152
8 PMI in Large Tokamaks
In burning plasma, both D and T impinge to PFM and both are simultaneously
retained. At the steady-state discharges, injecting fluxes of D and T to PFS must
be nearly the same as their recycling fluxes. However, D and T retention profiles
in PFM would not be the same, because the injecting fluxes of D and T must not
be even as discussed in Chap. 7, even if the energy of injecting D and T was the
same, their injecting depths are different, and trapping energy or binding energy in
PFM is also different for D and T. Probably larger diffusion or migration length
of lighter element, i.e. D, gives deeper penetration than T, while smaller trapping
energy or binding energy of T in PFM makes easier reemission of T than D. These
differences would give significant influence on controlling D/T ratio to keep the
optimized burning. Until now, no systematic studies have been done in this respect.
ITER would be the first test bed to examine D and T recycling for the optimization
of D-T burning by controlling D and T fueling. Most probably D and T must be
fueled separately. Unfortunately, it is not easy to determine D and T concentration
in burning plasma.
8.4.2.3 Recycling at Steady-State Plasma Operation
As discussed above, in most of the current tokamak experiments, low recycling
regime, is preferred because density control can be simply done by fueling. To attain
the low recycling, hydrogen retained in PFM should be minimized before discharges,
which is routinely done after certain numbers of discharges as wall conditioning.
However, after long discharge time or in the steady-state discharge, keeping low
recycling is difficult because PFS is saturated with the fuel except for a small fraction of penetrating in deep. Hence, plasma operation with saturated wall or under
nearly 100% recycling regime is not well established and remains as one of the most
important tasks in ITER.
The recycling coefficient or fractions of reflection, reemission and retention
change with the temperature of PFM. Because the saturation concentrations for both
of dissolution and trapping significantly decrease with the temperature above around
500 K. However, most of the present tokamaks operated at ambient temperatures
and their discharge duration are not long enough to increase the temperature of PFM
except the divertor region. Accordingly, changes of the recycling coefficient were not
significant. However, in JT-60U operated at higher temperature (around 400–500 K),
the recycling coefficient during a single long discharge (around 30 s) became near
unity; i.e. PFS was saturated during the discharge and then released flux was nearly
the same as the incident or no extra fueling was required [37]. (See Fig. 6.3)
8.4.3 Long Term Fuel Retention
Because of T safety and fuel self-sufficiency in a fusion reactor, it is necessary to
determine how much fuel is retained or fuel inventory in a reactor. Since integrated
8 PMI in Large Tokamaks
In burning plasma, both D and T impinge to PFM and both are simultaneously
retained. At the steady-state discharges, injecting fluxes of D and T to PFS must
be nearly the same as their recycling fluxes. However, D and T retention profiles
in PFM would not be the same, because the injecting fluxes of D and T must not
be even as discussed in Chap. 7, even if the energy of injecting D and T was the
same, their injecting depths are different, and trapping energy or binding energy in
PFM is also different for D and T. Probably larger diffusion or migration length
of lighter element, i.e. D, gives deeper penetration than T, while smaller trapping
energy or binding energy of T in PFM makes easier reemission of T than D. These
differences would give significant influence on controlling D/T ratio to keep the
optimized burning. Until now, no systematic studies have been done in this respect.
ITER would be the first test bed to examine D and T recycling for the optimization
of D-T burning by controlling D and T fueling. Most probably D and T must be
fueled separately. Unfortunately, it is not easy to determine D and T concentration
in burning plasma.
8.4.2.3 Recycling at Steady-State Plasma Operation
As discussed above, in most of the current tokamak experiments, low recycling
regime, is preferred because density control can be simply done by fueling. To attain
the low recycling, hydrogen retained in PFM should be minimized before discharges,
which is routinely done after certain numbers of discharges as wall conditioning.
However, after long discharge time or in the steady-state discharge, keeping low
recycling is difficult because PFS is saturated with the fuel except for a small fraction of penetrating in deep. Hence, plasma operation with saturated wall or under
nearly 100% recycling regime is not well established and remains as one of the most
important tasks in ITER.
The recycling coefficient or fractions of reflection, reemission and retention
change with the temperature of PFM. Because the saturation concentrations for both
of dissolution and trapping significantly decrease with the temperature above around
500 K. However, most of the present tokamaks operated at ambient temperatures
and their discharge duration are not long enough to increase the temperature of PFM
except the divertor region. Accordingly, changes of the recycling coefficient were not
significant. However, in JT-60U operated at higher temperature (around 400–500 K),
the recycling coefficient during a single long discharge (around 30 s) became near
unity; i.e. PFS was saturated during the discharge and then released flux was nearly
the same as the incident or no extra fueling was required [37]. (See Fig. 6.3)
8.4.3 Long Term Fuel Retention
Because of T safety and fuel self-sufficiency in a fusion reactor, it is necessary to
determine how much fuel is retained or fuel inventory in a reactor. Since integrated