Multichannel Tangential FIR Laser
Interferometer/Polarimeter for NSTX (FIReTIP)
CO2 and FIR Laser Systems
for FIReTIP
■ FIR lasers are most suitable for fusion
plasma diagnostics. To choose the appropriate probe beam wavelength, the
following needs to be considered:
· Availability of high power source
· Negligible beam diffraction due to plasma
inhomogenity
· Sufficiently large phase shifts and
Faraday rotation angle to be detected
· Beam size is as small as possible to
allow easy access to the fusion device and the use of reasonably small optical
components
These
effects are calculated using the predicted NSTX parameters, as shown in the
following table. It is seen that
l =
119 mm is the
optimum choice, as optically pumped Methanol lasers can produce high output
power at this wavelength.
■ Shown in the figure below is the CO2/FIR laser assembly. The CO2 laser has a unique four-electrode design, which is safe to operate, does not need Brewster windows, and has high output power (> 130 W @CW operation). It is grating tuned and will be operated on the 9P36 line (l = 9.695 mm) to pump the Methanol lasers, which will produce the desired output powers (~ 100 mW) at l = 119 mm. In the figure, the laser with white frames is the Stark-tuned laser; a DC voltage is applied to it so that it has a shifted gain curve, and operates at a frequency about 5 MHz away from that of the other two FIR lasers. It serves as the local oscillator, so that the IF frequencies are sufficiently high to avoid the 1/f noise of the mixers.
■ Shown in the figure below are the beam patterns of the FIR laser output in the far field.
■ The FIR laser beam propagation, collimation, and focusing diagrams are shown in the figure below. The beams are collimated by the Gaussian beam telescope consisting of f1 lens and f2 lens. The beam splitting and detection optics of all seven channels are arranged between f1 lens and f3 lens.
■ Further information is available:
· Introduction to
the FIR Tangential Interferometer/Polarimeter (FIReTIP)
· Principles
of Interferometry/Polarimetry
· 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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