Allen, S.L.

Coan, T.

Cowan, R.D.

Marmar, E.S.

Rice, J.E.

DSpace@MIT

Molybdenum and Chlorine X-Ray Emissionfrom Alcator AJ.E. -Rice, E.S. Marmar and T. CoanPlasna Fusion Center/Francis Bitter National MagnetLaboratory, M. I. T.S. L. Allen-Johns Hopkins UniversityR.D. CowanIos Alaros Scientific LaboratoryPFC/RR-80-5PFC/JA-80-1AbstractHigh resolution (AX/X : .015) x-ray spectra in thewavelength region 3.7 + 6.2 A (3.4 -+ 2.0 keV) have beencollected from the Alcator tokamak using a flat crystal Braggmonochromator. Molybdenum L lines and chlorine K lines havebeen observed and charge state identifications have been madeby comparison with calculations.I. Detection SystemA flat crystal monochromator with a pentaerythritol(PET, 2d = 8.742 A) crystal has been installed on theAlcator A tokamak. A proportional counter was used asthe photon detector and overall time resolution of thedetection system was about 100 ps. Spectra have been obtainedby scanning in wavelength on a shot to shot basis during asequence of identical discharges. Wavelength calibrationwas achieved by using the L series of Mo " produced by *a Henke tube. Accuracy of wavelength determinations is about+ .005 A.II. Observations on Alcator AFig. 1 shows the time history of a typical Alcator Adischarge at 60 kG. The top traces are the plasma currentand average electron density (50 kA/div and 1 x 10 14cm- 3fringe). The third trace is the monochromator signal at04.76 A (2.60 keV) and the bottom trace is the total softx-ray flux above 1.5 keV from a surface barrier diode (SBD)viewing along a chord through the center of the plasma.The time scale is 20 ms/div. During the steady state portionof this discharge, when the average electron density was14 -32.9 x 10 cm , the plasma current was 160 kA, and thesawtooth oscillations were well developed (between 70 and110 ms), the central electron temperature was % 900 eV. Atthe startup phase of the discharge, around 20 ms when the-2-0.0"-13 -3density was 5 x 10 cm , the electron distribution wasnon-thermal but (in the energy region of i-nterest) can becharacterized by a hot Maxwellian with T e 1800 eV. Noteethe large spike on the SBD and spectrometer signals atthis time.Fig. 2 is the spectrum at 100 ms for a series ofdischarges similar to the one shown in Fig. 1, obtained byscanning in wavelength on a shot to shot basis. Shown forcomparison are the gf values for the strongest 2p-3d23+ 24+transitions in Mo and Mo . These gf values were computedusing the HXR method of calculating radial wavefunctions,including the Slater-Condon theory of atomic structure1,2The bumps at 4.9 and 5.1 A are in good agreement with the23+ 24+predicted 2p-3d lines of Mo and Mo , supportive ofcoronal equilibrium calculations3 which predict the pre-dominant molybdenum charge states at 900 eV to be near Mo 2 3 +24+and MoFig. 3 is the spectrum at 20 ms, during the spike ofthe SBD signal. Shown for comparison are the predicted gfvalues for the 2 s- 3p, 2p-3d, and 2p-3s transitions of Mo31+ 30+and Mo as well as the 2p-3d transitions for MoAgain, the wavelength agreement is quite good. The relativeheights of the observed 2p-3s lines to the 2p-3d lines areconsiderably larger than the gf value ratio. This has been4 32+observed elsewhere for Mo2. These charge state identific-ations are again in agreement with coronal equilibrium3calculations which predict the predominant charge states30+ 31+ 32+at 1800 eV to be Mo , Mo , and Mo .However, this-3-is neither a steady state nor thermal portion of the dischargeand these particular coronal equilibrium calculations are notnecessarily a priori valid.There remains to explain the feature at 4.44 A (2.79 keV)in Fig. 2. Fig. 4 exhibits a more detailed scan of thisfeature for discharges similar to the one described above.Also shown are the predicted wavelengths for the K lines of515+heliumlike Cl+. The agreement is quite good. Again,coronal equilibrium calculations3 suggest that the predominant15+charge state for 400 eV < T e< 1500 eV is Cl . This isconsistent with the observation on Alcator A that this lineprevails for a range of central electron temperature between700 and 1200 eV. The molybdenum features between 4.6 and5.1 A shift as the electron temperature is altered as expectedfrom changes in charge states. These observations supportboth the wavelength predictions and the coronal equilibriumcalculations.Chlorine radiation has been seen in the UV and visibleregions as well. In particular, the 2s-2p transition of14+ 0 6+C1 at 384 A and the 3s-3p transitions of Cl at 813 and0801 A have been observed with a spatial imaging detection6system (SIDS)6. Fig. 5 is a spatial brightness profileof the 384 A line. The points outside + 5 cm representIV 0contamination from the bright C line also at 384 A butlocated at a larger radial position. The 4795 A line ofCl has been observed during discharge cleaning. The3 A7384 A line has been seen in TFR discharges.-4-A possible source for the chlorine is the solventtrichloroethane used in cleaning Alcator vacuum components.Chlorine may also be a contaminant of the stainless steelvacuum bellows. The source of the molybdenum is the Molimiter used in Alcator A. Further evidence that the 4.44 Afeature is not due to molybdenum radiation has been providedby a series of impurity injection experiments. In one suchcase Mo was introduced into the plasma by the laser blowoff8technique , and Fig. 6a demonstrates the result. The toptrace is the average electron density and the rise of thespike in the second trace indicates the time of the laserfiring which injected the molybdenum. The third trace isthe SBD flux (hv > 1.5 keV) which exhibits a bump similar tothat shown in Fig. 1. The fourth trace is the spectrometersignal at 5.20 A (2.38 keV) showing a factor of 8 or soincrease in signal after the injection. Fig. 6b is a blowupof Fig. 6a near the region of interest (2 ms/div). It seemsto take about 6 - 10 ms for the molybdenum to reach the centerof the discharge after the injection and the molybdenumremains in the plasma for about 15 ms. Fig. 7 shows a similartrace but with the spectrometer set on 4.44 A. The SBD tracedemonstrates the induced Mo radiation at "-' 50 ms as well asthe naturally occuring bump at 25 ms. Neither of thesefeatures is present on the 4.44 A trace indicating thatthis line is not due to molybdenum.On the bottom of Fig. 2, an X indicates an observedincrease in signal at that wavelength when molybdenum isinjected into the plasma and an 0 indicates no change.From the strength of the Cl K line above the continuumbackground, one may deduce an estimate of the chlorinedensity in Alcator A. If one equates the ratio of theobserved line to continuum to the ratio of the number of lsvacancies in 9 Cl 1 5  to the number of continuum photons1 0produced, one obtains (ignoring profile effects)1.6 x 10-5 ne nCl <g> nn* - AE/TI AE /TIcont -1 4  2 - - hv/T(1)9.58 x 10 n e g e Ahvhv VTewhere f is the oscillator strength of the transition ofenergy AE, <g> and g are gaunt factors, E is the bremsstrahlungenhancement factor, and A hv is the resolution of theinstrument. Solving for n and taking I C/I cont= 9,14 -3n = 3 x 10 cm , C = 1, <g> = .2, fnn* = .8, g = .7,T = 900 eV, A hv = 40 eV, and AE = hv = 2.79 keV, one obtains.ea chlorine density of 3 x 10 9 cm-3 or 10- 5 n .From this amount of chlorine, an estimate may be madeof its contribution to radiative power loss in the plasma.Taking the cooling rate of chlorine at 900 eV to be2.5 x 10- 27watt/cm3 (this is actually the value for sulphur 11),and electron and chlorine densities to be 3 x 10 14cm-3 and3 x 10 9 cm , respectively, one obtains a radiated power densityof .0023 watt/cm3 from the center of the plasma. This isa mere .015% of the central input ohmic power density for the-6-typical case of 15 watt/cm 3, and chlorine does not enter ina significant way to power balance in Alcator A..III. ConclusionsObservations of Mo L lines from Mo 22+ < Mo + < Mo +15+and Cl K lines from Cl have been made on the Alcator Atokamak, supportive both of wavelength predictions andcoronal equilibrium calculations. Estimates indicate thatnCl/ne is of order 10- 5, therefore not important in powerbalance considerations.IV. AcknowledgementsThanks are due Herb Schnopper for providing theinstrument used in these measurements, Dave Stallard fortechnical support, and Dave Overskei for operating Alcator A.References1. R.D. Cowan and D.C. Griffin, J. Opt. Soc. Am. 66, 1010(1976).2. R.D. Cowan, J. Opt. Soc. Am. 58, 808, 924 (1968).3. C. Breton, C. DeMichelis, M. Finkenthal, and M. Mattioli,EUR - CEA - FC - 948 (1978).4. P. Burkhalter, R. Schneider, C.M. Dozier and R.D.Cowan, Phys. Rev. A 18, 718 (1978).5. R.L. Kelly and L.J. Palumbo, NRL Report 7599 (1973).6. R.K. Richards, H.W. Moos, and S.L. Allen, Rev. Sci. Instr.51, 1 (1980).7. Equipe TFR CUR - CEA - FC - 984 (1979).8. E.S. Marmar, J.L. Cecchi and S.A. Cohen, Rev. Sci. Intr.46, 1149 (1975).9. R.C. Elton, Me-thods of Experimental Physics, Vol. 9,Part A, p.-115, Academic Press, 1970.10. J. Greene, Ap. J. 130, 693 (1959)WWN*WA0WWWW..k-11. D.E. Post, R.V. Jensen, C.B. Tartar, W.H. Grasbergerand W.A. Locke, Atomic Data and Nucl. Tables 20, No. 5,397 (1977) .-9 -Figure CaptionsFig. 1. Typical 60 kG Alcator A discharge. Top trace50 kA/div, second trace 1 x 10 14cm~ /fringe, time scale20 ms/div.Fig. 2. Brightness vs. wavelength at 100 ms for dischargessimilar to Fig. 1. Predicted gf values for 2p-3d transitions. 23+ 24+-in Mo and Mo .Fig. 3. Spectrum at 20 ms. Predicted gf values forMo30+, Mo 31+, and Mo L lines.Fig. 4. More detailed scan of C1 K lines near 4.44 A.Fig. 5. Radial brightness profile at 384 A.Fig. 6a. Molybdenum injection, 20 ms/div. 6b. Expandedview of injection, 2 ms/div.Fig. 7. Molybdenum injection showing no effect at 4.44 A.-10-A Discharge at 60 KGIe04.76 ASBDFig. 1.A AM A_qn MM'fT-qru -'\L ITypical AlcatorFC~JI I IrC)C\M C\J0 0S00/I'N* SSS0II I I I I I I I0 0 - CM S0 0<0K: S JJQ<a<:CLI'N100 0L0( iS0'S(Nj0oo-oCLa-iNS"I0.Firso0) -.co Z- r4)-oCjIQ- r.(,) C\j - M fl,- co n Q- ro C\jaloos *qJD) SSaU I L4 b I iI I I IIs 2-Is2p4.6CI00-IT--4.3 4.4 4.5X(A)Fig. 4.+a)4-00~C,,U,a)4--ccn+++±+++++++++-1-++ELOif)+±ocxeneInjection LoserSBD05.2 0 AMolybdenum Injection (a)(b)______________. __ __ __ ___________Fig. 6.OWWWWWOU-N.Mi-ieInjection LoserS4 D4.44 AID~T~1~17-I-Fig. 7

Molybdenum and chlorine x-ray emission from Alcator A

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Molybdenum and chlorine x-ray emission from Alcator A

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