Chapter 11
Closing Remarks
The main purpose of this book is to show that plasma-facing surfaces (PFS) respond
actively to the fusion plasma and that without understanding what is really happening
at PFS, the influence of PFS on plasma cannot be understood. PFS has been considered as passive to the plasma so that changes of plasma-facing material (PFM) would
not have a significant influence on plasma performance. Recent plasma material interactions (PMI) studies, however, indicate that plasma performance can be significantly
affected when PFM is altered or modified by plasma power and particle load.
Most of our knowledge about plasma confinement for fusion burning has been
obtained from plasma machines with a carbon (C) wall as PFM. Due to concerns
about T inventory and neutron effects, the C-wall has been changed to a high Z wall
and recent successes of the high Z wall in several tokamaks, like ASDEX-upgrade,
EEST, and JET with ITER like walls encourage the use of the high Z wall in a reactor.
However, in JET with ITER-like-wall experiments, the plasma character or properties, in particular, edge properties and hydrogen recycling behavior have clearly
changed from those observed with full carbon wall. Compared with current tokamaks,
the incident flux of energetic photons (radiation) and particles to PFS in a reactor is
so large that the current knowledge on boundary plasma and hydrogen recycling may
not be directly useful or applicable for understanding PMI in the reactor. Therefore,
it is quite important to know how different PMI will be in a reactor with low Z wall
and high Z wall. Due to concerns about W material damage, the power load in JETITER-like wall is kept rather low and does not simulate PMI with high Z wall in a
reactor. Therefore, main effort to study PMI at reactor-like conditions is given to heat
load tests with linear plasma machines or plasma guns. However, as described in this
book, the response to power load by radiation and particles are different depending on
PFM, accompanying different influence on plasma. Without comparisons of plasma
behaviors with low Z and high Z as PFM, for example, correct understanding of
PMI might be difficult. Furthermore, the power load to PFM is as high as the surface
of a rocket going into the sun is exposed to. Experimental apparatus to give such a
© 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_11
199
Closing Remarks
The main purpose of this book is to show that plasma-facing surfaces (PFS) respond
actively to the fusion plasma and that without understanding what is really happening
at PFS, the influence of PFS on plasma cannot be understood. PFS has been considered as passive to the plasma so that changes of plasma-facing material (PFM) would
not have a significant influence on plasma performance. Recent plasma material interactions (PMI) studies, however, indicate that plasma performance can be significantly
affected when PFM is altered or modified by plasma power and particle load.
Most of our knowledge about plasma confinement for fusion burning has been
obtained from plasma machines with a carbon (C) wall as PFM. Due to concerns
about T inventory and neutron effects, the C-wall has been changed to a high Z wall
and recent successes of the high Z wall in several tokamaks, like ASDEX-upgrade,
EEST, and JET with ITER like walls encourage the use of the high Z wall in a reactor.
However, in JET with ITER-like-wall experiments, the plasma character or properties, in particular, edge properties and hydrogen recycling behavior have clearly
changed from those observed with full carbon wall. Compared with current tokamaks,
the incident flux of energetic photons (radiation) and particles to PFS in a reactor is
so large that the current knowledge on boundary plasma and hydrogen recycling may
not be directly useful or applicable for understanding PMI in the reactor. Therefore,
it is quite important to know how different PMI will be in a reactor with low Z wall
and high Z wall. Due to concerns about W material damage, the power load in JETITER-like wall is kept rather low and does not simulate PMI with high Z wall in a
reactor. Therefore, main effort to study PMI at reactor-like conditions is given to heat
load tests with linear plasma machines or plasma guns. However, as described in this
book, the response to power load by radiation and particles are different depending on
PFM, accompanying different influence on plasma. Without comparisons of plasma
behaviors with low Z and high Z as PFM, for example, correct understanding of
PMI might be difficult. Furthermore, the power load to PFM is as high as the surface
of a rocket going into the sun is exposed to. Experimental apparatus to give such a
© 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_11
199