Boivin, R. L.

Brooks, N. H.

Galante, M. E.

Groebner, R. J.

Hill, D. N.

Magee, R. M.

McCarren, D.

Porter, G. D.

Scime, E. E.

English

The Research Repository @ WVU (West Virginia University)

Faculty Scholarship2012A Two Photon Absorption Laser InducedFluorescence Diagnostic For Fusion PlasmasR. M. MageeM. E. GalanteD. McCarrenE. E. ScimeR. L. BoivinSee 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 CitationMagee, R. M.; Galante, M. E.; McCarren, D.; Scime, E. E.; Boivin, R. L.; Brooks, N. H.; Groebner, R. J.; Hill, D. N.; and Porter, G. D.,"A Two Photon Absorption Laser Induced Fluorescence Diagnostic For Fusion Plasmas" (2012). Faculty Scholarship. 453.https://researchrepository.wvu.edu/faculty_publications/453AuthorsR. M. Magee, M. E. Galante, D. McCarren, E. E. Scime, R. L. Boivin, N. H. Brooks, R. J. Groebner, D. N. Hill,and G. D. PorterThis article is available at The Research Repository @ WVU: https://researchrepository.wvu.edu/faculty_publications/453REVIEW OF SCIENTIFIC INSTRUMENTS 83, 10D701 (2012)A two photon absorption laser induced fluorescence diagnostic for fusionplasmasa)R. M. Magee,1,b) M. E. Galante,1 D. McCarren,1 E. E. Scime,1 R. L. Boivin,2 N. H. Brooks,2R. J. Groebner,2 D. N. Hill,2 and G. D. Porter31Physics Department, West Virginia University, Morgantown, West Virginia 26506, USA2General Atomics, San Diego, California 92121, USA3Lawrence Livermore National Laboratory, Livermore, California 94550, USA(Presented 9 May 2012; received 7 May 2012; accepted 21 May 2012; published online 5 June 2012)The quality of plasma produced in a magnetic confinement fusion device is influenced to a largeextent by the neutral gas surrounding the plasma. The plasma is fueled by the ionization of neu-trals, and charge exchange interactions between edge neutrals and plasma ions are a sink of energyand momentum. Here we describe a diagnostic capable of measuring the spatial distribution of neu-tral gas in a magnetically confined fusion plasma. A high intensity (5 MW/cm2), narrow bandwidth(0.1 cm−1) laser is injected into a hydrogen plasma to excite the Lyman β transition via the si-multaneous absorption of two 205 nm photons. The absorption rate, determined by measurementof subsequent Balmer α emission, is proportional to the number of particles with a given velocity.Calibration is performed in situ by filling the chamber to a known pressure of neutral krypton andexciting a transition close in wavelength to that used in hydrogen. We present details of the calibrationprocedure, including a technique for identifying saturation broadening, measurements of the neutraldensity profile in a hydrogen helicon plasma, and discuss the application of the diagnostic to plasmasin the DIII-D tokamak. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4728092]I. INTRODUCTION: THE EVOLUTION OF TALIFActive spectroscopy with laser induced fluorescence(LIF) is a valuable plasma diagnostic technique, providinglocalized measurements of neutral and ion velocity distribu-tions. Since it was first demonstrated 35 years ago,1 it hasevolved into a workhorse plasma diagnostic. It has been usedto diagnose ion temperature anisotropy in a helicon plasma,2measure turbulence in a magnetic nozzle,3 and measure neu-tral densities in the edge of a hot fusion plasma.4 Its devel-opment has been driven largely by the development of lasertechnology. In fact, since reliable, high energy, narrow spec-tral width lasers have become commercially available, a majorstep in LIF evolution has been reached: the progression to twophoton absorption laser induced fluorescence, or TALIF.TALIF has several advantages over traditional LIF. First,because two photons are used for excitation, each needs onlyprovide half of the transition energy. This allows the directexcitation of the ground state with near-UV (λ = 205 nm)photons, which are less expensive to produce and easier totransmit than λ = 102.5 nm photons. Second, the signal isproportional to the square of the laser intensity, rather thanthe intensity, so the measurement can be made highly local-ized without a perpendicular viewing chord. By focusing theinjected beam with a lens, the TALIF signal is increased 105times at the focus relative to the unfocused beam. For com-parison, the LIF signal would only be 300 times larger atthe focus. (This is for the case used here: lens focal lengtha)Contributed paper, published as part of the Proceedings of the 19th TopicalConference on High-Temperature Plasma Diagnostics, Monterey, Califor-nia, May 2012.b)Email: richard.magee@mail.wvu.edu.f = 25 cm, beam divergence θ = 0.5 mrad, and initial beamradius r = 0.2 cm). Finally, the TALIF diagnostic has theability to make a Doppler-free measurement by employingtwo counter-propagating beams, where the energy of a pho-ton from each beam sums to the transition energy for all atomvelocities. Practically, this means that the time resolution ofdensity measurements is increased as there is no need to scanthe laser frequency.Here we employ the three level hydrogen scheme firstdemonstrated by Bokor et al.5 Ground state hydrogen is ex-cited from the n = 1 level to the n = 3 level by the simultane-ous absorption of two λ ∼ 205 nm photons. After excitation,a fraction of the hydrogen atoms decay from the n = 3 levelto the n = 2 level and emit photons at 656.3 nm which arecollected. Note that TALIF is absorption spectroscopy, so themeasured signal is related to the Doppler-shift of the absorbedlight, rather than the emitted light.The West Virginia University TALIF system builds onprevious work6, 7 by combining for the first time both highpower and narrow spectral width. This results in larger sig-nal levels, an important feature for measurements in fusionplasma where the background emission is high. It also meansthat the line is more easily saturated, which leads to artificialbroadening.II. LASER AND LIGHT COLLECTION HARDWAREA dye laser is pumped by a Spectra-Physics Quanta-RayPro-270 pulsed Nd:YAG laser with a maximum power of600 mJ per pulse at 532 nm at a repetition rate of 20 Hz.The pulses have been measured to be 10 ns wide (i.e., Pinst∼ 100 MW) with 2% energy stability.0034-6748/2012/83(10)/10D701/3/$30.00 © 2012 American Institute of Physics83, 10D701-1Downloaded 05 Jun 2012 to 157.182.27.238. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions10D701-2 Magee et al. Rev. Sci. Instrum. 83, 10D701 (2012)The dye laser is a Sirah CobraStretch optimized for615 nm light with a mixture of Rhodamine B and Rhodamine101. The grating (2400 l/mm) yields a measured linewidth of0.07 cm−1. The energy is ∼150 mJ/ pulse and the pulse widthis 12 ns (i.e., Pinst ∼ 10 MW).The output of the dye laser is frequency doubled to308 nm in a SHG-250 BBO crystal, mixed back with the615 nm fundamental, and passed through a SHG-206 BBOcrystal to generate a third harmonic beam at 205 nm. The205 nm light has a maximum power of 8.5 mJ/ pulse, atemporal FWHM of 7 ns (i.e., Pinst ∼ 1 MW), energy sta-bility of 5%-7% and a spatial profile very nearly Gaussianwith a width of σ = 1.7 mm. The linewidth of the 205 nmbeam can be calculated from the measured 615 nm beam:k205 =√3k615 = 0.12 cm−1.The collected light is filtered with a 656.3 ± 0.5 nm band-pass filter and focused onto a photomultiplier tube (PMT)(Hamamatsu H11526-20-NF). The PMT output is amplifiedwith a high speed amplifier (Hamamatsu C5594) that has anoperating range of 50 kHz–1.5 GHz and a gain of 36 dB. Theamplified PMT signal is then processed by a Stanford Re-search Systems Fast Gated Integrator and Boxcar Averager(Model SR250). The boxcar acquires data at twice the rep-etition rate of the laser, so that signal and background mea-surements are interleaved. The background is subtracted fromthe laser pulses, and they are integrated in time and aver-aged together. The result is recorded along with the measuredwavelength of the 615 nm beam, obtained with a High Fi-nesse WS7 wavelength meter that is accurate to 0.075 pm or60 MHz. The laser is scanned in wavelength across the dis-tribution in 1s or 10s of seconds. Typically, as shown in theinset of Figure 2, 400 data points are collected and smoothedby 10.III. TALIF CALIBRATION AND MEASUREMENTSThe TALIF signal amplitude as a function of laser fre-quency, omitting constants, isS(ν) ∼ I2n0√ν2d + 2ν2l. (1)It is proportional to the square of the laser intensity, I, andthe neutral density, n0. The Doppler and laser linewidths, νdand ν l, respectively, appear in the denominator, indicatingthat the signal amplitude drops with increased temperature ordecreased laser performance.The line shape of the TALIF signal is a convolution ofthe particle velocity distribution and the distribution of photonpairs (which is twice the laser linewidth). In the case whereboth of these are Gaussian, the width of the signal is given by,νs = 12√2ν2l + ν2d .Absolute calibration of the diagnostic is accomplishedby generating a TALIF signal from a known pressure ofkrypton gas.6, 9, 10 The excitation wavelength for the 4p61S0to 5p′[3/2]2 transition in ground state krypton is at 102.1nm (i.e., 2 × 204.2 nm photons) and the emission from the5p′[3/2]2 to 5s′[1/2]1 transition is at 826 nm, making it anideal calibration scheme.10 The absolute atomic hydrogenFIG. 1. Measured TALIF signal from KrI (symbols) scales very nearly asE2 (red curves) until the critical saturation intensity is reached. This occursfor E = 0.22 mJ with an f = 25 cm lens (squares) and E = 0.85 mJ for an f= 50 cm lens (triangles). The critical energy is four times larger in the lattercase because the spot size is twice as large.density in plasma is determined from the ratio of the TALIFsignals when the relative detection sensitivities for the twospecies is known.When the number of particles to be pumped in a laserpulse time exceeds the number of particles available in theemission volume, the line is saturated. Because this hap-pens first in the wings of the distribution, where there arefewer particles, exceeding the critical laser intensity broad-ens the line, a process referred to as saturation broadening.8If the laser pulse length is shorter than the relaxation time ofthe transition, the critical intensity can be approximated as,Ic ≈ ¯ωβσ2γ gτ , where ¯ is Planck’s constant, ω is the laser fre-quency, β is a statistical photon counting factor, σ 2γ is thetwo photon absorption cross section, g is the absorption lineshape, and τ is the laser pulse time. For the case of cold Kr,we expect the laser to saturate the line at Ic = 3 × 108 W/cm2.Shown in Figure 1 are the amplitudes of the Kr TALIFsignal as a function for laser energy for two cases: one withan f = 25 cm injection lens and one with an f = 50 cm injec-tion lens. In the f = 25 cm case, the functional dependenceof the amplitude changes from E2 to E at E = 0.22 mJ (cor-responding to an intensity of 2 × 108 W/cm2 assuming a fo-cused beam size of r = 70 μm). When the focal length ofthe injection lens is doubled, the radius of the focused beamspot is doubled, and the critical laser energy increases to E= 0.85 mJ, very nearly a factor of four. As the energy is in-creased beyond the critical energy the line is broadened. If thiseffect is not avoided or accounted for, one could interpret sat-FIG. 2. Neutral density radial profile across the plasma expansion chamberwith example spectrum inset.Downloaded 05 Jun 2012 to 157.182.27.238. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions10D701-3 Magee et al. Rev. Sci. Instrum. 83, 10D701 (2012)1.0 1.2 1.4 1.6 1.8 2.0R (m)-1.4-1.2-1.0-0.8Z (m)1.e+13 1.e+14 3.e+15 4.e+16 5.e+17 7.e+18 8.e+19-1.3 -1.2 -1.1 -1.0 -0.9Z (m)1.0e-031.0e-021.0e-011.0e+00SNR<= separatrixFIG. 3. The neutral density predicted for DIII-D by the UEDGE code in m−3 (left), and the expected signal-to-noise ratio (right). With averaging, SNR as lowas 1% have been resolved in the test plasma, implying that the neutral density can be measured up to the separatrix in DIII-D.uration broadening as Doppler broadening, and overestimateboth the temperature and density of the gas.Hydrogen plasma is generated by an RF discharge(∼600 W at ∼13 MHz) in a glass tube with D = 8 cm and L= 60 cm and allowed to expand into a stainless steel chamberwith D = 14 cm and L = 25 cm. There is an axial magneticfield of 10s-100s of G. Typical plasma parameters from Lang-muir probe measurements are ne ≈ 1010–1012 cm−3 and Te≈ 1–10 eV.The laser light is injected through a D = 1 cm hole drilledin a 45◦ turning mirror. The light is focused in the plasma,stimulating emission which is collected and collimated bythe same lens, and turned by the mirror into a focusing lensmounted in front of a PMT. This “confocal” configuration al-lows the measurement of the neutral density profile by simplyscanning the injection lens towards and away from the plasmasource. The result of such a scan is shown in Figure 2. In theseparticular plasmas the neutral hydrogen density profile wasfound to be relatively flat.IV. APPLICATIONS TO DIII-D PLASMASThe neutral particles at the edge of a magnetically con-fined plasma represent both a particle source (plasma fuelingvia ionization) and a particle, energy, and momentum sink(energetic particle loss through charge exchange). The neu-tral density is therefore an important parameter in calculationsof particle confinement time and plasma rotation. In tokamakplasmas, the neutral density profile is thought to play an im-portant role in pedestal formation and the L- to H-mode tran-sition. The neutral density is also important for interpretationof neutral particle energy analyzer data, where the velocitydistribution of plasma ions is inferred from measured neutralvelocities.Neutral density measurements are challenging for tworeasons. First, the background Dα levels tend to be high, dueto significant amounts of electron impact excitation. Second,the neutral density can vary by several orders of magnitudeover the desired measurement region. To quantify signal tobackground level, we investigate a specific case for the DIII-D tokamak.The neutral density and Dα emission are modeled withUEDGE (unified edge code), a 2-D fluid code, whichsolves the fluid equations for density, energy and parallelmomentum.11 Transport parallel to field lines is assumed tobe classical while perpendicular transport is assumed to beanomalous with an artificial radial diffusion coefficient. Theneutrals are treated as an inertial fluid that can interact (i.e.,exchange momentum) with ions via charge exchange.12 Theoutput of a UEDGE run are Te, ne, and n0. An example ofthe neutral density is show in Figure 3 for DIII-D discharge142 613 at t = 4500 ms.The outputs of the UEDGE code can be used to calcu-late the Dα emissivity, which has been benchmarked againstthe measured Dα emission to within a factor of 2. Becausethe TALIF measurement relies on a background subtractionto isolate the signal, the signal-to-noise ratio (SNR) dependson the size of the Dα fluctuations on the time scale of thissubtraction (t ∼ 100 ns). These data are not available, so weuse the fluctuation amplitude on the 500 μs time scale, likelyan upper bound. The result of the calculation is shown inFigure 3. We conclude that the signal is resolvable up to theseparatrix in DIII-D.ACKNOWLEDGMENTSThis work is funded by the US Department of Energy(DOE) through Grant No. DE-SC0004736.1R. A. Stern and J. A. Johnson, Phys. Rev. Lett. 34, 1548–1551(1975).2E. E. Scime, P. A. Keiter, M. W. Zintl, M. M. Balkey, J. L. Kline, andM. E. Koepke, Plasma Sources Sci. Technol. 7, 186–191 (1998).3F. M. Levinton and F. Trintchouk, Rev. Sci. Instrum. 72, 898–905 (2001).4P. Mertens and M. Silz, J. Nucl. Mater. 241-243, 842–847 (1997).5J. Bokor, R. Freeman, J. White, and R. Storz, Phys. Rev. A 24, 612–614(1981).6M. Boogaarts, S. Mazouffre, G. Brinkman, H. van der Heijden, P. Vankan,D. S. J. A. M. van der Mullen, and H. Döbele, Rev. Sci. Instrum. 73, 73–86(2002).7T. Kajiwara, T. Shinkawa, K. Uchino, M. Masuda, K. Muraoka, T. Okada,M. Maeda, S. Sudo, and T. Obiki, Rev. Sci. Instrum. 62, 2345–2349 (1991).8W. Demtröder, Laser Spectroscopy Volume 2: Experimental Techniques(Springer, 2008).9J. Jolly and J.-P. Booth, J. Appl. Phys. 97, 103305 (2005).10K. Niemi, V. Schulz-von der Gathen, and H. Döbele, J. Phys D 34, 2330–2335 (2001).11T. D. Rognlien, J. L. Milovich, M. E. Rensink, and and G. D. Porter,J. Nucl. Mater. 196-198, 347–351 (1992).12G. D. Porter, S. L. Allen, M. Brown, M. E. Fenstermacher, D. N. Hill,R. A. Jong, A. W. Leonard, D. Nilson, M. E. Rensink, T. D. Rognlien, G.R. Smith, and the DIII-D Team, Phys. Plasmas 3, 1967–1975 (1996).Downloaded 05 Jun 2012 to 157.182.27.238. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions

A Two Photon Absorption Laser Induced Fluorescence Diagnostic For Fusion Plasmas

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A Two Photon Absorption Laser Induced Fluorescence Diagnostic For Fusion Plasmas

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