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Review of Scientific Instruments

Issue 10 • Date Oct 1996

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Displaying Results 1 - 25 of 76
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Use of hydrogen negative ions in particle accelerators

    Page(s): 3393 - 3404
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    The use of negative hydrogen ions in different accelerators, especially in charge‐exchange injection of protons into orbits of high energy accelerators, is reviewed. The principles of charge exchange for particle acceleration and control and the benefits are discussed after which the performance and characteristics of selected accelerators are summarized. Recent trends in negative ion sources for charge‐exchange accelerators also are reviewed. © 1996 American Institute of Physics. View full abstract»

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  • Time‐resolved pulse‐counting lock‐in detection of laser induced fluorescence in the presence of a strong background emission

    Page(s): 3405 - 3410
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    We describe a time‐resolved pulse‐counting system well adapted for the detection of continuous laser induced fluorescence (LIF) signals in repetitive phenomena, when a strong background emission is present. It consists of 256 channels coupled to a first in first out memory and interfaced to a 486 DX 33 PC, for data storage. It accepts time‐averaged count rates up to 450 kcount/s. Time between channels can be set from 12.5 ns to several μs and the dead time between two consecutive cycles of the physical phenomena is less than 20 ns. In phase with a chopper, which modulates the laser beam, it adds the observed photon signal to the channel memories when the beam is on and substracts it when the beam is stopped, acting like a lock‐in amplifier which detect only the modulated part of the signal. The minimum detectivity on the LIF signal is only limited by the shot noise of the plasma induced emission signal. As an application, we studied the time variation of the Ar+*(2G9/2) metastable ions, detected by LIF, in two types of plasmas. Their radiative lifetime and collisional quenching frequencies were deduced from their decay rate in the afterglow of a pulsed Helicon reactor. We also observed the evolution of their density in a 455 kHz capacitively coupled argon discharge. © 1996 American Institute of Physics. View full abstract»

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  • Midinfrared cw difference‐frequency generation using a synchronous scanning technique for continuous tuning of the full spectral region from 4.7 to 6.5 μm

    Page(s): 3411 - 3415
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    We report in this article an experiment of laser‐based continuous‐wave (cw) midinfrared difference‐frequency generation using a synchronous scanning technique for continuous tuning of the complete spectral region from 4.7 to 6.5 μm (1550 to 2200 cm-1). A wavelength‐tuned, noncritically phase‐matched AgGaS2 crystal was used as the nonlinear optical mixing medium. Midinfrared output power of ∼10 μW was obtained by mixing two tunable lasers with ∼430 mW total pump power. The pump lasers were synchronously scanned during infrared wavelength tuning. The infrared frequency was deduced from the difference between the two laser frequencies which were simultaneously measured by wavemeters with an absolute accuracy of better than 0.007 cm-1. No reference cell was needed for frequency calibration. High‐resolution spectra of carbonyl sulfide (OCS) were recorded for an evaluation of the frequency calibration and high‐resolution spectroscopic characteristics. © 1996 American Institute of Physics. View full abstract»

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  • A multiple white light interferometer

    Page(s): 3416 - 3419
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    A multiple interferometer using the short coherence length of white light has been constructed and its operation demonstrated. With this apparatus, it is possible to count the number of light reflections between two parallel mirrors. The shift in fringe position resulting from differences in the optical thickness is proportional to the number of reflections between the two mirrors, enabling the resolution and measurement of a step difference of the order of 20 Å. This technique inherently has higher resolution than Michelson interferometers. As compared with Fabry–Perot interferometers, it offers an independent determination of the density of fringes, and has lower sensitivity to errors in minor parallelism. Further, the method does not require the extreme closeness in mirror spacing as compared to the Tolansky interferometer. © 1996 American Institute of Physics. View full abstract»

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  • Measurement of the width of a wedge boundary using phase change of interference with partially coherent light

    Page(s): 3420 - 3422
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    In this article, the phase change of interference in the shadow region of a wedge‐shaped object with partially coherent light is illustrated. It is shown that this phase change can be applied to the detection of the width of adjacent boundaries of small samples with partially coherent light. © 1996 American Institute of Physics. View full abstract»

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  • On the temporal resolution of multitau digital correlators

    Page(s): 3423 - 3427
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    This article treats a class of systematic errors in determining the intensity–intensity correlation function S(t) with a digital correlator. Errors in determining S(t) will lead to incorrect computations of apparent diffusion coefficients and size distributions for micelles, vesicles, and other supramolecular assemblies. Systematic errors in interpreting the spectrum Sd(tm) measured by real correlators, in terms of the true spectrum S(t), are minimized by optimum interpretations of the delay time tm of each correlator channel. This identification is more complex for multitau correlators than for simple linear correlators. For a multitau correlator with current and delayed sampling times T and θ, and a delay channel beginning τ after the beginning of the zero‐time sampling channel, the optimum choice is tm=τ-T/2+θ/2. The so‐called ‘‘half‐channel’’ correction tm=τ+θ/2 is erroneous. For the ‘‘zero delay channel’’ seen on some correlators, the most appropriate identification is tm=T/3. © 1996 American Institute of Physics. View full abstract»

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  • Stabilization and calibration of x‐ray wavelengths for anomalous diffraction measurements using synchrotron radiation

    Page(s): 3428 - 3433
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    An apparatus and technique are described for the characterization and energy stabilization of a monochromatic x‐ray beam from a synchrotron radiation source that is used for diffraction experiments. The energy of the beam between 6–30 keV can be absolutely measured to an accuracy of 0.3 eV and continuous stability can be guaranteed to 0.2 eV for the duration of an experiment. The particular case of multiple wavelength anomalous diffraction is considered in detail. © 1996 American Institute of Physics. View full abstract»

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  • Remote optical detection using microcantilevers

    Page(s): 3434 - 3439
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    The feasibility of microcantilever‐based optical detection is demonstrated. Microcantilevers may provide a simple means for developing single‐element and multielement infrared sensors that are smaller, more sensitive, and lower in cost than quantum well, thermoelectric, or bolometric sensors. Here we specifically report here on an evaluation of laboratory prototypes that are based on commercially available microcantilevers, such as those used in atomic force microscopy. In this work, optical transduction techniques were used to measure microcantilever response to remote sources of thermal energy. The noise equivalent power at 20 Hz for room temperature microcantilevers was found to be approximately 3.5 nW/√Hz, with a specific detectivity of 3.6×107 cm Hz1/2/W, when an uncoated microcantilever was irradiated by a low‐power diode laser operating at 786 nm. Operation is shown to be possible from dc to kHz frequencies, and the effect of cantilever shape and the role of absorptive coatings are discussed. Finally, spectral response in the midinfrared is evaluated using both coated and uncoated microcantilevers. © 1996 American Institute of Physics. View full abstract»

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  • Compact active quenching circuit for fast photon counting with avalanche photodiodes

    Page(s): 3440 - 3448
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    A compact and flexible circuit for operating avalanche photodiodes in Geiger mode was designed, fabricated, and tested. A new voltage driver stage, based on fast n‐channel double‐diffused metal–oxide–semiconductor (DMOS) transistors in a bootstrap configuration, makes it possible to obtain quenching pulses up to 25 V amplitude and fast active reset of the detector. At 20 V excess bias voltage above the photodiode breakdown level, an overall deadtime shorter than 36 ns is attained. The avalanche pulse charge is minimized by means of a mixed passive–active quenching approach, thus reducing self‐heating and afterpulsing effects in the photodiode. A user‐controllable hold‐off time is available for further reducing the afterpulsing effect. The saturated counting rate of the circuit exceeds 25 Mcounts/s, but, by working with avalanche photodiodes with high breakdown voltage (250–400 V) and high avalanche current (10–40 mA), a practical limit is set at about 9 Mcounts/s by thermal effects in the detector. Gated‐detector operation with gate times down to 10 ns is provided. The suitability of the new active‐quenching circuit for the development of compact, all‐solid‐state instruments for high‐performance photon counting was verified in experimental tests. © 1996 American Institute of Physics. View full abstract»

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  • Picosecond transient grating spectroscopy: The nature of the diffracted spectrum

    Page(s): 3449 - 3453
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    A ps transient grating setup using white light continuum for probing is presented. Measurements on an aromatic molecule in solution have been carried out with this system. The diffracted spectrum is analyzed using Kogelnik’s coupled wave theory. At short time delay after excitation, the diffracted spectrum is strongly dominated by absorption and in this case transient grating spectroscopy is equivalent but more sensitive to transient absorption spectroscopy. If some of the excitation energy is dissipated as heat, the diffracted spectrum is essentially the same as the dispersion spectrum of the transient species at time delays approaching half the acoustic period. The performances of this technique and of transient absorption spectroscopy are compared. © 1996 American Institute of Physics. View full abstract»

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  • Infrared–far‐infrared stabilized laser system with adjustable frequency offset for spectroscopic measurements

    Page(s): 3454 - 3457
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    A double resonance infrared–far‐infrared spectroscopic system is described. The variation of the 118.8 μm wavelength gain line shape of the methanol far‐infrared laser on the diameter of the pump CO2 laser beam due to the induced nonlinear waveguiding effect has been measured. The high‐frequency stability and continuous tunability of the pump CO2 lasers necessary for such measurement were achieved by frequency offset locking them to a molecular line center stabilized reference laser. The long term frequency stability achieved for the offset locked lasers is 24 kHz. © 1996 American Institute of Physics. View full abstract»

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  • Two highly sensitive microwave cavity spectrometers

    Page(s): 3458 - 3464
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    A description is presented for two unconventional highly sensitive spectrometers operating in the decimeter‐wave region (90–450 MHz) and millimeter‐wave region (90–150 GHz), respectively. In both cases a resonant cavity is used as an absorption cell. Either design combines high sensitivity with high resolution and wide spectral range. The radio frequency spectrometer allows measuring of extremely low absorption quantity 3×10-12 cm-1 in combination with high spectral resolution of order 1 kHz with the radio frequency–microwave double resonance method. The gas cell is a coaxial cavity (Q≊103) with a diameter of 40 cm and length about a wavelength. The millimeter‐wave spectrometer utilizes an orotron oscillator as the tunable, coherent source of radiation from 90 to 150 GHz. A gas cell is placed into a high quality (Q≊104) Fabry–Pérot resonator of the orotron, and the absorption signal is detected by variation of the orotron electron current in the collector circuit. The sensitivity limit of (2–5)×10-10 cm-1 is achieved with Stark modulation as well as source frequency modulation. Some results of successful application of both designs in molecular spectroscopy are presented, and potential use of the millimeter‐wave cavity spectrometer as a gas analyzer with a detection limit better than 1 ppm is briefly discussed. © 1996 American Institute of Physics. View full abstract»

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  • High sensitivity mass spectrometer system for contaminant measurement in high purity gases

    Page(s): 3465 - 3471
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    A mass spectrometer beam system using a double focusing sector analyzer and an electron impact ion source has been developed for trace analysis. The molecular beam, formed through a focusing glass capillary array, serves as the gas inlet of the system. Closed cycle cryopumps and ion pumps are used to generate the high vacuum. System roughing is achieved using sorption pumps. Clean and oil free vacuum was obtained by nonmechanical pumping. System normal mode sensitivity is about 107 counts/s/Torr. System low pressure mode sensitivity can be three orders of magnitude higher when sample pressures are below 30 mTorr. A parts‐per‐billion range system detection capability was accomplished. Slow response time for adsorptive species measurements is a major drawback of the sample inlet system. © 1996 American Institute of Physics. View full abstract»

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  • High sensitivity image intensifier tube device for hard‐x‐ray detection

    Page(s): 3472 - 3477
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    We present an x‐ray detector with a high sensitivity in the 1–20 MeV x‐ray range. This device consists of a scintillator coupled with a microchannel plate image intensifier tube working in the visible range. The performance of the hard‐x‐ray detector is described. The modulation transfer function, the characteristic curve, and the detection threshold were measured. Using an x‐ray flash generator in the 4 MeV x‐ray range, we have been able to record images of dense spheres for dose levels as low as 1.5 μGray. Similar experiments have been realized with a linear accelerator for different x‐ray spectra (10 and 20 MeV). Our results demonstrate that this device works in a range where the most sensitive intensifying screens and films, usually used on flash radiography, are inoperative. © 1996 American Institute of Physics. View full abstract»

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  • Mean secondary electron yield of avalanche electrons in the channels of a microchannel plate detector

    Page(s): 3478 - 3482
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    By modeling the statistical evolution of an avalanche created by 20 keV protons impacting the input surface of a z‐stack microchannel plate (MCP) detector, the mean secondary electron yield γC of avalanche electrons propagating through a MCP channel is measured to equal 1.37 for 760 V per MCP in the z stack. This value agrees with other studies that used MCP gain measurements to infer γC. The technique described here to measure γC is independent of gain saturation effects and simplifying assumptions used in the segmented dynode model, both of which can introduce errors when inferring γC through gain measurements. © 1996 American Institute of Physics. View full abstract»

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  • A 3D position controller for a charged particle suspended in a modified electrodynamic chamber

    Page(s): 3483 - 3490
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    A three‐dimensional (3D) position control system for a charged particle suspended in an electrodynamic chamber (EDC) was developed. The new controller is based on two major developments: (i) modification of the EDC that allowed the adjustment of the particle position at all three dimensions, by three dc electrodes and (ii) a special hardware that enabled the reading of the correct position of an arbitrarily shaped particle at high speed from its shadow, measured by a photodiode array (PDA). Three 128‐pixel linear PDAs were used to measure the trace of the particle at the three dimensions. The hardware extracted directly the digital value of the particle position at each dimension by determining the center of the particle shadow. The three coordinates of the particle position were fed into a computer that served as a proportional‐integral derivative and a storage device. The hardware allowed sampling of the particle position at frequency (7.8 kHz) and was able to handle particles at a wide range of sizes and various shapes (for example, with holes). The 3D position controller can maintain a particle at the center of the EDC, with a short characteristic time, under a wide range of disturbances, acting from arbitrary directions. The controller can also maintain the particle without an ac field. The results can be summarized as follows: (i) for a step disturbance, acting on a particle from an arbitrary direction, the controller can fully follow a force value as much as 10% of particle weight with a characteristic time of 100 ms; (ii) for a gradual disturbance, acting on the particle from an arbitrary direction, the controller can correct the disturbance with a changing force rate of 1 g/s with a delay time of less than 25 ms. © 1996 American Institute of Physics. View full abstract»

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  • Transient behavior of the terminal voltage in a recirculating electrostatic accelerator for a long‐pulse free‐electron laser

    Page(s): 3491 - 3493
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    For long‐pulse, single‐mode operation of free‐electron lasers, transient behavior of the terminal voltage in a recirculating electrostatic accelerator is analyzed. It is found that the rate of terminal voltage drop depends on the position where the current loss of electron beam occurs. The rate is measured to be 507 V/A μs for the current loss in the terminal and 100 V/A μs for the current loss in the deceleration tube. By increasing the capacitances between the adjacent electrodes of a deceleration tube, the accelerator becomes more stable with low rate of terminal voltage drop and increased recovery efficiency. © 1996 American Institute of Physics. View full abstract»

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  • A broad beam cold cathode Penning ionization gauge ion source for ion‐beam‐assisted deposition

    Page(s): 3494 - 3496
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    A broad beam cold cathode Penning ionization gauge ion source is developed for ion‐beam‐assisted deposition. The ion source is composed of several modules and it is 88 mm in diameter and 80 mm in height without any cooling system. The discharge voltage is 500 V. The multiaperture extracted ion beam diameter at the exit is 35 mm. It can be operated dc in oxygen, argon, nitrogen, and other gases. The ion energy can be adjusted from 50 to 1500 eV and the maximum extraction current density is above 200 μA/cm2. The extracted ion beam is matched to the arched rack in different vacuum coaters. Due to the small volume of the ion source, single or multiple sources can be installed according to the volume of the coaters. © 1996 American Institute of Physics. View full abstract»

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  • Technique for the removal of electrons from an extracted, pulsed, H- ion beam

    Page(s): 3497 - 3500
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    A small, permanent‐magnet insert structure for the removal of electrons from pulsed, extracted, negative ion beams has been developed at Lawrence Berkeley National Laboratory. The device was computer modeled and designed for an extraction field strength of 3 kV/mm. The testing was carried out with a rf driven multicusp ion source optimized for the production of H- ions and pulsed at a few Hz with pulse widths of several hundreds of μs. It is demonstrated that the insert structure together with a collar can remove over 98% of electrons from the extracted H- ion beam without any significant deterioration of the H- ion output. Application to other negative ion beams can be expected from this magnetic collar insert. © 1996 American Institute of Physics. View full abstract»

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  • Characteristics of AuSi liquid metal alloy ion source with coupling extractor electrodes

    Page(s): 3501 - 3506
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    Characteristics of AuSi liquid metal alloy ion source with single and coupling (double) extractor electrodes have been investigated both in the focused ion beam machine, and in the vacuum test bed system equipped with quadrupole mass spectrometer. Liquid metal ion source with coupling extractor electrode produce a stable total ion current which increases almost linearly when compared with a single extractor electrode. The isotope current of 197Au+ observed with quadrupole mass spectrometer is higher in the ion source with coupling extractor electrode than in a single extractor electrode, nonetheless, the pattern of its current–voltage characteristics remain the same for both single and coupling extractor electrodes. However, for lighter isotopes such as 28Si+ and 28Si++, the result is reversed (higher in ion source with single extractor electrode). The secondary electron image of copper grid pictures obtained from focused ion beam systems installed with an AuSi liquid metal alloy ion source with single and coupling extractor electrodes remain the same. © 1996 American Institute of Physics. View full abstract»

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  • Possible application of silicon microstrip detectors in solid state physics

    Page(s): 3507 - 3509
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    The possibility of utilizing silicon microstrip detectors for analyzing the angular correlations of positron annihilation γ rays is examined in some detail. It is found that suitable experimental arrangements, which allow exploitation of the high spatial resolution of these detectors, can be devised. © 1996 American Institute of Physics. View full abstract»

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  • Low‐energy Rutherford backscattering‐ion channeling measurement system with the use of several tens keV hydrogen and a time‐of‐flight spectrometer

    Page(s): 3510 - 3514
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    We have developed a low‐energy Rutherford backscattering spectrometry (RBS)‐ion channeling measurement system for the analysis of thin films and solid surfaces with the use of several tens keV hydrogen ions and a time‐of‐flight spectrometer which was originally developed by Mendenhall and Weller. The depth resolution of our system is better than that of a conventional RBS system with MeV helium ions and silicon surface barrier detectors. This measurement system is very small in size compared to the conventional RBS‐ion channeling measurement system with the use of MeV He ions, because of the small ion accelerator for several tens keV ions. The analysis of crystalline thin films which utilizes ion channeling effect can be performed with the use of this low‐energy RBS‐ion channeling measurement system. The in situ observation of the thermal reaction between iron and silicon substrate with the use of this measurement system is also demonstrated. © 1996 American Institute of Physics. View full abstract»

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  • Helium partial pressure measurement in a deuterium environment

    Page(s): 3515 - 3520
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    To study the removal of helium ash in present day, medium sized tokamaks, experiments with deuterium plasmas and helium puffs are conducted. Because D2 and He have almost the same mass, a spectroscopic method had been developed at TEXTOR where the He partial pressure in the exhaust line was measured by exciting the neutral gas in a Penning gauge and observing the characteristic emission lines of helium and deuterium. This method was limited to low pressures and concentrations because broad spectral lines from the deuterium molecules superimpose the He lines. By using a spectrometer and an intensified charge coupled device camera instead of optical filters and photomultipliers, it is now possible to record the interesting part of the spectrum with a time resolution of 20 ms. By subtracting the molecular spectrum of deuterium, helium concentrations from 2% on can be detected over about three orders of magnitude in pressure down to 2×10-6 mbar. He and D2 pressures are measured simultaneously. By another method the 58 nm resonance line of helium is observed and the crosstalk of the deuterium gas could be eliminated. Pressure measurements as low as 5×10-8 mbar and 1% He concentration are possible. A detection system utilizing a vacuum ultraviolet monochromator in combination with a channeltron has been tested. © 1996 American Institute of Physics. View full abstract»

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  • Helium I line intensity ratios in a plasma for the diagnostics of fusion edge plasmas

    Page(s): 3521 - 3529
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    Helium I line intensity ratios obtained by a collisional radiative model, including new recommended excitation rate coefficients and the effects of hot electrons, enable us to measure electron density (ne) and temperature (Te) in high density plasma. Measured ne and Te using 492.2 nm (4 1D→2 1P)/471.3 nm (4 3S→2 3P), 504.8 nm (4 1S→2 1P)/471.3 nm, and 492.2 nm/504.8 nm line intensity ratios are in good agreement with the Langmuir probe results in the helium discharge plasma in the NAGDIS‐I linear device (Nagoya University Divertor Simulator) for ne and Te regions of 1011–4×1012 cm-3 and 5–20 eV. Hot electrons in the plasma are important for the He I line emissions when Te is below the excitation energy ≊20 eV. A resonance scattering effect included in the calculation accounts for the experimental result of the enhanced 501.6 nm (3 1P→2 1S) line emission. © 1996 American Institute of Physics. View full abstract»

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Review of Scientific Instruments, published by the American Institute of Physics, is devoted to scientific instruments, apparatus, and techniques.

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Editor
Albert T. Macrander
Argonne National Laboratory