Far-infrared Interferometer Research Articles

A far-infrared Michelson interferometer for Tokamak electron density measurements using computer-generated reference fringes

A simple far-infrared interferometer which uses the 394 mu m laser line from optically-pumped formic acid vapour to measure tokamak electron density is described. This interferometer is unusual in requiring only one detector and a single probing beam since reference fringes during the plasma shot are obtained by computer interpolation between the fringes observed immediately before and after the shot. Phase modulation is provided by a rotating mirror modulator. Electron density has been measured with a phase resolution corresponding to +or-1/20 wavelength fringe shift, which is equivalent to a central density resolution of +or-0.1*1019 m-3 for an assumed parabolic density distribution in a plasma of diameter of 0.2 m, and with a time resolution of 0.2 ms.

Study of the detached plasma regime in TFTR using the MIRI far-infrared interferometer

Plasmas have been formed in the tokamak fusion test reactor (TFTR) which separate completely from the limiter and are called detached plasmas. The physical evidence for the separation from the limiter is that all the ohmic input power is radiated away at the plasma boundary which can be 10→30 cm inside the limiter. The 10-chord multichannel MIRI interferometer system has been used to diagnose this region and follows the formation of the detached state through a MARFE. MIRI indicates that a plasma ∼0.5→1×1013 cm−3 occurs in the region between the luminous boundary and the limiter. This work was supported by U.S. DOE Contract No. DE-AC02-76-CHO-3073.

MIRI: A multichannel far-infrared interferometer/polarimeter for TFTR (abstract)

A double path multichannel far-infrared interferometer/ polarimeter employing retroreflectors for density and current profile determination has been developed for TFTR. The system containing about 400 optical components is mounted on a 70 000-lb. vibration-free stand which encircles TFTR. Optically pumped cw CH3OH (λ=110 μm, p≂100 mW) twin lasers with a frequency separation of l MHz are employed as a source. Quasioptical Schottky diodes (NEP≂l0−9 W(Hz)1/2) are used as detectors. Considerable efforts are devoted to have remote control capabilities for future D–T operation. The detailed design characteristics of this system together with preliminary results of chord averaged density information will be presented. Also the theoretical aspects of Faraday rotation for TFTR plasma will be discussed.

Far-infrared laser diagnostic on ELMO Bumpy Torus and extreme far-forward laser scattering on the Impurity Study Experiment

A far-infrared (FIR) laser scattering diagnostic has been installed on the ELMO Bumpy Torus (EBT) experiment, and tests conducted on the system using a TPX Bragg cell show that the system is fully functional. Scattering experiments to date have shown only small detectable signal levels during normal operation and during ion cyclotron heating (ICH) fast-wave and slow-wave propagation experiments. On the Impurity Study Experiment (ISX), the output signal from the rf amplifiers of the FIR interferometer has been spectrum analyzed and shows frequency-shifted sidebands that correspond to extreme far-forward scattering of the laser beam as it passes through the plasma. Although the scattered signal cannot be resolved spatially, the scattered intensity provides information about density fluctuation levels and MHD activity during different modes of operation. Agreement has been found with measurements made by the x-ray diagnostic and the heavy-ion beam probe.

Multichannel far-infrared phase imaging for fusion plasmas

A 20-channel far-infrared imaging interferometer system has been used to obtain single-shot density profiles in the UCLA Microtor tokamak. This system differs from conventional multichannel interferometers in that the phase distribution produced by the plasma is imaged onto a single, monolithic, integrated microbolometer linear detector array and provides significantly more channels than previous far-infrared interferometers. The system has been demonstrated to provide diffraction-limited phase images of dielectric targets.

Capacitive-grid beam splitters for far-infrared and millimeter-wave interferometers

A beam splitter intended for operation in Fourier transform spectrometers is described. The beam splitter, composed of metallic dots evaporated onto a transparent substrate, acts as a capacitive grid. Construction of the beam splitter and spectra obtained using it are presented. The effect of variation of dot size and interdot distance is discussed. Advantages and disadvantages of using this device in interferometers are presented.

A far-infrared interferometer for the measurement of electron concentration in flames and plasmas with high spatial resolution

A far-infrared interferometer employing an HCN laser has been designed which uses only simple, inexpensive optical components. It can operate either as a lateral shearing or Mach-Zehnder-type interferometer. Spatial resolution down to 0.5 mm in the test space was achieved. Good agreement was found with the theoretical resolution limit calculated from diffraction theory. The interferometer was used to measure electron densities in premixed flames with and without cesium seeding. The results confirmed the high electron concentration in the reaction zone of an unseeded methane-air flame.

A Scanning Fabry-Perot Interferometer for the Oscillation-Frequency Measurement of Far-Infrared Lasers

A scanning Fabry-Perot interferometer with capacitive aluminium mesh mirrors, which can roughly estimate the oscillation frequency of far-infrared (FIR) laser for its direct frequency measurement, has been developed. The mesh-mirror separation is measured by using a visible laser interferometer (He-Ne 633nm) of frings counting type which is associated with the FIR interferometer. Using the Fabry-Perot interferometer, wavelengths of three FIR oscillation lines have been measured for evaluating its measurement error. It has been found that uncertainties in the wavelength measurements are estimated to be less than ± 0.1μm. This vale is small enough to carry out the direct frequency measurement.

Computerized Tomography for Sparse-Data Plasma Physics Experiments

Matrix inversion and least squares fitting have been used to recover two dimensional distribution functions from a small number of line integrals taken along chords across them. A self-consistent optimization procedure, free from reliance on comparison with a trial source function, for optimizing the configuration of these chords is described and used to demonstrate that an asymmetric arrangement usually leads to greater reconstruction accuracy than a regular array. Smoothing is incorporated by imposing auxiliary conditions relating to the second derivative ?2f of the source function f, and its effect on reconstruction accuracy and resolution is investigated. These methods are applied to the ten-channel far-infrared interferometer being prepared for use on JET. Electron denisty contour shapes can be identified rather sensitively if the source function contours belong to a predetermined family.

A Beat-Modulated 118.8-μm CH<SUB>3</SUB>OH Laser Interferometer System and Its Application to the Electron Density Measurement of JIPP T-II Plasma

A beat-modulated 118.8-μ m CH3OH laser interferometer system has been constructed and successfully operated for the electron density measurement of JIPP T-II plasma. A Michelson-type far-infrared (FIR) laser interferometer system with a corner-cube reflector has been adopted in order to compensate the mechanical vibration component of the system by means of a 632.8-nm He-Ne laser interferometer. The corner-cube reflector is directly attached to the JIPP T-II vacuum vessel because of simplicity of the interferometer system. It has been observed that the vibration component of the FIR laser interferometer is compensated completely and the electron density by the system agrees closely with the one by a 2-mm microwave interferometer system. The signal-to-noise ratios of the probe- and reference signals are 50 and 250, respectively. The minimum detectable line-averaged density of present system is 8 × 1011 cm-3 (that is, 1/40 fringes).

A vibrationally compensated far infrared laser interferometer for plasma density measurements

A modulated far-infrared laser interferometer which is presently operating on the PDX experiment at Princeton is described. The interferometer geometry permits the characterization of inside “D”, outside “D” as well as circular discharges. To achieve this versatility, a titanium corner cube reflector, mounted inside the PDX vacuum vessel is used in conjunction with a second visible wavelength interferometer for vibration corrections. In addition, the use of room temperature quasi-optical Schottky diodes in the far-infrared interferometer is reported. The minimum detectable line average density of the system is ∼5×1011 cm−3.

Theoretical studies of the frequency response of some far-infrared interferometers with wire-grid beam dividers

A method developed recently by the authors for calculating the scattering of electromagnetic waves by parallel-wire grids is used to compute some frequency response curves of three polarising far-infrared interferometers with wire-grid polarisers and beam dividers. Simple diffraction theory indicates that the highest critical frequencies are obtained by laying the wires 'as far as possible parallel' to the radiation. The best ratio of the diameter to the period is about one to three. Modulation by rotating the polariser is very good at low frequencies, but attempts to use this system for wideband spectroscopy may lead to undesirable background levels on the wings of the interferogram.

A twin optically-pumped far-infrared CH3OH laser for plasma diagnostics

A twin optically-pumped far-infrared CH3OH laser has been constructed for use in plasma diagnostics. The antisymmetric doublet due to the Raman-type resonant two-photon transition is reproducibly observed at 118.8 μm. With the 118.8-μm line, it is obtained from the frequency separation of the anti-symmetric doublet that CH3OH absorption line center is 16±1 MHz higher than the pump 9.7-μm P(36) CO2 laser line center. It is shown that the Raman-type resonant two-photon transition is useful in order to get several-MHz phase modulation for the far-infrared laser interferometer. Some preliminary performances of this twin laser for the modulated interferometer are described.

Fourier emission infrared microspectrophotometer for surface analysis—I. Application to lubrication problems

A Beckman-RIIC FS-720 far-infrared interferometer was converted into an emission microspectrophotometer for surface analysis. To cover the mid-infrared as well as the far-infrared the Mylar beamsplitter was made replaceable by a germanium-coated salt plate, and the Moiré fringe counting system used to locate the moveable Michelson mirror was improved to read 0.5 μm of mirror displacement. Digital electronics and a dedicated minicomputer were installed for data collection and processing. The most critical element for the recording of weak emission spectra from small areas was, however, a reflecting microscope objective and phase-locked signal detection with simultaneous referencing to a blackbody source. An application of the technique to lubrication problems is shown.

Balloon-borne far-infrared Michelson interferometer for atmospheric emission studies

A series of far-ir Michelson interferometers utilizing the rapid-scanning technique have been designed and built for balloon-borne measurement of the emission spectrum of the stratosphere. Design criteria for such instrumentation are presented, and details of the evolution of these designs are discussed, including liquid helium-cooled bolometer detector systems, internal blackbody calibration, and atmospheric limb-scanning. Stratospheric emission spectra between 30 cm(-1) and 110 cm(-1) at a resolution of 0.07 cm(-2) are presented as examples of results taken at balloon altitudes, and the merits of this type of instrumentation for monitoring stratospheric trace gas constituent concentrations are discussed.

High temperature amplitude-phase spectroscopy

A heater cell for use with a far-infrared asymmetric Michelson interferometer is described. The heater cell enables high temperature complex reflection coefficient measurements for the first time. Difficulties with measurements at temperatures greater than 500 K are discussed. Representative spectra are reported for Na β-alumina, an ionic conductor, and ferroelectric LiTaO 3. These experiments show that high temperature optical properties can be obtained without recourse to theoretical fitting models and Kramers-Krönig analysis.

Self-supporting thin-film beam splitter for far-infrared interferometers

Commercially available plastic films have been examined, in the spectral range up to 1200 cm−1, for use as a beam splitter in far-infrared Michelson interferometers. The materials reported include polyethylene terephthalate, polycarbonate, polyvinylchloride, polypropylene, cellulose and polyethylene.

The Performance of an Amplitude Fourier Spectrometer for Far-Infrared Solid-State Spectroscopy

A far-infrared Fourier interferometer for amplitude-phase reflection spectroscopy on solids is described. It can be used with small samples at low temperatures and with fairly high resolution (<0.1 cm/sup -1/). The spectral range presently is 10-1000 cm/sup -1/.

Experiments concerning the nature of trapped-plasma mode harmonic extraction from P+-N-N+avalanche diodes

An experimental investigation has clarified the nature of both fundamental and harmonic extraction from p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -n-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> avalanche diodes operated in the trapped-plasma mode. A standard coaxial cavity was found to be suitable for low-order ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n\leq5</tex> ) harmonic generation. Calculations revealed that the circuit behaved as a bandpass filter which allowed power to be extracted at a particular output harmonic while "reactively" terminating the diode at other harmonics. Measurements were made of external current and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dV/dt</tex> waveforms for illustrative cases of fundamental and low-order harmonic extraction. Integrated <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dV/dt</tex> waveforms exhibited overvoltages which increased with increasing peak displacement currents. In general, the duration of the plasma-removal period of high current increased with increasing overvoltage. Carroll's [16] suggested dichotomy of voltage waveforms into independent reactive and resistive components was found to be appropriate for optimum first and second harmonic extraction. For third, fourth, and fifth harmonic extraction, however, it was observed that optimum efficiency was associated with significant changes in the physical states of the diode. Fourier analysis of the integrated <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dV/dt</tex> waveforms for low-order harmonic extraction revealed that maximum voltage amplitudes occurred at the harmonic being extracted. Combination ridge waveguide coaxial cavities were used for high-order harmonic extraction in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</tex> band. A far-infrared interferometer detected components in the power spectrum from the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</tex> -band cavity at frequencies up to 40 GHz. It was found empirically that, to first order, the optimum observed extraction efficiency was equal to 50 percent/ (harmonic number).

A Pulsed Submillimeter Interferometer for Transient Plasma Diagnostics

The application of a far-infrared interferometer to transient plasma diagnostics is described. The experimental performance of the interferometer under varying plasma conditions is presented. Detailed discussion is devoted to such important system parameters as observable density range, spatial resolution, time response, calibration techniques, and gamma radiation effects.