ICRF Induced Argon Pumpout in H-D Plasmas in Alcator C-Mod

Transcription

1 ICRF Induced Argon Pumpout in H-D Plasmas in Alcator C-Mod C. Gao, J.E. Rice, M.L. Reinke, Y. Lin, S.J. Wukitch, L. Delgado-Aparicio, E.S. Marmar, and Alcator C-Mod Team MIT-PSFC, University of York, Princeton Plasma Physics Laboratory April 29, 215 C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

2 Overview 1 Key Results 2 Motivation 3 Experimental Observations 4 Possible Mechanisms 5 STRAHL Simulation 6 Summary C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

3 Key Results 1 Key Results Argon level is strongly reduced with ICRF in H D plasmas. Argon pumpout effect is maximal at n H /n D 42±5%. Argon pumpout effect has a MW ICRF power threshold. Molybdenum injection is coupled with argon pumpout. Impurity-wave interaction and edge effects are possible mechanisms. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

4 2 Motivation Motivation Motivation: Radiation & Impurity Control Impurity types: Type Example Intrinsic (non-recycling) B, Mo, C, W Non-intrinsic (non-recycling) Ca Non-intrinsic (recycling) Ne, Ar Impurities dilute plasmas. Energy loss by impurity radiation could affect the plasma performance, and even cause disruptions. Fully understadning of impurity transport is essential for fusion. Wave-impurity and impurity-impurity interactions may play roles in the argon pumpout effect, which is coupled with molybdenum injections. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

5 2 Motivation Diagnostics & ICRF Conditioning Diagnostics & ICRF Conditioning X-ray crystal spectroscopy (HiReX SR) Spatial resolution. Argon as diagnnsotic gas. H-like and He-like argon line emisivities. VUV spectroscopy (LoWEUS) Radially line-integrated emisivity through the core. A wider range of argon charge states. Visible Balmer-α emission spectrometer Edge H/D ratio. ICRF runs in the beginning of each campaign to condition antennas and reduce H/D ratio. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

6 3 Experimental Observations Argon pumpout during ICRF Argon Intensity Strongly Decreases during ICRF 1 MA a.u. a.u Ip & Argon Puff Ar 16+ w line intensity. No RF Ar 16+ w line intensity. No RF r/a ~ w/o RF r/a ~ w/o RF (a) (b) (c) a.u. (a) Argon puff at.3 s for 75 ms (blue line). (b, c) Without ICRF, core and edge argon intensity is steady (due to recycling). a.u. a.u Ar 16+ w line intensity. With RF Ar 16+ w line intensity. With RF time [s] r/a ~ w/ RF r/a ~ w/ RF (d) (e) MW MW (d, e) With ICRF (red line, MW ICRF), core and edge argon intensities strongly decrease (with decay time 5 ms). 1 Pre-physics operation ICRF conditioning experiments in 214. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

7 3 Experimental Observations Mo Injection and Non-Resonant Lines Mo Injection and Non-Resonant Lines Molybdenum intensity increases. Emission lines from non-resonant argon charge states also decrease with strength similar to resonant argon lines W - INT, LOC: Ar 16+ 1s2p->1s M - Int, LOC: 4-3 Mo 32+ 2p->4d RF Power (MW) & RF power [MW] [MW] 8 [AU] Ar XV Ar 14+ :1s 2 2s2p->1s 2 2s 2 ICRF Line Brightness 5 5 n H /n D H/D ratio Time [sec] time [s] C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

8 3 Experimental Observations preferred H/D ratio There is a Preferred n H /n D for the Pumpout Effect I(min)/I(max) Pumpout only in plasmas with relatively high H/D ratio. R [m] n(h)/n(d) Ar16+ 8MHz B =5.4T 3Ω I H-D MC layer 2Ω I n(h)/n(d) An optimum n H nd 42%±5%. n H nd = 46% H-D mode conversion layer overlaps with Ar nd harmonic resonance layer. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

9 3 Experimental Observations preferred H/D ratio There is a Preferred n H /n D for the Pumpout Effect 5.4 T, 8 MHz, H/D = 6 MC layer 2Ω Ar 16+ 2Ω Ar 17+ Pumpout only in plasmas with relatively high H/D ratio. An optimum n H nd 42±5% n H nd = 46% H-D mode conversion layer overlaps with Ar nd harmonic resonance layer. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

10 3 Experimental Observations Power Threshold A Power Threshold for Pumpout Effect Current I p (a) MA.9 MW ICRF power ICRF power (c) No pumpout when P ICRF < MW a.u. a.u Ar 16+ w line intensity (d) r/a ~ Ar 16+ w line intensity (e) r/a ~ time [s] Pumpout when P ICRF > MW C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

11 3 Experimental Observations Argon Intensity Modulation Argon Intensity Recovers when ICRF Turns Off MA 1.2 I p argon puffing , 1, 5, 16+ Ar w line intensity [a.u.] and RF power[mw] r/a ~ time [s] r/a ~ time [s] C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

12 3 Experimental Observations Plasma Profiles Profiles don t Change Significantly T e increases, then drops 1%. T I increases 1%. n e unchanged. Changes in neoclassical and turbulent particle transport should be small. MW RF Power T e T i n e 3 s 3 s.75 s s 3 s.73 s s 3 s.75 s a.u. 1.5 Ar 16+ W-line kev m t [s] r/a r/a r/a C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

13 Possible Mechanisms 4 Possible Mechanisms Possible Mechanisms 1. Impurity-RF wave interaction. A preferred n H /n D 42±5%. A power threshold. Argon intensity is modulated by ICRF. 2. Edge effect: changes in edge source/recycling. Plasma edge potential may change the impurity influx. Wall recycling could be modified by ICRF. 3. Transport effect: changes in T e, T i or n e. n e unchanged. T e increases then drops 1%. T i increases 1%. Gradient changes are small. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

14 4 Possible Mechanisms Impurity - Wave interaction 2 nd Harmonic Resonance with Finite k x π ρ L E + E + 1. Ar nd harmonic resonance layer overlaps with the H-D mode conversion layer when n H n D 46%. x x 2. Enhanced E r at H-D mode conversion layer. t = t = p/ω Figure from J. P. Freidberg. Plasma physics and fusion energy. 3. When Ω RF = 2Ω I,k x π ρ L, argon can be perpendicularly accelerated (v E) and deconfined? C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

15 4 Possible Mechanisms Impurity - Wave interaction Optimum H/D ratio: n H nd = 46% for Ar 16+ ) MC layer (n 2 = S for H D plasma: R MC = eb The impurity resonant layer: R resonant = n Z A R MC = R resonant : γ ( A Z+) = 1 (nz A) 2 2(n Z A) γ m pω RF eb m pω RF R 1+2γ R γ n H nd In typical C-Mod plasmas, Ar 16+ and Ar 17+ are dominant charge states. γ ( Ar 16+) = 46% γ ( Ar 17+) = 29% Charge State Density [a.u.] r/a Density profiles of different argon charge states with given density and temperature profiles calculated by STRAHL. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

16 4 Possible Mechanisms Impurity - Wave interaction B-dependence of Resonance Position Required n H /n D independ of B. Resonance layer in HFS at 5.4 T, on axis at 6.8 T..9 Ar16+ 8MHz B =5.4T 5.4 T, 8 MHz, H/D = 6 MC layer R [m].7 3Ω I H-D MC layer 2Ω Ar 16+ 2Ω Ar 17+ 2Ω I H/D ratio.9 H/D ratio=5 8MHz HD MC layer Ar16+ 1st 2nd 3rd - R [m] B T - C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

17 5 STRAHL Simulation STRAHL Introduction STRAHL: 1-D Time-Evolving Impurity Transport Code Charge State Density [a.u.] r/a Density profiles of different argon charge states with given density and temperature profiles calculated by STRAHL. He-like argon is the dominant charge state in C-Mod L-mode plasmas. Experimental n e (r,t),t e (r,t). Specify diffusion D(r,t), convection V (r,t). Ionization, recombination and charge exchange processes calculated with atomic data. Emissivity and (post-processed) line brightness profiles are compared with HiReX SR measurement. Calcium transport [N.T. Howard, 212]. Argon/moly transport with synthetic measurement of HiReX SR [M.L. Reinke, 213]. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

18 5 STRAHL Simulation Source Reduction Simulation with Source Reduction Simulation with a reduction in source qualitatively reproduces the evolution of argon brightness. STRAHL(normalized) D=1 m/s 2, Source t.3 Source t< Experiment (HiReX SR) Source time [s] time [s] time [s] r/a r/a C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

19 5 STRAHL Simulation Source Reduction Simulation with Source Reduction The evolutions of normalized brightness show agreement between STRAHL simulation and experiment. STRAHL(normalized) Experiment (HiReX SR) normalizedz line brightness r/a normalized Z line brightness r/a time [s] time [s] C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21

20 Argon level is strongly reduced with ICRF in H D plasmas. 1. A preferred n H /n D 42±5%. 2. A power threshold. 3. Argon intensity is modulated by ICRF. 4. Coupled with molybdenum injection. Impurity-wave interaction & source/edge effect are probable reasons. 1. Impurity-wave interaction: Preferred H/D. ICRF power threshold. 2. Source/edge effect: Molybdenum injection. Non-resonance argon pumpout. STRAHL simulation. 3. Change of other plasma parameters: Not relevant Future work (May 215) 1. Experiment with PCI is planned to identify the mode conversion strength and location. 2. Edge diagnostics (Chromex, scanning probe, etc.) will be employed to monitor edge effects. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21 Summary and Future Work 6 Summary Summary

21 6 Summary Higher Field Move Resonance Layer to Magnetic Axis Resonance layer close to magnetic aix at B = 6.5 T Within PCI viewing range for mode conversion measurement. C Gao etc., MIT-PSFC US TTF Workshop, Salem, MA April 29, / 21