T. Nakano , N. Asakura, H. Takenaga, H. Kubo, Y. Miura, S. Konoshima, K. Masaki, S. Higashijima

1. Impact of nearly-saturated divertor plates on particle control in long and high-power-heated discharges in JT-60U T. Nakano, N. Asakura, H. Takenaga, H. Kubo, Y. Miura, S. Konoshima, K. Masaki, S. Higashijima and the JT-60Team Japan Atomic Energy Research Institute, Ibaraki, Japan. 20th IAEA Fusion Energy Conference Vilamoura, Portugal, 1-6 November 2004

2. 15s => 65s a Background Long pulse, steady-state operation DEMO Day - Year ITER Min.~ Hour JT-60 Sec. JT-60NCT Min. twall Unconditioned Rrecycling ~ 1 or >1 (Wall saturation) Conditioned Rrecycling < 1 ISSUE: Experience and knowledge of operation with Rrecycling ~ 1

3. Baffle Baffle Divertor-Pumping Plates Divertor Introduction Pellet Particle control; For a constant density Fueled = Pumped aaaaaaaaaaaaaaaa Short pulse: Divertor-pumping + Wall-pumping Long pulse: Divertor-pumping + (Co-deposition?) Gas-puff NB Core Plasma First wall Particle control at wall saturated Long pulse in JT-60 Pheat~12 MW, 30s

4. Outline • Identification • of wall saturation • Density controllability w at wall saturation • Summary

5. No “carbon bloom” Energy Input : 350 MJ Div. surface temperature : ~ 1300 K

6. Identification of Wall Saturation

7. Saturation Wall Saturation identified in long pulse operation MARFE • Quasi-steady-state, • Vp dnp/dt ~ 0, • Vn dnD0/dt ~ 0, • Rwall = RP-NB + RN-NB • + RGas- Rpump • t = 20 - 27s, constant wall retention • Wall saturation • ELMy H-mode • Zeff ~3,H89PL~1.7 Then, • MARFE ( detach )

8. During 44020, • 3x1022 retained in walls. • Active wall-pumping capacity ~ 3x1022 Saturation Area, Wider than Divertor plates Particle Balance between pulses • Until 44018, • Wall retention increases • Wall saturation • After 44019, • 3x1022 are released. 3x1022 Particle Balance during 44020 Saturation level of D+ to C tile at 300eV = 1x1021m-2 Saturation area(Minimum) = 3x1022/ 1x1021m-2 = 30 m2 > Divertor plates ( 20 m2 )

9. Twall = 520 K Identical discharge condition; Ip,Bt,ne,PNB,H89PL,Tsurfdiv,,, Release-Rate from walls Significant Particle Release at Twall = 520 K Twall = 420 K Gas-Puff-Rate • The only difference : first-wall-temperature • Suggests particle release from first wall / Baffle plates

10. Viewing chord for Da 1ch 8ch 20ch 11ch 19ch Source of particles, Baffle Plates Da Brightness Main Chamber First wall Inner Baffle Outer Baffle Divertor Twall = 520 K No gas-puff Twall = 420 K Gas-Puff

11. Density Controllability byActive Divertor-Pumpingat Wall Saturation

12. Suggests; Large GAP => Increase of P0 => Increase of ne Density controllability of divertor-pumping Penning gage Wall saturation GAP Pumping With Gas-puff No gas-puf

13. High pumping-rate suppresses increase of plasma particles Condition : wall saturation Density : ne/neGW = 62-66% Decreasing Gap • Difficult to prevent undesirable density rise of • high d plasmas ( Large GAP ) • Limited period of high bN; ex. 22.3 s for bN = 2.3 Higher pumping-rate is required even for low d plasmas ( Small GAP )

14. Controllability of detachmentby divertor-puming at R = 1 SOLDOR simulation RGas 3x1021/s 3x1021/s 10 MW Rrecycle=1 RPump 6x1021/s e = i = 1 m2/s, D = 0.3 m2/s Indicates higher pumping speed by a factor of 2 - 3 can avoid MARFE at the end of long pulse discharges

15. Summary • Modification for Long Pulse Operation, 15 sec => 65 sec • ELMy H-mode plasma • ( ~30 s ,~ 12 MW, 350 MJ, No “carbon bloom”) • Wall saturation was identified • (Minor role of co-deposition) • Divertor plates • Wall/baffle plates <= important particle source at Rrecy > 1 • No sudden changes of plasma ( Zeff, H89PL ) • Undesirable increase of plasma density • => Confinement Degradation, MARFE • Higher divertor-pumping efficiency ( x 2 - 3 ) required to avoid MARFE NB 10 sec => 30 sec