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.VoroninIoffe Physico-Technical Institute, RAS, 194021, St. Petersburg, RussiaE-mail: vasily.gusev@mail.ioffe.ruV.G.Kapralov , I.V.Miroshnikov, V.A.Rozhansky, I.Yu.Senichenkov, A.S.Smirnov, I.Yu.VeselovaSt.-Petersburg State Polytechnical University, 195251, St. Petersburg, RussiaS.E.Bender, V.A.Belyakov, Yu.A.Kostsov, A.B.Mineev, V.I.VasilievD.V. Efremov Institute of Electrophysical Apparatus, 196641, St. Petersburg, RussiaE.A.Kuznetsov, V.N.Scherbitskii, V.A.YagnovSSC RF TRINITI, 142192, Moscow region, RussiaA.G.Barsukov, V.V.Kuznetsov, V.M.Leonov, A.A.Panasenkov, G.N.TilininNFI RRC “Kurchatov Institute”, 123182, Moscow, RussiaE.G.ZhilinIoffe Fusion Technology Ltd., 194021, St. Petersburg, Russia

Presented at 11th IST Workshop, 11-13 October, 2006, Chengdu, PR China

Outline1. 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 thermalizationElectrons heating – dependence on density and Ebeam

3. ICR heating at fundamental harmonic: General features, energy exchange, antenna spectrum, simulation and experimental result of CH 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

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 1.21020 m-

3

Pulse duration 200 ms 130 msSafety factor, edge4.5 2Toroidal beta 25% ~10%

ICRF power 1 MW 0.5 MW frequency 8 -30 MHz 7.5-30 MHz duration 100 ms 30-80 ms

NBI power 1.3 MW 1 MW energy 30 keV 30 keV duration 30 ms 30 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

<ne> ~ 1-1.21020 m-3 were achieved,

(n/nG)~1

Gusev NF 46 7 2006

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/nG)~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/nG~1 and approached is n/nMur~1

Average densities up to 1.21020m-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 temperatureTe(0) 0.95 keV is achieved at <ne>~1.51019m-3

Max electron density

Ne(0) 1.5 1020m-3

is achieved at <Te>~175 eV

0.2 0.3 0.40

200

400

600

800

1000

Te,

eV

R, m

#13802155ms

(1-(r/a)2)1.7

0.2 0.3 0.40.0

5.0x1018

1.0x1019

1.5x1019

2.0x1019

(1-(r/a)2)1

ne,

m-3

R, m

0.2 0.3 0.40

100

200

300

(1-(r/a)2)1.5

Te,

eV

R, m

#13727160ms

0.2 0.3 0.40.0

5.0x1019

1.0x1020

1.5x1020

2.0x1020(1-(r/a)2)1.1

ne,

m-3

R, m

NBI heating in Globus-M

NBI 130 140 150 160 170 180

100

200

300

400n

H/(n

H+n

D) = 20%

TD (

eV

)

t (ms)

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 RL 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)х1019m-3, for higher densities correction for plasma opacity should be done.

NB thermalization in Globus-M

0 5 10 15 20 25104

106

108

1010

Ecr EbEb/2

OH Ti=197 eV

NBI Ti=357 eV

(E3/2 + Ecr

3/2)-1

Beam slowing downparticles (trapped)

Thermalizedparticles

Globus-M2004.06.21 (#9191)t = 136.5 ms

cx /

E1/

2 (

eV3/

2cm

2 ster

s)-1

E (keV)

Eb/3

“Perpendicular’ spectrum of fast particles, measured by 12 channel NPA.

Above Ecrit (12-22.5 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-MShowed 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 ~0.3POH – Ti 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 χNEO) [Shcherbinin,…Leonov NF 46 7

2006]. 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).

-200 -100 0 100N z

Pra

d, a

rb.u

n

Spectrum of single loop antennae in Globus-M

The peaks correspond to resonator modes excited in the chamber. Short wavelength components (lNzl~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 46 7 2006

The elimination of second harmonic of

hydrogen resonance improves ICR heating

9MHz, 4T

-20 0 20x, cm

-40

-20

0

20

40

y, c

m

1D

2D, 1

H

3D

7,5 MHz, 4T

Specific Features of ICRH on ST

ωH2ωD

ωH2ωD 3ωD

2ωH

Several Resonances One Resonance

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 46 7 2006-20 -10 0 10 20

r,cm

dP/d

r, a

rb.u

n.

-20 -10 0 10 20

r,cm

dP/d

r, a

rb.u

n.

-20 -10 0 10 20

r,cm

-20 -10 0 10 20

r,cm

C H=50%|N z|<150

C H=50%|N z|<20

C H=10%|N z|<150

C H=10%|N z|<20

High absorption at r - 5cm – for protons at high CH is the consequence of short wave part of the exited spectrum

Ion heating improves withH-concentration increase

B0 = 0.4 T, f = 7.5 MHz, Pinp = 120 kW, ne(0) ≈ 3.1019m-3, IP = 195 – 230 kA.

The 2nd H-harmonic is absent in the plasma volume.

CH 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

CH 15 % 25 % 35 % 50 % 60 % 70 %

Ntail/Ntherm 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 CH rise.

The ratio of the tail proton concentration to the thermal proton concentration drops sharply with increase of CH (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 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. Criterion of penetration throughmagnetic field :

ρV2/2 > BT2/2μ0

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 2×1022m‑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 ρV2/2 < BT2/2μ0, 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

0.0 0.5 1.0 1.5 2.00.1

1

10

100 R (cm)

Te | z=

0 (eV

)t (s)

0 5 10 15 20

The TS measurements (left) show the jet penetration deep in plasma [Gusev NF 46 7 2006]

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 (5×1018 – 1×1019).

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 shotb-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 (1.1-1.2)×1020m-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 Ecr are insignificant. Regimes with overheated ions are achieved at densities below 2.5×1019m-3, heating of electrons is observed at densities higher (5-6)×1019m-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.