Cushman, A.

Greenwald, M.J.

Hubbard, A.E.

Lin, L.

Lin, Y.

Porkolab, M.

Reinke, M.

Rice, J.E.

Tsujii, N.

Wukitch, S.

DSpace@MIT

                      October 2008   Plasma Science and Fusion Center Massachusetts Institute of Technology Cambridge  MA  02139  USA               This work was supported by the U.S. Department of Energy, Cooperative agreement No. DE-FC02-99ER54512. Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted. * To be published on Phys. Rev. Lett.  PSFC/JA-08-31      Observation of Ion Cyclotron Range of Frequencies Mode Conversion Flow Drive in Tokamak Plasmas*  Lin, Y., Rice, J.E., Wukitch, S. J., Greenwald, M.J.,  Hubbard, A.E., Ince-Cushman, A., Lin, L.,  Porkolab, M., Reinke, M., and Tsujii, N   Observation of Ion Cyclotron Range of Frequencies ModeConversion Flow Drive in Tokamak PlasmasY. Lin,∗ J.E. Rice, S.J. Wukitch, M.J. Greenwald, A.E. Hubbard,A. Ince-Cushman, L. Lin, M. Porkolab, M.L. Reinke, and N. TsujiiMassachusetts Institute of Technology,Plasma Science and Fusion Center, Cambridge, MA 02139, USA(Dated: October 28, 2008)AbstractStrong toroidal flow (Vφ) and poloidal flow (Vθ) have been observed in D-3He plasmas with ioncyclotron range of frequencies (ICRF) mode conversion (MC) heating on the Alcator C-Mod toka-mak. The toroidal flow scales with the rf power Prf (up to 30 km/s per MW), and is significantlylarger than that in ICRF minority heated plasmas at the same rf power or stored energy. Thecentral Vφ responds to Prf faster than the outer regions, and the Vφ(r) profile is broadly peaked forr/a ≤ 0.5. Localized (0.3 ≤ r/a ≤ 0.5) Vθ appears when Prf ≥ 1.5 MW and increases with power(up to 0.7 km/s per MW). The experimental evidence together with numerical wave modelingsuggests a local flow drive source due to the interaction between the MC ion cyclotron wave and3He ions.PACS numbers: 52.55.Fa, 52.50.Qt, 52.35.Hr, 52.30.-q1Plasma flow and velocity shear can be important in stabilizing micro- and macro-instabilities (like drift wave turbulence [1, 2] or resistive wall modes [3]) in tokamak plasmas.For ITER and even more so for reactors, the input torque from neutral beam injection willbe low or non-existent, and beam penetration and accompanying deposition profile controlare limited. Plasma intrinsic rotation (spontaneous, without momentum input) may besufficient for resistive wall mode stabilization on ITER [4], but it does not allow simple ex-ternal control. Externally launched electromagnetic radio frequency (rf) waves may offer thepossibility of controlled rotation drive on ITER and reactors. RF poloidal flow drive mayalso offer a unique means for manipulating transport via flow shear stabilization in reactorrelevant regimes - thus affecting pressure profiles, bootstrap current, etc. As a result, rf flowdrive has been widely identified as a high leverage tool that, if successful, can produce greatbenefits to tokamak research.Ion cyclotron range of frequencies (ICRF) waves have been utilized to drive plasma flow.Kinetic Alfve´n waves were observed to drive plasma flow on the Phaedrus-T tokamak [5].Poloidal flow driven by direct launch ion Bernstein waves (IBW) was detected on TokamakFusion Test Reactor (TFTR) [6]. Using directed fast magnetosonic wave (fast wave, orFW), toroidal rotation was observed to be driven by wave momentum absorption by fastions on the JET tokamak [7]. Preliminary evidence of poloidal flow drive was observed onTFTR through mode conversion (MC) [8]. Here we report the first demonstration of efficienttoroidal flow (Vφ) drive and also observation of poloidal flow (Vθ) generation via the ICRFMC process. The experimental evidence together with numerical wave modeling suggestsa local flow drive source due to the interaction between the MC ion cyclotron wave (ICW)and 3He ions.On the Alcator C-Mod tokamak (R = 0.67 m and a = 0.22 m), fast waves at 80 MHzand 50 MHz are launched externally from the low field side (LFS), either separately orcombined [9]. For the results reported here, plasma current Ip ≃ 0.8 MA, and magnetic fieldBt0 ≃ 5.1 T. The 80 MHz fast wave heats the plasma via D(H) minority heating (MH) withresidual nH/ne ≤ 5%. The fast wave power is first deposited to the minority H ions viaion cyclotron (IC) resonance, RIC,H ≃ 0.65 m, slightly on the high field side (HFS) of themagnetic axis, and then the energetic H ions transfer their energy to the bulk electrons andmajority deuterium ions when they slow down through collisions. In D-3He plasma withmoderate 3He concentration (e.g., n3He/ne ≥ 5%), the 50 MHz fast wave (RIC,3He ≃ 0.712m) can be mostly converted to short wavelength slow waves, ICW and IBW, near the MCsurface (also called D-3He hybrid layer), and directly heat electrons and ions [10–14].For the first time in tokamaks, we have observed significant toroidal flow driven by theICRF MC process. In Fig. 1, we compare the central rotation velocity Vφ(r = 0) of twoupper-single-null plasmas in low confinement mode (L-mode) with the same electron densitybut with different ICRF applications: one using the 80 MHz wave in dipole phase (MH), andthe other using the 50 MHz wave (MC) phased at +90◦ (i.e., wave traveling in the co-currentdirection). The toroidal flow (rotation) velocity, derived from the Doppler shift of the x-rayspectra of Ar17+ impurity measured by high resolution x-ray spectroscopy (HIREX) [15], issignificantly higher in the MC plasma than that in the MH plasma as shown Fig. 1-(a). Inthe MC plasma, Vφ(r = 0) rises from −10 km/sec (counter-current direction), a typical valuein ohmic L-mode plasma, to +75 km/sec (110 kRad/s, and MA = Vφ/CA ∼ 0.013, whereMA is the Alfve´n Mach number and CA is the Alfve´n velocity) in the co-current directionwith 3 MW rf power. The velocity change ∆Vφ is approximately linear vs. Prf , indicating amechanism in which the rf power dissipation is important, rather than a mechanism directlyrelated to the rf field strength. In contrast, in the MH plasma at approximately the samePrf and heating history as the MC plasma, Vφ(r = 0) only rises to +20 km/sec. Strongco-current rotation has been observed in many MC plasmas where 5.0 ≤ Bt ≤ 5.2 T andboth IC resonance and MC surface are near-axis. The rotation is largely independent ofantenna phase. While the magnitude at +90◦ phase is larger than those in dipole and -90◦ phases, the difference is fairly small (< 10%). When compared with the empiricallydetermined intrinsic rotation scaling law ∆Vφ ∝ ∆Wp/Ip [16–18], the rotation change inthese MC plasmas is generally a factor ≥ 2 larger than that from MH plasmas with identicalplasma shape, magnetic field, current and density.Unlike the intrinsic toroidal rotation, which propagates inward from the edge throughmomentum pinch and diffusion [19, 20], the toroidal rotation in MC plasmas rises first nearthe plasma center. In Fig. 2 we compare the time traces of ∆Vφ at r/a ≃ 0, 0.3 and 0.7of an MC plasma. Both velocities at r/a = 0 and 0.3 rise to a steady value in less than80 ms after the application of the 50 MHz rf power, while ∆Vφ(r/a ≃ 0.7) is delayed andwith a much smaller magnitude. The difference in the response to rf power also createsa broad centrally peaked Vφ(r/a) profile in MC plasmas. In Fig. 3-(a), we compare thechord integrated rotation profiles obtained from Ar16+ of the two plasmas in Fig. 1. Vφ(r/a)3     -20020406080100     020406080100     012340.8 1.0 1.2 1.4 1.6t [s]01234(a)(b)(d)D(  He) Mode Conversion D(H) Minority Heating3V (r=0) [km/s]W  [kJ]<n  > [10   m  ]  e-320 T  [keV] P   [MW]perfφ(c)FIG. 1: (color online). (a) Change of plasma central rotation Vφ(r = 0) after the application ofrf power at 0.75 sec in an MC plasma (red solid lines) and an MH plasma (blue dashed lines).Plasma stored energy Wp, Te, 〈ne〉, and Prf are shown in (b), (c), and (d).0.6 0.7 0.8 0.9 1.0t (s)-1001020304050 MHz, 1.75 MW∆V  [km/s]φr/a = 0r/a = 0.3r/a = 0.7FIG. 2: (color online). Vφ traces at different r/a. The traces have been shifted to have 〈Vφ〉 ≃ 0for t < 0.8 sec.profiles before the rf application at t = 0.7 s are both flat. When heated with the rf powerat t = 1.4 s, the rotation profile in the MH plasma is slightly hollow, typical in L-modeplasmas [19, 20]. In contrast, Vφ(r/a) of the MC plasma is broadly peaked within r/a ≤ 0.5,but at a similar level to that of the MH plasma in the outer region (r/a ≥ 0.5). In Fig.3-(b), the Ar16+ ion temperature profiles show that the MC plasma has a higher Ti thanthat of the MH plasma for r/a ≤ 0.5, while it has a somewhat lower Te as seen in Fig. 1-(c).Concomitant to the strong toroidal rotation, a poloidal flow Vθ in the ion diamagnetic4      0204060800.0 0.2 0.4 0.6 0.8 1.0r/a0.00.51.01.52.02.5V  [km/s]φMCt = 0.7st = 1.4sMHt = 1.4st = 0.7sT [keV]i(a)(b)FIG. 3: (color online). (a) Chord integrated Vφ profile (systematic error are about ±10 km/s), and(b) Ti (Ar16+) profile of the MC and MH plasmas in Fig. 1 at two time points.direction is observed in the MC plasma. Since HIREX measures a combination of toroidaland poloidal components, Vθ is derived from the difference of the velocities measured by twochords viewing symmetrically above and below the mid-plane and tangential to a flux surfaceto remove the toroidal component. In Fig. 4-(a), ∆Vθ(r/a) at different rf power levels in theMC and MH plasmas are compared. In the MH plasma, there is no detectable poloidal flow,while in the MC plasma, a significant flow appears in the region of 0.3 ≤ r/a ≤ 0.5, and peaksat ∼ 0.7 km/s per MW rf power. In Fig. 5-(b), the peak poloidal velocity ∆Vθ(r/a = 0.44)is plotted vs. time, and it shows that the flow becomes significant when Prf ≥ 1.5 MWand increases with Prf (Fig. 5-(c)). Interestingly, the density fluctuation spectra, measuredby a phase contrast imaging (PCI) system [10], show a turbulence frequency broadeningfollowing the same trend as Vθ (Fig. 5-(a)), indicating a change in bulk ions flow as well.MC waves are detected by the vertical chords of PCI in the region of −4 ≤ R −Raxis ≤−2 cm. The wavenumber kR of the MC waves is ∼ 4 − 5 cm−1, close to previous resultsin D-3He plasmas at similar field and rf frequency [13]. During the MC process, part ofthe mode converted power heats the electrons directly through electron Landau damping(ELD). Fig. 6-(a) shows the direct electron heating profile, derived from the break-in-slopeof Te traces at t = 1.49 s (fast rf shutoff), Pe ≃32ne∆(dTedt) [12], of the MC plasma fromtwo independent electron cyclotron emission (ECE) Te diagnostics. The integrated powerto electrons is estimated to be ∼ 0.42 MW out of the total 3 MW input rf power.The two-dimensional (2-D) full-wave code TORIC [21] has been used to simulate the MC5     -0.50.00.51.01.52.02.50.0 0.2 0.4 0.6 0.8 1.0r/a-0.50.00.51.01.52.02.5∆V  [km/s]θ∆V  [km/s]θMC plasmaMH plasmaIon diamagnetic drift direction(a)(b)P   [MW]rf0.3 0.91.5 2.3 3.0 FIG. 4: (color online). Poloidal rotation velocity in (a) MC plasma and (b) MH plasma. Vθ beforethe rf application is subtracted. Uncertainties are approximately ±0.5 km/s.      -0.50.00.51.01.52.02.5∆V  (r/a = 0.44) [km/s]0.6 0.8 1.0 1.2 1.4 1.6024θP   [MW]rf0.80.60.40.201.0t (sec)freq. [MHz] PCI fluctuation spectralog scale(a)(b)(c)minmaxFIG. 5: (color online). (a) PCI turbulence power spectra in frequency and time, (b) Poloidalrotation vs time, and (c) Prf vs. time.plasma. In Fig. 6-(b), we plot power deposition profiles obtained from TORIC runs, weightedsumming over the entire antenna toroidal mode (nφ) spectrum of 90◦ phase (+1 ≤ nφ ≤+12). The species mix in the simulation is iteratively determined to be n3He/ne ≃ 8− 10%so that the peak electron deposition matches that in Fig. 6-(a). The location of the MCwave field also matches that from the PCI wave measurement. The total electron depositionfrom the simulation is 0.6 MW out of 3 MW, and 2.2 MW of the power is absorbed by the3He ions, with the rest 0.2 MW to D ions. The 3He deposition profile shown in Fig. 6-(b) isbroadly peaked within r/a ≤ 0.5, but decreases in the outer region.From the TORIC simulation, we find the power deposition to the 3He ions is mostly6      012345MW/m30.0 0.2 0.4 0.6 0.8 1.0r/a024681012MW/m3ECE 1 ECE 2  Curve fit Electron He ion 3(a)(b)Experimental electron power depositionSimulation power depostionFIG. 6: (color online). (a) Direct electron heating power deposition profile from ECE; (b) Powerdeposition to electrons and 3He ions from TORIC simulation.through the MC ICW, rather than through the fast wave as in MH plasmas. In Fig. 7,we show the 2-D power deposition profiles from the TORIC run with nφ = +6. In Fig.7-(a), the ion deposition by the MC ICW is shown to be more than an order of magnitudelarger than that by the FW. Fig. 7-(b) shows a weak electron deposition by the MC ICWbut strong electron heating by the MC IBW on the HFS of the MC layer. The MC ICWdeposition is within several cm on the LFS of the MC layer, and strongest at ∼ 7− 12 cmvertically below and above the mid-plane, where a larger poloidal B field makes the modeconversion to ICW possible [13, 14]. These peak deposition regions map to the flux surfacesof 0.3 < r/a < 0.5, the same location where local poloidal flow is observed (Fig. 4-(a)).Numerically solving the full electromagnetic wave dispersion equation finds that the ICWk|| is up-shifted from the FW level of 10 m−1 to k|| ∼ 40 − 50 m−1 [10, 14], and the ICresonance condition ω ≃ Ωc,3He + k||vt,3He can be satisfied in the vicinity of the MC surface,about 4 cm away on the HFS of the IC resonance. This is consistent with the TORICsimulation showing that significant ion damping of the ICW occurs near the MC surface.The involvement of slow wave and resonant wave-ion interaction is consistent with previ-ous theoretical work on rf flow drive [14, 22–24]. Analytical estimates for Vφ and Vθ generatedby direct-launch IBW at the diffusive limit were presented in Eqs. (45) and (43) in Ref. [23],Vφ[km/s] ∼48Prf [MW ]k||[m−1]R[m]f [MHz]D[m2/s]n20µ(1)Vθ[km/s] ∼0.4Prf [MW ]q2R[m]r[m]B[T ]D[m2/s]n20. (2)7Z [cm]15100-10-15-6  -4   -2   0    2    4    R - R      [cm]axis    R - R      [cm]axis484.8151.50.0.48-6  -4   -2    0    2    45-5MW/mper MW input3FWICWIBWICW(a) (b)MC         IC                           MC        ICICWElectronHe3ICWFIG. 7: (color online). 2-D power deposition from TORIC simulation: (a) Power to 3He, (b) Powerto electrons. The MC surface and 3He IC resonance are also labeled.Using the parameters of the MC plasma in Fig. 1, B ≃ 5.1 T, n20 ≃ 1.3, R ≃ 0.7 m, r ≃0.1 m, k|| ≃ 40 m−1, f = 50 MHz, local q ≃ 1.6 and effective µ =∑nimi/∑nimp ≃ 2.1 [25],and L-mode momentum diffusion coefficient D ≃ 0.2 m2/s [19, 20], we have Vφ ∼ 102 km/sand Vθ ∼ 2 km/s per MW. Assuming that the MC ICW (about half of the total input power)interacts with ions similarly to direct launch IBW, these estimates are comparable to ourexperimental observation.On ITER and other future burning plasma devices, ICRF mode conversion can be uti-lized in H-3He plasmas during the non-radioactive phase and in D-T plasmas during theradioactive phase. By carefully selecting the rf frequency, B field, antenna phase, speciesmix and other plasma parameters, it may be possible to drive substantial toroidal plasmarotation and also generate significant poloidal flow.The authors thank the Alcator C-Mod operation and ICRF groups, and E. Edlund forPCI system setup and operation. This research utilized the MIT Plasma Science and FusionCenter Theory Group parallel computational cluster. This work was supported at MIT byU.S. DoE Cooperative Agreement No. DE-FC02-99ER54512.∗ Email: ylin@psfc.mit.edu[1] G. G. Craddock and P. H. Diamond, Phys. Rev. Lett. 67, 1535 (1991).8[2] P. W. Terry, Rev. Mod. Phys. 72, 109 (2000).[3] E. J. Strait, T. S. Taylor, A. D. Turnbull, et al., Phys. Rev. Lett. 74, 2483 (1995).[4] J. E. Rice, A. Ince-Cushman, J. S. deGrassie, et al., Nucl. Fusion 47, 1618 (2007).[5] S. J. Wukitch, C. Litwin, M. Harper, R. Parker, and N. Hershkowitz, Phys. Rev. Lett. 77,294 (1996).[6] B. P. LeBlanc, R. E. Bell, S. Bernabei, et al., Phys. Rev. Lett. 82, 331 (1999).[7] L.-G. Eriksson, T. Johnson, T. Hellsten, et al., Phys. Rev. Lett. 92, 235001 (2004).[8] C. K. Phillips, M. G. Bell, R. E. Bell, et al., Nucl. Fusion 40, 461 (2000).[9] P. T. Bonoli, R. Parker, S. J. Wukitch, Y. Lin, M. Porkolab, et al., Fusion Sci. Technol. 51,401 (2007).[10] E. Nelson-Melby, M. Porkolab, P. T. Bonoli, Y. Lin, A. Mazurenko, and S. J. Wukitch, Phys.Rev. Lett. 90, 155004 (2003).[11] Y. Lin, S. J. Wukitch, P. T. Bonoli, et al., Phys. Plasmas 11, 2466 (2004).[12] Y. Lin, S. J. Wukitch, P. T. Bonoli, et al., Plasma Phys. Control. Fusion 45, 1013 (2003).[13] Y. Lin, S. J. Wukitch, A. Parisot, et al., Plasma Phys. Control. Fusion 47, 1207 (2005).[14] E. F. Jaeger, L. A. Berry, J. R. Myra, et al., Phys. Rev. Lett. 90, 195001 (2003).[15] A. Ince-Cushman, J. E. Rice, M. Bitter, et al., Spatially resolved high resolution x-ray spec-troscopy for magnetically confined fusion plasmas, submitted to Rev. Sci. Instrum.[16] J. E. Rice, P. T. Bonoli, J. A. Goetz, et al., Nucl. Fusion 39, 1175 (1999).[17] J. E. Rice, R. L. Boivin, P. T. Bonoli, et al., Nucl. Fusion 41, 277 (2001).[18] J. E. Rice, A. E. Hubbard, J. W. Hughes, et al., Nucl. Fusion 45, 251 (2005).[19] W. D. Lee, J. E. Rice, E. S. Marmar, et al., Phys. Rev. Lett. 91, 205003 (2003).[20] J. E. Rice, W. D. Lee, E. S. Marmar, et al., Nucl. Fusion 44, 379 (2004).[21] J. C. Wright, P. T. Bonoli, M. Brambilla, et al., Phys. Plasmas 11, 2473 (2004).[22] L. A. Berry, E. F. Jaeger, and D. B. Batchelor, Phys. Rev. Lett. 82, 1871 (1999).[23] J. R. Myra and D. A. D’Ippolito, Phys. Plasmas 9, 3867 (2002).[24] J. R. Myra, L. A. Berry, D. A. D’Ippolito, and E. F. Jaeger, Phys. Plasmas 11, 1786 (2004).[25] J. R. Myra, private communication. The definition of µ in Ref. [22] had an error and it shouldbe mi/mp.9

Observation of Ion Cyclotron Range of Frequencies Mode Conversion Flow Drive in Tokamak Plasmas

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Observation of Ion Cyclotron Range of Frequencies Mode Conversion Flow Drive in Tokamak Plasmas

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