Indication of bulk-ion heating by Energetic particle driven Geodesic Acoustic Mode on LHD NIFS M.Osakabe T.Ido １ , K.Ogawa １ , A.Shimizu １ , M.Yokoyama １ , R.Seki １ , C.Suzuki １ , M. Isobe １ , K. Toi １ , D. A. Spong 2 , K.Nagaoka １ , Y.Takeiri １ , H.Igami １ , T.Seki １ , K.Nagasaki 3 and LHD experiment group 1 National Institute for Fusion Science, Toki, 509-5292, Japan. 2 Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. 3 Institute for Advanced Energy, Kyoto University, Uji, 611-0011, Japan 1
CONTENTS 1. Background & Motivation 2. Experimental Observations a. Brief description of the phenomena b. Bulk-ion behaviors c. Fast-ion behaviors d. Mode structures/ Frequencies 3. Possible candidate to explain the phenomena a. Orbit topology change b. Enhancement of classical ion heating by the deformation of the energetic particle’s spectra with mode activities. c. Confinement improvement with the mode. d. Anomalous ion-heating with the mode. 4. SUMMARY 2
Background & Motivation • Zonal Flow(ZF) and Geodesic Acoustic Mode(GAM) get much interests recently, since it could be a knob to regulate turbulence in plasmas and to reduce anomalous transport. Moreover, additional heating by GAM is theoretically pointed out (GAM channeling).* • Energetic-particle(EP) induced GAMs are observed in several toroidal devices, such as JET, DIIID and LHD**. Effect of GAMs on the energetic particle behaviors needs to be investigated. • Recently, an influence of EP induced GAM on bulk-ions was observed on LHD. *M.Sasaki, et.al., PPCF 53 (2011)085017 **H.L.Berk, et.al., Nucl. Fusion 46 , S888, C.J.Boswell, et.al., Phys. Lett. A , 358 , 154, R.Nazikian, PRL 101, 185002, G.Y.Fu, PRL 101, 185002, K.Toi: 22 nd IAEA-FEC, EX P8-4, T.Ido : 23 rd IAEA-FEC 3
On LHD, increase of low energy neutrals are observed with up-sweeping n=0 modes by tangential NPA at low density plasmas. 14 a) se /2[sec.] 12 e ( ) T n e ( ) b) 10 • The mode was only excited during -3 ], se /2 8 17 m counter-NB injection phase. c) 6 e [10 • Superposition of ECH seems to be e [keV], n 4 effective to excite the mode. 2 d) • The typical initial frequencies of the T 0 0 0.2 0.4 0.6 0.8 1 mode are 50 -100kHz. The flux r/a increase was associated with e) relatively large amplitude modes. • No significant increase of H a -signals were observed. f) => Measured increase in neutral flux are due to the low energy ion behaviors. • Typical slowing-down time is g) estimated to be ~10[s] at the core 4
Bulk-ion behavior 3 a) 2 B -6 T] 1 [x10 0 -1 B -2 -3 10 22 • Neutral flux close to the bulk-ion b) 0.9keV 2.4keV NeutralFlux [a.u.] 1.3keV 1.9keV 10 21 energies are starts to increase after the 10 20 10 19 mode excitation. 10 18 • The effective ion temperature also 1 ~7.5ms c) 0.8 Decay time = ~79.3ms eff. [keV] starts to increase after the mode 0.6 0.26keV 0.43keV 0.4 i T excitation. 0.2 eff. T i 0 4.9 4.95 5 5.05 time[sec] 21 21 10 10 4.9325s 4.9325s 20 20 10 10 4.9475s Neutral flux [a.u.] Neutral flux [a.u.] Either confinement property of 19 19 10 10 bulk-ions or bulk-ion heating 18 18 10 10 mechanisms seem to be changed 17 17 d) d) 10 10 with the mode excitation. 16 16 10 10 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Energy[keV] Energy[keV] 5
Fast-Ion Behavior 2.00 a) Mirnov (ch.=0) The flux increase and its decay were observed with 1.00 dB /dt [T/s] the mode activity . 0.00 Decay times of the increased flux were order of -1.00 10-100 ms. -2.00 The fast-ions influenced by the mode activity were -0.005 0 0.005 0.01 0.015 t-t 0 [s] not promptly lost and were circulating on confined orbits. 10 20 Before (-0.005~0[s]) The decay times were smaller than the Beginning (0~0.005[s]) Neutral Flux ( )[a.u] Maximum (0.005~0.01[s]) 10 19 End (0.01~0.015[s]) expected slowing-down time of fast-ions and almost no-delays of decays were found at each 10 18 b) energy. 10 17 They were lost before they undergo slowing-down process. 10 16 1.00 10 18 140keV 151keV The decay times have energy dependence. c) 5.00 10 17 They became smaller as the energy decreases. [a.u.] 0.00 By comparing the decay time with charge exchange -5.00 10 17 cross section, the charge exchange loss was found to be the dominant loss process. -1.00 10 18 Beginning (0~0.005[s]) The charge exchange loss process produces the -1.50 10 18 Maximum (0.005~0.01[s]) End (0.01~0.015[s]) positive gradients in the energy spectra. -2.00 10 18 0.00 50.00 100.00 150.00 200.00 Source of instability drive. Energy [keV] Induces clump-hole formation in the energy spectra. 6
MODE DE ST STRU RUCT CTUR URE AN E AND ITS D ITS F FREQ REQUE UENCY NCY 7
Evaluation of poloidal mode structure by HIBP measurement. T.Ido, et.al. (The amplitudes are normalized by |Bp| to remove variation among events.) Upper side Symmetry Lower side n e • The spatial structures are Asymmetry consistent with the structures of the n GAM. ( : m ~ 0, : m ~ 1) e Lower side Upper side 8
T e dependence of the initial frequency of n = 0 mode T.Ido, et.al. v NB ,0 f b ,0 2 R 85 kHz, where E b = 175 keV, = 0.35, R = 3.75m 7 f T T GAM e i 4 dB dt p 1. One has the T e 0.5 -dependence of the mode frequency. (=> GAM) 2. The other mode has weak T e -dependence of the frequency. The mode frequency is much larger than the usual GAM, and is close to the orbital frequency of the fast-ions produced by the NBI. 9
The FI affects the GAM frequency T.Ido, et.al. The GAM frequency is modified by FIs. (G.Y.Fu, PRL,101,185002 (2008) ) 2 Λ = 𝜈𝐶 a P A 2 3 3 P A v v 1 0 f 3 exp exp 𝐹 2 0 c 0 f exp exp EP 3 v v e EP 3 3 3 3 v v v c v e c c 𝜶 ∙ 𝑲 𝑐𝑣𝑚𝑙 𝜶 ∙ 𝑲 𝑐𝑣𝑚𝑙 𝑓𝑝𝑒𝑓𝑡𝑗𝑑 + 𝑲 𝐺𝐽 𝑓𝑝𝑒𝑓𝑡𝑗𝑑 + 𝑲 𝑞𝑝𝑚𝑏𝑠𝑗𝑨𝑏𝑢𝑗𝑝𝑜 =0 𝑓𝑝𝑒𝑓𝑡𝑗𝑑 + 𝑲 𝑞𝑝𝑚𝑏𝑠𝑗𝑨𝑏𝑢𝑗𝑝𝑜 =0 (T.Watari, et al, PoP, 13, 062504 (2006) f1 g Growth < 0 Slowing-down 0.6 a = 6.8 distribution 0.4 0.2 20 v g Growth > 0 g Growth > 0 5 10 15 k V k V The freq. of FI-driven GAM can be // th ion , // th ion , much higher than the ordinary GAM freq. 10
Observed n=0 mode with weak Te-dependence is also identified as GAM T.Ido, et.al. EGAM n =0 mode (Experiment) More detailed analysis based on numerical simulation can be found at H.Wang TH/P1-12 11
Mechanisms which might explain this phenomenon 1. Change of orbit topology by the GAM. activities 2. Enhanced ion-heating by the classical collision process due to the deformation of fast-ion spectra by the GAM 3. Improvement in bulk-ion energy confinement with the mode 4. Enhanced ion-heating with the mode 12
Mechanisms which might explain this phenomena 1. Change of orbit topology by the GAM. activities 2. Enhanced ion-heating by the classical collision process due to the deformation of fast-ion spectra by the GAM 3. Improvement in bulk-ion energy confinement with the mode 4. Enhanced ion-heating with the mode These effects were examined by numerical calculations: The first one was examined by orbit calculations with perturbed electrostatic potential by delta5d. The effect was small. Thus, it was ruled out. The second one was examined by TASK-3D calculation with an artificial spectral deformation, which reproduces the experimental observation. The effect was also small. Thus, it was ruled out. 13
Mechanisms which might explain this phenomena 1. Change of orbit topology by the GAM. activities 2. Deformation of fast-ion energy spectra by the mode and the enhancement of ion-heating by classical collision process. 3. Improvement in bulk-ion energy confinement with the mode 4. Enhanced ion-heating with the mode 14
The effects of confinement improvement and enhanced ion- heating were evaluated by 0-D power balance model • To investigate the possibility of these 25 -3 ] 10 T e (0) a) 17 m effects, typical values of characteristic 20 8 T e [x10 e (0) [keV] 15 6 numbers, P i (ion-heating power density) n e (0) (Thomson) 10 4 and E n e (fir) eff. (effective ion energy confinement e (0), n 5 2 time) were examined in a simple power 0 0 n balance model with an assumption of 2 b) b) rms [x10 -6 T] B rms [T] constant ion – density. 1 𝑓𝑔𝑔. 𝑓𝑔𝑔. 𝑒𝑈 𝑗 𝑄 𝑗 − 𝑈 𝑗 50-150kHz = 𝜐 𝐹𝑓𝑔𝑔. B 𝑒𝑢 3𝑜 𝑗 2 0 1 n e =n i assumed ~7.5ms b) c) Original value of P i and E eff. can be 0.8 Decay time = ~79.3ms eff. [keV] evaluated from: 0.6 0.26keV 0.43keV (i) Ion heating power estimated by TASK3D: 0.4 i T P i = 170W/m 3 => E eff. = 364ms . eff. T 0.2 i (ii) Ion temperature decay: fitting E 0 eff. =~80ms => P i = 774W/m 3 4.9 4.95 5 5.05 time[sec] 15
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