latest results from the globus-m spherical tokamak
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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%.
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