Latest Results from the Globus-M Spherical Tokamak

1. 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, 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 TORIandTHE 11th INTERNATIONAL WORKSHOP ON SPHERICAL TORUS, 3 to 6 October 2005, St.Petersburg

2. Globus-M parameters Parameter DesignedAchieved Toroidal magnetic field0.62 T0.55 T Plasma current 0.3 MA0.36 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1020 m-3 1.51020m-3 Pulse duration 200ms 110ms Safety factor, edge4.5 2 Toroidal beta 25% ~10% ICRF power 1.0 MW 0.5 MW frequency 8 -30 MHz 7.5-30MHz duration 30 ms30 ms NBIpower 1.0 MW 0.7 MW energy 30 keV 30 keV duration 30 ms30 ms

3. 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.

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

5. Diagnostics. Thomson scattering Electron temperature Electron density • 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

6. 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

7. Diagnostics, Mirnov probes Provide MHD mode identification with m≤5, n≤4 New toroidal array 16 - 2D coils poloidal array 28 - 1D coils

8. High density OH operating Arrangements to obtain high density regime: • Vacuum vessel preparation • New toroidal limiter • Vertical equilibrium improvement • Experimental scenario optimization

9. 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

10. 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

11. 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

12. 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> ~ 11020 m-3 were achieved, (n/nG)~1

13. NBI heating experiments Neutral Beam Power Absorption • 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> ≥ 61019 m-3 • Electron heating • should be visible ASTRA code simulation of NBI power fraction absorbed by electrons and ions vs density V.M. Leonov

14. Plasma current, MA 0.30 0.15 0 Density (R=34.6 cm), 1020 m-3 2 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 NBI heating experiments Shot # 13727 • 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1020m-3 • The stored plasma energy (EFIT) approached 5.5kJ • βt~10% V.B. Minaev

15. NBI heating experiments • 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 Time (s) Time (s) Time (s) Temporal variation of the volume average density in OH and NB heated discharges with high density, TS data. 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

16. 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

17. 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 dischargeswere sawtooth oscillations • NB injection stabilizes IREs, which are specific for high current (low q953.5) OH discharges. Sawtooth fluctuations in NBI heated shot #13727 with ultimate plasma density

18. 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 ~ 31019 m-3. Penetration criterion: ρV2/2 > BT2/2μ0 Double stage plasma gun A.V. Voronin

19. 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.

20. Specific Features of ICRH on ST ωH 2ωD ωH 2ωD 3ωD 2ωH O.N. Shcherbinin Several Resonances One Resonance

21. 2nd H-harmonic effect on ICRH efficiency 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

22. CH effect on ICRH efficiency 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% . Bt/Bt0=1 In OH-regime TD=TH=180-200 eV O.N. Shcherbinin

23. Conclusions • High density target OH regime with n/nGr~1 was obtained due to improved equilibrium control, accurate wall conditioning and special experiment scenario. • Greenwald limit was exceeded at co-current NBI of 0.6 MW 28 keV. The record parameters were obtained: <ne(0)>= 21020 m-3, βt=10% at magnetic field of 0.4 T and low q953.5. Efficient heating of electrons was observed, the electron energy content at NBI exceeded the OH one by more than 30 %. • 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%.