Modelling the neutralisation process in neutral beam injectors
Fitzgerald, Niall J. (2009) Modelling the neutralisation process in neutral beam injectors. PhD thesis, Dublin City University.
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High power neutral beams currently play an important role in heating, fuelling and diagnosing magnetically confined thermonuclear fusion plasmas. At the Joint European Torus (JET) in Oxfordshire, England, the formation of such a beam involves passing a positive ion beam through a neutral gas target wherein beam electron-capture collisions result in a neutral beam component. The subsequent beam injection into the fusion plasma requires the sole use of this neutral component, since the charged component cannot penetrate through the large magnetic confinement fields of the tokamak. The observed failure to achieve near maximum theoretical neutralisation efficiency, has given motivation to those concerned to endeavour to understand the reason thereof. This neutralisation efficiency deficit is almost certainly due to gas target depletion, while the general consensus is that indirect heating of the neutraliser gas by the beam is its main cause. Paméla proposed a simplified analytical model of beam indirect gas heating over twenty years ago. The aim of this endeavour was to gain a more
thorough understanding of the interaction between the beam and the neutraliser gas (beam plasma), via electrostatic Particle-in-Cell (PIC) computer simulations incorporating Monte Carlo collisions (MCC). Results under varying beam & gas parameters include the calculation of plasma parameters and the resultant gas heating. The simulation results are qualitatively consistent with the experimental results from the Langmuir probe investigation of Crowley et al. (which includes spectroscopic measurements to estimate the gas temperature, and invokes the gas heating model developed by Paméla), while they predict the existence of four significant gas heating pathways not accounted for in the Paméla model i.e. direct kinetic energy transfer by H3+ ions, H2+ ions, H atoms (formed via H3+ formation) and electrons. However, the gas heating results do not account for the extent of the observed neutralisation inefficiency.
In agreement with Surrey, results from a similar simulation investigation of future (ITER) negative ion neutralisers predict insignificant gas heating effects. Beam composition simulations predict the existence of a specific gas line density pertaining to maximum neutralisation efficiency, as opposed to the generally assumed increasing asymptotic behaviour, while an experiment is proposed to verify this prediction.
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