Light Impurity Transport Studies in Alcator C-Mod*
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1 Light Impurity Transport Studies in Alcator C-Mod* I. O. Bespamyatnov, 1 W. L. Rowan, 1 C. L. Fiore, 2 K. W. Gentle, 1 R. S. Granet, 2 and P. E. Phillips 1 1 Fusion Research Center, The University of Texas at Austin 2 MIT-PSFC Poster Session PP6: Poster Session VI Wednesday, November 19, 2008 Marsalis A/B, 2:00pm - 5:00pm 50th Annual Meeting of the Division of Plasma Physics November 17 21, 2008; Dallas, Texas *Supported by USDoE Awards DE-FG03-96ER54373 and DE-FC02-99-ER54512
2 Summary Local, CXRS measurements of light impurity density and temperature profiles were measured in L-mode, H-mode, and ITB discharges in Alcator C-Mod along with impurity flow measurements and radial electric field. The basic results are Light impurity densities peak in ITB discharges but the transport is not neoclassical. Hollow impurity density profiles are observed in H-mode discharges The computed E R profiles demonstrated the large difference between the H-mode and ITB discharges. Linear gyrokinetic stability analysis (GS2) demonstrated that shearing rate ω E B prevails over the linear ITG growth rates γ max in the region where ITB forms These suggest new experiments Causes of impurity peaking and differences between light and heavy impurities Effect of turbulence on impurity transport These may lead to impurity control Some of these experiments would benefit from modulation of the impurity source and the possibilities on C-Mod are described. Impurity transport is a critical issue in burning plasmas due to fuel dilution and radiative cooling. The new experiments will provide information on the behavior of light impurities, on light impurity peaking, and may suggest means for impurity control.
3 Experiment n e (x10 14 cm -3 ) L-mode, H-mode, and ITB are all shown in a single discharge. EDA H-mode is triggered by ICRF at 0.7 s. H-mode evolves to ITB n e profiles are shown at the indicated times in the t-series plots B 5+ profiles, T e and T i profiles are on the next page.
4 Boron n and T profiles B 5+ density B 5+ temperature Fully stripped boron ion, B 5+ is the only boron ion present for R < 0.85 m. Impurity transport analysis using a fully stripped ion is much simpler that for other species. L-Mode. Relatively flat B 5+ profile. Z eff = and T i < T e H-Mode. Hollow B 5+ profile. Z eff = T i T e ITB. Peaked B 5+ profile. T i T e
5 Radial electric field E R Included for use in impurity transport analysis Inferred from momentum balance All terms are measured with CXRS Flow velocity terms usually dominate E R profile has similar shape for H-modes in LSN and USN ITB E R has an inner well and strong gradients Er behavior is similar to ITBs in JET and JT-60 Reminiscent of the E R well at the edge pedestal. E R Z ( VZϕ Bθ VZθ Bϕ ) pz = n TZ pz =, Zn Ζ
6 Poloidal and toroidal rotation Poloidal velocity Toroidal velocity Difference between measured and neoclassically calculated impurity poloidal rotation. (observed in other tokamaks: TFTR, DIII-D, JET) Possible effect of heating beams. J.Rice, Nuclear Fusion, Vol. 39, No. 9, (1999) C-Mod doesn t employ heating beams and observes the same type of difference. Turbulence generally considered as a main source of the anomaly. K.H. Burrell, Plasma Phys. Control. Fusion 36,(1994) Strong co-current toroidal rotation is associated with H-mode phase.
7 Peaked Impurity Profile (ITB) Simulation of the peaked profiles yields an inward convection Neoclassical yields a much stronger inward convection Turbulence persists, is consistent with density peaking and may reduce impurity peaking.
8 Impurity Peaking Experiments Impurity transport is a critical issue in burning plasmas due to possible fuel dilution and radiative cooling Low collisionality experiments reveal peaking of heavy impurities and not light. Weisen, et al., Plasma Phys. Contr. Fusion 48, A457 (2006) Comparison to the particle profile may answer the question: Does the main ion gradient act through neoclassical pinch to drive impurity peaking? Comparison to heavy impurity profile. Peaking differs in some ITB results and in low collisionality experiments. Mass and charge dependence needs study. Examples Dux, et al., Nucl. Fusion 44, 260 (2004) in itb discharges; Weisen, et al., Plasma Phys. Contr. Fusion 48, A457 (2006) in low collisionality discharges; Takenaga, et al., Nucl. Fusion 43, 1235 (2003). Ware pinch is one source for inward convection. LHCD may offer the opportunity to eliminate it at least briefly.
9 Comparison with other machines JET DIII-D Transition from L-mode to VH-mode. R. Guirlet, Plasma Phys. Control. Fusion 48 (2006) JT-60 RS ITB High-β p H-mode H. Chen., Nuclear Fusion, Vol. 41, No. 1 (2001) H. Kishimoto, Nucl. Fusion 45 (2005) Similar peaked and hollow impurity profiles are measured in other tokamaks: JET, DIII-D, JT-60
10 Hollow Impurity Profile Analysis of the EDA H-mode profile with this flux ansat Γ n = D + v r In H-mode in C-Mod, v Z /D Z > 0 Convection is outward leading to a hollow profile Neoclassical theory predicts an outward convection due to the temperature gradient D v Z Z = 1.25 = Z D q R P 1 ni m Z ni r TZ c BT Z Hollow profile good for energy confinement: minimies radiative loss e T Z n T r Z
11 Hollow Impurity Profile Turbulent Impurity Transport? Examples of impurity convection from turbulence theory Γ = D n r + v n pinch name dependence direction charge dependence curvature 1 q q q r r > 0 inward f Z thermodiffusion 1 T TEM inward 1 T r ITG outward Z parallel TEM outward Z compression ITG inward A mass dependence refs ( ) f ( A) 1,2,3,4,5 f ( A) 1,3,2,6 Z A 1,2,3 1 Guirlet, R., et al., 2006 PlasmaPhys. Control. Fusion 48 B63 2 Dubuit, N., et al., 2007 Phys. Plasmas Angioni C and Peeters A G 2006 Phys. Rev. Lett Isichenko M B et al 1995 Phys. Rev. Lett Baker D R and Rosenbluth M N 1998 Phys. Plasmas Coppi B and Spright C 1978 Phys. Rev. Lett
12 Hollow Impurity Profile Turbulent Impurity Transport? Quasilinear impurity transport flux Γ = n Quasilinear theory can be used to develop a description of impurity transport which fits the ansat n Γ = D r + n v Following Garbet, Phys. Plasmas 12, (2005), the impurity flux can be written as Γ = n, eq D turb v E, 1 n 2 1 T + Ccurv + CT n r R T r C curv = λ s λ s = cos( θ)+ sθ sin θ θ <<1, λ s ~ 1+ s 1 θ 2 2 ( ) θ,φ, s = r q q r C T = 1 Z ω ω D ς v D curv = 2λ s R v D T = 1 Z ω ω D ζ 1 T T r
13 Turbulent Impurity Transport (first results) ohmic-mode H-mode 1 n n r v D T v D curv 1 [ m ] 1 n n r v D T v D curv Shot # Ohmic-mode t = [ ] H-mode t = [ ] The full details of the curvature pinch and the thermopinch are not included in the calculation. In the case of the thermo pinch the details of the phase velocity are omitted, only a sign has been included. In the case of the curvature pinch, the spatial averaging over the angle is only estimated. Approximate evaluation of theory produces results similar to experiment Both curvature and thermodiffusion terms are negative and drive inward convection
14 Shear stabiliation Stabiliation criterion ω E B = r q RB θ E Ψ RB d d R θ ω E B > γ max γ max - linear ITG growth rate Suppression of the turbulent eddies in the inhomogeneous plasma flow The maximum linear ITG growth rates were calculated by GS2 code for several timephases of particular H-mode and ITB plasma discharges The distinctive feature of the ITB phase is the substantial increase in the E B shearing rate, due to the strong local E R gradients
15 Comparison with other machines TFTR RS to ERS comparison DIII-D VH-mode DIII-D ITB formation Suppression criterion: ω ExB > γ MAX DIII-D VH (improved confinement mode) is characteried by penetration of the H- mode edge transport barrier deeper into the plasma DIII-D ITB formation is preceded by the E B shearing rate increase TFTR transition RS to ERS mode is preceded by changes in the ω ExB, which proves that not only negative magnetic shear exhibit improved confinement
16 More Light Impurity Experiments Use turbulence to control the impurity profile shape? Example in C-Mod: Suppression due to ICRF On-axis heaing produced an increase in TEM turbulence diffusivity due to a unfavorable temperature scaling of turbulent transport D. R. Ernst, et al., Phys. Plasmas 11, 2367 (2004). Heavy impurity peaking suppressed as well Rice J.E. et al Nucl. Fusion (2002) What happens to the light impurities? Is suppression of impurity peaking due to a direct effect of turbulence or to the particle gradient acting through the neoclassical pinch term Additional options for experiments based on recent turbulence calculations which are displayed here.
17 E r in the ITB A local minimum in Er is observed in ITB discharges. JT-60 JET C-Mod There are hints of a similar feature in JET discharges as reported in K. Crombe et al. Phys. Rev. Lett. 95, (2005) and H. Shirai et al. Nuclear Fusion, Vol. 39 (1999) Suggests the same sort of E B dynamics may be at work in the ITB as in the edge barrier.
18 E r in the ITB Future experiments will increase the number of observations with improved sensitivity and spatial resolution to confirm this observation. The experiments will be expanded to Ohmic ITBs; that is, ITBs triggered by toroidal field ramps The poloidal and toroidal rotation measurements will benefit from improvements in spatial resolution and sensitivity.
19 Time Dependent Impurity Transport Experiments Impurity transport can be described by Γ = D n r + v n for both neoclassical transport and turbulent transport in the quasilinear limit The ratio v/d can be inferred from time independent measurements of the impurity profile. A time dependent measurement is required to separately infer both v and D Both D and v can be anomalous. The familiar tactic of taking v as neoclassical and the concentrating anomalous behavior in the diffusion coefficient is no longer the best approach to analysis since we are trying to understand peaking. There are two time-dependent methods CW: the impurity source is continuously varied Pulsed: the impurity source approximates a delta function.
20 Time Dependent Impurity Transport Experiments In time-dependent measurements, the source in the impurity transport equation is modified in a controlled way and is time dependent n j + Γ j = S j t CW Methods S is a repetitive waveform; square wave or sinusoidal, for example and accomplished by gas puffing Helium example: H. Takenaga, et al., Plasma Phys. Control. Fusion 40, 183 (1998). Impurity Pulse S approximates a delta function, accomplished by laser ablation. Some examples: H. Chen, et al, Nucl. Fusion 41, 31 (2001); J. E. Rice, et al., Fusion Sci. Technol. 51, 357 (2007).
21 Summary Local, CXRS measurements of light impurity density and temperature profiles were measured in L-mode, H-mode, and ITB discharges in Alcator C-Mod along with impurity flow measurements and radial electric field. The basic results are Light impurity densities peak in ITB discharges but the transport is not neoclassical. Hollow impurity density profiles are observed in H-mode discharges The computed E R profiles demonstrated the large difference between the H-mode and ITB discharges. Linear gyrokinetic stability analysis (GS2) demonstrated that shearing rate ω E B prevails over the linear ITG growth rates γ max in the region where ITB forms These suggest new experiments Causes of impurity peaking and differences between light and heavy impurities Effect of turbulence on impurity transport These may lead to impurity control Some of these experiments would benefit from modulation of the impurity source and the possibilities on C-Mod are described. Impurity transport is a critical issue in burning plasmas due to fuel dilution and radiative cooling. The new experiments will provide information on the behavior of light impurities, on light impurity peaking, and may suggest means for impurity control.
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