Comparison of Edge Fluctuation Measurements from PCI, BES, Langmuir Probes and Reflectometry on DIII-D

Phase contrast imaging (PCI), beam emission spectroscopy (BES), reflectometry, and Langmuir probes provide different but complementary measurements of the density fluctuations in the edge of the DIII‐D tokamak plasma. In particular, the PCI is sensitive to modes with k << kr, while the BES and probe response depends less on the angle of r k . On plasma discharges in which these diagnostics collect data from the same location at the same time, comparison of the signals gives more information about the turbulence than a single measurement. Comparing measurements from L‐ and H‐mode plasma phases, we found that the L‐mode frequency spectra (10‐250 kHz) from BES, PCI, and reflectometry have identical shapes. However, the H‐mode spectra differ significantly, with the k << kr modes suppressed the most. This suggests that the edge fluctuations are nearly isotropic in L‐mode but highly anisotropic in H‐mode. Other comparisons at the last closed flux surface in two similar low power plasma discharges differing by a factor of four in density showed power spectra with a more complicated structure. I. INTRODUCTION Understanding heat and particle transport via density fluctuations may be key in designing a plasma fusion reactor. Any single diagnostic can only measure a restricted set of the characteristics of the turbulence, including limits in frequency and wave number range and measurement location. Consequently, employing several of these diagnostics simultaneously gives a better picture of the turbulence than any of these alone. The advances in computer simulations of plasma transport are leading to mature models describing plasma fluctuations [1], including models specific to the plasma edge [2]. The type of comparisons presented here are needed to assess the validity of the modeling results. Furthermore, the general results of the computer models can improve our understanding of the diagnostic measurements and thus allow us to better interpret the observations. In this work, we compare measurements of density fluctuations in the plasma edge acquired with the phase contrast imaging (PCI) interferometer [3], the beam emission spectroscopy (BES) diagnostic [4], the reflectometer system [5], and fast scanning Langmuir probes [6] on the DIII‐D tokamak [7]. While the strengths and limitations of each diagnostic may be found in the references, the most important difference for this study is that BES and probes are equally sensitive to all r k wave number directions, while PCI responds to the k << kr range of wave number space, and reflectometry responds kr components but more so to k components. Note that this implies that the PCI data is minimally affected by Doppler shifts caused by plasma rotation.