ECRH Results during Current Ramp-up and Post-Pellet Injection in FTU Plasma

Transcription

1 18 th IAEA Fusion Energy Conference, Sorrento, Italy, October 4-10, 2000 ECRH Results during Current Ramp-up and Post-Pellet Injection in FTU Plasma Paper EX6/3 presented by G. Bracco FTU Group, ECRH Group* Associazione Euratom-ENEA sulla Fusione, Frascati *Associazione Euratom-ENEA-CNR, Istituto di Fisica del Plasma, Milano

2 Authors G. Bracco 1), A. Bruschi 1), P. Buratti 1), S. Cirant 2), F. Crisanti 1), B. Esposito 1), D. Frigione 1), E. Giovannozzi 1), G. Giruzzi 4), G. Granucci 2), V. Krivenski 5), C. Sozzi 2), O. Tudisco 1), V. Zanza 1), F. Alladio 1), B. Angelini 1), M.L. Apicella 1), G. Apruzzese 1), E. Barbato 1), L. Bertalot 1), A. Bertocchi 1), G. Buceti 1), A. Cardinali 1), S. Cascino 6), C. Castaldo 1), C. Centioli 1), R. Cesario 1), P. Chuillon 1), S. Ciattaglia 1), V. Cocilovo 1), R. De Angelis 1), M. De Benedetti 1), E. de la Luna 5), F. De Marco 1), B. Francioni 6), L. Gabellieri 1), G. Gatti 1), C. Gormezano 1), F. Gravanti 1), M. Grolli 2), F. Iannone 1), H. Kroegler 2), M. Leigheb 1), G. Maddaluno 1), G. Mafia 1), M. Marinucci 1), G. Mazzitelli 1), P. Micozzi 1), F. Mirizzi 1), S. Nowak 2), F.P. Orsitto 1), D. Pacella 1), L. Panaccione 1), M. Panella 1), F. Papitto 1), V. Pericoli Ridolfini 1), L. Pieroni 1), S. Podda 1), F. Poli 1), G. Pulcella 6), G. Ravera 1), G.B. Righetti 1), F. Romanelli 1), M. Romanelli 1), A. Russo 6), F. Santini 1), Sassi 1), S.E. Segre 3), A. Simonetto 2), P. Smeulders 1), S. Sternini 1), N. Tartoni 1), P.E. Travisanutto 1), A.A. Tuccillo 1), V. Vitale 1), G. Vlad 1), M. Zerbini 1), F. Zonca 1). 1) Associazione Euratom-ENEA sulla Fusione, CR Frascati, Frascati, Roma, Italy 2) Associazione Euratom-ENEA-CNR, Istituto di Fisica del Plasma, Milano, Italy 3) INFM and Dipartimento di Fisica, II Università di Roma Tor Vergata, Roma, Italy 4) Association Euratom-CEA sur la Fusion, DRFC/STPF, CEA/Cadarache, France 5) Asociación Euratom/CIEMAT para Fusión, CIEMAT, Madrid, Spain 6) ENEA Fellow

3 Motivation ECRH on current ramp-up produces high and in the plasma core in conditions characterized by a variety of current profiles, which is due to the various plasma start-up conditions and to the impurity content. ECRH in the post-pellet phase allows to test if the enhanced confinement observed in ohmic discharges can be obtained with additional electron heating for high density plasma. The relevance of profile resiliency, of critical models and the role of inward energy pinch velocity can be studied. The dependency on local parameters, such as magnetic shear, can be deduced.

4 Outline Current ramp ECRH scenario Global confinement Local analysis» On-axis heating: effect of different q profiles» Off-axis heating Discussion of measurement for high power density ECRH ECRH in post-pellet phase Conclusions

5 ECRH on current ramp-up: scenario #15020, on-axis heating peaked pre-ecrh (r) (A) I p Current ramp to 0.7 MA, di/dt~5 MA/s 140 GHz ECRH, 0.9 MW (m -3 ) (W) n(0) T i0 <nl> 0 On-axis/off-axis heating by changing B T or by tilting ECRH launchers Weak e-i coupling T i ~1 kev Hollow to peaked pre- ECRH & j profiles P ECRH t (s)

6 ECRH on current ramp-up: global confinement #15020, 5.3 T, on-axis ECRH (A) (V) (W) (J) P tot P rad Z eff W tot I p V L P ECRH L mode: the global energy confinament is close to ITER89-P scaling value Radiation losses: relevant in the outer part of the discharge Total ohmic power is of the order of P ecrh (s) ITER89P τ E In the plasma core P ecrh is the dominant term t (s)

7 ECRH on current ramp-up: local analysis # s s s s s Heat flux vs. n e for r/a<0.15 χ e =0.4 m 2 /s oh P ECRH t=0.095 s χ e =0.2 m 2 /s (w/m 3 ) P oh r (m) P rad The interpretative analysis indicates low χ e values in the plasma core, similar to the one obtained in ohmic discharges, despite high and

8 On-axis heating on hollow j profiles (1) When low Z impurities (C,O) content is low, hollow profiles are obtained in the startup phase. On-axis ECRH restores rapidly peaked profiles, in conditions of inverted magnetic shear. #17389 t=0.098 s s s s s (A) (m -3 ) (w) n e (0) #17389 I p <nl> Ti P ECRH oh r (m) t (s) A strong reconnection occurs at a time when q min becomes lower than 2

9 On-axis heating on hollow j profiles (2) q oh s (m 2 /s) χ e Comparison between # MW, peaked pre-ecrh (r) and # MW, hollow pre-ecrh (r) at the times of max (0) No clear differences in the value of χ e in the plasma core. In both cases the magnetic shear in the plasma core is low ( s < 0.5 ) r (m) r (m)

10 Radial scan of power deposition (kev) #17389 #17392 t=0.116 s The localization of the ECRH has been changed at fixed B t by tilting the ECRH launchers; all pulses have hollow pre-ecrh profiles. #17393 #17393 exp simul (w/m 3 ) #17389 p ECRH R (m) #17392 #17393 oh r (m) Diffusive behaviour found: the experiment can be simulated using an ad-hoc χ e in the range m 2 /s in the plasma core

11 Off-axis ECRH on peaked pre-ecrh profiles oh p ECRH exp simul t=0.096 s s s s #17386 Also in cases when the pre- ECRH profile is peaked a diffusive behaviour is found. Experimental data can be simulated by using the Bohm term of the mixed shear BgB model [ ] enhanced by a factor 2. (W/m 3 ) p oh t=0.096 s (OH) t=0.106 s Residual ohmic heating plays a role at the very centre of the plasma column. r (m)

12 Discussion of measurement at high values measurement relies on ECE analysis at >10keV due to TS design limitations in FTU. A small hump due to relativistic down-shifted emission from the central electrons is observed at low frequency: a simulation of the ECE spectra (2 nd, 3 rd, 4 th harmonics) requires a reduced 0 [Giruzzi]. A high poloidal resolution Fokker-Plank code shows that f e (E) is deformed at low energy (10 20 kev) by the high p ECRH : the proper ratio hump/peak is reproduced [Krivenski]. The effect is important only in the deposition layer. The unaffected region of the 2 nd harmonics is used to measure (r) for the transport analysis.

13 Off-axis ECRH in post-pellet phase (1) (m -3 ) (W) P TOT pellet #17839, 0.6 MA, q=4.7 <nl> n e0 P ECRH Constraints n cut-off =2.4x10 20 m -3 (140 GHz), B t = 5.6 T, ECRH timing [lower plasma characteristics than in the high n e, B t =8 T of the OH pellet injection experiments ] (s -1 ) (s) 0 Φ DD I sx τ E exp t (s) 2xneoc T i0 neoc ITER89P Results before the onset of m=1 MHD activity: Neutron yield is compatible with neoclassical (Chang-Hinton) ion transport Enhanced global confinement (1.5 ITER89-P)

14 Off-axis ECRH in post-pellet phase (2) (m -3 ) OH n e OH P ECRH r (m) t=0.595 s s t=0.595 s s s s Electron density is near to the cut-off density 2.4x10 20 m -3 at the ECRH deposition layer. Electron temperature profiles show a diffusive behaviour in the core region of high density plasma. Other similar experiments in FTU: on-axis ECRH in post-pellet phase: results in a fast onset of sawtooth activity and a strong density pump out. pellet injection on a ECR heated plasma: results in a broader n e (r), sawtooth is not supressed, no signs of enhanced energy confinement,

15 Conclusions Low χ e values are obtained in the plasma core of ECRH heated plasma during the ramp-up phase, despite the high and values, with no sign of power degradation. An enhanced confinement regime is transiently attained with ECRH in post-pellet phase, similarly to the results obtained in ohmic discharges. A variety of the electron temperature profile shapes can be obtained showing no profile resiliency in the plasma core and the local analysis demonstrate a diffusive behaviour far from a critical gradient limit. other experiments in FTU with a more peripheral localization of the ECRH heating, [C. Sozzi, Poster EXP5/13], show clear signs of profile resiliency; the difference could be correlated with a higher value of the magnetic shear in the deposition region in respect to the experiments illustrated in this presentation]