Recently, a strong up-down asymmetry in the poloidal distributions of diverted plasma flows has been observed in the l = 3/m = 9 Uragan-3M torsatron, in many features similar to what have been observed in the l = 2 Heliotron E heliotron/torsatron. With this asymmetry, the predominant outflow of the diverted plasma is directed with the ion toroidal drift. On this basis, the asymmetry can be related to the space non-uniformity of the charged particle loss. In the work reported, the magnitude of divertor flow in U-3M and the vertical asymmetry in its distribution are studied as functions of the heating parameter P/, P being the power absorbed in the plasma, and are juxtaposed with corresponding P-related changes in the density and fast ion content in the plasma. As P/ increases, an increase of fast ion content and of particle loss, on the one hand, and an increase of divertor flow magnitude and of vertical asymmetry of the flow, on the other hand, are observed. A mutual accordance between these processes validates the hypothesis on a dominating role of fast particle loss in formation of vertical asymmetry of divertor flows in helical devices

Bondarenko, V.N.

Chechkin, V.V.

Grigor’eva, L.I.

Konovalov, V.G.

Kulaga, A.Ye.

Litvinov, A.P.

Lozin, A.V.

Masuzaki, S.

Mironov, Yu.K.

Nazarov, N.I.

Slavnyj, A.S.

Smirnova, M.S.

Sorokovoy, E.L.

Tsybenko, S.A.

Volkov, E.D.

Yamazaki, K.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

EFFECTS OF PLASMA HEATING ON THE MAGNITUDE AND DISTRIBUTION OF PLASMA FLOWS IN THE HELICAL DIVERTOR OF THE URAGAN-3M TORSATRONV.V. Chechkin, L.I. Grigor’eva, E.L. Sorokovoy, M.S. Smirnova, A.S. Slavnyj, E.D. Volkov,  N.I. Nazarov, S.A.Tsybenko, A.V. Lozin, A.P. Litvinov, V.G. Konovalov, V.N. Bondarenko, A.Ye. Kulaga, Yu.K. Mironov, S. Masuzaki∗, K. Yamazaki∗Institute of Plasma Physics, National Science Center “Kharkov Institute of Physics and Technology”, Akademicheskaya st. 1, 61108 Kharkov, Ukraine∗National Institute for Fusion Science, Oroshi-cho 322-6, Toki-shi 509-5292, Japan     Recently, a strong up-down asymmetry in the poloidal distributions of diverted plasma flows has been observed in the  l = 3/m = 9 Uragan-3M torsatron, in many features similar to what have been observed in the  l = 2 Heliotron E heliotron/torsatron. With this asymmetry,  the predominant outflow of  the diverted plasma is directed with the ion toroidal drift. On this basis, the asymmetry can be related to the space non-uniformity of the  charged particle loss. In the work reported, the magnitude of divertor flow in U-3M and the vertical asymmetry in its distribution are studied as functions  of  the  heating  parameter  P/ en ,  P being  the  power  absorbed  in  the  plasma,  and  are  juxtaposed  with corresponding P-related changes in the density en  and fast ion content in the plasma.  As P/ en  increases, an increase of fast ion content and of particle loss, on the one hand, and an increase of divertor flow magnitude and of vertical  asymmetry of the flow, on the other hand, are observed. A mutual accordance between these processes validates the hypothesis on a dominating role of fast particle loss in formation of vertical asymmetry of divertor flows in helical devices. PACS: 52.55.Fa; 52.55.Rk1. INTRODUCTION     Experimental studies of spatial distributions of plasma flows in the natural helical divertor of the l = 2 Heliotron E  (H-E)  heliotron/torsatron  with  NBI  and  ECH  have shown  [1,2]  that  a  strong  vertical  asymmetry  of  these distributions  is  possible  in  helical  devices.  This conclusion  has  been  confirmed  by  measurements  of plasma flow distributions in the helical divertor of the l = 3 Uragan-3M (U-3M) torsatron with RF heated plasmas [3]. In many characteristics, the asymmetries in H-E and U-3M are similar, despite a substantial difference of these devices  in  magnetic  configuration,  plasma  heating methods and plasma parameters, this being an indication of  universality  of  this  effect.  Recently,  some manifestations of vertical asymmetry in the distribution of diverted plasma parameters have been also revealed in the l = 1 Heliotron J device with a helical magnetic axis [4].     The existence of many-fold difference in the values of particle  and  energy  fluxes  to  symmetrically  positioned target  plates  can  raise  serious  problems  with  the  heat removal  in  future  helical  devices  of  ITER  scale. Therefore,  a  search  for  the  nature  of  divertor  flow asymmetry should become an important issue of divertor research.     With magnetic  field reversal  in U-3M,  the larger divertor flux is always observed on the ion toroidal drift side. On these grounds it is supposed [3] that a substantial contribution to the asymmetry is made by those fast ions, which are trapped into helical magnetic field ripple wells and,  not  “filling”  the  rotational  transform,  left  the confinement volume due to the non-compensated toroidal drift  [5].  Such a  possibility  has been confirmed by the results of calculations of angular distribution of particle direct loss in U-3M [3].           The objective of this work is to find out a possible link between plasma heating in U-3M and the magnitude of divertor flow and the asymmetry of its distribution. In the studies reported, a qualitative correlation has been found between the heating power, the rate of particle loss and fast ion content, on the one hand, and the magnitude of divertor flow and degree of its vertical asymmetry, on the other hand.                    2. EXPERIMENTAL CONDITIONS     In the l = 3/m = 9 U-3М torsatron (Ro = 1 m, a  ∼ 0.1 m,  ι( a )  ≈ 0.4) an open helical divertor is realized. The magnetic field Bφ = 0.7 T is generated by the helical coils only. A hydrogen plasma is RF produced and heated (ω ≤ ωci). The line-averaged electron density  en  can attain  ∼1019 m-3. The RF power P absorbed in the plasma attains 240 kW in the 50 ms pulse.      The diverted plasma is detected by 78 plane 1.25×0.8 cm2  Langmuir  probes.  Six  probe  arrays  are  arranged poloidally in the spacings between the helical coils in two half-field-period  separated  symmetric  poloidal  cross-sections of the torus φ = 00 (A-A) and φ = 200 (D-D) as is shown in Fig. 1. The gap between the plates in an array (0.1 cm) is much less than the plate size in the polodal direction (1.25 cm).        Two operating regimes are used specified by the pressure  of  hydrogen  admitted  continuously  into  the vacuum  vessel.  In  the  “lower  pressure  regime”  (LPR, units  10-5 Torr),  the plasma occurs at  P ≈ 80 kW. The level of quasi-steady state density  en  is determined by the  balance  between  ionization  of  the  gas  entering  the confinement volume and plasma escape. At  P ≈ 80 kW en  takes  (2.5÷1.5)×1018 m-3.  As  P increases,  en  gradually falls up to (1.0÷0.7)×1018 m-3 at  P ≈ 240 kW (Fig. 2). The existence of a heating-related plasma loss is also evidenced by a short-time en  rise occurring after RF pulse switched off (Fig. 2, inset).48                        Problems of Atomic Science and Technology. 2002. № 4. Series: Plasma Physics (7). P. 48-50 80 120 160 200 2401.01.52.02.501230 20 40 60 80123   , 10   m_ e18-3I   RFtime, msdensity     , 10   m_e18-3RF power   , kW , a.u.n nP120 140 160 180 200 220 240012345678910P, I  /n  , 1575 eV450 eV135 eVn_ ekWrel. units1 3 5 7 9 11 13 15 17051015200513579111315I s, mANI s, mATOPBOTTOMNA-AFig. 1. Disposition of Langmuir probe arrays in the symmetric poloidal cross-sections A-A and D-D. Shown are  the helical coils I, II, III and the calculated edge structure of the magnetic field. The probe numbering of  interest in A-A: top spacing, 1-17; bottom spacing, 1-15.  in D-D: top spacing, 1-9; bottom spacing, 1-8.   Fig.  2. Quasi-steady state  line-averaged electron  density  en as a function of absorbed RF power P at a fixed  hydrogen  pressure.  In  the inset, time traces of  RF  current  in  the antenna  IRF  (envelope)  and density en .          In  the  “higher  pressure  regime”  (HPR,  units  10-4 Torr,),  en  attains  (7÷10)×1018 m-3.  When  studying  the power dependence of divertor flow magnitude, both ion saturation current Is and P are normalized by en . In LPR, the  range  of  P/ en values  (30÷320)  kW/1018 m-3 is covered. In HPR, P/ en can be reduced up to ∼10 kW/1018 m-3.        With P ≈ 240 kW in LPR, the ECE-estimated electron temperature attains Te(0) ≈ 300÷400 eV, while it does not exceed 20 eV in HPR.     The energy spectrum (ES) of plasma ions as measured by an CX neutral energy analyzer oriented normally to the torus midplane is distinguished by a slowly decaying high energy tail. For P/ en = 300 kW/1018 m-3 a “perpendicular temperature”  of  Ti1 ∼60  eV  can  be  assigned  to  the majority (∼90%) of ions. Also, two minor groups of faster ions with “temperatures”  Ti2 ≈ 326 eV and  Ti3 ≈ 900 eV can be conventionally separated. The presence of the  Ti1 and Ti2  groups is confirmed by the form of CV 227,1 nm profile. The high energy tail also remains for lower P/ en . However,  as  P increases,  the  relative  content  of  low energy  ions  (Ti1 group)  does  not  change  appreciably, while that of higher energy ions grows, with the growth rate increasing with energy (Fig. 3).     Fig. 3. Relative       contents In/ en  of ions     with three fixed     energies as functions               of heating power P.      The heating related behavior of  en  and ion energy can  cause  to  a  great  extent  the  pecularities,  which  are observed in the behavior  of  diverted plasma during the heating.3. EFFECTS OF PLASMA HEATING ON DIVERTOR FLOWS     The effect of plasma heating on the magnitude and spatial  distribution  of  diverted  plasma  flows  in  the symmetric cross-sections A-A and D-D is displayed most clearly in the flows, entering the top and bottom spacings between  the  helical  coils.  As  an  example,  the  polodal distributions of these flows are shown in Fig. 4 (A-A) as                                                                         Fig. 4. Ion saturation                current Is vs probe                             number N in the top and                            bottom spacings             between the helicalcoils  in the poloidal cross-  section A-A.            Is(N) plots taken in LPR with P ≈ 240 kW and Bφ directed counterclockwise.  The  higher  maxima  at  the  top  and bottom belong to the divertor legs located closer to the major axis. The  Is maxima in symmetric legs are several times different at the top and bottom, with the larger flow being directed upward, i.e., with the ion B×∇B drift.      In Fig. 5 presented are Is/ en  (maximum values) vs P/ en  plots for the top and bottom spacings in A-A. The data for both LPR (○) and HPR (●) are used. The same plots for D-D have a similar form. In both cross-sections Is/ en  tends to  grow with  P/ en on the ion toroidal  drift side. The maximum Is/ en  values are comparatively closeat the top and bottom for the smallest P/ en  values,   49A-AIIIIIIIIφ  =  2 0 oφ  =  0o11791BOTTOMOUTBOARD811234040-4040-4040cmcmcmcmINBOARDBOTTOMTOPIIID-DTOP-40I-40611150 100 200 3000510/    , kW/10    mPne 18 -3asymmetry index, D-DA-A_3 12 21 30 39 48 57 66 75 84 93714212835424956637077ion energy, 10 eVdensity, 10   cm11-3νbplsbFig. 5. Maximum values of  en - normalized  ion saturation current Is vs  heating parameter P/ en  in the top and bottom spacings between the  helical coils in the poloidal cross-section A-A.10 – 20 kW/1018 m-3, their ratio α not exceeding ∼2. As P/en  increases, the flow grows more rapidly at the top, and α can attain ∼10 for P/ en  ∼ 300 kW/1018 m-3. The same holds for D-D. Thus, the degree of vertical asymmetry of divertor  flows  is  a  rising  function  of  heating  power (Fig. 6).                  Fig. 6. The asymmetry index α  as a function of  heating parameter P/ en  in the poloidal cross-sections A-A and D-D. The lower pressure regime.4. SUMMARY AND DISCUSSION     In summary, the following processes are observed at the same time as the absorbed RF power increases.In the confinement volume: - charged particle loss increases; this is displayed in a reduction of quasi-steady state value of en     (Fig. 2); - relative fast ion content increases, with the growth rate being a rising function of ion energy (Fig. 3).In the divertor region:- the total flow of diverted plasma increases (Fig. 5);- a predominant rise of the diverted plasma flow in the ion toroidal drift direction, with the asymmetry index α rising with power (Fig. 6).         In view of ideas having been developed in [3], the following  causality  can  be  suggested  for  the  processes observed.  The heating power increase results in a rise of fast  ion  relative  content  (Fig.  3).  Due  to  a  poor confinement of these particles, particle loss increases and the density en  decreases (Fig. 2), on the one hand, while the diverted plasma flow increases (Fig. 5), on the other hand.  A dominating  contribution  to  the  particle  loss  is made by those fast banana ions,  which become trapped into the  helical  ripple  wells  and leave  the  confinement volume due to the toroidal drift. Therefore, a dominating rise  is  undergone  by  plasma  flows  in  the  top  spacings between the helical coils (Fig. 5). In turn, this results in a rise of vertical asymmetry of divertor flow (Fig. 6).      The range of P/ en  values used in these studies allows to  cover  a  wide  range  of  characteristic  collision frequencies,  which  govern  the  character  of  charged particle loss from the confinement volume (Fig. 7). In the higher  density  discharges  (small  P/ en ,  ●  in  Fig.  5) predominant orbit losses are limited by the plato (pl) or banana  (b)  diffusion  and,  therefore,  the  asymmetry  of divertor flow is small. With  P rise and  en  decrease, the diffusion  regime  of  ion-ion  collisions  shifts  into  the region  of  lower  frequencies  with  higher  transport coefficients. Since the diffusion in the velocity space is of stochastic character in its nature, it  results in a uniform scattering of particles in the co-ordinate space. Therefore, the rise of diffusion (non-direct) ion loss can lead to only total value of the divertor flow increase without changing its asymmetry. In the lower density discharges (large  P/en , ○ in Fig. 5), the diffusion in the 1/ν collisional regime typical for most of  >250 eV ions,  does not  prevent the banana ions to be trapped into helical ripple wells. As a result, a considerable fraction of these particles enters the divertor region in the upper half of the torus, increasing the vertical asymmetry of the divertor flow.           Fig. 7. Ion-ion collision time vs density and ion  energy. The boundaries of  correspondingdiffusion regimesare indicated bybold lines.      This work was carried out in collaboration with NIFS (Toki, Japan) by the Program LIMEREFERENCES [1] Mizuuchi T., Voitsenya V.S., Chechkin V.V. et al       1999 J. Nucl. Mater. 266-269 1139 [2] Chechkin V.V., Voitsenya V.S., Mizuuchi T. et al        2000 Nucl. Fusion 40 785 [3] Chechkin V.V., Grigor’eva L.I., Smirnova M.S. et al         2002 Nucl.  Fusion 42 192 [4] Mizuuchi T., Ang W.L., Kobayashi T. et al 2002        Asymmetric divertor plasma distribution observed in        Heliotron J ECH discharge 15th International        Conference on Plasma Surface Interactions in         Controlled Fusion Devices Gifu, Program and Book         of Abstracts  Paper P1-64 (Toki, Japan: NIFS).  [5]  Gurevich А. V., Dimant Ya. S. 1990 Kinetic theory         of fast particle  transport in tokamaks, in Reviews of 500 50 100 150 200 250 300 3500510152005100 50 100 150 200 250 300 350In /    , mA/10     m_ e18-3P/    , kW/10    msne_ 18 -3A-ATOPBOTTOMN =  4N =  13(a)(b)         Plasma Physics, Vol. 16 (Kadomtsev, B.B., Ed.),         Consultants Bureau, New York, p.351

Effects of plasma heating on the magnitude and distribution of plasma flows in the helical divertor of the Uragan-3M torsatron

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Effects of plasma heating on the magnitude and distribution of plasma flows in the helical divertor of the Uragan-3M torsatron

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)