Abstract
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 (1996)], fast waves (∼60 MHz, ∼5 W) 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 (∼100 MHz, 20 mW) 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 CURRAY) 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.
Highlights
In a previous experiment,[1] an ion cyclotron resonance frequencyICRFheating antenna was used to launch a 60 MHz, 5 W signal into the plasma
The wave travelled across the torus at the Alfven speed and was received by small loops mounted behind the protective graphite tile wall of the torus
The current diagnostic only differs from the previous diagnostic by having modified graphite tiles and smaller launched power, the effectiveness of the new antenna design is over an order of magnitude better than that of the previous design
Summary
In a previous experiment,[1] an ion cyclotron resonance frequencyICRFheating antenna was used to launch a 60 MHz, 5 W signal into the plasma. The inability to function for nonlimiter plasmas was attributed to the antennae being mounted behind the graphite wallsFig. 1͑a͒ of the tokamak This required the wave to penetrate through the graphite tiles, resulting in significant power loss. The hydrogen–deuterium relative concentrations can be measured by calculating the time of flight for the wave as it travels from the launching antenna, reflects off the ion–ion hybrid cutoff layerdetermined by Eq ͑2͔͒, and is picked up by the receiving antennaFig. 2͒. This diagnostic can apply to any ion–ion species mix, providing that the charge to mass ratios of the two species are different
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