Alcator C-Mod. Double Transport Barrier Plasmas. in Alcator C-Mod. J.E. Rice for the C-Mod Group. MIT PSFC, Cambridge, MA 02139

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1 Alcator C-Mod Double Transport Barrier Plasmas in Alcator C-Mod J.E. Rice for the C-Mod Group MIT PSFC, Cambridge, MA 139 IAEA Lyon, Oct. 17,

2 Outline Double Barrier Plasma Profiles and Modeling Conditions for ITB Formation ITB Control Interpretation Conclusions Alcator C-Mod: R =.67 m, r.5 cm, κ 1.8 ICRF: ICRF: 3 MW, 8 MHz, D(H), 5.3 T on-axis absorption MW, 7 MHz, D(H), 4.6 T on-axis absorption Reactor Relevant: T e T i, No Direct Momentum Input

3 ITB Formed with Off-axis ICRF Heating Held Steady with On-axis ICRF kj kev 1 m B T =4.5T W p n e () T i () mw/cm /sr MW 1 4 m/s ICRF Power D a V Tor () Off-axis, 8 MHz On-axis, 7 MHz t (s)

4 Frequency ICRF Arrests Density Rise r/a s s Electron Density (1 /m 3 ) s.575 s R (m)

5 Off-axis ICRF Increases Central Ion Temperature 14 r/a ITB 1 T i (ev) 8 6 Ohmic L-mode r (cm)

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7 ITB Forms with ICRF Resonance Outside of r/a=.5 4 Resonance Location (r/a) V Tor (1 4 m/s) MHz ICRF n e ()/n e (.7) B T (T)

8 V Tor (1 4 m/s) Velocity Profiles Hollow during ITB r/a s 1. s.7 s 1.1 s.55 s n e (1 /m 3 ) s.85 s 1.1 s.7 s 1.55 s R (m)

9 Other C-Mod ITB Observations Target Plasma Must Be EDA H-mode ITB Foot at r/a.5, near B P maximum Also Seen in Some Purely Ohmic Discharges Impurity Peaking for r/a.5 Core E r Changes from +1 kv/m to kv/m q 1, Constant I P, w/ Sawteeth With Additional On-Axis ICRF Heating Arrests Impurity Peaking Steady State ITBs, over 1 τ E Density Control References J.E.Rice et al., Nucl.Fusion41 (1) 77 C.L.Fiore et al., Phys. Plasmas 8 (1) 3 J.E.Rice et al., Nucl.Fusion4 () 51 S.J.Wukitch et al., Phys. Plasmas 9 (1) 149

10 Conclusions C-Mod ITBs ITB Formed with Off-Axis ICRF Heating No Momentum or Particle Source T i =T e RotationSuppressionandITBFormationwithResonanceLocation r/a.5, Both HFSand LFS With No On-Axis Heating, Core Temperature Increases with ITB Formation TRANSP Modeling and Heat Pulse Propagation Analysis Confirm Substantial Drop in χ eff Density Peaking via Ware Pinch and Large Drop in Core D Additional On-Axis ICRF Heating Arrests Density Peaking Mechanism/Trigger for ITB Formation Unknown Role of Rotation

11 ASDEX Upgrade Max-Planck-Institut für Plasmaphysik Dependence of Particle Transport on Heating Profiles in ASDEX Upgrade J. Stober 1), R. Dux 1), O. Gruber 1), L. Horton 1), P. Lang 1), R. Lorenzini ), C. Maggi 1), F. Meo 1), R. Neu 1), J.-M. Noterdaeme 1), A. Peeters 1), G. Pereverzev 1), F. Ryter 1), A.C.C. Sips 1), A. Stäbler 1), H. Zohm 1), and the ASDEX Upgrade Team 1) MPI für Plasmaphysik, EURATOM Association, D Garching, ) Consorzio RFX, Associazione EURATOM-ENEA sulla Fusione, I-3517 Padova, Italy. slow density peaking with NBI does not occur with central ICRH but well with off-axis ICRH can be well modelled assuming D. χ_turb and v_in v_ware Effect of heat flux profile also on impurities Significant impact on performance and machine operation Jörg Stober, 7.Oct., /u/jks/draw/iaea_rap/title

12 Slow density peaking with NBI 1 m n e T (kev) e n e,ped (a.u.) Gas puff loss of saw teeth H-factor (ITERH-98P) core pedestal Soft-X (a. u.) 1 / s #13476 n e { ρ=. q=1. top of pedestal. ρ min.5 NBI Power (MW) time (s) process stopped by loss of sawteeth and NTM at βn n e (1 m ) n e Thomson Scattering Interferometry and Li-Beam { Saw tooth inversion 3.5 s 4.5 s 5. s 3.5 s 4.5 s ASDEX Upgrade # normalized poloidal flux radius density profile peaks for 1 τe fast equilibration of pedestal density T e, T i profiles stay constant confinement improves { (similar observations for DIII-D and JET) Jörg Stober, 7.Oct., /u/jks/draw/peaknbi

13 a peaked density profile does not develop with central ICRH - but well with off-axis ICRH comparison of NBI (5 MW), on-axis ICRH (.1 T, 3 MHz, 5MW), one-to-one mixture of both (5 MW) and off-axis ICRH (. T, 4 MHz, 4. MW) (1 m ) 8 q 1 n e 3 kev T e T i v tor q 1 q 1 q 1 6 ρ 1 ρ 1 ρ 1 ρ 1 tor tor tor tor 3 kev 6 km / s (suppressed zero) NBI and off-axis ICRH lead to similar density profiles but different toroidal rotation with on-axis ICRH, density is flat. Intermediate toroidal rotation. Toroidal rotation is not correlated to density peaking. Jörg Stober, 7.Oct., /u/jks/draw/peakall

14 19 3 n (1 m ) e Good description assuming D. χ and neoclassical Ware pinch χ (m / s) eff s 4. s ρ tor χ eff 4.5 s dashed: data solid: calculated.5 MW ICRH +.5 MW NBI 5 MW NBI (4.5 sec) ρ tor Jörg Stober, 7.Oct., /u/jks/draw/model 19 3 n (1 m ) e turb NBI NBI + central ICRH off-axis ICRH.5 MW ICRH +.5 MW NBI dashed: data solid: calculated...4 ρ tor Main effect: χ follows q heat -profile due to stiff T-profiles electron density ( m ) n_e from TS : - 4. s s - 3. s -.5 s ρ tor v and D can be separated due to time evolution off-axis heating: central D close to neoclassical also for other scenarios (pellets, L-mode,...)

15 Trasport Coefficients of Si in ICRH Heated H-Mode Discharges Method: fit v and D to Si-density (from Soft-X) after Silicon Laser-blow-off Discharge with central ICRH heating has higher central diffusion coefficient for Si Heating at Center/Edge n e (1 19 m -3 ) ICRH #1561 #151 the strong inward pinch in the core disappears for the case of a flat electron density profile D (m /s) 1..1 D neo v/d consistent with neo-classical calculation v/d (m -1 ) v neo D ρ pol Ralph Dux, /u/jks/iaea_rap/impurities

16 Impact on plasma performance and operation Steady state operation with NBI only is hindered by: high τ_p (high δ), low q_95, off-axis heating operation at δ.3 and q_95 = 3 requires central ICRH with counter-nbi (more off-axis) this is necessary also for higher q_95 Peaked density profiles are less stable against NTMs: Application of central ICRH can increase achievable β Impurity control by central heating concentration of heavy impurites may be significantly reduced Jörg Stober, 7.Oct., /u/jks/draw/operation

17 ASDEX-U and C-Mod Observe Core Confinement Improvement During Off-axis Heating Regime Nomenclature Parameter Space C-Mod: ITB ASDEX-U: peaked density operation ASDEX-U C-Mod R 1.69 m.67 m B T. T 4.5 T n e 1.3x1 /m 3 7.x1 /m 3 ICRF off-axis off-axis also NBI (ECRH) Ohmic Observed Responses during Improvement ASDEX-U C-Mod n e (r) peaks peaks strongly T i (r), T e (r) same peaks slightly D e reduced to.1 χ eff reduced to.1 χ eff v e v Ware 1. v Ware χ reduced in core reduced in core D impurity reduced to NC reduced to NC w/ Off-axis ICRF reduced core rotation core rotation countercurrent Effect of Adding On-axis ICRF to Improved Regime ASDEX-U C-Mod n e (r) stops peaking stops peaking Impurities stops accumulation stops accumulation density control Conclusion: Off-axis ICRF results are very similar on both devices. ASDEX-U interpretation of general phenomenology is that stiff temperature profiles lead to reduced central χ for off-axis heating, and with D tied to χ, the density profile peaks.