Edge radiation control in stochastic magnetic field and with RMP application in LHD

Transcription

1 2nd Technical Meeting on Divertor Concepts 13 to 16 November 217, Suzhou, China Edge radiation control in stochastic magnetic field and with RMP application in LHD M. Kobayashi 1,2, S. Masuzaki 1,2, S. Morita 1,2, H. Tanaka 3, B.J. Peterson 1,2, Y. Narushima 1, K. Mukai 1,2, T. Kobayashi 2, and the LHD experimental group 1 National Institute for Fusion Science, Oroshi-cho, Toki city, Gifu-ken , Japan 2 Department of Fusion Science, Graduate University for Advanced Studies, Oroshi-cho, Toki-city, Japan 3 Nagoya University, Furo-cho, Chikusa-ku, Nagoya, , Japan 1

2 Introduction: Helical DEMO design & divertor power load FFHR-d1 (LHD based) R c =15.7 m B c =4.7 T V p =15 m 3 P fusion = 3 GW Helical divertor HELIAS 5-B (W7-X based) R c =22 m B c =6 T V p =14 m 3 P fusion = 3 GW Island divertor Heat flux in SOL for stellarator (LHD) cases At upstream, with q = q λ q st q l PSOL q =, l ~ 2πRq 2 // 4π ar q // = PSOL 2π R λ q st B B P sep ~4 MW, P sep /R~25 MW/m p // // ~ It may follow the similar scaling as tokamaks Needs detachment operation (Scaling of λ qst?) J. Miyazawa, IAEA DPW

3 Introduction : Control of enhanced radiation at divertor region Necessity of divertor heat load reduction for future devices: Divertor detachment, radiative divertor is prerequisite to meet the engineering limit of PFC heat load (< several MW/m 2 ). Control of enhanced radiation region in its location & intensity is one challenging issue. Involved issues in the physics: A/M processes with strong non-linearity in cold plasma Energy transport in parallel/perpendicular to field lines Impurity transport, plasma recycling with volume recombination, momentum loss process 3D magnetic field configuration: Seeking divertor optimization in helical devices, RMP application to tokamaks Effects of three dimensionality/symmetry breaking on radiating edge plasma are not yet fully understood. This contribution reports experimental results of detachment control with RMP application to the edge stochastic layer of LHD. 3

4 Contents of the talk 1. Edge magnetic field structure of LHD 2. Time traces of plasma parameters of the controlled detachment discharge 3. Radiation enhancement and modification of radiation distribution by RMP Density dependence of radiation EMC3-EIRENE prediction for radiation modification, AXUV measurements EUV measurements of CIII CVI emission profiles 4. Divertor flux distribution with RMP Toroidal asymmetry Reduction during detached phase Rotation of the asymmetry by RMP phase shift 5. Operation space and simple model analysis 6. Core confinement of the RMP assisted detachment 7. Summary 4

5 Magnetic field structure of LHD: RMP (m/n=1/1) application remnant island in stochastic region R = 3.9 m, a ~.7 m, 1 field periods (toroidal) Divertor : carbon, Fist wall : Stainless steel Connection length (m) RMP coils (m/n=1/1) Helical coils 1..5 Divertor legs Edge surface layers Plasma shape Z (m) Without RMP (m/n=1/1) Rotational transform ι 2 1 ~ br coil B Resonance value.1% Without RMP R(m) Stochastic region R (m) Magnetic island O-point 5

6 Stable sustainment of radiative divertor operation (RMP assisted RD) Without RMP Radiation collapse due to thermal instability Radiative divertor Divertor power load (MW/m 2 ) ne (1 m ) 2 Radiation (a.u.) 1.6 a 99 (m) W p (kj) -3 Without RMP time (s) M. Kobayashi et al., Nucl. Fusion 53 (213) Stable operation around density limit Radiation increase by a factor of ~ 3 Reduction of divertor power load by a factor of 3 ~ 1 Plasma shrinks at RD phase due to radiative energy loss and RMP penetration No significant degradation of main plasma confinement Intensity (a.u.) No noticeable high Z impurity (Fe) emission at high density range. Fe XXI Fe XIX Fe XXII Fe XXII Fe XXI Fe XXI Fe XXIII FeXX Detach (6.6x1 19 m -3 ) Fe VIII Fe IX Fe X Fe X Fe X Fe XII Fe XI Attach (n e = 2.x1 19 m -3 ) Pixel number 6

7 Increased volume of low T e region (~1 ev) at remnant island with RMP leads to enhanced carbon radiation n e dependence of radiation power (bolometer) 3 Radiation collapse T e, n e profiles at outboard midplane (Thomson scattering) Resonance layer Without RMP Radiation Power (a.u.) 2 1 Detach Radiation enhanced Without RMP T e flattening at island n (1 m ) e Carbon radiation (Estimated with n carbon =.1n e n e τ = 1 17 m -3 s) M. Kobayashi et al., Nucl. Fusion 53 (213) R (m) 7

8 Modification of 3D edge radiation structure by RMP : 3D numerical simulation Carbon radiation distribution by EMC3-EIRENE Inboard side MW/m 3 2.x1 2.x1-1 2.x1-2 2.x1-3 Z R Without RMP Outboard side Poloidal angle (deg.) Without RMP Radiation peak at inboard side Without RMP Radiation peak appears at inboard side. X-point of m/n=1/1 island is selectively cooled. 8 Poloidal angle (deg.) Toroidal angle (deg.) Outboard Inboard Outboard Outboard Inboard Outboard

9 Radiation profile : Comparison between experiments & simulation Carbon radiation distribution by EMC3-EIRENE Intensity (mw/m 2 ) Inboard side Ch16 LOS of measurements Experiments Channel The qualitative change of profiles due to RMP application roughly agrees between experiments & simulation. Without RMP Ch1 Simulation Results implies selective cooling at X-point of m/n=1/1 island in experiments. Imaging bolometer shows similar effect. Split to two peaks Channel The well-structured magnetic field catches the radiation and prevents it from penetrating inward?! Poloidal asymmetry of radiation is enhanced. Intensity (a.u.) 9

10 Vertical profiles of impurity emission with EUV spectrometers CIII (386.2 Å) CIV (384.2 Å) CV (4.3 Å) CVI (33.7 Å).6.5 O-point.4.2 Z (m) Z (m) Poloidal angle (deg.) EMC3-EIRENE Location of measurement X-point R (m) Toroidal angle (deg.) Outboard Inboard Outboard 1 CIII.5 4 CIV 2 W/O RMP.5 CV 5 CVI Intensity (1 15 phs./cm 2 /s) CIII, CIV, CV are enhanced by RMP application Profiles of CIII & CIV affected significantly by RMP Up-down asymmetric CVI is slightly reduced by RMP good indication of impurity screening (?!) H. Zhang et al., POP 24 (217)

11 CII emission and H LCFS γ/hβ : Comparison with connection length (LC) plot Connection length (LC) Stable detachment (with RMP) Divertor leg Div. plate Detach. fr15, 4.6 s LC (m) 1 Hγ/Hβ Island 1 LCFS Island LCFS fr18, 5.5 s fr25, 6.1 s Div. plate 1 Divertor leg Divertor leg CII (426.7 nm) 1 Island Island LCFS fr15,lcfs 4.6 s fr18, 5.5 s fr25, 6.1 s CII radiates at LC~1 m region & along divertor legs. Clear upstream shift of emission at detachment transition. : In detach. phase, CII is stabilized outside of island (LCFS). Hγ/Hβ (index for volume recombination) : increase after detachment transition along LCFS. : Formation of very cold plasma around (outside) of island Comparison with 3D modeling is underway.. 11 M. Kobayashi et al., NME 12 (217) 143.

12 Magnetic field configuration & divertor probe arrays in LHD Open field lines RMP coils (m/n=1/1) Helical coils One field period (Δφ=36 ) Plasma shape Helical coils Top view of torus Divertor probe arrays at inboard midplane Midplane 12

13 Toroidal asymmetry of divertor flux caused by RMP application.6 Higher flux is attributed to increased wetted area induced by RMP L div. 3D model 1.6 R div. 1.4 Exp Wetted area increases with RMP. RMP Toroidal section 4L No RMP s (mm) L No RMP Wetted area remains same. RMP s (mm) Toroidal section R RMP Wetted area increases with RMP. No RMP s (mm) Analysis of power load is underway (mostly same trend as the particle flux) 13

14 The toroidal asymmetry can be rotated by toroidal phase shift of RMP.6 L div. 3D model 1.6 3D model Exp. 5 Shift by Exp Toroidal section Toroidal section Shift by 36.6 L div. 1.6 L div Toroidal section Toroidal section 14

15 Toroidal asymmetry of divertor particle flux changes at detachment phase At certain sections, the flux even increases. The asymmetry during detachment phase can also be shifted by RMP phase shift..6 L div. Attach.6 Attach R div..4 Detach.4 Detach Toroidal section RMP toroidal phase shift by Toroidal section.6 L div..6 R div Toroidal section Toroidal section 15

16 Significant plasma response to external RMP Attach Detach Reattach ΔΦ r in plasma (1-4 Wb) Island poloidal phase Shift (degree) #11585 Shielded Relative to vacuum RMP Time (s) Amplified Attached phase: RMP is shielded, island poloidal phase shift ~ 3 deg. Detached phase: RMP is amplified, island poloidal phase shift ~ 15 deg. Hysteresis in plasma response at detach reattach transition ΔΦ ex Effect on divertor particle/power deposition 16

17 Quantitative comparison of temporal evolution of radiated power with EMC3-EIRENE EMC3-EIRENE Experiments Indication of the study: 1. Larger D imp is closer to experiments. Needs drifts (electric field)?, or turbulence? 2. Impurity source should be reduced after detachment. 3. The modeling still overestimates radiated power as compared to experiments. Sputtering coefficient or atomic data base? 17 S. Pandya et al., NF 56 (216) 462.

18 Comparison of radiation profiles with EMC3-EIRENE Radiation distribution by bolometer from top view port Experimental results are much broader than the simulations D imp should be greater than the bulk plasma at least by a factor of 4 18 S. Pandya et al., NF 56 (216) 462.

19 Lower threshold of perturbation strength for sustained detachment n e (1 19 m -3 ) ~ b / B (%) Upper bounds for density 5 Radiative collapse Detachment transition Sustained detach I RMP (ka) Density limit of radiative collapse seems independent of perturbation field strength. Upper bounds for density operation range Detachment transition density decreases with increasing I RMP. Easy access to detachment & enough margin for operation (but at the expense of Wp.) No sustained detachment has been realized so ~ far at I coil < 1.9 ka ( b / B <.6 %) 19

20 Operation domain of stable detachment in LHD ~ Key geometric parameters: Δ x, /, LCFS island b r B Stable detach (m).1.5 Stable detach. Radiation collapse. 2.x1-4 coil ( b ~ 1.x1-3 2.x1-3 /B ΔxLCFS island Healing r ) vac Remnant island of m/n=1/ ~ coil ( br /B w ).1% w ~ coil ( br /B ).5% R(m) Separation between radiation region (island) & confinement region is important factor for stable detachment vac Radiation collapse No T e flattening due to plasma healing vac LCFS Confineme nt region M. Kobayashi et al., Nucl. Fusion 53 (213) Threshold for RMP amplitude to overcome plasma screening. 2

21 Main plasma confinement : Recovery of energy confinement after detachment transition due to pressure profile peaking Confinement enhancement factor vs n e exp ISS4 τ E / (fren τ E ) Without RMP (attach) Sudo n / e n c (detach) RD transition p e profiles with & without RMP (detach) (attach) R(m) Without RMP Increase of n e leads to confinement degradation without RMP. Significant degradation in RMP attached phase due to large magnetic island in the edge. Energy confinement recovers after RD transition with RMP due to pressure peaking. The cause of the pressure peaking is under investigation. 21

22 Fluctuation of magnetic probe & Isat Fluctuation of magnetic probe Frequency (khz) 1 Magnetic probe High frequency components (several tens khz) disappears after detach transition Low frequency (~5kHz) component Detach Time (s) 6 Div probe Peaked at 6~9Hz after detach transition. Strong correlation with radiation oscillation. 7 Fluctuation of Isat -4 6I 1.5U Radiation Isat Attach Detach 1 Frequency(Hz) Frequency(Hz) 1 Isat Time(s)

23 Summary Effects of RMP application on the detachment is being investigated in LHD. 1. The RMP (m/n=1/1) application leads to stable sustainment of detached plasma. 2. Radiation is enhancement by RMP application EMC3-EIRENE prediction: Poloidal asymmetric radiation due to island AXUV line integrated profiles consistent with the modeling EUV measurements of CIII CVI : Enhancement at X and O-points Visible CII and H γ /H β : Enhancement outside/around of LCFS (signature of volume recomb.) 4. Divertor flux distribution with RMP Toroidal asymmetry according to toroidal mode number Reduction during detached phase: asymmetric pattern changes from the attached phase The asymmetry can be rotated by RMP phase shift 5. Operation space Key geometric parameters : RMP amplitude and separation between edge island and confinement region 6. Geometrical effect on energy transport and radiation distribution Poloidally asymmetric radiation Energy flow from O to X-point can help to sustain the localized radiation 7. Confinement of the RMP assisted detachment Confinement is recovered after detachment transition due to pressure peaking Issues to be investigated further Effects of different impurity species: Ne, N, Ar etc in relation to the island Te Mode number of RMP Decoupling effect between SOL & confinement plasma in terms of neutral penetration (RMP thicker SOL decoupling) 23