Review of Scientific Instruments · 1988 · 111 citations · 4 references
EngineeringMeasurementOptical TestingInterferometryLaser ApplicationsOptical MetrologyFiber OpticsLaser FabricationMagnetic Confinement FusionCalibrationOptical DiagnosticsControlled Nuclear FusionLine DensityInstrumentationOptical SystemsPhysicsDiii-d TokamakTime MetrologyHigh SpeedQuadrature Heterodyne InterferometerNatural SciencesSpectroscopyInstrument ScienceApplied PhysicsElectronic Instrumentation
A real‑time, vibration‑compensated two‑color quadrature heterodyne interferometer measures DIII‑D line density at 1 MHz with 2π/256 resolution over ±8193 fringes, using CO₂ and He‑Ne lasers digitized at 6 MHz, zero‑crossing counting for quarter‑fringe accuracy, and a PROM‑based lookup for 1/256 resolution at 350 kHz, with digital error monitoring for low amplitude and excessive fringe rate. The system delivers analog line density at 80 kHz with a noise‑equivalent phase shift of ±2/256, and a coating‑prevention method for in‑vessel mirrors during plasma and cleaning discharges has been developed.
A multichannel, two-color, quadrature heterodyne interferometer is used to measure the line density in the DIII-D tokamak. The unique feature of this real-time vibration-compensated interferometer is the combination of high speed (1 MHz), high resolution (2π/256), and wide range (±8193 fringes). Quadrature phase information from a CO2 laser (10.6 μm) and a He–Ne laser (0.63 μm) are digitized with high-speed (6 MHz) flash digitizers. Zero crossings of the signals are counted with digital circuitry yielding quarter fringe resolution with a 4-MHz bandwidth. Further fringe resolution of 1/256 is provided at 350 kHz by a PROM which uses the digital signals as input to a look-up table. Analog line density is presently available at 80 kHz with a system noise equivalent phase shift of ±2/256. Error monitoring is provided for low signal amplitude and exceeding the maximum fringe rate. In addition, a method to prevent coating of in-vessel mirrors due to plasma and vessel wall cleaning discharges has been developed.
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CO2 interferometer operation in doublet III
D. R. Baker · Review of Scientific Instruments · 1980 · 17 citations