimental results from the TORTUS tokamak have indicated that the density increase might be eliminated by electrostatic screens. In their case, two AWH experiments were performed. In the first, an unshielded OFHC copper loop antenna was excited at a given power and, in the second, the same antenna was excited at the same power after installation of an aluminium, TiN coated, slotted screen. The density increase in the first case was shown to be completely eliminated in the second, although spectroscopic measurements revealed a difference in the plasma O(II) and Cu(I) content for each case. (author) 2 refs., 3 figs

[en] Short communication

[en] In the presence of finite plasma current, the axisymmetric (m=0) magnetoacoustic wave resonance exhibits a frequency splitting for finite toroidal mode number (n ≠ 0) between oppositely directed travelling waves. Calculations are presented which demonstrate that Δ(1/f), the difference between the inverse resonance frequencies, is directly proportional to known moments of the plasma current profile and, in the limit where the radial wavenumber is much larger than the parallel wavenumber, is independent of the plasma mass density and its profile. Some aspects of the implementation of a current profile diagnostic based on the excitation of these resonances are also considered. (author) 16 refs., 6 figs., 1 tab