Multichannel Tangential FIR Laser
Interferometer/Polarimeter for NSTX (FIReTIP)
Principles of
Interferometry/Polarimetry
■ Plasmas have a refractive index depending
on the plasma density (n) and wavelength (l) of
the probe beam. For an O-mode polarized wave propagating through plasma, the
refractive index is:
where
is the plasma
frequency. Thus, the phase shift induced by the plasma is:
Plasmas are also optically active media. In the presence of a parallel magnetic field component (Bp), the polarization of the wave is Faraday rotated by an angle:
An FIR interferometer measures the phase shift of the laser beam to obtain the plasma density information, while a polarimeter measures the Faraday rotation angle. In a combined interferometer/polarimeter, both the phase shift and the Faraday rotation angle of the laser beam are measured, so that the plasma density and magnetic field information can be obtained simultaneously.
With a sufficient number of chords spanning the plasma, time-resolved FIR interferometry data can be inverted to obtain the temporal density profiles, and the FIR polarimetry data can be subsequently inverted to obtain the temporal magnetic field profiles.
■ For the FIReTIP system, a triple FIR laser configuration will be adopted, as shown in the figure below:
Two of the FIR laser beams (linearly polarized)
are combined and transformed into left- and right-hand circularly polarized
probe beams with a small frequency difference (
). These two waves are the eigenstates of magnetized plasmas
along the magnetic field direction. They have the following refractive indices:
where 
is the electron
cyclotron frequency. It is seen that the R- and L-waves have a small difference
in the refractive indices due to the parallel magnetic field. This is the cause
of the Faraday rotation of a linearly polarized probe beam. Thus, we can
measure the Faraday rotation angle by comparing the phase difference between
the two circularly polarized probe beams. Also, it is easy to see that the
average of the plasma induced phase shifts of the two circularly polarized
probe beams will give the interferometer phase 
.
■ Further information is available:
· Introduction to
the FIR Tangential Interferometer/Polarimeter (FIReTIP)
· The CO2/FIR
Lasers for the FIReTIP System
· Optical
Layouts of the FIReTIP System
· Phase
Detection Electronics for the FIReTIP System
· Computer
Simulations of Anticipated Performance
· Recent Progress
and Physics Results
Send comments to: cwdomier@ucdavis.edu.
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