J. Plasma Fusion Res. SERIES, Vol.2 (1999) 324-327

Experimental Study of Shielding Layer Plasma Radiation atHigh Power Plasma-Material lnteraction
LITUNOVSKY Vladimir N., OVCHINNIKOV Ivan B.,
TITOV Vladimir A. and LYUBLIN Boris V.
D.V. Efremov Scientific Research Institute of Electophysical Apparatus
189631, St. Petersburg, Russia(Received: 8 December 1998 / Accepted: 13 May 1999)

Abstract
Some results of the study of radiation of shielding layer plasma in visible and VUV regions ofspectrum during high power (P1.-l0MWcm2) plasma irradiation to targets (graphite and tungsten) are

given in the report. It is observed the difference of some radiation characteristics in dependence ofresistration direction.

It is concluded that visible radiation power flux on the tarset surface can be characterised bv
quasistationary level during inadiation.

Keywords:
plasma-material interaction, shielding layer, radiation.

1. Introduction

The sum (P1 + P,) includes components that provide

The dense plasma Shielding Layer (SL) - productof material evaporation - appeared near the surface ofshielding effect itself. So, the shielding coefficient K,n=(Pi+ P,)lPt* represents the portion of irradiation power

the target at high heat plasma flux-material interactionthat is dissipated by SL and does not influence on

protects effectively the material against irradiation
power. The study of this phenomenon is an important
problem taking into account the wide spectrum ofpresent and future applications of plasma-materialinteraction (fusion technology, plasma processing ofirradiated target.
The performed experimental measurements of SL
radiation have shown the ability of SL to convert thegreater part of plasma irradiation power into backgroundradiation with high efficiency. Typically the values ofshielding coefficient K,6 are around -0.8 in experiments

structural materials, space technology etc.).
The power balance at the interaction process can be
described schematically by the following expression:

with plasma irradiation power Pi..-l0MWcm2 [].
Other part of power flux-P1n"- is responsible for thematerial damage. This power flux is determined by SL

P1. = P;n" f (Pt+ Pr),

thermal radiation and material plasma electron heat
conduction [2]. It is absent up to now the direct
measurements of P1n". So, any knowledge about

where P1. - plasma irradiation power flux, Ptn. - power
flux reached the exposed material surface, Pt - power
flux spent for the growth of SL internal energy, P. power flux emitted from SL as a background radiation.

radiation flux to the target surface will be useful for the
study of P;n" parirm€torS.

@1999 by The Japan Society of Plasma

Corresponding author's e-mail: vlitun@ niiefa. spb.su

Science and Nuclear Fusion Research324

Litunovsky V. N. er al., Experimental Study of Shielding Layer Plasma Radiation at High Power Plasma-Material Interaction

2. Experimental Gonditions

The main difference of SL radiative characteristics(for tungsten and graphite targets) was observed in theThe experiments were performed at the VIKA
facility [3]. Long-pulse coaxial plasma accelerator - thesource of plasma high heat flux in the VIKA facility - isable to generate hydrogen plasma flows with power flux

visible region. It indicates that main variations in the SL
structure for different target materials consist of both thechange of the thickness of the plasma layer that emitted

continuum radiation and variation of continuumon the target up to -10MWlcm2 at pulse duration up to
Tp = 0.36ms. The use of power supply - Pulse Forming
Network (5 kV, 100 kJ) - allows to form plasma flows
with quasistationary parameters during pulse duration.

radiation intensity. So, for tungsten target the continuum
is radiated from a very thin layer above the surface(-0.5mm), for carbon this thickness is 3-4mm [5].
The registration of the time- and space-integrated
SL plasma spectra for graphite target in visible, UV andThe effective diameter of plasma stream is do-4cm.
The diagnostic complex to determine the main
parameters of both incident plasma stream and SL isused in experiments. The spectroscopic apparatus allows

to be studied plasma radiation in a wide range of
wavelength - from visible region to deep UV-vacuum
region [4].

Two schemes of the observations of SL radiation
were used in described experiments: l. side on view
through quartz window or vacuum ttbe;2. view throughthe hole in target (graphite) with use of quartz fiber(registration of SL "back side" (BS) radiation). For
second case the registered spectral range was L= 400-&800nm.

v

I

It was used also the data of the measurements ofprobing laser beam ()" = 632.8nm) absorption in SLplasma. The measurements were performed in theRarodirection parallel to target surface for several distancesfrom the target.

20

il*" n"*,#50Fig. 1 The fragment of typical space resolved VUVspectrum of SL radiation. Target - tungsten.

3. Experimental Results
The performed earlier study of SL plasma

lrradiation power fl,,-5MWcm2.

properties during irradiation of tungsten and graphitetargets has shown that this dense (n"-l02am-3) plasma
layer can be optically thick for visible radiation fordeveloped phase of SL formation [4]. In this connection
the study of SL radiation in VUV region of spectrum(for tungsten and graphite targets) was continued. It wasobserved that carbon, oxygen and nitrogen ions lines
give the main contribution in the line emission of nearsurface plasma zone for both targets (Fig.l.) ThisIb

spectral region is characterised by the maximal intensityof radiation in neighbourhood of wavelength 1= l5nm.
As it is seen the thickness of intensively radiated plasma
layer is of -l5mm.
The comparison of observed spectra data with thecalculations of the plasma ionisation state for the LTE
approximation (Saha-equilibrium) shows that the valuesof the electron temperature should lie in the range fromT"= 8-l2eY (in the central region of SL) up to T" = 34eV (in a periphery).

Wavcl.ng6, AFig. 2 The typical observation spectrum of SL radiationin visible and UV regions of spectrum. Target graphite. lrradiation power f,,-SMWcm'.
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Litunovsky V. N. er a/., Experimental Study of Shielding Layer Plasma Radiation at High Power Plasma-Material Interaction

near VUV spectral ranges reveals the presence ofcarbon

was observed also some impurities (Al, Mn, Na, Cu)
and hydrogen spectral lines. The most probable sourceof the impurities is due to plasma gun. One can supposethat the observed structure of visible radiation can be
distorted by the opacity of the near-surface plasma layer
with the density n.) l}2am-3. Really, one can see thatspectral lines CII, CIII and CIY, the most intensive
being the lines CII (z = 1) in visible range (Fig. 2). It

dynamics of observed SL radiation is modulated
sufficiently by high absorption in the plasma layer'6<aclosed to the target surface (Fig. 3, bottom curve of Fig.
4).

H@T-l'-

With the aim to exclude this effect it wasI_=<c-.+- -.-1, ,,50

IOO I i)l) 2oo 25O :too:t50

4ll04t,O

performed the registration of visible radiation throughthe hole in the target. Its difference from SL backgroundradiation (in a corresponding region of spectrum) was
observed: practically full absence of impurities spectral
lines and increased intensities of lines and continuum
radiation. Besides one can see the absence of strong
modulation of intensity even for more long-wave
radiation (upper curve of Fig. 4).

SOllt.' ;r--4. Discussion and Gonclusions
The observed spectra of background radiation inVUV region have shown practical absence of target
material spectral lines. As to graphite target, the absenceof carbon lines in the spectrograms of the far VUVrange can be explained by the lack of the intensivel-Z---)-)---) '--J50 roo l5o ?00 250 300 3!-,t) 400 ,ll-r0o ()"spectral lines CIII and CIV in this range. Ions with z = 4t' P.

(CV-lines) are not emit the spectral lines at evaluatedelectron temperature. As to tungsten target, one canFig.3 a, b. The dynamics of the continuum (a) andabsorption (b) intensities. Target - tungsten.

assume that the "tungsten plasma" layer is very thin, andbesides its radiation is masked by the intensive

lrradiation power ff,,-SMWcm2.

continuum radiation.
Taking into account all abovementioned data, one

can assume that SL has an intricate structure in\\r/ttl\?
r0b

described experiments. The estimation shows that bulkof irradiation power can be reradiated by the impurities
in the upper plasma layer with composition determined\

mainly by the compressed incident plasma stream. This\

mixed layer has the highest electron temperature,
determined by the thermalisation of kinetic energy (e1 =0.2keV [ ]) of the incident plasma flow. The observed
parameters and temporal behaviour of this radiation aredetermined by this plasma layer properties that can be\aoo
2Co 3ooqr|sstrongly changed in this open system under both
irradiation conditions and target material response.

Fig. 4 The dynamics of the relative radiation intensities

As to SL BS radiation one can believe that its
structure is determined by the radiative properties ofcold and very dense material plasma in a considerable
extent. The observed quasistationary character of BSradiation intensity during irradiation reflects the true

for two directions of registration. Target - graphite. lrradiation power f.,-SMWcmz. bottom curve- CIII carbon line (1. = 229.7nm1, side-on registra-tion; upper curve - CII resonance line (1 =426.7nm!, registration through the hole in the target.

326

Litunovsky V. N. e/ a/., Experimental Study of Shielding Layer Plasma Radiation at High Power Plasma-Material Interaction

dynamics of visible radiation power flux on the target
surface (at the absence of opaque plasma layer between
this radiation source and the target surface). Assuming
the sufficient role of the radiation in the level of powerUSA) for fruitful discussions.

The work is performed under partial support of
ISTC (Project #539).

flux reached the material P;ns on€ can suppose that last

References

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[3] V.M. Kozhevin, V.N. Litunovsky, B.V. Ljublin,
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34-35,359 (1997).

t5l V.N. Litunovsky, A.A. Drozdov, V.E. Kuznetsov,

structure of SL background radiation determined by thefeatures of studied open plasma system - SL, theradiation flux reached the surface of target isquasistationary.

B.V. Lyublin, I.B. Ovchinnikov and V.A. Titov,
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t6l V.N. Litunovsky, V.E. Kuznetsov, I.B.
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Acknowledgements
The authors would like to thank Drs. I.
Konkashbaev (TRINITI, RF) and A. Hassanein (ANL,Symposium on Fusion Technology (FUSIONTECHNOLOGY 1998), v.l, p. 59.

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