Neutralisation of intense ion beams at JET
Neutralization is an important issue in the design of neutral beam injection (NBI) systems for heating and fueling fusion plasmas. It has long been established that the neutralization efficiency of the JET injectors falls short of the theoretical value as determined by the expected gas target thickness and the well-known reaction cross-sections. This problem is even more acute in the recently upgraded 130 kV/60 Amp injectors at JET. An illustration shows the measured deuterium power delivered to JET as a function of extracted beam power. It has been proposed that the efficiency deficit is caused by a reduction in the line integrated gas density in the injector's neutralizer. This density reduction could be due to a number of factors including wall pumping and modification of the flow regime, but recent spectroscopic measurements of the neutralization system suggest that the density depletion is due to heating of the neutralizer gas. The mechanism responsible for this is thought to be indirect energy transfer from the beam to the neutral gas via the low temperature plasma that is formed in the neutralization process inside the neutralizer cell. This paper presents measurements of neutralization efficiency based on calorimetric data from the JET injectors. We also present some results from an experimental investigation at the JET Neutral Beam Test Bed including measurements of the low temperature neutralizer plasma. The plasma measurements are used as input parameters for a gas heating model and the outputs of this model are compared to a spectroscopic measurement of the neutralizer gas temperature. Overall, the results support hypothesis that the neutralization deficit is caused by gas density depletion due to indirect heating of the neutralizer gas.
Experimental studies of the JET NBI neutraliser plasma
Abstract Neutralisation efficiency is an important issue in the design of neutral beam injection (NBI) systems for heating and fuelling fusion plasmas. The JET Neutral Beam Injectors system does not reach the efficiency that is expected on the basis of current understanding of the neutralisation process. If this neutralisation efficiency deficit could be eliminated by re-design of the neutralisers, additional neutral beam heating power would become available. The interaction of the high-energy beam ions and the gas in the neutraliser cell results in the partial neutralisation of the beam and also in the formation of a low temperature plasma inside the neutraliser. The interaction between this plasma, the neutral gas and the walls of the cell can give rise to various phenomena capable of depleting the neutral gas molecular density hence reducing the effective neutralisation target. This paper presents the results of a number of experiments that were carried out in order to measure the plasma and gas parameters inside the neutraliser. The plasma parameters were obtained using a combination of planar Langmuir probes and a novel electrical diagnostic called a ‘plasma eater’. The gas pressure was measured using hot cathode ionisation gauges and capacitance manometers. An attempt has been made to measure the neutral hydrogen gas temperature using a spectroscopic technique that requires the measurement of the intensities of the spectral lines of the hydrogen Fulcher-α system [Plasma Sources Sci. Technol. 10 (2001) 17].
Studies in Conservation
The Journal of the International Institute for the Conservation of Museum Objects, Vol. 1, No. 1, October, 1952. (London : Thomas Nelson & Sons, Ltd.) Price 21s. each number. In introducinga new journal something should be said to justify an addition to the ever growing list now confronting any scientist. For Studies in Conservation it may at the outset be claimed that it is the only journal covering specifically the application of science to the study of archaeological objects and works of art, for which there has been no medium since Technical Studies in the Field of the Fine Arts ceased to appear in 1942.