S9-5
International Collaborative Study on
Edge Plasma and Plasma-Wall Interaction by Using Linear Plasma Device
toward Steady State Operation
直線型装置を用いた定常運転のための周辺プラズマ・PWI 国際連携研究
Noriyasu Ohno
大野哲靖
*Graduate school of Engineering, Nagoya University, Furo-sho, Chikusa-ku,Nagoya, Aichi 464-8603, Japan
名古屋大学工学研究科,	 〒464-8603	 名古屋市千種区不老町	 	 
Linear plasma devices (LPDs), sometimes called divertor plasma simulators, have contributed greatly tounderstanding of egde plasma physics and plasma-wall interactions (PWI) in fusion devices toward a steady state
operation. In addition, research in LPDs aimed at ITER and future DEMO is already underway. This presentation
describes the present status of international collaboration study by using LPDs especially under IEA Implementing
Agreement on the Development and Research on Plasma Wall Interaction Facilities for Fusion Reactors (PWI IA), andsome of the key contributions of the research performed in LPDs. Further, the next-steps facilities based on LPDs aimed
at obtaining long-tem solutions for future fusion reactor will be reported.

1. Introduction
As nuclear fusion research continue to progress
toward burning plasma, ITER and DEMO, moreand more stress is placed on the systems
surrounding the confined plasma. However, present
fusion devices are not able to solve all criticalissues, which are relevant to ITER and DEMOdevices.
Linear plasma devices (LPDs), sometimes called
divertor simulators, are able to bridge this gap
between present plasma confinement devices andfuture ITER and DEMO. This presentation
describes the benefit of LPDs including key
contributions, and the present status ofinternational collaboration studies.
2. Property of Linear Plasma Devices
LPDs has good properties as follows:- Ability to produce plasma relevant to future
devices at relatively low cost
- Steady-state plasma production allowing
equilibration between the plasma and the material- Ample diagnostic access to the plasma-materialinterface- Ability to easily vary individual exposure
parameters to allow systematic investigations- Ability to control plasma composition and add
controlled amounts of impurities
Fig. 1 shows operated or planned LPDs in theworld. Excellent research on divertor simulation

and PWI have been done in LPDs by utilizing theircharacteristics based on uniqueness and innovativeideas. For examples, PISCES-B in UCSD canhandle beryllium (Be) with in-situ surface analysis
and Be effusion cell for impurity seeding.
MAGNUM-PSI in DIFFER has high B field
generated by superconducting coils and producestransient heat load like ELMs. TPE in Idaho
National Lab. can generate tritium plasma andhandle radioactive materials. GAMMA 10/PDX in
Tsukuba Univ. can produce plasma with high ion
temperature and investigate edge-core plasma
coupling.
International collaborative studies using LDSs
have been started under new implement agreement:"Implementing Agreement on the Development andResearch on Plasma Wall Interaction Facilities forFig.1 Linear plasma devices in the world[1].

Fusion Reactors" (in short: PWI IA) under theumbrella of the International Energy Agency(IEA)[2]. The PWI IA aims at contribution to awide rage of activities in the fusion program,including the topics: (1) characterization of plasmaedge properties, (2) development and qualificationof first wall materials, (3) surface characteristics
under intense particle and heat loads, (4)development of diagnostics for edge plasma andmaterial surface characterization, (5) modelling ofplasma wall interaction processes.
3. Key Contributions
Some of the key contributions of the research
performed in LPDs are described. In order toovercome the difficulty to reduce heat loads onto
the divertor plate, the gaseous divertor concept wasproposed. LPDs have contributed extensively to theinvestigation of gaseous divertors associated withplasma detachment [3], because LPDs can generatea steady-state detached plasma with a simple
geometry. It should be mentioned that the QED
LPD at PPPL was the first to verify the feasibilityof the gaseous divertor concept [4]. Fig. 2 shows
the detached plasma observed in the NAGDIS-II
device in Nagoya University. Molecular Assisted
Recombination (MAR) has been firstly observed
under Japan-US collaboration [5].
LPDs also offer the opportunity to begin
investigations related to long-term solutions to PWI
issues. One typical example of this is the surface
temperature dependent response of tungsten(W)material exposed to a plasma containing helium. At
temperature
above
1000
K,
fiber-form
nanostructure W, so called “W fuzz”, was found tobe formed in LPDs as shown in Fig. 3[6]. The W
fuzz becomes one of the critical issues in the next
fusion device, such as ITER, which is intended touse W as a plasma-facing material.

Fig. 4 SEM photos of W fuzz structure formed in
the LPD[6]4. Advanced PWI Study
Transient plasma events in steady state plasma
such as Edge Localized Modes (ELMs) determines thelifetime of plasma-facing materials. To produce
transient plasma events in steady state plasma, thepossibility to superimpose transient plasma pulses tothe steady-state plasma was studied on the NAGDISand Pilot-PSI devices. A Magnetized Co-axial PlasmaGun (MCPG) was mounted on the NAGDIS-I
device[7]. On the other hand, in the PILPO-PSI, thecascaded arc source used to produce the steady-state
discharge is connected in parallel to a pulsed power
supply and the current regulated DC supply[8]. The
discharge power is pulsed in a sub-ms timescale fromthe DC level (~30 kW) to up to 1 MW creating heat
fluxes in excess of 1 GWm-2.
Further, toward DEMO, systematic studies ofhydrogen isotope retention in neutron-damaged
materials will be required. In order to perform thisexperiment, PMTS (Plasma Material Test Stand) at
ORNL is planned and JULE-PSI (Jülich LinearExperiment for PSI studies in a hot cell) in Jülich isunder construction. C_DPS/IG-TDS (CompactDivertor Plasma Simulator/Ion Gun with TDS) at
Tohoku Univ. will be operated in the radiation
controlled area and perform TDS analysis without
air exposure in near future.
References[1] M. Sakamoto., 2nd IAEA DEMO PROGRAMME
WS 2013.
[2] http://www.pwi-ia.org/index.html.
[3] G. Matthews, J. Nucl. Mater., 220 (1995) 104.
[4] W. Hsu et al., Phys. Rev. Lett., 49 (1982) 1001.
[5] N. Ohno et al., Phys. Rev. Lett., 81 (1998) 818.
[6] S. Takamura et al. Plasma Fusion Res. 1 (2006) 051.
[7] S. Kajita et al., J. Nucl. Mater., 438 (2013)S707.
[8] G. De Temmerman et al., Appl. Phys. Lett., 97(2010) 081502.

Fig. 2 Plasma detachment observed in NAGDIS-II in
Nagoya University.