Latest Results from the Globus-M Spherical Tokamak Yu.V. Petrov 1, A.G. Barsukov 2, V.K. Gusev 1, F.V. Chernyshev 1, I.N. Chugunov 1, V.E. Golant 1, V.V. Dyachenko 1, L.A. Esipov 1, V.G. Kapralov 3, S.V. Krikunov 1, V.M. Leonov 2, R.G. Levin 1, V.B. Minaev 1, A.B. Mineev 4, I.V. Miroshnikov 3, E.E. Mukhin 1, A.N. Novokhatskii 1, M.I. Patrov 1, K.A. Podushnikova 1, V.V. Rozhdestvenskii 1, N.V. Sakharov 1, O.N.Shcherbinin 1, A.E. Shevelev 1, A.S. Smirnov 2, A.V. Sushkov 2, G.N. Tilinin 2, S.Yu. Tolstyakov 1, V.I. Varfolomeev 1, M.I. Vildjunas 1, A.V. Voronin 1, G.S. Kurskiev 1, B.B. Ayushin 1 1 A.F. Ioffe Physico-Technical Institute, St. Petersburg, Russia 2.NFI RRC Kurchatov Institute, Moscow, Russia 3 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 and THE 11th INTERNATIONAL WORKSHOP ON SPHERICAL TORUS, 3 to 6 October 2005, St.Petersburg
Globus-M parameters Parameter Designed Achieved Toroidal magnetic field 0.62 T 0.55 T Plasma current 0.3 MA 0.36 MA Major radius 0.36 m 0.36 m Minor radius 0.24 m 0.24 m Aspect ratio 1.5 1.5 Vertical elongation2.2 2.0 Triangularity 0.3 0.45 Average density 1 10 20 m -3 1.5 10 20 m -3 Pulse duration 200 ms 110 ms Safety factor, edge4.5 2 Toroidal 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 10 19 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 10 19 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 Electron density Electron temperature Central column 2,50E+020 Central column 500 400 2,00E+020 300 1,50E+020 1,00E+020 N e, m -3 200 100 T e, ev 0,2 0,3 R, m 0,4 0,5 0,6 160 155 150 145 140 135 t, ms 130 5,00E+019 0,00E+000 170 165 0,2 0,3 R, m 0,4 0,5 0,6 0 170 165 160 155 150 145 140 135 t, ms 130 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
Diagnostics, 32 channel SXR pinhole camera 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 Made in Kurchatov Institute by A.Sushkov & D.Kravtsov
Diagnostics, Mirnov probes Provide MHD mode identification with m 5, n 4 23 22 21 Z (m) 24 25 26 27 28 1 2 3 4 20 19 18 17 16 15 14 13 12 5 6 7 8 9 11 10 R (m) poloidal array 28-1D coils New toroidal array 16-2D coils
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 200 0 C 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 Plasma current ( ka) shot # 13735 300.0 200.0 100.0 0.0 D-alpha (a.u.) 3.0 2.0 1.0 0.0 150.0 100.0 50.0 0.0 2.0 1.0 0.0 Radiation power (kw) SXR 7 (a.u.) 600.0 400.0 200.0 0.0 Electron density at R=38,6cm (10E20 me-3) 2.0 1.0 0.0 3.0 2.0 1.0 0.0 Electron temperature at R=38,6cm (ev) Gas puffing (a.u.) 110 180 120 130 140 150 160 170 Time (ms) 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 <n e > ~ 1 10 20 m -3 were achieved, (n/n G )~1
NBI heating experiments P/P NBI, % 100 80 60 40 20 0 Neutral Beam Power Absorption P NBI -P shth P abs P bm_ei E o =30 kev E o =20 kev P bm_e P bm_i 2 4 6 8 10 n e, 10 19 m -3 ASTRA code simulation of NBI power fraction absorbed by electrons and ions vs density V.M. Leonov 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 <n e > 6 10 19 m -3 Electron heating should be visible
NBI heating experiments 0.30 0.15 0 Plasma current, MA 2 Density (R=34.6 cm), 1020 m -3 Shot # 13727 1 0 Electron temperature, kev 0.6 0.3 0 Bolometer, a.u. 1 0 D-alpha, a.u. 2 0 SXR, a.u. 2 0 Ion temperature, kev 0.4 0.2 0 Plasma stored energy, kj 6 3 NBI, 0.55MW, 28 kev 0 115 130 145 160 175 Time, ms 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 2 10 20 m -3 The stored plasma energy (EFIT) approached 5.5 kj β t ~10% V.B. Minaev
NBI heating experiments Maximum electron density at NBI is 20% higher than in OH, n/n G ~1.2 Electron component stored energy increased by 30% with NBI at high density Density decrease by 25% nearly cancels the effect Time (s) Temporal variation of the volume average density in OH and NB heated discharges with high density, TS data. Time (s) Electron energy content in the plasma during NB heating, (red) and OH high density regimes Time (s) Electron energy content in the plasma during NB heating, (red) and OH moderate density regimes
MHD activity Locking of 2/1 (Mirnov signal) and 1/1 (SXR emission) toroidally coupled modes in OH discharge #13532 In our previous experiments at average densities higher than 5 10 19 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. M.I. Patrov
MHD activity Sawtooth fluctuations in NBI heated shot #13727 with ultimate plasma density 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) 10 20 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 q 95 3.5) OH discharges.
Plasma jet injection Vacuum shutter Plasma gun Jet parameters: density up to 10 22 m -3 total number of accelerated particles - (1-5) 10 19 flow velocity of 50-110 km/s Shot parameters: Bt=0.4 T, Ip= 0.2 MA initial central electron density ~ 3 10 19 m -3. Penetration criterion: Double stage plasma gun A.V. Voronin ρv 2 /2 > B T2 /2μ 0
Plasma jet injection 6 3 6 3 6 3 4 2 2 1 Plasma density, 10 13 cm -3 R = 38.6 cm Shot 13969 Gun current Plasma density, 10 13 cm -3 R = 30.6 cm Plasma density, 10 13 cm -3 R = 25.6 cm Plasma density, 10 13 cm -3 R = 21.1 cm Plasma density, 10 13 cm -3 R = 17.6 cm 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 10 19 to 1 10 19 ) 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 Time, ms
Specific Features of ICRH on ST ω H 2ω D 3ω D ω H 2ω D 2ω H O.N. Shcherbinin Several Resonances One Resonance
2nd H-harmonic effect on ICRH efficiency TD, ev 350 300 250 200 150 100 0.8 0.9 1 Bt/Bt0 In OH regime T D =180-200 ev O.N. Shcherbinin B t0 = 0.4 T, Ip= 190-210 ka C H =n H /(n H +n D )=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 21
C H effect on ICRH efficiency 500 T D,T H, ev 400 300 200 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%. 100 0 B t /B t0 =1 - T H - T D 0 20 40 60 80 C H, % In OH-regime T D =T H =180-200 ev O.N. Shcherbinin
Conclusions 1. High density target OH regime with n/n Gr ~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: <n e (0)>= 2 10 20 m -3, β t =10% at magnetic field of 0.4 T and low q 95 3.5. 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%.