The extreme ultraviolet spectra of C VI have been studied for a Neutral Beam Injection (NBI) plasmas in Large Helical Device (LHD). A strong distortion in the population distribution over the excited levels was observed and we conclude that is caused by charge-exchange recombining (CXR) processes between C^6+ ion and recycling neutral hydrogen. Spatially resolved measurements show that the C^6+-H CXR processes take place in the plasma peripheral region in LHD. We have taken a CXR pan of C VI 1s-4p line using the result of a calculation code

Experimental Group LHD

GOTO  Motoshi

K. Kawahata

K. Sato

KAWAHATA  Kazuo

KOBUCHI  Takashi

LHD Experimental Group

M. Goto

MOTOJIMA  Osamu

N. Ohyabu

O. Motojima

OHYABU  Nobuyoshi

S. Sudo

SATO  Kuninori

SUDO  Shigeru

T. Kobuchi

National Institute for Fusion Science (NIFS-Repository)

J. Plasma Fusion Res. SERIES, Vol.5 (2002) 167-170Observation of Gharge-Exchange Spectra on C6*+Hin Low-Energy GollisionKOBUCHI Takashi, sATo Kuninori, Goro Motoshi, oHyABU Nobuyoshi, KAWAHATA Kazuo,SUDO Shigeru, MOTOJIMA Osamu and LHD Experimental GroupNational Institute for Fusion Science, Toki 509-5292, Japan(Received: 1 I December 2001 / Accepted: 23 August 2002)AbstractThe extreme ultraviolet spectra of C VI have been studied for a Neutral Beam Injection (NBI)plasmas in Large Helical Device (LHD). A strong distortion in the population distribution over theexcited levels was observed and we conclude that is caused by charge-exchange recombining (CXR)processes between C6* ion and recycling neutral hydrogen. Spatially resolved measurements show thatthe C6+-H CXR processes take place in the plasma peripheral region in LHD. We have taken a CXR panof C VI 1s-4p line using the result of a calculation code.Keywords:charge exchange recombination (CXR), low-energy collision, C6*+H, collisional-radiative model (CRM),LHD1. IntroductionCharge-exchange recombination (CXR) cross-sections of highly-ionized ions with neutral hydrogen,which can have the order of 10-15 cm2, are one of thelargest atomic collision processes in plasmas. This CXRprocess: A+q + H0 -, A+(q-l)* + H' and the subsequenttransitions (termed as charge-exchange spectroscopy(CXS)) have been intensively studied in atomicphysicists and plasma spectroscopists [-3]. Especiallyas a plasma diagnostics in fusion plasma, CXS usingNeutral Beam Injection (NBI) provides capabilities ofDoppler ion temperature, plasma rotation, impuritydensity and transport measurements [4].Besides the high-energy neutral beam, there existneutral hydrogen having low energy (E < a few tens eV)in plasmas. These hydrogen atoms can be classified intotwo kinds [5,6] as follows. (1) Cold particles by gaspuffing. Since most ofthe neutral hydrogen is producedby Frank-Condon dissociation, this energy is about 3eV. (2) Recycling particles. These are mainly producedby charge-exchange neutrals reflecting on the wallsurface or by plasma bombardment. These recyclingparticles may have higher energies (- a few tens eV)than Frank-Condon neutral and can deeply penetrateinto the plasma [5].In the case of CXR in low-energy collisions, theelectron is selectively captured into a single low-excitedlevel of ions. State-selective single-electron capturebetween hydrogen atom and fully-stripped ion has beenso far investigated experimentally and theoretically [7].It is known that the principal quantum number n of theions, to which the electrons are selectively captured inlow energy collisions, is determined by the charge 4 andis irrespective of the ion species [8]. For C6* + Hcollisions, the dominant level of electron capture is n =417).In this paper, we discuss CXR on C6* + H in low-energy collision. We have carried out spatially resolvedmeasurements, and it is confirmed that the reacting@2O02by The Japan Society of PlasmaScience and Nuclear Fusion ResearchC o r r e s p o ndin g aut ho r' s e - mail : ko b uc hi @ nifs. ac j p167Kobuchi T. et al., observation of charge-Exchange spectra on c6*+H in Low-Energy Collisionregion of CXR processes is in the plasma peripheryregion. Next, we have calculated C VI intensitydistribution from the computer code, and have comparedthem with the experimental results. Finally, we discuss amechanism occurring the CXR in the low-energycollision.2. Gollisional-Radiative Model CalculationThe emission line intensity distribution of C VILyman series has been calculated with the COLRADcode [9]. COLRAD code is a collisional-radiative modelcode for hydrogen-like ions which solves the coupledrate equations about the time derivatives of the excitedlevel populations. Assuming the quasi-steady-stateapproximation, the population densities of the excitedlevels are obtained for a siven set of electrontemperature and density.Figure I shows the C VI line intensity ratiosI(Ly B )/ I(Ly a) and I(Ly y)/ I (Ly B ) calculated usingCOLRAD code for the plasma parameter range of n"=1012-1014 cm-3 and T" = 10-1000 eV. Only the ionizingplasma component, which is proportional to the densityof ground state of Be-like ion, is shown. These resultslittle depend on n", and in the Z" range of 100 eV to1000 eV, ?" dependence is also small: the intensityratios show I(Ly B ) l I(Ly a) = 0.09+0.03, I(Ly y)l I (Ly B )= 0.26+0.03. The parameters in the emission region ofC5* ions are thought to be in the above ranges, so theintensity ratios of Lyman series are expected to beindependent on both of 7" and n" in our experiment.100l0-r10-210-3I(rvv/I(LvF)\I(Lyp)/I(Lyct)lol2 - lola cm33. Experimental ArrangementLHD is a large heliotron type device (l/m = 2ll0)using super conducting coils with a nominal majorradius R of 3.9 m, a minor radius a of 0.6 m andmagnetic field strength of - 3 T. The electron density n"of - 1.2 x 10ra cm-3 and electron temperature T"of - 4.5keV have been achieved with NBI of 3 MW and pelletsinjection.In LHD, an impurity is monitored using a 2mgrazlng incidence spectrometer (Schwob-Fraenkel typespectrometer), which is equipped with two multichanneldetectors. The wavelength coverage is 0.5-34 nm with aresolution of 0.02 nm using a 600 grooves/mm grating.This XUV spectrometer is installed at the end ofmanifold for the vacuum pumping system of LHD. Theneutral beams do not cross the field of view of thespectrometer.To move the field of view, spectrometer is rotatedaround a pivot located 1 meter ahead from the entranceslit. Figure 2 shows the line-of-sight for spatiallyresolved measurements using the XUV spectrometer.The last closed flux surface (LCFS) and the ergodiclayer in vacuum magnetic field on magnetic axisposition R* = 3.75 m are shown in Fig. 2. The fields ofview for the measurements at the plasma center andedge region are also shown. In the case of center view,the line-of-sight crosses the X-points and the thick partof the ergodic layer. The line-of-sight of 500 mm heightmonitors mainly the ergodic layer.Divertor leg500m highSOXMOSHolizontal-.-fimer wallLmt Closed Flux Surface (LCFS)Fig. 2 The line-of-sight of spectrometer.x:.1Fl10-410 100 1000T" [eVlFig. 1 Emission ratio l\LyFlll(Lya) and /(Lyy)//(Ly0 calcu-lated using COLRAD code at n. = 1012 -1014 cm-3and[=10-1000eV.168Kobuchi r. et al., observation ofcharge-Exchange Spectra on ce+H in Low-Energy collision4. Results and Discussion4.1 A "distortion" in the regular decrement ofthe Lyman seriesTypical spectra of C VI Lyman series observedwith the line-oI-sight at the plasma center with exposuretime for 0.4 sec are shown in Fig. 3. Figure 3(b) shows a"distortion" in the regular decrement for Ly7 line.Though the relative intensity of LyB to Lya is consistentwith the theoretical prediction, the intensity rutio of LyytoLyP reaches to 0.9, it deviates far from the expectedvalue of 0.3. The result suggests an additional electronpopulating processes to n = 4 level of C5* ion.3 10130 200 400 600 800 1000pixel numberFig. 3 Typical spectra of C Vl Lyman series.0 100 200 300 400 s00 600height [mm]Fig. 4 Spatially resolved distribution of Lyman seriesspectra.4.2 Spatially resolved intensity measurementIn order to investigate the reacting region for C6*-Hcharge-exchange collision, spatially resolved intensitymeasurements were tried shot by shot for C VI Lymanseries spectra, and an intensity distribution of C VIseries is shown in Fig. 4. A chordal radius, theperpendicular distance from the magnetic axis to thechordal view of the spectrometer at magnetic axis, wasscanned from 0 to 500 mm. The measurements weremade in a series of nominally identical NBI heatingplasma with the total stored plasma energy IVo - 0.4 MJ,the line averaged electron density n. - 2-3 x 1013 cm-3,the total radiation power P.u6 - 0.5 MW, magnetic fieldBt= 2.75 T and magnetic axis Ru* = 3.6 m. Solid lines inthe figure are fitted results of the measurement. Thefitted lines of I(Ly B) and I(Lyy) are within the errors ofthe measurement, but this is not the case for I(Lyu).The relatively smaller values at 250mm might be causedby the slightly different plasma conditions, so we don'tthink that they indecate a change in the emission profile.The spatial profiles of Lya and Lyp intensitiespresent hollow shapes, and both intensities steeplydecrease in the plasma peripheral region, at around 350mm height in chord radius. On the other hand, theintensity distribution ofLyy is different from the otherlines. An intensity inversion between LyB and Ly7 isseen at around the chordal view of 450 mm height.In order to make clear the region where the C6+-HCXR processes predominantly take place, the Abelinversion is carried out using the fitting lines in Fig. 4.Intensity contour is defined using the vacuum magneticNormalized Minor RadiusFig. 5 Radial profiles of observed emission intensity ofLya, LyB and Ly7. These are calculated using Abelinversion method. Ly7"" has been estimated fromthe relation between LyB and Ly7 on CRM.oxIo2 2 totto,9 11gtr()q040f-i:o;3zovGgilrh6-iqF>ootlIIIIII(b) qJ'r(a)EE()Qlltlt(tltttlIItl"Lya(xo.r)Lyd (xl0-r)t69Kobuchi T. et aI., Observation of Charge-Exchange Spectra on C6*+H in Low-Energy Collisionsurface in magnetic axis position Ru* = 3.6 m. In theoutside region of LCFS (p > 1.0), it is simply assumedthat a closed magnetic surface still exists. Figure 5shows the Abel-inverted radial emission profiles ofLyman series. The radial profiles of Lya and LyB havethe same shape and peak position at normalized minerradius p = 0.9. The peak position of LyTis located atoutside those of Lya andLyB.We have tried to pick up the CXR component(LyTc) from the measured Ly7 intensity. We defineI(Lyyc) as follows:I(LvYc) = I(LvT) - 0.26 x I(Lvb, (1)where the value of 0.26 is the normal ratio (see. Fig. 1).This assumption is reasonable for our experiment be-cause the electron temperature is a few hundreds eV atthe ergodic layer and the peak of the radial profile ofC5* exists at the inside the ergodic layer. The right-handside of Eq. (1) represents the difference between themeasured Lyy and the intensity calculated with CRM.The emission profile of Lyygt is also shown in Fig. 5.This differs from the emission profile of Lya and Ly B,and peak position is located at outside those of Lyu andLyB. The emission profile of Lyygy suggests the react-ing region of the CXR processes.5. GonclusionWe have shown the characteristics of the CXR ofcarbon ions in low-energy collisions and have foundCXR spectrum which originates from the low-energycollision on C6* + H system. Using the spatiallyresolved measurements and Abel inversion technique,the reacting region in the low-energy collision on C6* +H system is found at peripheral region. Comparing theresults of COLRAD code with the relative intensity ofLyB, we estimate CXR component in the measured Ly7line intensity.In order to investigate a behavior of recyclingparticles and the influence to plasma confinement, wemust take a plasma parameter condition in which theCXR process occurs in the low-energy collision on C6*+ H system. We try to reproduce the measureddistribution in decrement of C VI Lyman seriesemission from CORLAD code with CXR process.Assuming that the condition from CORLAD code withCXR process is taken, we may understand a mechanismin which the CXR occurs in low-energy collisions indetail.In order to extend the discussion, we have toinvestigate a wide range of plasma parameter and otherelements. We expect to observe the same charactersabout other elements, oxygen etc. If other elements arealso observed with the same characters, themeasurement of such as a distinctive feature may beused for the observation of a behavior and density ofrecycling particles by comparing the results of a modelcalculation with the CXR process at more central region.AcknowledgementsWe are grateful to technical staffs for their help inoperating the LHD, NBI, ECRH and diagnostics. Andwe also wish to thank for the other members of LHDgroup.References[l] R.C.Isler, Phys. Rev. Lett.38, 1359 (1977).[2] R.K. Janev, D.E. Belie and B.H. Bransden, Phys'Rev. A 2E,1293 (1983).t3l R.C. Isler, Plasma Phys. Control. Fusion 36, l7l(1994\.t4l K. Ida and S. Hidekuma, Rev. Sci. Instrum 60,876(1989).t5l Y. Nakashima et al., J. Phys. Soc. Jpn. 52, 4166fl983).t6l H. Takahashi et al., J. Phys. Soc. Jpn. 44, 1363(1978).t7l N. Toshima and H. Tawara, NIFS-DATA-26.t8l M. Kimura et al., J. Phys. Soc. Jpn. 53,2224(1984).t9l N.N. Ljepojevic, R.J. Hutcheon and J. Payne, Com-puter Physics Communications 44,157 (1987).t'to

Observation of charge-exchange spectra on C6+ +H in low-energy collision

Observation of charge-exchange spectra on C6+ +H in low-energy collision

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