Effect of beam-attenuation modulation on fluctuation measurements by heavy-ion beam probe

Beam-attenuation modulation arising from density fluctuations along the orbit of the heavy-ion beam probe (HIBP) in a plasma can distort the local amplitude, coherence, and phase derived from one- and two-point correlation measurements. Path-integral expressions for these effects are derived and applications to the TEXT tokamak are discussed. The work is part of an effort to account for previously reported wave-number data. However, the analysis is general and bears on any correlation measurement in turbulent media that depends on beam propagation or might otherwise be affected by spurious common-mode signals. In the HIBP case the effects depend critically on the ratio of the average fluctuation amplitude ñe along the beam path to the local ñe at the sample volume. Because the fluctuation amplitude is small in the core and rises sharply toward the plasma edge, the contamination effect is negligible in a radial zone near the edge but rises sharply to the interior of a critical radius. With increasing average plasma density n̄e the interior contamination increases strongly and the critical radius moves outward. The conclusion is that beam-modulation fluctuations affect the data but do not fully explain the disagreement with theoretical predictions of drift waves. The effects are expected to decrease with increasing beam energy.