120
7 Fundamentals of Hydrogen Recycling
7.4 Reflection, Reemission, and Retention
Figure 7.4 schematically shows time sequences of reemission flux of injected fuels
from the plasma-facing wall (a) and retention in the wall (b). Except direct reflection
of the incident particle, the figure is quite similar to Fig. 4.14 in Chap. 4. The reemission flux increases with time and finally comes to the steady state where the impinging
particle flux balances with the reemitted flux plus the permeation flux. Generally, the
permeation flux is quite small and not considered in hydrogen recycling but does
matter for T safety as discussed later. Hatched area in Fig. 7.4a corresponds to the
amount of the hydrogen particles (H) retained (dissolved and trapped) (W(t)) in PFM,
W (t) =
t
0
(ϕ 0 − ϕ(t))dt,
(7.3)
where φ 0 and φ(t) are incident flux and reemitted flux, respectively, and plotted in
Fig. 7.4b. After the termination of H injection, if there is no H trapped, all retained
are subsequently released by diffusion. While if some are trapped as indicated as
bold dotted lines in the figure, the reemission after the termination of the H injection
decays earlier than that without the trapping and some of injected H remain in PFM.
Because energetic H injection produces new defects in PFM, which can trap
H subsequently injected, the true steady state is hardly attained. Nevertheless, the
retention rate or difference between the injected flux and reemitted one becomes less
and less toward the saturation. Hence, after a certain time of the injection, the remitted
flux becomes constant nearly equivalent to the incident flux. However, the integrated
Fig. 7.4 Schematics for time sequences of a reemission flux of injected fuels from PFM and
b retention in PFM. Lines and dotted lines correspond to remissions without and with trapping,
respectively
7 Fundamentals of Hydrogen Recycling
7.4 Reflection, Reemission, and Retention
Figure 7.4 schematically shows time sequences of reemission flux of injected fuels
from the plasma-facing wall (a) and retention in the wall (b). Except direct reflection
of the incident particle, the figure is quite similar to Fig. 4.14 in Chap. 4. The reemission flux increases with time and finally comes to the steady state where the impinging
particle flux balances with the reemitted flux plus the permeation flux. Generally, the
permeation flux is quite small and not considered in hydrogen recycling but does
matter for T safety as discussed later. Hatched area in Fig. 7.4a corresponds to the
amount of the hydrogen particles (H) retained (dissolved and trapped) (W(t)) in PFM,
W (t) =
t
0
(ϕ 0 − ϕ(t))dt,
(7.3)
where φ 0 and φ(t) are incident flux and reemitted flux, respectively, and plotted in
Fig. 7.4b. After the termination of H injection, if there is no H trapped, all retained
are subsequently released by diffusion. While if some are trapped as indicated as
bold dotted lines in the figure, the reemission after the termination of the H injection
decays earlier than that without the trapping and some of injected H remain in PFM.
Because energetic H injection produces new defects in PFM, which can trap
H subsequently injected, the true steady state is hardly attained. Nevertheless, the
retention rate or difference between the injected flux and reemitted one becomes less
and less toward the saturation. Hence, after a certain time of the injection, the remitted
flux becomes constant nearly equivalent to the incident flux. However, the integrated
Fig. 7.4 Schematics for time sequences of a reemission flux of injected fuels from PFM and
b retention in PFM. Lines and dotted lines correspond to remissions without and with trapping,
respectively