Dermott, F.S.

Fiore, C.L.

Moody, J.D.

Porkolab, M.

Squire, J.

Takase, Y.

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PFC-/JA-86-60A Study of Directly Launched IonBernstein Waves in a TokamakY. Takase, J. D. Moody, C. L. Fiore,F. S. McDermott, M. Porkolab, and J. SquirePlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139November 1986Submitted to: Physical Review LettersThis 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.By acceptance of this article, the publisher and/or recipient acknowledges the U. S. Govern-ment's right to retain a non-exclusive, royalty-free license in and to any copyright coveringthis paper.A Study of Directly Launched Ion Bernstein Waves in a TokamakY. Takase, J. D. Moody, C. L. Fiore, F. S. McDermott,M. Porkolab, and J. SquirePlasma Fusion Center, Massachusetts Institute of TechnologyCambridge, MA 02139Excitation, propagation, and absorption of directly launched ion Bern-stein waves (IBW) were studied with C02 laser scattering in theAlcator C tokamak. Optimum excitation is obtained when an ion-cyclotron harmonic layer is located just behind the antenna. Waveabsorption at the w = 3ID = 1.5f0m layer was observed. Ion heatingand the scattered signal from waves in the plasma interior are bothmaximized at a density of R, ~ 1 x 1014 cm- 3 .PACS numbers: 52.40.Db, 52.35.Hr, 52.50.Gj, 52.55.FaRecently, plasma heating by directly launched IBW was proposed by Ono,1and encouraging results have been obtained at moderate power levels.2', However,the excitation, propagation, and absorption of IBW in a hot tokamak plasma are stillnot well understood. In this Letter, we report the results of the first experimentalstudy of such characteristics obtained by collective scattering of C02 laser radiationfrom the directly excited IBW in the Alcator C tokamak. The details of the heatingexperiment will be reported in a separate publication.During the IBW experiments, reduced size molybdenum limiters of a = 12.5 cm,12.0 cm, or 11.5cm defined the plasma miinor radius while the major radius was1RO = 64 cm. The transmitter frequency was w/(27r) = 183.6 MHz and up to 150kWof rf power was coupled into the plasma from a Faraday shielded toroidal loop an-tenna located on the low magnetic field side of the torus.4 The majority ion specieswas hydrogen, with varying concentrations (0-20%) of minority deuterium. Thetoroidal magnetic field was varied in the range 5 < B < 11 T which corresponds to2.4 > W/11 > 1.1 at the plasma center, where fH is the hydrogen ion-cyclotronfrequency.The scattering arrangement was similar to that described in Ref. 5. The scat-tering diagnostic was located at the same toroidal location as the IBW antenna.The horizontal resolution (defined as 1/e' in power) ranged from ±8mm to ±3mmdepending on the optical geometry. The measurements were essentially chord aver-aged in the vertical direction. The location of the scattering volume is denoted bythe horizontal distance from the plasma center, x = R - RO. The range of wavenum-bers accessible by the present scattering arrangement was 10 < k < 140 cm- 1 withresolutions of ±5 cm- 1 to ±7 cm- 1 .A wavenumber spectrum of IBW can be obtained from a shot-by-shot scanof the scattering angle. As shown in Fig. 1(a), the wavenumber spectrum is welldefined in spite of poor vertical resolution. The dispersion relation can be mappedout by scanning the scattering volume horizontally while keeping the plasma pa-rameters constant. The result is displayed in Fig. 1(b). The solid curve representsthe theoretical dispersion curve obtained by solving IIl = 0, where 6 is defined byA. x (il x A) + . -, i c k/w, and k is the hot plasma dielectric tensor.'The value of the wavenumber corresponding to the peak in Pc was used, and thehydrogen Larmor radius pH was evaluated from the measured hydrogen ion tem-perature at the midplane of the torus. The agreement between the experimentallyobtained dispersion relation and the theoretical dispersion relation suggests that2most of the scattered signal originates from a region close to the plasma midplane,in agreement with ray tracing calculations. 4 The scattered power varied linearlywith the injected rf power.The spectral power density of the IBW, defined as, 'i(k) = -(c/4r)f*(8V/8n±) - E ~ -(c/47r)(8K../ni) 1E 12, can in principle be obtained from thescattered signal, which is proportional to M,(k). The fluctuating electron densityassociated with the wave is given byf, . e W2 .W K,,V= - 2 2 - 2 n-E., (1)ne mecw w we K -iln -ni n, - KzZwhere the z-direction is parallel to the magnetic field. In this paper the scatteredpower is displayed because the conversion of the scattered power to the wave poweris subject to uncertainties in nil of the IBW, which cannot be determined experimen-tally by scattering. Nevertheless, the scattered power still represents the variationof the actual wave power well (within factors of two) under the conditions reportedin this paper.First we show the scattering results from the case of second harmonic launch-ing in a hydrogen plasma at B ~ 7.6 T. The wave power is expected to be absorbednonlinearly7 '8 at the w = 1.5011 (= 3D) layer located at x/a ~ -0.3. In thepresence of a deuterium minority species, the wave can also damp linearly on thedeuterium ions. The scattered power measured at x/a = +0.4 is plotted as afunction of the toroidal magnetic field in Fig. 2(a). In agreement with theoreticalexpectations, the scattered signal is maximized at B ~ 7.6 T, when w/0H = 1.9 infront of the antenna and the w/AH = 2 layer is located just behind the antenna. Fordirect excitation of IBW, launching is expected to be optimized when k_ is smallest[see Fig. 1(b)]. As shown in Fig. 2(b), such a resonant peak is also observed on theantenna loading resistance, and corresponds to the direct excitation of the IBW. In3addition, an almost field independent loading of 1M was also observed.The scattered power observed at z/a = +0.4 is plotted as a function of densityin Fig. 3(a). The scattered power decreases rapidly as the density is raised above avalue of i, ~ 1 x 1014 cm- 3 . If the scattered power were dominated by low (high)njj components (the boundary is around n0 ~ 1 for typical Alcator parameters), theactual wave power density would fall off more rapidly (more slowly) with increasingdensity than the scattered power plotted in Fig. 3(a). The ion temperature increase3also exhibits a sharp drop above R, ~ 1 x 10 1 4 cm- 3 and no clear evidence of heatingis observed above R. ~ 2 x 10 1 4 cM~ 3. On the other hand, the loading resistance(not shown) increases with density up to R, ~ 3 x 104 cm-3 .The disappearance of ion heating at high plasma density can in principle be ex-plained by the scaling of the threshold power for nonlinear cyclotron damping withdensity.7 Even at high density, however, linear damping of the IBW on the minoritydeuterium should still occur. The scattering result suggests that no ion heating oc-curs at high plasma density because the IBW is not able to penetrate to the plasmacenter [see Fig. 3(a)]. The cause for the decrease in IBW power with plasma densityis not fully understood at present. It may be that the present antenna does not cou-ple efficiently to the IBW at high plasma density. Another mechanism that couldprevent penetration of wave power to the plasma interior is scattering of IBW fromthe low-frequency density fluctuations near the plasma edge.' In Fig. 3(b) the scat-tering data from the low-frequency density fluctuations, integrated up to 5 cm-1in wavenumber and 400kHz in frequency, and observed along a chord located atz/a = +0.83, is shown as a function of plasma density. The quantity plotted on thevertical axis is P.c/R, which is proportional to (i/,/n,)2 . Here fi refers to the am-plitude of the low-frequency density fluctuation. The absolute magnitude of fi./ncould not be determined with accuracy, but based on previous measurements 10 it4is estimated to be in the range 0.1 to 0.5 at fl, ~ 2 x 1014 cm-3. The correlationbetween Figs. 3(a) and (b) suggests that scattering of IBW by low-frequency den-sity fluctuations may be important in determining wave accessibility to the plasmainterior.The radial profile of the scattered power from the IBW is plotted in Fig. 4.The strong radial attenuation of wave power near the plasma edge is observed atall densities. This may be caused by a spatial spreading (especially in the toroidaldirection) of the waves, 4 and possibly by scattering from low-frequency density fluc-tuations. A fairly significant attenuation of the scattered power is observed on thehigh field side of the plasma center, near the w = 1 .50, nonlinear resonance, whichmay be an indication of nonlinear absorption since the deuterium concentration wasnegligibly small (nD/n, < 1%).In order to investigate the effects of local plasma parameters at the antennasurface on excitation of IBW, the antenna was moved radially with respect to theplasma surface (defined by the limiter) under fixed plasma conditions. Both the iontemperature increase and the scattered signal reached a maximum when the frontsurface of the antenna Faraday shield was located 4mm in the shadow of the limiterregardless of the limiter size (the density scrape-off length is typically 2-3mm inAlcator). However, the loading resistance increased monotonically as the antennawas moved in radially, possibly indicating an increased power loss near the antennawhen the local density is excessively high.At magnetic fields of 9 < B < 10T, the w = 30D = 1. 5 0H layer is locatedin the outer plasma edge region in front of the antenna. A strong attenuation ofthe scattered signal was observed when the w = 3 0D layer was located between theantenna and the scattering volume, which indicates a nearly complete absorptionof the IBW power; either linearly by deuterium minority ions, or nonlinearly by5hydrogen ions. We note that the signal level detected under these conditions ismuch smaller (two orders of magnitude) than the case of second harmonic resonantlaunching, and no scattered signal was detected in the plasma interior. Nevertheless,significant central ion heating and centrally peaked deuterium minority tail wereobserved even in this regime.3 It is also noted that the ion temperature at r/a ~0.85, deduced from the IBW dispersion relation,11 correlated well with the centralion temperature increase. At present this phenomenon is not understood.In summary, excitation, propagation, and damping of the directly launchedIBW in a tokamak plasma were studied for the first time, using C02 laser scatteringtechniques. The IBW was identified by mapping out the dispersion relation, andthe scattered power varied linearly with the injected rf power. Wave launchingwas optimized when an ion-cyclotron harmonic layer was located just behind theantenna, and when the antenna was placed 4mm into the shadow of the limiter.Wave damping at the w = 3 0D = 1. 5flH layer was observed. Ion heating andthe scattered signal both decrease above a density of ii, ~ 1 x 1014 cm-3 . Thecorrelation with the increase of low-frequency edge density fluctuations suggests thatthe scattering of IBW by these fluctuations may be preventing the wave penetration.The local plasma parameters near the antenna surface may also play an importantrole in determining the wave excitation. Direct observation of IBW by scattering isa powerful tool for investigating wave physics. This technique can also be used tomeasure the ion temperature profile in hot plasmas.We would like to thank the members of the Alcator group for their contribu-tions, especially the engineering support of D. Griffin. This project was supportedby the U.S. Department of Energy, Contract No. DE-AC02-78ET51013.6IM. Ono, K. L. Wong, G. A. Wurden, Phys. Fluids 26, 298 (1983).2 M. Ono, et al., Phys. Rev. Lett. 54, 2339 (1985); J. Hosea, et al., PlasmaPhys. and Controlled Fusion 28A, 1241 (1986).3 M. Porkolab, et al., in Plasma Phys. and Contr. Nucl. Fusion Res. (IAEA11th Int. Conf., Kyoto, 1986) proc. to be published.4 J. Moody, et al., Bull. Am. Phys. Soc. 31, 1586 (1986).'Y. Takase, et al., Phys. Rev. Lett. 53, 274 (1984).'T. H. Stix, The Theory of Plasma Waves (McGraw-Hill, New York, 1962).7M. Porkolab, Phys. Rev. Lett. 54, 434 (1985).8H. Abe, et al, Phys. Rev. Lett. 53, 1153 (1984).9 M. Ono, Phys. Fluids 25, 990 (1982).10R. L. Watterson, R. E. Slusher, C. M. Surko, Phys. Fluids 28, 2857 (1985).1G. A. Wurden, M. Ono, K. L. Wong, Phys. Rev. A 26, 2297 (1982).7Figure CaptionsFIG. 1.' (a) A typical k-spectrum of the IBW. Hydrogen, B = 7.6 T, Ip=250 kA, W, = 2 x 1014 cm-3 , x/a = -0.16. (b) The dispersion relation determinedfrom CO 2 scattering data. Hydrogen, B = 7.6 T, Ip = 290 kA, e = 2.3 x 104 cm- 3 .Data shown as circles were obtained from a spatial scan of the scattering volume.The solid line represents the theoretical dispersion curve.FIG. 2. (a) The scattered power and (b) the loading resistance as functionsof the toroidal magnetic field. Hydrogen, 4 = 290 kA, W. = 2.3 x 1014 cm-3,x/a = +0.4.FIG. 3. (a) The scattered power as a function of density. Hydrogen, B = 7.6 T,4 = 200-300 kA, x/a = +0.4. (b) The scattering data from the low-frequencydensity fluctuations as a function of density. Same parameters, z/a = +0.83.FIG. 4. A radial profile of the scattered power. Hydrogen, B = 7.6 T, 4 =290 kA, W. = 0.8 x 1014 cm- 3 , a = 12.5 cm.8m **~' ~ * U U U I *-*UUUUUUUUUUma ' I a15* *U U U r---I----u----I-----u----u I * ~0* 8 SS--- a - ~- I - - -30-F-a- a a ' a - - - - -.- - ' a20 25 35k (cm-1). . .. . . . . .. .(b)- -0.5 1.0 1.5k i pHFig. 120515<v100 502.010040x.51 l.U0.0 2.0..1510502.01.51.00 . 5- :':' liii 1111111111 I 5lfT~ aaaa~EUIu -(a)l.. 3go * A, am..~a a a La a a~I * a* *I am a a-I I I l og I -I 1 IA ON K 0-c--U-. -- m -- - ------------------to-i SEAIsmU.U m -6.0 7.0 8.0 9.0-(b)-Leg see,s.o 1 s .B (T)Fig. 2SC,U'aI- i l - A I -- g-aA .. v91 AA.AMI I U10IS.0 00I * ~* ** I I U* ~~LDJ* a0 1- 0* 0 0* .00*p I a p* U2iie (10 14 cm- 3)3 4Fig. 3m40*30<2000_5o,0 1(a)5 I Ir-17~ B U U I I U U U I U U U EU0Ir0rUUUrUU3 3 II I I I p I a I I I 1 -1 2 1 1 1 1 20-5:00005 10x (cm)Fig. 4U'IUU103MCE)101100101-10 15- w>= 1.5f)H

A study of directly launched ion bernstein waves in a tokamak

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A study of directly launched ion bernstein waves in a tokamak

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