Dodhy, A.

Kühner, G.

Ringler, H.

Sardei, F.

MPG.PuRe

639NEUTRAL GAS TRANSPORT AND PARTICLE RECYCLINGIN THE W VII-AS STELLARATURF. Sardei, H. Ringler, A. Dodhy, G. KUhner,w VII-AS Team“Max-Planck-Institut fflr PlasmaphysikAssociation EURATOM-IPP, D-80N6 Garching, FRGECRH Group”Institut fflr Plasmaforschung der Universitat StuttgartD-7000 Stuttgart, FRGIntroductionIn the w VII-AS stellarator plasma particle and power balance will bestrongly affected by the distribution of the ion sources and chargeexchange losses (similar as in w VII-A [1]), which are poorly known fromexperimental data. Furthermore, the control of the plasma density criti-cally depends on the recycling processes in the plasma boundary region andat the limiters and walls.Results for a model plasmaBefore the w VII-AS operation was started, model plasmas were used tostudy the neutral gas transport behaviour with the 3D DEGAS code [2]. Fora neutral source due to limiter recycling a simulation was performed toinvestigate to what extent the neutral density distribution is affected bythe asymmetries of the flux surfaces and wall structures. The Te, Ti and heprofiles were taken from model calculations with the TEMPL plasma transportcode (Te(0) = 1.9 keV, Ti(0) = 500 ev, ne(0) = 2 x 1013 cm‘3 for an ECFinput power of 720 kw) [3].The typical cross section of the w VII—AS toroidal sector modelled forthe neutral transport calculations is shown in Fig. 1. The limiter. thebulge for the N81 ports and the geometry of the magnetic surfaces (see Fig.1) introduce asymmetries in the torus, which directly affect the neutraldensity distribution. Figure 2 shows the radial profiles of the neutraldensity at the poloidal angles 9 = 0°, 90°, 180°, 270°. As shown by theB = 90° profile, the neutral density has a maximum at the limiter face anddrops by 2 orders of magnitude to the plasma centre. In the oppositedirection with respect to the limiter. e = 270°, the density issignificantly lower, reaching a maximum of 1/3 of the limiter value. Alongthe horizontal chord, e = 0° and e = 180°, the neutral density profilesare fairly symmetric up to a distance of :12 cm from the magnetic axis.The relatively high gradients of these profiles reflect the smaller plasmaradius along this chord, associated with the asymmetry of the magneticsurfaces (see Fig. 1). Due to this asymmetry, the central neutral densityis mainly determined by the neutrals penetrating the plasma in thehorizontal direction. At r > 15 cm, well inside the scrape—off region, theprofiles diverge from each other, reflecting the asymmetry introduced bythe bulge. Within the bulge, the neutral density is fairly constant andamounts to roughly 1/” of the limiter value.4—”..4640“mIn the following, results fromneutral transport simulationsare discussedfor a typical deuterium discharge from the first months of HVII-ASoperation.Discharge parameters and Hc-monitoring systemThe analysis is based on a ECF (70GHz) heated deuterium discharge atthe 2nd harmonic, with 1.25 Tat the plasma centre and the externalrotational transform # being 20.53 at the plasma edge. Electron densityand temperature profiles, measured by Thomson scattering, were available at200 ms (Fig. 3). Stationary plasmaconditions were maintained by onegyrotron heating with 150 kwinput power and by external gas input linkedto a feedback control of the plasma density. A plasma current inducedeither by ECF and/or by plasma pressure [U] was controlledand kept below= 500 A by the OH-transformer. Themeasured Te, he profiles covered aplasma radius extending up tothe separatrix, which was located at reff =17.5 cm, about 2.5 cm outsidethe last flux surface not intersected by thelimiter. A clear—cut separation of the plasma from the scrape-off region isnot possible for this particular discharge type with edge tvalues close tothe 5/9 resonance, because ofthe strong poloidal deformationof the fluxsurfaces crossing the limiter. A more effective limiting of the plasma isprovided in this case by theseparatrix, outside of whichthe open fieldlines lead to a flattening ofthe temperature and density profiles.An absolutely calibrated Hamonitoring system [5] installed atW VII-AS yields information about particle recycling at both limiters (twotoroidal positions for the bottom and one for the top limiter) and at two"triangular" magnetic surfacecross sections close to and far from a gasfeed inlet, respectively.Results for first H VII-AS dischargesThe DEGAS code simulation atthe "triangular" cross section near thegas inlet was used to find estimates for Te, ne between separatrix andwall, which were consistent with both the measured gas input rate and Hasignal close to the gas inlet. These estimates were thenused in thelimiter and wall recycling simulations. The time histories of the measuredHd-signals from the limitersand the "triangular" cross sections are shownin Fig. Ma,b. The neutral fluxes and densities were estimated by scalingthe calculated Ha emission rates to agree with the measured ones.The calculated total refuelling rate of the plasma insidetheseparatrix due to limiter andwall recycling and to external gas feedamounts to 2.5 x lO?° ions/s. 83% of this ionization source areprovidedby the limiter, 13 i by thewall and u 1 by the gas puffing. Thiscorresponds to a global recycling coefficient of 0.96. From the calculatedrefuellihg rates and a plasma ion content of 8.9 x 1018particles insidethe separatrix, as resultingfrom the measured electron densities (Fig. 3)and an assumed Zeff of 3, a global particle confinement time of 36ms isestimated for this time of the discharge. The plasma refuellingefficiencies as fractions of the neutrals ionizing inside the separatrixare 0.65 for the limiter, O.HN for the wall and 0.29 for the external gassource. The small values for the wall and gas puff sources are directly64]related to the large volume of the boundary region between separatrix andwall (more than twice the volume of the confined plasma). 0n the otherhand, this has the advantage of improving the decoupling between the plasmaand the wall, thus reducing the wall loading by high energetic chargeexchange neutrals.The given refuelling rates and efficiencies from wall recycling andgas puffing may have a considerable error margin due to their sensitivityto the unknown plasma density and temperature in the boundary region.Furthermore, the flux of ions incident to the wall at the position of theHa viewing line may not represent a good average of the ion fluxes overthe entire torus wall. However, these uncertainties do not strongly affectthe global recycling coefficient and ’U , since recycling from the limiteris the main refuelling mechanism to the bulk plasma.Experimental indication of a recycling coefficient clOSe to one forthe discharge type discussed here is given after a second gyrotron isswitched on for about 100 ms (see Fig. Nb). During that time a constantplasma density is maintained without any external gas feed and the Hasignals at the triangular cross sections close to and far from the gasinlet coincide (see Fig. Nb). The amplitude of all signals (Figs. Ua,b) isconsiderably higher than before the 2nd gyrotron was added. Consideringthat at stationary conditions the externallv supplied neutral flux equalsthe fluxes absorbed by wall and limiters and that the absorbed fluxeslinearly scale with the recycling fluxes, gas puffing proves to be a veryefficient density control mechanism for high recycling ECF discharges.For the same discharge, the calculated average neutral densities atthe limiters and wall due to the recycling sources are 2.9 x 109 cm"3 and3.9 x 10B cm", respectively. The total ion power loss due to chargeexchange with all neutrals is 3.1 kw. The total electron power loss due toionization, excitation and dissociation is 2.0 kw.REFERENCES[1] H. Ringler et al., Plasma Phys. and Contr. Nucl. Fusion, lAEA-CN-UY/D—V—1 (1987), Vol. 2, 603[2] 13.13. Heifetz et al., J. Comput. Phys. N6 (1982), 3093 H. Wobig, Notes on the W VII-A and w VII-AS Stellarators, Ringberg(1986), Internal Report[N] U. Gasparino et al., this conference5 T. Uckan, ORNL/TM—10698.itii ., : see U. Gasparino et al., this conference{Ev 1: Vertical cross—section of the modelledN V[ -/\S torus at the position of the modulecunnncting flanges, including the wan. the bulgefur the HBI ports. the flux surfaces and one ofthe limiters.W VII-AS:642[a u] limiter0 20 40 60distance [mm magnetic and: (cm)Fig. 2: Radial distributions of the neutral th‘llSliK‘at the puloidal angles 0 = o”, 90". H30". 270“ in [IN'vertical cross-section shown in Fig. 1.Shots 1294 — 1513—Jcm1.5no“1.0”e 0.5 lFig. 3: Radial Te and n, profiles v5. aliective radius atmeasurements an n. side of (he nrnfile.0.0p’ crnAt — 0.2 5 (ram Thomson scattering. run circles represent., a WZAS5.H:G.'P.'|9.9|99I19§£La A t. . a WZASA.Hzala'iejiqgfwfils ._9r: , I)!“ g 7 UIH_ I alpha nholon rlux . 1 alpha phulun uuxn u UlUlIUM [cu m cm I 25 lESLA ,. )L‘QHZRIUM [on 70 012 I 25 Itsu i‘A";‘- I"i \ ' ‘\ m/v f)“ \A ’i \\ _ NW.. ,A v" ~ ,3" {‘ ‘i £w JV" V. f ‘15w \ ,J Y’ 3w} ,‘nW “'m ‘ 25m \ J ‘l 9"-" i . .NV " ‘ .E V 25w ‘3“?:? E {“i”T . 'M15l7< 7 lap rimncy (u) »~ 'u-Isn mum" Iiv (“3' ‘ ~ w .u - plasma "inllqulm (u)-u n 14551 p hollnm h , (Hy . . ' l qns inpul (Hr, .. _. . WWW—wwimfi—rv _, W. i:00 3 0,: u 5 04 u ,um». (a) " nmu (s)Fig. ha: Hmphoton fluxes vs. time from ECE discharge Fig. 4b: Ho—photnn fluxes vs time at two triangularlooking at top and bottom limiter, plasma cross*section elliptical (a second gyrotron was switchedml between 0.3 and 0.43 5).Flasma crossnsections. with and without gas inlet.The large spikes reflect the pulse sequence from thegas inlet.

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