Line Density Measurements Research Articles

Stitching Measurement of Line Density of Variable-Line-Spacing Gratings with Long Trace Profiler

Stitching Measurement of Line Density of Variable-Line-Spacing Gratings with Long Trace Profiler

Measurement of the free electron line density in a spherical theta-pinch plasma target by single wavelength interferometry

A single wavelength heterodyne interferometer has been set up to investigate the free electron density integrated axially along the line of sight (line density) in a theta-pinch plasma to determine its applicability as a plasma target for ion beam stripping. The maximal line density reached in this experiment was cm−2 at 80 Pa and 20 kV. The findings demonstrate the pulsed character of the line density and its increase by raising the load voltage and the working gas pressure. Additionally, the results were compared with spectroscopic free electron density estimations, which were carried out by H-line broadening and peak separation. The time behavior of the line density indicates that its peak value is delayed by about 10 µs compared to the spectroscopic results. This effect is due to the formation of an extended, magnetically compressed plasma column in the vicinity of the current maximum, although the highest volumetric free electron density is reached near the current zero crossing. Since the line density is an essential parameter in describing the stripping capabilities of the plasma target, the interferometric diagnostic is superior to a spectroscopic diagnostic, because it directly provides integrated values along the line of sight. Furthermore, the measurements of the line density in this experiment partially show nonphysical negative values, which is due to gaseous effects and residual shot vibrations.

Millimeter-wave interferometry and far-forward scattering for density fluctuation measurements on LTX-β.

The λ ≈ 1 mm (f = 288 GHz) interferometer for the Lithium Tokamak Experiment-β (LTX-β) will use a chirped-frequency source and a centerstack-mounted retro-reflector mirror to provide electron line density measurements along a single radial chord at the midplane. The interferometer is unique in the use of a single source (narrow-band chirped-frequency interferometry) and a single beam splitter for separating and recombining the probe and reference beams. The current work provides a documentation of the interferometry hardware and evaluates the capabilities of the system as a far-forward collective scattering diagnostic. As such, the current optical setup is estimated to have a detection range of 0.4 ≲ k ⊥ ≲ 1.7 cm-1, while an improved layout will extend the upper k ⊥ limit to ∼3 cm-1. Measurements with the diagnostic on LTX are presented, showing interferometry results and scattered signal data. These diagnostics are expected to provide routine measurements on LTX-β for high frequency coherent density oscillations (e.g., Alfvénic modes during neutral beam injection) as well as for broadband turbulence.

Sigma phase detection in duplex stainless steel by induced magnetic field

Duplex stainless steels when subjected to temperatures above 600 ° C have its tenacity decreased by the formation of sigma phase. This phase has high hardness and is rich in chromium and reduces the matrix of this element. In this study, field line density measurements, obtained in the reversibility region of magnetic domains, and application of artificial neural networks are used to monitor the formation of this undesirable phase. Samples of a stainless steel SAF 2205 were subjected to aging at temperatures of 800 ° C and 900 ° C, in order to obtain different amounts of sigma phase. The amount of this phase was obtained by image processing and the density of field lines through a Hall Effect sensor. Charpy impact tests were performed. The field lines densities were used for training of an artificial neural network and correlated with the presence of sigma phase and embrittlement of the material. The results showed that the method was able to correlate the parameters studied with the presence of the sigma phase and toughness of the material studied in both temperatures.

Distortion of meteor count rates due to cosmic radio noise and atmospheric particularities

Abstract. The determination of the meteoroid flux is still a scientifically challenging task. This paper focusses on the impact of extraterrestrial noise sources as well as atmospheric phenomena on the observation of specular meteor echoes. The effect of cosmic radio noise on the meteor detection process is estimated by computing the relative difference between radio loud and radio quiet areas and comparing the monthly averaged meteor flux for fixed signal-to-noise ratios or fixed electron line density measurements. Related to the cosmic radio noise is the influence of D-layer absorption or interference with sporadic E-layers, which can lead to apparent day-to-day variation of the meteor flux of 15–20%.

Imaging sensor constellation for tomographic chemical cloud mapping

A sensor constellation capable of determining the location and detailed concentration distribution of chemical warfare agent simulant clouds has been developed and demonstrated on government test ranges. The constellation is based on the use of standoff passive multispectral infrared imaging sensors to make column density measurements through the chemical cloud from two or more locations around its periphery. A computed tomography inversion method is employed to produce a 3D concentration profile of the cloud from the 2D line density measurements. We discuss the theoretical basis of the approach and present results of recent field experiments where controlled releases of chemical warfare agent simulants were simultaneously viewed by three chemical imaging sensors. Systematic investigations of the algorithm using synthetic data indicate that for complex functions, 3D reconstruction errors are less than 20% even in the case of a limited three-sensor measurement network. Field data results demonstrate the capability of the constellation to determine 3D concentration profiles that account for ~?86%? of the total known mass of material released.

Phase error correction method for a vibration compensated interferometer

In order to compensate for vibrations while simultaneously measuring plasma line-averaged density, the existing real-time interferometer on DIII-D uses both a HeNe and CO2 laser operating at 10.59 and 0.632 μm, respectively. The line density resolution is 1018 m−2 which is predominantly determined by the measurement electronics. In particular, errors resulting from detection of the CO2 phase shift and finite bit size in the processing electronics are the major limiting factors. This article describes a method for postshot error correction generally applicable to two-color interferometers that utilize analog quadrature phase detection. The error correction scheme exploits the facts that error introduced by the phase comparator is periodic and that measurements can be made while the density is zero and only the errors are recorded. Application of this method will reduce unphysical density oscillations which are purely a result of vibration induced phase shifts. Application to DIII-D data shows a 25% reduction in noise amplitude on line density measurements.

Prototype tests of a combined CO2 laser interferometer/polarimeter for ITER (abstract)

Line density measurements on ITER are expected to be provided by a conventional vibration-compensated CO2 laser interferometer. This type of interferometer relies on maintaining an accurate measurement of hundreds of vibration fringes during the entire time history of the plasma. Because of the very long pulse lengths (1000 s) and the possibility of fringe counting errors, a second method that measures the Faraday rotation of a laser beam with a tangential laser beam path through the plasma will be used to provide a history-independent measurement of the line density with somewhat reduced accuracy. These two measurements can be accomplished in a combined interferometer/polarimeter system that uses a single CO2 laser and detector. We have constructed a prototype of this type of system, which uses two acousto-optic modulators to generate two beams, shifted from the laser frequency by 40 and 45 MHz. These probe beams are converted to left- and right-hand circular polarization and combined into a single probe beam. Vibrations are simulated by moving a mirror in the reference beam path, and the Faraday rotation is simulated by rotating the linear polarized probe beam. Frequency analysis of the interference signal from a single detector is used to separate the phase of the interferometer and the phase of the polarization rotation. Limits on the phase resolution of the two methods and possible sources of error will be presented.

Design and Operation of a Passively Switched Repetitive Compact Toroid Plasma Accelerator

The design and operation of a spheromak-like compact toroid (SCT) plasma accelerator is described. As an example application, some principles are presented for using the device as a plasma injector to fuel a tokamak plasma. The device forms and accelerates an SCT plasma. The SCT is a self-contained structure of plasma with embedded poloidal and toroidal magnetic fields and their associated currents that provide plasma confinement and structural integrity. The SCT is formed in a magnetized coaxial plasma gun and then accelerated within coaxial electrodes. The typical mass of an SCT for tokamak fueling is from several tens to several hundreds of micrograms and is accelerated up to a velocity of ~2 × 105 m/s. Larger-mass SCTs can be produced, and higher velocities are possible. This is important for other applications such as space propulsion, X-ray generation, fast-opening plasma switches, and low-temperature high-density plasma simulators.The novel features of the device are as follows: (a) it can be operated in a repetitive mode, (b) the high-energy capacitor bank to form the SCT is switched by initiating breakdown with fast gas injection, (c) the required delay between formation and acceleration is achieved passively with saturable inductors that switch the high-energy accelerator capacitor bank, and (d) a drift section has been added within the toroidal field region to study cross-field propagation prior to tokamak penetration.With the device installed on the Davis Diverted Tokamak (DDT), measurements are taken to study tokamak fueling. Typical rep-rated parameters are as follows: the SCT poloidal magnetic field at the outer electrode = 0.4 T, the stored formation bank energy = 1600 J, and the stored accelerator bank energy = 3600 J. The lower bound on SCT kinetic energy leaving the accelerator = 40 J (inferred from electron line density measurements). Typical SCT velocity is 15 to 20 cm/μs. The maximum rep-rate achieved so far with the device is 0.2 Hz and is currently limited by vacuum pumping capacity. Reliable operation has been demonstrated for 1000 consecutive shots. Higher-energy single shots have also been taken to study SCT propagation through an open guide tube and to study penetration of the SCT into the DDT tokamak vessel with both tokamak plasma discharges and vacuum toroidal magnetic field only.

Hybrid-kinetic simulation of the generation of mass flow due to plasma-wall interaction in reversed-field pinches

Flux loop and line density measurements in the reversed-field pinch ZT-40M indicate the existence of poloidally symmetric excitations in the edge that move in the direction anti-parallel to the toroidal current with a velocity of the order of the thermal ion speed. A recently proposed mechanism of momentum transfer between the plasma and the wall explains these observations by showing that plasma flow near the wall can be sustained over magnetohydrodynamic time scales as a result of unidirectional transfer of momentum during a collision with the wall of an ion with finite gyroradius. The present hybrid-kinetic simulations substantiate the proposed mechanism by showing that within a few to ten microseconds flow of the required direction and order of magnitude results from collisions of ions with a momentum-absorbing wall.

Mesure de densit lectronique l'aide d'un interf rom tre am lior

A Mach Zehnder interferometer is described which is used for electron line density measurements of a rapidly varying plasma. Fringe detections is made with a system of two photo-multipliers which allows one to follow the sense of variation of the electron line density in an unambiguous manner. The sensitivity of the device (approximately 0.01 fringe) allows precise measurements to be made in experimental situations where electron line density variations produce only fractions of a fringe shift. This interferometer is used to measure the electron line density of an axial current discharge in 25 mtorr H2. The density profile of the plasma is obtained by scanning a 3 mm wide He-Ne laser beam across the diameter of the discharge tube. Complete ionisation of the gas is found after 9 μsec.