UC Davis PDG: FIR Interferometry and Polarimetry

Real-time density profile measurements have been identified as essential for advanced fusion tokamak operation. Multichannel Far Infrared (FIR) Interferometry is a proven method for measuring density profiles. Unfortunately, access restrictions on advanced magnetic fusion tokamak devices, such as ITER, prevent the utilization of a conventional vertically-viewing multichord interferometer system such as the FIR Interferometer and polarimeter system on TEXT-U.

The UCD Plasma Diagnostics Group is researching various multichannel interferometer configurations for plasma density measurements in current plasma devices as well as next generation devices such as ITER. Two approaches under investigation are (a) a multichannel Far Infrared Tangential Interferometer (FIReTIP) system that will be installed on the National Spherical Torus Experiment (NSTX) device, and (b) a multiple fan beam poloidal interferometer system that is still in the conceptual design phase.

Principles of Operation


The index of refraction of a plasma depends on the plasma density. As radiation passes through a plasma, the speed at which it travels differs from that of radiation passing through vacuum. An interferometer measures the time difference between two ray paths, one which travels through the plasma, and one which does not (see the figure above). This difference, measured as phase, depends upon the integrated plasma density along the ray path.

The index of refraction is also influenced by the presence of magnetic fields within the plasma. In particular, magnetic fields directed along the path of an electromagnetic wave will change the index of refraction slightly between left and right hand polarized radiation. This effect, known as Faraday rotation, is measured by a polarimeter. In a similar manner as that utilized by the interferometer, integrated polarimetry data may be inverted to reconstruct magnetic fields within the plasma, and from that, the plasma current profile.

FIReTIP System for NSTX

The UCD Plasma Diagnostics Group, in collaboration with researchers from the Princeton Plasma Physics Laboratory (PPP), are developing a 3-channel Far Infrared Tangential Interferometer/Polarimeter (FIReTIP) system for the National Spherical Torus Experiment (NSTX).

Follow these links for more information:

         FIReTIP Introduction

         Principles of Interferometry/Polarimetry

         Current Status and Projected Progress

         The CO2/FIR Lasers for the FIReTIP System

         Phase Detection Electronics for the FIReTIP System

         Recent Progress and Physics Results

         Computer Simulations of Anticipated Performance (reference only)

         Optical Layouts of the old FIReTIP System (reference only)

Fan Beam Approach

A fan beam viewing geometry requires more complex methods of image reconstruction. An inversion technique has been developed to generate high resolution 2-D images of both the plasma density, and current profiles from interferometer and polarimeter data collected at arbitrary positions and viewing angles through a tokamak plasma.

Follow these links for more information:

*The inversion technique
*Interferometry and polarimetry on TEXT-U
*Multiple fan beam imaging on tokamak devices (ITER)

email  Comments to: Calvin Domier