Utilizing the dependence of the fast Alfven wave upon density, the mass density evolution of a plasma can be tracked via interferometry. In previous measurements on the DIII-D tokamak ͓H. Ikezi et al., Phys. Plasmas 3, 2306 ͑1996͔͒, fast waves ͑ϳ60 MHz, ϳ5 W͒ were launched from an antenna at the outer midplane. Detection was hampered by the poor sensitivity of the receiving antennas that were mounted behind protective graphite tiles on the inner wall. New antennas were installed where the graphite tiles were converted to be part of the receiving antenna, increasing reception by at least one order of magnitude. Density evolution measurements with these new antennas ͑ϳ100 MHz, 20 mW͒ were made for the first several hundred milliseconds until tracking was lost. The plasma shape and an evanescent layer are the main factors for this loss. Changes in wave propagation ͑as determined by the ray tracing code CURRAY͒ are less important. When tracking was successful, the density evolution observed from the new antennas show reasonable agreement with existing diagnostics. In addition, by placing receiving antennas on the same wall as the launching antenna and launching a frequency near the ion-ion hybrid frequency, it may be possible to make an ion species mix ratio measurement using the same diagnostic.

A superheterodyne reflectometer could provide a direct and inexpensive measurement of ion species mixes with different charge-to-mass ratios. Using the cold plasma dispersion relation, the ion-ion hybrid cutoff frequency is uniquely determined by the density ratio and cyclotron frequencies of the two different species. The phase of a 20 MHz wave that travels from the launching point to the cutoff layer to the receiving antenna provides a direct measure of the hydrogen : deuterium species mix. In the first experiment, a fast Alfvén wave is launched perpendicular to a hydrogen-deuterium plasma from the low field side of the DIII-D tokamak. Quantitative measurements observe a hydrogen concentration range of 3-67% and a maximum penetration depth of 0.60 m. Corroborative values are obtained from two independent diagnostics. In the second experiment, the fast Alfvén wave is launched from the high field side (HFS) during a hydrogen puffing experiment. The results suggest that a wave launched from the HFS is able to tunnel through the resonance layer and reflect back to the receiving antenna.

A superheterodyne reflectometer can provide a direct and inexpensive measurement of the concentrations of ion species with different charge to mass ratios. The ion-ion hybrid cutoff frequency is uniquely determined by the cyclotron frequencies and concentrations of the different species. The phase of a ϳ20 MHz wave that travels from a launching antenna on the low-field side of a tokamak, reflects off the cutoff layer, then travels to a receiving antenna provides a direct measure of the species mix. Hydrogen concentrations between 3% and 67% are measured in DIII-D using this technique. In theory, the technique can measure the spatial profile of the tritium concentration in the International Thermonuclear Experimental Reactor. Possible practical difficulties include attenuation of the wave in the evanescent layer near the antenna.

Since fast Alfvén waves propagate across a plasma at the Alfvén speed, the plasma mass density can be determined through interferometry. In previous measurements on the DIII-D tokamak, 1 fast waves ͑ϳ100 MHz, ϳ5 W͒ were launched from an antenna at the outer midplane, but detection of the signal was hampered by poor sensitivity of the receiving antenna, which was mounted behind protective graphite tiles on the inner wall. We modified several graphite tiles to act as more sensitive receiving antennas. At lower frequencies ͑ϳ25 MHz͒, fast waves can reflect from the ion-ion hybrid cutoff layer. The position of this layer is sensitive to the ratio of hydrogen to deuterium in the plasma. Receiving antennas on the outer wall will measure the hydrogen concentration through reflectometry. Launching other frequencies may yield impurity density ratios as well. These techniques may be useful for measuring relative densities if D, T, and ␣ particles in burning plasmas.