Assessment of the use of injected impurities for heat flux mitigation in JET ELMy H-modes. P. Monier- Garbet (1) , J. Hogan (2) , Y. Corre (3) , Ph. Andrew (4) , D. Coster (5) , P. Dumortier (6) , Th. Eich (5) , A. Huber (7) , R. Koslowski (7) , K. Lawson (4) , A. Messiaen (6) , M.F. Nave (8) , J. Ongena (6) , J. Rapp (9)(7) , J. Stober (5) , and JET EFDA contributors. (1) Association EURATOM-CEA, CEA/DSM/DRFC CEA-Cadarache, 13108 Saint-Paul-lez-Durance, France (2) Fusion Energy Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA (3) Association EURATOM-VR, KTH-Physics, 10691 Stockholm, Sweden (4) Association EURATOM-UKAEA Culham Science Center, Abingdon, OX14 3DB, UK (5) Association EURATOM-IPP, MPI fur Plasmaphysik, 2 Boltzmannstrasse, Garching, Germany (6) LPP-ERM/KMS, EURATOM-Belgian State Association, TEC, Brussels, Belgium (7) Association EURATOM-TEC, Forschungszentrum Jülich GmbH, Germany (8) Association EURATOM-IST, Lisbon, Portugal (9) EFDA-CSU Culham. 1. Introduction. One of the most severe problems for the operation of next step fusion devices in H-mode is the transient power load to the divertor target plates due to Type I ELMs [1]. The purpose of this paper is to assess the use of impurity seeding to reduce Type I ELM heat fluxes through enhanced radiation, either in a narrow region just inside the separatrix, or in the SOL and divertor regions, while maintaining good core confinement. An analysis of the divertor heat load during Type I ELMs in argon seeded H-modes on JET has already been reported [2]. In these discharges, with Ar and P rad /P tot ≈ 0.6 , the peak heat flux due to Type I ELMs was reduced by a factor of ≈ 2 , and the confinement was ≈ 5% less than in the deuterium reference pulse. This effect is due to a reduced pedestal energy, and hence a reduced ELM energy release. From the limited set of data analysed in [2], no clear evidence of any additional radiative dissipation in the SOL and divertor region was observed. In the present paper, the previous analysis of argon seeded H-modes is compared with new experiments where nitrogen is used as radiating species as a means to change the localisation of the radiation from the region just inside the separatrix to the SOL and divertor, the underlying physics idea being that, in that case, some fraction of the ELM energy might be radiated before reaching the target plates. The results are modelled using the time-dependent SOLPS5.0 code. Previous calculations of this possible buffering effect have been reported for JET [2] and ITER [3], both predicting it to be large (> factor 2) only for small ELMs (Δ W dia <20kJ in JET). 2. Experiment. a. Experimental scenario and reduction of the transient divertor energy load during ELMs. Low levels of N or Ar impurity have been injected in H-mode plasmas with Type I ELMs in the Diagnostic Optimized Configuration (DOC-L, δ≈ 0.26 ) with q 95 =3.8 (2.0MA/2.4T) or q 95 =3.0 (2.5MA/2.4T). The low q 95 experiments correspond to a higher electron density (n e ped = 5.0 × 10 19 m -3 for N experiments, 7.0 × 10 19 m -3 for Ar experiments, 30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-1.95