Volume 77, Issue 8, August 2006
Index of content:
- NUCLEAR PHYSICS, FUSION AND PLASMAS
Compact multichannel neutral particle analyzer for measurement of energetic charge-exchanged neutrals in Alcator C-Mod77(2006); http://dx.doi.org/10.1063/1.2238519View Description Hide Description
A four-channel compact neutral particle analyzer (CNPA) based on operating small Si diode detectors in pulse-height analysis (PHA) mode is used to measure energetic hydrogen minority ions with energies between and stemming from ion-cyclotron range-of-frequency heated D(H) Alcator C-Mod plasmas with both active and passive charge exchange (CX). First core minority ion distribution results from Alcator C-Mod discharges and a detailed description of the diagnostic are presented. The diagnostic employs integrated electronics and fast digitization of the shaping amplifier voltage. The digitized data are stored for postshot PHA, which removes the constraints of real-time PHA and allows for improved performance via elimination of base line shift effects and potentially relieving pileup through Gaussian fitting routines. The CNPA is insensitive to the large gamma and neutron background in Alcator C-Mod discharges but is susceptible to the plasma’s soft x-ray flux. The soft x-ray flux limits the CNPA energy resolution to . A simple model is used to interpret the active CNPA data which permits rapid estimates of the core hydrogen minority temperatures and anisotropy with a time resolution of . Hydrogenlike boron is identified as an important electron donor for the CX signal.
Abel inversion of a holographic interferogram for determination of the density profile of a sheared-flow pinch77(2006); http://dx.doi.org/10.1063/1.2336082View Description Hide Description
A holographicinterferometer is used to determine the radial electron number density profile of a sheared-flow pinch. Chord-integrated density information is recorded during a plasma pulse using the expanded beam of a pulsed ruby laser and holographic techniques. An Interactive Data Language (IDL) computer routine that requires only minimal user interaction is used to measure the resulting fringe shift in the reconstructed interferogram. This chord-integrated density information is inverted using an Abel inversion to determine the radial electron density profile. The density profiles obtained show a radially symmetric plasma column with an electron density of above the background plasma density.Holographicmeasurements are made at different times on separate plasma pulses to track the evolution of the density profile over time. These measurements are corroborated by time-dependent measurements made using a He–Ne interferometer.
77(2006); http://dx.doi.org/10.1063/1.2336769View Description Hide Description
Laser interferometry is an extensively used diagnostic for plasma experiments. Existing plasma interferometers are designed on the presumption that the scene and reference beam path lengths have to be equal, a requirement that is costly in both the number of optical components and the alignment complexity. It is shown here that having equal path lengths is not necessary, instead, what is required is that the path length difference be an even multiple of the laser cavity length. This assertion has been verified in a heterodyne laser interferometer that measures typical line-average densities of with an error of .
Reconstruction of quasimonochromatic images for multispectral x-ray imaging with a pinhole array and a flat Bragg mirror77(2006); http://dx.doi.org/10.1063/1.2336194View Description Hide Description
We have developed a software package for reconstruction of quasimonochromatic images from a multiple monochromatic x-rayimager for inertial confinement fusion implosions. The instrument consists of a pinhole array, a multilayer Bragg mirror, and an image detector. The pinhole array projects hundreds of images onto the detector after reflection off the multilayer Bragg mirror, which introduces spectraldispersion along the reflection axis. The quasimonochromatic images of line emissions and continuum emissions can be used for measurement of temperature and density maps of implosion plasmas. In this article, we describe a computer-aided processing technique for systematic reconstruction of quasimonochromatic images from raw data. This technique provides flexible spectral bandwidth selection and allows systematic subtraction of continuum emission from line emission images.
Application of wavelet multiresolution analysis to the study of self-similarity and intermittency of plasma turbulence77(2006); http://dx.doi.org/10.1063/1.2336754View Description Hide Description
Multiresolution property of wavelet analysis makes it a powerful tool in describing turbulent signals. Plasma turbulence similar to its fluid turbulence counterpart shows a degree of self-similarity, intermittency, and multifractal. The multiresolution analysis technique based on a biorthogonal wavelet was applied to the Langmuir probe data to study the self-similarity and the intermittency of plasma fluctuations. The data were collected in the plasma edge region of the HT-7 superconducting tokamak. It is found that plasma turbulence is composed of multiscale eddies; these eddies are similar in structure between different scales; the coexistence of multiscale eddies results in the self-similarity in plasma fluctuations. Large-scale turbulenceeddies intermittently burst out; the superposition of these large-scale eddies with those small ones is found responsible for the intermittency behavior. Success of utilizing wavelet multiresolution analysis in the HT-7 tokamak suggests it is a promising technique in the understanding of the turbulence structures.