Kallne, E.

Kallne, J.

Marmar, E.S.

Rice, J.E.

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PFC/JA-85-33PRECISION MEASUREMENT OF THE iS LAMBSHIFT IN HYDROGEN-LIKE ARGONE.S. Marmar, J.E. RiceMIT Plasma Fusion CenterE. Killne, J. KAllneJET Joint UndertakingR.E. LaVillaUS National Bureau of StandardsPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139October, 1985This work was supported by the U.S. Department of Energy ContractNo. DE-AC02-78ET51013. Reproduction, translation, publication, useand disposal, in whole or in part by or for the United States govern-ment is permitted.By acceptance of this article, the publisher and/or recipientacknowledges the U.S. Government's right to retain a non-exclusive,royalty-free license in and to any copyright covering this paper.Submitted for publication in: Phys. Rev. A.Precision Measurement of the Is Lamb Shift in Hydrogen-like ArgonE. S. Marmar, J. E. RiceMIT Plasma Fusion CenterCambridge, MA, USAandE. Killne, J. KillneJET Joint UndertakingAbingdon, UKandR. E. LaVillaUS National Bureau of StandardsGaithersburg, MD, USAPrecision measurements of the absolute wavelengths of the Lyman a doublet(2p'py _ is 2S) in hydrogen-like argon (Ar 1 7 +), as well as those of some strongdielectronic satellites, are reported. The Alcator C tokamak plasma was seeded with traceamounts of argon and the emission spectra were taken with a high resolution, crystal X-rayspectrometer. In situ wavelength calibration was achieved by utilizing the potassium Kalines from a KCl fluorescence source. For the Lya 2 line, a precision of 11 ppm was achieved.Comparisons of these data with QED predicted wavelengths show good agreement, yield-ing a test of the is Lamb shift at the 3% level. The precision of these measurements waslimited by uncertainties in the wavelength calibration, with the uncertainties due to theinfluence of unresolved satellites being of secondary importance.I. INTRODUCTIONThis paper presents precision measurements of the absolute wavelengths of the Lyman-a doublet (2p 2 Pa k -+ 1s 2Sf) in H-like argon (Ar 17+) emitted from Alcator C tokamakplasmas. Previous measurements of absolute is energy levels in medium Z ions (16 < Z <36) have come primarily from beam-foil experiments 1-5, where corrections for substantialDoppler shifts limit the attainable precision. More recently, Beyer, et al.6 used an argonion recoil technique, eliminating most of the Doppler shift effects. In their work, relativelystrong satellites, due to the decay of doubly excited levels in ionization states lower than H-like, limited the accuracy of the measurements to a precision of 5 ppm. Tokamak plasmasare particularly well suited as a source for spectroscopic studies of highly charged ions.In the plasma conditions of this experiment, satellite lines are not very strong, and theions have relatively low temperatures (- 1 keV). The effects of Doppler shifts due tobulk plasma motions are very small in these Ohmically heated discharges. The Alcator Ctokamak plasma has previously been used as the source in an experiment to obtain thewavelengths of the Lya doublet in H-like chlorine, and results with precision at the 34 ppmlevel were achieved.7 The present work constitutes a continuation of those measurements.Several improvements have been made in the instrumentation,8 and consequently, theresults presented here, for argon, approach the 10 ppm level of precision, providing a testof the Lamb shift for the ground state energy of Ar17+ at the 3% level.2II. EXPERIMENTAL PROCEDUREIn this experiment, the main parameters for the hydrogen working gas plasmas were:central electron temperature of 1500 eV; central ion temperature of 1100 eV; line averagedensity of 2.4 x 1014 cm-3 ; toroidal magnetic field of 10 Tesla; plasma current of 460kA. The discharge was physically limited by molybdenum rings located at the edge ofthe plasma. Argon was introduced into the plasma through a fast piezoelectric valve,which was opened 50 msec after the beginning of each discharge, for a period of 20 msec.Subsequently, Ar entered the plasma and was increasingly ionized as it diffused into the hotcore of the discharge, where the H-like state had its density maximun. The excited statesof Ar17 + are populated primarily by electron impact excitation from the ground state, andthen decay radiatively back to the ground state. Spectra near A = 3.7 X, which includethe Lyal, 2 doublet, were obtained during the approximately 300 msec quasi-steady-stateportion of the discharges. The data were stored in time bins of 20 msec. The Braggcrystal spectrometer, which is in the Von Hamos geometry, has been described in detailpreviously.8 Absolute wavelength calibration of the spectrometer was achieved with theuse of the potassium Ka lines from a KCl fluorescence source. The KCl was excited witha primary X-ray beam from a Cu-anode source, operated at an anode voltage of 45 kVwith an emission current of 20 mA.The experimental procedure was as follows. The argon measurements were taken dur-ing the steady state portions of 5 consecutive discharges, with the time between dischargesbeing 8 minutes. Then the spectrometer was turned towards the fluorescence source, andthe potassium calibration was performed, with the accumulated data being read out fromthe histogramming memory every 8 minutes. The total collection time for the calibrationwas 170 minutes. Finally, Ar measurements were again taken for 5 consecutive discharges.In this way, any possible effects due to drifts could be monitored and minimized. The totalaccumulation time for recording Ar spectra was 3.0 seconds.3III. RESULTS AND DISCUSSIONAll the measured potassium Ka spectra have been added together, and Fig. 1shows these data, along with the fit used to obtain the absolute wavelength calibration.Lorentzian line shapes have been fitted to the Kaj,2 doublet data, along with Ka" onthe short wavelength side of Kal and a straight line background. The positions of bothKal and Ka 2 have been used to obtain the absolute wavelengths in the H-like Ar spec-trum. The wavelengths of the potassium lines are:6 A(Kai) = 3741.296(5) mA; A(Ka 2 ) =3744.405(5) mA; A(Ka") = 3739.1(2) mA.The results of the argon measurements are shown in Fig. 2, which depicts the Lyadoublet, as well as several satellite lines due to doubly excited He-like argon; the standardnotation from reference 9 is used to designate the lines. Figure 3 shows the results of aniterative least-squares fit to the data of Fig. 2, in the region around the Lya doublet. Theline widths are dominated by thermal Doppler broadening (A/A AD e 1000). To accountfor the presence of satellites, five lines, in addition to Lya 1 and Lya 2 , were fitted to thespectrum, with the positions and heights of each as free parameters. The line widths werealso free parameters, except that the two Lya lines were forced to have a single width, alsoa free parameter. Once again a straight line background has been included. All lines aretaken to be Gaussian in shape, consistent with the facts that the intrinsic Lorentzian linewidth is negligible, and that the instrumental spectral response (A/A A1 = 3000) has onlya small influence on the line shapes. The weak line near A = 3739 mA is due primarily to a2p-4d transition in Mo 2+. Spectra taken under similar plasma conditions, but without Arin the plasma, show that this is the only Mo line which appears in this spectral region. It iswell resolved from the Lya doublet, and so has negligible effect on the measured positionsof the argon lines.The results of the measured argon Lya wavelengths are summarized in table 1. Themain uncertainties come from fitting the potassium calibration lines and from extrapolat-ing the results to the positions of the Ar lines. These errors lead to estimated uncertainties4of ±0.035mA for Lya 2 and t0.07mA for Lya 1 . The next most important error contribu-tion is the uncertainty in the fits to the argon lines themselves, which is due primarily tothe influence of unresolved satellite lines combined with the Doppler widths, and is at thelevel of ±0.02mL. The overall uncertainty is at the level of ±0.04mA, or about 11 ppm, forLya 2. One further possible influence on the line positions from the tokamak source mustbe considered, namely Doppler shifts due to bulk plasma convection. All measurementsreported here were made during Ohmically heated discharges; no neutral beam heating hasbeen used. As such, it can be expected that any toroidal or poloidal drifts should be small(vT < 106 cm/sec, vp < 10 5cm/sec).11 Since the spectrometer views the plasma along amajor radius in the horizontal midplane, toroidal rotation would primarily contribute asecond order Doppler shift, which is negligible at these velocities. Furthermore, the instru-ment can be scanned vertically, on a shot-by-shot basis, to look for any evidence of theinfluence of poloidal flows on the line positions. This has been done, and shifts have notbeen detected, at least down to the level of .02 mA, corresponding to vp < 2 x 10 5cm/sec.The precision of the results presented here represents approximately a threefold im-provement over the previous Cl results from Alcator C,7 and approaches that of the resultsof Beyer et al.6 in Ar, obtained using the ion recoil method. In the latter case, the overallquoted precision is 5 ppm, with the dominant uncertainty due to the modelling and sub-traction of the fairly strong satellites. Satellites play a much smaller role in the resultsreported here. Although the overall precision is about a factor of two less than in therecoil measurements, the intrinsic uncertainty in the X-ray line positions due to satellitesis probably somewhat smaller. The comparison is therefore an interesting one, and the re-sults from Ref. 6 are also shown in Table 1. The present results are in excellent agreementwith theory10 for the positions of each of the individual lines; the agreement for the finestructure splitting is also reasonable.Table 2 lists the fitted wavelengths for the strongest satellites to the Lya lines, as wellas theoretical wavelengths for these lines from Ref. 12. Note that there is a systematic5shift of a little less than 1 mA for each of the lines. In the work of Ref. 12, the QEDeffects have not been included in the wavelength calculations. From table 1, the measuredshift in Lya 2 is 1.287(40) mA.IV. CONCLUSIONSIn summary, the absolute wavelengths of the Lyman-a doublet in Arl7+ have beenmeasured on the Alcator C tokamak. The line positions are in excellent agreement withthe predictions of QED theory; the measured fine structure splitting is also in reasonableagreement with theory. The strongest satellites, due to doubly excited Arl+, also show ashift to longer wavelength, when compared with predictions which ignore the QED effects.ACKNOWLEDGEMENTSWe wish to thank M. St6ckli for useful discussions, and D. Gwinn and B. Lipschultzfor expert operation of the tokamak during these experiments. This work was supportedby the US Department of Energy under Contract No. DE-AC02-78ET51013.6Table 1. Comparison of experimental measurements for the Ar17 + Lya doublet withrelativistic quantum theory, including reduced mass effects (Dirac) and QED theory, bothfrom Ref. 10. The results of Beyer et al. (Ref. 6) are also included for comparison. Allwavelengths are given in mA.Transition A(Dirac) A(QED) A(this experiment) A (Ref. 6)Lyal : 2 pj -+ 1s 3729.827 3731.101(1) 3731.142(70) 3731.105(19)2 2Lya 2 : 2pj -+ 1si 3735.227 3736.516(1) 3736.514(40) 3736.522(19)2 2Fine Structure Splitting 5.400 5.415 5.372(40) 5.417Table 2. Wavelengths (in mA) of some strong satellites near Lyman-a. The theoreticalwavelengths come from Ref. 12, and do not include any QED corrections.Transition A (this experiment) A (theory)T : 2s2p IP 1 -+ s2s 1 So 3755.26(10) 3754.4Q : 2s2p 3 P 2 -+ 1s2s 3 Sj 3761.06(15) 3760.3J : 2p 2 1D2 -- 1s2p IPi 3771.79(20) 3770.97References:1L. Schleinkofer, F. Bell, H. D. Betz, G. Trollmann, and J. Rothermel, Physica Scripta25(1982)917.2 J. P. Briand, M. Tavernier, P. Indelicato, R. Marrus, and H. Gould, Phys. Rev. Lett.50(1983)832.3 J. P. Briand, et al., Phys. Rev. A 28(1983)1413.4 H. D. Dohmann, D. Liesen and E. Pfeng, GSI Scientific Report 1982, GSI 83-1, Darm-stadt, 155 (1983).5 P. Richard, et al., Phys. Rev. A 29(1984)2939.6 H. F. Beyer, R. D. Deslattes, F. Folkmann and R. E. LaVilla, J. Phys. B 18(1985)207.7 E. Killne, J. Kfillne, Patrick Richard and M. Stockli, J. Phys. B 17(1984)L115.8E. Killne, J. K0llne, E. S. Marmar and J. E. Rice, Physica Scripta 31(1985)551.9U. I. Safronova, A. M. Urnov and L. A. Vainshtein, in Physics of Atomic Collisions andSpectroscopy of Plasma, Nauka, Moscow, 1980 (Ed. N. Basov).10 P. J. Mohr, At. Data Nucl. Data Tables 29(1983)453.11K. Brau, et al., Nucl. Fusion 23(1983)1643.12 L. A. Vainshtein and U. I. Safranova, At. Data Nucl. Data Tables 21(1978)49.81000800-cIo 600-c4002003730 3740 3750Wavelength (miA)Figure 1. The measured potassium Ka fluorescence spectrum with Lorentzian line fits ofKai, Ka2 and the satellite Ka".600050004000300020001000I ILya 1( )4,U-3720Lya2J" M32+ T KQBR A3740 3760Wavelength (mA)Figure 2. The measured argon spectrum with the Arl 7 + Lyman-a doublet and severalstrong satellites identified. The satellite identification follows the conventions of reference9.C)0C)03780I I I I4I446000500040003000200010000- -- -3720 3730 3740 3750Wavelength (mA)Figure 3 (a). A portion of the spectrum in the vicinity of the Ar1 7+ Lya doublet togetherwith the Gaussian line fits.C)0C)3104c103V)210 -++t-103720 3730 3740 3750Wavelength (mA)Figure 3(b). Same as Fig. 3(a) but on a semi-logarithmic scale.

Precision measurement of the 1S lamb shift on hydrogen-like argon

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Precision measurement of the 1S lamb shift on hydrogen-like argon

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