42
3 Power Load on Plasma-Facing Materials
As noted in Fig. 1.4, the energy of radiation distributes vary widely from characteristic X-rays of impurities in plasma, bremsstrahlung from burning plasma
which is also widely distributed from X-ray to infrared, UV-Viz and infrared range
photons from boundary plasma, and visible radiation from high-temperature material
surfaces.
Material response to the radiation changes with its photon energy. Some uncertainty remains in the material response to high-energy photons with high flux.
Powerful photon sources are hardly available to simulate the power load in a fusion
reactor. In particular, photon sources with energy ranging from soft X-ray to UV are
scarce. The photons with such an energy range interact strongly with any elements,
resulting in secondary phenomena, emissions of secondary ions, electrons, and
photons. Consequently, PMI phenomena in plasma boundary must be significantly
modified by such high-energy photon irradiation. Particularly opacity change in
the boundary plasma caused by the secondary phenomena would influence plasma
confinement and power load to PFS. Therefore, in some sense, ITER divertor is a
good testbed for the investigation of PSI in a fusion reactor.
Power loads by ions, electrons, and charge exchanged neutrals are controlled by
electric–magnetic field or plasma configuration, and hence depending on the location
of PFS, PSI phenomena appear differently. In normal operation, the highest power
is loaded at the divertor target position and nearby the outer divertor target, and the
power load to the inner first wall surface is higher than that of the outer first wall
surface.
Although some very high energy of T and He produced by D-T reactions are
escaping from burning plasma owing to ripple loss or orbital loss, their power would
not be large enough to influence the total power load to PFS. Nevertheless, they could
induce radiation damage in near surface regions of PFM and influence fuel recycling
as discussed in Chap. 7.
In addition to the above described steady power loads, periodically changing
power load is added, they are given by saw tooth activity, blobs, and ELM. Although
the power load given by saw tooth activity is not likely very large, ELM activity is
found to be sometimes very large in high-density and high-temperature plasma. Now
giant ELM concerns seriously [6, 7], which appears rather irregularly and discussed
in the next section.
3.4 Transient Power Load
Typical transient power load is given by ramp-up and ramp-down phase of plasma,
particularly in the later, energy used for confinement is dissipated to PFM, and control
of slower rump-down is quite important. It is well known that during the ramp-up
phase, lots of impurities (mainly water) adsorbed on PFS are released because some
of the input power for plasma heating is dissipated to PFS to heat up its surface
temperature. Sometimes when wall conditioning is not well done, plasma collapses
during the ramp-up phase.
3 Power Load on Plasma-Facing Materials
As noted in Fig. 1.4, the energy of radiation distributes vary widely from characteristic X-rays of impurities in plasma, bremsstrahlung from burning plasma
which is also widely distributed from X-ray to infrared, UV-Viz and infrared range
photons from boundary plasma, and visible radiation from high-temperature material
surfaces.
Material response to the radiation changes with its photon energy. Some uncertainty remains in the material response to high-energy photons with high flux.
Powerful photon sources are hardly available to simulate the power load in a fusion
reactor. In particular, photon sources with energy ranging from soft X-ray to UV are
scarce. The photons with such an energy range interact strongly with any elements,
resulting in secondary phenomena, emissions of secondary ions, electrons, and
photons. Consequently, PMI phenomena in plasma boundary must be significantly
modified by such high-energy photon irradiation. Particularly opacity change in
the boundary plasma caused by the secondary phenomena would influence plasma
confinement and power load to PFS. Therefore, in some sense, ITER divertor is a
good testbed for the investigation of PSI in a fusion reactor.
Power loads by ions, electrons, and charge exchanged neutrals are controlled by
electric–magnetic field or plasma configuration, and hence depending on the location
of PFS, PSI phenomena appear differently. In normal operation, the highest power
is loaded at the divertor target position and nearby the outer divertor target, and the
power load to the inner first wall surface is higher than that of the outer first wall
surface.
Although some very high energy of T and He produced by D-T reactions are
escaping from burning plasma owing to ripple loss or orbital loss, their power would
not be large enough to influence the total power load to PFS. Nevertheless, they could
induce radiation damage in near surface regions of PFM and influence fuel recycling
as discussed in Chap. 7.
In addition to the above described steady power loads, periodically changing
power load is added, they are given by saw tooth activity, blobs, and ELM. Although
the power load given by saw tooth activity is not likely very large, ELM activity is
found to be sometimes very large in high-density and high-temperature plasma. Now
giant ELM concerns seriously [6, 7], which appears rather irregularly and discussed
in the next section.
3.4 Transient Power Load
Typical transient power load is given by ramp-up and ramp-down phase of plasma,
particularly in the later, energy used for confinement is dissipated to PFM, and control
of slower rump-down is quite important. It is well known that during the ramp-up
phase, lots of impurities (mainly water) adsorbed on PFS are released because some
of the input power for plasma heating is dissipated to PFS to heat up its surface
temperature. Sometimes when wall conditioning is not well done, plasma collapses
during the ramp-up phase.