12
1 Introduction
plasma confinement as shown in Fig. 1.6. The first break in the early ’80s was
given by titanium (Ti) gettering to give a good vacuum and hence confinement was
significantly improved. Covering all PFS by deposited carbon layers (Carbonization)
[22] further improved plasma performance, and plasma temperature over 1 keV was
attained. With this, in the late ’90s, the temperature barrier owing to strong radiation
of oxygen around 600 eV to inhibit plasma heating was overpassed. Introduction of
boron (B) [23] and beryllium (Be) [24, 25] which capture oxygen and radiate less
results in a significant increase in plasma temperature. As shown in Fig. 1.7 [26],
the advance of plasma confinement in the 80’s is quite parallel to these first wall
changes.
When plasma density and temperature became sufficiently high, a new problem
appeared; the difficulty of density control owing to reemission of hydrogen (recycled
hydrogen) from PFS and temperature rise of PFM. In recent days, for easy density
control and good plasma performance, wall pumping is encouraged, which means
PFS retains most of the injected hydrogen during a discharge with less recycling of
hydrogen. To attain wall pumping conditions, wall cleaning or hydrogen removal
from the wall by various methods was employed. Today discharge cleaning, which
uses glow or ECH discharges with an inert gas like He as working gas, is routinely
used in most tokamaks. With these successes, plasma temperature has raised over
several keV in three large tokamaks in the world, JET, TFTR, and JT-60U and burning
plasma looked like within our hands. On the other hand, heat load to PFM in such
high-temperature plasma became so large to enhance evaporation and melting of
PFM.
To reduce heat load to PFS and to exhaust He, a divertor structure was introduced.
To realize a fusion reactor as an energy source, one of the hardest tasks is to establish
a divertor system that tolerates high-power load (remove heat load) and exhausts He
quite effectively. Since T fuel of a fusion reactor is radioactive and its resources are
limited, keeping T safety in construction and operation and having enough margin in
T breeding (reduce T inventory in divertor) are required in addition to management
of power load given by radiation and particles in divertor and hydrogen recycling
[26]. Hopefully, this book will guide to or hint at that.
In ITER, tungsten (W) and beryllium (Be) are selected as PFM of divertor and first
wall, respectively. Although Carbon was initially selected as PFM, it was excluded
due to the concerns of T retention and neutron damage. In a reactor, W is a candidate
PFM, while Be is not likely used because of its low melting temperature. In this book,
PMI is discussed considering both W and C as candidate plasma-facing materials.
Although an advanced concept with using liquid metals has been proposed, little
data is available on plasma and liquid metal surface interactions. In principle, there
is no significant difference in PSI between the liquid surface and solid surface, except
cooling efficiency.
1 Introduction
plasma confinement as shown in Fig. 1.6. The first break in the early ’80s was
given by titanium (Ti) gettering to give a good vacuum and hence confinement was
significantly improved. Covering all PFS by deposited carbon layers (Carbonization)
[22] further improved plasma performance, and plasma temperature over 1 keV was
attained. With this, in the late ’90s, the temperature barrier owing to strong radiation
of oxygen around 600 eV to inhibit plasma heating was overpassed. Introduction of
boron (B) [23] and beryllium (Be) [24, 25] which capture oxygen and radiate less
results in a significant increase in plasma temperature. As shown in Fig. 1.7 [26],
the advance of plasma confinement in the 80’s is quite parallel to these first wall
changes.
When plasma density and temperature became sufficiently high, a new problem
appeared; the difficulty of density control owing to reemission of hydrogen (recycled
hydrogen) from PFS and temperature rise of PFM. In recent days, for easy density
control and good plasma performance, wall pumping is encouraged, which means
PFS retains most of the injected hydrogen during a discharge with less recycling of
hydrogen. To attain wall pumping conditions, wall cleaning or hydrogen removal
from the wall by various methods was employed. Today discharge cleaning, which
uses glow or ECH discharges with an inert gas like He as working gas, is routinely
used in most tokamaks. With these successes, plasma temperature has raised over
several keV in three large tokamaks in the world, JET, TFTR, and JT-60U and burning
plasma looked like within our hands. On the other hand, heat load to PFM in such
high-temperature plasma became so large to enhance evaporation and melting of
PFM.
To reduce heat load to PFS and to exhaust He, a divertor structure was introduced.
To realize a fusion reactor as an energy source, one of the hardest tasks is to establish
a divertor system that tolerates high-power load (remove heat load) and exhausts He
quite effectively. Since T fuel of a fusion reactor is radioactive and its resources are
limited, keeping T safety in construction and operation and having enough margin in
T breeding (reduce T inventory in divertor) are required in addition to management
of power load given by radiation and particles in divertor and hydrogen recycling
[26]. Hopefully, this book will guide to or hint at that.
In ITER, tungsten (W) and beryllium (Be) are selected as PFM of divertor and first
wall, respectively. Although Carbon was initially selected as PFM, it was excluded
due to the concerns of T retention and neutron damage. In a reactor, W is a candidate
PFM, while Be is not likely used because of its low melting temperature. In this book,
PMI is discussed considering both W and C as candidate plasma-facing materials.
Although an advanced concept with using liquid metals has been proposed, little
data is available on plasma and liquid metal surface interactions. In principle, there
is no significant difference in PSI between the liquid surface and solid surface, except
cooling efficiency.