Marmar, E.S.

Rowan, W.L.

Terry, J.L.

Wootton, A.J.

DSpace@MIT

PFC/JA-96-13Diagnostic Neutral Beam andActive Spectroscopy Requirementsfor the Alcator C-Mod TokamakE.S. Marmar, J.L. Terry, W.L. Rowan', A.J Wootton'May 1996'Fusion Research Center, The University of Texas at Austin, Austin, TX 78712.Submitted to Review of Scientific Instruments.This work was supported by the U. S. Department of Energy Contract No. DE-AC02-78ET51013. Reproduction, translation, publication, use and disposal, in whole or in partby or for the United States government is permitted.Diagnostic Neutral Beam and Active Spectroscopy Requirements for the Alcator C-Mod TokamakE.S. Marmar, J.L. TerryMIT Plasma Fusion Center, Cambridge, MA 02139 USAW.L. Rowan, A.J. WoottonFusion Research Center, The University of Texas at Austin, Austin, TX 78712AbstractConsiderations are presented on the application of a diagnostic neutral beam to theAlcator C-Mod tokamak for active profile measurements of ion temperature, poloidal andtoroidal rotation, impurity and minority species densities, and internal magnetic field.Calculations indicate that, for a typical 50 keV, 0.13 MW hydrogen beam, chargeexchange recombination measurements for plasmas with line average densities up to2.5x 1020 m3 will yield, with good signal to noise and ~1cm spatial resolution,measurements of helium and low Z impurity density profiles with better than 1 ms timeresolution, and ion temperature and rotation profiles with -1 ms time resolution. MotionalStark effect measurements appear feasible over most of the operating density range of thedevice (at least up to 5x 1020 m 3), also with ms time resolution. Spatial resolution of theMSE measurements will depend on the toroidal width of the beam and the angle of theview with respect to the beam; for a 5 cm wide beam with a 600 viewing angle, 2.5 cmspatial resolution is expected.1I. INTRODUCTIONThe use of neutral beams, combined with visible spectroscopy, is widely applied tothe diagnosis of tokamak plasmas. Local plasma parameters which have been measuredwith these techniques include low Z impurity densities' 3 and impurity transportcoefficients (e.g. reference 4), ion temperature and rotation (e.g. reference 5), internalmagnetic field, (and thus current density and safety factor),6-8 and electron densityfluctuations (e.g. reference 9). Recently, the first measurements of the energy distributionof slowing-down D-T fusion alpha particles have been reported.'0Alcator C-Mod is a high field, compact tokamak (B 9T, R = .67m, a = 0.22m),which operates over a large range of plasma density, up to IX 1021 m73. In considering thepossible applications for active spectroscopy with a neutral beam on this device, severalissues must be addressed. In particular, because of the very high densities, combined withrelatively large line integral density (up to 2x1020 m72 for perpendicular injection along thehorizontal midplane), beam attenuation and competition from bremsstrahlung continuumto measured spectroscopic signals are particularly important. In many ways, this is similarto the situation which will be encountered on devices such as ITER, where similar orslightly higher line integral densities would be expected, albeit at much lower density andlarger minor radius. A treatment of many of the issues relevant to this topic for ITER canbe found in reference 11.On Alcator C-Mod, the addition of a diagnostic neutral beam and the associatedspectroscopic and neutral particle diagnostics should lead to the enhancement of profilemeasurement capabilities for ion temperature, plasma rotation, and low Z impurity2concentrations, including He, B, C, N, 0 and Ne. Ion temperature profiles in the plasmacore are presently available from high resolution X-ray Doppler spectroscopy,12 usinginjected argon. Charge exchange recombination spectroscopy (CXRS) measurementspromise better spatial and temporal resolution, along with increased rotation sensitivity.Low Z impurity core density profiles cannot be obtained by any means at present on C-Mod, and their measurements will be important adjuncts to many experimentalinvestigations, including divertor impurity screening (He, N, Ne) and ICRF heating ('Heminority concentration). Two additional important applications will be to measure internalmagnetic field (and thus q profiles) via the Motional Stark effect (MSE) and to measurefluctuations via beam emission spectroscopy (BES). In both of these cases, it is theradiation resulting from excitation of beam neutrals which is observed. For CXRS, thecross section for population of the desired upper level in the H-like ion is a stronglydecreasing function of interaction energy. As a result, even though beam attenuation issignificant, the optimum energy for CXRS, on C-Mod, is in the range from 50 keV/AMUto 60 keV/AMU for hydrogenic beams. The excitation cross sections for MSE and BESare weaker functions of neutral energy than for charge exchange recombination, and as aresult, somewhat higher energy would be optimum for measurements into the core of thehighest density plasmas (-100 keV). Nevertheless, good signal to noise MSEmeasurements are feasible at 50 keV. C-Mod current density profiles are presentlydiagnosed using lithium pellet imaging and polarimetry; this has the disadvantage of beingvery perturbative to the subsequent plasma evolution, and measurements are at most3available at 1 or 2 time points in a discharge. The potential for BES measurements offluctuations on C-Mod are discussed elsewhere in these proceedings."II. CHARGE EXCHANGE RECOMBINATION SPECTROSCOPYWe first consider CXRS measurements of impurity density, ion temperature androtation profiles. In all cases presented in this paper, the plasma has been modeled asfollows: R = .67m, a = .22m, Ti = Te = 4-[l-(r/a)2 2 keV, ne = no-[1-(r/a) 2]'m, Zef = 1.5.Figure I shows the results of penetration calculations for a range of neutral hydrogenbeam energies, assuming perpendicular injection along the midplane. The plasmaparameters used are those of an intermediate density case, with central densityno = 2.5x1020 m". For the beam attenuation calculations, ionization due to electronimpact," proton impact,1-,16 and charge exchange 7 have been taken into account. Heliumis often injected into C-Mod plasmas, both for impurity studies and for the primary ICRFheating scheme at 8 tesla (3He minority at 80 MHz). As a result, we will consider fullystripped helium to be the ion which captures an electron from the neutral hydrogen in thebeam. The emissivity in the H-like He line of interest is given by:E = fiefH, (av)ce phlm 3/s/nm, (1)VIL, AADwhere (a v)cx is the effective rate for emission of the n = 4 -+ 3 transition atX = 468.6 nm, taken from reference 5, v&i is the neutral velocity and AXD is the Doppler4width due to the ion thermal distribution. The neutral beam is assumed to deliver anequivalent current density of 103 amp/m2 in the full energy component (before plasmaattenuation), independent of beam energy, with a cross-section of .05x.05 m2. At 50 keV,this corresponds to a beam power of 0.13 MW. These parameters are similar to those ofthe neutral beam which was formerly used at the TEXT tokamak facility, and plans arebeing made to transfer this beam to MIT for installation at C-Mod. The main difference isthat the Texas beam has a slightly larger foot print (-8 cm wide at the focus), as describedin more detail in reference 13. For a detection geometry where the optics view the beamtangentially at the plasma axis, the line brightnesses which would result are shown infigure 2. For comparison, the continuum brightness, due to free-free bremsstrahlung,18 isalso shown. In the center of the plasma, the continuum will be more than 1 order ofmagnitude brighter than the CXRS signal. In order to pull the signal out of thisbackground, it will be necessary to modulate the DNB.The signal to noise ratio of the measurement is fundamentally limited by thefluctuations in the signal.3 The minimum level of these fluctuations is determined byphoton statistics. In order to estimate this, it is assumed that the optical system has anetendue of 2x10 7 m2 .st (determined by the spectrograph), and an overall transmission anddetection efficiency (mirrors, fibers, lenses, detectors) of 1%. To measure the density ofhelium in the discharge, we integrate over the Doppler broadened spectrum of the line; atT,= 4 keV, the full width at half maximum will be -1 nm. In general, the signal to noiseratio (SNR) is given by:5SNR = , (2)2(2 Svb + SCx)where Sex is the charge exchange signal, S is the continuum signal (both in photon/s) andt is the integration time. The factors of 2 in the denominator result from the need tomodulate the beam, assuming a 50% duty factor. To measure the ion temperature, as wellas rotation along the viewing direction, it is necessary to measure the spectral width andshift; this requires an increase in the spectral resolution of about a factor of 10, and acorresponding reduction of the signal in each spectral channel. Figure 3 shows the resultsof calculated signal to noise ratios (from equation 2) for an ion temperature measurementassuming that the helium density is 2% of the electron density. The plot shows theexpected SNR at 5 different minor radii, from the axis out to r/a = 0.8. Note that even forthe core measurement, reasonable signal to noise can be expected with an integration timeof 1 ms. The rapid improvement with increasing r/a results from two effects: both thebeam attenuation and the visible continuum brightness are decreasing. While helium isused in this example, the measurements could also use intrinsic boron or carbon, which arethe dominant intrinsic low Z impurities in C-Mod. Typical densities of carbon are 0.5% ofne; the decrease in ion density is just about balanced by the combination of decreasedDoppler width and increased charge exchange population of the upper level.519 As aresult, the signals for He or C will be approximately the same.III. MOTIONAL STARK EFFECTThe final diagnostic application of the C-Mod DNB considered here is themeasurement of internal magnetic field, and thus q profiles, using the motional Stark effect6(MSE). As the high energy neutrals traverse the plasma, crossing the magnetic field, theatoms, in their rest frame, experience an electric field, E = v x B. The resulting Stark effectshifts the energy levels in the atom, and line emission (e.g. from Balmer-a) is split intomultiple components. The various lines are also polarized, depending on the relativedirections of v x B and the optical view.3 '.'7In order to see a component of the poloidal magnetic field, as well as to provide aDoppler shift to move the lines spectrally away from the edge neutral emission, it isnecessary that the view of the beam must be neither tangent to the toroidal direction norperpendicular to beam. Since beam attenuation remains as an issue for MSE, and C-Modport access also dictates it, we again assume perpendicular injection along a major radiusin the midplane. The viewing angle with respect to the beam is taken to be 60. Figure 4shows the results of a simulation, similar to those done for CXRS, giving the brightness inthe central, unshifted a component of H,. In this case, a central density of 3x10 20 is used.The spectral width is determined by the Doppler broadening; here we assume this isdominated by the viewing cone for each spatial channel, taken to be ±0.8 degrees. Theexcitation cross-section, which for these energies is dominated by charge exchange andproton impact, does not fall as rapidly with increasing neutral energy as the CXRS rates.Consequently, because of the improving beam penetration, it would be advantageous togo to energies somewhat higher than 50 keV. However, even for 50 keV, at this densitythe central signal is a factor of 2 higher than the bremsstrahlung continuum for the chordintersecting the beam at r/a = 0. The measurement itself requires splitting the light intomultiple polarization components, reducing the effective signal size (here taken to be a7factor of 0.25). Figure 5 shows the resulting SNR for the on-axis measurement as afunction of central density, assuming 1 ms integration time. Measurements thus appearfeasible for MSE, even up to very high densities, with excellent time resolution. Onecaveat is that these calculations do not take into account the collisional depopulation ofthe upper level before radiative decay. According to calculations by Zinov'ev andKorotkov,' 9 this will be about a factor of 5 effect in going from 100 to 10" m-.IV. SPATIAL RESOLUTIONSpatial resolution is also an important consideration for all of the measurements.Because the CXRS measurements can be made viewing nearly tangentially, or vertically,the spatial resolution that results is fundamentally limited by the ratio of the beam width tothe major radius (for the tangential view) or by the ratio of the beam height to the minorradius (for the vertical view). Both views are necessary in order to measure the toroidaland poloidal components of rotation. For MSE, the situation is complicated by the factthat, as already discussed, the toroidal view cannot be tangent to the flux surfaces, sincethe angle of the total field with respect to toroidal, and thus the poloidal field, could notthen be measured. For a given beam width, w, with perpendicular injection, the spatialresolution is limited to w cot(e), where e is the angle between the view and the beamdirection. Thus, for 60 degrees, the resolution will be 0.58w, which, for a 5 cm beam, isabout 10% of the minor radius. Going to larger angles would improve the spatial8resolution, but at the cost of having the measurable component of field angle becomingtoo small.In principal, it might be possible to infer the poloidal field from a measurement ofthe total local field profile, which is available from the Stark shifts, rather than thepolarization measurements. Because the field magnitude is given by B+B , therequired accuracy for the total field measurement is very demanding. For typical C-Modplasmas, B, is in the range from 0 on axis, to a maximum of about 0.2-BT. Measurement ofa detailed q profile requires field angle profile measurements with roughly 0.2 degreeaccuracy, implying that the total field must be measured with an uncertainty of only a fewparts per thousand. While this may be possible, there is a further complication. Thediamagnetic effect of the plasma reduces the toroidal field. The magnitude of this effect,which is proportional to the plasma #, is very similar to that of the poloidal field on themagnitude of B. Measurements using MSE, of the total field magnitude profile on JET,have been reported by Wolf, et al.20 In that case, nominal current profiles were assumedand the measurements were used to infer the pressure profile through the plasmadiamagnetism. However, with sufficiently accurate kinetic pressure profile measurements,it may be possible to use this approach to measure, or at least to constrain, the currentdensity profiles in equilibrium reconstructions. If this is possible, the measurements can bemade with a purely tangential view, and significant improvement in spatial resolutionwould result.9ACKNOWLEDGMENTSThis work supported by U.S.DoE Contract No. DE-AC02-78ET51013 and Grant No.DE-FG03-94ER-54241.REFERENCES1. R.C. Isler, Phys. Rev. Lett. 38(1977) 1359.2. A. Boileau, et al., Plasma Phys. Contr. Fusion 31(1989)779.3. M.G. von Hellerman and H.P. Summers, in Atomic and Plasma-Material InteractionProcesses in Controlled Thermonuclear Fusion, R.K. Janev and H.W. Drawin (editors),Elsevier Science Publishers, Amsterdam, 1993, page 45.4. E.J. Synakowski, et al., Phys. Rev. Lett. 65(1990)2255.5. R.J. Fonck, et al., Phys. Rev. A 29(1984)3288.6. F. Levinton, et al., Phys. Rev. Lett. 63(1989)2060.7. D. Wroblewski and L.L. Lao, Rev. Sci. Instrum 63(1992)5140.8. R.C. Wolf, et al., Nucl. Fusion Lett. 33(1993)663.9. R.D. Durst, et al., 1992, Rev. Sci. Instrum. 63(1992)4907.10. G. McKee, et al., 1995, Phys. Rev. Lett. 75(1995)649.11. E.S. Marmar, Active Spectroscopic Diagnostics for ITER Utilizing Neutral Beams,Proceedings of the International School of Plasma Physics "Piero Caldirola" Workshop10on Diagnostics for ITER, in Diagnostics for Experimental Thermonuclear FusionReactors, Edited by Peter E. Stott, Guiseppe Gorini and Elio Sindoni, Plenum Press, NewYork, 1996, page 281.12. J.E. Rice and E.S. Marmar, Rev. Sci. Instrum. 61(1990)2753.13. W.L. Rowan, et al., I 1th High Temperature Plasma Diagnostics Conference,Monterey, CA, 1995, paper P1.07, submitted to Rev. Sci. Instrum.14. R.K. Janev, et al., Elementary Processes in Hydrogen-Helium Plasmas, Springer-Verlag, Berlin, 1987.15. W.L. Fite, et al., Phys. Rev. 119(1960)663.16. H.B. Gilbody and J.V. Ireland, Proc. Roy. Soc. A-277(1964) 137.17. E.L. Freeman and E.M. Jones, Atomic Collision Processes in Plasma PhysicsExperiments I, UKAEA Report CLM-R137, Culham Laboratory, Abingdon, England,1974.18. K. Kadota, et al., Nucl. Fusion 20(1980)209.19. A.N. Zinov'ev and A.A. Korotkov, JETP Lett. 50(1989)306.20. R.C. Wolf, et al., Nucl. Fusion 33(1993)11835.1110C?~E oxO 6-LOc JII-o EC 1" Z-0U))10C0 00 0 oO Co C\l TUQ!SS!WSUBJ I jeIOijdFigure 1. Neutral hydrogen beam transmission as a function of minor radius for anintermediate density plasma. The curves are parameterized by beam energy in keV.12E0C|-0C cChl,Co1CO)ECaU_U_1 oLL , 1, .10LO1. II..(01(wu/is/s/,w/qd) sseujq6!jSFigure 2. Brightness in the He' (n = 4 to n = 3) transition at X = 468.6 nm, as a function ofminor radius, for 4 different neutral energies. The continuum due to free-freebremsstrahlung at the same wavelength is shown by the dashed curve.13ICOC'46CDNCo W0.. 4 E .LO1o.I I I,-0*--Cz0cc00:0OE EmI-)o EEcE i DCO000 Eo~0E~ 0O I-00cCD CD0161e-d 9s!G0N 01 POu$!SFigure 3. Signal to noise ratios as fuinctions of integration time for ion temperaturemeasurements using He*. The curves are parameterized by the normalized minor radius.14I ~ I I ~-..- I----------I/~~ LUEEU-L:CI,E0xCf)Io 0(wu/js/s/,w/qd) sseujq~pqFigure 4. Calculated Balmer-a brightnesses as functions of minor radius for 6 differentbeam energies. Note that even for this relatively high density (3x10 20 m-3 ), the signal witha 50 keV beam is comparable to the bremsstrahlung even at the plasma axis.15. # II1O06500C')0IC;0CDJLO-6r05651 T I ILOCO,E> 0CM-So eCOLOlC'Ja 01uMCo 0 0.o1 0 p1B021 GSINFigure 5. Signal to noise ratio for central MSE measurements as a function of central density.16

Diagnostic neutral beam and active spectroscopy requirements for the Alcator C-Mod tokamak

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Diagnostic neutral beam and active spectroscopy requirements for the Alcator C-Mod tokamak

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