Tangentially viewing phase contrast imaging for turbulence measurements in toroidal fusion devices

Phase contrast imaging (PCI) is an established and powerful technique for measuring density fluctuations in plasmas and has been successfully applied to several fusion devices. Rooted in a concept first developed for microscopy, PCI belongs to the category of internal-reference interferometers and has been shown to possess superior qualities among such techniques, particularly in terms of spatial linearity. In essence, it produces a true image of fluctuations in the plane perpendicular to the propagation direction of the probing laser beam, provided their characteristic spatial scale is smaller than the beam width. The measurement in itself is line-integrated and thus not spatially resolved longitudinally to the beam. However, the properties of the turbulence itself can be exploited to achieve longitudinal resolution, particularly when the beam propagates nearly tangentially to the magnetic field. This assertion has been recently rigorously tested through numerical modeling, which has revealed significant additional complexity while confirming the general principle. Tangential PCI has been employed extensively in the TCV tokamak and has resulted in a rich body of work on broadband microturbulence in the ion-temperature-gradient/trapped-electron-mode range and on geodesic acoustic modes. A similar diagnostic arrangement is also at an advanced planning stage for the new superconducting tokamak JT-60SA.