Recent Development inthe Operational Regimeof Large Helical Device
Toward a Steady-State
Helical Fusion Reactor
Yasuhiko Takeiri
on behalf of the LHD Experimental Group
National Institute for Fusion ScienceJoint Conference of 17th International Toki Conference and16th International Stellarator/Heliotron Workshop 2007
Ceratopia Toki, Japan, October 15 – 19, 2007

1/28

Large Helical Device Heliotron configuration ofl=2/m=10 field period(LHD)
All superconducting coil system
Plasma major radius 3.42-4.1 m
Plasma minor radius 0.6 m
Plasma volume30 m3
Toroidal field strength 3 T
negativeNBI(BL1)
positiveNBI(BL4)

negativeNBI(BL2)

Heating Systems and Achieved Powernegative-NBI (H-inj. 180keV) 14MW
positive-NBI (H-inj. 40keV)
7MW
ICRF (25 – 100MHz)
2.7MW
ECH (84 & 168GHz)
2.1MW

negativeNBI(BL3)

Plasma vacuum vessel

2/28

LHD plasma performance has progressed
together with improvement of heating devices.
- Plasma stored energy is beyond 1.4 MJ,
comparable to large tokamaks.
- Ion and electron temperatures exceed 10 keV,
respectively.
- Plasma discharge duration is extended toaround 1 hour.

Increase in heating power is a major
contribution to the steady progress.
N-NBI : 14 MW P-NBI : 7 MW
ECH : 2.1 MW ICRF : 2.7 MW

3/28

Highlights in the 10th Campaign(FY2006: After the IAEA Conference)- High β : Achievement of the reactor-relevant value<β>=5.0% (transient by pellet)<β>=4.8% (stationary by gas-puffing)- High ne : Extension of the parameter regime in HD configurationne(0)=1x1021m-3 with IDB
P(0)>130kPa (above an atmospheric pressure)- High Ti : Achievement in the hydrogen plasmaTi(0)=5.2keV at ne=1.2x1019m-3 (H-plasma)- Other achievements
Wp=1.44MJ (with high-power NBI heating)Te(0)=15keV at B=2.931T (Sub-cooling of the helical coil)Observation of 2.5MeV of the ICRF-accelerated ionsLong-pulse plasma sustainment of 1MW-500sec
4/28

Outline- Characteristic features with regard to the magnetic axis
position- High-β plasma characteristics relevant to the helical
reactor- High β : <β>=5.0% (transient by pellet)<β>=4.8% (stationary by gas-puffing)- Plasma confinement of the IDB plasmas formed in thehelical divertor configuration- High ne : ne(0)=1x1021m-3 with IDB- Ion transport in the high-Ti plasmas- High Ti : Ti(0)=5.2keV at ne=1.2x1019m-3 (H-plasma)- New approach to ignition- High-density scenario to the ignition based on theSDC/IDB plasma- Summary and Future project
5/28

LHD plasma characteristics depend
strongly on magnetic axis position.
Good particle orbit
Theoretical prediction
In the inward-shifted
configurations, theparticle orbit and thetransport are better
although the ideal linear
MHD mode (interchangemode) is unstable dueto the presence of themagnetic hill region.

Orbit of deeply trapped particle

In the outward-shifted
configurations, the MHD
mode is stable although
the particle orbit andthe transport are worse.
Compatibility
Amplitudes
ofbetween
MHD
instabilities
found tostability
andwereconfinement
beresolved
small evenin theinwardis
in theinwardshifted configuration.
Magnetic hill

Good
Magnetic well stability
6/28

Strategy for the access to high-β plasmas.
Control of magnetic axis and plasma aspect ratio
Shafranov shift is better for stability due to the spontaneous
well formation while it deteriorates the transport and the NBI
heating efficiency.

Control of magnetic axis position
GoodMHD stability

Optimum Rax invaccum is 3.6m.

Pabs, τE, Orbit(inward) Magnetic axis (outward)- In outward-shifted configuration,
MHD stability is better whiletransport and NBI heating efficiency
are degraded.
- Magnetic axis is shifted outward by
Shafranov shift as the β increases.
7/28

Strategy for the access to high-β plasmas.
Control of magnetic axis and plasma aspect ratio
Shafranov shift is better for stability due to the spontaneous
well formation while it deteriorates the transport and the NBI
heating efficiency.

Control of plasma aspect ratio
GoodPabs, Orbit

Optimum
Ap is 6.6.

MHD stability(small) Aspect ratio (large)- Shafranov shift is restrictedwith an increase in the plasma
aspect ratio.
- Restricted Shafranov shift
suppresses the spontaneous
formation of magnetic well.
8/28

Plasma of <β>=4.8% is stationary sustained
without any disruptive phenomena
tduration(>4.5%) > 50τEGas Puff

- No crucial MHD instability is observedin a period of 50 times the confinement
time during which <β> is over 4.5%.
- Shafranov shift normalized by theeffective plasma minor radius, Δ/aeff, is
as large as around 40%, near theequilibrium beta limit of 50%.

(ι/2π=1)bθ/B0(x10-5)- Dominant MHD modes are moving
outward as β increases, and peripheral
MHD modes (ρ>0.9), resonated withm/n=2/3 and 1/1, are only observed.
#69910,t=1.23s

0.5(ι/2π=1.5)

2.0

ι

0.4

1.5

0.3(ι/2π=2)n0.2

T (keV)eTS(AU)

e

0.5

0.1

Raxvac=3.6m, Ap=6.6
B=0.425T
Pabs=11MW

1.00- Flattening ofthe Te profile
causing theinstability is
not observed.

0.000.2

0.4a (m)
eff

0.6

9/28

Reactor-relevant <β> value of 5% isachieved transiently by pellet injection.
Pellet Inj.

tduration(>4.5%) > 10τE6~5.0%

104522<βe-kin>

00 .6

00 .4

Δ R a x/a p0 .2
0 .0

0#1 +2+3

10NBI

p o r t - te m p

00 .5//#451t im e ( s )⊥01 .5- Tangential NBI power is reduced at thepellet injection for effective penetrationof the pellets.
- Shafranov shift is rapidly reduced atthat time, followed by theperpendicular NBI together with therestoration of the tangential NBI.
- Due to the reduction of the Shafranov
shift, the perpendicularly injected beameffectively heats the plasma, and the<β> is rapidly increased, then, reaching5%.
- High-β plasma greater than 4.5% of <β>is maintained for 10τE without crucial
MHD instability.
- Te and ne profiles are nearly the sameas those in the gas-puffing shot.

Raxvac=3.6m, Ap=6.6
B=0.425T, Pabs=11MW

10/28

5- <β> is increased as P0.25, and is not
limited by the MHD instabilities.

4- Considering the energy confinement
scaling of Wp depending on P0.4,confinement degradation caused by
the β increase is suggested.

3dia<β > (%)<β> is still increased with an increase in theheating power without saturation.

210Exp.FY2006-Exp.FY2005

024- It is thought that the core MHD mode
is not correlated with the possible
confinement degradation, because ofno observation of it.

6 8 10 12P (MW)
abs

11/28

Transport enhancement in the peripheralregion seems to be responsible for theconfinement degradation in high-β plasmas.
- Global energy confinement isgradually degraded with increasing β.
- Accordingly, the peripheral transport
is also degraded with increasing β.
- Observed MHD modes in the
peripheral region are resistive
interchange mode, and the resistive
g-mode turbulence would cause thetransport degradation.

χGRB=ρ*χB, ρ*=ρs/ap- Analysis with magnetic Reynolds
number, S, indicates that theamplitude of resistive interchange
modes is expected to be reduced at
higher temperature and at higher
magnetic field.
K.Y. Watanabe, I-13 on Thursday
12/28

Internal Diffusion Barrier (IDB) is formed in
LID-controlled plasmas core-fueled by pellets.

coreplasma

Separatrix(disappeared)

Previously observed in FY2005- LID realizes a low recycling plasma
condition by strong particle control
at plasma edge.

LID
headtopump
m/n=1/1 island
separatrix- Direct core fueling by repetitivepellet injection results in a superdense-core (SDC) plasma ofne(0)=5x1020m-3, accompanied byformation of IDB.
t=1.36s2.0
t=1.136s

5ne41.5eT (keV)

3T

1.0e

2e

LCFSbafflen (1020m-3)

Hydrogen pellet

0.5

10-1.0

-0.5

0.0

ρ

0.5

0.0
1.0

13/28

IDB is realized even in helical divertor (HD)configuration with well-conditioned wall.
- Edge plasma control is a key issue to form the SDC/IDB plasmas.
- In the outward-shifted configuration, localization of neutrals is small, andthe recycling is suppressed due to weaker plasma-wall interaction.
- Since the low edge-density can be maintained by the wall-pumping
effect in the outward-shifted configuration, the SDC/IDB plasma isrealized without the LID at Raxvac > 3.7m.

M. Kobayashi, I-07 on Tuesday

14/28

SDC/IDB plasma formed in HD configuration
with core density collapse (CDC).
- Density is rapidly increased by thecore fueling with repetitive pelletinjection.
- After the pellet injection, theperipheral density in ‘mantle’ regionis rapidly decreased during thedensity relaxation with the recoveryof temperature, and then theSDC/IDB plasma is formed.
- Central pressure continues to beincreased and, occasionally, thecore density collapse (CDC) occurs
with an abrupt decrease in thecentral pressure.
J. Miyazawa, P1-087 on Tuesday- Impurity accumulation is notobserved, and Zeff is around 1.5
during the discharge.
15/28

Large Shafranov shift with a steep pressure
profile is observed in SDC/IDB plasmas withimproved confinement.
- SDC/IDB plasma has a peaked density profilein the core region while a low density is
maintained with a steep Te-gradient in the‘mantle’ region.
- Compared with the normal gas-puffing
discharge, the peripheral Te rises toward thecore due to the low density in the ‘mantle’region, and, as a result, the core Te is higher.
- Central pressure is extremely high with a steep
gradient, and large Shafranov shift is observed,
approaching the equilibrium limit of Δ=a/2.
- Confinement
improvement issuggested in theSDC/IDB plasmas.

16/28

Central pressure exceeds an atmosphericpressure, and reaches 130kPa.
- Formation of the IDB in the HD
configuration is observed in outwardshifted configuration at Raxvac>3.7m.
- Achieved ne(0) is increased as theRaxvac is shifted more outward, and themaximum ne(0) is 1x1021m-3 obtainedat Raxvac = 3.9m.
- Peripheral density in the mantle
region is maintained low,independent of the Raxvac.
- Central pressure, P(0), jumps up at theIDB formation, and the maximum P(0)is 130kPa observed at Raxvac=3.85m.

Preset vacuum Rax- High P(0) and the steep pressure
gradient cause large Shafranov shiftnear the equilibrium limit, and,occasionally, the core density
collapse (CDC) occurs.
17/28

Particle confinement is maintained well incore region while it is degraded inperipheral region.
- Core particle diffusion is maintained
low for the steep density gradient in
the IDB formation.
- On the other hand, peripheral
particle diffusion in the ‘mantle’
region is enhanced with an increasein the density gradient.
- As a result, the SDC/IDB plasma isformed with an extremely high coredensity and a low peripheral density.

R. Sakamoto, I-04 on Tuesday
18/28

Te-gradient can exist in the magnetic surfaces
distorted by the large Shafranov shift.
Raxvac 74968t0.9s

108

β dia (%)

IDB

610Raxvac 69910t1.23s

8high-βRaxvac=3.75m
4 Baxvac=1.0T

622β dia (%)

40

Raxvac=3.6m
Baxvac=0.425T

00
33.5

4

4.5

3R(m)

3.5

4R(m)

4.5

- Compared with the high-β
plasmas at low B, the SDC/IDB
plasma has a similar β(0), asteeper pressure gradient, and alarger Shafranov shift.
- Finite β equilibrium obtained withHINT2 code shows distorted
magnetic surfaces in the‘mantle’ region in the SDC/IDB.
- Connection lengths of themagnetic field lines are longer
than the electron mean freepaths.

Connection lengthElectronmean free path

Te- Thus, Te profile in the ‘mantle’
region has a gradient even in thedistorted region of the magneticsurfaces.
- SDC/IDB plasmas should becompatible with high-β plasma.
Y. Suzuki, P2-043 on Tuesday

19/28

SDC/IDB plasmas pioneer a new approachto the ignition with a high-density scenario.
- Based on the SDC/IDB plasma parameters, high-density ignition scenario
is proposed.
- Compared with the high-temperature ignition scenario, the engineering
demand is much reduced, due to a moderate operational temperature.
- Helical system, in which the current drive is not required, can access thehigh-density operation, and the neoclassical ripple transport is mitigated.

LHD

High density operation

Density (1020-3m )5 ● ●Self-ignition

43

High temperature
operation

2ITER

10

Contour lines:
PexBreakeven30
1020
Temperature (keV)

20/28

LHD-type helical fusion reactor is being
designed based on the high-density ignition
scenario.
IDB profile

Ignition condition:n(0) = (5 -7) x 1020 m-3T(0) = 7 – 10 keV
τE = 1.2 τEISS95

FFHR
1,000 MW
6Tesla25,000 ton

A. Sagara, I-22 on Friday
21/28

Low-energy NBI system with perpendicular
injection has been installed for the ion heating.
Also used for the Ti-profile measurement
along a toroidal line of sight with CXRS

180keV-tangential
NB injector- Ion heating is dominant with lowenergy NBI.
- Electron heating is dominant withpresent high-energy NB injectors.
- Particle fueling into core plasma- Profile measurement of Ti and Er(CXRS)- Confinement study of perpendicular
injected high-energy particles- Injection energy : 40keV

Toroidalline of sightfor CXRS

40keV-radial - Injection power : 6MW(with 4 positive-ion sources)
NB injector- Perpendicular hydrogen
injection

22/28

Ion temperature exceeds 5keV
in H-main plasmas by combination oflow- and high-energy NB injections.
65CXRS

4Ti(0)=5.2±0.6keV

3(H-main plasma)- High ion-temperature of 5.2keV has beenachieved in H-main plasma at 1.2x1019m-3,
and high-Ti regime has been extended
toward higher density plasmas.
- Large toroidal rotation and its shear isobserved in association with the Ti rise.

iT [keV]lhdcxs3_tir@74282_001

210
3.6 3.8 4.0 4.2 4.4 4.6 4.8

R [m]
60

lhdcxs3_vpr@74282_001

5432
120

Ti(0)
Te(0)

0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5t

V [km/s]40

Ti(0), Te(0) [keV]6ne=1.2×1019m-3

19 -30Toroidal rotation-20
3.6 3.8 4.0 4.2 4.4 4.6 4.8

R [m]n (0)_fir_ne_rho [10 m ]eK. Nagaoka, O-11 on Thursday
M. Yoshinuma, O-12 on Friday
M. Yokoyama, P1-036 on Tuesday

23/28

Ion transport is improved in the high-Ti plasma
with a reduction of the anomalous transport.
- Ion thermal diffusivity divided by the gyro-Bohm factor, χi /Ti3/2, isreduced with the Ti rise.
- Anomalous transport should be reduced in the high-Ti plasma.
- Electron transport is not changed.

23/2
exp 3/2/T(m /s/keV )ii

t=1.35st=1.75s

5432
10
3.610

lhdcxs3_tir@75235_001 10:14:59 2007/04/10

ne=1.6x1019m-3

3.8

4.0 4.2 4.4 4.6
Major Radius (m)lhd_075235_001366_v001_funaba_001_chi_exp 9:30:05 2007/10/09

t=1.35st=1.75s

1

χIon Temperature (keV)

6

4.8

0.1
0.0

0.2

0.4

ρ

0.6

0.8

1.0

24/28

Enhanced ripple transport is suppressed in ion
root with negative Er in the high-Ti plasma.
- Neoclassical ambipolar calculation indicates negative Er in the Ti rise.
- Ripple transport of ions would be greatly enhanced by the Ti-rise without
consideration of the Er effect.
- With strong reduction of the ripple transport by the negative Er,
neoclassical χi is not so changed with the Ti rise.
- Considering the experimental results, the improvement of ion
confinement is due to reduction of the anomalous transport in the ion root.
Calculation

41075235-1.35s.ambEr 15:01:36 2007/05/10

-1

310
NC
2(m /s)i-2-3-4-5-6
0.0

Calculation

210

1

t=1.35st=1.75st=1.35s (Er=0)10χ

Radial Electric Field (kV/m)01

t=1.35st=1.75s0.20.4

ρ

0.6

0.8

1.0

0.1
0.0S. Matsuoka, P-013 on Wednesday

0.20.4

ρ

0.6

0.8

1.0Er25/28

Achieved plasma parameters areapproaching to the final targets in LHD.
Final Targets

Achievements

Fusion triple product: nτT
Ion temperature (average)
Density
Energy confinement time

5 x 1019 keVm-3s
2.5 keV : <Ti>
1 x 1020 m-3
0.1 - 0.2 s

4.4 x 1019 keVm-3s
0.8 keV : Ti(0)5 x 1020 m-3
0.11 sElectron Temperature: TeCentral Te
Density

10 keV2 x 1019 m-3

10 keV5 x 1018 m-3

15 keV2 x 1018 m-3

Ion temperature: Ti
Central TiDensity

10 keV2 x 1019 m-3

13.5 keV
3 x 1018 m-3

5.2 keV
1.2 x 1019 m-3(Ar gas)(H gas)

Beta: βMagnetic field strength

β=5%
1-2T

β = 5.0 %0.425 T

Steady state operation
Pulse length

3600 s at 3 MW

3900 s at 110 kW1905 s at 680 kW
3268 s at 490 kW(Input Energy: 1.6GJ) 26/28

Now, proposing the next experimental stage.
Deuterium experiments with upgrading the heating facilities- Plasma performance is expected to be improved by a factor of 1.5 – 2 indeuterium experiments.
- Plasma parameters are also pushed up by increasing the heating power.
- By systematic investigations of high-performance plasmas with plasma
parameters of the LHD goal, the helical fusion reactor can be designed.
Achievements [Final target]

Commercial
reactor

Ignition

ITER

Central Ti 13.5 keV
Density
3×1018m-3 (Ar gas)
> 10 keV : reactor
Electron Temperature
condition
Central Te 10 keV [10 keV]
Density
5×1018m-3 [2×1019m-3]
Volume Averaged β
5.0 % (B=0.425T) [≥ 5 % (1-2 T)]
Steady State Operation
31min.45sec. (680 kW) 1.3GJ [1 hour (3,000 kW)]
54min.28sec. (490 kW) 1.6GJ
LargestHigh Density
input energy
21-3
1×10 mbreak-even101(1020m-3 keV s)

Fusion tripple productIon Temperature
Central Ti 5.2 keV [10 keV]
Density
1.2×1019m-3 [2×1019m-3 ]102

LHD goal
10010-1 LHD achieved Upgrade
10-2
10-3

Existing
Tokamaksmin. hour day month
10-4
10-1 100 101 102 103 104 105 106 107

Holding time (s)27/28

Summary- LHD has much progressed for the 9 year’s operation, andextended the parameter regime toward a helical fusion reactor.
- Reactor relevant <β> value of 5% was achieved without any
disruptive MHD instabilities.
- IDB plasmas were formed in the helical divertor configuration,and high central density of 1x1021m-3 was observed.
- High-Ti of 5.2keV was obtained in H-plasmas at ne=1.2x1019m-3.,
and the ion transport improvement was observed with areduction of the anomalous transport in the neoclassical ion root.
- High-density ignition scenario based on IDB/SDC plasmas isproposed for the helical fusion reactor, which reduces theengineering demand and mitigates the helical ripple transport.
- Upgrade of LHD is proposed, including deuterium experiments
and power-up of the heating systems to the further extension of
the operational regime for designing the steady-state helicalfusion reactor.
28/28