Aleksey Beletskii

Electrostatic edge plasma turbulence in the Uragan-3M torsatron

Outline Introduction Electrical probes setup. General discharge

conditions Plasma density fluctuations in SOL and

divertor Er shear formation. Its influence on edge

turbulent transport Contribution to International

Stellarator/Heliotron Edge Turbulence Data Base

Future plans Conclusion

Introduction

FUSION PHYSICS. Edited by M. Kikuchi, K. lackner, M. Quangtran Vienna : International Atomic Energy Agency, 2012, p. 368

• Edge plasmas play an important role and often even dominate central plasma processes • The understanding of the plasma edge and the control of the edge conditions critically depend on measurements of the local plasma parameters • Electrical probe measurements are one of the earliest approaches in plasma diagnostics. In plasmas in which probes can survive, this diagnostic remains the easiest and most accurate way to measure the local plasma parameters: Te, Ti, ne, Vfl, Vpl • Electrostatic turbulence driven transport, as well as local particle fluxes can only be fully evaluated with electrical probes • Electrical probes in general include the variety of diagnostics with one or several electrodes contacting with a plasma and using an electrical measurement technique. Ion temperature, ion and electron energy distribution and impurity ions content can be obtained with more complicated and sophisticated kinds of electrical probes

G R Tynan, A Fujisawa and G McKee, Plasma Phys. Control. Fusion 51 (2009) 113001, 77pp FUSION PHYSICS. M. Kikuchi, et al. Vienna : International Atomic Energy Agency, 2012, p. 368

H-mode (1982)

Edge plasma play an important role and often even dominate central plasma processes

Electrostatic turbulence driven transport, as well as local particle fluxes can be fully evaluated with electrical probes

X. Garbet / C. R. Physique 7 (2006) 573–583

Introduction

• Torsatron: • A large vacuum chamber, 70 m3

• Open natural helical divertor • Magnetic islands

• Continuous hydrogen admission, 10-5÷10-4 Torr

• РRF ≲ 150 kW, tpulse ~ 50 ms

A7 D7 VG8

03, 9, 100( ) / 2 0.3, 12 , 0.72ϕ

= = =

ι π ≈ ≈ =

m R cma a cm B T

Electrical probes setup

III

II

I

III

III

I

II

III

movableprobes

(VG8, VG7)ion energyanlyzers(A8, D7)

divertorprobes (D7, A7)

ion energyanlyzers

(D5,A6,D6)

RF antenna

G. G. Lesnyakov, E. D. Volkov, A. V. Georgievskij, et al., Nucl. Fusion 32, 2157 (1992); V. N. Kalyuzhnyj and V. V. Nemov, Fusion Sci. Technol. 46, 248 (2004).

U-3M Magnetic system

Magnetic field Poincaré cross-sections where probes were disposed

The Langmuir probes array (LP) was moved parallel to the torus midplane (z = 1 cm) from 9.4 cm (-0.6 cm inside LCMS, edge plasma) to 12.0 cm (2 cm outside the LCMS, SOL). As a recording facility, a 12 bit ADC with 1.6 µs sampling rate/channel was used.

Electrical probes setup

• radial turbulent particle flux

• wavenumber-frequency spectrum

• mean poloidal phase velocity

• skewness, kurtosis

3 2 1( ) ( )[ ( ) ( )]υΓ = ∝ − −

r s f ft n I t V t V t

( , )stat locP kθ ω

phθυ

E.J. Powers, Nucl. Fusion, 1974, 14, P. 749-752 S.J. Levinson, J.M. Beall, E.J. Powers and R. D., Nucl. Fusion 24, 527 (1984)

Plasma density fluctuations

below torus midplane

0 20 40 60 80 100kHz

0

0.2

0.4

0.6

0.8

1

pow

er, a

.u.

0 20 40 60 80 100kHz

0

0.2

0.4

0.6

0.8

1

pow

er, a

.u.

above torus midplane

cross-section Across-section D

0 20 40 60 80 100kHz

0

0.2

0.4

0.6

0.8

1

pow

er, a

.u.

0 20 40 60 80 100kHz

0

0.2

0.4

0.6

0.8

1

pow

er, a

.u.

close to the LCMSVG7VG8

more distant from LCMS

SOL

divertor

V. Chechkin et al., Nucl. Fusion, 2002, V. 42, P. 192; T. Mizuuchi et al., J. Nucl. Mater. 313–316 (2003) 947. A. A. Beletskii et al. Plasma Phys. Reports 2009, 35, 10, p. 818-823

In the SOL to more (less) distantly located points relative to the LCMS, higher- (lower-) frequency fluctuations are inherent. A similar spectral splitting in two subranges occurs in the diverted plasma flows (DPF) too, depending on flow disposition relative to the torus midplane.

A manifestation of the up-down asymmetry of diverted plasma flows in U-3M

Er shear formation

The fast ion content passes over a maximum and a short-time burst of fast ion outflow to the divertor takes place. Simultaneously a hard bifurcation of the floating potential and, correspondingly, radial electric field shear increase are observed

I. M. Pankratov et al. Contrib. Plasma Phys. 2010, 50, 6-7, p. 520–528

0.9 1.0 1.1-20

020406080

100120140

0.9 1.0 1.1-20

020406080

100120140

∇Er weak Er,Volt/cm -0.005 mc -0.004 mc -0.003 mc -0.002 mc -0.001 mc 0.0 mc

Vfl, Volt

∇Er strong

0.001 mc 0.002 mc 0.003 mc 0.004 mc 0.005 mc

ρ

-300-200-1000100200300400

-300-200-1000100200300400

∇Eh weaker ∇Eh stronger wavenumber-frequency power spectrum

Fluctuations poloidal phase velocity

Turbulent particle flux

Er shear formation

9.0 9.5 10.0 10.5 11.0-4-3-2-10123456 LCFS

υ ph,

105 c

m/c

h, cm9.0 9.5 10.0 10.5 11.0

-4-3-2-10123456

υ ph,

105 c

m/c

h, cm

LCFS

A strong radial electric field shear and, consequently, a higher E×B velocity shear result in decrease of turbulence-induced anomalous transport in the URAGAN-3M torsatron Density fluctuations statistics changes from Levy to Gaussian after ∇Eh increase Temperature fluctuation yet have to be taken into account B.Nold, New J. Phys. 14 (2012) 063022

International Edge Turbulence Data Base

The U-3M electrical probes measurements are included in the International Stellarator/Heliotron Edge Turbulence Data Base. A universal parabolic relation between S and K occurs in the case of non-diffusive transport when non-Gaussian distribution of turbulent fluctuations take place (F. Sattin et al., Phys. Scr. 79 (2009) 045006). The relation is true for edge plasma fluctuations in U-3M. Scaling and transport analyses based on the international edge turbulence database have been carried out by P.Simon

Nold B. et al., 37th EPS Conference on Plasma Physics (Dublin, Ireland 2010); 7th Coordinated Working Group Meeting (Greifswald 2010); P Simon, M Ramisch, A Beletskii et al., Plasma Phys. Control. Fusion 56 (2014) 095015 (8pp)

https://ishpdb.ipp-hgw.mpg.de

Future plans

0 2 4 6 8 10 12 14 160

2

4

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14

16

Is

Bϕ

V+ Is

VflVfl

84 12

8

6

BPP

Bθ

BPP

mm

BN

W

A new combined probe will be utilized in future experiments in order to: • Direct measurements of edge plasma potential • Electron temperature fluctuations • Radial turbulent particle flux • The parallel heat flux

R. Schrittwieser, J. Ad´amek et al., Czech. J. Phys. 56 (2006) C. Silva et al., Plasma Phys. Control. Fusion 57 (2015) 025003 (9pp) J. Adamek et al., Nucl. Fusion 57 (2017) 022010 (9pp)

Conclusion

Asymmetric loss of particles with higher energies results in spectral splitting of plasma density fluctuations

The radial electric field shear strongly affects edge turbulent transport in the URAGAN-3M torsatron

Fast ion loss is the cause of the strong Er shear formation

In U-3M, the edge transport is strongly non-diffusive. Universal statistical relation K=1.54S2 + 2.68 is satisfied

Electron temperature fluctuations should be included to the International Stellarator/Heliotron Edge Turbulence Data Base

Investigations of electrostatic turbulence in U-3M will be continued with use of the combined probe