Latest Results from the Globus-M Spherical

Tokamak

Yu.V. Petrov1, A.G. Barsukov2, V.K. Gusev1, F.V. Chernyshev1, I.N. Chugunov1, V.E. Golant1, V.V. Dyachenko1, L.A. Esipov1, V.G. Kapralov3, S.V. Krikunov1, V.M.

 Leonov2, R.G. Levin1, V.B. Minaev1, A.B. Mineev4, I.V. Miroshnikov3, E.E. Mukhin1, A.N. Novokhatskii1, M.I. Patrov1, K.A. Podushnikova1, V.V. Rozhdestvenskii1,

N.V. Sakharov1, O.N.Shcherbinin1, A.E. Shevelev1, A.S. Smirnov2, A.V. Sushkov2, G.N. Tilinin2, S.Yu. Tolstyakov1, V.I. Varfolomeev1, M.I. Vildjunas1, A.V. Voronin1,

G.S. Kurskiev1, B.B. Ayushin1

1 A.F. Ioffe Physico-Technical Institute, St. Petersburg, Russia2.NFI RRC “Kurchatov Institute”, Moscow, Russia3 Saint-Petersburg State Politechnical University, St. Petersburg, Russia 4 D.V. Efremov Institute of Electrophysical Apparatus, St. Petersburg, Russia

THE 3rd IAEA TECHNICAL MEETING ON SPHERICAL TORI andTHE 11th INTERNATIONAL WORKSHOP ON SPHERICAL TORUS,

3 to 6 October 2005, St.Petersburg

Globus-M parameters

Parameter Designed Achieved

Toroidal magneticfield 0.62 T 0.55 TPlasma current 0.3 MA 0.36 MAMajor radius 0.36 m 0.36 mMinor radius 0.24 m 0.24 mAspect ratio 1.5 1.5Vertical elongation2.2 2.0Triangularity 0.3 0.45Average density 11020 m-3 1.51020 m-

3

Pulse duration 200 ms 110 msSafety factor, edge4.5 2Toroidal beta 25% ~10%

ICRF power 1.0 MW 0.5 MW frequency 8 -30 MHz 7.5-30 MHz duration 30 ms 30 ms

NBI power 1.0 MW 0.7 MW energy 30 keV 30 keV duration 30 ms 30 ms

Motivation

• One of the most attractive fusion relevant scenarios is a high plasma density regime as the fusion power depends squarely on density.

• Density limit obtained in our previous OH experiments was <n> ~ 5∙1019 m-3 which is 0.75 of the Greenwald limit.

• No progress in the density limit was obtained with NBI.• In spite of favorable predictive ASTRA simulations no plasma

heating by NBI, either electrons or ions were observed at high plasma densities.

• MHD instabilities seemed to restrict the density rise.• The densities higher 5∙1019 m-3 were beyond the interferometer

measurement possibility.

• The task was to improve the situation in all points.

Contents

• Diagnostics improvement• High density OH operating • NBI heating experiments• MHD activity• Plasma jet injection• ICR heating experiments• Conclusions

Diagnostics. Thomson scattering

0,6

0,5

0,4

0,3

0,20,00E+000

5,00E+019

1,00E+020

1,50E+020

2,00E+020

2,50E+020

130135

140145

150155

160165

170

t, ms

0,6

0,5

0,4

0,3

0,20

100

200

300

400

500

130135

140145

150155

160165

170

Te

, e

V

t, ms

Nd-glass laser Thomson scattering system was used in experiment to measure Te(R,t) and Ne(R,t)

• 5 spatial points along the major radius

• up to 20 temporal points for one tokamak shot

S.Yu. Tolstyakov

Electron density Electron temperature

Diagnostics, 32 channel SXR pinhole camera

Made in Kurchatov Institute by A.Sushkov & D.Kravtsov

• 32 DMPX detector (Duplex Multi-wire Proportional X-ray detector)• provides a good value of the gain factor • permits temperature profile measurements by the foil method• permits observation of the internal MHD activity

Diagnostics, Mirnov probes

poloidal array

28 - 1D coils

New toroidal array

16 - 2D coils

Provide MHD mode identification with m≤5, n≤4

High density OH operating

Arrangements to obtain high density regime:

• Vacuum vessel preparation• New toroidal limiter• Vertical equilibrium improvement• Experimental scenario

optimization

Vacuum vessel preparation

Steps:• Vacuum pumping system

exchange for oil free pumps with higher pumping rate

• Permanent vessel baking at 2000C for several days

• Careful wall conditioning with glow discharge in He for 40-50 hours

• Standard boronization procedure with carboran

Result:• Residual gas pressure

decrease• More clear mass-spectrum

New toroidal limiter

• Intercept a fraction of the particle flow to the outer wall

• Prevent the plasma contact with the lower dome at vertical displacement of the plasma column

Graphite toroidal limiter

Vertical equilibrium improvement

EFIT reconstruction of high density discharge with NBI

• EFIT permits the plasma shape reconstruction between tokamak shots.

• Vertical plasma displacement due to CS asymmetry was indicated, which led to the plasma current termination. A dipole vertical displacement sensor was not sensible for it.

• The sensor was replaced by a new quadrupole one. The situation has been improved, but still needs further perfection.

R.G.Levin

High density OH operating

Listed above steps and:• Density control by inner wall gas

puff (contribution of the walls could be neglected)

• Experiment scenario, when high density shot was followed by several low density shots to prevent wall saturation by deuterium.

Results:• Stable operating at currents in

excess of 230 kA at high average densities in the target OH regime.

• Line average densities <ne> ~ 11020 m-3 were achieved,

(n/nG)~1

0.0100.0200.0300.0

Plasma current ( kA) shot # 13735

0.01.02.03.0

D-alpha (a.u.)

0.050.0

100.0150.0

Radiation power (kW)

0.01.02.0

SXR 7 (a.u.)

0.0200.0400.0600.0

Electron temperature at R=38,6cm (eV)

0.01.02.0

Electron density at R=38,6cm (10E20 mE-3)

0.01.02.03.0

Gas puffing (a.u.)

110 180120 130 140 150 160 170Time (ms)

NBI heating experiments

2 4 6 8 100

20

40

60

80

100

Eo=20 keV

Eo=30 keV

Pbm_i

Pbm_e

PNBI

-Pshth

Pbm_ei

Pabs

P/P

NB

I , %

ne, 1019m -3

• Power absorbed by electrons at low and moderate densities is small.

• It becomes a considerable fraction of OH power only at high average densities

At <ne> ≥ 61019 m-3

Electron heating

should be visible

Neutral Beam Power Absorption

ASTRA code simulation of NBI power fraction absorbed by electrons and ions vs density

V.M. Leonov

NBI heating experiments

NB co-injection

0.55 MW, 28 keV, 30ms

• Optimization of the NBI start point was made in 130–150 ms time range

• Highest heating efficiency was achieved at early beam injection (135 ms)

• Central electron density reached the value of 21020 m-3

• The stored plasma energy (EFIT) approached 5.5 kJ

• βt~10%

0

0.15

0.30

0

1

2Density (R=34.6 cm), 1020 m-3

0

0.3

0.6Electron temperature, keV

0

1Bolometer, a.u.

0

2D-alpha, a.u.

0

2SXR, a.u.

0

0.2

0.4Ion temperature, keV

0

3

6Plasma stored energy, kJ

115 175130 145 160

Time, ms

NBI, 0.55MW, 28 keV

Plasma current, MA

V.B. Minaev

Shot # 13727

NBI heating experiments

Electron energy content in the plasma during NB heating, (red) and OH high density regimes

Electron energy content in the plasma during NB heating, (red) and OH moderate density regimes

• Maximum electron density at NBI is 20% higher than in OH, n/nG~1.2• Electron component stored energy increased by 30% with NBI at high density • Density decrease by 25% nearly cancels the effect

Temporal variation of the volume average density in OH and NB heated discharges with high density, TS data.

Time (s) Time (s) Time (s)

MHD activity

• In our previous experiments at average densities higher than 5×1019 m-3 strong instability of coupled 1/1 (“snake”) and 2/1 modes developed, which seemed to create a density limit

• The both modes have common frequency that evidences of their toroidal coupling.

• Locking of the modes leads to an internal reconnection event (IRE), manifesting it self in a characteristic spike on the plasma current trace. Locking of 2/1 (Mirnov signal) and 1/1

(SXR emission) toroidally coupled modes in OH discharge #13532

M.I. Patrov

MHD activity

• In our recent experiments, global plasma column stability is conserved for the whole duration of the discharge at much higher average plasma densities (1-1.5)1020 m-3

• “Snakes” did not occur in high density discharges

• The level of external MHD fluctuations, measured by Mirnov coils was low.

• The only instability observed in this type of discharges were sawtooth oscillations

• NB injection stabilizes IREs, which are specific for high current (low q953.5) OH discharges.

Sawtooth fluctuations in NBI heated shot #13727 with ultimate plasma density

Plasma jet injection

Jet parameters:• density up to 1022 m-3

• total number of accelerated particles - (1-5)1019

• flow velocity of 50-110 km/s Shot parameters:• Bt=0.4 T,• Ip= 0.2 MA• initial central electron density

~ 31019 m-3. Penetration criterion:

ρV2/2 > BT2/2μ0

Double stage plasma gun

A.V. Voronin

Plasma jet injection

• Thomson scattering demonstrates density increase in all spatial points for 0.5 ms after a plasma gun shot

• Plasma particle inventory increased by 50% (from 0.65×1019 to 1×1019) in a single gun shot without target plasma parameter degradation.

• Penetration mechanism is not clear yet, but preliminary data show that it occurs trough recombination to a relatively fast neutral jet.

150 152 154 156 158 160 162 164 166 168 170

1

2

Time, ms

2

4

Plasma density, 1013cm-3 R = 38.6 cm Shot 13969

Plasma density, 1013cm-3 R = 30.6 cm

3

6

Plasma density, 1013cm-3 R = 17.6 cm

Plasma density, 1013cm-3 R = 21.1 cm

Plasma density, 1013cm-3 R = 25.6 cm

3

6

Gun current3

6

Specific Features of ICRH on ST

ωH

2ωD

ωH

2ωD3ωD

2ωH

Several Resonances One Resonance

O.N. Shcherbinin

21

2nd H-harmonic effect on ICRH

efficiency

0.8 0.9 1Bt/Bt0

100

150

200

250

300

350

TD

, eV

Bt0 = 0.4 T, Ip= 190-210 kA

CH=nH /(nH+nD)=15%, 30%

f= 7.5 MHz

Pinp= 200 kW

presence of 2nd H-harmonic in front of the antenna diminishes efficiency of on-axis ion heating

In OH regime TD=180-200 eV

O.N. Shcherbinin

CH effect on ICRH efficiency

Bt/Bt0=1

In OH-regime TD=TH=180-200 eV

0 20 40 60 80

C H , %

0

100

200

300

400

500

TD

,TH,

eV

- T H

- T D

O.N. Shcherbinin

2nd H-harmonic outside the vacuum chamber

The experiments with hydrogen-deuterium plasma have shown slight improvement with increase of hydrogen fraction from 10% to 70% .

Conclusions

1. High density target OH regime with n/nGr~1 was obtained due to improved

equilibrium control, accurate wall conditioning and special experiment scenario.

2. Greenwald limit was exceeded at co-current NBI of 0.6 MW 28 keV. The record

parameters were obtained: <ne(0)>= 21020 m-3, βt=10% at magnetic field of 0.4 T

and low q953.5. Efficient heating of electrons was observed, the electron energy

content at NBI exceeded the OH one by more than 30 %.

3. The plasma density limit manifested in our previous experiments has been

overcome without loss of the global stability. The toroidally coupled MHD modes

1/1 and 2/1, which seemed to restrict the density rise, was not observed at ultimate

densities exceeding the Greenwald limit.

4. A double stage plasma gun with increased up to 110 km/s jet velocity was used for

plasma feeding. Fast density increase (during the time less than 0.5 ms) in the

center of the plasma column was registered by Thomson scattering.

5. The role of the 2nd cyclotron hydrogen harmonic at ICRH is shown to be negative

when it is located in front of the antenna. Effective ion heating takes place at

concentration of light ion plasma component up to 70%.