<RESEARCH REPORT>High Intensity Li Beams for Beam Probe Laser Spectroscopy
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概要
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High intensity Li beams which include the laser blow-off beam and the fast beam have newly been developed for plasma diagnostics with the combination of beam probe and laser spectroscopy, especially for the measurement of magnetic field strength. Characteristics of the beam are investigated for checking their applicability as diagnostic beams. A novel technique has been developed to measure the magnetic field strength based on the laser blow-off Li beam probing combined with Dopper-tuned laser spectroscopy. A low energy beam is produced with a simple laser blow-off method. The density of Li beam produced by blowing off a thin Li film is of the order of 10^<10>cm^<-3>, which is measured by laser-induced fluorescence method. It is the highest density as a diagnostic Li beam. The beam energy is found to have a nearly Maxwellian velocity distribution which has a translational energy of a few eV. The time duration of the beam is a few hundred μs. Applicability of the newly developed method combing the blow-off beam with the laser spectroscopy to the absolute magnetic field measurement has been demonstrated. The measurable field strength is from a few tens Gauss to 1Tesla with an error of 10%. The time resolution is a few tens μs. Considering the attenuation of this beam it is applicable for plasma where the line-integrated electron density is less than 5×10^<13>cm^<-2>. A Li ion source of a discharge type was used to produce a high energy and high current beam. A Li beam of a few mAcm^<-2> with the minimum beam divergence of 1.2°is produced at the beam energy of 12keV. By measuring beam intensity and divergence it is found that a beam of good quality with the minimum divergence and the maximum current can be obtained simultaneously for a given energy when the negative potential is set to an optimum value. The divergence decreases with the beam energy, but increases after it takes the minimum value. A neutralization efficiency of about 80% can be obtained with Na vapor cell without difficulty. Applicability of the fast beam to the measurements of impurity ion densities is discussed. For the measurement of the magnetic field strength the same principle as the case of the laser blow-off beam with Doppler-tuned laser spectroscopy is applicable to the fast beam by slightly modulating the beam velocity. The energy modulation of about a few % does not deteriorate the quality of the beam. Limits of the measurements are also described.
- 核融合科学研究所の論文
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