8.4 Recycling and Retention of Fuels
149
In metallic walls, the fuel particles are retained either as adsorbed water, dissolved
or trapped at defects or impurities and molecules in surface blisters or babbles.
Different from the C-wall for which most of the fuel inventory is caused by deposited
layers, the fuel inventory in metals is caused by diffusion into deeper region showing
square root dependence on plasma exposure time which is a typical diffusioncontrolled retention and the retention in W obeys this. However, the retention in early
times is owing to trapping at various defects in near-surface regions produced by the
injected fuel particles. Later, trapping at defects produced by neutron irradiation
becomes additional inventory.
In case of Be, it is known that chemical sputtering making beryllium hydride
produce deposited layers which increases the fuel retention [27]. Therefore, erosion,
deposition and fuel retention schemes for Be would be similar to those for C.
Irrespective of C or metals, the wall inventory is larger by several orders of magnitude than the total number of fuel particles confined in plasma. Hence only tiny change
in the wall retention strongly influences plasma performance and, accordingly, the
density control of plasmas becomes difficult for the saturated walls. This is the
reason why the low recycling or wall pumping scheme, i.e. small reemission before
the establishment of the steady state, is preferred to obtain good plasma confinement.
Since long pulse or steady-state operations exclude the low recycling regime, it is
one of the most important tasks of ITER to confirm the steady-state operation with
the saturated wall being possible.
8.4.2.2 Isotopic Replacement and Appearance of the Lightest
Hydrogen Isotope (H) as an Impurity
Fuel dilution by the lightest hydrogen isotope (H) which is not the fuel cannot be
avoided. Water vapor is ubiquitous and is adsorbed on any material surfaces. H atoms
are included in any materials as an impurity with its concentration more than ppm
order. It is also well known that water vapor always remains as a residual gas in any
vacuum systems. During D fueled discharges, the reemitted D reacts with H and H 2 O
adsorbed on PFS and release H. Accordingly, H concentration in the D discharges
decreases with discharge time. However, the release of H from PFM continues owing
to isotopic replacement of H retained in near-surface regions. Since depletion of H
in near-surface region induced diffusion of H dissolved in deep diffusion, emission
of H from the wall continues very long.
Therefore, it has been hardly possible to make pure D discharges and some H
always remains in present tokamaks. Figure 8.11 [35] shows a typical example
showing how isotopic changeover proceeded when fueling gas was changed from H
to D or vice versa in JET plasmas with C-wall and ILW for succeeding discharges.
Although initially the isotopic changeover is first, a few % of minor isotope remains
quite long. Thus in “D discharges” in present tokamak always unintentionally
includes a few % of H. There is a tendency that for lower wall temperatures H
concentration is higher because absorbed H 2 O and dissolved H are larger.
149
In metallic walls, the fuel particles are retained either as adsorbed water, dissolved
or trapped at defects or impurities and molecules in surface blisters or babbles.
Different from the C-wall for which most of the fuel inventory is caused by deposited
layers, the fuel inventory in metals is caused by diffusion into deeper region showing
square root dependence on plasma exposure time which is a typical diffusioncontrolled retention and the retention in W obeys this. However, the retention in early
times is owing to trapping at various defects in near-surface regions produced by the
injected fuel particles. Later, trapping at defects produced by neutron irradiation
becomes additional inventory.
In case of Be, it is known that chemical sputtering making beryllium hydride
produce deposited layers which increases the fuel retention [27]. Therefore, erosion,
deposition and fuel retention schemes for Be would be similar to those for C.
Irrespective of C or metals, the wall inventory is larger by several orders of magnitude than the total number of fuel particles confined in plasma. Hence only tiny change
in the wall retention strongly influences plasma performance and, accordingly, the
density control of plasmas becomes difficult for the saturated walls. This is the
reason why the low recycling or wall pumping scheme, i.e. small reemission before
the establishment of the steady state, is preferred to obtain good plasma confinement.
Since long pulse or steady-state operations exclude the low recycling regime, it is
one of the most important tasks of ITER to confirm the steady-state operation with
the saturated wall being possible.
8.4.2.2 Isotopic Replacement and Appearance of the Lightest
Hydrogen Isotope (H) as an Impurity
Fuel dilution by the lightest hydrogen isotope (H) which is not the fuel cannot be
avoided. Water vapor is ubiquitous and is adsorbed on any material surfaces. H atoms
are included in any materials as an impurity with its concentration more than ppm
order. It is also well known that water vapor always remains as a residual gas in any
vacuum systems. During D fueled discharges, the reemitted D reacts with H and H 2 O
adsorbed on PFS and release H. Accordingly, H concentration in the D discharges
decreases with discharge time. However, the release of H from PFM continues owing
to isotopic replacement of H retained in near-surface regions. Since depletion of H
in near-surface region induced diffusion of H dissolved in deep diffusion, emission
of H from the wall continues very long.
Therefore, it has been hardly possible to make pure D discharges and some H
always remains in present tokamaks. Figure 8.11 [35] shows a typical example
showing how isotopic changeover proceeded when fueling gas was changed from H
to D or vice versa in JET plasmas with C-wall and ILW for succeeding discharges.
Although initially the isotopic changeover is first, a few % of minor isotope remains
quite long. Thus in “D discharges” in present tokamak always unintentionally
includes a few % of H. There is a tendency that for lower wall temperatures H
concentration is higher because absorbed H 2 O and dissolved H are larger.