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Jun 2000

Volume 71, Issue 6, pp. 2263-2611

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back to top NUCLEAR PHYSICS, FUSION and PLASMAS

Doppler-free two-photon excitation of Lyman-α fluorescence for the diagnostics of magnetically confined fusion plasmas

D. Voslamber

Rev. Sci. Instrum. 71, 2334 (2000); http://dx.doi.org/10.1063/1.1150620 (19 pages) | Cited 2 times

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Doppler-free two-photon excitation of hydrogen Lyman-α fluorescence is investigated as a possible laser-induced fluorescence (LIF) technique for the diagnosis of magnetically confined fusion (MCF) plasmas. A formal analysis is presented of the underlying atomic and plasma physics as well as of various practical aspects, such as parameter optimization and experimental precision. The latter is analyzed with regard to the photon noise and to the sensitivity of the fluorescence signals to the plasma and laser parameters. The diagnostic potential of the LIF technique described relies on its high spectral resolution. Thus, the absorption lines of the hydrogen isotopes H, D, and T are clearly separated from each other and can serve for isotope-selective density measurements. In addition, using a tunable laser system with small bandwidth, various plasma parameters can be inferred from the spectral line shapes, such as the neutrals’ temperatures or the effective charge number Zeff. The polarization of the fluorescence can, under favorable circumstances, be exploited for magnetic field measurements. The photon statistics impose neutral densities above 1014 m−3 and thus make the diagnostic suitable primarily for the plasma edge. However, previous work has shown that it is applicable even in the plasma bulk of large machines if a neutral beam is used that generates sufficient neutral densities by charge exchange with the plasma ions. Variations or insufficient knowledge of the neutrals’ temperatures are found to seriously affect the precision of absolute isotope density measurements. They are rather uncritical, however, for the determination of the H/D/T density ratios, which are of prime importance for the burn control of large MCF devices. A notable sensitivity is also found to variations of the laser frequency detuning, which should be known and stable to better than 100 MHz. The perspectives demonstrated in the present study and the success of a first experiment on the plasma generator PSI 1 at IPP Berlin are considered to be sufficient motivation for testing the diagnostic on a tokamak or other MCF device. © 2000 American Institute of Physics.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
32.50.+d Fluorescence, phosphorescence (including quenching)
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

Development of a novel fast frequency modulation scheme for the Joint European Torus multichannel reflectometer

N. Deliyanakis

Rev. Sci. Instrum. 71, 2353 (2000); http://dx.doi.org/10.1063/1.1150660 (7 pages)

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A novel frequency modulation scheme has been developed for the multichannel reflectometer used to measure density profiles and density fluctuations on the Joint European Torus (JET) tokamak. This reflectometer normally uses slow frequency sweeping (6 ms), combined with fixed-frequency operation, to measure the group delay, as well as plasma fluctuations, at ten different microwave frequencies. The novel scheme uses continuous frequency modulation on a time scale much shorter (1/65 μs) than that of plasma fluctuations, the main aim being to make the group delay measurement more robust against plasma fluctuations. This article discusses the theoretical background of the scheme, gives a detailed description of the system, and presents results from plasma measurements. Although the implementation used suffered from an excessive power-dependent group delay distortion (resulting in a positional error of 10 cm), the main objectives have been achieved. © 2000 American Institute of Physics.
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52.55.Fa Tokamaks, spherical tokamaks
52.70.Gw Radio-frequency and microwave measurements
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment

Measurements of poloidal rotation velocity using charge exchange spectroscopy in a large helical device

K. Ida, S. Kado, and Y. Liang

Rev. Sci. Instrum. 71, 2360 (2000); http://dx.doi.org/10.1063/1.1150621 (7 pages) | Cited 38 times

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Absolute measurements of poloidal rotation velocity with the accuracy up to 1 km/s (2 pm in wavelength) were done using charge exchange spectroscopy in a large helical device. Radial profiles of the absolute Doppler shift of charge exchange emission with a beam are obtained from spectra measured with four sets of optical fiber arrays that view downward and upward at the poloidal cross section with and without neutral beam injection. By arranging the optical fiber from four arrays close to each other at the entrance slit, the apparent Doppler shift due to aberrations of the spectrometer and due to interference of the cold component (the charge exchange between He-like oxygen and thermal neutrals 8 pm from the charge exchange emission with a beam) can be eliminated from the measurements. The measured poloidal rotation velocity is 1–3 km/s in the electron diamagnetic direction at half of the plasma minor radius. © 2000 American Institute of Physics.
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52.55.Jd Magnetic mirrors, gas dynamic traps
52.70.-m Plasma diagnostic techniques and instrumentation
52.25.Fi Transport properties

Calibration of a multiple microchannel plate detectors system by α-induced secondary electrons

J. Villette, M. Barat, and P. Roncin

Rev. Sci. Instrum. 71, 2367 (2000); http://dx.doi.org/10.1063/1.1150622 (4 pages) | Cited 1 time

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α particles emitted from an 241Am radioactive source at energies of 5.4 MeV generate bursts of about ten electrons when passing through an aluminized Mylar foil. Besides the typical surface barrier electrons, the energy spectra of the secondary electrons clearly reveal two additional peaks. One at 66 eV is ascribed to aluminum LVV Auger electrons and another at 10.5 eV is attributed to the decay of aluminum volume plasmons. The well-resolved angular and energy distributions of these secondary electrons are used to calibrate the relative detection efficiencies of a large set of individual detectors of a complex multicoincidence system. © 2000 American Institute of Physics.
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29.40.Gx Tracking and position-sensitive detectors
06.20.F- Units and standards
29.30.Aj Charged-particle spectrometers: electric and magnetic
29.30.Ep Charged-particle spectroscopy
07.81.+a Electron and ion spectrometers
79.20.Hx Electron impact: secondary emission

A proposed new microstructure for gas radiation detectors: The microhole and strip plate

J. F. C. A. Veloso, J. M. F. dos Santos, and C. A. N. Conde

Rev. Sci. Instrum. 71, 2371 (2000); http://dx.doi.org/10.1063/1.1150623 (6 pages) | Cited 26 times

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A new type of microstructure device for a gas radiation detector is proposed. This microstructure, the microhole and strip plate structure, merges the structures of a gas electron multiplier and a microstrip plate in one single plate. This design allows two-multiplication stages and a separation of the sensitive and the detection regions, with full optical positive feedback suppression. Simulations for gas gain and electron transparency of the microstructure are presented. Different applications are discussed. © 2000 American Institute of Physics.
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29.40.Cs Gas-filled counters: ionization chambers, proportional, and avalanche counters
29.40.Gx Tracking and position-sensitive detectors
07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors

Nitrogen-filled tube as a sensor of ionizing radiation

Momčilo M. Pejović and Goran S. Ristić

Rev. Sci. Instrum. 71, 2377 (2000); http://dx.doi.org/10.1063/1.1150662 (3 pages) | Cited 9 times

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The influence of secondary electrons released by gamma radiation from the cathode of a nitrogen-filled tube, on probability of electrical breakdown and memory curve, has been investigated. The results show that the nitrogen-filled tube is sensitive to gamma radiation only at longer afterglow periods in the case of overvoltage values grater than 50%. For shorter afterglow periods, the lower values of overvoltages can be used to separate the contribution of gamma rays. © 2000 American Institute of Physics.
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29.40.Cs Gas-filled counters: ionization chambers, proportional, and avalanche counters

Miniature photoconducting capacitor array as a source for tunable THz radiation

D. Hashimshony, A. Zigler, and K. Papadopoulos

Rev. Sci. Instrum. 71, 2380 (2000); http://dx.doi.org/10.1063/1.1150624 (6 pages) | Cited 1 time

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The prospects of a miniature photoconducting capacitor array as a source for THz radiation are discussed. The device consists of an alternately biased capacitor array built on a semiconductor substrate and illuminated by a short laser pulse. The laser pulse creates a propagating plasma front inside the crystal volume by side illumination, and triggers the discharging of the capacitor by shorting out the photoconducting material between each capacitor plate. The sequential discharge of the capacitor array inside the expending plasma region can be considered as an interaction between a static wave and a superluminous ionizing front. In the present article we have demonstrated production of two cycle pulses with a center frequency of up to 1.5 THz. The device combines elements from the well-known “frozen-wave” generator and from the photoconducting switches array. The underlying physics of this radiation mechanism is very similar to the dc-to-ac conversion mechanism inside a gas-filled capacitor array which generates coherent microwave radiation. © 2000 American Institute of Physics.
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07.57.Hm Infrared, submillimeter wave, microwave, and radiowave sources
42.72.Ai Infrared sources
85.60.-q Optoelectronic devices
84.32.Tt Capacitors
07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
72.40.+w Photoconduction and photovoltaic effects
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