6.2 Material Response to Power Load …
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photons, and they cause changes in boundary plasma, which is referred to as plasma–
surface or plasma–wall interactions (PSI or PMI). The emission of charged particles
including reflected and secondary ions, and electrons directly correlates to the formation of plasma sheath, which in turn modifies properties of the boundary plasma.
Various photoemissions which include reflection, de-excitation of excited atoms and
molecules, and radiation from heated PFM modify the boundary plasma. Higher
energy photons interact with particles in the boundary plasma and consequently
secondary electrons and photons are emitted. Because no plasma machines realize
reactor like power load, many processes expected in reactor divertor are uncertain
and remain unsolved or not understood well. High-power linear machines have been
constructed or being under construction for divertor simulation [13]. However, the
liner machines are hard to use as a divertor simulator, owing to the lack of strong
magnetic field and rather large angle of incident particles.
As an example of phenomena not well understood, Balmer lines emission is
discussed in the following. This emission is often used to determine the hydrogen
recycling coefficient at PFM assuming it originated from recycled (released)
hydrogen from PFS. The recycled hydrogen includes various species, reflected ions
and neutrals, reemitted atoms and molecules, and some product given by chemical sputtering like water and hydrocarbons. Considering cross sections of electron
excitation of these particles to be in excited states, released hydrogen flux can be
estimated. However, without knowing what kinds of hydrogen particles (molecules
and atoms, and those in excited states and ionized) with their energy distributions
are emitted from PFS, the estimation of the recycling coefficient would include large
error. In addition, as noted in Figs. 4.7 and 4.8, direct release of excited atoms from
PFS also gives the Balmer line emission, which is often not considered.
6.2.2 Melting and Sublimation
When power load to PFM exceeds cooling power, the temperature of PFM easily
escalates over its melting point. Generally, PFM is designed to be tolerant to steady
state or normal power load. In a tokamak type fusion reactor, however, transient or
off-normal power loads such as ELM and disruption seem unavoidable. Because of
its huge effects, the disruption should be avoided as seldom as possible and will not
be discussed here. PFM melting and sublimation under ELM appeared in current
tokamaks giving significant impacts on both plasma performance and lifetime of
PFM [14, 15].
Once the surface is melted, resulted liquid moves on the surface and some are
released as droplets. Strong electromagnetic field in a reactor controls the melt layer
motion and the direction and moving speed of the droplets. The droplets are ionized
and move according to E x B force. In tokamak discharges, white small particles are
often observed as line traces in the plasma. At the same time, various particles are
sublimated or desorbed. If the sublimation/desorption is large enough to contaminate
the plasma, plasma performance would be degraded.
97
photons, and they cause changes in boundary plasma, which is referred to as plasma–
surface or plasma–wall interactions (PSI or PMI). The emission of charged particles
including reflected and secondary ions, and electrons directly correlates to the formation of plasma sheath, which in turn modifies properties of the boundary plasma.
Various photoemissions which include reflection, de-excitation of excited atoms and
molecules, and radiation from heated PFM modify the boundary plasma. Higher
energy photons interact with particles in the boundary plasma and consequently
secondary electrons and photons are emitted. Because no plasma machines realize
reactor like power load, many processes expected in reactor divertor are uncertain
and remain unsolved or not understood well. High-power linear machines have been
constructed or being under construction for divertor simulation [13]. However, the
liner machines are hard to use as a divertor simulator, owing to the lack of strong
magnetic field and rather large angle of incident particles.
As an example of phenomena not well understood, Balmer lines emission is
discussed in the following. This emission is often used to determine the hydrogen
recycling coefficient at PFM assuming it originated from recycled (released)
hydrogen from PFS. The recycled hydrogen includes various species, reflected ions
and neutrals, reemitted atoms and molecules, and some product given by chemical sputtering like water and hydrocarbons. Considering cross sections of electron
excitation of these particles to be in excited states, released hydrogen flux can be
estimated. However, without knowing what kinds of hydrogen particles (molecules
and atoms, and those in excited states and ionized) with their energy distributions
are emitted from PFS, the estimation of the recycling coefficient would include large
error. In addition, as noted in Figs. 4.7 and 4.8, direct release of excited atoms from
PFS also gives the Balmer line emission, which is often not considered.
6.2.2 Melting and Sublimation
When power load to PFM exceeds cooling power, the temperature of PFM easily
escalates over its melting point. Generally, PFM is designed to be tolerant to steady
state or normal power load. In a tokamak type fusion reactor, however, transient or
off-normal power loads such as ELM and disruption seem unavoidable. Because of
its huge effects, the disruption should be avoided as seldom as possible and will not
be discussed here. PFM melting and sublimation under ELM appeared in current
tokamaks giving significant impacts on both plasma performance and lifetime of
PFM [14, 15].
Once the surface is melted, resulted liquid moves on the surface and some are
released as droplets. Strong electromagnetic field in a reactor controls the melt layer
motion and the direction and moving speed of the droplets. The droplets are ionized
and move according to E x B force. In tokamak discharges, white small particles are
often observed as line traces in the plasma. At the same time, various particles are
sublimated or desorbed. If the sublimation/desorption is large enough to contaminate
the plasma, plasma performance would be degraded.