1. 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 21 st IAEA Fusion Energy Conference 16-21 October 2006, Chengdu China

2. 2 Contents 1)A brief description of 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. 3 LHD picture LHD A super conducting large helical device (l=2, M=10) R ax = 3.5-3.9 m, a  0.5-0.6 m, B = 3T

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

5. 5 Internal Diffusion Barrier (IDB) n(0) = 4.6  10 20 m -3, T(0) = 0.85 keV, W p = 1.1 MJ at P = 10 MW, n o  E T o = 0.44  10 20 m -3 keVm -3 s  (0) = 4.4 % at B = 2.64 T Outer region (Mantle) Pellet Island Separatrix IDB

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

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

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

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

10. 10 Dense core expands up to LCFS for outward shifted configuration (R ax = 3.85m). = 1.38 % = 0.63 % LCFS Dense core expands with beta and Rax. n n 1x 10 20 m -3

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

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

13. 13 Quasi-steady state operation of IDB mode has been demonstrated. Pellet injection tends to fuel the particle in the region with high  n. Continuous pellet injection n o = 2.0E20m -3

14. 14 Summary We have discovered Internal Diffusion Barrier which maintains a high density core plasma (n(0) = 4.6  10 20 m -3, T(0)=0.85 keV,  (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. 15 End

16. 16 Particle Balance core mantle n- profile Core  pellet = n c V c /  c  c = 0.4 s n c = 3.3 x 10 20 m -3  pellet = 0.5 x 10 22 s -1 V c = 6 m 3 Mantle  pump = V /  p *  p * = 0.5 s, = 8.3 x 10 19 m -3  pellet  recycled  pump Role of LID * Pumping of the recycled particles low  p * low n mantle * With intensive wall conditioning, IDB is maintained by wall pumping (without LID). * For longer pulsed operation, divertor pumping is essential.

17. 17 A New Ignition Scenario (SDC reactor design) n o = 5-7  10 20 m -3, T o = 7-9 keV (Conventional reactor) n o = 1.5  10 20 m -3, T o = 30 keV Internal Diffusion Barrier +Pellet maintain high density core. Achievement of ignition with core temperature as low as possible.