S9-4
Plasma wall interaction in long duration plasmas on ST
STにおける長時間維持プラズマのプラズマ・壁相互作用
Kazuaki Hanada
花田和明
Research Institute for Applied Mechanics, Kyushu University
6-1, Kasuga-Koen, Kasuga, Fukuoka, 812-8580, Japan
九州大学応用力学研究所 〒812-8580 福岡県春日市春日公園６－１
Steady state operation of spherical tokamaks is one of the key objectives in realizing cost-effective fusion
power plants and establishing steady power and particle balance are indispensable. Plasma-wall interaction(PWI) plays an essential role in the balance. Fuel recycling is a key to resolve complexity of PWI.
Modeling to combine global balance with post-mortem analysis is required for a quantitative discussion
and QUEST-wall model is established. The model indicates solution and recombination play a dominantrole in the growth of fuel recycling rate during discharges.

1. Introduction
Spherical tokamaks (STs) have a capability torealize cost-effective fusion power plants becauseof their compactness and high  stability. Steadystate operation (SSO) of STs is one of the key
objectives and establishing steady power andparticle balance are indispensable. ST magnetic
geometry gives an inherently large figure of merit
P/R, and leads to higher divertor heat fluxes, raisingconcerns over divertor operation in the proposed
ST-based fusion power plant concepts [1], where P
is the input power and R is the major radius.
Substantial experimental work has been carried out
on heat load studies during ELM on MAST [2-4],
power accountability study on NSTX [5], partial
detached divertor [6] and well-structured magnetic
geometry [7, 8]. Therefore, power accountability
studies on long duration ST yield an important
information on conceptional design of ST-based
future fusion power plants.
While, ST research activities have not focused onparticle balances yet. The world record for plasma
duration for tokamaks of more than 5 h wasachieved in TRIAM-1M [9], where an accurate
power balance of the discharge was investigated[10]; a particle balance was also studied in regard toglobal particle balance [11]. The result indicatesthat even when the power balance of the discharge
seems to be complete, the global particle balance
was not in a steady state [11]. Indeed, the longest
duration plasma was terminated unpredictably, one
reason being a slight shift in particle balance, due touncontrollability of fuel recycling. Thus, tounderstand particle balance is still important inestablishing SSO. Plasma-wall interaction (PWI)
plays an essential role in their balances. Fuelrecycling is a key to resolve complexity of PWI.

2. PWI effects on core plasma
Many ways of wall-conditioning have beendeveloped to control wall behavior and one of themost effective ways is lithium (Li) conditioning
done in NSTX [12]. Lower collisionality has beenobtained in H-mode plasmas via Li conditioning ofplasma facing components (PFCs). An increase inenergy confinement time, E, has been reported
shown in Fig. 1. The escaling is provided by the
gyrokinetic simulation, which gives the dependenceof electron thermal transport on electron
collisionality, ei, is examined for plasmas withdifferent underlying microinstabilites. Reduction offuel recycling due to Li conditioning plays anessential role in improvement of core plasmatransport.

Fig. 1. BTE as the function of e [12]

Density behavior related on fuel recycling wasobserved in long duration discharge on QUEST,
which is a medium size of ST and has a capability
to make a SSO. Recently, a series of more than 10
minutes discharges has obtained with non-inductive

microwave current drive technique. The density
was controlled in a feedback manner of H signal
and a lower threshold value was decided to puff
predetermined quantity of hydrogen molecule asfuel gas. The response of electron density to thefuel puffing is gradually modified with plasma
duration shown in Fig. 2. The duration between
higher density and return has clear relation toamount of wall-stored fuel particle, which indicates
fuel recycling plays an intrinsic role in thephenomena.
These results suggest that fuel recycling is a key
to resolve complexity of PWI.
60sec

6220sec17-2

ne (10 m )84460 sec

200

1020
Time (sec)3040Fig. 2. Density response on gas puffing in a long
duration discharge. The elapsed time from thebeginning of the discharge shows in each sub-figure.

3. Investigation of fuel recycling
Global fuel balance has been investigated inmany devices and many of the post-mortem
analyses are also applied to specimens exposed toplasmas. Their investigations into the quantitative
consistency of each measurement are still lacking.
Inherently, any post-mortem analysis shows only
local information. Therefore, the results obtained
are difficult to apply directly to global fuel balance.
This indicates that modeling to combine bothactivities is required for a quantitative discussion
and QUEST-wall model [13] is established.
Based on the D2+ implantation experiments, theQUEST-wall model is devised that included
prescriptions for describing several physicalprocesses, such as reflection, diffusion, solution,recombination, trapping, and plasma induced
desorption that would occur in the QUEST wall.
Clear evolutions of Rrec are reconstructed by themodel in Fig. 3, where Rrec is defined as a ratio of
the out-going flux to in-coming flux of H on thewall. The model can reconstruct the growth of Rrec
and the time evolution of Rrec for a 100 nm thicklayer and a H flux of 0.2×1018 H/m2/s is the mostsuitable for the experiment. The model indicatessolution and recombination play a dominant role in

the growth of Rrec.

Fig. 3. Time evolutions of Rrec are calculated basedon the QUEST-wall model. The solid, dotted, anddashed-dotted lines correspond to the cases of 100,50, and 20 nm at H flux = 2 x 1017 H /m2/s. The
hatched area shows the region of H flux from 1 to 3 x
1017 H/m2 in the case of 100 nm. The thick solid line
show the experimental result.

Acknowledgments
I sincerely appreciate for cooperation of theQUEST team to write this proceedings.
This work was supported by Grant-in-Aid for
JSPS Fellows (KAKENHI Grant Nos. 24226020
and 24656559) and performed with the support andunder the auspices of the NIFS Collaboration
ResearchProgram(NIFS13KUTR085
andNIFS13KUTR093). This work was also supported
in part by the Collaborative Research Program ofResearch Institute for Applied Mechanics, Kyushu
University.
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