(e.g. see [64, 67] and the references therein), the IC following by the dissociative
recombination (DR)
H
þ
2 þ e ! H þ H,
ð2:29Þ
turns out to be a significant plasma recombination sink in high density, low temperature, H divertor plasmas. However, in D (and T) divertor plasma, effective recombination through the DA channel followed by mutual neutralization of negative and
positive hydrogen ions,
H
À
þ H
þ
! H þ H,
ð2:30Þ
becomes more important [68].
We note that the products of the reactions (2.29) and (2.30) are in excited
quantum states (from n ¼ 2 to n ¼ 4) and their further fate can be described by
proper modification of the CRM.
The sequence of reactions (2.27), (2.28), (2.29) and (2.30) is known in magnetic
fusion community as the Molecular Assisted Recombination (MAR) (e.g. see
[28, 68–71]).
Original estimates for the MAR rate [28, 69] performed for H 2 molecules
demonstrated that MAR can be significantly faster than the EIR channel (see
Fig. 2.5) at temperatures ~few eV, where the EIR rate constant is small, and
contribute significantly to the overall plasma recombination rate. However, unlike
the EIR rate, which is not sensitive to the particular hydrogen isotope, the MAR
rate is. This is because (i) the vibrational excitation cross-section of a hydrogen
molecule by electron impact decreases with an increase of the reduced mass e
M,
σ v!v 0 e
M
/ e
M
ÀjvÀv
0 j=2 [62]; and (ii) the vibrational level corresponding to endothermic reactions (2.27) and (2.28) v th / e
M
1=2 increases with an increase of e
M .
Therefore, heavy isotopologues of a hydrogen molecule should encounter more
interactions with electrons to reach v th . As a result, one could expect a weaker effect
of MAR for the case of tritium and deuterium plasmas in comparison with the
hydrogen one. However, we notice that the overall contribution of MAR to plasma
recombination depends on both spatial distribution of the plasma parameters and
transport of molecules in low-temperature divertors and, therefore, is machine
sensitive. The current version of MAR built into the neutral transport code EIRENE,
which is used, in particular, for the simulations of edge plasma in ITER, assumes the
CRM for the population of the vibrational states of hydrogen molecules (e.g. see [68]
and the references therein).
In fusion-related experiments, the MAR effects were identified in the experiments
on Alcator C-Mod [48] and linear divertor simulators [72–74]. Recent experimental
data from TCV tokamak show that in detached divertor regime MAR can account for
up to 40% of the overall plasma recombination sink [75].
2.4 Application of CRM to Edge Plasma Relevant Species
37
recombination (DR)
H
þ
2 þ e ! H þ H,
ð2:29Þ
turns out to be a significant plasma recombination sink in high density, low temperature, H divertor plasmas. However, in D (and T) divertor plasma, effective recombination through the DA channel followed by mutual neutralization of negative and
positive hydrogen ions,
H
À
þ H
þ
! H þ H,
ð2:30Þ
becomes more important [68].
We note that the products of the reactions (2.29) and (2.30) are in excited
quantum states (from n ¼ 2 to n ¼ 4) and their further fate can be described by
proper modification of the CRM.
The sequence of reactions (2.27), (2.28), (2.29) and (2.30) is known in magnetic
fusion community as the Molecular Assisted Recombination (MAR) (e.g. see
[28, 68–71]).
Original estimates for the MAR rate [28, 69] performed for H 2 molecules
demonstrated that MAR can be significantly faster than the EIR channel (see
Fig. 2.5) at temperatures ~few eV, where the EIR rate constant is small, and
contribute significantly to the overall plasma recombination rate. However, unlike
the EIR rate, which is not sensitive to the particular hydrogen isotope, the MAR
rate is. This is because (i) the vibrational excitation cross-section of a hydrogen
molecule by electron impact decreases with an increase of the reduced mass e
M,
σ v!v 0 e
M
/ e
M
ÀjvÀv
0 j=2 [62]; and (ii) the vibrational level corresponding to endothermic reactions (2.27) and (2.28) v th / e
M
1=2 increases with an increase of e
M .
Therefore, heavy isotopologues of a hydrogen molecule should encounter more
interactions with electrons to reach v th . As a result, one could expect a weaker effect
of MAR for the case of tritium and deuterium plasmas in comparison with the
hydrogen one. However, we notice that the overall contribution of MAR to plasma
recombination depends on both spatial distribution of the plasma parameters and
transport of molecules in low-temperature divertors and, therefore, is machine
sensitive. The current version of MAR built into the neutral transport code EIRENE,
which is used, in particular, for the simulations of edge plasma in ITER, assumes the
CRM for the population of the vibrational states of hydrogen molecules (e.g. see [68]
and the references therein).
In fusion-related experiments, the MAR effects were identified in the experiments
on Alcator C-Mod [48] and linear divertor simulators [72–74]. Recent experimental
data from TCV tokamak show that in detached divertor regime MAR can account for
up to 40% of the overall plasma recombination sink [75].
2.4 Application of CRM to Edge Plasma Relevant Species
37