Influence of Toroidal and Vertical Magnetic Fields on Ion Cyclotron Wall Conditioning in Tokamaks

1. Influence of Toroidal and Vertical Magnetic Fields on Ion Cyclotron Wall Conditioningin Tokamaks Presented by A.Lyssoivan LPP-ERM/KMS, Brussels With contribution from G.Sergienko, V.Rohde, V.Philipps, G.Van Wassenhove, M.Vervier, V.Bobkov, J.Harhausen, R.Koch, J.-M.Noterdaeme, D.Van Eester, M.Freisinger,H.-U.Fahrbach, H.Reimer, A.Kreter, D.A.Hartmann, J.Hu, R.Weynants, O.Gruber, A.Herrmann, D.Douai, Y.D.Bae, H.G.Esser, J.G.Kwak, E.Lerche, O.Marchuk, V.Mertens, R.Neu, U.Samm, A.Scarabosio, C.Schulz, S.J.Wang, TEXTOR Team and ASDEX Upgrade Team A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

2. Motivation ICRF Plasma / Antenna Coupling Characterization ICWC in TEXTOR and ASDEX Upgrade ICWC Extrapolation to ITER Conclusions Outline A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

3. ICRF discharge has a high potential for wall conditioning (tritium retention, surface isotope exchange, wall cleaning/coating) in the presence of permanent high magnetic field. Ion Cyclotron Wall Conditioning (ICWC) was approved for integration into the ITER baseline using ITER ICRF heating system. Further development of the ITER relevant ICWC scenarios with conventional ICRF antennas is an important and urgent task. Motivation A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

4. Plasma Production with Standard ICRF Antennas RF Field/Waves excitation RF Power e-collisional absorption Neutral Gas e-collisional ionization TEXTOR ICRF antennas AUG ICRF antennas f=30.0; 36.5 MHz, BT=1.0-2.4 T, p=(1-8 )10-2 Pa f=25-38 MHz, BT=0.25-2.5 T, p=(1-10 )10-2 Pa A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

5. ICWC Optimization ICRF Plasma Production Removal Mechanisms Antenna Coupling Plasma Homogeneity / Extension Fast Ions Generation 1. High Ion Cyclotron Harmonics, =nci, n>>1 2. Mode Conversion, = ci BT+BV, BV<<BT Fundamental Ion Cyclotron Resonance  = ci A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

6. TEXTOR: ICRF Plasma Characterization ne, Te and Ppl vs BT • ICRF plasma can be produced at any BT-field • =10cH+(BT0.2 T): High coupling (0.8), density (>21017 m-3) and homogeneity • =cH+(BT2.3 T): improved coupling (0.5) and homogeneity A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

7. AUG: ICRF Plasma Characterization (He+H2)-plasma vs He-plasma • Mode conversion scenario in (He+H2)-plasmas: • Higher antenna coupling(up to 3 times) • Better homogeneity and extension in radialdirection • Better performance at two frequencies He+H2, f1=30 MHz+f2=36.5 MHz He, f=30 MHz He+H2, f=30 MHz BT+BV vs BT Vertical magnetic field improves plasma homogeneity in poloidal dirction and extends it towards divertor BV BT BT BT=2.4 T, BV=0 BT=2.4 T, BV0.02 T A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

8. ICWC in TEXTOR(C-coated wall)  see G.Sergienko, P2-45, 27/05/2008 Removal rate: Measured removal rate for m=3 vs BT Calculated absorbed power vs BT • =10cH+(BT0.2 T):Effective conditioning due to high antenna coupling and homogeneity possible in both, low and high the BT-fields • =cH+(BT2.3 T):Mode conversion in (He+H2)-plasmas is the best scenario for ICWC (coupling + homogeneity + fast particles) • Applied BV-field (BV << BT)  increased ICWC yield A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

9. ICWC in ASDEX Upgrade(W-coated wall) Measured removal rate for m=40 vs BT Fast particles energy/power vs BT • Benefit from mode conversion in (He+H2)-mixture with ICR (=cH+) location closer to the antenna • ICWC output correlates with fast particles energy and power absorbed by protons • BV-field improves the ICWC effect • Major concern – ICWC homogeneity (efficient cleaning from ~25% of the AUG surface) A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

10. ICWC Extrapolation to ITER:scenario for operation 4 3 2 1 row 1 & row 2: /3, f=40 MHz row 3 & row 4: /6, f=48 MHz 0.32 m TOMCAT modeling (rpl2.4 m, R0=6.2 m, BT=3.6 T, ne0=3x1017 m-3, Te0=5 eV): - Mode conversion in (He+H2)-plasmas at two frequencies A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

11. ICWC Extrapolation to ITER:power for operation Modeling with 0-D plasma/transport code • 0-D Plasma/Transport code: • ne(1-4)1017 m-3, Te~1.5 eV,ioniz=1-2%, p=(2-8)10-2 Pa • PRF-pl(ITER)= 0.2-1.5 MW (coupl0.40) PRF-G (ITER)0.5-3.8 MW • Extrapolation from TEXTOR data (assuming similar power density and coupl0.40): • PRF-pl (TEXTOR)  12-30 kWP RF-pl (ITER) 1.0-2.5 MW  P RF-G (ITER) 2.5-6.0 MW A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008

12. Inter-machine (TEXTOR, ASDEX Upgrade) ICWC studies: Wall conditioning in the mode conversion scenario in the presence of toroidal and vertical magnetic fields (BV<<BT) may be considered as the most promising candidate for application in ITER using the main ICRF antenna. Better radial/poloidal homogeneity of the ICRF plasma and its ability to accelerate ions at the fundamental ICR may contribute to improving the conditioning effect. ICWC at high cyclotron harmonics appears also to be attractive mainly due to very high antenna-plasma coupling (80%) and plasma homogeneity. However, the scenario needs operating at high generator frequencies for the nominal magnetic fields and does not produce fast ions. Modeling with the 1-D RF and 0-D plasma codes and extrapolation from the existing machines give a good evidence for the feasibility of using ICWC in ITER with the ICRF heating system. Conclusions A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008