Adriany, K.

Chakraborty Thakur, S.

Gosselin, J. J.

McKee, J.

Scime, E. E.

Sears, S. H.

Tynan, G. R.

English

The Research Repository @ WVU (West Virginia University)

Faculty Scholarship2016Laser Induced Fluorescence Measurements OfAxial Velocity, Velocity Shear, And Parallel IonTemperature Profiles During The Route To PlasmaTurbulence In A Linear Magnetized Plasma DeviceS. Chakraborty ThakurK. AdrianyJ. J. GosselinJ. McKeeE. E. ScimeSee next page for additional authorsFollow this and additional works at: https://researchrepository.wvu.edu/faculty_publicationsThis Article is brought to you for free and open access by The Research Repository @ WVU. It has been accepted for inclusion in Faculty Scholarshipby an authorized administrator of The Research Repository @ WVU. For more information, please contact ian.harmon@mail.wvu.edu.Digital Commons CitationChakraborty Thakur, S.; Adriany, K.; Gosselin, J. J.; McKee, J.; Scime, E. E.; Sears, S. H.; and Tynan, G. R., "Laser InducedFluorescence Measurements Of Axial Velocity, Velocity Shear, And Parallel Ion Temperature Profiles During The Route To PlasmaTurbulence In A Linear Magnetized Plasma Device" (2016). Faculty Scholarship. 579.https://researchrepository.wvu.edu/faculty_publications/579AuthorsS. Chakraborty Thakur, K. Adriany, J. J. Gosselin, J. McKee, E. E. Scime, S. H. Sears, and G. R. TynanThis article is available at The Research Repository @ WVU: https://researchrepository.wvu.edu/faculty_publications/579REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11E513 (2016)Laser induced fluorescence measurements of axial velocity, velocity shear,and parallel ion temperature profiles during the route to plasma turbulencein a linear magnetized plasma deviceS. Chakraborty Thakur,1,a) K. Adriany,1 J. J. Gosselin,1 J. McKee,2 E. E. Scime,2S. H. Sears,3 and G. R. Tynan11Center for Energy Research, University of California at San Diego, San Diego, California 92093-0417, USA2Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA3Department of Physics, University of Wisconsin at Madison, Madison, Wisconsin 53706, USA(Presented 8 June 2016; received 10 June 2016; accepted 24 June 2016;published online 2 August 2016)We report experimental measurements of the axial plasma flow and the parallel ion temperature in amagnetized linear plasma device. We used laser induced fluorescence to measure Doppler resolvedion velocity distribution functions in argon plasma to obtain spatially resolved axial velocities andparallel ion temperatures. We also show changes in the parallel velocity profiles during the transitionfrom resistive drift wave dominated plasma to a state of weak turbulence driven by multiple plasmainstabilities. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4959275]I. INTRODUCTIONPlasma flows in fusion devices are critically important forseveral reasons. The lifetime of plasma facing components in afusion device is governed by material erosion, migration, andredeposition, and hence, accurate measurements of the scrape-off-layer (SOL) flows are necessary.1,2 However, scarcity ofmulti-point measurements makes impurity studies in the SOLplasma flows difficult. Moreover, these flow measurementsare generally made with Mach probes,1 whose interpretationis subject to considerable debates.3–5 Hence, here we usednon-perturbative laser induced fluorescence (LIF) for Dopplerresolved spectroscopic measurements of ion flows.6Plasma flows in fusion devices are also the key to under-stand plasma turbulence. Simpler geometry and better diag-nostics access allow detailed drift turbulence and shear flowstudies in linear plasma devices.7–10 Strong shear flows alsogive rise to velocity shear driven instabilities.11 Toroidal rota-tion is beneficial for both MHD control and suppression ofmicro-instabilities. Intrinsic rotation,12,13 driven by the re-sidual turbulent stresses, is also of prime interest and linearplasma devices provide an ideal test bed to study intrinsic flowsin a magnetic shear free regime.14Previous studies15 in the Controlled Shear DecorrelationeXperiment (CSDX) have demonstrated that with increasingmagnetic field (B), density gradient driven drift wave fluctu-ations evolve from narrow-band coherent azimuthal modesto a state of weak turbulence characterized by broadenedfrequency and wave number spectra. Nonlinear energy trans-fer analyses16 have shown that for B ∼ 1000 G, energy istransferred from the higher frequency ( f ∼ 10 kHz, m = 3mode) turbulent fluctuations to a low frequency ( f < 1 kHz,Note: Contributed paper, published as part of the Proceedings of the 21stTopical Conference on High-Temperature Plasma Diagnostics, Madison,Wisconsin, USA, June 2016.a)Author to whom correspondence should be addressed. Electronic mail:saikat@ucsd.edu.m = 0 mode) azimuthally symmetric shear flow, thus havingcharacteristics of zonal flows. More recent experiments17 onan upgraded CSDX (higher B, larger antenna with m = 1helical source) have shown that a global transition occursat a threshold magnetic field Bth (which depends on thesource parameters). Below Bth, the plasma is dominated bydensity gradient driven collisional drift wave instabilities. Anew global equilibrium is found for B > Bth, with coexis-tence of three radially separated plasma instabilities: coherenthigh azimuthal mode number fluctuations in the central core;collisional drift waves in the density gradient region; andstrong, turbulent, shear-driven instabilities at the edge. Thisnew regime shows very rich plasma dynamics including inter-mittency, formation and propagation of blobs, formation ofa radial internal transport barrier which explain the blue-core formation in helicon sources, inward particle flux againstdensity gradients, spontaneous self-organized transportbifurcation, etc.17–21 In this paper, we investigate the axialvelocity and the velocity shear, and parallel ion temperaturesin CSDX, which relate to both studies of plasma impurityentrainment22 and generation of intrinsic flows.14II. EXPERIMENTAL APPARATUSThe experiments were carried out in CSDX, a linearmagnetized plasma device, 2.8 m long with 0.2 m diameter(see Fig. 1). Argon plasma is produced by a 13.56 MHz, 1.8kW, m = 1 helicon antenna with diameter 0.15 m. CSDX isimmersed in a magnetic field up to 2400 G. At the standardoperating conditions (3.2 mTorr of neutral fill pressure at 25SCCM of neutral argon flow), typical electron temperaturesand plasma densities in CSDX, as measured by RF compen-sated cylindrical Langmuir probes, areTe ∼ 3 eV and n ∼ 1013cm−3, respectively. More details of the experimental device,the operating regimes, the plasma diagnostics, and typicalradial profiles of the standard plasma parameters can be found0034-6748/2016/87(11)/11E513/3/$30.00 87, 11E513-1 Published by AIP Publishing. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  157.182.253.87 On: Tue, 02 Aug2016 15:44:5411E513-2 Chakraborty Thakur et al. Rev. Sci. Instrum. 87, 11E513 (2016)FIG. 1. The experimental device CSDX.elsewhere.17,18,23 Insulating end boundaries were used as theyenhance turbulence levels in CSDX.24,25 All data shown areobtained at the port named α in Fig. 1, ∼75 cm downstream ofthe plasma source.III. DIAGNOSTICSFor the three level LIF scheme used here, the laser lightat 668.6138 nm is absorbed to induce a transition from theAr-II metastable 3d4F7/2 state to the 4p4D5/2 state causing aphoton to be re-emitted at 442.72 nm, through a transition tothe 4s4P3/2 state.10 The magnetic field causes this transitionto split into one group of six linearly polarized π transitions(m = 0) and two orthogonal, circularly polarized groups of sixσ transitions each (m = ±1) due to Zeeman effect. We used aToptica TA100 tunable diode laser (Fig. 2(a)) with a line widthof ∼1 MHz and a mode hop free range of up to 30 GHz. Abeam splitter was used to redirect 8% of the beam into a BristolInstruments 621-VIS wavemeter, to measure the wavelengthwith an accuracy of ±0.000 05 nm which corresponds to aninstrumental error of ∼±25 m/s. Another 8% of the beam isdirected to an iodine cell. A quarter wavelength plate was usedto choose only the m = 1 circularly polarized σ branch, whichconsist of six Zeeman split lines.FIG. 2. Details of the LIF diagnostics: (a) the injection setup where 1 isthe diode laser; 2 is the wavemeter; 3 and 4 are 8% beam splitters; 5 and6 are turning mirrors; 7 is a quarter wave-plate, 8 is an iodine cell; 9 is aphoto-multiplier tube; 10 is a mechanical chopper; 11 is the CSDX chamber;and 12 is the plasma column. The dotted lines represent a box to cover theiodine cell and photo-multiplier. (b) Collection setup.FIG. 3. Axial velocities measured at r = 0 cm, as B is varied.Laser light centered at 668.6138 nm was injected parallelto the magnetic field through a window at the pump end ofCSDX. The laser frequency was scanned over 10-20 GHzto capture the full ion velocity distribution function (IVDF).The emitted florescence was collected through a side portusing a focusing optic and 1 nm width bandpass filter centeredat 443 nm in front of a photo multiplier tube (PMT) (seeFig. 2(b)). The overlap of the focused collection optics and thelaser beam forms a collection volume of ∼3 mm3, the spatialresolution of the measurements. Since this transition can alsobe collisionally excited by electron impact, phase synchronousdetection was performed by modulating the beam intensitywith a chopper at 1 kHz and a lock-in amplifier. The injectionand the collection optics were moved in tandem to accessmultiple spatial locations to measure radial profiles.For this argon ion transition, the Doppler and Zeemanbroadening are the only two relevant line broadening mech-anisms.26 The relative magnitudes and the shifts of the Zee-man splitting of the lines can be calculated from quantummechanical considerations.27 Since the Zeeman splitting andDoppler broadening are of the same order, the data need to befit with a convolution of six Doppler broadened Maxwellianfunctions (since we choose only the m = 1 circularly polar-ized σ branch). The measured intensity is fit to the followingequation:FIG. 4. Radial profiles of axial velocities below and above Bth. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  157.182.253.87 On: Tue, 02 Aug2016 15:44:5411E513-3 Chakraborty Thakur et al. Rev. Sci. Instrum. 87, 11E513 (2016)FIG. 5. Parallel ion temperatures at r = 0 cm, as B is varied.I(ν) =nIn exp(κν (ν − δν − νn)2Ti), (1)where In and νn represent the relative amplitudes and Zeemanshifts for each transition line, Ti is the ion temperature, theDoppler shift is given by δν, and κν is a constant (0.092 495eV/GHz2). The best fit of Eq. (1) to the measured IVDF usinga nonlinear least squares fitting routine gives the Doppler shiftand the ion temperature.IV. RESULTS AND DISCUSSIONSThe radial profiles of the axial flows and ion temperaturesare measured for B = 400 G to B = 2200 G, to complementother studies of the transition at Bth ∼ 1300 G. In Fig. 3,we see that the axial velocity, measured at the center of theplasma device, remains almost constant with increasingB untilB ∼ Bth. ForB > Bth, the core axial velocity decreases rapidly.Recent experiments show plasma detachment at the end forB > Bth and this could be related to the drop of the axialvelocities in this regime.In Fig. 4, we show the radial profiles of the axial velocity,for B above and below theBth. We find that for lowB, the flowprofiles are almost flat with little to no radial shear in the axialvelocities. However as we approach Bth and go beyond, weobserve the development of a strong radial shear in the axialvelocities. We are currently investigating if the energy fromthe parallel shear flow instability plays any role in the globaltransition atBth. Recent theoretical work has also shown14 thatin the absence of magnetic shear, as in CSDX, parallel velocityshear can lead to a new physical mechanism of driving intrinsicrotation via symmetry breaking and these preliminary resultsfrom CSDX agree with these arguments.Fig. 5 shows that the parallel ion temperature, measuredat the center of the device (r = 0 cm), increases with B (forB < Bth), reaches a maximum forB ∼ Bth and forB > Bth, theion temperature drops. The edge ion temperature near the wallof the device is almost constant for all values of B. Hence thisindicates increased ion temperature gradients (ITGs) near Bth.Existence of multiple instabilities suggests multiple energysources involved and the ion temperature gradient (ITG) driveninstability might be relevant. In tokamaks, it has been seenthat the nature of instabilities has a crucial role in decidingthe direction of rotation for intrinsic rotation along the toroidaldirection12,13 and effects of parallel ITG on turbulence dy-namics are being studied in CSDX.In conclusion, we measured parallel flows, radial shearin the parallel flows, and parallel ion temperatures in CSDX.These studies complement our efforts in understanding theglobal transition from drift wave dominated plasmas to a multi-instability regime with several interesting characteristics.17–21The axial flow measurements have also resolved several earlierissues with impurity transport studies on CSDX28 and theapplicability of Mach probes.22 We figured out the physicalreasons for a systematic error in the interpretation of Machprobes. Comparison to LIF results helped us to introduce acorrection term in the analysis of Mach probe data to fix thesystematic error due to perturbative probe shadows in magne-tized plasmas.221R. A. Pitts et al., Plasma Phys. Controlled Fusion 47, B303 (2005).2G. F. Matthews, J. Nucl. Mater. 337-339, 1 (2005).3L. Oksuz, M. A. Khedr, and N. Hershkowitz, Phys. Plasmas 8, 1729(2001).4I. H. Hutchinson, Phys. Plasmas 9, 1832 (2002).5K.-S. Chung, Plasma Sources Sci. Technol. 21, 063001 (2012).6G. D. Severn, D. A. Edrich, and R. McWilliams, Rev. Sci. Instrum. 69, 10(1998).7G. R. Tynan, C. Holland, J. H. Yu, A. James, D. Nishijima, M. Shimada, andN. Taheri, Plasma Phys. Controlled Fusion 48, S51 (2006).8Y. Nagashima et al., Phys. Plasmas 16, 020706 (2009).9T. Windisch, O. Grulke, and T. Klinger, J. Nucl. Mater. 390, 395(2009).10T. A. Carter and J. E. Maggs, Phys. Plasmas 16, 012304 (2009).11E. Thomas, Jr., J. D. Jackson, E. A. Wallace, and G. Ganguli, Phys. Plasmas10, 1191 (2003).12J. Rice et al., Nucl. Fusion 44, 379 (2004).13B. LaBombard et al., Phys. Plasmas 12, 056111 (2005).14J. C. Li, P. H. Diamond, X. Q. Xu, and G. R. Tynan, Phys. Plasmas 23,052311 (2016).15M. J. Burin, G. R. Tynan, G. Y. Antar, N. A. Crocker, and C. Holland, Phys.Plasmas 12, 052320 (2005).16P. Manz, M. Xu, S. C. Thakur, and G. R. Tynan, Plasma Phys. ControlledFusion 53, 095001 (2011).17S. C. Thakur, C. Brandt, L. Cui, J. J. Gosselin, A. D. Light,and G. R. Tynan, Plasma Sources Sci. Technol. 23, 044006(2014).18A. D. Light, S. C. Thakur, C. Brandt, Y. Sechrest, G. R. Tynan, and T.Munsat, Phys. Plasmas 20, 082120 (2013).19S. C. Thakur, C. Brandt, L. Cui, J. J. Gosselin, and G. R. Tynan, IEEE Trans.Plasma Sci. 43, 2754 (2015).20L. Cui, G. R. Tynan, P. H. Diamond, S. C. Thakur, and C. Brandt, Phys.Plasmas 22, 050704 (2015).21L. Cui, A. Ashourvan, S. C. Thakur, R. Hong, P. H. Diamond, and G. R.Tynan, Phys. Plasmas 23, 055704 (2016).22J. J. Gosselin, S. C. Thakur, S. H. Sears, J. McKee, E. E. Scime, and G. R.Tynan, Phys. Plasmas 23, 073519 (2016).23S. C. Thakur, C. Brandt, A. Light, L. Cui, J. J. Gosselin, and G. R. Tynan,Rev. Sci. Instrum. 85, 11E813 (2014).24S. C. Thakur, M. Xu, P. Manz, N. Fedorczak, C. Holland, and G. R. Tynan,Phys. Plasmas 20, 012304 (2013).25D. A. D’Ippolito, D. A. Russell, J. R. Myra, S. C. Thakur, G. R. Tynan, andC. Holland, Phys. Plasmas 19, 102301 (2012).26R. F. Boivin, “Study of the different line broadening mechanisms for the laserinduced fluorescence diagnostic of the HELIX and LEIA plasmas, PL-039,West Virginia University,” Morgantown, WV, 2002.27R. F. Boivin, “Zeeman splitting for LIF transitions and de-convolutiontechnique to extract ion temperatures,” PL-050, West Virginia University,Morgantown, WV, 2002.28J. J. Gosselin, Ph.D. thesis, University of California at San Diego, 2016. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  157.182.253.87 On: Tue, 02 Aug2016 15:44:54

Laser Induced Fluorescence Measurements Of Axial Velocity, Velocity Shear, And Parallel Ion Temperature Profiles During The Route To Plasma Turbulence In A Linear Magnetized Plasma Device

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Laser Induced Fluorescence Measurements Of Axial Velocity, Velocity Shear, And Parallel Ion Temperature Profiles During The Route To Plasma Turbulence In A Linear Magnetized Plasma Device

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