Advancements in Dense Plasma via Internal Diffusion Barrier in LHD

1. Super dense core plasma due to Internal Diffusion Barrier in LHD N. Ohyabu 1), T. Morisaki 1), S. Masuzaki 1), R. Sakamoto 1), M. Kobayashi 1), J. Miyazawa 1), M. Shoji 1), T. Akiyama 1), N. Ashikawa 1), M. Emoto 1), H .Funaba 1), P. Goncharov 1), M. Goto 1), J.H. Harris 2), Y. Hirooka 1), K.Ichiguchi 1) T. Ido 1), K. Itoh 1), H. Igami 1), K. Ikeda 1), S. Inagaki 1), H .Kasahara 1), T. Kobuchi 1), S. Kubo 1), R. Kumazawa 1), S. Morita 1) S. Muto 1), K. Nagaoka 1), N. Nakajima 1), Y. Nakamura 1), H. Nakanishi 1), K. Narihara 1) Y. Narushima 1), M. Nishiura 1), T. Notake 1), S. Ohdachi 1), N. Ohno 1), Y. Oka 1), M. Osakabe 1), T. Ozaki 1), B.J. Peterson 1), K. Saito 1), S. Sakakibara 1), R. Sanchez 2), H. Sanuki 1), K. Sato 1), T. Seki 1), A. Shimizu 1), H. Sugama 1), C. Suzuki 1), Y. Suzuki 1), Y. Takeiri 1), K. Tanaka 1), N. Tamura 1), K. Toi 1), T. Tokuzawa 1), S. Toda 1), K. Tsumori 1) I. Yamada 1), O. Yamagishi 1), M.Yokoyama 1), S. Yoshimura 1), Y. Yoshimura 1), M. Yoshinuma 1), K. Ida 1), T. Shimozuma 1), K.Y. Watanabe 1), Y. Nagayama 1), O. Kaneko 1), T. Mutoh 1), K. Kawahata 1), H. Yamada 1), A. Komori 1), S. Sudo 1), O. Motojima 1) 1) National Institute for Fusion Science, Toki, Gifu-ken, Japan presented by N. Ohyabu for LHD team at 21st IAEA Fusion Energy Conference 16-21 October 2006, Chengdu China

2. Contents • A brief descriptionof LHD, LID 2) Observation of Internal Diffusion Barrier（IDB) in the LID divertor Discharge Features of IDB mode Time Evolution of IDB LID divertor function+ Pellet injection Location of IDB Foot High (o) plasma at high B Steady State operation of IDB mode 3) Summary

3. LHD A super conducting large helical device (l=2, M=10) Rax = 3.5-3.9 m, a  0.5-0.6 m, B = 3T • LHD picture

4. Pellet Pumping Duct Island Divertor Chamber LID Head Main Plasma Vacuum Pump Local Island Divertor A closed divertor with high pumping efficiency Objectives i) to develop island divertor concept. Of LID experiment ii) to study island related physics iii) to explore confinement enhancement mode. Pellet core fueling Powerful particle control

5. Internal Diffusion Barrier (IDB) Island Separatrix IDB Pellet Outer region (Mantle) n(0) = 4.6  1020m-3, T(0) = 0.85 keV, Wp = 1.1 MJat P = 10 MW, noETo = 0.44  1020m-3keVm-3s  (0) = 4.4 %at B = 2.64 T

6. Time evolution of IDB Time constant of n(0) decay is 1sec.

7. Dense core plasma Low mantle densityHigh T in the mantleHigh core temperature Avoidance of radiative collapse IDB + Pellet injection High confinement Pumping Confinement Improvement Mechanismsin IDB discharges T Low T High T IDB discharge: high core density + low mantle density Gas puff discharge: flat n profile n High n Edge Density limit Low n SDCIDBMantle

8. In the outer region (mantle), T increases with P/nedge q = - nT

9. Dense core expands with beta and Rax. Standard configuration (Rax=3.75m) Optimum core Inward shifted configuration (Rax=3.65m). Small, but clear core

10. LCFS Dense core expands up to LCFS for outward shifted configuration (Rax = 3.85m). <> = 1.38 % n 1x 1020m-3 Dense core expands with beta and Rax. n <> = 0.63 % 1x 1020m-3

11. “Reheat” raises the core beta upto 5.1 %(B=1.5T) “Reheat” starts Large Shafranov shift. n profiles before and during “reheat” Te n

12. Pellet Pumping Duct Island Divertor Chamber LID Head Main Plasma Vacuum Pump Role of LID * Pumping of the recycled particles low nmantle * With intensive wall conditioning, IDB is maintained by wall pumping (without LID). * For longer pulsed operation, divertor pumping is essential.

13. Quasi-steady state operation of IDB mode has been demonstrated. Pellet injection tends to fuel the particle in the region with high n. no= 2.0E20m-3 Continuous pellet injection

14. Summary • We have discovered Internal Diffusion Barrier which maintains ahigh density core plasma (n(0) = 4.6 1020m-3, T(0)=0.85 keV, b(0)=4.4 in the LHD divertor discharge fueled by pellets. • Radial location of IDB foot increases with beta and magnetic axis. • Function of the LID is pumping of the recycled particles. This leads to low density in the outer region and hence increase in temperature there. • We propose a novel ignition scenario at high density and relatively low temperature in the helical device.

15. End

16. n- profile core mantle Particle Balance Core pellet = ncVc / c c = 0.4 s nc = 3.3 x 1020m-3 pellet = 0.5 x 1022s-1 Vc = 6 m3 Mantlepump= <nouter> V / p* p* = 0.5 s, <nouter> = 8.3 x 1019m-3 pellet recycled pump Role of LID * Pumping of the recycled particles low p* low nmantle * With intensive wall conditioning, IDB is maintained by wall pumping (without LID). * For longer pulsed operation, divertor pumping is essential.

17. A New Ignition Scenario • Internal Diffusion Barrier +Pellet maintain • high density core. • Achievement of ignition with core temperature • as low as possible. (SDC reactor design) no = 5-7 1020m-3, To = 7-9 keV (Conventional reactor)no = 1.5 1020m-3, To = 30 keV