Spectroscopic Measurements of Temporal and Spatial Profiles of Electric Fields in a Discharge Plasma Fusion Neutron Source

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For a highly accurate measurement of electric field with an improved spatial resolution by the laser-induced fluorescence (LIF) method, a CCD of 7 mm square with 256 x 256 pixels has introduced. Also, we have introduced a converter of transverse shape of optical beam from a circle to a line, i.e. a sheet beam. For the measurements with a high spatial resolution by making use of these equipments, it is also necessary to develop a method based on the laser-induced fluorescence method by use of Stark effect, to adapt the wide dynamic range of the electric field strength in the discharge plasma fusion neutron source.In 2000 in this study, for a higher sensitivity, we studied an LIF for n = 4 forbidden transition (4^1D →2^1P, 492.2nm), which is theoretically expected to have 10 times higher transition probability, and adaptable to a much lower electric field range of 0.05〜0.5 kV/cm, as well, than the n = 3 transition previously we used. For verification, the present method was applied to a … More U-shaped hollow cathode discharge plasma, within which the electric field profile is well known, and was found to show a excellent accuracy in detecting a weak field of around 0.1 kV/cm or less.In 2001, we applied the above highly sensitive LIF method to the discharge plasma fusion neutron source with both a photo-multiplier and, then, the CCD. Particularly with the CCD, however, the S/N ratio was found too small to identify the existence of electric field, exclusively due to its higher spatial and temporal resolution than the photo-multiplier. The poor S/N ratio is found to result, also, from a too small fraction of 2^1S state (approx. 20.6 eV above the ground level) helium atoms in the neutron source, especially at high-voltage mode operations. To overcome this problem, we, thus, started a study to develop an injector of 2^1S state helium atom beam into the LIF observation point.The CCD with an extremely high resolution, on the other hand, was found to be very efficient in measuring the spatial profile of the spontaneous emission intensity from the discharge plasma, and the energy distribution of the fast neutral helium atoms by use of Doppler shift spectroscopy, as well, with a high accuracy. Also, we achieved an enhanced neutron yield of 1.1 x 107 n/sec with an improved high-voltage holding capability with a design refinement of the neutron source. Less