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KINETIC SIMULATIONS OF RETARDING FIELD ANALYZERMEASUREMENTS IN TOKAMAK EDGE PLASMASF. VALSAQUE and G. MANFREDILaboratoire de Physique des Milieux Ionisés, Unité CNRS 7040,Université Henri Poincaré,B.P. 239, 54506 Vandœuvre - lès - Nancy, FranceAbstractRetarding Field Analyzers (RFA) provide an integral of the ion velocity distribution in tokamak edge plasmasleading to an estimate of the ion temperature. However, the presence of the RFA itself perturbs the ambientplasma, thus limiting the accuracy of the temperature measurements. The purpose of this study is to examine theresults obtained from RFA in light of numerical simulations with a kinetic code. We investigate themodifications of the plasma characteristics (e.g. temperature, density or electric potential) induced by the probe.Systematic numerical studies are performed for different plasma parameters, such as the ion-to-electrontemperature ratio and the parallel drift speed. These results have potential implications for the interpretation ofRFA measurements, in order to extract the value of ion temperature without the probe perturbation.I. IntroductionA good knowledge and control of edge plasma conditions are necessary to maintain a steadystate in tokamaks. In particular, ion temperature and distribution in the scrape-off layer areimportant parameters to assess the high power flux released on the limiters or divertor plates,which generate impurities contaminating the plasma by physical sputtering. As hydrogenisotopes do not emit photons, spectroscopy measurements do not provide directly iontemperature. The latter is estimated from the neutral temperature and an ion-neutral couplingmodel is required. The alternative technique using a RFA device 1-2 , which measures directlythe ion energy distribution, is investigated here by means of a kinetic model and numericalsimulations.unperturbed plasma (f 0 ) flowing at the speed U 0magneticfield linesRFApresheath (f i )W f iL ⊥Fig. 1: RFA in a flowingplasma, in the presence ofa strong magnetic field.sheathsentrance slitW f 0particleexchangeII. Model: probe in a strong magnetic fieldIn order to model the plasma-probe interaction in the scrape-off layer of tokamak plasmas, weconsider the interaction of a collisionless flowing plasma with a fixed wall, in the presence ofa strong uniform magnetic field, following the approach of Chung and Hutchinson 3 . Thedensity perturbation caused by the probe can be characterized by two regions: an electricallycharged Debye sheath and a quasineutral presheath. The Debye sheath is found in theimmediate vicinity of the wall and is typically only a few Debye lengths thick. Due to thepresence of a strong magnetic field, an anisotropy is introduced and the presheath regionextends only along the field lines and inside the flux tube surrounding the probe.Nevertheless, exchanges between the unperturbed plasma outside the flux tube and the1

AFMETINGEN: ONDERDELENBESCHRIJVING MATERIAAL1. Schroefdeksel RVS 3042. O-ring EPDM3. PG9 kabelwartel4. Elektronica behuizing RVS 3045. 1" BSP draaibare nippel, SW 41 RVS 3166. Sanitair procesdeel RVS 3167. Sensor (∅19 mm) RVS 3168. Insolatiedeel (∅19 mm) PEEK9. Sensortip (∅19 mm) RVS 316Schematische functie aanduiding:SPECIFICATIES:Gevoeligheid : Instelbaar(= 33 to 1.5 µS)Hysteresis : ±10% van ingesteld bereikStroomwaarde :

BESCHRIJVING ONDERDELENART NR.1. Schroefdeksel 102412. O-ring 11029 *3. PG9 Kabelwartel 10138 *4. Behuizing elektronica 102345. 1" BSP schroefdraad, SW 41 102726. Sanitair procesdeel 102437. Contcactdeel (∅19 mm) 102358. Isolatiedeel (∅19 mm) 200409. Sensortip (∅19 mm) 1023910. Draadstuk 1024911. O-ring (2x) 11041 *12. O-ring 11043 *13.Tussenstuk 1024414. Aardpen 1025115. O-ring 1104216. Elektronica 192 *17. Bevestiging elektrode 2003718. Seal 2002519. Type sticker 2052320. Lasnippel(1" BSP) 10197Aanbevolen onderdelen*)Elekctronica module met wisselcontact (standaard module):Opmerking:1. Posititie 4 en 5 altijd in serie.2. Wanneer instelling voor gevoeligheid metpotmeter niet toereikend is, kan eenweerstand worden geplaatst tussen 4 en 53. Potmeter P2 kan optioneel wordengemonteerd bij specifieke media oftoepassing.HNL/FLX/02-09/04 Klay Instruments B.V. Pagina 3

Niveau module MNV (optionele module):Status aanduidingMassa (m)Elektrode (E)Uitgang (A)Voeding (+/-)PotmeterJumper instelling: Middels status LED: aansluiting referentie elektrode: sensor tip: PNP (n.c of n.o): 15-34 Vdc: instelling gevoeligheid: keuze voor uitgang n.o. of n.c.Producent:KLAY INSTRUMENTS B.V.Nijverheidsweg 5 7991 CZ Dwingeloo TEL.: 0521-591550Postbus 13 7990 AA Dwingeloo Nederland FAX.: 0521-592046www.klay.nlemail: info@klay.nlHNL/FLX/02-09/04 Klay Instruments B.V. Pagina 4