Chapter 6
Material Modification by High-Power
Load and Its Influence on Plasma
6.1 Power Load to PFM
As noted in Chap. 3, power load to plasma-facing materials (PFM) can be divided
into two, steady and transient loads. Figure 6.1 summarizes five main power loads in
terms of their time durations and appearing frequencies in ITER [1]. In the figure, as
transient heat loads, bursts of edge localized modes (ELM), disruptions, and vertical
displacement events (VDE) are separated from steady heat load to divertor and first
wall (FW).
With advances of plasma confinement, i.e. increasing of plasma temperature and
density to satisfy the Lawson condition for D-T burning, heat load under steady
operation becomes very large. Allowable power load to plasma-facing materials is
depending on not only the materials themselves but also cooling power. In current
techniques, the maximum heat removal by a water-cooling system would be around
20 MW·m
−2 as described in Chap. 3. Therefore, reduction of power load to be less
than 20 MW·m
−2 is necessary and the power exhaust to divertor is one of the hardest
tasks in ITER and reactors [2, 3].
Due to such high-power load, only limited high-temperature materials can be used
as PFM or plasma facing armor tiles covering structure materials. They should have
high melting point, low vapor pressure, high thermal conductivity, and heat shock
resistance. Only W, Carbon-based materials, and some composite materials like SiC
are PFM candidates. Still material erosion due to sputtering, sublimation and particle
release due to surface cracking caused by thermal heat shock cannot be avoided.
As already discussed in Chap. 2, concerned power loads to PFM from plasma
consist of radiations and particles from plasma core and edge, while the power load
by neutron is not significant. However, the continuous load of 14 MeV neutrons
produces various kinds of radiation damages and will degrade all materials used in a
fusion reactor. This is the subject of an extensive long-term materials test program [4].
The expected power load by giant ELM is of the order of 1 GW·m
−2 within a submillisecond time scale [5, 6]. Up to now, only limited information is available on the
material performance under these events. VDE is transient events that may deposit
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
T. Tanabe, Plasma-Material Interactions in a Controlled Fusion Reactor, Springer Series
in Plasma Science and Technology, https://doi.org/10.1007/978-981-16-0328-0_6
95
Material Modification by High-Power
Load and Its Influence on Plasma
6.1 Power Load to PFM
As noted in Chap. 3, power load to plasma-facing materials (PFM) can be divided
into two, steady and transient loads. Figure 6.1 summarizes five main power loads in
terms of their time durations and appearing frequencies in ITER [1]. In the figure, as
transient heat loads, bursts of edge localized modes (ELM), disruptions, and vertical
displacement events (VDE) are separated from steady heat load to divertor and first
wall (FW).
With advances of plasma confinement, i.e. increasing of plasma temperature and
density to satisfy the Lawson condition for D-T burning, heat load under steady
operation becomes very large. Allowable power load to plasma-facing materials is
depending on not only the materials themselves but also cooling power. In current
techniques, the maximum heat removal by a water-cooling system would be around
20 MW·m
−2 as described in Chap. 3. Therefore, reduction of power load to be less
than 20 MW·m
−2 is necessary and the power exhaust to divertor is one of the hardest
tasks in ITER and reactors [2, 3].
Due to such high-power load, only limited high-temperature materials can be used
as PFM or plasma facing armor tiles covering structure materials. They should have
high melting point, low vapor pressure, high thermal conductivity, and heat shock
resistance. Only W, Carbon-based materials, and some composite materials like SiC
are PFM candidates. Still material erosion due to sputtering, sublimation and particle
release due to surface cracking caused by thermal heat shock cannot be avoided.
As already discussed in Chap. 2, concerned power loads to PFM from plasma
consist of radiations and particles from plasma core and edge, while the power load
by neutron is not significant. However, the continuous load of 14 MeV neutrons
produces various kinds of radiation damages and will degrade all materials used in a
fusion reactor. This is the subject of an extensive long-term materials test program [4].
The expected power load by giant ELM is of the order of 1 GW·m
−2 within a submillisecond time scale [5, 6]. Up to now, only limited information is available on the
material performance under these events. VDE is transient events that may deposit
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
T. Tanabe, Plasma-Material Interactions in a Controlled Fusion Reactor, Springer Series
in Plasma Science and Technology, https://doi.org/10.1007/978-981-16-0328-0_6
95