Millimeter-wave reflectometry for electron density profile and fluctuation measurements on NSTX
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A millimeter-wave reflectometry system for electron density profile and fluctuation measurements is being developed and installed on the National Spherical Torus Experiment. The initial frequency coverage will be in the bands 12–18, 20–32, and 33–50 GHz, provided by frequency-tunable solid-state sources. These frequencies correspond to O-mode cutoff densities ranging from 1.8×1012 to 3.1×1013 cm−3, which will span both the plasma core (ρ=r/a<0.8) and edge (ρ>0.8) regions. Operated as a broadband swept-frequency (frequency-modulated continuous-wave) reflectometer, the diagnostic is expected to provide routine (shot-to-shot) time- (⩽50 μs) and spatially resolved (∼1 cm) density profiles. The previous hardware can be easily reconfigured as a fixed-frequency reflectometer for density fluctuation measurements. The combination of measurements would be valuable for studying phenomena such as possible L- to H-mode transitions and edge-localized modes.Keywords:
Reflectometry
Extremely high frequency
A new spectroscopic method for the determination of the "bulk-electron temperature" of non-Maxwellian plasmas containing highly charged ions is proposed. Even for extremely high fractions (up to some 10%) where the energetic electrons carry most of the energy the new approach permits one to diagnose the "bulk temperature". An analytical model is derived which approximates the numerical calculations very well for a wide range of plasma parameters and elements (Zn=6-42). The method is applied to an experimental soft X-ray H- and He-like argon spectrum from a high density plasma.
Electron temperature
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Reflectometry
Extremely high frequency
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The application of large collection optics to image millimeter waves on a detector array improves the spatial resolution and enables the recovery of small-scale structures and fluctuation characteristics of the plasma electron temperature and density from radiometer and reflectometer measurements. Both diagnostic techniques are combined into a single instrument on the TEXTOR tokamak.
Reflectometry
Radiometry
Thermonuclear Fusion
Extremely high frequency
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A method to detect the parameters of electronegative plasmas with bi-Maxwellian electrons using a Langmuir probe is proposed by applying the test function. The radial model for the positive ion collection is extended and found to be more appropriate for density detection than the orbital motion limited model. The parameters of a diffused Ar/SF6 plasma are obtained by cylindrical and spherical probes for various Ar/SF6 ratios over a pressure range from 0.5 to 10 mTorr. The density ratio of bulk to hot electrons decreases from about 50 in Ar plasma to less than one for Ar/SF6, of which the SF6 flow is 0.05 standard cubic centimeters per minute. The hot electron group cannot be neglected for density ratios of negative ions to electrons lower than 100, though approximation of Maxwellian electrons is valid for larger density ratios.
Langmuir Probe
Electron temperature
Torr
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A millimeter wave imaging diagnostic for simultaneous density and temperature fluctuation measurements has been developed for operation on the TEXTOR Tokamak. The system is based on the quasi-optical imaging of the radiation from the plasma onto state-of-the-art millimeter wave mixer/receiver arrays. The temperature (fluctuation) measurements are based on the observation of electron cyclotron emission from a 2D plasma area with 8/spl times/16 sample volumes of about 1 cm/sup 3/. The density fluctuation measurements are based on active probing by means of reflectometry. In this case reflections from 16 sample volumes, arranged along the cutoff surfaces are diagnosed. The combined millimeter wave imaging diagnostic offers distinct advantages above the commonly used non-imaging diagnostics.
Reflectometry
Extremely high frequency
Electron temperature
Imaging spectrometer
Microwave Imaging
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Significant advances in millimeter wave and THz receiver and array technology have led to new approaches for the visualization of density and temperature fluctuations inside the core of high temperature plasmas. Electron cyclotron emission imaging collects millimeter wave electron cyclotron emission from the plasma and images it onto a planar mixer array to form 2-D images of electron temperature fluctuations. Illuminating the plasma with millimeter waves, microwave imaging reflectometry images the plasma cutoff layer onto a planar mixer array and thereby spatially resolves density fluctuations on the extended plasma cutoff surface; the use of multiple frequency illumination results in the formation of 2-D density fluctuation images. Collective scattering approaches at THz frequencies can resolve density fluctuations deep into the plasma core that are not accessible via reflectometry.
Reflectometry
Extremely high frequency
Microwave Imaging
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Abstract The effect of discharge voltage on bi-Maxwellian electrons in the diffusion region of a double plasma device has been studied. The increase in discharge voltage enhances the flux of ionizing electrons to the diffusion region separated by a mesh grid. This energetic electron flux in turn affects other important parameters such as density, electron temperature, plasma potential and floating potential in the diffusion region. Furthermore, the dependence of density and temperature of both ionizing and plasma electrons on discharge voltage is investigated. The electron energy probability function obtained from probe data also indicates the bi-Maxwellian nature of electrons.
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The number and energy distribution of suprathermal electrons produced in a laser-heated plasma can be quantitatively obtained directly from the experimental x-ray spectrum with only the assumption that the fast electrons lose energy by bremsstrahlung and electron-electron collisions. The result is independent of the spatial and temporal distribution of electron density and temperature.
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Energy distribution
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In this paper,the time-resolved emission spectrum of Ni atom in laser induced Ni plasma was measured in the wavelength region from 350 nm to 600 nm.The electron temperature and the electron density of plasmas were calculated from the measured intensity and stark broadening of emission spectral line respectively.The temporal evolution property of the electron temperature and the electron density were finally discussed according to the experimental results.
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