OV/5-5

26th IAEA Fusion Energy Conference
Kyoto International Conference Center, Kyoto, Japan, 17-22 Oct. 2016

Overview of Spherical Tokamak Research in Japan
Y. Takase1, A. Ejiri1, T. Fujita2, N. Fukumoto3, K. Hanada4, H. Idei4, M. Nagata3, Y. Ono1,
H. Tanaka5, A. Fukuyama5, R. Horiuchi6, S. Tsuji-Iio7, Y. Kamada8, Y. Nagayama6, Y. Takeiri6
1U. Tokyo, 2Nagoya U., 3U. Hyogo, 4Kyushu U., 5Kyoto U., 6NIFS, 7Tokyo Inst. Tech., 8QST, Japan

Nationally Coordinated Research Using Various ST Devices**TST-2, TS-3, TS-4, UTST, TOKASTAR-2, LATE, HIST, QUEST + theory/modelingContents• Ip Start-up

Directions of Japanese ST research• RF Waves: ECW/EBW, LHW• CHI• AC OH• Advanced Fueling• CT Injection• Steady-State Operation• Particle Control by High Temperature Metal Wall

• Ultra-High-b Operation• Reconnection Heating by Plasma Merging• Low-A Tokamak-Helical Hybrid• Stability Improvement by Helical Field
1CS-less Ip Start-up by ECW/EBW

EX/P4-45 H. Tanaka, et al.

Non-inductive Production of Extremely Overdense ST Plasma
by EBW Excited via O-X-B Method in LATE
Extremely overdense ST plasmas are produced non-inductively with EBW
excited by O-X-B mode conversion when EBW is excited in the 1st
propagation band (between wce and 2wce layers). Density reaches ~ 6 times
the cutoff density for 5 GHz microwave.

2

Density and soft X-ray emission increases when the 2wce layer is located on
the outboard side of the upper hybrid resonance (UHR) layer, i.e., when
EBW is excited in the 1st propagation band. Such Bt dependence isobserved at two different microwave frequencies.
5GHz Microwave

2.45GHz Microwave

31-D Kinetic Full-Wave Analysis of
O-X-B Mode Conversion by TASK/W1

Wave structurevs. major radius

Integral-operator form of
dielectric tensor to describe
finite Larmor radius effects
Parameters of LATE
B = 0.08 T, ne0 = 1017 m-3

Fukuyama, et al.

Perpendicular wave number
vs. major radius

Antenna loading resistance vs k//
Peakcorresponds tothe optimum
injection angle

f = 2.45 GHz
k// = 32 m-1

4

Non-inductive ECH/CD with Waves at Two
Frequencies (8.2 GHz / 28 GHz) in QUEST

EX/P4-50 H. Idei, et al.

Microwave power was injected at two frequencies (8.2
GHz and 28 GHz) simultaneously for fully non-inductive Ip
start-up and sustainment in QUEST.
Gas puff timings are indicated with dP increments(t = tg1, tg2, tg3 and tg4).
Ha intensity increased at gas puff timings. Density
sometimes increased spontaneously without gas puff.
Spontaneous density jumps (SDJs) were observed twice( t = t SDJ1 and tSDJ2 ) in this discharge, and the density
exceeded the cutoff density for 8.2 GHz.

Equilibrium electron pressure
profile was bell-shaped.
Flux surfaces are densely spacedon the low-field side.

0.4 sOverdense 25 kA plasma withcentral high energy electronpressure was built up noninductively and sustained for 0.4 s.

EFIT

5

High bulk Te was observed near 2wce layer for 28 GHz, while a
local Te peak was observed near wce layer for 8.2 GHz only
before the 1st SDJ at t=1.75 s.
Measured bulk pressure Pe profile was hollow, resulting from theoff-axis heating at the 2wce layer and radiation cooling after
SDJs.

bulk
electron

Current density profile and equilibrium pressure profile (EFIT)
are centrally peaked.
without inboard side

Hard X-ray emission athigh energies (>200 keV)
was observed from thecentral overdense region,
while bulk Pe was larger on
the high-field side off-axis
region near the 2wce layerfor 28 GHz.

tangential view with inboard side

There were abundant
energetic electrons in thecentral overdense region.

Bulk Te and Pe increased in the overdense region where there
are Doppler-shifted wce resonance layers for large N// .
6

Local ECH/CD Non-inductive Ip Start-up in QUESTFull Wave
Simulation
Kirchhoff integral code was used for mirror design.
Mirror performance at 28 GHz was checked by 3-D full-wave simulation.
Sharply focused beam with small beam size (0.052 m) was obtained.
The 2nd focusing mirror can steer the beam in the toroidal direction.
Non-inductive Ip of 70 kA wasachieved using the new launcher.

Focused beammeasured
at low power

Polarization
control for
2nd ECH/CD

28 GHz140 kW
N// = 0.78

7CS-less Ip Start-up by LHW

EX/P4-48 Ejiri, et al.

Two capacitively-coupled combline (CCC) antennas• Traveling LHW is excited directly• Sharp n|| spectrum & high directivity

TST-2

Top-launchIp [kA] PRF [kW] Bt [T](installed in March 2016)
Good coreaccessibility
and deposition
are expectedeven at high ne.

0.2

ne [1018m-3] VL [V]Outboard-launch

Ip = 25 kA achieved withoutboard-launch antennaalone (improved from 16 kA)
0.1
0
80
40
0
20
10
0
0.1
0
-0.1
0.8
0.4
0

LHW
10xECH20

collaboration with Moeller (GA, US)

40
60
Time [ms]808Sustained Ip increases with ne and Bt

Comparison of discharges
sustained by outboard-launch
and top-launch antennas
Ip of up to 13 kA is sustained by
the top-launch antenna alone.
However, precise plasma positionand size control during the initial
Ip ramp-up phase is difficult.

Use outboard-launch for start-up
and top-launch for ramp-up.
(initial experiment in progress)
9EX/P5-21 Nagata, et al.

CS-less Ip Start-up by CHI

Coaxial Helicity Injection (CHI) These ST devices in US and Japan
aim to develop and understand noninductive CHI start-up/ramp-up that isnecessary for the viability of ST
reactors.

HIT-II(U. Washington)

NSTX(PPPL)

HIST

QUEST

(U. Hyogo)(Kyushu U.)

 The successful current generation up
to 0.3 MA in NSTX (PPPL) validated
the capability of CHI for start-up
followed by inductive ramp-up to 1 MA.
 A new CHI start-up experiment with analternate electrode and insulator configuration, combined with ECH, will
start on QUEST (Kyushu U.) under
US-Japan collaboration.

 Primary purpose of CHI experiments
on HIST (U. Hyogo) is to examine thephysics of flux closure and current
profiles which still remain key issues of
CHI.
10Characteristics of ST Plasmas Generated by CHI on HIST
low bias flux
high bias flux
It : toroidal
plasma current

Comparison of CHI generated
plasmas between high and low
bias flux operations.

 Peak Ip of 80-120 kA wasgenerated by CHI. A stable closed
jt(R) : toroidal
current density
flux formation was achieved in thehigh bias case.
high bias flux
low bias flux

 In the low bias case, the toroidal
current density is concentrated on
Ti.D : Doppler ion
the inboard side, leading to kinktemperature
instabilities at a later time.
Te : electron
temperature Ti.D during the Ip rise phase in the
low bias case is higher than Te ~ 10eV, indicating ion heating.

ne : electron
density ne was ~1×1020 m-3.
11

Plasmoid Formation and Flux Closure in CHI Plasmas

HIST
2-D contour plots
Poloidal flux Ypt

Plasmoids

X point

Toroidal current density JtExperimental results frominternal magnetic probe
measurements
1. Formation of closed flux surfaces(c) was vefified. The ratio of theclosed flux to the total flux is 2530%.

2. Small-scale plasmoids (a), (b)are generated in the elongatedtoroidal current sheet (d), (e) in
Elongated current sheet
the presence of a strong Bt.
3. The plasmoid grows in size due
to inward diffusion (e), (f) of thetoroidal current in the open flux
Inward diffusion
region during the decay phase .

12

Ip Start-up by AC Ohmic Heating

EX/P4-48 Ejiri, et al.

AC Ohmic Heating Experiments on TST-2(for pre-ionization and DC current drive)
VL

Y

t

Ip ?

t

Small flux swing
Resistive V.V.
Inconel 625 (t=1.57mm)
1/e frequency: 133 kHz
DC
CapacitorPS
IGBT
H-bridget Cumulative effect~to~increase ne (pre-ionization)?
 Heating power: <IpVL> → DC current drive?

13

DC current drive

Pre-ionization#137292

DC Ip component (<1.9 kA)
appears when BZ is applied

Ip [kA]

0.4
0

+10

VL [V]VL [V]

-0.4
1

0

0

-10
+10

0

-1

Radiation & Visible[a.u.]

Ip [kA]100

Vis PD

10-2

Rad
Vis. PMT

10-4
10-6
20

22

24
26
Time [ms]2830

Exponential growth in emission and Ip
 The growth rate and saturation in Ip ne
can be explained qualitatively by amodel based on Townsend’s a.
 Successful pre-ionization with
|Vloop| ≥ 0.5 V (~0.6 V/m).

DC component

-10
2
1
0

Bz [mT]

10

20
30Time [ms]40Time averaged
visible camera
image shows an
ST configuration.
0.26 mInboardboundary
14

Advanced Fueling by CT Injection

PD/P-16 Fukumoto, et al.

CT Injection (CTI) as Advanced Fueling on QUEST
Compact toroid (CT) injection experiments havebeen conducted to develop an advanced fuelingmethod.
CT
injection

A CT plasma with high density (up to
the order of 1021 m-3) was injected
perpendicularly from the outboard
midplane along the major radius at a
speed > 200 km/s.

UH-CT injector used in this experiment iscapable of injecting a CT plasma that canpenetrate to Bt = 0.8 T.

BT=0.8 T
WB = 255 kJ/m3← JFT-2M← QUEST
BT = 0.5 T
WB = 99 kJ/m3
for pulse mode15

CT Injection into OH ST Plasma
CT plasma injection:(VCT_form.= 17 kV and VCT_acc.= 25kV)♦ CTI has no adverse effect on Ip.
♦ ne increases just after CTI.
» Non-disruptive CTI was obtained.
Thomson scattering measurement:♦ ne is observed to increase on peripheral
channels 0.5 ms after CTI.
Diffusion and equilibration times of CT injectedparticles in the vicinity of CT deposition:• 50~100 s for central CT deposition• 1.4~2 ms for peripheral deposition» Peripheral particle deposition by CTI wasobserved.

16

Steady-State Operation

QUEST

EX/P4-49 Hanada, et al.
Center-stack vessel

Top Flat Plate

Hot wall was installed in 2014

Flat Divertor

Top Conical Area

Radiation
ShieldLimiter
Heater
Cooling panel

Mid-plane
Area
Center-stack
Vessel Cover
BottomConical Area

ThermalIsolator

CHI electrode

Bottom Flat Plate
Surface of APS-W at
Top WallB

Hot wallHot
wall Vacuum Vessel
316L stainless steel
coated with 0.1 mm thick
APS-W, operated with
Twall = 393-523 K

Cross-Sec. at A
surface

Ion Drift

APS-W

50nm

Cross-Sec. at B

Electron Driftsurface

APS-W

A

20µm

50nm

17

Progress Towards Steady State:
115 min Discharge Achieved with PRF = 40 kW, Twall = 393 K

10LimiterIBnullDivertor

3

Ha
2

10

1

18

10

S/S

2011
A/W

10000

S/S

A/W

15000

2012
S/S

A/W

20000
Shot No.

2013
S/S

A/W

2014

2015

16

S/S A/W S/S A/W S/S

25000

30000

4

2x10
10

(c) ne

-6

-6

1.5

1.0

0.5

0.0

0.1
-0.5

2

(d) neSOL TeSOL

2.5
0.0

1

0

2000

4000
Time (sec)

6000

0

Te SOL(eV)

2010
-1

(b) Ha pH2

-3

10

10

0
1.2
0.8
0.4
0.0
0.2

0.0
15
5.0x100#327375-3

ne (10 m )

Hot
wall

ne SOL (m )Pulse Duration (s)10(a) Ip

Z(m)

4

10

H2 Pressure (Pa)Ip_Hall (kA)

Progress towards SSO(2010-2016)

-1.0

-1.5

0.0

0.5

1.0
R(m)

1.5

Microwave systems:
2.45 GHz : < 50kW
8.2 GHz: < 400kW
28 GHz: 300kW
18

Time Evolution of Wall Pumping Rate can be
Reproduced by Hydrogen Barrier Model
3.5x1018#32700#32737

Wall pumping rate (H/sec)

3.0
2.5

473 K

2.0

HW

393 K

1.5

1.0

HT

0.5
0.0
0

2

4

6

8x10

20

Wall Stored H (H/sec)d ( HW  H T )k in S  2 HW2
dt
Sd R
 H 
dH T
 a HW 1  T0    H T
dt
 HT 

dHW/dt : wall pumping rate for H
HW : number of H dissolved in wall
HT : number of H trapped in defects
HT0 : upper-limit of H trapped in defects
S : surface area
Gin : net influx per unit of area into wallk : surface recombination coefficient of HdR : thickness of deposition layera : H trapping rateg : H de-trapping rate

#32737 (Twall = 393 K: blue dots)#32700 (Twall = 473 K: red dots)
Solid lines indicate calculated
results predicted by thehydrogen barrier model.

19

EX/P3-38 Y. Ono, et al.
PD/P-11 Inomoto, et al.

Ultra-High-b Operation

Merging/ Reconnection Heatingfor Direct Access to Fusion Reaction
Ion Temperature
106 K

A fusion reactor can maintainreaction, once D-T plasma
with nt > 1020 m-3 s is heated to> 10 keV.

The key is how to increase
Ti to over 10keV.

1

It
NBI,
RF

In conventional tokamaks:
1st : Ohmic heating
W = hIt2 ∝ Te-3/2 It2+ 2nd : Additional heating(NBI or RF)

Merging of two toroidal
plasmas can increase
their Ti .

20High Ti
jt
Outflow

Brec

.1

.2

.3

Radius [m]Magnetic Field Lines Ion Temperature Profile

20

Brec2 Scaling of Reconnection Heating Indicates Direct Access to a-heating
without NBI, Leading to New High-Brec Experiments: ST-40 & TS-U

Voutflow ~ VpA
 Bp / n1/2

high Bt rec. (tokamak)low
Bt rec.Regime(spheromak)
ITER
WmB/W
Dzero
m~100%
rec.
(counter)a heatingt

MAST

DWth~DWion
 D(nTi)  DTi

DWrec  Brec2

ST40, TS-U
(2016 ~)

DWm/Wm=1%(n ~ constant) Brec2  Bp2
TS-3

ne ~ 1.5 x 1019 m-3

Construction of ST40 &TS-U:
high-Brec merging devices started

Brec[T]
ST40 : FIP/P7-19 M. Gryaznevich, et al.

21

T. Fujita, et al.

Low-A Tokamak-Helical Hybrid

Stability Improvement by Helical Field
TOKASTAR-2

w/o Helical Field

w/ Helical Field

Bz(CH2)Bz(CH2)

Ip

Ip

tokamak-helicalhybrid device

Tokamak discharges withand without helical field
were compared.
Suppression of horizontal
position oscillation by helicalfield application was observed
by a magnetic probe inserted
to the plasma core under aweak vertical field condition.
UEDA, T., et al., Plasma Fusion Res. 10 (2015) 3402065.

22

• The radial movement of tokamak plasma was studied
without probe insertion by a high-speed camera.
• The plasma touched the outer wall repeatedly in adischarge without helical field. In a discharge withhelical field, the plasma position was stable andtouched the inner wall during the discharge.
• The stabilization effect of the external helical field on
the tokamak plasma horizontal position was confirmedin the TOKASTAR-2 experiment.

SAKITO, T., et al., Plasma Fusion Res. 11 (2016) 2402074.

23

For Further Details (at FEC2016)• Ip Start-up– ECW/EBW (EX/P4-45 Tanaka, et al., EX/P4-50 Idei et. al., )– LHW (EX/P4-48 Ejiri, et al.)
– CHI (EX/P5-21 Nagata, et al.)– AC OH (EX/P4-48 Ejiri, et al.)• Advanced Fueling– CT Injection (PD/P-16 Fukumoto, et al.)• Steady-State Operation– Particle Control by High Temp. Wall (EX/P4-49 Hanada, et al.)

• Ultra-High-b Operation– Reconnection Heating (EX/P3-38 Y. Ono, et al., PD/P-11 Inomoto, et al. )24