Study of sublevel population mixing effects in hydrogen neutral beams

Abstract

Intensities of hydrogen Balmer-series lines are sensitive to distribution of populations of sublevels of excited state due to difference of radiative decay branching ratio for sublevels with different orbital quantum number. Since these sublevels are degenerate transitions between sublevels and mixing of its population may be caused by very weak action of environment. A beam of fast hydrogen atoms is a system in which these fine effects of population mixing can be observed. Experimental data of H α line radiation intensities in hydrogen neutral beams are compared with calculations assumed different models of sublevel population. The comparison most probably point out that population mixing effects cause transitions from long-lived 3s to 3p sublevel with corresponding changes in H α radiation intensities. Discussed effects may influence to results of Doppler-shift spectroscopy measurements of the neutral beam parameters such as species and impurities content and neutralizing efficiency. INTRODUCTIONWell-known feature of energy level structure of hydrogen atom is degeneration of sublevels in orbital quantum number. Since a splitting of sublevels due to spin-orbital interaction and relativistic effects is in order of 10 -5 eV only, sublevel population distribution may be sensitive to very weak action of environment. In turn, this distribution affect to a rate of radiative de-excitation of the level, and that is intensity of line radiation. Consequently, in some cases intensity of radiation of hydrogen spectral lines is determined by fine effects of mixing of sublevel populations (so-called l-mixing).In particular, effect of sublevel population mixing can be manifested in the measurements of intensities of Doppler-shifted hydrogen Balmer-α lines in the beams of fast hydrogen atoms. Such beams are widely used for plasma heating and diagnostics in nuclear fusion oriented experiments. As a rule, such beams contain species with fractional energies (E/2, E/3, E/18) originated from acceleration and follows dissociation of hydrogen-contained molecular ions (H 2 + , H 3 + , H 2 O + ). Routine method for controlling of species content is Doppler-shift spectroscopy consisting of measurements of intensities of Doppler-shifted Hα spectral lines appeared in collisions of beam particles with background gas [1-4].Interpretation of Doppler-shift spectroscopy measurement of neutral beams species content requires an assumption about distribution of populations of thin structure of excited state (hydrogen n=3). The reason is that in contrast to long-lived 3s and 3d sublevels that decays with H α emission, the 3p sublevel predominantly decays directly to ground state with emission of L β line. Transitions between sublevels would change effective branching factor that is ratio of de-excitation via H α and L β emission.Several fine processes in the beamline may cause mixing of sublevel population and accordingly emissivity of H α radiation from beam atoms. Mentioned processes are collisions of fast atoms with background plasma particles ...

Assessment of neutron production in neutral beam injector of TCV tokamak

“…Due to presence of deuterium molecular ions in the plasma emitter the ion source, the beam composed of several fractions of particles with energies equal to full, half, and one-third of accelerating voltage. The ratio of fluxes of different fractions, measured by Doppler Shift Spectroscopy system [14][15][16][17][18], was F E / F E/2 / F E/3 = 0.5/0.3/0.2, that corresponds to the relative current of full energy ions equal to I F =0.7·I. Deuterium gas, puffed to a plasma emitter of the ion source, outflows via ion-optical system to neutralizer cell, and then was captured in the cryopump of the vacuum tank.…”