Chapter 7
Fundamentals of Hydrogen Recycling
7.1 Introduction
Hydrogen recycling at plasma-facing surface (PFS) is quite important for density
control or maintaining plasma in good condition. In present tokamaks, low recycling
scheme, i.e. most of impinging hydrogen to plasma-facing materials (PFM) being
retained with little reemission (referred to as wall pumping), is favored for easy
density control by fueling only. In order to attain the large wall pumping, in most
of the present tokamaks, “wall conditioning” to remove hydrogen retained in PFM
such as He glow discharges has been routinely made after certain numbers of main
plasma discharges [1]. However, a long pulse or steady-state operation in a fusion
reactor would not keep the low recycling scheme, and fuel accumulation in PFM
continues to increase up to the saturation of fuels in PFM. In addition, impinging
hydrogen flux to PFS in a reactor is so high that retention would be saturated within
short discharge time. This means that in a fusion reactor, plasma operation in high
recycling scheme is dispensable, which is not well established yet and one of the
important research targets in ITER.
Even after the surface saturation, hydrogen diffuses into deep to increase T retention continuously during the plasma operation, which are concerns for T safety and
fuel self-sufficiency. Although techniques developed for the wall conditioning could
be used to reduce the T inventory, processing or refinement of removed T is required.
For example, He grow discharge results in huge amount He gas containing T, which
gives undesired load on the refinement system of exhausted fuels.
Therefore, investigation and understanding of H recycling and its influence on
plasma confinement are urgent tasks. However, no tokamaks having high-temperature
PFS and high H flux are currently available. Although linear plasma apparatus having
high H flux are available, they are mostly used for high heat flux tests of PFM
and do not seem suitable for simulation of the H recycling in a reactor divertor
owing to their different geometry from that of a rector divertor and lack of magnetic
field. Therefore, currently available data on H recycling should be extrapolated to
reactor environment. This chapter is devoted to summarize current knowledge and
© 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_7
115
Fundamentals of Hydrogen Recycling
7.1 Introduction
Hydrogen recycling at plasma-facing surface (PFS) is quite important for density
control or maintaining plasma in good condition. In present tokamaks, low recycling
scheme, i.e. most of impinging hydrogen to plasma-facing materials (PFM) being
retained with little reemission (referred to as wall pumping), is favored for easy
density control by fueling only. In order to attain the large wall pumping, in most
of the present tokamaks, “wall conditioning” to remove hydrogen retained in PFM
such as He glow discharges has been routinely made after certain numbers of main
plasma discharges [1]. However, a long pulse or steady-state operation in a fusion
reactor would not keep the low recycling scheme, and fuel accumulation in PFM
continues to increase up to the saturation of fuels in PFM. In addition, impinging
hydrogen flux to PFS in a reactor is so high that retention would be saturated within
short discharge time. This means that in a fusion reactor, plasma operation in high
recycling scheme is dispensable, which is not well established yet and one of the
important research targets in ITER.
Even after the surface saturation, hydrogen diffuses into deep to increase T retention continuously during the plasma operation, which are concerns for T safety and
fuel self-sufficiency. Although techniques developed for the wall conditioning could
be used to reduce the T inventory, processing or refinement of removed T is required.
For example, He grow discharge results in huge amount He gas containing T, which
gives undesired load on the refinement system of exhausted fuels.
Therefore, investigation and understanding of H recycling and its influence on
plasma confinement are urgent tasks. However, no tokamaks having high-temperature
PFS and high H flux are currently available. Although linear plasma apparatus having
high H flux are available, they are mostly used for high heat flux tests of PFM
and do not seem suitable for simulation of the H recycling in a reactor divertor
owing to their different geometry from that of a rector divertor and lack of magnetic
field. Therefore, currently available data on H recycling should be extrapolated to
reactor environment. This chapter is devoted to summarize current knowledge and
© 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_7
115