Spectroscopic Study of Interaction of High Power Plasma Stream with Lithium-Carbon Composites

at Multimirror Trap GOL-3

A.A. Shoshin, A.V. Burdakov, I.A. Ivanov, K.N. Kuklin,S.V. Polosatkin, V.V. Postupaev

Budker Institute of Nuclear Physics SB RASNovosibirsk State University

Li in Tokamaks

FTU limiter (J. Nucl. Materials 390–391 (2009) 876–885)

T11M limiter2002 Plasma Phys. Control. Fusion 44 955

Li Capillary-Pore SystemJournal of Nuclear Materials 390–391 (2009) 876–885

CDX-U “Try limiter” (Journal of Nuclear Materials 390–391 (2009) 876–885)

Li pellets in NSTX (Journal of Nuclear Materials 363–365 (2007) 791–796)

LiD pellets in GOL-3

Li vacuum evaporation in stellarator TJ-II (Journal of Nuclear Materials 390–391 (2009) 852–857)

Electron beam generator U-2

Ribbon beam diode with beam compression system

Solenoid with corrugated magnetic field

Plasma exhaust, materials test station

Multimirror trap GOL-3

Electron beam• 0.8-1 MeV • 30 kA • 8-12 s• up to 300 kJ

Plasma • length ~12 m• 1020- 1022 m-3

• temperature ~1-4 keV• 1 ms

Plasma stream parameters

2

0.1 1 10 100 1000energy, keV

0.01

0.1

1

10

100po

wer

den

sity

, M

W/c

m

/keV

Plasma

Suprathermalelectrons Beam

electrons

Energy density in the plasma stream 2 MJ/m2

Main power contained in the energetic (1-10 keV) electrons

Specific energy release is below volumetric destruction threshold (10 MJ/m2 )

Calculated energy deposition over depth of lithium under action of

electron stream in GOL-3 facility (power density 2 MJ/m2, target

inclined at 30º). Horizontal solid and dashed lines correspond to

start and end of phase transition, correspondingly. Initially target

was at room temperature.

0 1 2 3 4 5

0

200

400

600

800

1000

1200

z,mm

J/g

Melting point (181 C)

Li-graphite phase diagram

Physical Rev.B. 2010, v.82, n.12, p.125416

Physical Rev.B. 1984, v.30, n.12, p.7092

Known different binary compounds Li4C, Li6C2, Li8C3, Li6C3, Li4C3, Li4C5 …They are mainly thermodynamically metastable and after heating dissociate to metallic Li and Li2C2

SIBUNIT - SYNTHETIC CARBON MATERIAL

Sibunit is a new class of porous carbon-carbon composite materials combining advantages of graphite (chemical stability and electric conductivity) and active coals (high specific surface area and adsorption capacity).

Patents were granted in the Russian Federation (1990) and the United States (1992).

Boreskov Institute of Catalysis SB RAS, Novosibirsk

Compared to active coals, Sibunit has the following advantages:

• high mechanical strength; • chemical and thermal stability; • high purity.

Sibunit synthesis

Several lithium-carbon composite samples was made by different methods:

1. Lithium together with graphite or sibunit was heated in argon atmosphere to the temperature 700 ºС. In the experiments these samples were held in special graphite box.

2. Lithium was heated on graphite sample at temperature about 250-300ºС and after melting lithium was smearing for producing homogeneous layer of 0.5 – 1 mm depth.

Li-Sibunit

Li film on graphite

Plasma stream

Magnetic coils

Target

Diaphragm

Target holder

Vacuum chamber

Layout of target irradiation experiments in the GOL-3 exit unit

Beam

Plasma

Surveyspectrometerresolution 0.3 nmrange 100 nm

High-resolutionspectrometer

2D imagingsystemTarget

Optical diagnostics of target plasma

CCD

CCD

MDR-12 or DFS-24resolution 0.08 nm 0.005 nmexposure 7-300 microseconds

Plasma radiation in narrow spectral band

of graphite target (top) and sibunit-lithium

composite in 4cm graphite box (bottom)

under action of hot plasma stream moving

from the right.

Spectrum of surface

plasma produced near

lithium-graphite target

under action by power

plasma stream.

580 600 620 640 660 680

0

500

1000

1500

Li I

H

C II

C II

Li I

Intensity, a.u.

W avelenghts, nmOne can determine the plasma electron temperature by relation of intensities of lines Li I 610,36 и 670,78 nm. In different shots temperatures varied within 0.7 - 1.2 eV. Temperature of lithium plasma is less than temperature of surface plasma near graphite targets (was measured by ratio of C II lines). It corresponds to smaller first ionization potential and higher transmissibility of lithium with respect to carbon.

508 510 512 514 516 518 5200

0.2

0.4

0.6

0.8

1

1.2

Вт/см2*нм

длина волны, нм

Filter transition coefficientSpectrum С2

2D Spectral selective optical system

objectivelenses

Object

Narrow band filter

CCD камера

wavelength, nm

Consider a flux of atoms ФA, along a line-of-sight r from surface into a fully ionized plasma. If we assume all the incoming atoms are ionized by electron collisions, between r1 and r2

(1)

where nA(r) and ne(r) are the density of atoms and electrons. Ionization rate coefficient <ionization> is a function of the electron temperature Te(r). Electron impact excitation of the atom leads to photon emission with the intensity IA

(2)

where <excitation> is the electron impact excitation coefficient for the excitation of the upper level of the radiating state, and B is the branching ratio for the radiative decay which leads to appearance of the observed photons. Equations (1) and (2) give a relation between the particle fluxes and intensities. Provided the rates do not vary much over the observation volume we may write

(3)

Equation (3) enables conversion of the photon flux ФA (photons/сm2s) into the particle flux. The inverse photon efficiency S/XB is the ratio between ioniSation rate and the product of eXcitation rate and Branching ratio for the observed electronic transition. For 670.8 nm Li I lines S/XB = 1/8.6 was calculated with data from Aladdin database. Estimated atomic lithium flux from the surface was 11020 atoms/(cm2s), it is 3 times smaller with respect to atomic carbon fluxes from graphite targets.

XB

SI

hdrv

drvI

BhФ A

eexcitation

eionizationAA

44

2

1

)()(4

r

r

eexcitationeAA drvrnrnBh

I

2

1

)()(

r

r

eionizationeAA drvrnrnФ

Li-Sibunit (carbon) before and after irradiation 5 shots per 2MJ/m2

Li on graphite surface before and after 6 shots per 2 MJ/m2

View of carbon target with lithium films under action of cold plasma

in exit unit of GOL-3 facility.

Several lithium-carbon targets (including Li-Sibunit) were

designed, produced and tested under action of plasma stream.

Set of diagnostics was developed and used for investigation

parameters of surface plasma near targets.

It was shown that lithium erosion depth corresponds to melting

depth.

Temperature of surface plasma about 1 eV was measured.

Atomic lithium flux from surface was determined. The flux 1020

atoms/(cm2s) cannot explain the erosion value.

Conclusion