Bondarenko, V.N.

Konovalov, V.G.

Shapoval, A.N.

Slavny, A.S.

Voitsenya, V.S.

Volkov, E.D.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

TEMPERATURE OF IMPURITY IONS IN A RF HEATED PLASMA OF THE U-3M TORSATRON AS MEASURED BY MEANS OF THE DOPPLER SPECTROMETRYV.G.Konovalov, V.N.Bondarenko, A.N.Shapoval, A.S.Slavny, V.S.Voitsenya, E.D.VolkovInstitute of Plasma Physics, NSC KIPT, 61108 Kharkov, UkrainePACS: 52.55.Hc; 52.70.Kz1. INTRODUCTIONThe  spectroscopic  methods  of  measurements  of  ion temperature,  Ti,  are  routinely  used  in  magnetic  fusion experiments. However some strong requirements must be satisfied as for the quality of spectroscopic equipment and reproducibility of plasma parameters when spectral  line profile  is  obtained  shot  by  shot.  Besides,  the  radial location of ions of the given ionization state in the plasma volume  should  be  known.  The  correctness  of  Ti measurement results  has to be controlled by comparing them  with  temperature  Ti  of  the  main  plasma  ion component (H+, D+) by calculation of energy balance for impurity  ions.  Important  is  also  the  requirement  to estimate  the  role  of  other  possible  mechanisms  of broadening of  the impurity ion line profiles.     The goal of this paper is to determine the plasma ion temperature  using  data  on  profiles  of  spectral  lines  of intrinsic impurity, i.e.,  carbon ions (lines CV 227.1 nm and CIII 229.6 nm) in the plasma confinement volume of U-3M torsatron and to compare these results with data of Ti measurements  based  on  the  energy  distribution  of charge  exchange  atoms  (CXA)  obtained  by  neutral particle analyzer (NPA).2. EXPERIMENTAL CONDITIONS AND RESULTS     The experiments were carried out on the fusion device Uragan-3M (U-3M) which is a l=3, 9 magnetic field period torsatron  with  major  radius  100  cm  and  mean  plasma radius 12.5 cm. The hydrogen plasma was produced and maintained during ~50 ms by RF fields in the frequency range  ω≤ωci corresponding  to  a  multimode  Alfven resonance regime. The spectroscopical measurements were provided for  the  following  conditions:  toroidal  magnetic field 0.7T, PRF ≤ 240kW, mean plasma density ~1⋅1018 m-3.     Fig.1 shows the relative positions of diagnostic ports Fig.1 Disposition of diagnostic channels.relating the present work: 1. NPAII  – for measuring CXA fluxes, i.e., the H+ ion energy distribution along the toroidal direction with the line of sight passing near the RF antenna; 2. NPA⊥ for measuring the CXA flux in vertical direction was  positioned  at  ~90° from  the  RF  antenna  along  the toroidal direction; 3. Spectroscopy for measuring the chord distributions of different spectral lines in vertical direction of  the  cross  section  (DD),  where  plasma  column  is symmetrical relatively the central plane of U–3M; 4. Spectroscopy along the toroidal direction of the plasma confinement volume.For spectroscopy measurement of ion temperature the C4+ and C2+ ions  were  used.  The  potentials  of  excitation  of these ions differ very strongly each other what gives chance to obtain some data on the Ti radial distribution. During these experiments,  the spectral  range chosen for spectral measurements, was practically free of other spectral lines.   Problems of Atomic Science and Technology. 2002. № 4. Series: Plasma Physics (7). P. 53-55                          51Fig.2. Time behaviors of RF power supplied to antenna and integrated along lines of sight: the plasma density ne and spectral line intensities of indicated lines.     In  Fig.2  the  waveforms  are  shown for  RF power supplied to antenna and integrated along the line of sight: the plasma density ne and spectral line intensities of carbon and hydrogen lines. The chord distributions of intensities of these lines in the D-D cross-section are presented in Fig. 3. Fig. 3. Chord distributions of intensities of lines in the D-D cross-section.The technique of obtaining the radial distribution of line intensities  from measured  chord  distribution  is  minutely described in [5].  It  is  seen that the chord profile  of CV radiation was limited by size 14-15 cm, CIII radiation – 24-25  cm and Hα radiation  –  26-27 cm. The intensities  of indicated lines of carbon ions in a poloidal cross section were not enough for measuring the line profiles if one takes into account the requirements on the widths of entrance and exit  slits  of  the  spectrometer  in  use.  Therefore  for  line profile  measurements  we  used  the  port  for  the  tangent plasma observation. With this, the center of the observation line was antipode to the position of the RF antenna center (Fig.1) and according to geometry of measurements,  the volume of  observation was 5-6 cm in diameter.  For  the spectrometer MDR-23 with grating 1200 lines per mm the inverse linear dispersion was 1.3 nm/mm in the first order, but to work with better resolution we used the 3rd order of the  spectrum.  The  total  number  of  points  on  the  line contour was ~30 (obtained shot by shot). The instrumental contour was measured by means of Hg lamp (line 253.6 nm) operating in a cold regime and its  halfwidth was  ≈0.015  nm  what  corresponds  to  the  “instrumental temperature”  ~8  eV  (for  carbon).  The  contribution  of Zeeman  and  Stark  effects  into  broadening  of  indicated carbon lines may not be taken into account, [1,2].      The probable Doppler width of spectral line contours were  estimated  with  taking  into  account  the  previously obtained NPA data, [3,4].Fig.4. Energy distribution of H+ ions obtained by NPAII.    In Fig.4 the energy distribution of H+ ions obtained by NPAII are  presented  (note,  that  results  of  NPA⊥ measurement are very much similar). These data indicate that  there  are  three  groups  of  H+ ions  in  the  confined plasma with “temperatures” equal to: T1≅70eV, T2≅300eVFig. 5. Equipartition time for C4+ and C2+ ions in the background H+ plasma.and  T3≅500eV.  The  estimated  relative  composition  of every this component is: n1≈85-90%, n2≈6-12%, n3≈4-6%. However,  for  the  temperature  of  impurity  ions  to correspond to the H+ ion temperature, the particle life time has to be much longer than the equipartition time. Thus, the life time of H+ ions, τp≅1.5÷2 ms, found by measuring Hα line intensity, has to be compared with the equipartition time [6] for both, C4+ and C2+ ions, in a wide energy of H+ ions,  comparable with  “temperatures”  shown in Fig.  5. From  data  of  this  figure  one  can  conclude  that  the favorable  conditions  for  correct  Ti measurements  are fulfilled for C4+ ion component, and measurements of ion temperature by use of the CIII line can also be correct for Ti(H+)≤100eV. The results  of  measurements  of  the  CV and CIII line contours are shown in Fig.6 (note, that these data are obtained by integration along the line of sight). In Fig.  6a  the  experimental  points  are  presented  together with a Gaussian fitting contour of the same halfwidth as the experimental halfwidth. It is evident that at the wings there is a very big difference of measured points and the fitting  curve.  However,  if  the  experimental  contour  is depictured as a sum of two Gaussian profiles with different 54200 400 6000481216Temperature of H+ ions, eVτ eq, msCVCIII  0 2000 4000481216.ln (1/E1 / 2*dn/dE)E,eV504eV300eV76eVhalfwidth, much better fitting to the experimental profile can be obtained, as Fig. 6b demonstrates.Such  a  presentation  indicates  that  in  the  plasma confinement volume there are two groups of C4+ ions with energy distribution corresponding to temperatures ~40 eV and ~300 eV. This result is in good qualitative agreement with  measurements  by  the  NPA  method.  In  contrast, contour of the C2+ ion line is reasonably depicted by only one  Gaussian  curve.  The  halfwidth  of  this  contour corresponds to the temperature value ~30 eV, Fig.6c.The “temperature” of H atoms found from Hά line width was  estimated  as  ~2ev  without  accounting  other broadening  factors.  This  value  is  in  accordance  with typical data for the energy of Franck - Condon H atoms.3. SUMMARY     1.  The spectroscopic data on ion temperature from measurement  of  CV  227nm  line  structure  showed  the existence of double-energy distributions of C4+ ions. This is in a quite good agreement with results of NPA data for energy  distribution  of  the  main  plasma  component  (H+ ions).      2. To our regret, the intensity of CIII and CV lines was not  enough to provide similar measurements in the D-D cross section and thus to obtain the information on radial distribution of  Ti.  Such measurements are planned to be provided in the next campaign when plasma parameters ne, Te will be sufficient for this.REFERENCES[1]    S.E.Frish.  Opticheskie  spectry  atomov.   Moscow, 1963, p.41 (in Russian).[2]   Hans R. Griem. Plasma spectroscopy. Moscow, 1969, p.77 (in Russian).[3]   Volkov E. D., Arsen’ev A. K.,  et al. Generation  of fast  particles in  RF discharge plasma in the Uragan-3M torsatron, Proc. 25th  EPS Conf. Control.  Fusion Plasma Physics. Prague. 1998, 22C, p.455.[4]   V.V.Chechkin, et al.  Plasma flow asymmetries in the natural  helical  divertor  of  an l =  3  torsatron  and  their relation to particle losses. Nucl. Fusion, 42 (2002) p.192.[5]    Bondarenko  V.N.  et  al.  Investigation  of  radial distributions  of  spectral  line  radiation  emissivities  in torsatron “Uragan-3M”. These Proceedings.[6]   L. Spitzer. Physics of Fully Ionized Gases. Moscow, 1965, p.179 (in Russian).Fig. 6a. Experimental points together with a Gaussian fitting contour.Fig. 6b Two Gaussian profiles fit with different halfwidth of every component.Fig.6c. Calculated contour of the C2+ ion line and the experimental points.55229,65 229,70 229,750,00,51,0CIIINorm. IntensityWavelength, nm227,025 227,100 227,1750,00,51,0 Norm. IntensityTi~300eVTi~40eVWavelength, nm227,025 227,100 227,1750,00,51,0 Norm. intensityWavelength, nm

Temperature of impurity ions in a RF heated plasma of the U-3M torsatron as measured by means of the Doppler spectrometry

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Temperature of impurity ions in a RF heated plasma of the U-3M torsatron as measured by means of the Doppler spectrometry

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)