Author：Hiroaki Ohwada1, K. HANADA2, N. Yoshida2, K. Nakamura2,, K. Yamazaki2, Hao LONG2, QUEST group2

1. Interdisciplinary Graduate School of Engineering Sciences IGSES, Kyushu University

2. Research Institute for Applied Mechanics RIAM, Kyushu University 6-1 Kasugakoen, Kasuga, P.R. Japan

Investigation of plasma wall interaction based on plasma radiation measurement in long duration discharge on QUEST

1

For SSO realization…

SSO realization has two problems

• Power balance • Particle balance

• Global gas balance

• Plasma wall interaction(PWI)

• Tritium inventory

• Deposition layer

• Non-inductive plasma

maintenance.

• Effective energy exhaust

2

Plasma is terminated due to the break of particle balance

“”The plasma density increase was due to water out- gassing,

probably coming from components located far away from the plasma

(such as ports) heated by radiated flux, and not from the actively

cooled main PFCs.””

E. Tsitrone, et al, 2007 J. Nucl. Mater. 363–365, no. 1–3, pp. 12–23 E. Tsitrone, et al, 2007 J. Nucl. Mater. 363–365, no. 1–3, pp. 12–23

3

Wall temperature is responsible for wall pumping

Wall material

plasma

Injected flux (Pa・m3)

Extracted Flux

(Pa・m3) Vessel Inventory

E. Tsitrone, et al 2003 J. Nucl. Mater., vol. 363–365

4

Another factor: Deposition layer on the wall

Deposition Layer

Plasma

Wall metal

Deposition Layer may plays an important role in PWI

Amount of absorbed hydrogen increases

with the width of deposition layer

M. Miyamoto., 2005. J. Nucl. Mater., vol. 337–339, no. 1–3 SPEC. ISS., pp. 436–440

5

Motivation: clarify the mechanism of wall pumping

Deposition Layer

Plasma

Wall metal

Γ𝑤

Γ𝑟𝑒𝑙

reflection

In order to understand the wall pumping

mechanism, absorption/desorption behavior

of the deposition layer have to be revealed.

Proposal of

Hydrogen Barrier model

Γin Γ𝑊 = Γ𝑖𝑛 − Γ𝑟𝑒𝑙

6

Hydrogen is mainly absorbed within the deposition layer

5x1026

0

D d

ensi

ty (

m-3

)

1.20.80.40.0depth (m)

重水素分布

分解能150nm

Deposition-layer thickness = 20nm

Deposition layer

Penetration to the bulk metal is negligible

Bulk metal

Film thickness resolution =

0.15μm

7

Hydrogen Barrier model (HB model)

Deposition Layer

Plasma

Wall metal

Γ𝑝𝑢𝑚𝑝

Γ𝑟𝑒𝑙

reflection Γin Γ𝑊 = Γ𝑝𝑢𝑚𝑝 − Γ𝑟𝑒𝑙

𝑑(𝐻𝑊 + 𝐻𝑇)

𝑑𝑡 = Γ𝑖𝑛𝑆 −

𝑘

𝑆𝑑𝑅2 𝐻𝑊

2

𝑑𝐻𝑇

𝑑𝑡= 𝛼𝐻𝑊 1 −

𝐻𝑇

𝐻𝑇0 − 𝛾𝐻𝑇

• Hw: the number of H dissolved

• HT: the number of H trapped

• HT0: the upper-limit number of H trapped in defect

• k: surface recombination coefficient

• S: surface aria

• dR: thickness of deposition layer

• α: H trapping rate

• 𝛾: de-trapping rate

Γ𝑝𝑢𝑚𝑝 Γ𝑟𝑒𝑙

8

QUEST spherical tokamak device

Parameters

Size: R ~ 0.64 m, a ~ 0.36 m

Aspect ratio: R/a ~ 1.78

Magnetic field: Bt ≤ 0.25 T

RF power: P8.2 ≲ 25 kW×8

Plasma current: Ip ≲ 75 kA

Duration: t ≲ 120 minute

9

Hot wall install to QUEST

Hot Wall

Hot Wall

・k : recombination coefficient

K depends on wall temperature

Wall temperature must be controlled to control k

Hot wall install to QUEST

Hot wall can be heated up to 200℃

“S”: PFW surface area [m2]

[m2]

APS-W(Hot wall, etc.) 14.1

SUS316L 5.6

(All) 19.7

𝑑(𝐻𝑊 + 𝐻𝑇)

𝑑𝑡 = Γ𝑖𝑛𝑆 −

𝑘

𝑆𝑑𝑅2 𝐻𝑊

2

𝑑𝐻𝑇

𝑑𝑡= 𝛼𝐻𝑊 1 −

𝐻𝑇

𝐻𝑇0 − 𝛾𝐻𝑇

10

Thickness of deposition layer 𝑑(𝐻𝑊 + 𝐻𝑇)

𝑑𝑡 = Γ𝑖𝑛𝑆 −

𝑘

𝑆𝑑𝑅2 𝐻𝑊

2

𝑑𝐻𝑇

𝑑𝑡= 𝛼𝐻𝑊 1 −

𝐻𝑇

𝐻𝑇0 − 𝛾𝐻𝑇

100nm

Deposition layer t=70nm

element (keV) at%

C K 0.277 80.39

O K 0.525 11.81

Cr K 5.411 0.73

Fe K 6.398 1.41

W M 1.774 5.66

total 100

TEM image

SEM image

11

How to estimate Γin

Wall saturated term

Gas balance at wall saturated term is …

𝑑𝑁𝐻

𝑑𝑡= 0 = Γ𝑟𝑒𝑙 − Γ𝑖𝑛 − Γ𝑒𝑥ℎ𝑢𝑛𝑠𝑡

∴ Γ𝑟𝑒𝑙 = Γ𝑖𝑛 + Γ𝑒𝑥ℎ𝑢𝑛𝑠𝑡

𝑑𝑁𝐻

𝑑𝑡= 0 = Γ𝑟𝑒𝑙 − Γ𝑒𝑥ℎ𝑢𝑛𝑠𝑡

∴ Γ𝑟𝑒𝑙 = Γ𝑒𝑥ℎ𝑢𝑛𝑠𝑡

STOP discharge during wall saturation

A characteristic peak appears

𝑑(𝐻𝑊 + 𝐻𝑇)

𝑑𝑡 = Γ𝑖𝑛𝑆 −

𝑘

𝑆𝑑𝑅2 𝐻𝑊

2

𝑑𝐻𝑇

𝑑𝑡= 𝛼𝐻𝑊 1 −

𝐻𝑇

𝐻𝑇0 − 𝛾𝐻𝑇

12

How to estimate Γin

Wall saturated term

From wall saturated term

∴ Γ𝑟𝑒𝑙 = Γ𝑖𝑛 + Γ𝑒𝑥ℎ𝑢𝑛𝑠𝑡

∴ Γ𝑟𝑒𝑙 = Γ𝑒𝑥ℎ𝑢𝑛𝑠𝑡

From peak

Γexhaust can be estimated from QMS ↓

Γin can be estimated

𝑑(𝐻𝑊 + 𝐻𝑇)

𝑑𝑡 = Γ𝑖𝑛𝑆 −

𝑘

𝑆𝑑𝑅2 𝐻𝑊

2

𝑑𝐻𝑇

𝑑𝑡= 𝛼𝐻𝑊 1 −

𝐻𝑇

𝐻𝑇0 − 𝛾𝐻𝑇

13

Γin is proportional to Hα intensity Γ 𝑖

𝑛 [N

um

/s]

𝐻α [a.u]

• We evaluated the dependence of Hα

intensity on Γin

• It is observed that the Γin is proportional

to Hα intensity

• Hα ray intensity is a good monitor for Γin

𝑑(𝐻𝑊 + 𝐻𝑇)

𝑑𝑡 = Γ𝑖𝑛𝑆 −

𝑘

𝑆𝑑𝑅2 𝐻𝑊

2

𝑑𝐻𝑇

𝑑𝑡= 𝛼𝐻𝑊 1 −

𝐻𝑇

𝐻𝑇0 − 𝛾𝐻𝑇

14

Hα feedback system = keep Γin constant

Feedback system

measure

control

Hα

[a.u

]

Since Γin is proportional to Hα intensity, Γin can be kept constant by

Hα feedback control.

𝑑(𝐻𝑊 + 𝐻𝑇)

𝑑𝑡 = Γ𝑖𝑛𝑆 −

𝑘

𝑆𝑑𝑅2 𝐻𝑊

2

𝑑𝐻𝑇

𝑑𝑡= 𝛼𝐻𝑊 1 −

𝐻𝑇

𝐻𝑇0 − 𝛾𝐻𝑇

15

393K (#32737)

473K (#32700)

Wall stored(H)

HB model breaks in the termination phase (Phase III)

Wall pumping rate and the amount of the stored hydrogen shows good agreement with HB

model when Hα intensity is kept constant(phase I,II), though the relation deviates from the

phase III.

Phase I Phase II Phase III

0 15 30 45 60

Time(min) H

α [

a.u

]

Mas

s F

low

[m

l/m

in]

16

Out gas dose not depend on Twall in QUEST

MP

1day 2day

Hot wall (APS-W) Mid plane(SUS316)

During discharge

Twall control using heater

Twall range at Phase III

Release rate is not affected by the wall temperature.

17

One candidate : Deposition layer thickness profile

Fig 2B TEM cross section image of

deposition layer at lower side

Fig 2B TEM cross section image of

deposition layer at lower side

50nm 50nm

HB model have assumed the thickness of the

deposition layer to be constant inside the vacuum

vessel. However, the thickness is found to be different

in upper and lower region.

18

Deposition layer thickness difference : Particle flux assymetry

Fig 2B TEM cross section image of

deposition layer at lower side

Fig 2B TEM cross section image of

deposition layer at lower side

50nm

50nm

19

Thin deposition layer may show different behavior from the present HB model

Fig 2B TEM cross section

image of deposition layer at lower side

50nm

Retention dominant region

• The lower side deposition layer is thick

→High retention capacity

The HB model is applicable to this region

20

Fig 2B TEM cross section

image of deposition layer at lower side

50nm

Release dominant region

• The lower side deposition layer is thin

→ Low retention capacity

• The Ionization particle collide with this region

→Plasma Induced desorption is likely to occur

Thin deposition layer may show different behavior from the present HB model

21

plasma emission intensity is different in upper and lower region

Upper

Lower

Upp

er

Low

er

Upper region is bright but lower region is dark. →It may indicates that neutral particle density is different in upper and lower region due to the difference in deposition layer thickness.

22

Summary

• Hydrogen Barrier model (HB model) is tested in long duration discharge on

QUEST.

• HB model is consistent until the termination phase.

• It is observed that the deposition layer is different in upper and lower region.

• Absorption/Desorption behavior may be different depending on the thickness

of the deposition layer.

Future Work

Clarify the absorption / release behavior in the APS-W wall without deposition

layer

Thank you For Your Listening

24

In order to realize SSO, We challenging to continuously and stably long–term tokamak plasma.

Fuel injection rate [Num/s]

Hα[V]

Stored particle[N]

Ip[kA]

※fuel is Hydrogen

Target Hα level = 0.35

25

26

Hα

[a

.u]

𝑑(𝐻𝑊 + 𝐻𝑇)

𝑑𝑡 = Γ𝑖𝑛𝑆 −

𝑘

𝑆𝑑𝑅2 𝐻𝑊

2

𝑑𝐻𝑇

𝑑𝑡= 𝛼𝐻𝑊 1 −

𝐻𝑇

𝐻𝑇0 − 𝛾𝐻𝑇

27

My purpose is…

I design and install device for measure light

intensity vertical asymmetry

I try to measurement in the experiment where the

toroidal magnetic field is reversed

Final purpose is to elucidate the reasons for the

non-uniformity of deposition layer and the vertical

asymmetry of plasma light intensity.

28

Upper

Low

er

The vertical light intensity distribution is reversed (Hα:λ = 656nm)

29

Upper

Low

er

Normal

Up

per

Low

er

Toroidal magnetic field

reversed

The vertical light intensity distribution is reversed (Hα:λ = 656nm)