Impurity transport of ion ITB plasmas on LHD

Transcription

1 EX9- Impurty transport of on ITB plasmas on LHD M.Yoshnuma, K.Ida, M.Yokoyama,.Suzuk, M.Osakabe, H.Funaba, K.Nagaoka, S.Morta, M.Goto, N.Tamura, S.Yoshmura, Y.Taker, K.Ikeda, K.Tsumor, H.Nakano, O.Kaneko, and LHD Experment Group Natonal Insttute for Fuson Scence (NIFS), Tok, Japan e-mal contact of man author: Abstract. Impurty hole, whch s an extremely hollow profle of carbon mpurty, s observed assocated wth ncrease of on temperature gradent n the Large Helcal Devce (LHD). Dfference of convecton velocty n the magnetc axs poston s clearly observed. It s suggested that the mpurty hole becomes strong as the magnetc axs s shfted outward. The dependence of the mpurty hole on the charge number, Z, s observed and the profle of hgher Z mpurty becomes more hollowed.. Introducton Smultaneous achevement of mproved energy confnement and low mpurty confnement s one of the crucal ssues to realze the plasma relevant to nuclear fuson because mpurtes cause reducton of the fuson power densty by enhancng the coolng of the plasma wth radaton and also by dlutng the hydrogen fuel. Impurtes tend to accumulate n the plasma wth an mproved confnement mode such as H-mode and nternal transport barrers (ITB) n tokamaks. Although mpurty accumulaton can be avoded n the ELMy H-mode dscharges, mpurty accumulaton s stll a problem n the dscharges of the ELM-free H-mode and the dscharges wth an nternal transport barrer. On the other hand, temperature screenng effect due to an on temperature gradent s expected n tokamaks and the outward convecton of mpurtes was confrmed n the mproved confnement plasma wth a weak densty gradent and a strong on temperature gradent on DIII-D []. Impurty accumulaton can be avoded by densty profle control wth an ntensve gas puff, and hgh energy and low mpurty confnement tmes are demonstrated n hgh densty H-mode (HDH) plasma n the Wendelsten 7-AS stellarator []. Radal electrc feld s strongly affected to the mpurty transport n non-axsymmetrc system. The mpurty convecton can be outward and the mpurty densty can be hollow due to the postve radal electrc feld n the electron-itb plasmas, where the electron temperature s much hgher than the on temperature. Although nward convecton of mpurtes s expected from neoclasscal theory n the plasma wth a hgh on temperature gradent because of the negatve radal electrc feld, extremely hollow mpurty profles due to an outward convecton s observed n the plasma wth a steep gradent of on temperature s observed n LHD.. Ion ITB dscharge on LHD LHD s a helotron-type devce equpped wth three hgh energy beam lnes that have the beam energy of 8keV wth negatve on sources (N-NBI) and a low energy beam lne that has the beam energy of kev wth postve on sources (P-NBI). The hgh energy beams have the advantage n the electron heatng, and these are njected to the plasma tangentally. The low energy beam has the advantage n the on heatng and also n the charge exchange spectroscopy and njected to the plasma perpendcularly. The on temperature, torodal flow velocty, polodal flow velocty, and ntensty of the charge exchange emsson from carbon

2 EX9-, -grad T (kev/m) = A B N-NBI njecton P-NBI njecton (.9s~) tme (s) 6 =.8 ITB nsde =.8 ITB outsde N-NBI njecton tme (sec), Intensty of VI Emsson (a.u.). T t=.s B T t=.s t=.s (b) t=.s A T t=.s A t=.s B t=.s t=.s t=.s t=.s (d) FIG.. Tme evoluton of on temperature and electron temperature. (b) Radal profles of on temperature and electron temperature. Tme evoluton of on temperature gradent. (d) Radal profles of charge exchange emsson of carbon. mpurty (VI, λ=9.nm) are measured wth the charge exchange spectroscopy usng the P-NBI as a probe beam. In order to subtract the background emsson, whch s cold component from the plasma perphery, the beam s modulated on and off. The radal electrc feld can be derved from the polodal flow velocty. arbon densty can be derved from the ntensty of the emsson from carbon mpurty by charge exchange between the fully onzed carbon n plasmas and the neutral beam. Fgures and (b) show the tme evoluton and radal profles of on and electron temperature n the plasma wth the magnetc axs =.6m, the magnetc feld strength B=-.7T, the col ptch parameter γ=., and the cancelng rate of the quadrupole fled B q =%, respectvely. Steep gradent of on temperature s produced at the md-rad, and the on temperature becomes hgher than the electron temperature at the center of the plasma. The hgher on temperature plasma s produced by njecton of N-NBI nto the plasma sustaned by the P-NBI. After the N-NBI njecton, the on temperature profle s changed from the hll shape, whch s moderately ncreased toward the center, to the bell shape, whch s a peaked profle wth a steep gradent at the mddle of the plasma rad, whle the electron temperature s moderately ncreased toward the center as shown n Fg. (b). We are nterested n the poston where the transport barrer exsts. It s dffcult to defne the ITB foot n the on ITB observed n the LHD plasma, because the barrer s not localzed and the on temperature gradent s seemed to be gradually ncreased toward the plasma center. The tme evoluton of the on temperature gradent shows clear dfference between nsde and outsde of the mproved confnement regon as shown n Fg.. The gradent s ncreased wth the rate of kev/m/s before the N-NBI njecton both near the md rad (=.8) and near the edge (=.8). The rate changes to kev/m/s after the N-NBI njecton near the md rad whle the rate s not changed near the edge. The boundary between the regon where the

3 EX9- n c ( 9 m - ) -grad (kev/m) n c ( 9 m - ) =.96 arbon =.96 t=.s (e) arbon t=.s =. =. t=.6s tme (sec) n c ( 9 m - ).. arbon t=.6s (f) n e ( 9 m - ) -grad (kev/m) n e ( 9 m - ) - - =.96 Electron t=.s =.7 =. =.8 t=.6s (b) (d) tme (sec) t=.s Electron (g) n e ( 9 m - ). Electron t=.6s (h) FIG.. Overvew of the dscharge wth the mpurty hole from Ref.[]. Tme evolutons of carbon densty, (b) electron densty, on temperature gradent, and (d) electron temperature gradent. Radal profles of carbon densty (e) before and (f) after the mpurty hole developed. Radal profles of electron densty (g) before and (h) after the mpurty hole developed. gradent changes to large and the regon where the gradent kept constant s the boundary of the mproved confnement regon. Although we have dscharges that the gradent near the edge decreases after the on ITB formaton, the dfferences across the boundary s clearly observed. Fgure (d) shows the profle of the ntensty of charge exchange emsson from carbon mpurty. The ntensty of carbon mpurty at the center of the plasma decreases as the on temperature ncreases, and t becomes one dgt smaller than that before the on temperature rse, whle the ntensty changes small at the edge as shown n Fg. (d). Ths observaton shows that the hollow profle of the carbon densty s produced n the hgh on temperature plasma wth steep gradent of the on temperature. We denote that the hollow profle observed wth the steep gradent of on temperature as Impurty Hole.. Impurty hole.. Extremely hollow profle of carbon mpurty The mpurty hole s characterzed by an extremely hollow profle of mpurty observed wth the steep gradent of on temperature n the confnement mproved mode. There s a dscharge n whch the mpurty hole s clearly observed []. A plasma wth steep on temperature gradent n the dscharge s produced wth njecton of a cylndrcal carbon pellet nto the plasma wth B=.676T, =.6m, γ=., and B q =%. The plasma s sustaned wth the N-NBI. The P-NBI s modulated on and off wth Hz as a probe beam for the charge exchange spectroscopy. The P-NBI also contrbutes to the on heatng. An extremely hollow profle of the carbon densty s observed n the decay phase after the carbon pellet njecton whle the on temperature gradent ncreases due to both the ncrease of the deposton power

4 EX9- and the mprovement of the on transport. Fgures -(d) show the tme evolutons of the carbon densty, electron densty, the gradent of on temperature and the gradent of electron temperature, respectvely. The carbon pellet s njected at t=.8s and the sze of the carbon pellet s φ. mm x.mm. The carbon and electron densty s ncreased wth the carbon pellet njecton and then the denstes decrease, whle the hydrogen partcles are suppled wth only the neutral beams. The decay speed of the carbon densty at the edge s comparable to the decay speed of the electron densty. The decay speed of the carbon densty near the plasma center s faster than that at the edge. The gradent of on temperature ncreases and reaches kev/m, whle the gradent of the electron temperature s weak (~ kev/m) durng the decay phase of the electron densty. The densty of the carbon near the center s smaller than that at the edge at earler tme of the dscharge. The densty of the carbon near the center becomes larger than that at the edge after the on temperature gradent becomes large. Ths observaton suggests that the on temperature gradent plays an mportant role of the mpurty transport whch enhances the decay speed of the carbon near the plasma center. Fgures (e)-(h) show the radal profles of the carbon densty and electron densty before and after the on temperature gradent rse. The profle of the carbon s peaked at the plasma center just after the carbon pellet njecton and becomes extremely hollow n the tme scale of a few hundred mllseconds durng the decay phase. On the contrary, the electron densty profle stays peaked and does not become hollow. Ths observaton ndcates that the bulk on profle s always peaked durng the decay phase because of the fuelng of the neutral beams. The central carbon densty drops to.x 7 m -, whch s only.% of the central electron densty at t=.6s. It should be noted that the hollow profle of the carbon densty s due to the faster decay of the carbon densty near the center than that at the edge and not due to the ncrease of the nflux. The transport analyss has been performed wth the tme slces of the carbon n the dscharge. It s well known that there s a sgnfcant contrbuton by off-dagonal terms of the transport matrx n the partcle transport, and the radal partcle flux Γ normalzed by the densty can be wrtten wth a sum of dffusve (dagonal) terms and non-dffusve (offdagonal) terms as Γ n = D + V, () n n r where n s the densty of the ons, D s the dffuson coeffcent, and V s the convecton velocty drven by the gradent of other parameters. The radal partcle fluxes of carbon and proton and the densty gradents of carbon and proton are calculated from the tme slces of the carbon and proton densty profles whch are obtaned expermentally. The radal profles of the partcle source due to the beam fuelng are calculated usng the FIT code based on the measured temperature and densty profles. The transport analyss has shown that the extremely hollow profle of carbon s attrbuted to the large postve convecton velocty and small dffuson coeffcent n the plasma core. The dependence of the convecton velocty of carbon on the gradent of suggests that the outward convecton of carbon mpurty drven by the on temperature gradent... Dependence on the poston of magnetc axs Radal poston of the magnetc axs,, s one of the mportant parameters, whch changes the helcal rpples and affects to behavours of helcal plasmas. It has been known that the change of the magnetc axs poston affects not only the neoclasscal transport but also the anomalous transport. There s a smulaton result whch shows turbulent transport s reduced wth enhancng zonal-flow generaton n the plasma wth nward shfted confguraton []. Thus t s mportant to nvestgate the dependence of varous phenomena n helcal plasmas on

5 EX9- T n ( 9 m - ) t=.6s t=.s t=.s w/o ITB R ax =.m t=.s t=.s t=.6s t=.6s R =.m ax wth ITB ITB regon (d) n e ( 9 m - ) n ( 9 m - ) T t=.s t=.s w/o ITB =.6m t=.s t=.s t=.s R =.6m (b) t=.s ax t=.s ITB regon the magnetc axs poston. We have nvestgated the dependence of the mpurty hole on the magnetc axs poston n hgh on temperature dscharges. Fgures - show the radal profles of on temperature n the case of =.m (nward shfted confguraton),.6m and.6m (outward shfted confguraton), respectvely. The strength of the magnetc feld are.789t,.7t, and.77t for =.m,.6m, and.6m, respectvely. There are no sgnfcant dfferences n the on temperature profle and the gradent. The on temperature gradent s.kev/m wth the ITB and t s kev/m wthout the ITB at =. n these plasmas. Fgure (d)-(f) show the radal profle of carbon and electron densty n the dscharges wth and wthout the on ITB. Development of hollow profle of carbon (mpurty hole) s observed assocated wth the ncrease of on temperature gradent due to the on ITB. The ft curves of the measured electron densty are also plotted wth dashed lne and t kept peaked or flat durng the dscharges. The depth and wdth of the hollowed carbon profles becomes deeper and wder n the plasma wth outward shfted confguraton than n the plasma wth the nward shfted confguraton. The dfferences n mpurty holes are also observed n the tme evoluton of the carbon densty profles. The tme t n Fg. (d)-(f) s the tme untl the end of the profle changes. The t s smaller n the plasma wth outer shfted confguraton than that n the plasma wth nner shfted confguraton. The hollow profle s always nsde the ITB regon n these dscharges because the steep gradent of on temperature s nsde the ITB regon. Fgure shows the dependences of convecton veloctes of carbon on the on temperature gradent for the varous magnetc axs postons at =. where the convecton velocty become large. hange of the convecton velocty s more senstve to change of the on temperature gradent n the plasma wth outward shfted confguraton than that n the plasma wth nward shfted confguraton. Ths result suggests that the mpurty hole becomes strong as the magnetc axs s shfted outward nsde the ITB regon where the on temperature gradent becomes large. Fgure (b) shows the convecton velocty at the on temperature gradent of -kev/m plotted as the functon of the strength of the helcal rpple effect whch can be changed wth the magnetc axs poston. The outward convecton (postve sgn) wth ITB (e).... n e ( 9 m - ) T n ( 9 m - ) t=.s t=.s w/o ITB =.6m t=.s t=.s t=.s =.6m t=.s t=.s wth ITB ITB regon FIG.. Radal profle of on temperature n the plasma wth Rax=.m, (b).6m, and.6m. Radal profle of carbon densty n the plasma wth (d) Rax=.m, (e).6m, and (f).6m. Dashed lnes n (d)-(f) shows the ftted lne of electron densty profle. (f).... n e ( 9 m - )

6 6 EX9- V (m/s) =.7m R =.6m ax R =.6m ax =.7m =. V (m/s) (b) -grad T = kev/m =..6m R =.6m.7m ax.m =.m grad T (kev/m).7m FIG.. Dependences of convecton velocty of carbon on the on temperature gradent n the plasma wth dfferent magnetc axs poston. (b) The convecton velocty when the on temperature gradent s -kev/m plotted as the functon of effectve rpple strength. ε eff. ncreases assocated wth the ncreasng the effect of rpples. Thus the clear dependence of the mpurty hole on the magnetc axs poston s observed... Dependence on charge number of mpurtes It s expected that the mpurty hole depends on the charge number, Z, by assumng that the radal electrc feld connects to the outward convecton of mpurty transport whch produces the mpurty hole. The densty profles of helum (Z=, λ=68.6nm), carbon (Z=6, λ=9.nm), and neon (Z=, λ=.9nm) s measured wth charge exchange spectroscopy n hgh on temperature dscharge. The plasma s produced wth magnetc feld strength of -.8T and magnetc axs poston of.6m The carbon mpurty s suppled wth a carbon pellet njecton at t=.8s. The helum and the neon are suppled by the gas puffng before the XS measurement. Fgure shows the radal profles of the on temperature and ft curve of the electron densty at t=.7s and.7s. The on temperature s already ncreased up to larger than the electron temperature at the plasma centre at t=.7 and kept the profle durng few hundred mllseconds. The profle of the mpurtes at earler tme and at later tme are shown n Fg. (b) and, respectvely. It s clearly shown that the profles of mpurtes become hollowed. The hollow profle of carbon and neon become deeper near the plasma centre. In order to clarfy the dfference of the hollowness between three mpurty profles, the normalzed gradent of densty s evaluated. Fgure 6 shows the radal profles of the normalzed gradent of densty of helum, carbon, and neon before and after the mpurty hole developed n the plasma wth the on ITB. The on temperature and the gradent are acheved kev and 6keV/m at =.6, respectvely. The normalzed gradent for the three mpurtes are about m - at =.6 and the profles are smlar before the mpurty hole developed. The normalzed gradent of the carbon and neon s ncreased up to m - and 8m -, respectvely, by developng the mpurty hole wthn three hundred-mllseconds, whle there s no sgnfcant change n the gradent of the helum profle. The result shows that the profle of hgher Z on speces becomes more hollowed. Although the development of the hollow profle s not observed n the helum profle, no accumulaton of helum s also observed n the hgh on temperature dscharges. The Z dependence of the mpurty hole s clearly observed.

7 7 EX9- t=.7s t=.7s electron (b) electron T, T e. Dscusson There s a wonder that the mechansms of the mpurty hole. The neoclasscal transport s a frst FIG.. canddate Radal for profles the domnant of on temperature player on the and mpurty electron temperature. hole. The plasmas Radal profles n whch of electron, observed the helum, mpurty carbon, hole and s neon n low densty collsonalty (b) before regon and due after to the hgh mpurty on and hole electron developed. temperature and low densty. Neoclasscal predcton and measurement wth heavy on beam prove show the dn/dr/n (/m) t=.7s t=.7s t=.s t=.s Helum (Z=) N ( 9 m - ) dn/dr/n (/m) (b) carbon helum neon t=.7s arbon (Z=6) - t=.7s dn/dr/n (/m) t=.7s t=.7s Neon (Z=) FIG.6. Radal profles of normalzed densty gradent of helum, (b) carbon, and neon profle before and after the mpurty hole developed. N ( 9 m - ) helum carbon neon negatve radal electrc feld near the plasma centre []. Neoclasscal predcton wth GSRAKE code [6], whch can calculates partcle transport n stellarator, shows the nward convecton of mpurty due to the negatve radal electrc fled near the plasma center. The predcted nward convecton s opposte to the observed mpurty convecton whch drects outward []. It should be note that the predcted partcle flux lacks the contrbutons of the momentum transfer between the bulk on and mpurtes. Ths contrbuton suggests that mpurty profle can be more peaked wth peaked densty profle of bulk on, and more hollowed wth peaked temperature profle of bulk on []. The contrbuton of the temperature gradent s well known as the temperature screenng. The observed outward convecton of mpurty ncreases assocated wth the ncrease of on temperature gradent. Although the contrbuton s seemngly domnant to the outward convecton of mpurtes, the contrbuton can not be explaned the clear dependence on the magnetc axs poston. A non-lnear phenomena that the carbon densty suddenly decrease after the on temperature gradent reaches a crtcal value whle the carbon densty ncreased just before the crtcal value has been observed [7]. Thus the observatons of the mpurty hole can not be explaned unformly by the neoclasscal transport. The turbulence transport therefore can be a next canddate for the domnant player of the mpurty transport n the hgh on temperature dscharges n LHD.

8 8 EX9-. Summary An extremely hollow profle of carbon mpurty s observed n the plasma wth the steep gradent of the on temperature. We denoted the hollow profle as Impurty hole. Large outward convecton and small dffuson of the mpurty s a key to produce the mpurty hole. Dependence of the mpurty hole on the magnetc axs poston s clearly observed n the plasma wth steep gradent of on temperature. It s suggested that the mpurty hole becomes strong as the magnetc axs s shfted outward. Dependence of the mpurty hole on the charge number, Z, s also observed. The densty profle of hgher Z on speces becomes more hollowed. Acknowledgement We would lke to thank techncal staff for ther effort to support the experment n LHD. Ths work s partly supported by NIFSULHH. References [] WADE, M. R. et al., Expermental onfrmaton of Impurty onvecton Drven by the Ion-Temperature Gradent n Torodal Plasmas Physcal Revew Letters 8 () 8. [] MORMIK, K et al., New Advanced Operatonal Regme on the W7-AS Stellarator Physcal Revew Letters 89 (). [] YOSHINUMA, M. et al., Observaton of an mpurty hole n the Large Helcal Devce, Nuclear Fuson 9 (9) 6. [] WATANABE, T.-H. et al., "Reducton of Turbulent Transport wth Zonal Flows Enhanced n Helcal Systems" Physcal Revew Letters (8)9. [] IDO, T. et al., "Expermental study of radal electrc feld formaton and electrostatc potental fluctuaton n the Large Helcal Devce", n 7th EPS onference on Plasma Physcs (submtted to Plasma Phys. ontrol. Fuson) (). [6] BEIDLER,. D. et al., A general soluton of the rpple-averaged knetc equaton (GSRAKE), Plasma Physcs and ontrolled Fuson 7 (99) 6-9. [7] IDA, K. et al., Observatons of an mpurty hole n a plasma wth an on nternal transport barrer n the Large Helcal Devce, Physcs of Plasmas 6 (9) 6.