Dong, C. Z.

Fritzsche, S.

Fu, Y. B.

Surzhykov, A.

Wu, Z. W.

Zhang, Y. Z.

GSI Repository

Dielectronic recombination of rubidium-like tungsten ionsZ. W. Wu1,2, Y. Z. Zhang2, Y. B. Fu2, A. Surzhykov1, S. Fritzsche1,3, and C. Z. Dong21Helmholtz Institute Jena, Germany; 2Northwest Normal University, Lanzhou, China; 3University of Jena, GermanyOwing to its high melting point, low sputtering rateand low absorption of tritium, tungsten has been utilizedas a plasma-facing material in magnetically confined fu-sion facilities. Most recently, it was considered as a lead-ing candidate for the divertor and main chamber regionsof the International Thermonuclear Experimental Reactor(ITER) tokamak. Inevitably, tungsten ions are expected tobe prominent impurities in fusion plasmas. The emitted ra-diation from excited tungsten leads to substantial plasmacooling that has to be well controlled in order to maintainconditions for nuclear fusion. In addition, dielectronic re-combination (DR) is an important atomic process in suchplasmas in which an initially free electron is captured by atarget ion under simultaneous excitation of one of its boundelectrons. This leads to a doubly excited state of the ionthat may subsequently stabilize by photon emission. Ac-curate DR cross sections and rate coefficients are essentialfor simulating ionization balance of highly charged ions inhigh-temperature plasmas. Therefore, studies on the DR oftungsten ions have attracted much attention.In the isolated-resonance approximation, if we assumethat the electron velocity obeys the Maxwellian distributionin plasmas, DR rate coefficients can be given byαDR(kTe) =( 2pi~2mekTe)3/2 gj2giAaji Brj,f exp(− EijkTe). (1)Here, kTe is the electron temperature, Eij the resonanceenergy, gi and gj the statistical weights of initial and inter-mediate states, respectively. Moreover,Brj,f denotes the ra-diative branching ratio and is defined in terms of the Augerdecay rate Aaji and the radiative decay rate Arjf .In order to learn importance of different subshell excita-tions, we perform ab initio calculations for DR of Rb-likeW37+ through the intermediate doubly excited configura-tions [Ne](3s23p63d10)−14s24p64dnl n′ l′ with n′ ≤ 16and l′ ≤ 9, and [Ne]3s23p63d10(4s24p64d)−1 nl n′ l′ withn′ ≤ 18 and l′ ≤ 12, while the contribution from otherconfigurations with larger n′ l′ is estimated by using the ex-trapolation procedures [1]. In Figure 1, we display total DRrate coefficients of initially W37+ ions together with partialcontributions as associated with excitations of the 3s, 3p,3d, 4s, 4p, and 4d subshells as functions of the electrontemperature. These coefficients are calculated for electrontemperatures from 1 eV to 5× 104 eV. Each partial contri-bution to total DR rate coefficients is given here by the sumof rate coefficients for the doubly excited configurations[Ne](3s23p63d104s24p64d)−1nln′ l′ with all permitted nland n′ l′ combinations. As seen clearly in Figure 1, the ex-1 10 100 1000 1000010-1310-1210-1110-1010-910-83d3p3s4p4s4dtotal   kTe (eV)Rate coefficient (cm3 /s)Figure 1: Total DR rate coefficients of initially Rb-likeW37+ ion and the partial contributions associated with theexcitations of 3s, 3p, 3d, 4s, 4p and 4d subshells as func-tions of the electron temperature kTe. Each partial contri-bution gives the sum of rate coefficients for all DR channelscorresponding to the excitations from nl subshells.citation of 4p subshell dominates total DR rate coefficientsin the whole region of electron temperature.With respect to the excitation of 3l subshells, it is foundthat the DR rate coefficients for the excitation from 3d sub-shell are the largest and that the corresponding DR rate co-efficients decrease with the quantum number l. For elec-tron temperatures lower than 100 eV, the excitations of3l subshells are much less significant than the ones of4l, and so its contributions to total DR rate coefficientscan be neglected. However, as the electron temperatureincreases, the DR rate coefficients for the excitations of3l subshells start to compete with their counterparts forthe 4l. This behavior arises mainly because of the influ-ence of near-threshold states, since their resonance ener-gies are quite small and since the DR rate coefficients havean exp(−Eij/kTe) dependence. Regarding the excitationof 4l subshells, the rate coefficients for the 4p excitationare the largest, while the contribution from 4s and 4d sub-shells cannot be neglected. The largest contributions to thetotal DR rate coefficients can reach 8% and 21% for theexcitations from 4d and 4s subshells, respectively, even forelectron temperatures below 100 eV.References[1] Z. W. Wu et al., Eur. Phys. J. D, submitted (2014).GSI SCIENTIFIC REPORT 2014 APPA-MML-AP-07DOI:10.15120/GR-2015-1-APPA-MML-AP-07 221

Dielectronic recombination of rubidium-like tungsten ions

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Dielectronic recombination of rubidium-like tungsten ions

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