1 Elimintion of the Non-Axisymmetric inter-elm Het Flux Generted y Resonnt Mgnetic Perturtions in High Density Divertor Conditions A.R. Briesemeister 1, J.W. Ahn 1, J.M. Cnik 1, M.E. Fenstermcher 2, H. Frerichs 3, C. J. Lsnier 2, J.D. Lore 1, A.W. Leonrd 4, M.A. Mkowski 2, A.G. McLen 2, W.H. Meyer 2, O. Schmitz 3, M.W. Shfer 1, E.A. Untererg 1, J. G. Wtkins 5 1 Ok Ridge Ntionl Lortory, Ok Ridge, TN, USA 2 Lwrence Livermore Ntionl Lortory, Livermore, CA, USA 3 University of Wisconsin-Mdison, Mdison, WI, USA 4 Generl Atomics, Sn Diego, CA, USA 5 Sndi Ntionl Lortories, Aluquerque, NM, USA E-mil contct of min uthor: riesemeister@fusion.gt.com Keywords: detchment Astrct. It is found tht symmetries in het flux rising from resonnt field perturtions re eliminted s density is rised to pproch detched divertor conditions on DIII-D. It is shown tht pplying RMPs reduces the core density nd increses the inter-elm pek het flux to oth the inner nd outer trgets in the divertor. Using gs puffing to return the core density to the pre-rmp levels reduces the inter-elm het flux t the outer strike point to vlue just elow the pre-rmp level; the het flux profile is rodened slightly reltive to the pre-rmp profile. It is lso shown tht further incresing the plsm density elimintes the non-xisymmetric het flux rodening, even efore the ttinment of detched conditions. Both experimentl evidence nd modeling re presented showing tht neither detchment nor the elimintion of the 3D field structure is needed to eliminte the non-xisymmetric het flux rodening. Mesurements showing tht secondry pek in the ion sturtion current, oserved t modest densities, is not present t high densities. Mesurements re presented showing tht in cses where the ppliction of RMPs leds to n increse in the core electron temperture, higher densities re needed to induce detchment of the outer strike point. These results help llevite concerns over potentil het nd prticle lod symmetries tht my rise with the use of 3D ELM control techniques, nd re prticulrly encourging for future rectors where ner detched conditions will e required in ny cse to reduce divertor het nd prticle lods. 1. Bckground Scling studies for ITER nd future devices generlly show tht tools will e needed to reduce or eliminte oth the stedy stte het flux nd the trnsient het flux from ELMs. RMPs hve een shown to effectively suppress, mitigte or trigger ELMs on severl devices [1][2][3][4]. RMPs re therefore eing considered s tool to control ELMs in ITER [5]. However, RMPs lso led to loclized hot spots on plsm fcing components of the divertor, nd thus concerns remin out the comptiility of the stedy stte het flux footprint with the divertor structure [6]. Detchment hs een shown to e n importnt tool to control stedy stte het nd prticle flux to the divertor. Concerns out the comptiility of detchment nd RMPs hve een rised ecuse of results showing tht incresing density cn increse het flux to regions further from the strike [7,8]. Although the het flux to these regions is generlly lower thn the pek het flux in ttched conditions, nd therefore this spreding of the het flux cn e eneficil, modeling shows tht in ITER RMPs my increse the het flux to more vulnerle, highly shped, regions of the divertor [6]. Additionlly t DIII-D it hs een oserved tht higher levels of gs puffing re needed to detch the inner strike point when RMPs re pplied [9] nd t NSTX it hs een oserved tht dding RMPs cn cuse detched divertor to re-ttch [10].
2 Additionlly, it hs een oserved tht the RMP induced non-xisymmetries cn pper in the prticle flux structure while the het flux structure remins xisymmetric. At low densities prticle flux profiles in DIII-D hve shown splitting while the het flux profile remins reltively unpertured y RMPs [11]. At higher densities in DIII-D, nd t NSTX, the prticle nd het flux profiles hve similr structure [8][12]. Simultions hve shown tht chnges in fueling my e responsile for the differences etween the het nd the prticle flux structure, [13] reinforcing the need to etter understnd the reltionship etween het nd prticle flux structures nd their reltionships to density nd fueling. Since chnges in core conditions will generlly ffect divertor conditions [14] it is importnt to understnd how RMP induced chnges in the core conditions will impct divertor conditions. A reduction in core density, known s pumpout, typiclly occurs when RMPs re pplied to low collisionlity plsms. At DIII-D pumpout hs een oserved in low collisionlity plsms, ut is not typiclly seen in high collisionlity plsms even in cses with ELM suppression [8]. No pumpout in high collisionlity ELM mitigted dischrges ws oserved in ASDEX Upgrde [15], ut RMP induced pumpout ws seen t low density [15]. 2. Using Fueling to Eliminte Pumpout Induced Increse in Het Flux It is shown in this section tht pplying RMPs cn increse the pek het flux etween ELMs to the divertor in cses with core density pumpout, ut this increse is lrgely eliminted when the core density is rised to the pre-rmp level. FIG. 1.() shows tht when RMPs re pplied the core density s frction of Greenwld density [16] is reduced from n e /n GW =0.37 to n e /n GW =0.26. Electron density here is the line verged density mesured y the interferometer, [17] nd n GW =I p /π 2 where I p is the totl plsm current nd is the verge minor rdius of the plsm. For ll the dt used in this pper RMPs were generted using the upper nd lower I-coils in n n=3 even prity configurtion. FIG. 1. The () line verged electron density s frction of Greenwld density, () gs puffing rte, (c) Deuterium emission line strength nd (d) pedestl electron collisionlity re shown s function of time. Verticl lines indicted the time slices chosen for detiled nlysis. c d The reduction in core density is eliminted when deuterium is dded y gs puffing t the crown of the plsm. Gs puffing is initited 300 ms fter the RMPs re pplied nd incresed until the core density equls nd then exceeds the pre-rmp level. In this dischrge fter the RMPs were pplied the ELM size ws reduced y the ppliction of the RMPs. This chnge cn e seen in the deuterium Blmer-α line signl mesured in the divertor y the filterscopes [18] nd shown in FIG.1.(c). The reduction in deuterium emission ws correlted with reduction in the energy relesed y the ELMs mesured y the fst mgnetics. The electron collisionlity is mesured t the top of the pedestl, nd drops to minimum vlue of ν e =0.35. The reduction in ELM size nd temporry gp in ELMs shortly fter 2.8 s re consistent with the typicl onset of ELM suppression [11]. Collisionlities elow ν e =0.3 re typiclly needed for roust ELM
3 suppression with even prity RMPs [4], ut were not chieved in this experiment. The dischrges shown throughout this pper hd n edge sfety fctor of q 95 =3.5 which is within the rnge where low-collisionlity ELM suppression hs een chieved in DIII-D. A typicl lower single null shpe ws used with the ion B B drift direction towrds the divertor. The toroidl field ws B t =1.9 T nd neutrl em heting of 6 MW ws used. In this section shpe with moderte lower tringulrity, which puts the outer strike point ner the inlet of the cryopump, ws chosen to mximize pumpout. Dt in susequent sections ws collected in shpe with reduced lower tringulrity to llow etter dignostic mesurements of the outer strike point. FIG. 2. The electron () density nd () temperture profiles re shown for the three time slices indicted in Fig. 1. Three time-slices from the dischrge, indicted y verticl lines on FIG. 1., were chosen for detiled nlysis. Dt t 2.1 s ws chosen to represent pre-rmp plsm conditions. The second slice t 2.9 s ws chosen to e in the middle of the ELM-free time window. At 3.2 s the core density ws found to mtch the pre-rmp density. At time slices where the line-verged densities re similr the core electron density nd temperture profiles re similr regrdless of whether RMPs re pplied. In FIG. 2.() it cn e seen tht the densities re similr in the pre-rmp time slice nd the cse in which gs puffing hs een used to rise the plsm density cross the entire profile. During the RMP induced density pumpout the density profile from the SOL to the top of the pedestl is nerly identicl to tht seen in the other two time slices. The pedestl height in the cse with density pumpout is reduced y just over 25%. The density grdients inside the pedestl region re similr to those in the other two cses. FIG. 2.() shows neither the RMPs nor the dded gs hd mesurle effect on the electron temperture. Het flux to the divertor is mesured y the IRTV system [19,20]. Integrtion times of 62 µs re used. Ech profile shown in FIG. 3. is the verge of multiple frmes cptured during 200 ms intervl. In intervls with ELMs, only frmes cptured during the lst 20% of the ELM cycle were used. As shown in FIG. 3.() efore the RMPs re pplied there is no mesurle het flux to the inner strike point. During the RMP induced pumpout the het flux to the inner strike point is incresed, ut dding deuterium elimintes the het flux to the inner strike point. This indictes tht the reduction in core density is lrgely responsile for the increse in het flux. The outer strike point shows similr trend, ut s is typicl for dischrges with the field direction configured so tht the B B drift is directed towrds the divertor, the pek het flux to the outer trget ws much lrger thn the het flux to the inner strike point. As shown in FIG. 3.(), pplying the RMPs rises the pek het flux to the outer strike point. When the core density with RMP is mtched to the pre-rmp level the pek het flux is slightly reduced, nd the het flux profile is rodened y the presence of the RMPs. These mesurements show tht RMPs cn reduce the inter-elm het flux t the outer strike point when compred to cses with comprle core densities. Het flux mesured t the outer strike point t
4 single toroidl loction hs een found to e lrgely xisymmetric oth in modelling nd in experiments, such s those presented in section 3, where the RMPs were moved reltive to the het flux dignostic. Awy from the outer strike point lrger toroidl vritions my exist in the het flux. FIG. 3. The het flux etween ELMs to the () inner nd () outer strike points re shown for the three time slices indicted in FIG.1. The mgnetic configurtion used for the mesurements shown in this section limits the outord rdil extent of het flux seen y the IRTV cmer due to shdowing from the divertor shelf, which limits nlysis of the het flux outord of the outer strike point. This configurtion ws chosen ecuse it is the configurtion in which low collisionlity ELM suppression is typiclly chieved (due to proximity to cryopump tht ids low collisionlity ccess) nd in which mximum RMP induced pumpout is oserved. The next section fetures dt from configurtion with more extensive divertor dignostic coverge. 3. Incresing Density cn Eliminte Non-xisymmetric Structure in Het nd Prticle Flux Since het flux profile mesurements re currently only ville t single toroidl loction, the non-xisymmetric structure is mesured y moving the pplied RMP field reltive to the dignostics. By reversing the current in ll the coils used to crete the RMP fields, the mgnetic structures cn e effectively shifted 60 degrees toroidlly round the mchine. FIG. 4. shows het flux mesurements with the coils in oth the 0 nd 60 degree configurtion. Although the loe structures re not well seprted in this dischrge the effects of the RMPs on the het flux structure cn e seen in the lower density cse, with core density of n e /n GW =0.4, shown in FIG.4.(). When the coils re moved from 0 degree phsing to 60 degree phsing locl chnges of up to 0.4 MW/m 2 cn e seen in the het flux profile, indicting nonxisymmetric structure. FIG.4.() shows tht t higher density, n e /n GW =0.75, the het flux to the tiles is reduced nd shows no mesurle non-xisymmetric structure is seen. c FIG. 4. The het flux profile ner the outer strike point is shown for () moderte upstrem density nd () high upstrem density. (c) The ion sturtion current mesured t the trget ner the outer strike point is shown for lower density (lue) nd higher density (red) cse with RMPs.
5 Simultions hve shown tht incresed divertor density cn reduce the non-xisymmetric het flux while rising the prticle flux to the stritions [13]. The Lngmuir proes in the divertor trget tiles [21] re used to mesure the prticle flux. To increse the sptil resolution of these mesurements the plsm is swept rdilly reltive to the proe re nd the dt is rempped to single equilirium. In FIG. 4.(c) the re-mpped profiles for oth the low nd high-density cses re shown. A secondry pek cn e seen in the lower density cse, ut t higher densities there is no pprent secondry pek. 3.1. Chnges Seen in Divertor Electron Temperture nd Density The divertor Thomson scttering system mesures chnges in the electron temperture consistent with loe structures induced y the RMPs. As shown in FIG. 5., when RMPs re pplied loe in the electron temperture ppers to extend from the x-point outwrds towrds point on the trget out 10 cm outord of the outer strike point. No such structure ppers in the cse without RMPs [22]. The temperture in this loe is out 10 ev ove the plsm tht surrounds it. The plsms shown in FIG. 5.() nd () re ttched with n upstrem density of Greenwld density is n/ngw=0.4. Although some structure is seen in the electron density when the RMPs re pplied it is much less pronounced thn the structure seen in the electron temperture. c FIG. 5. The electron temperture in the divertor is shown ) without RMPs, ) with RMPs t lower density nd c) with RMPs t higher density. As the density is incresed, the loes in the electron temperture move wy from the trget plte. As shown in FIG. 5.(c), lthough structure remins in the electron temperture, there is no longer cler secondry pek t the trget. The electron density lso shows no secondry pek t the trget. At this higher density the divertor is not yet detched. 4. Comprisons of Mesurements with EMC3-Eirene Modeling The EMC3-Eirene 3D edge fluid modeling code [23,24] shows tht when upstrem density is incresed while ll other quntities re held fixed, the experimentlly oserved chnges in the divertor electron temperture re reproduced. In the lower density clcultions shown in FIG. 6.() the loes in the electron temperture in the divertor extend to the trget plte. As the density t the upstrem simultion oundry is incresed the loes in the electron temperture, shown in FIG. 6.(), re preferentilly cooled ner the trget plte. This chnge in structure is qulittively similr to tht shown in FIG. 5.
6 In these simultions the mgnetic field structure nd perpendiculr trnsport coefficients re held constnt while the density t the core simultion oundry, chosen to e t the normlized enclosed flux surfce Ψn=0.7, ws vried. A vcuum model for the 3D mgnetic field structure is used this does not include plsm response fields to RMPs. Since the plsm response cn chnge with density, the vcuum field ws chosen to test the effects of incresing core density in the sence of chnges in the mgnetic field structure. c FIG. 6. The electron temperture in the divertor clculted for () lower nd () higher upstrem density conditions. (c) The electron temperture t the trget plte clculted y the EMC3-Eirene code is shown for vriety of upstrem densities. Exmining the clculted electron temperture t the trget plte, shown in FIG.6., it cn e seen tht incresing the core density cuses reduction in oth the temperture t the outer strike point nd reduction in the locl peks wy from the outer strike point. The electron temperture pek t the outer strike point is lrgely xisymmetric, showing less thn 10% vrition toroidlly. The secondry peks show much lrger toroidl vritions in the lower density clcultions, ut these peks re grdully eliminted t ll toroidl ngles s the upstrem density is incresed. 5. RMPs Increse the Detchment Onset Threshold Exmining two high-density cses shows evidence tht RMPs rise the detchment onset threshold for the outer strike point. FIG. 7. shows the ion sturtion current profiles mesured y Lngmuir proes ner the outer strike point for two reltively high-density plsms. The cse without RMPs shows strong reduction in the sturtion current t the outer strike point nd the pek in the sturtion current is moved rdilly outord. This structure is chrcteristic of detchment t the strike point. The profile with RMPs shows pek in the sturtion current ner the outer-strike point. This profile is chrcteristic of the outer strike point eing in the high-recycling regime. FIG. 7. The sturtion current t the floor shows tht the divertor plsm is in the high-recycling regime with RMPs (red), ut detched t slightly lower upstrem density without RMPs (lck). A slight reduction in the core temperture possily cused y the presence of rotting MHD mode in the cse with no RMPs my e responsile for the differences in detchment etween the cses with nd without RMPs. The electron temperture is 10 to 20 percent higher cross
7 the core profile in the cse with RMPs. In the cse with no RMPs rotting mode with toroidl numer of 2 is present for the stedy stte portion of the dischrge. In the dischrge with RMPs this mode only ppers fter the RMPs re turned off. This drmtic modifiction of the MHD modes y the RMPs ws not oserved in the other dischrges presented in this work nd ppers to e correlted with the high densities used in this set of dischrges. The differences in the level of divertor detchment etween the cses with nd without RMPs cnnot e explined y differences in core density. The line-verged density in the cse without RMPs is found to e 8.1x10 19 m -3 while in the cse with RMPs it is 8.3x10 19 m -3. Notle, just inside the seprtrix t Ψ n =0.99 the density is 20 percent higher in the cse with RMPs nd there is no mesurle difference in the density t the seprtrix with nd without RMPs. Other prmeters such s plsm current, mgnetic field strength nd uxiliry het power were held fixed etween the two cses. 6. Conclusion It hs een shown tht while the reltionship etween core nd divertor conditions re lrgely similr to wht is oserved in xisymmetric plsms some notle modifictions re oserved when RMPs re pplied. In cses where RMPs cuse density pumpout, ut no mesurele chnge in the core electron temperture, the ssocited increse in het flux is lrgely eliminted y rising the core density. When the ppliction of RMPs cuses n increse in core electron temperture n increse in the divertor detchment threshold s function of density is seen. RMPs cn cuse mesurle rodening of the inter-elm het flux t moderte densities, ut this rodening is eliminted t high densities. Loes in the electron temperture structure re induced y the RMPs nd extend to the floor t moderte densities, ut t high densities these structures move wy from the floor. The cooling of these structures is consistent with cooling seen in the simultions in which the core density is rised. Modeling shows tht the electron temperture ner the trget plte ecomes lrgely xisymmetric s the density is rised, consistent with the mesurements which show the het flux ecomes xisymmetric t high densities. This work is suported y the U.S. Deprtment of Energy under DE-AC04-00OR22725 1, DE- AC52-07NA27344 2, DE-FG02-08ER54999 3, DE-F02-04ER54698 4 nd DE-AC04-94AL85000 5. DIII-D dt shown in this pper cn e otined y following the links t https:// fusion.gt.com/glol/d3d_dmp. 7. References [1] SUTTROP, W. et l., First Oservtion of Edge Loclized Modes Mitigtion with Resonnt nd Nonresonnt Mgnetic Perturtions in ASDEX Upgrde, Phys. Rev. Lett. 106 22 (2011) 225004. [2] CANIK, J.M. et l., On Demnd Triggering of Edge Loclized Instilities Using Externl Nonxisymmetric Mgnetic Perturtions in Toroidl Plsms, Phys. Rev. Lett. 104 4 (2010) 45001. [3] CANIK, J.M. et l., ELM destiliztion y externlly pplied non-xisymmetric mgnetic perturtions in NSTX, Nucl. Fusion 50 3 (2010) 34012. [4] KIRK, A. et l., Understnding the effect resonnt mgnetic perturtions hve on ELMs, Plsm Phys. Control. Fusion 55 12 (2013) 124003. [5] LANG, P.T. et l., ELM control strtegies nd tools: sttus nd potentil for ITER, Nucl. Fusion 53 4 (2013) 43004. [6] SCHMITZ, O. et l., Three-dimensionl modeling of plsm edge trnsport nd divertor fluxes during ppliction of resonnt mgnetic perturtions on ITER, Nucl. Fusion 56 6 (2016) 66008.
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