Collisional radiative recombination

Neutral stabilized collisional-radiative recombination is also quite slow under typical afterglow conditions. In his review, Flannery4 gives the simple formula,... 

Thus, collisional-radiative recombination will not make an important contribution to the deionization in typical low-density afterglow experiments, except in cases where dissociative recombination is extremely slow, e.g. for He . However, collisional-radiative recombination may not be slow compared to the partial dissociative recombination coefficient for a particular product branch. For instance, a product channel that accounts for only 1% of the total may very well arise from collisional-radiative recombination. 

Since the atom A is preferentially formed either directly or in subsequent electron interactions in an excited state which can radiate, this process is often designated as collisional-radiative recombination. For most laboratory gas discharges in which ne — 1010 cm."3, this process is not of consequence. 

Collisional radiative recombination important at high pressure May be important at high pressure... 

Nonlinear Thomson scattering can only be observed for the highest laser intensities, as shown in Fig. 11.7. As expected from the theory, when ao < 1, the nonlinear Thomson scattering vanishes and the collisional radiative processes from the thermal plasma prevail (Bremsstrahlung and radiative recombination). For ao > 1, these latter processes are still effective, as shown by the quadratic dependency on the electronic density of the plasma observed for 9 = 40°. However, this isotropic thermal emission remains less intense than the collimated nonlinear Thomson scattering emission and becomes detectable only at a large angle of observation (> 40°). 

From inspecting the atomic database of the EIRENE code [31], which is used in many applications to a large number of different tokamaks, including for the ITER design, in particular its collisional-radiative models for molecules, it was clear that matters can be more complicated. The relaxation time for establishing a vibrational distribution of the ground state molecule is comparable to the transport time of the molecule, hence the applicability of local collisional-radiative approximations is questionable. Furthermore, one of the two atoms created in dissociative recombination is electronically excited, and, hence, can be ionized very effectively even at low divertor plasma temperatures (instead of radiative decay). In this case, the whole chain of reactions would be just an enhanced ( molecular activated ) dissociation (MAD, i.e., dissociative excitation of those H]]", which have been produced... 

State selective dielectronic recombination rate coefficients from Li-like ions to Be-like ions (C, O, Ne, Fe ions) and for carbon L-shell ions have been calculated [10-13]. These data are used to develop collisional-radiative models including dielectronic recombination to excited states [14-17]. The population kinetics of L-shell ions and atoms have been developed and their results have been applied to plasma diagnostics. 

Data is archived under SCCS version control and all data sets contain a tail section giving attribution and update history. A number of ADAS data sub-directories have a year number, such as 89 associated with the name. The year number is often used in ADAS to give the year of introduction of a new approximation and is not necessarily the year of production. Thus 89 in the adfll data format denotes the baseline parametric form of stage to stage recombination and ionization data widely used in fusion laboratories. 96 denotes the much higher precision generalized collisional-radiative data which is valid at all densities and is metastable resolved. 

The ionization structure of the wind is also very important in determining the chemistry. Since the photospheric temperature is so low, the initial ionization is simply taken to be that of material in LTG at z = 3 to 5 where the ejecta temperature is at a maximum, so that the ionization is simply given by the balance of radiative recombination with the collisional ionization rates. Once the ejecta cools below about 15000 K, recombination occurs. For T Tauri ejecta temperatures and densities (at z = 10 where T 15000 K) this recombination will occur over a characteristic recombination length of Az 6. Thus the conditions within the ejecta are changing from being hot, dense and ionized to being relatively cool, diffuse and neutral. 

The k and j satellites are the strongest dielectronic satellites to the He-like lines, the q, r, s and t satellites have strong contributions due to inner-shell excitation from the Li-like ground state. Besides the dominant collisional excitation of He-like ions in the ground state, recombination processes (radiative, dielectronic and charge exchange) of H- and He-like ions, inner-shell excitation of the Li-like ions and, in the case of the z line, also inner-shell ionization process contribute to the intensity of the He-like lines. We will discuss these processes in detail. 

For He, two-body removal of He(2 5) and three-body removal of He(2 iS) have been observed. Collisional deactivation of 2 5 to 2 5 (except by electrons ) has been ruled out, so that two-body radiative combination to He2(/4 j ) is probably the dominant mechanism. This has received support from Sando, whose calculations of the intensity of two-body He(2 S) -l- He(l S) emission closely match the observed quenching rate constant. The absence of two-body removal of He(2 iS) by He is consistent with the metastable nature of He2(a S+). As the kinetics of growth of a in the He afterglow match those of the decay of He(2 5), the three-body process can be assigned to recombination into the molecular state. ... 

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