Tangentially viewing phase contrast imaging for turbulence measurementsin toroidal fusion devices

Abstract

Phase contrast imaging (PCI) is an established and powerful techniquefor measuring density fluctuations in plasmas and has been successfullyapplied to several fusion devices. Rooted in a concept first developed formicroscopy, PCI belongs to the category of internal-reference interferometersand has been shown to possess superior qualities among such techniques,particularly in terms of spatial linearity. In essence, it produces a true imageof fluctuations in the plane perpendicular to the propagation direction of theprobing laser beam, provided their characteristic spatial scale is smaller thanthe beam width. The measurement in itself is line-integrated and thus notspatially resolved longitudinally to the beam. However, the properties of theturbulence 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 numericalmodeling, which has revealed significant additional complexity while confirmingthe general principle. Tangential PCI has been employed extensively in the TCVtokamak and has resulted in a rich body of work on broadband microturbulence inthe ion-temperature-gradient/trapped-electron-moderange and on geodesic acoustic modes. A similar diagnosticarrangement is also at an advanced planning stage for the new superconductingtokamak JT-60SA.