ContributorsEuratom Associations L. Carraro, M. Mattioli, M.E. Puiatti, P. Scarin, B. Zaniol Consorzio RFX, Padova, Italy P.DuMortier, A. Messiaen, J OngenaEcole Royal Militaire, Brussels, Belgium R.Dux, IPP-Euratom Assoziation, Garching Germany M.F.F NaveCentro de Fusão Nuclear, 1096Lisbon, Portugal J.Rapp, B. UnterbergIPF Jülich GmbH, Jülich, Germany L. Gabellieri,D. Frigione, L. Pieroni ENEA, Frascati, Italy Impurity Transport in High Density Plasmas in JET and FTU 9th EU-US Transport Task Force Workshop Cordoba, Spain - Sept / 2002 Presented by M. Valisa Task Forces S1 and T/impurity transport

Content High density regimes (relative to the Greenwald limit) of good confinement quality can be obtained in several ways Here we concentrate the impurity transport analysis on the high density Radiatively Improved Modes experiments carried out in JET (ELMy H mode) and FTU (Ohmic) JET: injection of ICRH on top of NBI heating changes transport in the core and avoids impurity accumulation in Ar seeded quasi stationary D discharges with high density (n e /n G ~ 0.9), good confinement (H 98 ~ 1) and high power radiated fraction (> 50 %). FTU : Ne seeding of D plasmas avoids saturation of confinement with density and the radiation belt at the edge reduces significantly the metal influx, with no major modification of the impurity transport.

Increasing interest in High Density regimes - around Greenwald limit - because reactor relevant. In this context impurities are an important issue: - radiative effectiveness /core power dissipation [ Prad ≈ n e n imp L(T e..) ] - risk of accumulation in the core when confinement improves - beneficial effects in accessing high density regimes w/o confinement degradation (e.g. RI-modes ) - beneficial effects as a heat exhaust channel Same impurity transport model used to analyze the two different experiments Motivation

Background - 1: Radiatively Improved mode Integrated scenario combining - high confinement ( increasing with density) - high density - good heat exhaust capability (edge radiating belt) - acceptable Zeff. Obtained in Textor-94 ( ISX results of 1984) by seeding the plasma with impurities (Ne, Ar, Si) and then reproduced in several experiments ( Asdex-UG, TFTR, D III-D, JT-60, FTU, JET). For an overview see J. Ongena et al., Physics of Plasmas 8 (2001) 2188

Background 2: Impurity accumulation Accumulation of impurities depends on the combination of various processes Transport Processes Anomalous transport - Typically flattens profiles Neoclassical transport Edge transport/ ELM’s/ screening PWI Impurity production mechanisms Impurity net influx

The analysis method : 1 D impurity transport model (M.Mattioli’s) Ionisation, recombination and radial transport of the ions of charge Z: Radiative, dielectronic, charge-exchange recombination Impurity influx is given as boundary condition, its time evolution is determined by tracking the brightness of peripheral lines. The transport coefficients D and v, radius and time dependent, are chosen in such a way as to obtain the best ‘global’ simulation of the available experimental data: Emission line spectra SXR Bolometry.

Radiatively improved modes in JET Elmy H mode Radiatively improved modes obtained in Jet in various configurations, heating schemes and puffing rates. Example : Shot Low triangularity (  ~ 0.22) X-point on septum. Ar Puffing. ITER ref. Scenario : H 98 =1,  N =1.8, n/n G =0.85 J. Ongena et al., Phys.of Plas. 8 (2001) 2188

JET Elmy H mode / After puff/ Ar accumulation The after puff phase features higher particle confinement time and density peaking. With strong Ar puffing -q(0) increases, - sawtooth amplitude decreases - Ar accumulates - confinement degrades - sometimes radiative collapse is reached -. W. Suttrop et al., Phys.of Plas.9 (2002) 2103

JET Elmy H mode / After puff/ Effect of ICRH Moderate (2-3 MW against MW of NBI) ICRH power deposited in the center: Heats the plasma core (Te peaks)-> Screens impurity Increases diffusion ( ne flattens) -> Opposes impurity peaking Keeps q(0) below 1 - maintains sawteeth -> Contribute to expel Ar Altogether sustains the anomalous transport -> Reduces impurity accumulation M.F Nave et al. To be published

JET Elmy H mode / After puff/ Effect of ICRH Ar density profiles reconstructed by a 1-D Collisional Radiative Transport Code (Mattioli’s) Septum, low  w/o ICRH Septum, low  with 2 MW ICRH

JET Elmy H mode / After puff/ Effect of ICRH EHT, Continuous D2 Puffing, with 2 MW ICRH Best radiation belt. Possible contribution from CX

JET Elmy H mode / After puff/ Effect of ICRH D’s and V’s (from Mattioli’s impurity transport model) Accumulation - Strong inward convection No accumulation : convection may become outward M.E. Puiatti et al.Plas. Phys.Contr. Fus. 44(2002)1863 In shots in which accumulation is avoided Anomalous transport increases Inward convection decreases and may become outward

JET Elmy H mode / After puff/ Effect of ICRH Neoclassical transport parameters In both cases, with and without accumulation, transport is anomalous, but in the shot with accumulation the empirical peaking factor is “closer” to the neoclassical one than in the case w/o accumulation.

JET Elmy H mode / After puff/ Effect of ICRH

JET Elmy H mode / After puff/ Effect of Sawteeh Impurity transport model results : Sawteeth contribute to the expulsion of the impurities from the core M.Mattioli et al.EPS meeting Montreaux 2002

JET Elmy H mode / After puff/ Effect of Sawteeh However their sole contribution does not justify the absence of Ar accumulation : other mechanisms are present

JET Elmy H mode / After puff/ Effect of continuous modes Other MHD activity in the form of continuous modes - m=1 n=1 and others -helps increasing the anomalous transport. M.Mattioli et al.EPS meeting Montreaux 2002

Radiatively improved mode in FTU In FTU ohmic Ne seeded plasmas RI-Mode avoids saturation of confinement with density. Typical signatures Ne profiles peak Electron and ion temperature increase (for the same input power) As a consequence, confinement improves (x1.4)

Radiatively improved mode in FTU D.Frigione, L. Pieroni et al. EPS Montreaux, 2002

Radiatively improved mode in FTU

In Ne seeded shots metal concentration (Fe, Ni, Mo) decreases This appears to be due to a reduced sputtering associated with the reduced convected /conducted power through the edge (  rad ~.85). FTU has TZM (Mo alloy) limiters L.Carraro et al. EPS Montreaux 2002

Radiatively improved mode in FTU Impurity transport does not change significantly (same v’s and D’s) give satisfactory simulation results in both shots with and without seeding) Impurity transport is anomalous: neoclassical diffusion in the core ~ 0.02 m 2 s -1 Accumulation is avoided by a reduction of the influx

Conclusions In High density regimes impurity seeded discharges impurity accumulation can be avoided. IN JET: The risk of impurity accumulation with Ar seeding is avoided by modifying transport. Adding central deposited ICRH on top of NBI heats the core and maintains q(0) below 1 and flat. IN FTU : The radiation belt in Ne seeded D plasmas avoids the risk of impurity accumulation by reducing significantly the metal influx, with no major modification of the impurity transport. FUTURE WORK 1) Extend the analysis to other High Density scenarios 2) Investigate detailed transport mechanisms