Book of Abstracts
4th International Workshop on Models and Data for
Plasma-Material Interaction in Fusion Devices(MoD-PMI 2019)18 – 20 June 2019, National Institute for Fusion Science, Toki, Japan

This workshop is organized in cooperation with the International Atomic Energy Agency(IAEA).

MoD-PMI 2019

International Scientific Committee
Jean-Paul Allain (USA)
Bastiaan J Braams (Netherlands)
Gregory de Temmerman (France)
Maria Ganchenkova (Russia)
Yves Ferro (France)
Daiji Kato (Japan)
Predrag Krstic (USA)
Christian Linsmeier (Germany)
Duc Nguyen-Manh (UK)

Joerg Neugebauer (Germany)
Kai Nordlund (Finland)
Takuji Oda (Korea)
Klaus Schmid (Germany)
Manoj Warrier (India)
Brian Wirth (USA)
Hong-Bo Zhou (China)
Kalle Heinola (IAEA, Austria)

Local Organizing Committee in NIFS
Daiji Kato (Chair)
Suguru Masuzaki
Katsuyoshi Tsumori
Atsushi M. Ito
Arimichi Takayama
Yukinori Hamaji

Hiroaki Nakamura
Izumi Murakami
Naoko Ashikawa
Hiroyuki A. Sakaue
Masayuki Tokitani
Miyuki Yajima

Local organizers are grateful to financial support by the IAEA and the NINS program of
Promoting Research by Networking among Institutions (Grant Number 01411702).

MoD-PMI 2019 program
2019/6/18 (Tue)registration

8:40

opening

9:30
Takeo Muroga

9:35(Deupty Director General of NIFS)

Welcome address and introduction of NIFS
session 1 (Chair: Yves Ferro)

9:50
10:20

I-1

Sergei Dudarev

I-2

Tomoaki Suzudo

What Drives the Evolution of Microstructure of Plasma Facing Materials: Energies ofDefects or Elastic Stresses and Strains?
First Principles Modeling on Radiation Defects in W and W-based Alloys
break

10:50
11:00
11:30

I-3

Tsuyoshi Miyazaki

I-4

Johann Bouchet

Linear-scaling First-principles Molecular Dynamics Simulations of Complex Nanostructured Materials with the CONQUEST CodeVibrational properties of the actinides

12:00
lunch

session 2 (Chair: Hong-Bo Zhou)
13:00
13:30

I-5

Chris Van de WalleFirst-principles modelling of defects and hydrogen in oxides

I-6

Christoph Freysoldt

Modelling of charged point defects with density-functional theory
break

14:00
14:10
14:40

I-7

Byeongchan Lee

O-1

Duc Nguyen-Manh

15:00

Atomistic calculations on random nature of irradiated damage in single crystalline
tungstenOrigin of outstanding radiation resistance in W-based high-entropy alloys
group photosession 3 (Chair: Duc Nguyen-Manh)

15:30
16:00
16:30

I-8

Ryo Kobayashi

Construction of efficient machine-learning potential for W-H system

I-9

Tommy Ahlgren

Corrections to sink strengths used in rate equation simulations of defects in solids

O-2

Hong-Bo Zhou

17:20
17:40

investigating the interaction between dispersed/aggregated-Re and H
break

16:50
17:00

Towards understanding the influence of Re on H dissolution and retention in W by

O-3

Shin Kajita

O-4

Atsushi M. Ito

O-5

Zhangcan Yang

Deposition effects on He induced fiberform nanostructure growth
Sputtering and Re-deposition Effects during Fuzz Growth Process Simulated by BCAMD-KMC Multi-hybrid Simulation
Helium Bubble Growth In Tungsten Nano-Tendrils

19:00 -

Workshop dinner at Yamagami Onsen (hotspring house)

21:00*Bus for Yamagami Onsen will depart at 18:30 from NIFS

2019/6/19 (Wed)session 4 (Chair: Bastiaan J. Braams)
8:40
9:10

I-10

Predrag Krstic

I-11

Paul Erhart

Quantum-Classical Molecular Dynamics Simulation of Synthesis of Boron-Nitride
Nano-Structures at High-Temperature, High-Pressure Plasmas
Tools for the construction of atomic scale models for materials
break

9:40
9:50
10:20

I-12

Thomas Morgan

I-13

Kenzo Ibano

I-14

Jianmin Yuan

Liquid metal vapour shielding in linear plasma devices
Weighted PIC simulation for vapor shielding at wall under transient heat loads
Modifications to the Photonic and Electronic Impact Ionization Cross Sections of

10:50

Ions in Dense plasmas due to the Transient Localization of Continuum Electron
Wavefunctions
break

11:20

session 5 (Chair: Hiroaki Nakamura)
11:30

I-15

Yonggang Li

O-6

Yves Ferro

12:00

Effects of Microstructures on Surface Damage and H/He Retention in W: A multiscale Modelling
Surface coverage dependence of the diffusion/desorption mechanisms of hydrogenfrom the W(110) and W(100) surfaces: a model based on DFT and thermodynamics

12:20
lunch

session 6 (Chair: Katsuyoshi Tsumori)
13:20
13:50
14:10
14:30

I-16

Seiki Saito

O-7

Mizuki Sakamoto

Impact of a high temperature target on hydrogen recycling

O-8

Takato Hirayama

Potential Sputtering from Rare Gas Solid Surface by Multiply-Charged Ion impact

O-9

Miroslaw Zlobinski

Molecular Dynamics Simulation for Hydrogen Recycling on Carbon Divertor

D retention in bulk Be and D co-deposited in Be layers studied by 3 different thermal

14:50

desorption techniques and their modelling by CRDS
break

15:00

Recent approaches to machine learning of interatomic potentials seen from asession 7 (Chair: Takuji Oda)

15:30
16:00

I-17

Bastiaan J. Braams

I-18

Linfeng Zhang

I-19

Utkarsh Bhardwaj

perspective of plasma material interaction and primary radiation damage
Deep Learning for Multi-Scale Molecular ModelingClassification of Clusters in MD Simulations of Collision Cascades
break

16:30

session 8 (Chair: Izumi Murakami)
16:40
17:00

O-10

Noboru Yoshikawa

O-11

Yugo Nagata

Microstructural Change of Au Thin Films by Microwave Irradiation
A monoenergetic energy-tunable positronium beam and its applications
break

17:20

poster session

17:30 18:20

P-1

Daiji Kato

P-2

Hyung-Gyu Lee

P-3

Jie Nan

P-4

Harsh Hemani

Vacancy clustering and stabilization in tungsten by hydrogen trapping
The defect formation mechanisms and change of the mechanical properties under
the irradiation in pure W and W-Re alloy: Molecular dynamics study
Molecular dynamics study of materials properties of pure W and W-10Re alloy
before and after irradiation
Inclusion of Electronic Stopping and Validation in the Open Source Large-scale
Atomic/Molecular Massively Parallel Simulator (LAMMPS)

17:30 18:20

Investigation of temperature and ion energy threshold for tungsten fuzz growth by

P-5

Quan Shi

P-6

SangHyuk Yoo

P-7

Woonghee Cho

P-8

Lukas Pichl

P-9

Izumi Murakami

NIFS Atomic and Molecular Numerical Database for Collision Processes in Plasmas

P-10

Naoko Ashikawa

Observation of trapped hydrogen in coated tungsten on titanium specimen

Monte Carlo Simulation
Study of proton irradiation in wurtzite GaN with a-type prism-edge dislocation using
molecular dynamics simulations
Development of Potential Model with Three-body Interaction for Tungsten
Recent Machine Learning Algorithms for Classification of Time Series: Recurrent
Deep Learning Networks with Potential for Plasma Applications

2019/6/20 (Thu)session 9 (Chair: Daiji Kato)
8:40
9:10
9:40
10:00

Displacement damage stabilization by hydrogen presence under simultaneous W ion

I-20

Sabina Markelj

I-21

Thomas Schwarz-Selinger

Influence of the presence of deuterium on displacement damage in tungsten

O-12

Kazuhito Ohsawa

Anomalous Properties of Vacancy in Tungsten and Interaction with Hydrogen

O-13

Takuji Oda

damaging and D ion exposure

Molecular Dynamics Simulation on Interactions between Hydrogen and Vacancy
Clusters in Tungsten
break

10:20

session 10 (Chair: Suguru Masuzaki)
10:30
11:00
11:20
11:40
12:00

Deuterium trapping at irradiation-induced defects in tungsten studied by positron

I-22

Takeshi Toyama

O-14

Yuji Hatano

Deuterium retention in W and binary W alloys irradiated with high energy Fe ions

O-15

Yoshio Ueda

Hydrogen isotope exchange between H and D in damaged tungsten

O-16

Toshikio Takimoto

annihilation spectroscopy

Development of functional material for deuterium permeation observation under
divertor plasma exposures
closingI-1
What Drives the Evolution of Microstructure of Plasma Facing Materials:
Energies of Defects or Elastic Stresses and Strains?
S.L. Dudareva,*, P.-W. Maa, D. R. Masona, and F. Hofmannba UK Atomic Energy Authority, Culham Centre for Fusion Energy, Oxfordshire OX14 3DB, UKb Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK

Density functional theory (DFT) calculations have now provided extensive information
about the stable structures of elementary defects formed in pure metals and alloys
under irradiation. For many types of defects, DFT simulations have also identified
defect migration pathways, achieving strikingly good agreement with experimental
observations and resolving fundamental questions that, in the absence of DFT
models, remained outstanding for decades.
Recently, DFT simulations, combined with atomistic simulations, have also been able
to offer insight into phenomena traditionally thought not to be accessible to ab initio
methods. The notions of dipole tensor and relaxation volume tensor of defects [1],directly computable from DFT [2], enable relating the data on the structure of defects
derived from DFT simulations to strains and stresses that they produce in materials.
Also, through the dipole tensor formalism, it is now possible to compute energies andforces acting between defects, and between defects and dislocations. Furthermore,using relaxation volume tensors of defects derived from DFT simulations, it ispossible to predict, at a fairly high level of accuracy, the volumetric swelling ofmaterials resulting from the accumulation of radiation defects. Taking advantage ofthe scaling properties of elasticity equations, we can also interpolate DFT results tolarger defect structures, for example vacancy clusters, cavities, or mesoscopic
dislocation loops [3], and even extend the treatment to complex entangled dislocation
configurations, offering a way of “mechanistically” linking the information about thedefect structures observed in a material using an electron microscope, with thedeformations and stresses that such defect structures produce on the macroscale.
Recent applications of the method include the interpretation of data on strains andstresses produced by the accumulation of helium in ion-implanted tungsten [4], theformation of vacancy-dominated microstructures in materials exposed to self-ion
irradiation, and the use of first principles approaches to the derivation of models forfinite element simulations of macroscopic reactor components.
[1] S.L. Dudarev and P.-W. Ma, Elastic fields, dipole tensors, and interaction between
self-interstitial atom defects in bcc transition metals, Phys. Rev. Mater. 2 (2018)033602[2] P.-W. Ma and S.L. Dudarev, Universality of point defect structure in body-centeredcubic metals, Phys. Rev. Mater. 3 (2019) 013605[3] D.R. Mason et al., Relaxation volumes of microscopic and mesoscopic irradiationinduced defects in tungsten, https://arxiv.org/abs/1812.06874[4] F. Hofmann et al., Lattice swelling and modulus change in a helium-implanted
tungsten alloy, Acta Materialia 89 (2015) 352*Corresponding author: sergei.dudarev@ukaea.uk (S.L. Dudarev)

I-2
First Principles Modeling on Radiation Defects in W and W-based Alloys
T. Suzudoa,*, and T. Tsurub,ca Center for Computational Science and e-Systems, Japan Atomic Energy Agency, 2-

4 Shirakata, Tokai-mura, Ibaraki 319-1195, Japanb Nuclear Science and Engineering Center, Japan Atomic Energy Agency, 2-4

Shirakata, Tokai-mura, Ibaraki 319-1195, Japanc Elements Strategy Initiative for Structural Materials (ESISM), Kyoto University,

Yoshida, Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Tungsten (W) metals and alloys are considered as a promising candidate for plasmafacing materials for future nuclear fusion devices because of its high melting
temperature, high resistance to sputtering, and high thermal conductivity. However,radiation-induced effects, such as swelling and increase in tritium retention seem tothreaten its actual applicability. Thus, understanding the radiation-induced defects are
critical to the application of these materials.
Rhenium (Re) and Osmium (Os) are produced from W through nuclear transmutation,and they would naturally be solute elements of W crystals under neutron irradiation.
In such alloys, stable solvent-solute mixed dumbbells appear because the SIA andRe and Os substitutional atoms are attractive. We analyzed the atomistic migration
patterns of these two transmuted elements and indicated that the mixed dumbbells
composed of Re (or Os) and W atoms occasionaly become separated, and the Re(or Os) atom jumps to the 1NN site forming another mixed dumbbell. Because therotation of these mixed dumbbells is easy, these solute elements can be carried forlong distance by iteration of jumping and rotation events, letting them be capable offast three-dimensional (3D) migration [1,2,3].
We also examined stability and mobility of various solute interstitials in W crystals.
Through the examination, Ti, V, Cr and Mo interstitials were found to form mixeddumbbells in addition to Re and Os. Further detailed first-principles study suggested
that Ti, V, and Cr are also capable of fast 3D migration [4].
It is commonly recongnized that the most enegetically-favored SIA configuration in W
would be <111> dumbbells and <111> crowdions in W. However, some DFT results
show that they are not necessariliy the most favored. We revisited stability andmobility of SIAs based on accurate DFT calculations.
[1] T. Suzudo, et al., Modeling and Simulations Materials Science Engineering 22(2014) 075006.
[2] T. Suzudo, et al., J. Nucl. Mater. 467 (2015) 418-423.
[3] T. Suzudo, et al., Scientific Reports 6:36738 (2016) 1-6.
[4] T. Suzudo, et al., J. Nucl. Mater. 505 (2018) 15-21.

*Corresponding author: suzudo.tomoaki@jaea.go.jp (T. Suzudo)

I-3
Linear-scaling First-principles Molecular Dynamics Simulations of Complex
Nano-structured Materials with the CONQUEST Code.
T. Miyazaki*
International Center for Materials Nanoarchitectonics (WPI-MANA),
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0031, Japan.

First-principles simulations based on the density functional theory (DFT) are
powerful tools to clarify the structural and physical properties of various materials at
atomic scale. However, usual DFT methods cannot treat large systems containing
many thousands of atoms, since the calculation cost increases very rapidly,
proportional to the cube of the number of atoms N in the system. To overcome thissize limitation, we have developed a linear-scaling[1] or O(N) DFT code CONQUEST.
Using a recently-developed approach (multi-site support functions or MSSF[2]), thecode is capable of performing high accuracy calculations on up to 10,000 atoms,
while with a linear scaling approach we can treat million-atom systems[3].
In this talk, I will give an overview of the methods used in the code and present ourrecent applications of nano-scale materials[4,5]. I will also demonstrate that the code
can perform robust linear-scaling first-principles molecular dynamics simulations oflarge complex systems[6,7]. The combination of using machine-learning forces[8,9]with the code will be also discussed.
This work is done in collaboration with Prof. D. R. Bowler (UCL), Dr. J. Lin, Dr. A.
Nakata and Dr. R. Tamura (NIMS). The CONQUEST code[10] is developed jointly by
the groups of University College London (Prof. D. R. Bowler), National Institute forMaterials Science (First-principles simulation group), and the University of Bordeaux(Dr. L. Truflandier). The code will be open to public soon with the MIT license.
[1] D. R. Bowler and T. Miyazaki, Rep. Prog. Phys. 75 (2012) 036503.
[2] A. Nakata, D. R. Bowler, and T. Miyazaki, Phys. Chem. Chem. Phys. 17 (2015)31427[3] M. Arita, S. Arapan, D. R. Bowler, and T. Miyazaki, J. Adv. Simul. Sci. Eng. 1(2014) 87.
[4] A. Nakata, Y. Futamura, T. Sakurai, D. R. Bowler, and T. Miyazaki, J. Chem. Theor.
Comput., 13 (2017) 4146.
[5] C. O’Rourke, S. Mujahed, C. Kumarasinghe, T. Miyazaki, and D. R. Bowler, J.
Phys.:Condens. Matter, 30 (2018) 465303.
[6] M. Arita, D. R. Bowler, and T. Miyazaki, J. Chem. Theor. Comput.,10 (2014) 5419.
[7] T. Hirakawa, T. Suzuki, D. R. Bowler, and T. Miyazaki, J. Phys.:Condens. Matter,
29 (2017) 405901.
[8] T. Suzuki, R. Tamura, and T. Miyazaki, Int. J. Quantum Chem. 117 (2017) 33.
[9] R. Tamura, J. Lin and T. Miyazaki, J. Phys. Soc. Jpn. 88 (2019) 044601.
[10] http://www.linear-scaling.org*Corresponding author: MIYAZAKI.Tsuyoshi@nims.go.jp (T. Miyazaki)

I-4
Vibrational properties of the actinides
J. Bouchet1,*, F. Bottin1, and B. Dorado3
1 CEA/DAM/DIF

Density functional theory (DFT) has been an extraordinary successful tool tounderstand the ground state properties of the actinides and to show the fundamentalrole played by the 5f electrons. The inclusion of temperature effects in thecalculations is even more challenging but crucial for studying the rich phase diagram
exhibited by these elements.
Usually, thermal vibrations of atoms can be taken into account via the socalled quasiharmonic approximation (QHA). In this framework, the phonon dispersion
relations are calculated at 0 K using density functional perturbation theory (DFPT)
and the temperature is included only via the thermal dilatation, i.e., by computing thephonon spectrum at different volumes. This method has been applied with success to
Th. However with only one f electron and a fcc structure, Th does not show all thecomplexity found in the heavier actinides. In particular, this method cannot be usedon structures stabilized at high temperature by anharmonic effects and therefore
dynamically unstable at 0 K such as the bcc structure adopted by the whole actinide
series before the melting point.
Here we show the results of a new method based on ab-initio molecular
dynamics simulations to obtain the phonon spectrum of a material at hightemperature and capture the anharmonic effects [1]. Whereas all previous attempts
based on the QHA failed to reproduce the behavior of the phonon modes of uranium
as a function of temperature, this method can capture the effects responsible for the
CDW phase transition in α-U [2]. We also show the phonon spectrum of the γ bcc
structure at high temperature. Using the phonon density of states we calculate thefree energies of the α and the γ structure to obtain the transition line in pressure andtemperature [3].
We also present new results for the phonon spectra of the α, δ and ε phasesof plutonium and we discuss the different mechanisms governing the stability and thephase transitions of these structures [4]. We will also show recent results obtained on
the U-Mo system.
[1] O. Hellman, I. A. Abrikosov, and S. I. Simak, Phys. Rev. B 84, 180301 (2011)[2] J. Bouchet and F. Bottin, Phys. Rev. B 92, 174108 (2015)[3] J. Bouchet and F. Bottin, Phys. Rev. B 95, 054113 (2017)[4] B. Dorado, F. Bottin and J. Bouchet, Phys. Rev. B 95, 104303 (2017)*Corresponding author: johann.bouchet@cea.fr (J. Bouchet)I-5
First-principles modelling of defects and hydrogen in oxides
Chris G. Van de Walle*
Materials Department, University of California, Santa Barbara, California 93106-5050, USA

First-principles calculations based on density functional theory can provide accurate
results for the formation and migration of point defects and impurities. I will first
discuss the underlying methodology and computational advances [1]. Then I will
illustrate the type of results that can be obtained with the example of Al 2 O 3 , for which
we have calculated point defects [2,3] and impurities such as nitrogen and carbon (4].
I will devote particular attention to the behavior of hydrogen in oxides. I will describe
some general principles [5], and again illustrate the results with the example of Al 2 O 3[6].
Finally, I will comment on how these calculations can be used to assess radiation
hardness, and determine which point defects are most likely to result from irradiation.
Work performed in collaboration with M. Choi, C. Freysoldt, A. Janotti, J. Lyons, H.
Peelaers, J. Varley, and J. Weber.

[1] C. Freysoldt et al., Rev. Mod. Phys. 86 (2014) 253.
[2] J. R. Weber et al., J. Appl. Phys. 109 (2011) 033715.
[3] M. Choi et al., J. Appl. Phys. 113 (2013) 044501.
[4] M. Choi et al., Appl. Phys. Lett. 102 (2013) 142902.
[5] C. G. Van de Walle and J. Neugebauer, Annu. Rev. Mater. Res. 36(2006) 179.
[6] M. Choi et al., ACS Appl. Mater. Interfaces 6 (2014) 4149.

*Corresponding author: vandewalle@mrl.ucsb.edu

I-6
Modelling of charged point defects with density-functional theory
C. Freysoldt*, J. Neugebauer
Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany

Point defects such as vacancies, interstitials, or impurities play an important role indiffusion-driven transformations and ageing. In insulating or semiconducting materials,
defects may carry a charge and therefore are crucial for the electrical behaviour ofsuch materials, even if their concentrations are low. Density-functional theory
calculations have proven invaluable for assigning experimental signatures to specificdefect types, and for further characterizing properties that are often challenging toobtain from experiment.
Yet, the simulation of charged defects is a challenge in its own, because typicalsimulation setups such as the supercell approach (in which the defect is modelled ina periodic structure) are biased by artificial defect-defect interactions that are absent
if the defect occurs at low concentrations. We have shown how Coulomb interactions
can be addressed systematically to extrapolate to the isolated-defect limit [1]. The
case of Coulomb interactions (related to charge) can be contrasted to the elastic
interactions (related to the elastic dipole tensor), which are relevant also for theinterplay with extended defects. Last, I will then discuss the relevance of defectdefect interactions at finite concentrations.

[1] C. Freysoldt, C. Van de Walle, J. Neugebauer, Phys. Rev. Lett. 102 (2009) 016402.

*Corresponding author: freysoldt@mpie.de (C. Freysoldt)

I-7
Atomistic calculations on random nature of irradiated damage in single
crystalline tungsten
Y. Shina, Y. Joa, H. Leeb, K. Kangb, and B. Leea, *a Kyung Hee University, 1732 Deogyeong-daero, Yongin, Gyeonggi 17104, Republic of Korea
b Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea

Tungsten is one of the high-temperature materials, and in particular, a candidate
material for divertors in fusion reactors, which experience high-energy neutron
irradiation. The microstructural evolution during irradiation damage is often
understood with atomistic calculations, in which a few approximations as well as
numerical techniques are used. Here we try to visit some of these approximations
and techniques to find rooms for improvement.
The first phase of the improvement involves rethinking of the system size, ensemble,
and boundary conditions, and the results can be compared with microstructures inthe tungsten sample prepared with the state-of-the-art processes, which contains
micro-meter-size grains. Nevertheless, a complex interplay between various factors
such as temperature and grain orientation is yet to be understood in detail from afurther study.
From a micro-meter-scale single crystalline tungsten, the grain orientation dominates
the irradiation damage, but because of that and because of many potential pathways
with equal probability, the overall damage profile appears random. Last, we briefly
discuss the way experiments and calculations are compared, and talk about ways toimprove the fidelity of atomistic calculations.

*Corresponding author: airbc@khu.ac.kr (B. Lee)

I-8
Construction of efficient machine-learning potential for W-H system
Ryo KOBAYASHIa,b
aDepartment of Physical Science and Engineering, Nagoya Institute of Technology, Gokiso-cho,Nagoya, 466-8555, JapanbNational Institute for Materials Science, 1-2-1 Sengen, Tsukuba, 305-0047, Japan

Tungsten (W) is a promising candidate as a plasma facing material (PFM) in next
generation fusion reactors. Thus a lot of experimental and computational researches
have been done to understand the response of W to harsh environment such as highenergy neutron and/or helium irradiation and hydrogen isotope contamination. Amongsome study approaches, molecular dynamics (MD) is a powerful simulation tool since
it can provide information and insights that cannot be obtained by the other research
approaches. The reliability of the MD simulation results depend strongly oninteratomic potentials used. Recently a lot of machine learning (ML)-type interatomic
potentials have been developed such as neural-network (NN)[1], linear-regression
with LASSO[2], gaussian approximation potential (GAP)[3] and so on, that canachieve quantum-mechanical (QM) level accuracy by learning from big data of QM
calculation. Since ML potentials usually requires high dimension descriptor to
precisely differentiate atomic environment, the cost of ML potentials are much higher
than those of classical interatomic potentials, which is limiting the application of ML
potential to very large-scale and/or long-time simulations that are required to study
defect evolution under harsh environment in PFM.
We here focus on the construction of efficient NN potential for W-H system. A small
number of descriptors relevant to the system are selected from a lot of candidates
without loosing the accuracy using a greedy algorithm. This approach is comparedwith some previous methods that are aimed to reduce dimensionality of descriptors
such as LASSO and orthogonal series [4], and it is shown that this approach
outperforms the other methods in terms of cost performance.

[1] J. Behler and M. Parrinello, Phys. Rev. Lett. 98, 146401 (2007).
[2] A. Seko, et al., Phys. Rev. B 90, 024101 (2014).
[3] A.P. Bartók, et al., Phys. Rev. Lett. 104, 136403 (2010).
[4] N. Artrith, et al., Phys. Rev. B 96, 33176 (2017).
*Corresponding author: kobayashi.ryo@nitech.ac.jp

I-9
Corrections to sink strengths used in rate equation simulationsof defects in solids

T. Ahlgren*, and K. Heinola
Department of Physics, University of Helsinki,Finland

Mean-field rate equations have proven to be a versatile method to simulate defectdynamics and temporal changes in the micro-structure of materials. However, thereliability and usefulness of the method depends critically on the defect interaction
parameters used.
We will show that the sink strength depends also on the detrapping or dissociation
process. The sink strength is much larger, than is usually used, for defects that are
detrapped. Theory how to determine the appropriate sink strength is presented.
Test simulations with kinetic Monte Carlo method is used to check the new theory.
Results show that the rate equation method, in some cases, gives wrong results if thedetrapping dependence on the sink strength parameter is neglected.

I-10
Quantum-Classical Molecular Dynamics Simulation of Synthesis of BoronNitride Nano-Structures at High-Temperature, High-Pressure Plasmas
P. S. Krstića,*, L. HanaaInstitute for Advanced Computational Science, Stony Brook University, Stony Brook, NY 11749, USA

Boron-Nitride (BN) Nanostructures can be synthetized in various forms like fullerenes(hBN), nanocages, nanococoons, nanoflakes and nanotubes, similarly to the carbon
nanostructures. The diamond-like forms (cBN) are also seen. The BN nanomaterial
has exceptional features and its synthesis is carried out experimentally by a numberof methods, with ultimate goals to reach high-rate production of impurity and defectfree nanostructures. However, complete understanding of the BN nanosynthesis
process at the atomic level has been missing so far.
With use of the DFTB {1, 2] and DC-DFTB-K [3] based BO Quantum-ClassicalMolecular Dynamics (QCMD) we were able to synthesize various BN nanomaterials
by self-organization of atoms at high temperatures and pressures, mimicking partiallythe experiments on the BNNT nanosynthesis in the plasma environment.
Interestingly, we could not obtain these structures by classical molecular dynamics
simulations. We observed that hydrogen may have a profound effect in the BN
nanosythesis process, altering the nanosynthesis outcomes that could lead tocreation of an ultra-hard cubic BN material [2]. Based on the results of thesesimulations we are able to recommend the optimal precursors, temperatures andpressures for synthesis various BN material in plasma [3].
[1] Han, Longtao, and Predrag Krstić, "A Path for Synthesis of Boron-Nitride
Nanostructures in Volume of Arc Plasma", Nanotechnology 28 (7), 07LT01 (2017).
[2] Krstić, Predrag, and Longtao Han, „Can Hydrogen Catalyze Transitions Between
h-BN and c-BN in Volume Plasma?“, J. Phys. Chem. C. 122, (2018) 936[3] Krstić, Predrag, Longtao Han, Stephen Irle and Hiromi Nakai, „Simulations ofsynthesis of the boron-nitride nanostructures in a hot, high pressure gas
volume“, Chemical Science 9 , (2018) 3803

*Corresponding author: predrag.krstic@stonybrook.edu

I-11
Tools for the construction of atomic scale models for materials
F. Erikssona, M. Ångqvista, E. Franssona, W. A. Muñoza, M. Rahma,
C. Durniakb, P. Rozyczkob, T. H. Rodb, and P. Erharta*
a Chalmers University of Technology, Department of Physics, Gothenburg, Swedenb Data Management and Software Centre, European Spallation Source, Copenhagen, Denmark

Modeling the thermodynamic and kinetic properties of materials commonly requires
taking into account both vibrational and compositional degrees of freedom. Whileelectronic structure calculations strictly speaking can provide the necessary
information, they are computationally too demanding for any practical purposes. One
therefore requires models that can reproduce the relevant parts of the potential
energy landscape at a much smaller computational cost. Here, I will present a set ofsoftware packages that we developed over the course of the last years that allow one
to rapidly construct and sample models that map vibrational and/or compositional
degrees of freedom. These tools can be integrated in a homogeneous Python based
workflow, which combines structure generation, electronic structure calculations,model construction, thermodynamic sampling, and subsequent analysis. I will mostly
discuss the hiphive and icet packages, which respectively have been developed toobtain anharmonic force constant potentials and alloy cluster expansions. Example
applications include the prediction of alloy phase diagrams, temperature driven
phase transitions, thermal conductivities, and ordering phenomena.
[1] F. Eriksson, E. Fransson, and P. Erhart, Advanced Theory and Simulations, Early
View (2019) https://doi.org/10.1002/adts.201800184[2] M. Ångqvist, W. A. Muñoz, J. M. Rahm, E. Fransson, C. Durniak, P. Rozyczko, T.
H. Rod, and P. Erhart, Advanced Theory and Simulations, accepted (2019)https://arxiv.org/abs/1901.08790*Corresponding author: erhart@chalmers.se (P.Erhart)

I-12
Liquid metal vapour shielding in linear plasma devices
T.W. Morgana,*, G.G. van Edena, P. Rindtb, V. Kvona, D. U. B. Aussemsa, M. A. vanden Berga, K. Bystrova, N.J. Lopes Cardozob and M. C. M. van de Sandenaa Dutch Institute for Fundamental Energy Research- DIFFER, De Zaale 20, 5612 AJ
bEindhoven University of Technology, 5612 AZ Eindhoven, The Netherlands

Liquid metals used in plasma facing components are attractive for DEMO due to theliquid’s ability to self-heal against erosion, immunity to most of the negative impact ofneutrons and ability to go beyond conduction-only based cooling. One extension ofthis cooling is vapour shielding, occuring under high heat loading of the liquid-metal
surface by the plasma leading to strong evaporation. In such a situation theinteraction between the vapour and the plasma leads to additional energy andmomentum loss which reduce the power to the surface from the plasma by these
additional volumetric processes. A series of experiments carried out in DIFFER’slinear devices Magnum-PSI and Pilot-PSI have explored this phenomenon in detailfor the first time. These machines are able to recreate the high density (>1020 m-3)
low temperature (<5 eV) conditions expected close to the partially detachedstrikepoints of ITER or DEMO. It was found that the surface temperature of Sn based
substrates becomes locked when the plasma pressure and vapour pressure become
similar, and that this temperature locking persists over a power input range of 1-20
MW m-2. Due to ion-neutral friction and electron ion collisions the plasma is strongly
cooled from 2-3 eV to <0.5 eV driving strong recombination. The plasma cooling andradiation leads to an overall reduction of ~one third in the conducted power.
Additionally the process appears oscillatory in nature due to the mutual interaction of
the thermal cooling (slow) with the plasma processes (fast). For Li similar behaviour
is also observed and appears a general phenomenon which would be expected to bepresent in tokamaks. The talk will discuss vapour shielding for liquid metals and theneeds for atomic data to give a deeper physical understanding and predictive
capabilities for these processes.

*Corresponding author: t.w.morgan@differ.nl (T.W. Morgan)

I-13
Weighted PIC simulation for vapor shielding at wall under transient heat loads
K. Ibanoa,*, Y. Kikuchib, Y. Uedaa, and T. TakizuakaaaGraduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, 565-0871,

Japan bGraduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo, 671-2201,
Japan

Erosion caused by transient heat loads is a serious concern for plasma facing
components (PFCs) of the future fusion reactor. An inherent mitigation of the erosion,
vapor shielding, is expected during the high heat loads. Melting and evaporation
occurs on the PFC surface under the heat loads, and emitted vapor from the surface
interacts with incoming plasma. Atomic-molecular reactions, i.e., elastic collision,
ionisation/recombination, and radiation, cause a dissipation of incoming plasmaenergy. As a result, total absorbed heat flux at the wall can be reduced, andconsequently the erosion is mitigated.
Question is how much mitigation can be expected from the vapor shielding. It is very
difficult to experimentally study a transient heat loads equivalent to those for reactor
condition. Thus, we have been developing a numerical simulation code, PIXY [1,2]. A
particle-in-cell (PIC) model is applied for the plasma simulation because the plasma
considered in the vapor shielding is not likely in Maxwellian. One issue of the PIC
simulation is its expensive calculation cost due to huge numbers of super-particles. If
a constant weight (number of real-particles in a super-particle) is used for all particles,
the number of super-particles representing vapor particles increases linearly inaccordance with the vapor emission rate, which is dependent strongly on the surface
temperature. To solve this issue, a weighted particle model is applied for the PIXY
code. Using different weight for each super-particle, the number of super-particles
can be much reduced, and then the calculation cost becomes realistic. In the wallside, a 1-d heat transfer calculation is taken to determine the vapor emission rate asa function of the surface temperature. Combining plasma and wall models, the PIXY
code simulates the plasma-wall interaction at the transient loads.
The PIXY code successfully reproduced experimental results of a plasma gun [1].
Then, the code was applied to simulations of the mitigated erosions of Be and W
walls for reactor condition, where a rectangular ELM pulse was injected by assuming
a fixed pulse width (0.2 ms) and a fixed plasma energy (2keV) [2]. In this study, we
examine the sensitivity to the pulse width and the plasma energy. We also study theeffect of pulse shape, such as a triangle pulse closer to the realistic transient loads ina reactor. These results can be applicable to estimate the erosion by ELM anddisruption in ITER. In addition, the estimated erosion rates are beneficial information
for the future DEMO reactor designs.

[1] K. Ibano, Y. Kikuchi, S. Togo, Y. Ueda, and T. Takizuka, Contrib. Plasma Phys. 58(2018) 594-601[2] K. Ibano, Y. Kikuchi, S. Togo, Y. Ueda, and T. Takizuka, Nucl. Fusion, in press*Corresponding author: kibano@eei.eng.osaka-u.ac.jp (K. Ibano)I-14
Modifications to the Photonic and Electronic Impact Ionization Cross Sectionsof Ions in Dense plasmas due to the Transient Localization of Continuum
Electron Wavefunctions
Pengfei Liu1, Cheng Gao1, Yong Hou1, Jiaolong Zeng*1, and Jianmin Yuan†,1,2
1

Department of Physics, National University of Defense Technology, China
Graduate school of China Academy of engineering Physics, Beijing 100193, China

2

Photonic and electronic impact ionization processes of ions in warm/hot dense
plasmas play a key role in the determination of ionization balance, equation of state,
and opacity. Here we propose the notion of a transient space localization of electrons
produced during the ionization of atoms immersed in a hot dense plasma, which cansignificantly modify the fundamental properties of ionization processes. A theoreticalformalism is developed to study the wavefunctions of the continuum electrons thattakes into consideration the quantum de-coherence caused by coupling with theplasma environment. The method is applied to the photoionization of Fe16+
embedded in hot dense plasmas. We find that the cross section is considerablyenhanced compared with the predictions of the existing free-atom model, and thereby
partly explains the big difference between the measured opacity of Fe plasma [1] andthe existing standard models for short wavelengths.
We also proposed that the transient space localization of the one electron states
involved in the collision processes significantly modifies the wavefunctions of thescattering and ionized electrons resulting in big enhancements of these parameters.
The theoretical formalism is applied to study the electron-ion collision processes in asolid-density magnesium plasma. The results show that not only the collision
dynamics and the energy correlation of the two continuum electrons are greatly
modified, but also the integrated cross sections and transition rates are dramaticallyincreased in hot dense plasmas. Compared with the isolated ion model, theintegrated cross section can be increased by one order of magnitude and thetransition rate by two orders of magnitude, which supports the recent experimental
evidences that the electronic impact ionization cross sections and collision rates inthe solid-density Al [2] plasmas produced using X-ray free electron lasers (FEL) were
underestimated by more than one order of magnitude.
[1]J. E. Bailey et al., A higher-than-predicted measurement of iron opacity at solar
interior temperatures. Nature (London) 517, 56 (2015).
[2] S. M. Vinko et al., Investigation of femtosecond collisional ionization rates in asolid-density aluminium plasma. Nat. Commun. 6, 6397 (2015).
*Corresponding author: jlzeng@nudt.edu.cn (Jiaolong Zeng)†Corresponding author: jmyuan@gscaep.ac.cn; jmyuan@nudt.edu.cn (Jianmin Yuan)

I-15
Effects of Microstructures on Surface Damage and H/He Retention in W:
A multi-scale Modelling
Y.G. Lia,b,*, L.M. Weia, and Z. Zenga,ba Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences,

Hefei, 230031, Chinab Science Island Branch of Graduate School, University of Science and Technology of China, Hefei,

230026, China

Microstructures greatly affect the behaviours of surface damage and H/He retention
in plasma-facing materials (PFMs) during plasma-material interaction (PMI) in fusion
devices [1]. Two new models are thus developed to investigate the key issues incomplex materials under ion/neutron irradiation, i.e., a universal, computationallyefficient and massively parallel 3D Monte Carlo code, IM3D [2,3], based on thebinary-collision approximation for estimating ion implantation, sputtering and primarydamage, as well as a spatially resolved cluster dynamics (CD) model, IRadMat [4,5],
based on the rate theory for describing defect generation, diffusion, reaction andaccumulation. A sequential multi-scale modelling framework is then constructed bycoupling these two new models with atomistic approaches [5]. The effects of surface
roughness and grain sizes on surface damage and H/He retention in PFMs like W
are systematically investigated and quantitatively compared with experiments.
1) Influence of surface roughness on H/He implantation and sputtering [3]: Using the
IM3D code and a random rough surface model, ion implantation and sputtering of W
with a surface roughness varying between 0-2 μm have been studied for irradiation
by 0.1-1 keV D+, He+ and Ar+ ions. It is found that both ion backscattering andsputtering yields decrease with increasing roughness, due to the direct, line-of-sightdeposition of a fraction of emitted atoms onto neighbouring asperities.
2) Effect of grain size on H/He retention and neutron irradiation damage [5,6]: By
introducing the cellular sink strength of grain boundaries (GBs) into the CD model,
defect accumulation and H/He retention in W with different grain sizes are studied
under various ion/neutron irradiation conditions. It indicated that both the low vacancy
diffusivity and high diffusion bias suppress the radiation tolerance of nanocrystalline
W. H/He retention increases dramatically with grain size decreasing due to theenhancement of H/He capture ratio by GBs. Thus, we suggested that coarse-grained
crystals should be selected for W-based PFMs in practice.

[1] K. Nordlund, C. Bjorkas, T. Ahlgren, and et al., J. Phys. D: Appl. Phys. 47 (2014)
224018[2] Y.G. Li, Y. Yang, M.P. Short, and et al., Sci, Rep. 5 (2015) 18130[3] Y.G. Li, Y. Yang, M.P. Short, and et al., Nucl. Fusion 57 (2017) 016038[4] Y.G. Li, W.H. Zhou, R.H. Ning, and et al., Commu. Comput. Phys. 11 (2012) 1547
[5] Z. Zhao, Y.G. Li, C.G. Zhang, and et al., Nucl. Fusion, 57 (2017) 086020[6] Z. Zhao, L.M. Wei, Y.G. Li, and et al., Effect of diffusion bias on radiation toleranceof iron and tungsten with different grain sizes, to be published (2019)*Corresponding author: ygli@theory.issp.ac.cn (Y.G. Li)

I-16
Molecular Dynamics Simulation for Hydrogen Recycling on Carbon Divertor
S. Saitoa,*, H. Nakamurab,c, K. Sawadad, M. Kobayashib,G. Kawamurab, and M. Hasuoe
a Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japanb National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japanc Nagoya University, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japand Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japane Kyoto University, Kyoto daigaku-katsura, Nishikyo-ku, Kyoto 615-8530, Japan

Plasma facing materials are bombarded by hydrogen plasma in magnetic
confinement plasma devices such as Large Helical Device (LHD) in National Institutefor Fusion Science, Japan. In the case of LHD, the divertor which directly contacts toplasma is made of carbon materials. When the divertor plates are irradiated byhydrogen ions, some hydrogen atoms reflect and back to the plasma while the otherhydrogen atoms retain in the divertor.
Because plasma particles are neutralized by the recycling process on the divertor,
many neutral particles of fuel exist in the edge plasma. These neutral particles affectthe plasma parameters such as the electron and ion temperature and the plasma
density by the interaction with plasma such as ionization, charge-exchange, andrecombination processes. Moreover, the temperature of the neutral particles affects
the electron temperature of detached plasma near the divertor. Therefore, theunderstanding of the transport of the neutral particles in the plasma is one of the criticalissues for the achievement of effective fusion devices.
The transport of the neutral particles can be numerically simulated by neutraltransport code. However, when neutral-transport code is performed, the parameters ofemitted neutral particles such as the energy
Incident
distribution and the form (atom or molecule) ofhydrogeemitted neutral particles are usually decidednartificially without logical reason because theseparameters are not clearly obtained fromexperimental results. The effects of neutral
particles on plasma parameter may depend on
the parameters emitted neutral particles fromthe plasma facing material.
In this paper, therefore, as shown in Fig. 1, wedevelop an MD simulation model [1, 2] ofhydrogen injection into an amophous carbon
material to investigate the distribution oftranslational, vibration, and rotation energy of40Å
emitted hydrogen atoms and molecules.
Figure 1. Simulation
system of hydrogen
ング
injection ito amorphous carbon material.
White and green ball denotes hydrogen[1] H. Nakamura, K. Sawada, M. Hasuo, and S. and carbon atoms, respectively.
Saito, “Angular Distribution of Hydrogen
Generated from Plasma-Facing Material”, 18th ICPP, Taiwan, 2016.
[2] S. Saito, H. Nakamura, K. Sawada, M. Kobayashi, G. Kawamura, and M. Hasuo,“Molecular Dynamics Simulation of Ion Implantation with Heat Transfer into Bulk”,
Proc. JSST, 2018.
*Corresponding author: saitos@yz.yamagata-u.ac.jp

I-17
Recent approaches to machine learning of interatomic potentials seen from aperspective of plasma material interaction and primary radiation damage
Bastiaan J. Braamsa,*
a Centrum Wiskunde & Informatica (CWI), Science Park 123, 1098XG Amsterdam, The Netherlands

The starting point for my presentation is that for the study of material microstructure(as it is affected by radiation and as it affects hydrogen retention and transport) the
fitted interatomic potential or force field is everything. Work in a recent International
Atomic Energy Agency (IAEA) Coordinated Research Project on Plasma-Wall
Interaction with Irradiated Tungsten and Tungsten Alloys in Fusion Devices [1] (asone of many applications for fusion materials) provides motivation to develop betterinteratomic force fields. One may think of systems such as W-H-He immediately after
a primary knock-on atom (PKA) event; there is a local melt region and simulations(using molecular dynamics on a fitted force field) must provide accurate statistics forthe microstructure after resolidification. The simulations are essential in connection
with experiments that rely on surrogate (charged particle) irradiation or on irradiation
by neutrons from fission, spallation or a stripping reaction and they are also essential
to interpret the rather indirect measurements of microstructure.
Recent big data (machine learning) approaches to the development of atomistic force
fields include use of Gaussian process approximation (kernel ridge regression) [2],spherical wavelet expansions [3] and deep neural networks (e.g. [4], [5]). At the sametime, work done for the application domain of interatomic force fields is influencing
research in computer science environments through the concept of deep neuralnetworks that are invariant or covariant (equivariant) with respect to groups of
discrete or continuous transformations: the point group of translations, rotations andreflections and the permutational symmetry group associated with the labeling ofchemically identical atoms. Key words in the machine learning community include
Point Cloud Convolutional Networks, Deep Sets, Spherical CNNs, Tensor Field
Networks and Gauge Equivariant Neural Networks [6]. I will survey some of thesedevelopments in interatomic force fields and related machine learning research.
[1] See https://www-amdis.iaea.org/CRP/.
[2] Bartók, Albert P., and Gábor Csányi. "Gaussian approximation potentials: A brief
tutorial introduction." International Journal of Quantum Chemistry 115, no. 16 (2015):
1051-1057.
[3] Eickenberg, Michael, Georgios Exarchakis, Matthew Hirn, Stéphane Mallat, andLouis Thiry. "Solid harmonic wavelet scattering for predictions of molecule
properties." The Journal of chemical physics 148, no. 24 (2018): 241732.
[4] Schütt, Kristof T., Huziel E. Sauceda, P-J. Kindermans, Alexandre Tkatchenko,
and K-R. Müller. "SchNet–A deep learning architecture for molecules and materials."
The Journal of Chemical Physics 148, no. 24 (2018): 241722.
[5] Zhang, Linfeng, Jiequn Han, Han Wang, Wissam Saidi, Roberto Car, and Weinan
E. "End-to-end symmetry preserving inter-atomic potential energy model for finite andextended systems." In Advances in Neural Information Processing Systems, pp.
4441-4451. 2018.
[6] Detailed references will be provided in the presentation.

I-18
Deep Learning for Multi-Scale Molecular Modeling
Linfeng Zhanga*a Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey

08544, USA

Deep learning has emerged as a promising tool for a variety of applications in scientificmodeling. However, while deep learning is a powerful tool for fitting data, constructing
reliable and practical deep learning-based physical models is still a very non-trivial
task. In this talk, we discuss the important issues and illustrate the relevant ideas in
the context of constructing potential energy and free energy surfaces for molecularmodeling. In particular, we will discuss the Deep Potential [1-3] and Deep Potential
Generator scheme [4] for boosting ab-initio molecular dynamics and the Reinforced
Dynamics [5] for enhanced sampling and free energy calculation. Important
applications to various materials science problems will be introduced and used toillustrate these methods.

[1] Jiequn Han, Linfeng Zhang, Roberto Car, and Weinan E, Communications in
Computational Physics 23.3 (2018): 629-639.
[2] Linfeng Zhang, Jiequn Han, Han Wang, Roberto Car, and Weinan E, PhysicalReview Letters 120 (2018): 143001.
[3] Linfeng Zhang, Jiequn Han, Han Wang, Wissam Saidi, Roberto Car, and Weinan
E, Advances in Neural Information Processing Systems (2018): 4441-4451.
[4] Linfeng Zhang, De-Ye Lin, Han Wang, Roberto Car, and Weinan E, Phys. Rev.
Materials 3 (2019): 023804.
[5] Linfeng Zhang, Han Wang, and Weinan E, J. Chem. Phys. 148 (2018):124113.
*Corresponding author: linfengz@princeton.edu (L. Zhang)I-19
Classification of Clusters in MD Simulations of Collision Cascades
U. Bhardwaja,*, A. Sandb, and M. Warriera,ca Computational Analysis Division, Bhabha Atomic Research Centre, Visakhapatnam, India - 531011
b Department of Physics, P.O. Box 43, FI-00014 University of Helsinki, Finlandc Homi Bhabha National Institute, Anushaktinagar, Mumbai, Maharashtra, India - 400 094

The structure of defect clusters formed in a displacement cascade plays a significant
role in the micro-structural evolution during irradiation [1]. We present a novel method
based on machine learning for pattern matching and classification of defect clusters
from Molecular Dynamics simulations of collision cascades. The methods are applied
on a database of collision cascades in Fe and W at energies ranging from 10 keV to
200 keV. The results show twenty six classes of cluster shapes providing new
insights and parameters that can be used in simulations at higher scales. We discusseach step, starting from efficient identification of defects from simulation output toreduction of physics problems to machine learning stages viz. feature engineering,
dimensionality reduction and unsupervised classification.
We characterised a cluster by the geometrical histograms of angles and distances
between neighbouring defects in the cluster. We show the use of these histograms
as similarity metric to search for similar cluster shapes. The dimensionality reduction
helps in plotting each cluster as a point in a two or three dimensional space. The
algorithm when used on the designed features (histograms) places similar clusters
together. We show that clusters that are placed together also have similar types(interstitial / vacancy), sizes and dimensionality. The unsupervised classification
separates out many already known categories of clusters such as crowdions, planar
crowdion pairs, rings, etc. The distribution of different classes of shapes among
cascades of different elements and energies shows the exclusivity of shapes toelements and energies. The cluster shape preferences for Fe and W agree with priorstudies [2, 3]. The results also show new cluster shape classes and quantitative
study of the sizes, dimensionality and preferences to elements and energies for eachclass. We discuss the key points and computational efficiency of the algorithms alongwith the various prominent results of their application.
We present an open-source software [4] implementing the methods and providing
additional tools for analysis and interactive visualisations. The distribution of cluster
shapes and structures along with the shape properties like diffusivity, stability, etc.
can be used as input to higher scale models in a multi-scale radiation damage study.
The machine learning methods can also be further extended to study shapes ofsubcascades and collision cascades.
[1] C. Becquart, Souidi, Domain, Hou, Malerba, Stoller, J. Nucl. Mater 351 (1) (2006)[2] A. Sand, S. Dudarev, K. Nordlund, EPL 103 (4) (2013) 46003.
[3] L.Dzerald, Marinica, Ventelon, Rodney, Willaime, J. Nucl. Mater 449(1) (2014)[4] csaransh : https://github.com/haptork/csaransh*Corresponding author: butkarsh@barc.gov.in (U. Bhardwaj)

I-20
Displacement damage stabilization by hydrogen presence under simultaneous
W ion damaging and D ion exposure
S. Markelja,*, T. Schwarz-Selingerb, M. Pečovnika, M. Kelemena,c, W. Chromińskid,
L. CiupinskidaJožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
Max-Planck-Institut für Plasmaphysik, Boltzmannstrasse 2, D-85748 Garching, GermanycJožef Stefan International Postgraduate School, Jamova cesta 39, 1000 Ljubljana, SloveniadWarsaw University of Technology Faculty of Materials Science and Engineering, Wołoska 141, Warsaw, Polandb

In a future thermonuclear device such as DEMO the wall material will be irradiated by
14 MeV neutrons produced in the D-T nuclear reaction and at the same time exposed
to a high flux of ions and neutrals of hydrogen isotopes (HIs) as well as helium. In
most experiments where fuel retention in displacement-damaged tungsten (W) was
studied by ion or plasma loading, the high energy ion damaging was performed as asurrogate for neutron irradiation and sequentially exposure to deuterium was done todetermine the retention and properties of HI trapping. However, in real fusion devices
creation of displacement damage and HI irradiation will take place simultaneously.
This difference in the conditions at which damage is created, meaning with or without
the presence of HIs, could be important when studying fuel retention. It was shownby positron annihilation spectroscopy that vacancy-hydrogen complexes have ahigher barrier associated with their migration in metals like tantalum and niobium [1]compared to hydrogen free vacancies.
Here we will present experimental results of simultaneous exposures wherepolycrystalline W samples were irradiated by 10.8 MeV W ions and exposed to 300
eV deuterium (D) ions at different temperatures ranging from 450 K to 1000 K. Afterthe simultaneous W/D exposure the samples were additionally exposed to low
energy D ions at 450 K in order to populate all the defects created beforehand. The
amount of damage created was evaluated by measuring D depth profiles and D
thermal desorption spectra. Results are compared with data obtained in a sequential
experiment where samples were first irradiated by 10.8 MeV W ions and only
afterwards exposed to 300 eV D ions at 450 K to populate the created defects. At 450
K we observe a two times higher maximum D concentration for the simultaneouscase as compared with the sequential case. At 600 K and 800 K the ratio between
simultaneous and sequential decreases to about 1.6 and 1.2, respectively, andincreases again to a factor of two at 1000 K. We attribute this temperaturedependence to the change in the concentration of mobile and trapped D during thesimultaneous exposures, which is in line with theoretical calculations predicting thattrapped D in a vacancy prevents vacancy annihilation with self-interstitials [2]. Fromthe comparison with the previous experimental series with D atoms [3] conclusions
about the influence of retained D on damage stabilization will be drawn.
[1] P. Hautojärvi, H. Huomo, M. Puska and A. Vehanen, Phys. Rev. B 32 (1985),
4326-4331.
[2] D. Kato, H. Iwakiri , Y. Watanabe, K. Morishita and T. Muroga, Nucl. Fusion 55(2015) 083019.
[3] S. Markelj et al., Nucl. Mater. Energ. 12 (2017) 169-174 & E. Hodille et al. Nucl.
Fusion 59 (2019) 016011.
*Corresponding author: sabina.markelj@ijs.si (S. Markelj)

I-21
Influence of the presence of deuterium on displacement damage in tungsten
T. Schwarz-Selingera,*, J. Bauera, M. Pečovnikb S. Markeljb
a Max-Planck-Institut für Plasmaphysik, Boltzmannstrasse 2, D-85748 Garching, GermanybJožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia

Among many other favourable properties intrinsically low fuel retention makes
tungsten one material of choice as plasma-facing material. However, during
operation of a fusion device defects in the tungsten lattice will evolve that will traphydrogen isotopes. Experiments with neutron-irradiated samples as well as MeVenergy ion-irradiated samples showed that hydrogen isotope retention can reachvalues of the order of 1 at.% due to the displacement damage [1]. However, all theseexperiments were done sequentially: First damage was produced and only thentransport and deuterium retention in this material was studied. First beam
experiments where displacement damage was created with MeV tungsten ions while
simultaneously dosing the samples with either atomic deuterium [2] or low energy
deuterium ions [3] revealed increased retention by up to a factor of two compared tothe sequential exposures.
Here a different experimental approach will be presented that allows studying theinfluence of hydrogen isotopes on damage creation. In this approach first 20 MeV
tungsten is implanted into recrystallized tungsten to a level where damage cascades
overlap and defect density saturates within the 2 μm deep implantation zone. Second,the created defects are decorated with deuterium with a low temperature plasmaachieving a homogenous deuterium concentration of 1.8 at.%. In a third stage, thesesamples are again irradiated with 20 MeV tungsten. In this step not only tungsten
atoms are displaced and defects are generated but also the retained deuterium
atoms are de-trapped and hence mobilized. SDTrimSP calculations reveal that for
the implantation parameters used, on average each D atom is recoiled at least once.
Nuclear reaction analysis shows that during this second implantation no deuterium islost from the sample, but it is only de-trapped and is effectively re-trapped again.
When samples are again decorated with deuterium after the second self-damaging
step, NRA and TDS reveals that deuterium retention exceeds the initial saturation
value by nearly a factor of two [4]. These experiments clearly support the calculations
by Kato et al. [5] and Middleburgh et al. [6] which predicted that hydrogen isotopes
either stabilize vacancies or lower the vacancy formation energy which both leads toan increase in trap density.
[1] Y. Hatano, J. Nucl. Mater. 438 (2013) S114–19[2] S. Markelj et al. Nucl. Mater. Energy 12 (2017) 169[3] S. Markelj et al. submitted to Nuclear Fusion (2019)[4] T. Schwarz-Selinger et al. Nucl. Mater. Energy 17 (2018r) 228–34[5] D. Kato et al., Nucl. Fusion 55 (2015) 083019[6] S.C. Middleburgh et al., J. Nucl. Mater. 448 (2015) 270–75*Corresponding author: thomas.schwarz-selinger@ipp.mpg.de (T. Schwarz-Selinger)

I-22
Deuterium trapping at irradiation-induced defects in tungsten studied bypositron annihilation spectroscopy
T. Toyamaa,*, K. Amib, K. Inouea, Y. Nagaia, K. Satoc, Q. Xuc and Y. Hatanoba Tohoku Universit, Narita 2145-2, Oarai, Ibaraki 311-1313, Japanb University of Toyama, Toyama 509-5292, Japanc Kyoto University, -6 Oroshi-cho, Toki, Osaka 509-5292, JapanThe development of fusion reactors like ITER has been improving. In plasma-facing
components, energetic hydrogen is induced together with radiation damage due toexposure to fusion plasma. Since tritium is an essential fusion reaction source, it isnecessary to constrain its retention in the plasma-facing components. Such
constraints are also of great importance from a safety perspective. Tungsten is a
primary candidate material for the plasma-facing components due to its high melting
point and high sputtering resistance to energetic particles. The solubility of tritium intungsten is very low (the lowest among metals), which is an advantage in reducing
tritium retention during the operation of fusion reactors. However, recent studies havereported that irradiation cause significant enhancement of hydrogen retention intungsten. This is ascribed to hydrogen trapping at irradiation-induced defects such asvacancies and vacancy clusters, however, it has not yet been clarified. We here
employed positron annihilation spectroscopy (PAS) to reveal both irradiation-induced
defects and hydrogen in neutron-irradiated tungsten [1].
Pure tungsten (4N) was neutron-irradiated at about 573 K for about 40 days to thedose of about 0.3 dpa, followed by post-irradiation annealing (PIA) at 573 K for 100
hours. The PIA was performed in a vacuum or in deuterium gas at about 0.1 MPa. In
the as-irradiated state, the formation of vacancy clusters with the size of about V 40
was revealed by positron lifetime measurement. After the PIA in vacuum, positron
lifetime was almost the same as that in the as-irradiated state, indicating no change
in the vacancy clusters. On the other hand, after the PIA in deuterium gas, positron
lifetime was significantly shorter than that in the as-irradiated state, strongly
suggesting positron annihilation with deuterium electrons at the vacancy clusters. To
analyze chemical environment at the positron trapping sites, coincidence Doppler
broadening (CDB) measurements were performed. CDB measurements clearly
indicated positron annihilation with deuterium electrons, thus deuterium trapping at
irradiation-induced vacancy clusters was revealed. This gives the direct observationof deuterium trapping at irradiation-induced defects, which is suggested to be themain origin of significant increase in hydrogen isotope retention in irradiated-tungsten.

[1] T. Toyama, K. Ami, K. Inoue, Y. Nagai, K. Sato, Q. Xu and Y. Hatano, J. Nucl.
Mater 499 (2018) 464.
*Corresponding author: ttoyama@imr.tohoku.ac.jp (T. Toyama)O-1
Origin of outstanding radiation resistance in W-based high-entropy alloys
D. Nguyen-Manha,*, D. Sobierajb, J.S. Wrobelb, O. El Atwanic,
A. Deverajd and E. Martinez Saeze
a CCFE, UK Atomic Energy Authority, Culham Science Centre, Oxfordsire, OX14 3DB, UK
b Faculty of Materials Science and Engineering, Warsaw University of Technology, Warsaw, Poland
c Materials Science and Technology Division, Los Alamos National Laboratory, NM, USA
d Materials Science Group, Northwest Pacific National Laboratory, WA, USA
e Theoretical Division T-1, Los Alamos National Laboratory, NM, USA

Understanding the phase stability under irradiation of alloys with multiple principal
elements is one of the great chanllenges in developing materials engineering
components such as the divertor or plasma-facing materials for fusion devices.
Recently, a body-centered cubic tungsten-based refractory high-entropy alloy (HEA)
with outstanding radiation resistance has been developed [1]. It was found that nosign of irradiation-created dislocation loops, even after 8 dpa, was observed.
Furthermore, nanomechanical testing shows a large hardness of 14 GPa in the asdeposited samples, with near negligible irradiation harderning.
In this work, an ab-initio based constrained thermodynamic formalism for driven
alloys, initially proposed by G. Martin, has been developed to model multi-component
alloy system under irradiation for wich point defects are being considered as the
additional elements in the system. It is found that the formalism can bemathematically respresented within matrix formulation for the K-component alloys via
many-body cluster correlation functions which in turn can be computed efficiently
from Monte-Carlo simulations in a combination with first-principles cluster-expansion
Hamiltonian [2]. Applying the theory for bcc W-Ta-Cr-V based HEAs, it is predictedthat there is a strong phase decomposition between W, Ta and Cr, V as a function ofcomposition and temperature. The formation of Cr and V rich phase found from theMC simulations is in an excellent agreement with the precipitates oberserved with
Atom Probe Tomography analysis for W 38Ta36Cr15V11 alloy irradiated at T=1050K. The
comptetion between phase segretation and radiation effects is discussed. The
absence of irradiated damage in material ties the outstanding radiation resistance tomaximum recombination probabilities between vacancy and interstitial defects in theinvestigated HEAs.
[1] O. El-Atwani, N. Li, M. Li, A. Devaraj, J. K. S. Baldwin, M. M. Schneider, D.
Sobieraj, J. S. Wróbel, D. Nguyen-Manh, S. A. Maloy, E. Martinez, Science
Advances, 5 (2019) eaav2002[2] A. Fernandez-Caballero, M. Fedorov, J.S. Wrobel, P.M. Mummery, D. NguyenManh, Entropy, 21 (2019) 00068*Corresponding author: duc.nguyen@ukaea.uk (D. Nguyen-Manh)O-2
Towards understanding the influence of Re on H dissolution and retention in W
by investigating the interaction between dispersed/aggregated-Re and H
Hong-Bo Zhou*, Fang-Fei Ma, Fang-Ya Yue, Yu-Hao Li and Guang-Hong Lu
Department of Physics, Beihang University, Beijing 100191, China

Tungsten (W) and W alloys are considered as the most promising candidates forplasma facing materials (PFMs) in future fusion reactors. Rhenium (Re) is not only
the typical alloying element but also the main production of transmutation in W-PFMs.
The microstructure and mechanical properties of W as well as the behaviors of
impurities in W will be influenced by the presence of Re. The deuterium retention indamaged W-3%Re at 750 K is two of magnitude lower than that in damaged Re-free
W. Therefore, one can expect that Re should have significant effect on the behaviorof H isotopes in W. However, little work has focused on this aspect so far.
Here, we have systematically investigated the effects of dispersed/aggregated Re on
the behaviors of H in W as well as their interaction with point defects using a firstprinciples method in combination with thermodynamic models. It has been
demonstrated that the influence of Re on H is strongly related to the distribution of Re
in W. Re will aggregate and form clusters/ Re-rich precipitation phases under highenergy ions/neutrons irradiation in W. The influence of Re clusters on H isextraordinary stronger than that of a single Re. The retention of H in W can besignificantly suppressed by Re clusters, and their influences will be enhanced with
the increasing of the number of Re atoms. On the contrary, it is found that thesolution energy of H at most interstitial sites (> 80%) in W-Re sigma phase is much
lower than that in pure W. Specifically, the H solution energy at most stable interstitialsite in W-Re σ phase is only 0.47 eV, ~ 54% lower than that in pure W. This can be
attributed to that W-Re sigma phase provides the larger available volume forinterstitial H than the pure W, weakening the W-H repulsive interaction. Consequently,
our calculations reveal that the Re-rich precipitation can serve as the strong trapping
centers for H in W, while dispersed-Re/small Re clusters can be used to suppress H
retention. These results provide an important reference to evaluate the influence ofRe and other alloying elements on the behaviors of H isotopes in W-PFMs under
future fusion conditions.

*Corresponding author: hbzhou@buaa.edu.cn (Hong-Bo Zhou)

O-3
Deposition effects on He induced fiberform nanostructure growth
S. Kajitaa,*, T. Nojimab, T. Okuyamab, N. Yoshidac, Y. Yamamotoa, and N. Ohnob
a Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 464-8603, Japanb Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japanc Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

Helium (He) plasma irradiation to metals leads to various morphology changes
accompanied by He bubble formations. In particular, fiberform nanostructures called‘fuzz’ can be grown when the incident ion energy and the surface temperature satisfy
a certain condition [1]. To understand the mechanisms for the He induced
morphology changes, several models have been proposed and various simulations
have been conducted. However, the growth process has yet to be fully understood. In
particular, the growth process of a recently found enhanced growth process to large
scale structures such as nanotendril bundles (NTBs) and mm-thick large-scale
fiberform nanostructures (LFNs) [2,3] is a mystery. In this study we show LFN growth
on several metals (W, Mo and Re) while metal particles (ions and neutrals) are
precipitated on the samples which is exposed to He plasmas. When metal particles
are precipitated on the surface, the growth process is not limited by some diffusion
process. The height of nanostructure building block increased exponentially with
irradiation time. Based on detailed transmission electron microscope (TEM)
observations, the crystal orientation of the growth direction of linearly grown fiberform
nanostructures identified on some HCP metals were identified. It is shown that HCP
metals have a preferential growth orientation in c-axis direction. The results
suggested that adatoms formed by He ion irradiation or metal ion precipitations
diffuse on nanofibers and contribute to the growth at the tip of nanofibers in epitaxial
crystal growth process.
[1] S. Kajita, W. Sakaguchi, N. Ohno, N. Yoshida, T. Saeki, Nucl. Fusion 49 (2009)
095005.
[2] K. Woller, D. Whyte, G. Wright, Nuclear Fusion 57 (6) (2017) 066005.
[3] S. Kajita, S. Kawaguchi, N. Ohno, N. Yoshida, Sci. Rep. 8 (2018) 56.

*Corresponding author: kajita.shin@nagoya-u.jp (S. Kajita)

O-4
Sputtering and Re-deposition Effects during Fuzz Growth Process Simulatedby BCA-MD-KMC Multi-hybrid Simulation
A. M. Itoa,*, A. Takayamaa, and H. Nakamuraa,ba National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japanb Department of Electrical Engineering, Graduate School of Engineering, Nagoya University, Furo-cho,

Chikusa-ku, Nagoya 464-8603, Japan

Helium plasma irradiation induces the fuzzy nanostructure, ‘‘fuzz‘‘, on a tungsten
surface[1,2]. The helium plasma is the ash of fusion reaction and the tungsten isused for the divertor plate in magnetic confinement fusion reactors. The heat lead ofthe divertor is regarded as a critical problem in the fusion reactor. Because thethermal conductivity of the tungsten material with fuzz is smaller than that of bulk[3],the generation of the fuzzy nanostructure on the divertor plate is a key issue of theresearch in plasma-wall interaction.
The growth mechanism of the fuzzy nanostructure has not been clarified, and therelation with the helium bubble, which is generated in the case that the fluence of
helium ion is lower than the fluence to generate the fuzzy nanostructure, has been
considered. Although, the formation mechanism of the helium bubble was well
explained by using density functional theory(DFT)[4] and the molecular dynamics(MD)[5,6]. However, those simulations could not reproduce the generation of the
fuzzy nanostructure because they cannot achieve the fluence necessary for fuzzy
nanostructure formation, 1024 m-2. We developed the BCA-MD-KMC triple hybrid
simulation[7]. The hybrid simulation achieved the fluence of 1024 m-2 while theincident flux is kept low at 1022 m-2s-1, which is comparable with an experimental
condition. As a result, the fuzzy nanostructure growth was reproduced. From thissimulation, we clarified the transport of tungsten atoms for the growth of the fuzzy
nanostructure is caused by the knocking out by the incident helium ion and the redeposition of the tungsten atom knocked out.
We focused that the experiment requires that an incident energy of 6 eV or more isnecessary for helium bubble growth[8] and an incident energy of 30 eV or more isnecessary for fuzz growth[9]. From the DFT calculation[4], it is clarified that theincident energy required for helium atom to penetrate into tungsten is 6 eV. In thepresent work, we investigate incident energy dependence of the growth of the fuzzy
nanostructure by using the BCA-MD-KMC multi-hybrid simulation. We consider thatthe incident energy threshold of 30 eV for fuzz growth is caused by the recoil(knocking out) threshold energy of tungsten on rough surfaces.
[1] S. Takamura, et al., Plasma Fusion Res. 1, 051 (2006).
[2] M. J. Baldwin and D. P. Doerner, Nucl. Fusion 48, 035001 (2008).
[3] S. Kajita, et al., Results Phys. 6, 877 (2016).
[4] T. Tamura, et al, Modelling Simul. Mater. Sci. Eng. 22, 015002 (2014).
[5] F. Sefta et al., Nucl. Fusion 53, 073015 (2013).
[6] R. Kobayashi, et al., J. Nucl. Mater. 463, 1071-1074 (2015).
[7] A. M. Ito, et al., Plasma Fusion Res. 13 (2018) 3403061.
[8] D. Nishijima, et al., J. Nucl. Mater. 313-316, 97 (2003).
[9] W. Sakaguchi et al., J. Nucl. Mater. 390-391, 1149 (2009).
*Corresponding author: atsushi.m.ito@nifs.ac.jp (A. M. Ito)

O-5
Helium Bubble Growth In Tungsten Nano-Tendrils
Yingzhao Hea, Zhangcan Yanga,*a Department of Nuclear Engineering and Technology, School of Energy and Power Engineering,

Huazhong University of Science and Technology, Wuhan 430074, China

It is known that fuzz nanostructures will form when tungsten surfaces are exposed to
low-energy helium plasma. These nanostructures appear as a tangled mass ofdisordered nano-tendrils with up to micrometer in length and several tens ofnanometer in diameter. It is still unclear what continues to drive the formation oftungsten fuzz up to several micrometers in thickness. Recently, several studies haverevealed that both helium bubbles and intratendril grain boundaries (GB) are present
in the nano-tendrils. It may provide some insight to understand the mechanism offuzz growth by investigating the growth of helium bubbles in the vicinity of GBs in
nano-tendrils.
In this study, molecular dynamics (MD) simulations have been performed tosystematically study the growth and bursting of helium bubbles near different GBs in
nano-tendrils. The results show that the growth behavior of helium bubbles is very
different from the cases in the bulk or near surfaces. Prismatic dislocation loops are
rarely formed in the nano-tendrils. The results have also indicated that the resulting
surface morphology depends on GB structures (see Figure 1). The movement of
dislocations punched out by helium bubbles is also studied in details by analyzing theshear stress-field nephogram around the helium bubbles.

Figure 1. The evolution of surface morphology for 4 different GBs structure after He
bubble bursts. (a) Σ3<111>{112} GB structure, (b) Σ5<100>{310} GB structure, (c)Σ7<111>{123} GB structure, (d) Σ17<100>{410} GB structure.
[1] K. Wang, M.E. Bannister, F.W. Meyer, and C.M. Parish, Acta Mater. 124 (2017)[2] K. Wang, R.P. Doerner, M.J. Baldwin, and C.M. Parish, J. Nucl. Mater. 509 (2018)679[3] C.M. Parish, K. Wang, R.P. Doerner, and M.J. Baldwin, Scr. Mater. 127 (2017)
132–135*Corresponding author: Yang_zhangcan@hust.edu.cn (Zhangcan Yang)

O-6
Surface coverage dependence of the diffusion/desorption mechanisms ofhydrogen from the W(110) and W(100) surfaces: a model based on DFT andthermodynamics
Z. A. Piazzaa, M. Ajmalghana, Y. Ferroa,* and R. D. Kolasinski b
a Aix Marseille Univ, CNRS, PIIM, Marseille, Franceb Sandia National Laboratories, Energy Innovation Department, Livermore, California 94 551, USA

The purpose of the present work is to establish the mechanisms for the diffusion ofhydrogen in the sub-surface of tungsten and the mechanism for the recombination ofmolecular hydrogen on top of the W(100) and W(110) surfaces depending on thesurface coverage in hydrogen.
Indeed, the interaction of tungsten with hydrogen is the aim of many theoretical andexperimental modellings. In Laboratories, Temperature Program Desorption (TPD) is
widely used to determine the behavior of hydrogen in tungsten depending on thetemperature. These experiments are assisted by numerical simulation to understandthe trapping mechanisms and to determine the type of defects with which hydrogeninteracts. Nevertheless, the surface effects are currently not well understood andtheir dependence with the state of the surface in terms of hydrogen coverageremains unclear.
In this work, we first computed by DFT the energies of the most stable configurationsof hydrogen adsorbed on the W(110) and W(100) surfaces from low coverages up to
saturation. Based on these energies and on the vibrational properties of the system,
we built a thermodynamic model that provides the hydrogen coverage of the W(110)and W(100) surfaces depending on the experimental conditions, which are thepressure and the temperature. These results are compared with experimental resultsfrom Low Energy Ion Scattering and Direct Recoil Spectroscopy.
The diffusion of hydrogen in the sub-surface and the recombination of H 2 on theW(110) and W(100) surfaces was then investigated. The related activation barriers
were determined depending on the hydrogen coverage of the surface. We were
finally able to determine the range of temperature and pressure in which therecombination mechanism becomes the rate limiting step during TPD experiments.
This corresponds to temperature above 450K to 500K and pressure around 10-8 Pa,
which falls in the conditions where the main of desorption peak is observed during
TPD of hydrogen from tungsten.
This work has been carried out within the framework of the EUROfusion Consortium and hasreceived funding from the Euratom research and training program 2014-2018 under grant
agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of
the European Commission.

*Corresponding author: yves.ferro@univ-amu.fr

O-7
Impact of a high temperature target on hydrogen recycling
M. Sakamotoa,*, A. Terakadoa, K. Nojiria, N. Ezumia
a Plasma Research Center, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan

Hydrogen recycling is one of the most important issues for stable steady state
operation. Wall temperature plays a critical role on the hydrogen recycling [1]. In thetandem mirror GAMMA 10/PDX, the hydrogen recycling study has been carried out
by using a temperature-controlled tungsten target which is exposed to the end loss
plasma [2-3]. One of the features of GAMMA 10/PDX experiments is enough low
neutral pressure during the plasma exposure to measure emissions of the recycling
particles.
A device which is utilized in this study is installed in the west end region of GAMMA
10/PDX. It is named D-module and consists of a rectangular box (0.5 m square and
0.7 m in length) with an inlet aperture at the front panel and a V-shaped tungsten
target (0.3 m in width and 0.35 m in length) inside the box. The end loss plasma flows
through the inlet, and the target is exposed to the plasma. The target can be heated
up to 573 K by sheath electric heaters. The open-angle of the V-shaped target was
45 degrees. Langmuir probes are installed on the upper target plate. The Balmer line
and Fulcher-α band emissions of the plasma in front of the V-shaped target aremeasured by spectroscopies. Two dimensional image of the H α line intensity is alsomeasured by a fast camera with an interference filter (656 nm ± 10 nm).
When the target temperature (T target ) increased from room temperature to 573 K, theH α and H β intensities increased by a factor of two although the electron density
increased by ~13 % (from ~2.3 × 1016 m−3 to ~2.6 × 1016 m−3), indicating enhancedhydrogen recycling due to increasing the target temperature. The electron
temperature was almost constant (~30 eV). The significant increase in the Balmer
intensities are not caused by electron impact excitation, since the mean free path of
excitation from n=1 to n=3 is ~100 m, which is much longer than the plasma size. A
possible candidate of the mechanism of the increase in the Balmer intensities is
production of excited hydrogen atoms by dissociation of vibrationally excitedhydrogen molecules. The vibrational temperature which was evaluated with a
Fulcher-α band spectrum was ~3400 K and constant relative to the target
temperature, indicating the hydrogen molecules were vibrationally excited. The
vibrationally excited molecules are considered to be produced by hot atom
recombination on the tungsten target [4].

[1] M. Sakamoto et al., Nucl. Fusion 44 (2004) 693.
[2] M. Sakamoto et al., AIP Conf. Proc. 1771 (2016) 060001.
[3] A. Terakado et al., Plasma and Fusion Research 13 (2018) 3402096.
[4] S. Markelj et al., J. Chem. Phys. 134 (2011) 12.

*Corresponding author: sakamoto@prc.tsukuba.ac.jp (M. Sakamoto)O-8
Potential Sputtering from Rare Gas Solid Surface
by Multiply-Charged Ion impact
H. Sawaa, and T. Hirayamaa,b,*
aDepartment of Physics, Rikkyo University, Tokyo 171-8501, Japan
bResearch Center for Measurements in Advanced Science, Rikkyo University, Tokyo 171-8501, Japan

The potential sputtering phenomenon, widely observed on the surfaces of insulators,
such as alkali-halides and oxides, is a desorption mechanism induced by the transferof potential (internal) energy from incident multiply-charged ions (MCIs) to the solid
surface [1].
Dynamic processes of sputtering can ideally be studied using rare gas solid (RGS),
as it has the simple electronic structure similar to isolated atoms, along with very
unique characteristics, such as very small cohesive energy (e.g., 0.02 eV/atom for
Ne), large band gap energy (e.g., 21.6 eV for Ne), and long diffusion length of
excitons (e.g., ∼ 200 nm for Ne), which set it apart from the other materials.

We reported the relative ion yields of potential sputtering from solid Ne by Arq+ (q = 1− 7) impact and demonstrated the proportionality of the sputtering ion yields to thepotential energy of the incident MCI [2]. Our absolute measurements of the ion
sputtering yields revealed that the yields depend not only on the potential energy but
also on the kinetic energy of the incident MCIs. In order to explain our results, wehave proposed a new desorption mechanism, where ions are created in the solid by
the charge transfer with the incident MCIs, and part of them desorb with surrounding
neutral atoms [3].
To acquire more detailed and quantitative information on ion desorption process, wehave developed a system for coincidence measurements of the ions desorbed andprojectiles scattered from the surface of RGSs by MCI impact [4]. Recent progressincluding some preliminary results obtained by the new system will be presented at
the workshop.
This work was partly supported in part by MEXT - Supported Program for theStrategic Research Foundation at Private Universities, 2014 - 2018 (S1411024).
[1] F. Aumayr, and HP. Winter, Phil. Trans. Roy. Soc. Lond. A362, (2004) 77, andreferences therein.
[2] K. Fukai, S. Fujita, T. Tachibana, T. Koizumi, and T. Hirayama, J. Phys. Cond. Matt.
22, (2010) 084007.
[3] K. Ban, M. Akiwa, H. Ueta, T. Tachibana, and T. Hirayama, Low Temp. Phys.,
accepted for publication.
[4] H. Sawa, S. Uchida, H. Ueta, and T. Hirayama, X-Ray Spectrom., accepted for
publication.
*Corresponding author: hirayama@rikkyo.ac.jp (T. Hirayama)O-9
D retention in bulk Be and D co-deposited in Be layers studied by 3 different
thermal desorption techniques and their modelling by CRDS
M. Zlobinskia,*, M. Eichlera, D. Matveeva, T. Dittmara, M. Rasinskia, S. Möllera,
G. Sergienkoa, Y. Martynovaa, B. Unterberga, S. Brezinseka, Ch. Linsmeiera,
C. P. Lungub, C. Porosnicub, P. Dincab, and G. De Temmermanca Institute of Energy and Climate Research – Plasma Physics, Forschungszentrum Jülich GmbH (FZJ),

Partner in the Trilateral Euregio Cluster, D-52425 Jülich, Germanyb National Institute for Laser, Plasma and Radiation Physics (INFLPR), Atomistilor 409, Magurele,

Jud Ilfov, 077125, Bucharest, Romania
c ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St Paul Lez Durance Cedex,

FranceHydrogen isotope retention in beryllium is an important issue for ITER especiallyconcerning tritium due to the licence-limited inventory. Moreover, hydrogen retention
has effects on the wall and the global fuel cycle. Therefore, understanding of theretention and release mechanisms for hydrogen implantation and co-deposition in Be
layers is crucial. We present recent deuterium retention studies in Be/D co-deposits
on W, D implanted in single and poly-crystalline Be and Be/W mixed materials(Be 12W). Modelling was performed by a Coupled Reaction Diffusion System (CRDS)[1], while the experiments were performed in two devices at FZJ that can handle Be:
The ARTOSS device combines many techniques of surface science in one UHV
device: D implantation (1-2 keV/D) by a mass and energy separated ion source,
Thermal Desorption Spectrometry (TDS) providing insight into the D binding states,
Nuclear Reaction Analysis (NRA) for D inventory and X-ray Photoelectron Spectroscopy (XPS) to study chemical surface states.
The new FREDIS device [2] contains a TDS system for even larger samples and a
Laser-Induced Desorption (LID) system that can scan full JET tiles with rapid sample
heating by laser pulses. Both parts share the same Quadrupole Mass Spectrometer(QMS) system for quantification of the released gases.
ARTOSS recently revealed low energy binding states in Be for higher D implantation
fluences (~ 1022/m2). Very slow heating ramps of 0.01 K/s allowed to resolve three
low energy binding states of D in Be for the first time. Secondary Electron Microscopy(SEM) imaging of the Be surface after D implantation showed fluence dependent
blistering that might be correlated to these states.[3]FREDIS showed similarly sharp low-temperature and broad high-temperaturedesorption peaks for co-deposited D in Be layers of 1, 10 and 20 µm thickness
produced by High Power Impulse Magnetron Sputtering (HiPIMS) on ITER grade W
and even sharper, instantaneous release in some cases. Additionally, it could bedemonstrated that LID can desorb the D inventory completely under certain circumstances, opening a possibility for in situ desorption in the fusion device.[4][1] D. Matveev et al., Nuc. Inst. Meth. Phys. Res. B 430 (2018) 2330, doi:10.1016/j.nimb.2018.05.037[2] M. Zlobinski et al., Fusion Eng. Des. (2019) in press, doi:10.1016/j.fusengdes.2019.02.035[3] M. Eichler, Nucl. Mat. En. 19 (2019) 440-444, doi:10.1016/j.nme.2019.03.018[4] M. Zlobinski et al., Nucl. Mat. En. 19 (2019) 503-509, doi:/10.1016/j.nme.2019.04.007*Corresponding author: m.zlobinski@fz-juelich.de (M. Zlobinski)

O-10
Microstructural Change of Au Thin Films by Microwave Irradiation
N. Yoshikawaa,*, T. Igarashib, H.Taguchic, A.Nagatac and S. Komarova
a Graduate School of Environmental Studies, Dept. of Materials Science and Engineering,Tohoku Univeristy, 6-6-02, aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japanb Graduate student at Tohoku University, c Bachelor student at Tohoku University

For the purpose of fabricating interconnects in integrated electronic circuits, metalthin films are vapor-deposited and play their roles. In order to avoid thermal damages
to the substrates, low temperature deposition is favored in the film fabrication(metallization) process. As a result, these films consist of small (nano) crystal grains,therefore, the films do not possess sufficient electric conductivity. Post annealing is
conducted in order to improve their conductivity, which is again required to beminimized the thermal damage. Lamp heating is one of the currently-conducted
processes in the electronic industry.
In our research group, microwave post annealing is investigated [1,2] because of its
chracteristics of rapid and selective heating. In this paper, it is intended to present
some results on the microstructural variations caused by microwave irradiation incomparison with the conventional (furnace) annealing.
Au films were sputter-deposited on SiO 2 substrates. Microwave at 5.8GHz was
utilized for heating in a sigle mode microwave cavity at maxmum power 100W. The
specimens were placed at the magnetic field maximum position in the cavity.
The films are also heated in an electric furnace for comparison. Temperature
measurement was performed using the same optical method for the both heating
methods. The post annealed specimens were evaluated their microstructures bymeans of AFM, SEM/EBSP, TEM and XRD.
Differences were demonstrated in microstructural evolution process of Au thin filmby two anneling methods. It was observed that fine roughness of the film surface was
reduced more by microwave than by furnace under conditions of low temperatureannealings. However, by the higher temperature annealing, similar microstructures
were obtained in both methods, eventually.
Considering various factors having possible influences on the film roughness
variations, alteration of surface energy and/or surface diffusivity of atoms is one of
the major effects for evolution of the film microstructures and the morphologies.
Micro-plasma generated on the film surface is inferred to be related with alteration of
the surface states.

[1] Z.Cao, N.Yoshikawa and S.Taniguchi, Mater. Chem. Phys.,117,(2009),14-17[2] N.Yoshikawa, IOP Conference Series: Mater. Sci. Eng., 424,(2018),012041-*Corresponding author: yoshin@material.tohoku.ac.jp (N. Yoshikawa)O-11
A monoenergetic energy-tunable positronium beam and its applications
Y. Nagataa,*, K. Michishiob and Y. Nagashimaaa Department of Physics, Tokyo Univsersity of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601,

Japan
b National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba,

Ibaraki 305-8568, Japan

The positronium (Ps) atom is a hydrogen-like neutral system composed of anelectron and a positron. The ground state splits into singlet (para-Ps) and triplet(ortho-Ps) spin states. They decay into gamma rays due to pair annihilation with very
short lifetimes of 125 ps and 142 ns in a field free region, respectively [1]. Ps atoms
are usually produced in materials such as gases, polymers, and liquids when
positrons are injected into them. When metal surfaces are bombarded with positrons,
they penetrate into the bulk, lose their energy and diffuse back to the surface. Someof the positrons are then converted to Ps atoms at the surface by capture of electronsand emitted.
For the detailed study of interactions between Ps atoms and materials,
monoenergetic and energy-tunable Ps beams are needed. However, Ps atoms
cannot be accelerated because they are electrically neutral. So far monoenergetic Ps
beams have been produced in the energy range of 7 - 400 eV by using a chargeexchange reaction of energetic positron beams with gaseous atoms or molecules [2].
Unfortunately, extension to higher energy range has been limited because of
decrease of the charge-exchange cross section.
Recently a high quality energetic Ps beam in the energy range of a few keV has been
available by using photo-detachment of positronium negative ion (Ps-) [3,4]. Ps- is athree-body system composed of two electrons and a positron and can be acceleratedusing an electric field easily. This achievement was made possible by dramatic
improvement of the efficiency for Ps- production [5]. When positrons are injected into
a tungsten surface coated with alkali-metal, the production efficiency of Ps- increases
to around 200 times larger than that obtained for uncoated surface. This method wasapplied for the first observation of the shape resonance of Ps- [6]. In our present
system, positrons from a 22Na source are moderated and trapped by a buffer-gas
trap [7]. The positrons are extracted from the trap as pulsed slow positrons
synchronized with the laser for the photo-detachment [4].
In this talk, the details of our Ps beam system and its possible applications will bepresented.
[1] M. Charlton and J. W. Humberston, Positron Physics, Cambridge Monographs on
Atomic, Molecular and Chemical Physics (2008)[2] G. Laricchia and H. R. J. Walters, La Rivista del Nuovo Cimento 35 (2012) 305[3] K. Michishio et al., Appl. Phys. Lett. 100 (2012) 254102[4] K. Michishio et al., Rev. Sci. Instrum., 90 (2019) 023305[5] Y. Nagashima et al., Phys. Rep. 545 (2014) 95[6] K. Michishio et al., Nature Communications 7 (2016) 11060[7] R. G. Greaves and J. Moxom, AIP Conf. Proc. 692 (2003) 140*Corresponding author: yugo.nagata@rs.tus.ac.jp (Y. Nagata)O-12
Anomalous Properties of Vacancy in Tungsten and Interaction with Hydrogen
K. Ohsawaa,*, T. Toyamab, Y. Hatanoc, M. Yamaguchid, and H. Watanabea
a Institute for Applied Mechanics, Kyushu University, Kasuga-koen, Kasuga, Fukuoka, 816-8580,

Japan
bInstitute for Materials Research, Tohoku University, Ibaraki, 311-1313, JapancHydrogen Isotope Research Center, Organization for Promotion of Research, University of Toyama,

Toyama, 930-8555, Japan
dCenter for Computational Science and e-Systems, Japan Atomic Energy Agency, Tokai-mura, Ibaraki,

3191195, Japan

Tungsten (W) is a plausible candidate as the divertor materials of the ITER and
MEDO reactors which would be exposed to high-flux plasma particle and neutron
irradiation. W is considered to be a promising plasma facing materials (PFMs)
because of the high melting point, high thermal conductivity, and low hydrogen (H)solubility etc. H or H isotopes used in the fusion reactors as fuels usually do notaccumulate in W specimens due to the low H solubility. However, a large amount of
deuterium (D) retention were observed in the damaged zones of irradiated W
materials. Tritium (T) retention in the PFMs will be a serious problem for the future
fusion reactors. One of the candidates of traps for the H isotopes is vacancy-typelattice defects nucleated under the irradiation circumstance. So, we investigated theinteraction of monovacancy with H atoms in terms of first-principle calculations basedon density functional theory. Then, we found and examined two anomalous
properties associated with a monovacancy in a bulk W.
H atoms can be accommodated in the vacancy-type lattice defects in bcc transition
metals. Firstly, we investigated abnormal stable configurations of H atoms trapped ina W monovacancy [1, 2]. An interstitial H is located at a tetrahedral interstitial site (Tsite). On the other hand, H atoms trapped in a monovacancy in bcc metals arelocated close to the octahedral interstitial sites (O-site) next to the monovacancy.
Therefore, a maximum of 6 H atoms can be accommodated in the monovacancy in
bcc lattice because 6 O-sites exist next to the monovacancy. However, stableconfigurations of H atoms trapped in a W monovacancy are remarkably reorganized
as the number of H atoms increases. H atoms are largely shifted from the O-sites
and they are finally located in the vicinity of T-sites. As a result, a maximum of 12 H
atoms can be accommodated in the W monovacancy. This abnormal stableconfigurations of H atoms greatly affects the binding energy of H atoms to themonovacancy and zero-point energy of H isotope.
Secondly, we examined the stability of a divacancy in a bulk W [3]. Usually, a
divacancy is supposed to be energetically stable than two separated monovacancies
in metals. However, a repulsive or only slightly attractive interactions between twomonovacancies is predicted in W by first-principle calculations. We confirmed therepulsive interaction by the calculations performed in a large supercell. On the other
hand, the divacancy is considerably stabilized by H atom trapping.
[1] K. Ohsawa, J. Goto, M. Yamakami, et al., Phys. Rev. B 82 (2010) 184117.
[2] K. Ohsawa, K. Eguchi, H. Watanabe, et al., Phys. Rev. B 85 (2012) 094102.
[3] L. Ventelon, F. Willaime, C.-C. Fu, et al., J. Nucl. Mater. 425 (2012) 16.
*Corresponding author: ohsawa@riam.kyushu-u.ac.jp (K. Ohsawa)

O-13
Molecular Dynamics Simulation on Interactions between Hydrogen and
Vacancy Clusters in Tungsten
Takuji Odaa,*a Seoul National University, Republic of Korea,

Tritium retention and leakage behaviors in fusion reactor materials such as
tungsten and steels are relevant with the safety and sustainability of fusion reactors.
In order to accurately predict the tritium behaviors during reactor operation,
understanding of interactions between hydrogen isotopes and irradiation defects isneeded. Since vacancies basically act as strong traps of hydrogen in metals, many
computational studies have been devoted to quantify the trap effects of vacancies,
especially of mono-vacancies [1]. However, the knowledge on vacancy clusters is still
limited, probably due to the fact that the first-principles calculation cannot easily deal
with a large vacancy cluster and the accuracy of available potential models for H-V
interaction is limited in molecular dynamics (MD) calculation.
In the present study, to investigate interactions between hydrogen andvacancy clusters in tungsten, we first developed a potential model with putting a highpriority on reasonable description of H-V interaction energies. The potential model
was fitted to energies, forces and stresses calculated by first-principles calculation
using VASP code. Subsequently, with a developed potential model, MD calculations
were performed using LAMMPS code. Systems of various hydrogen and vacancy
concentrations were simulated. Two sorts of initial vacancy configurations wereprepared: (1) vacancies are agglomerated as a single cluster and (2) vacancies are
isolated as multiple mono-vacancies. The effective diffusivities of hydrogen, which isa good index to quality the strength of H-V interaction [2], and the H/V ratios in H-V
complexes were evaluated,
The MD simulations showed that the hydrogen trapping energy in a H-V
complex decreases as the number of trapped H atoms increases and increases as
the number of involved vacancies increases. The trapping energy increases up toaround the hydrogen trapping energy on a surface. The effective diffusivity ofhydrogen is significantly reduced by vacancies. The reduction is often moresignificant when vacancies are isolated, not clustered. The trapping energy isreasonably correlated with the ratio of the number of trapped H atoms to the numberof effective trapping sites in a vacancy cluster, which is calculated from the surface
area of the vacancy cluster, rather than the number of vacancies involved in thevacancy cluster.

[1] For example, K. Ohsawa et al., Phys. Rev. B 82 (2010) 184117.
[2] D. Zhu and T. Oda, J. Nucl. Mater. 469 (2016) 237.
*Corresponding author: oda@snu.ac.kr (T. Oda)

O-14
Deuterium Retention in W and Binary W Alloys Irradiated withHigh Energy Fe Ions
Y. Hatanoa,*, J. Wanga, T. Hinokib, A. V. Spitsync, N. P. Bobyrc, S. Kondod,
T. Toyamae, H. T. Leef, Y. Uedaf
aHydrogen Isotope Research Center, Organization for Promotion of Research, University of Toyama,

Toyama 930-8555, JapanbInstitute of Advanced Energy, Kyoto University, Kyoto 611-0011, Japan
cNRC “Kurchatov Institute”, Kurchatov sq. 1, Moscow 123182, Russia
dInstitute for Materials Research, Tohoku University, Sendai 980-8577, JapandInstitute for Materials Research, Tohoku University, Oarai 311-1313, Japan
fGraduate School of Engineering, Osaka University, Suita 565-0871, Japan

Trapping of hydrogen isotopes at radiation-induced defects in W is an important
problem in evaluating tritium inventory in a vacuum chamber of future fusion reactor.
In our previous study, deuterium (D) retention in W-5Re alloy after irradiation with Fe
ions to 0.5 dpa was compared with that in pure W [1]. The addition of Re to W
strongly reduced the concentration of vacancy-type defects and consequently D
retention at irradiation temperatures ≥ 773 K [1]. According to DFT calculations by
Suzudo et al. [2] a Re atom can form a dumbbell cluster with a W self-interstitial atom
(SIA) to reduce the SIA’s effective mobility and enhance recombination with avacancy. However, the dependence of D retention on Re concentration and damagelevel has not been understood. The effects of other elements have not been
examined. In this study, D retention in W-xM alloys (M = Cr, Ta, Mo and Re, and x is
the concentration in at.%) after Fe ion irradiation to 0.5–2.5 dpa was examined
systematically. These alloying elements were selected according to the DFT
calculation of Suzudo et al. [2]. According to their predictions, the interaction of a W
SIA with an atom of Mo and Ta is far weaker than that of Re, while a Cr atom forms adumbbell cluster with a SIA as a Re atom.
Plates of W-(0.1-5)Re, W-(0.5-2.5)Mo, W-(1-5)Ta and W-3Cr alloys were irradiated at1073 K with 6.4 MeV Fe ions to 0.5-2.5 dpa at the damage peak. The irradiatedsamples were exposed to D 2 gas at 0.1 MPa and 673 K, and the retention of D was
measured by thermal desorption spectrometry (TDS).
No clear reduction in D retention was observed by Re addition at/below 1 at.%, whilestrong enhancement of vacancy annihilation was observed at 5 at.% up to 2.5 dpa.
The addition of Mo and Ta resulted in no significant influence in D retention even at
the highest concentrations examined, as expected from the above-mentioned resultsof DFT calculations by Suzudo et al. [2]. The experiments for W-Cr alloy are in
progress and the results will be reported in the presentation.
[1] Y. Hatano et al., Nucl. Mater. Energy, 9(2016)93-97.
[2] T. Suzudo et al., J. Nucl. Mater., 505(2018)15-21.

*Corresponding author: hatano@ctg.u-toyama.ac.jp (Y. Hatano)O-15
Hydrogen isotope exchange between H and D in damaged tungsten
Yoshio Uedaa*, Heun T. Leea, Makoto Oyaba Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871 Japanb Faculty of Engineering Science, Kyushu University, Kasuga, Fukuoka 816-8580 JapanIn ITER, full tungsten divertor will be used due to its low tritium inventory as well as
low sputtering yield compared with carbon based materials. However, tritium
retention during a DT phase could be significantly increased because neutron
irradiation will make strong trap sites uniformly distributed in the bulk of W [1]. One ofpossible methods to remove the retained tritium would be hydrogen isotope
exchange during plasma operation with deuterium [2]. In order to correctly evaluate a
tritium removal rate by isotope exchange with deuterium, understanding of themechanism and appropriate modeling are necessary. But few studies have been
performed about isotope exchange at various tritium trapping sites. In this study, byusing high energy Fe ion damage as a proxy of neutron damage, isotope exchange
between deuterium and hydrogen was investigated.
Recrystallized tungsten specimens annealed at 2073 K for an hour with very low
intrinsic trapping sites (concentration of less than 10-4) were used. Ion damage wasmade at 473 K by 6.4 MeV Fe ion irradiation to about 0.2 dpa (average value within
an ion range), corresponding to trap site concentration of about 10-2. Implantation of
1 keV D ions or H ions at 473 K was performed by an ion irradiation device, HiFIT.
Ion flux and fluence were 1.3 x 1020 m-2s-1 and 1.5 x 1024 m-2, respectively. Under thetemperature (473 K) and the fluence conditions, implanted D or H can fill all the ion
damaged traps. In addition, new traps with their implantation were created. After D(H) implantation, H (D) ions were successively implanted also at 473 K with thefluences up to 1.5 x 1024 m-2 (the same fluence as the first implanted ions). Retentionof D and H was measured by thermal desorption spectroscopy (TDS).
TDS spectra have a broad peak from 473 K (irradiation temperature) to about 950 K.
In these spectra, two recognizable peaks were observed at around 550 K and 750 K,
probably corresponding to vacancies (activation energy of~1.4 eV) and voids(activation energy of~1.8 eV), respectively. It is noted that even at irradiation
temperature of 473 K isotope exchange took place in the whole TDS spectra (up to
950 K). In more detail, as the second ion fluence increased isotope exchange was
firstly observed at low temperatures (weak trap sites), then it extended to hightemperatures (strong trap sites). At the second ion fluence of 1.5 x 1024 m-2 (thesame fluence as first ions), about 75% of first implanted species were removed. No
clear isotope effects were observed, namely D to H exchange and H to D exchange
behaved similarly. Considering difference in vibrational energies between hydrogenisotopes in lattices sites and defects sites, lighter ions may have higher trapping
energies at defects, which may predict that isotope exchange from lighter ions toheavier ions (H to D) is slower than that from D to H. But our results did not showsuch tendency.
[1] Y. Hatano, et al., J. Nucl. Mater. 438 (2013) S114–S119.
[2] J. Roth, et al., J. Nucl. Mater. 432 (2013) 341–347.
*Corresponding author: yueda@eei.eng.osaka-u.ac.jp (Y. Ueda)

O-16
Development of functional material for deuterium permeation observation
under divertor plasma exposures
T. Takimotoa,*, N. Ashikawab,c, D. Morid, K. Katayamad, V. Rohdee, Y. Matsumuraa,
A. Tonegawaa, K.N. Satof, and K. Kawamuraaa Tokai University, 4-1-1 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292, Japanb National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi, Gifu 509-5292, Japanc SOKENDAI, 322-6 Oroshi-cho, Toki-shi, Gifu 509-5292, Japand Kyushu University, 6-1 Kasuga-Koen, Kasuga-shi, Fukuoka 816-8580, JapaneMax-Planck-Institut für Plasmaphysik, 85748 Garching b. München, Germany
fTokyo University of Science,1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

It is known that plasma-induced permeation of hydrogen occurs at the plasma-facing
wall in fusion devices. Accordingly, a quantitative evaluation on a hydrogen transfer
to primary cooling system is necessary for the design of the DEMO. However, there
is no precedent on a direct evaluation of hydrogen permeation in metallic walls underhigh heat flux plasma exposure in fusion devices.
Therefore, in order toquantitatively evaluate the amount of hydrogen permeation in metallic walls during
plasma exposure, the sample was newly developed that combines tungsten (W) andtitanium (Ti).
We have produced the sample which is an integrated structure in which W is coatedon palladium (Pd) coated side on Ti bulk considering the experimental environment infusion devices. W is a prime candidate for divertor plates and plays a role as aplasma-facing material also in this sample. The reason for selecting the coated W is
to facilitate the permeation of deuterium due to its thinness (~0.1 mm). The hydrogen
permeated through W is collected by Ti that is a hydrogen storage material. This is
the mechanism as a key of this new sample to measure the hydrogen permeation.
Pd is a catalyst to facilitate a hydrogen transfer from W to Ti. The main role is to
reduce the dissociative energy required in the process of hydrogen becoming an
atom from a molecule at the interface of Ti.
In the initial samples, blisters on the surface and pores between Pd layer and Ti bulk
by the cross-sectional images were observed. In order for Pd to perform its own role,
it is necessary to strengthen the contact between Pd and Ti. Accordingly, for the
purpose to improve the coating characteristics of Pd, heat treatment was applied tothe sample after Pd coating. However, an issue was arisen that the Pd layer
becomes unclear after heat treatments of 650 ℃. Therefore, we are promoting theimprovement by temperature adjustment. We plan deuterium plasma exposures tothe improved samples in the linear plasma device TPD-Sheet IV [1]. Retained
deuterium permeated through coated W to Ti is evaluated by Glow discharge opticalemission spectrometry (GD-OES).
In this presentation, we report that the improvement of Pd layer by heat treatments
and retained deuterium after plasma exposure to the improved samples.

[1] T. Hayashi, T. Takimoto, et al., Fusion Engineering and Design, 136 (2018) 545.

*Corresponding author: 2bsp1208@hope.tokai-u.jp

P-1
Vacancy clustering and stabilization in tungsten by hydrogen trapping
Daiji Katoa,b,*
a National Institute for Fusion Science, Toki, Gifu 509-5292, Japana Department of Advanced Energy Engineering Science, Kyushu University, Fukuoka 816-8580, Japan

Tritium retention in tungsten is a key issue to qualify the full-tungsten option for
DEMO reactors. Tungsten is known as a highly non-occluder metal to hydrogen,which is one of advantages for divertor materials. However, tritium will be trapped byvacancies created by plasma irradiations and fusion neutrons resulting in an increaseof tritium retention. Nano-blistering of tungsten surfaces exposed to high-flux
deuterium plasmas and enhanced deuterium retention at elevated temperatures areobserved [1, 2]. Recently, deuterium retention in tungsten is investigated by self-ion
damage experiments with co-injection of deuterium atoms [3]. Influence of hydrogenon vacancies and on its evolution in tungsten is a key issue to predict the tritium
retention of DEMO reactors.
In this study, hydrogen (H) mediated vacancy (V) clustering and stabilization intungsten (W) is investigated based on first-principle calculations of mutualinteractions of W-H-V ternary system. The calculations based on density functional
theory using VASP code [4,5] are corrected by adding surface exchange-correlationenergy correction. The first-principle calculations predict that the di-vacancy
formation in tungsten is energetically unfavourable. However, the di-vacancy
formation is facilitated remarkably by trapping hydrogen atoms [6]. The formationenergies for the di-vacancies trapping single hydrogen atom coincide with anexperimental value for the di-vacancy in the first-nearest-neighbour configuration [7].
Similar calculations are conducted for Tri-vacancies [8]. Our calculations predict alsothat hydrogen clusters stacking with a mono-vacancy suppresses annihilation of thevacancy-SIA (111-crowdion) pair in tungsten [9].
[1] V.Kh. Alimov et al, J. Nucl. Mater 375 (2008) 192[2] L. Buzi et al, J. Nucl. Mater 455 (2014) 316[3] S. Markelj et al., Nucl. Mater. Energ. 12 (2017) 169[4] G. Kresse and J. Furthmüller, Phys. Rev. B 54 (1996) 11169[5] G. Kresse D. and Joubert, Phys. Rev. B 59 (1999) 1758
[6] D. Kato, H. Iwakiri and K. Morishita, J. Nucl. Mater 417 (2011) 1115
[7] J.Y. Park et al, Philos. Mag. A 48 (1983) 397[8] D. Kato, 3rd MoD-PMI, 22-24 May 2017, Forschungszentrum Jülich, Germany.
[9] D. Kato et al., Nucl. Fusion 55 (2015) 083019*Corresponding author: kato.daiji@nifs.ac.jp (D. Kato)

P-2

The defect formation mechanisms and change of the mechanicalproperties under the irradiation in pure W and W-Re alloy: Molecular
dynamics study
Hyung Gyu Leea, Byeongchan Leeb, Keonwook Kanga,*a Department of Mechanical Engineering, Yonsei University
b Department of Mechanical Engineering, Kyung Hee University

Tungsten is used plasma-facing material, because of good material properties forwithstanding extreme operating condition. Under the irradiation condition, defects
nucleation and transmutation into Re or Os occur, which causes the material properties
changes. So, it is need to understand the material properties change and defectformation mechanisms of W and W-Re alloys under the irradiation. In this study, we
performed molecular dynamics simulations to investigate the defect formation
mechanisms and changes of the mechanical properties in bulk W and W-Re alloys.
Three different temperatures (400, 700, 1000 K) are chosen to analyse the temperatureeffect on the displacement cascades. In all trials, irradiation dose reaches up to about
0.1 dpa with a PKA energy of 2 keV. As increasing the irradiation dose, Young’s
modulus and bulk modulus decreases. Also, formation mechanisms of 1/2 <111> or<100> dislocation loop are observed under the cascade overlapping simulation.

Acknowledgement
This research was supported by Nuclear Fusion Research Program through the National
Research Foundation of Korea (NRF) (2017M1A7A1A01016221)

Presenting author: cptp123@nate.com*Corresponding author: kwkang75@yonsei.ac.kr

P-3
Molecular dynamics study of materials properties of pure W and W-10Re alloy
before and after irradiation
Jie Nana,Hyung Gyu Leea ,Byeongchan Leeb, Keonwook Kanga,*a Department of Mechanical Engineering, Yonsei University
b Department of Mechanical Engineering, Kyung Hee University

Tungsten is expected to be used as plasma facing material (PFM) of fusion reactors,
because W(tungsten) has high melting point, thermal conductivity and sputtering
resistance. But some tungsten atoms transmute into the Re atoms and form some
defects after the neutron irradiation. These lead to changes in material properties.
Therefore it is necessary to understand the changes of material properties withtemperature and irradiation. In this study, we calculate mechanical properties(Young’s modulus and bulk modulus), thermal properties (thermal expansion
coefficient, melting temperature) using molecular dynamics simulations of pure W
and W-10Re alloy from 0K to 3000K. Before irradiation, pure W has larger Young’s
modulus and higher melting point than W-10Re alloy. After irradiation, pure W haslower melting point and smaller Young’s modulus.
Acknowledgement
This research was supported by Nuclear Fusion Research Program through the
National Research Foundation of Korea(NRF) (2017M1A7A1A01016221)
Presenting author:nanjie@yonsei.ac.kr*Corresponding author:kwkang75@yonsei.ac.kr

P-4
Inclusion of Electronic Stopping and Validation in the Open Source Large-scale
Atomic/Molecular Massively Parallel Simulator (LAMMPS)
H. Hemania,*, U. Bhardwaja, A. Majaleea, K. Nordlundc and M. Warriera,ba Computational Analysis Division, Bhabha Atomic Research Centre, Visakhapatnam, Andhra Pradeshb- 531011, India
Homi Bhabha National Institute, Anushaktinagar, Trombay, Mumbai – 400085, Indiac Accelerator Laboratory, Pietari Kalmin katu 2 P.O.Box 43 Helsinki, Finland

Electronic stopping (ES) of energetic atoms is not taken care of by the interatomic
potentials used in molecular dynamics (MD) simulations. Several methods forinclusion of ES in MD simulations have been developed [1,2,3,4,5]. We use theLindhard-Scharf (LS) formula [6] for electronic stopping and include it as a drag term
for energetic atoms in the open source code LAMMPS. In order to validate themodified code, the total loss due to electronic stopping during the MD simulations iscompared with the electronic stopping losses predicted by the NRT model [7] which
also uses the LS formula. MD simulations of collision cascades at primary knock-on
atom (PKA) energies of 5, 10 and 20 keV are carried out in W and Fe in 100 random
directions. It is seen that the total ES losses from the MD simulations averaged overthe 100 directions matches the loss predicted by the NRT model very well. The root
mean square error from the MD implementation is around 10 % for both W and Fe in
the PKA energy range explored.
An important parameter in the MD implementation is the cut-off energy (E CutES ) below
which there is no electronic stopping. We expect this to be of the order ofdisplacement energy (E D ) of the material, since ES occurs only if there is a physicaldisplacement of atom from its lattice site to another location straddling a neighbouringatom/atoms. We have earlier obtained direction averaged displacement probability(P(E D )) for atoms in W [8] and Fe by carrying out MD simulations in 1000 random
directions in the PKA energy range 30-500 eV. It is seen that there exists a minimum
energy, E dmin, below which the probability of displacement is 0 and a maximum
energy, E dmax, above which the probability of displacement is 1. We show that the ES
losses from MD matches the ES losses from the NRT model when E cutES equals theaverage value of E DMin and E dmax , for both W and Fe.
[1] O.B. Firsov, Soviet Physics JETP, 36 (9) (1959) 1076
[2] M.W. Finnis, P. Agnew and A.J.E. Foreman, Phys. Rev. B, 44 (2) (1991) 567[3] A. Caro and M. Victoria, Phys. Rev. A, 40 (5) (1989) 2287
[4] K. Nordlund, et al., Phys. Rev. B, 57 (13) (1998) 7556
[5] D. R. Mason et al, NIMPR-B, 269 (2011) 1640
[6] J. Lindhard and M. Scharff, Phys. Rev. 124 (1) (1961) 128[7] M.J. Norgett, M.T. Robinson, I.M. Torrence, Nucl. Engg. and Design, 33 (1975) 50[8] M. Warrier, U. Bhradwaj, BARC report, BARC/2018/I/005 (2018), Submitted for
publication.

P-5
Investigation of temperature and ion energy threshold for tungsten fuzz growth
by Monte Carlo Simulation
Q. Shia,*, and S.Y. Daiba Graduate School of Engineering, Nagoya University, Furo-cho, Nagoya 464-8603, Japanb Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Ministry of Education),

School of Physics, Dalian University of Technology, Dalian, 116024

Tungsten has been considered as the primary candidate for the wall material of thenext generation fusion devices. The advantages of low sputtering yield, high melting
point and excellent thermal conductivity are crucial factors for fusion operation
environment. However, in recent decades, nanostructures on tungsten surface called‘fuzz’, which is known to degrade the good properties of tungsten, have been
discovered under helium plasma irradiation in many experiments [1,2]. Numbers ofstudies has been done to investigate the mechanism of fuzz growth in bothexperiment and simulation method. Ion inducement and migration is wide believed as
the main reason of fuzzy structure generation [3]. A window of temperature andincident ion energy for tungsten fuzz growth has been rivaled [4]. The mechanism offuzz growth can partly explain this parameter window, but the comprehension ofthese thresholds which fuzz can grow is still deficient.
We investigate this issue by using the three dimensional Monte Carlo code named
SURO-FUZZ, which has simulated the early stage of tungsten fuzz growth in 1073K[5]. In this study, surface morphology is presented at different surface temperatureand incident ion energy compared with experimental observations. The results
indicate that diffusitive of helium particles and the capacity of tungsten for holding
the bubble presure play important roles in the various behaviour of sungsten surface(nanobubbles, fuzz, large voids).

[1] S. Tamura, N. Ohno, D. Nishijima and S. Kajita, Plasma Fusion Res. 1 (2006) 051[2] S. Kajita, S. Takamura, N.Ohno, D. Nishijima, H. Iwakiri and N. Yoshida, Nucl.
Fusion 47 (2007) 1358-1366[3] A.M. Ito, A. Takayama, Y. Oda, T. Tamura, R. Kobayashi, T. Hattori, S. Ogata, N.
Ohno, S. Kajita, M. Yajima, Y. Noiri, Y. Yoshimoto, S. Saito, S. Takamura, T.
Murashima, M. Miyamoto and H. Nakamura, Nucl. Fusion 55 (2015) 073013[4] G. De Temmerman, T. Hirai and R.A. Pitts, Plasma Phys. Control. Fusion 60(2018) 044018[5] Q. Shi, S.Y. Dai and D.Z. Wang, Fusion Eng. Des. 136 (2018) 554-557*Corresponding author: shi.quan@k.mbox.nagoya-u.ac.jp

P-6
Study of proton irradiation in wurtzite GaN with a-type prism-edge dislocation
using molecular dynamics simulations
S. Yooa, S. Oha, and K. Kanga,*a School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro Seodaemun-gu, Seoul, 03722,

Repulbic of Korea

LED and HEMT (High Electron Mobility Transistor) based on gallium nitride (GaN)
was introduced in the aerospace industry decades ago because of outstanding
features including large bandgap and high electron mobility. GaN also shows highradiation hardness, which is a major property considered in aerospace applications.
In order to investigate radiation damage on GaN during operating in the space,researchers conducted experiments [1, 2]. From these studies, the relation between
the degradations of properties in applications and non-ionizing energy (NIEL) wereproved. Simulations were also carried out to improve the comprehension of themechanism of radiation damages [3, 4]. However, dislocations in the GaN, which
occurred necessarily due to the lattice mismatch in the experiments, were notconsidered in the simulation.
In this study, Primary Knock-on Atom (PKA) simulations with and without a-type
prism-edge dislocations are conducted in order to predict displacement damage onbulk wurtzite GaN at 300K. The PKA energies coming from protons are selectedbased on the results of GEANT4 simulation. PKAs are chosen near the dislocations
under 10 Å and its velocity is applied in the directions of the dislocation core. The
defects statistics, recovery rate and changes of dislocation will be compared anddiscussed.
Acknowledgement
This research was supported by Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT& Future Planning (2017M1A3A3A02015033). And This research was partially
supported by the Graduate School of YONSEI University Research Scholarship
Grants in 2018
Reference[1] Xinwen, H., et al., IEEE Trans. Nucl. Sci. 50 (2003) 1791
[2] Khanna, S. M., et al., IEEE Trans. Nucl. Sci. 51 (2004) 2729
[3] Nord, J., et al., Nucl. Instrum. Methods Phys. Res. B 202 (2003) 93[4] Chen. N., et al., IEEE Trans. Nucl. Sci. 65 (2018) 1108

*Corresponding author: kwkang75@yonsei.ac.kr (K. Kang)

P-7

Development of Potential Model with Three-body Interaction for Tungsten
Woonghee Choa,*, Takuji Odaaa

Department of Nuclear Engineering, Seoul National University, Korea

Tungsten (W) is considered as one of the candidate materials for plasma facing
components in fusion reactors. Molecular dynamics (MD) simulation can be used inunderstanding the behavior of tungsten during the reactor operation. MD simulations
are categorized into two types, classical MD (CMD) and first-principles MD (FPMD),
which are different in the way they calculate the potential energy and atomic forcesneeded to simulate time evolution of a system. In the present study, we propose amethod to systematically construct an accurate potential model for CMD simulationby using calculation results of FPMD, and construct a potential model for tungsten.
In the proposed potential model, we explicitly treat up to three body interaction andtreat many body interactions above four body with an embedded-atom method (EAM)
potential [1]. The coefficients of each term are optimized by minimizing the leastsquare error from a set of energy, force and stress data calculated by FPMD. The
VASP code is used for FPMD based on the density functional theory (DFT). The
Perdew-Burke-Emzerhof (PBE) exchange correlation functional is utilized.
After construction of the potential model, physical quantities are calculated by CMD
using the LAMMPS code. Subsequently, the quality of the potential model isevaluated by comparing the calculation results with experimental data and firstprinciples calculation results. To be specific, equilibrium lattice constant, cohesiveenergy, surface energies, bulk modulus, vacancy formation energy, and the melting
point are examined. The comparison shows reasonable correspondence for allproperties tested. The constructed potential model can reproduce (100) surface
reconstruction [2].
Finally, the calculation cost of the developed potential model is compared with thecosts of other existing potential models for tungsten. The developed potential model
has one-fourth speed of a bond-order potential [3], which also uses many body andangular interaction. This comes from the interpolation scheme that is used for threebody terms in the present potential model, which we plan to improve in a future work.
[1] M. S. Daw and M. I. Baskes, Phys. Rev. B 29 (1984) 6443.
[2] K. Heinola and T. Ahlgren, Phys. Rev. B 81 (2010) 073409.
[3] X.-C. Li et al., J. Nucl. Mater. 426 (2012) 31.

*Corresponding author: yeschowh@snu.ac.kr (Woonghee Cho)

P-8
Recent Machine Learning Algorithms for Classification of Time Series:
Recurrent Deep Learning Networks with Potential for Plasma Applications
L. Pichla,* and T. Shintatea
a International Christian University, Osawa 3-10-2, Mitaka, Tokyo, 181-8585, Japan

Applications of deep learning have recently become common and achieved
substantial interest in various fields of science, such as plasma disruption modelling[1] and solar flares prediction [2].
In the proposed poster we review the recent recurrent neural network architecturesfrom deep learning, such as the LSTM and GRU models [3] and focus on the
regression and classification performance of related machine learning architectures
for the case of time series. In particular, we illustrate the performance of our recent
method [4] for autoregressive processes using the example of non-stationary
cryptocurrency time series data. Preliminary results may also be shown for simple
time series related to plasma applications.
We hope to establish possible collaboration in using the recurrent deep learningneural network techniques for plasma modelling applications.

[1] Kates-Harbeck, J., Svyatkovskiy, A., and Tang, W., Nature (2019), DOI:
10.1038/s41586-019-1116-4[2] Huang, X., Wang, H., Xu, L. et al., Astrophysical Journal, 856 (2018) 7[3] Subramanian, V., Deep Learning with PyTorch: A practical approach to building
neural network models using PyTorch, Packt Publishing, Birmingham, UK, 2018[4] Shintate, T. and Pichl, L., J. Risk Financial Manag. 12(1) (2019) 17*Corresponding author: lukas@icu.ac.jp (L. Pichl)

P-9
NIFS Atomic and Molecular Numerical Database for Collision Processes in
PlasmasI. Murakamia,b*, M. Katoa, M. Emotoa, H.A. Sakauea, and D. Katoa,caNational Institute for Fusion Science, Toki, Gifu 509-5292, JapanbDept. of Fusion Science, SOKENDAI, Toki, Gifu 509-5292, Japan
cDept. of Advanced Energy Enginnering Science, Kyushu University, Fukuoka 816-8580, Japan

We have constructed and opened NIFS Atomic and Molecular Numerical
Database for collision processes for public at http://dbshino.nifs.ac.jp/. Atomic andmolecular (AM) data compilation started in 1970s [1] and the AM database system
was first built in computer in 1980s [2]. Web accessible system with oracle relational
database managing system was started in 1997 [3]. The database system was rebuiltwith PostgreSQL and Ruby on Rails with JavaScript in 2017. We improved userinterfaces introducing simple table skipping complex query form. This allows to findavailable data easily especially for molecular data. Numerical data can be seen inbrowser and graphical output is also available.
The database consists of 9 sub-databases, i.e. AMDIS ION (electron-impact
ionization cross sections/rate coefficients, 2,943 records stored), AMDIS EXC(electron-impact excitation cross sections/rate coefficients, 725,718 records), AMDIS
REC (recombination rate coefficients, 27,534 records), AMDIS DIO (dissociation
cross sections, 202 records), CHART (ion-atom collision cross sections, 7,618
records), AMDIS MOL (electron – molecule collision cross sections/rate coefficients,
3,187 records), CHART MOL (atom – molecule collision cross sections, 2,139
records), SPUTY (sputtering yields, 2,349 records), and BACKS (back scattering
coefficients, 485 records). Almost all data are compiled from publications, andinformation on papers are attached to the data records. Such publications with AMdata are searched by human and extracting data to make data files are also done byhuman. Thus man-power is needed for data update. We tried to use data mining
concept to search such publications many years ago, but there were many problems.
We need such tool with a new technic to find publications to extract valid AM data.
Recently we have started to implement our database to Virtual Atomic andMolecular Data Centre (VAMDC) [4]. At the moment AMDIS ION is linked to VAMDC
portal. Other sub-databases will be linked in future. This new activity will help users
who need AM data for their researches.
Acknowledgements: This work is partly supported by NINS program of
Promoting Research by Networking among Institutes (Grant Number 01411702). We
acknowledge Prof. M.-L. Dubernet and Dr. N. Moreau for their help to implement the
database to VAMDC.
[1] K. Takayanagi, IPPJ-DT-50, report or IPPJ, Nagoya University (1974).
[2] T. Kato et al., Phys. Scr. 23 (1981) 198.
[3] I. Murakami et al., NIST Special Publication 926, p.57 (1997)[4] VAMDC, http://www.vamdc.org/*Corresponding author: tel.: +81 572 58 2262, e-mail: mizumi@nifs.ac.jp (I.
Murakami)

P-10

Observation of trapped hydrogen in coated tungsten on titanium
specimen
Naoko Ashikawaa,b*, Toshikio Takimotoc, Akira Tonegawac,
Yoshihito Matsumurac, Kazunari KatayamadaNational Institute for Fusion Science, Toki, 5095292, JAPANbSOKENDAI, Toki, 5095292, JAPANcTokai University, Hiratsuka, 2591292, JAPANdKyushu University, Kasuga, 8128581, JAPAN

Estimations of leakage tritium amounts from plasma facing side to the coolant isimportant issue in DEMO [1]. In general, hydrogen isotope permeation is measured
by a quadrupole mass spectrometry (QMS), which can detect widely integrated atregions around target areas [2]. Permeation measurements are not sufficient data
evaluated directly the hydrogen permeation amount in metallic walls under high heatload plasma irradiation in magnetic confinement fusion devices.
Therefore, in order to quantitatively evaluate the amount of hydrogen permeationin the metallic wall during high heat load plasma irradiation, it is designed that a
specimen, which was combined titanium (Ti) as a hydrogen trapping material, andtungsten (W) as one of candidate materials for plasma facing walls in DEMO. For
high efficiency of hydrogen permeation from the W to Ti, palladium (Pd) is used as ahydrogen permeation catalyst between Ti and W.
Hydrogen isotope observations are planned from a plasma facing side and abackside of the specimen, which is Ti plate without connected to Pd layer, using
sputtered analytical method such as glow discharge optical emission spectroscopy(GD-OES). It is required that optimizations of thickness of W, Pd layers, and Ti forsufficient intensities of deuterium.
In the case of thin Ti plate about 0.1 mm, a surface of the specimens show
distortion for subsequent Pd and W coatings. Hence, the current design uses Ti plateof 0.5 mm. After deuterium plasma exposure in a linear plasma facility, TPD-Sheet IV,
in Tokai University [3], deuterium at Pd side on Ti was detected. In this presentation,observations of deuterium at the backside of the specimen are shown, andcharacterizations of the specimen are discussed.
[1] K. Katayama, et al., Fusion Science and Technology, 71 (2017) 261.
[2] H.S. Zhou et al., Journal of Nuclear Materials, 455 (2013) 470.
[3] T. Hayashi, T. Takimoto, et al., Fusion Engineering and Design, 136A (2018) 545.
*Corresponding author: ashikawa@nifs.ac.jp (N. Ashikawa)