Overview of Globus-M Spherical Tokamak Results V.K.Gusev, B.B.Ayushin, F.V.Chernyshev, I.N.Chugunov, V.V.Dyachenko, L.A.Esipov, D.B.Gin, V.E.Golant, N.A.Khromov, S.V.Krikunov, G.S.Kurskiev, M.M.Larionov, R.G.Levin, V.B.Minaev, E.E.Mukhin, A.N.Novokhatskii, M.I.Patrov, Yu.V.Petrov, K.A.Podushnikova, V.V.Rozhdestvensky, N.V.Sakharov, O.N.Shcherbinin, A.E.Shevelev, S.Yu.Tolstyakov, V.I.Varfolomeev, M.I.Vildjunas, A.V.Voronin Ioffe Physico-Technical Institute, RAS, , St. Petersburg, Russia V.G.Kapralov, I.V.Miroshnikov, V.A.Rozhansky, I.Yu.Senichenkov, A.S.Smirnov, I.Yu.Veselova St.-Petersburg State Polytechnical University, , St. Petersburg, Russia S.E.Bender, V.A.Belyakov, Yu.A.Kostsov, A.B.Mineev, V.I.Vasiliev D.V. Efremov Institute of Electrophysical Apparatus, , St. Petersburg, Russia E.A.Kuznetsov, V.N.Scherbitskii, V.A.Yagnov SSC RF TRINITI, , Moscow region, Russia A.G.Barsukov, V.V.Kuznetsov, V.M.Leonov, A.A.Panasenkov, G.N.Tilinin NFI RRC “Kurchatov Institute”, , Moscow, Russia E.G.Zhilin Ioffe Fusion Technology Ltd., , St. Petersburg, Russia Presented at 11 th IST Workshop, October, 2006, Chengdu, PR China

Outline 1.Density limits in Globus-M: Technology and scenario, features of high density regimes (MHD etc.) 2.NB heating: Ions heating – beam specie AM influence, temperature measurement, beam thermalization Electrons heating – dependence on density and E beam 3.ICR heating at fundamental harmonic: General features, energy exchange, antenna spectrum, simulation and experimental result of C H dependence, fast particles (tail) temperature and fraction 4.Plasma jet injection with high velocity: Density control by injection at stationary stage, plasma jet penetration through magnetic field, penetration into hot plasma – experiment and simulations, discharge initialization by plasma jet 5.Summary

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 ms130 ms Safety factor, edge4.52 Toroidal beta25%~10% ICRF power1 MW0.5 MW frequency8 -30 MHz MHz duration100 ms30-80 ms NBI power1.3 MW 1 MW energy30 keV30 keV duration30 ms30 ms

Improvement in vacuum technology, plasma control, and experimental scenario gave synergetic effect in density limit increase Until last year high B/R ratio potential of Globus-M was not realized Oil free pumping Vertical plasma control improvement Belt limiters made possible to operate routinely with high plasma currents (up to 0.25MA) with the small gap 3-4cm between plasma and vessel wall

High density OH operating without current stabilization Listed above steps and: Careful wall conditioning and boronization improved 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 high average densities in the target OH regime. Near the density limit no radiation collapse – current degradation Line average densities ~  m -3 were achieved, (n/n G )~1 Gusev NF

High density OH operating with current stabilization Listed above steps and: Density control by inner and outer wall gas puff (contribution of the walls could be neglected). Plasma current feedback stabilization Results: Stable operating at high average densities in the target OH regime. Radiation collapse at density limit (n/n G )~1 was achieved Petrov 33 EPS conf Roma 2006

Features of high density OH operating with current stabilization Density limit depend neither on the gas puff position (inner – outer wall) nor on gas puffing rate (high, moderate, small) Main MHD instability is saw-tooth oscillations, amplitude and period increase with gas puffing rate. Saw-tooth seems not restrict the density limit Sometimes benign m=1/n=1 tearing mode (snake) develops ( no saw-teeth in this case), but does not restrict the density limit

Operational space of Globus-M increases due to higher densities Achieved is n/n G ~1 and approached is n/n Mur ~1 Average densities up to 1.2  m -3 obtained at 0.4T Radiation collapse at density limit restrict further density rise Density limit is easily accessed at lower plasma currents

Operational space of Globus-M increases due to higher densities and temperatures Max electron temperature Te(0)  0.95 keV is achieved at ~ m -3 Max electron density Ne(0)  m -3 is achieved at ~175 eV

NBI heating in Globus-M Principal difficulties due to small size of the target plasma compared to the beam dimensions and small size of the vacuum vessel compared to fast particle orbit extent - in Globus-M R L  a/2 for 30keV deuterons at outboard edge. Moreover in Globus-M plasma is tightly fitted into the vacuum vessel. Ion NB heating efficiency weakly depends on of the beam specie AM. D-beam is slightly more effective due to lower atoms velocity at the same beam energy. Minaev 33 EPS conf 2006 Roma

NBI ion heating in Globus-M Ion temperatures measured by NPA in principle coincide with first CHERS measurements at densities < (3-3.5) х m -3, for higher densities correction for plasma opacity should be done.

NB thermalization in Globus-M “Perpendicular’ spectrum of fast particles, measured by 12 channel NPA. Above Ecrit ( keV) electron drag predominates, pitch angle scattering is poor. Below Ecrit (0–12 keV) collisions with plasma ions provides pitch angle scattering. Specrtum coincides with Fokker-Plank predictions ( no losses). Beam ion slowing down is well described by classical Coulomb scattering theory and the particle losses at least in the energy range below Ecr are insignificant.

NBI heating at high densities in Globus-M Ion heating vanishes at high densities at beam energy 25keV, (low P=0.5MW, opaque plasma), contrary electrons heating is improved with density rise and increase of NB energy and power. Agree with ASTRA simulations

Ion cyclotron resonance heating experiments and simulations in Globus-M Showed that: Ion heating is effective in the low frequency range (fundamental IC resonance for protons as “minority” in deuterium plasma). At low RF power input ~0.3P OH – T i increases two times. Energy exchange between plasma components is classical, i.e. deuterons is mostly heated through energy exchange with protons. Ion energy confinement is neoclassical, or even better (ASTRA gives 0.7 χ NEO ) [ Shcherbinin,…Leonov NF ]. Electron heating is small at such power level.

Ion cyclotron resonance heating in STs has specific features Simultaneous existence of several IC harmonics in the plasma cross- section. Low width of resonance absorption layers (much smaller than excited wave length) and lower efficiency of single-pass absorption of FMS waves. Wave propagation similar to a resonator kind, when the whole tokamak vessel plays the role of a multimode resonator of low quality. Significant non-resonance absorption ( high plasma dielectric constant). Spectrum of single loop antennae in Globus-M The peaks correspond to resonator modes excited in the chamber. Short wavelength components (lN z l~150) are strong enough. Dashed - the idealized spectrum if all excited waves are completely absorbed in the plasma without any reflection from inner plasma layers. Shcherbinin NF

The elimination of second harmonic of hydrogen resonance improves ICR heating 9MHz, 4T 7,5 MHz, 4T

Simulated RF Energy Absorption Profiles Left - calculated absorption for the whole excited wave spectrum (|Nz|≤150). Right - calculated absorption for the narrow part of wave spectrum (||≤20). Green – electrons (TTMP, Landau damp) Red – protons (cyclotron) Blue – deuterons (cyclotron, Bernstein wave abs) Shcherbinin NF High absorption at r  - 5cm – for protons at high C H is the consequence of short wave part of the exited spectrum

Ion heating improves with H-concentration increase B 0 = 0.4 T, f = 7.5 MHz, P inp = 120 kW, n e (0) ≈ m -3, I P = 195 – 230 kA. The 2nd H-harmonic is absent in the plasma volume. C H increases from 10% to 70% Sensible ICRH efficiency improvement with increase of hydrogen fraction may be explained by strong short wavelength component of antennae spectrum Triangles – deuterons Circles – protons

Fast proton population approximately constant in the wide range of H- concentration during ICRH CHCH 15 %25 %35 %50 %60 %70 % N tail /N therm 15,7%11,4%11,5%7 %4,8 %3,5 % The effective hydrogen “tail” temperature (measured in the energy range 1.7 – 4 keV) decreases with C H rise. The ratio of the tail proton concentration to the thermal proton concentration drops sharply with increase of C H (see the Table). The total quantity of fast proton population in the plasma remained approximately the same in the course of experiment.

Plasma jet injection with double stage plasma gun 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. Criterion of penetration through magnetic field : ρV 2 /2 > B T 2 /2μ 0

Plasma jet injection into steady state discharge period Injection from the equatorial plane, along the major radius from the low field side. The distance between the plasma gun output and plasma was ~ 0.5 m Magnetic field at the center of the vessel was 0.4 T The jet speed 110 km/s. The jet density 2×10 22 m ‑ 3 at the gun edge. Comparison with low speed (~2 km/s) gas jet injection Time constant of density increase with plasma jet injection is much smaller, than with gas jet. Discharge is not disturbed

Plasma jet penetration through the magnetic field If ρV 2 /2 < B T 2 /2μ 0, how the plasma jet penetrates through the magnetic field? Study of jet penetration between poles of DC magnet of 0.3 T induction. The jet specific kinetic energy at the velocity of 75 km/s is less than magnetic pressure of 0.3 T field. The pressure signal varies as the magnet is moved from the gun towards detector from zero level ( the jet is blocked) to full pressure (~0.5 Atm – the jet passes freely). Time-of-flight recombination of dense cold (1 eV) plasma jet into the jet of neutrals with the same density and velocity makes the jet insensitive to the magnetic field. Outside plasma - Time-of-flight recombination into dense neutral jet occur

Plasma jet penetration into the plasma The TS measurements (left) show the jet penetration deep in plasma [Gusev NF ] Simulations inside plasma show – ionization of jet by hot electron influx and braking due to emission of Alfven waves (ionization is very fast -0.5mks). grad B drift accelerates jet towards LFS. Resulting effect allows the jet deposition beyond separatrix, unlike the case of molecular supersonic beam, which is definitely deposited outside separatrix [ Rozhansky 33 EPS 2006 Roma ]

Discharge initialization by plasma jet injection Injection at maximum loop voltage (UHF preionozation and prefill of the vacuum vessel are off). Number of injected particles is comparable with total number of the particles in tokamak (5×10 18 – 1×10 19 ). Plasma current ramps up faster than with traditional method. D-alpha and CIII start earlier. Plasma current is higher which confirms more intensive plasma heating at the initial stage of the discharge.

Gas generator of the plasma gun modification Two versions of gas generating stage: a- fresh grains loaded before each shot b-fresh grains loaded before series Double stage plasma gun A.V. Voronin 21 IAEA FEC 2006 Dependences of number hydrogen molecules on shot number a b

Summary New results were obtained during the reported period practically at all main direction of Globus-M tokamak research program. Greenwald limit densities are obtained both in OH and NBI heating regimes. Average densities, obtained in low field of 0.4 T reaches ( )×10 20 m -3 with gas puffing. NBI thermalization was studied. Slowing down of beam ions is well described by classical Coulomb scattering theory and the particle losses at least below E cr are insignificant. Regimes with overheated ions are achieved at densities below 2.5×10 19 m -3, heating of electrons is observed at densities higher (5-6)×10 19 m -3. The ICR heating study at the fundamental harmonic range were continued on Globus-M tokamak to make clear and self consistent the picture of RF heating in ST. ICR heating efficiency improvement with hydrogen minority concentration increase in the wide range of 10 – 70% was recorded experimentally and confirmed by simulations. The negative role of second harmonic hydrogen resonance positioning was outlined in the experiments. The reliability of the plasma gun as the source for plasma feeding and the instrument for the discharge initialization was confirmed. Numerical simulations of plasma jet interaction with core tokamak plasma were started, giving first results in tolerable agreement with experiments.