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 ParameterDesignedAchieved Toroidal magnetic field 0.62 T0.55 T Plasma current0.3 MA0.36 MA Major radius0.36 m0.36 m Minor radius0.24 m0.24 m Aspect ratio Vertical elongation Triangularity Average density1  m  m -3 Pulse duration200 ms110 ms Safety factor, edge4.52 Toroidal beta25%~10% ICRF power1.0 MW0.5 MW frequency8 -30 MHz MHz duration30 ms30 ms NBI power1.0 MW0.7 MW energy30 keV30 keV duration30 ms30 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 ~ 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 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 D coils New toroidal array D 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 C for several days Careful wall conditioning with glow discharge in He for 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 ~ 1  m -3 were achieved, (n/n G )~1

NBI heating experiments 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 ≥ 6  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 2  m -3 The stored plasma energy (EFIT) approached 5.5 kJ β t ~10% 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/n G ~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)

MHD activity 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. 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)  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. Sawtooth fluctuations in NBI heated shot #13727 with ultimate plasma density

Plasma jet injection Jet parameters: density up to m -3 total number of accelerated particles - (1-5)  flow velocity of km/s Shot parameters: Bt=0.4 T, Ip= 0.2 MA initial central electron density ~ 3  m -3. Penetration criterion: ρV 2 /2 > B T 2 /2μ 0 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 0.65×10 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.

Specific Features of ICRH on ST ωH2ωDωH2ωD ωH2ωDωH2ωD 3ωD3ωD 2ωH2ωH Several ResonancesOne Resonance O.N. Shcherbinin

21 2nd H-harmonic effect on ICRH efficiency B t0 = 0.4 T, Ip= 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 In OH regime T D = eV O.N. Shcherbinin

C H effect on ICRH efficiency B t /B t0 =1 In OH-regime T D =T H = eV 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/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: = 2  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%.