A. Ichihara

D.F. Duechs

R.K. Janev

T. Kato

Janev,  R.K.

Duechs,  D.F.

Ichihara,  A.

Kato,  T.

NIFS-Repository (National Institute for Fusion Science)

§6. Isotope Effect in Divertor Plasma Volume Recombi nation Janev, R.K. (MacedonianAcd. Sci., Skopje), Duechs, D.F. (Max-Planck-Inst. Plasma Phys., Garching), Ichihara, A. (Tokai Res. Establ., JAERI, Tokai-mura), Kato, T. The theoretical analysis [1] of volume recombination of hydrogen divertor plasmas in the temperature range 0.3 - 2 eV is the result of two molecule assisted recombination (MAR) mechanisms [2,3] H+ + H- ----?H + H (n=3) (lb) e + H/ ----?H + H (n~2) (2b) where v is the vibrational state of H2 molecule and n is the principal quantum number of excited H- atom product. It has been known since mid-sixties that reaction (1 a) exhibits a pronounced isotope effect for the low v-values[ 4], which, however, diminishes with increasing v [5]. Reaction (2a) should also exhibit an isotope effect in the thermal energy region at least for v~4 when it becomes exothermic and when it is governed by the Langevin (intermediary compound formation and decay) mechanism. The question arises whether the isotopic effect in the reaction (1 a) survives after averaging over the distribution of vibrational levels in a plasma with temperature in the range 0.3 - 2 eV, and after the coupling of negative ion recombination MAR channel (1 a) - (1 b) with the isotopic less sensitive (in the range 0.3 -2 eV) ion-conversion MAR channel (2a) - (2b). We have analyzed this question for a HID divertor plasma adopting a 0 - D transport model and stationary plasma conditions. Apart from the reactions (1 a,b) and (2a,b) for the H- and D- plasma components, we have included 22 additional processes in the reaction kinetics, including the isotope exchange reactions which establish an efficient mixing of the two MAR channels. The full reaction kinetics is presented elsewhere [6]. For the most important reactions, such as (Ia) and (2a) coefficient were then averaged over a Boltzmann distribution of the vibrational populations of H2(V) and D2(V) at the plasma temperature T=leV. After solving the rate equations for the concentrations of relevant species entering the Eqs. (1) - (3), the ratio RH-D = [e,H+ J I [e,D+ J of the recombined electron-ion pairs, resulting from the reaction kinetics, was evaluated. For the HID divertor plasma at T=leV, the value ofRH-o was found to vary in the interval 1.015 - 1.03, reflecting the uncertainties of the used data. By this factor, the H+ ions recombine faster with the plasma electrons than the D + ions. A similar analysis 'of a Drr plasma gives for the analogons ratio RO-T a value between 1.005 - 1.01. It should be noted that the non-zero value of RA_A' (A,A' = H,D,T) results exclusively from the ion-conversion MAR channel. It turns out that the kinetics associated with the negative-ion MAR channel has an "isotopic symmetry", even after the "mixing" of this channel with the ion-conversion channel through the isotope exchange reactions (3). The isotope effect in the overall volume divertor plasma recombination is a consequence of the isotopic shift of the vibrational energy levels in the two isotopes and the associated differences in the isotope level populations at a given plasma temperature. For this reason, the isotope effect cannot in principle be removed from the ion-conversion MAR channel. In order to get a feeling for the size of the effect on a macroscopic level, one should estimate the time t2 required for the concentration ratio of produced (e, H+) and (e,' D+) pairs to double. Assuming a step-wise sequence of recombination / re-ionization processes, with a time step r = (ne KH,ion)-I, one obtains t2=nr, where n is determined by (RH_D)n = 2. For RH-D = 1.02, n = 35. If the electron concentration is fIe == 1014cm-3 and we take the value for the electron-impact ionization of Hat T == l.5eV (KH,ioll == 10-13 cm3/s) , then r =0.35, resulting in t2=3.5s. References [1] Reiter, D.et aI., l_Nuci. MateL 2441-243 (1997) 342. [2] lanev, R.K. et at, J. NucL MateL ill (1984) 10. [3] Krasheninnikov, S_ et at, Phys_ Lett. A 214 (1996) 295_ [4] Rapp, D_ et aI., Phys. Rev. Lett 11 (1965) 533_ [5] Wadehra, J.M_, Appl. Phys_ Lett. 35 (1979) 917_ [6] Janev, R.K_ et aI., (to be published) 359 

Isotope Effect in Divertor Plasma Volume Recombination

Isotope Effect in Divertor Plasma Volume Recombination

Abstract

Similar works

Full text

Available Versions

NIFS-Repository (National Institute for Fusion Science)