10
1 Introduction
interactions with materials are different. Photons in the energy range of 1 eV to
several eV are visible, 10 eV to several tens change from ultraviolet (UV) to vacuum
ultraviolet (VUV), and above 100 eV changes from Soft X-rays to keV range Xrays as indicated in the figure. This means that depending on the atomic numbers
of the elements, not only their radiation power but also the character of their radiation is different. Higher Z elements like Tungsten (W) are most probably used as
PFM in a fusion reactor. When they are accumulated in the plasma center, they give
stronger radiation as the X-rays and hence their concentration in core plasma should
be suppressed to be below 10
−5 to 10
−6 .
Interactions of VUV and Soft X-ray with materials are so intense that they can
penetrate the materials only in very short distances, in other words, their energy
deposition on the material is so intense to exhibit strong PSI. Since the radiation for
the high Z elements is in the X-ray region, PSI for the high Z wall must be dominated
with the interaction with strong X-ray. However, no such high-intensity sources of
Soft X-ray and X-rays emitted from burning plasma are available, the interaction
of the strong radiation from the accumulated high Z elements in the plasma center
with materials is not easy to study and lots of subjects remain, for examples, plasma
opacity (radiation from plasma center does not come out to PFS to inhibit power
exhaust), the effect of energy deposition limited within very thin surface layers,
vapor shielding, and so on..
Particle confinement time in plasma is limited within a few seconds and they
escape from the plasma with diffusion across the magnetic field. Since the surface
temperature of the PFS should be below their melting temperature, injection of
high-energy plasma particles to PSF should be avoided. To realize this, boundary
plasma or scrape off layers are constructed with the installation of limiters or divertor
outside of the last closed magnetic field lines. The energy conversion from 10 keV
in core plasma to 100 eV in boundary plasma occurs with various processes together
with complex mass transfer. Furthermore, boundary plasma with the energy of 10–
100 eV contacts with PFS, resulting in plasma–material interactions (PMI) including
complex physical and chemical processes. Studies of the physical and chemical
processes occurring in the energy range of 1–100 eV are quite difficult mainly because
atoms and molecules having these energies are in excited states. Moreover, emitted
photons and electrons accompanied with energy loss process in this energy range
have quite short escaping (penetration) depths in materials even in gases. This makes
observation or monitoring of the occurring process in boundary plasma difficult.
Thus, PMI has not been well diagnosed or understood yet.
1.4 Brief History of the Development of Plasma-Facing
Materials
The first step in fusion research was to make plasma in vacuum, or to confine plasma
in a vacuum system that was initially composed of glass and turned to stainless
steel. Historically, improvement of plasma confinement relied on the improvement
1 Introduction
interactions with materials are different. Photons in the energy range of 1 eV to
several eV are visible, 10 eV to several tens change from ultraviolet (UV) to vacuum
ultraviolet (VUV), and above 100 eV changes from Soft X-rays to keV range Xrays as indicated in the figure. This means that depending on the atomic numbers
of the elements, not only their radiation power but also the character of their radiation is different. Higher Z elements like Tungsten (W) are most probably used as
PFM in a fusion reactor. When they are accumulated in the plasma center, they give
stronger radiation as the X-rays and hence their concentration in core plasma should
be suppressed to be below 10
−5 to 10
−6 .
Interactions of VUV and Soft X-ray with materials are so intense that they can
penetrate the materials only in very short distances, in other words, their energy
deposition on the material is so intense to exhibit strong PSI. Since the radiation for
the high Z elements is in the X-ray region, PSI for the high Z wall must be dominated
with the interaction with strong X-ray. However, no such high-intensity sources of
Soft X-ray and X-rays emitted from burning plasma are available, the interaction
of the strong radiation from the accumulated high Z elements in the plasma center
with materials is not easy to study and lots of subjects remain, for examples, plasma
opacity (radiation from plasma center does not come out to PFS to inhibit power
exhaust), the effect of energy deposition limited within very thin surface layers,
vapor shielding, and so on..
Particle confinement time in plasma is limited within a few seconds and they
escape from the plasma with diffusion across the magnetic field. Since the surface
temperature of the PFS should be below their melting temperature, injection of
high-energy plasma particles to PSF should be avoided. To realize this, boundary
plasma or scrape off layers are constructed with the installation of limiters or divertor
outside of the last closed magnetic field lines. The energy conversion from 10 keV
in core plasma to 100 eV in boundary plasma occurs with various processes together
with complex mass transfer. Furthermore, boundary plasma with the energy of 10–
100 eV contacts with PFS, resulting in plasma–material interactions (PMI) including
complex physical and chemical processes. Studies of the physical and chemical
processes occurring in the energy range of 1–100 eV are quite difficult mainly because
atoms and molecules having these energies are in excited states. Moreover, emitted
photons and electrons accompanied with energy loss process in this energy range
have quite short escaping (penetration) depths in materials even in gases. This makes
observation or monitoring of the occurring process in boundary plasma difficult.
Thus, PMI has not been well diagnosed or understood yet.
1.4 Brief History of the Development of Plasma-Facing
Materials
The first step in fusion research was to make plasma in vacuum, or to confine plasma
in a vacuum system that was initially composed of glass and turned to stainless
steel. Historically, improvement of plasma confinement relied on the improvement