Abstract
Abstract
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.