148
8 PMI in Large Tokamaks
ones. And the fraction of the retuning fuel flux to the plasma compared to the incident fuel particle flux to PFS which is referred to as a recycling coefficient or rate is
critically important for plasma density control.
8.4.2.1 Changes of Recycling Coefficient with Discharge Time
As described in Fig. 7.4, in early discharge time, most of the fuel particles injected to
PFS is retained in PFM so that the recycling coefficient is very small. Soon reemitted
fuel particles appear and increase with time. Reflection stays nearly constant except
small changes caused by the fuel retention in PFS. When the fuel particle concentration in PFS layers exceeds its saturation concentration, or attained steady-state
discharge, most of injected fuel particles are reemitted, i.e. the recycling coefficient
becomes nearly 1.0.
Although after long discharge time or under the steady-state discharge, the recycling coefficient becomes nearly one, some small part of the incident flux continues
to be retained owing to diffusion in deeper region and piled up in PFM with discharge
times as fuel inventory. In principle, the inventory could be determined with the integration of flux difference between throughput and exhaust. However, the difference is
not large enough to make accurate measurements. Compared to the throughput which
can be determined rather accurately, the measurement of the exhaust includes large
error. The fuel inventory is separately described later. Such recycling and retention
scheme are basically similar for the C-wall and metal wall, although dependencies
on discharge time or particle fluence are different from each other.
The retention is not limited to PFS or PFM, but that in the redeposited layers
irrespective of facing or shadowed to plasma. For the C-wall, the fuel retention in
deposited layers on plasma shadowed area dominates the whole retention. Because
the hydrocarbons produced by the chemical sputtering enter in boundary plasma and
impinge again to make deposited carbon layers. Since the deposited carbon layers
are active to H, incident H onto them or residual H are retained until the layers
are saturated with H. The saturation concentration in the deposited carbon layers
on the plasma shadowed area is higher than those on the plasma-facing surface.
Accordingly, fuel retention in the deposited layers on plasma shadowed or remote
dominates in-vessel fuel inventory as discussed in the next section.
For the C-wall, the fuel particles are mostly retained making chemical bond with
C atoms either C-D or C-T until the concentration attains a saturation level. The
saturation concentration in C at RT is as high as ~ 0.4 in D(T)/C atom ratio. At
higher temperature, it decreases significantly (see Fig. 9.3). Although the top surface
is immediately saturated after plasma exposure, the saturated layers grow deeper
zone with time. Hence full coverage of plasma-facing surfaces with C would result
in significantly large T inventory which is one of the reasons for excluding the C-wall
in ITER. However, it should be noted that since any hydrocarbons are not stable at
elevated temperatures, which is the reason for the reduction of chemical sputtering
above around 800 K, the saturation concentration at elevated temperatures decreases
appreciably and becomes similar level to that of metals (see Fig. 9.7).
8 PMI in Large Tokamaks
ones. And the fraction of the retuning fuel flux to the plasma compared to the incident fuel particle flux to PFS which is referred to as a recycling coefficient or rate is
critically important for plasma density control.
8.4.2.1 Changes of Recycling Coefficient with Discharge Time
As described in Fig. 7.4, in early discharge time, most of the fuel particles injected to
PFS is retained in PFM so that the recycling coefficient is very small. Soon reemitted
fuel particles appear and increase with time. Reflection stays nearly constant except
small changes caused by the fuel retention in PFS. When the fuel particle concentration in PFS layers exceeds its saturation concentration, or attained steady-state
discharge, most of injected fuel particles are reemitted, i.e. the recycling coefficient
becomes nearly 1.0.
Although after long discharge time or under the steady-state discharge, the recycling coefficient becomes nearly one, some small part of the incident flux continues
to be retained owing to diffusion in deeper region and piled up in PFM with discharge
times as fuel inventory. In principle, the inventory could be determined with the integration of flux difference between throughput and exhaust. However, the difference is
not large enough to make accurate measurements. Compared to the throughput which
can be determined rather accurately, the measurement of the exhaust includes large
error. The fuel inventory is separately described later. Such recycling and retention
scheme are basically similar for the C-wall and metal wall, although dependencies
on discharge time or particle fluence are different from each other.
The retention is not limited to PFS or PFM, but that in the redeposited layers
irrespective of facing or shadowed to plasma. For the C-wall, the fuel retention in
deposited layers on plasma shadowed area dominates the whole retention. Because
the hydrocarbons produced by the chemical sputtering enter in boundary plasma and
impinge again to make deposited carbon layers. Since the deposited carbon layers
are active to H, incident H onto them or residual H are retained until the layers
are saturated with H. The saturation concentration in the deposited carbon layers
on the plasma shadowed area is higher than those on the plasma-facing surface.
Accordingly, fuel retention in the deposited layers on plasma shadowed or remote
dominates in-vessel fuel inventory as discussed in the next section.
For the C-wall, the fuel particles are mostly retained making chemical bond with
C atoms either C-D or C-T until the concentration attains a saturation level. The
saturation concentration in C at RT is as high as ~ 0.4 in D(T)/C atom ratio. At
higher temperature, it decreases significantly (see Fig. 9.3). Although the top surface
is immediately saturated after plasma exposure, the saturated layers grow deeper
zone with time. Hence full coverage of plasma-facing surfaces with C would result
in significantly large T inventory which is one of the reasons for excluding the C-wall
in ITER. However, it should be noted that since any hydrocarbons are not stable at
elevated temperatures, which is the reason for the reduction of chemical sputtering
above around 800 K, the saturation concentration at elevated temperatures decreases
appreciably and becomes similar level to that of metals (see Fig. 9.7).