Multipole Vortices in an ECR Plasma

Transcription

1 J. Plasma Fusin Res. SERIES, Vl. 4 (2001) 'l Multiple Vrtices in an ECR Plasma OKAMOTO Atsushi*, ISHIHARA Tatsuz, NAGAOKA Kenichi, YOSHIMURA Shinjit and TANAKA Masayshi y.r Nagya University, Nagya , Japan lnatinal Institute fr Fusin Science, Tki , Japan (Received: 5 December 2000 / Accepted: 27 September 2001) Abstract Spntaneus frmatin f multiple vrtices in an ECR plasma is experimentally examined t investigate self-rganizatin and structure frmatin in a rtating plasma. These vrtices are characterized by steep density prfiles with the peak densities 4-5 times higher than that f backgrund plasmas. The vrtex pattern des nt mve with time, while the plasma rtates in the directin f electrn diamagnetic drift. It is fund that there exists axial flw reversal in the rim f vrtices. Keywrds: multiple vrtices, self-rganizatin, structure frmatin, E x B drift, shear flw, ECR plasma 1. Intrductin Vrtex, a lcalized vrticity distributin, is a cmmn entity f self-rganizatin in rtating fluids, and many wrks have been dne in rdinary fluids and superfluids. Recently, vrtex frmatin and interactin in a nn-neutral plasma have been attracting much attentin [1,2], and discussed in relatin t 2- dimensinal Eulerian fluid. On the ther hand, a few experimental wrks have been dne in neutral plasmas, in which interplay between a variety f prcesses such as cllective effect, nnlinear effect, viscsity and cmpressibility etc. is expected t be imprtant. Thus, in the present wrk it is f primary interest t understand the inherent features f vrtex generated in a neutral plasma [3,4]. Here, we cncentrate ur attentin t characterize the multiple vrtices bserved in an ECR plasma. The experimental results n the spatial structure, density prfile, and flw velcity field f the multiple vrtices are presented. In Sec. 2, the experimental setup is described, and the results and discussins are presented in Sec. 3, where the mst simplest structure, a pairedvrtices is described in detail. It is fund that the tw * Crrespnding authr' s e -mail : n fs. ac.jp vrtices have the same plarity in vrticity, and that the ins flw in the directin f electrn diamagnetic drift. It is als fund that there exists an axial flw reversal in the rim f vrtices. 2. Experimental Setup The experiments have been perfrmed in the High Density Plasma Experiment (HYPER-I) device ar Natinal Institute fr Fusin Science [5]. A schematic view f the HYPER-I device is shwn in Fie. 1. The Cllng Water Fig. 1 Schematic f HYPER-I device by The Japan Sciety f Plasma Science and Nuclear Fusin Research 363

2 Okamt A. et al., Multiple Vrtices in an ECR Plasma HYPER-I device cnsists f ten magnetic cils and a cylindrical chamber with the inner diameter f 30cm and axial length f 200 cm. The magnetic field lines are weakly diverging alng the chamber axis, and make a s-called magnetic beach cnfiguratin. Plasmas are prduced and sustained by electrn cycltrn resnance (ECR) heating with a 2.45 GHz micrwave injected frm an pen end f the vacuum chamber. Under the typical experimental cnditin, the ECR pint is lcated at 111 cm frm the micrwave launching psitin. The rati f micrwave frequency t electrn cycltrn frequency, al)ee, at the injectin psitin is 1.1. The density and electrn temperature are typically n < 10r3 cm-3 and Z" = 5 ev, respectively, with the peratin pressure p = 3 x 10-2 Trr (Argn) and 5-10 kw micrwave input pwer. In flw velcity is measured with mvable directinal Langmuir prbes [6] frm the radial prts. A three-dimensinal (3-D) mtr drive is equipped at an pen end f the chamber t measure the density prfile. A CCD camera is als installed t btain the end view image f vrtices. 3. Results and Discussins 3.1 Multiple Vrtices Frmatin Figure 2 shws the end view images f the bserved structures fr different magnetic field intensities. When the peratin pressure is higher than 2 x 10-2 Trr, lcalized structures are spntaneusly frmed in the plasma. These structures have been fund t be rtating plasma clumns, and are called as multiple vrtices hereafter. The pattern f structures is statinary during the whle perid fdischarge, and the number f ples (bright regins in the figure) changes with the external magnetic field. When the cil current is 1"11 = 100 A (Fig' 2(a))' the lwest magnetic field intensity in this experiment, three large vrtices are frmed in the center f the chamber surrunded by many small vrtices. When the cil current is increased (1""u = 103 A), the center vrtex disappears, while tw vrtices in the upper side and the bttm side grw as seen in Fig. 2(b). Althugh the number f large vrtices increases frm three t fur, the small vrtices merge in the peripheral regin s that the ttal number fvrtices decreases. Fr higher magnetic field (Fig. 2(c) and (d))' Fig.2 End-view images f multiple vrtices: (a) 4.r = 100 A, (b) /ir = 103 A, (c) /"",r = 106 A, (d) ["u = 110 A, (e) l.r= 120 A, and (f) /"ir = 130 A. 364

3 Okamt A. et al., Multiple Vrtices in an ECR plasma further merging f vrtices ccurs and small vrtices in the peripheral regin disappear. The simplest structure bserved in the present experiment cnsists f tw large vrtices as shwn in Fig. 2(e), which is referred t as vrtex pair hereafter. The lcalized structures disappear fr the cil current abve 1.1 = 130 A (Fig. 2(f)), and unifrm plasmas are prduced. All these structures presented in Fig. 2(a)-(e) have a cmmn feature. i.e.. rtatinal symmetry with z-radian rtatin arund the center axis. A change in the external magnetic field crrespnds t a shift f ECR pint alng the chamber axis. Increasing the magnetic field, the ECR pint mves away frm the micrwave injectin windw. The experimental bservatin shws that the number f vrtices and the cnfiguratin are very sensitive t the lcatin f ECR pint. This means that the frmatin f multiple vrtices and generatin f glbal cnvectin are strngly affected by the lcatin f plasma prductin regin. The density prfile and in flw structure have been measured with a Langmuir prbe and a directinal Langmuir prbe, respectively, the results f which are presented in the next subsectin. 3.2 Density Prfile and Flw Field Structures f Vrtex Pair In this subsectin, the characteristic features f the vrtex pair are presented. Density cntur fthe vrtex pair measured with a Langmuir prbe is shwn in Fig. 3. In accrdance with the tw bright val regins in the CCD image (Fig. 2(e)), there are tw density humps. The full width at half maximum f the density hump is abut 10 cm x 5 cm n the crss sectin perpendicular t the magnetic field. The peak density is 6 x 1012 cm-3, and is 4-5 times higher than that f the backgrund plasma with a flat prfile ver the whle crss sectin. T examine the axial structure f the vrtices, we have measured the density prfile at axially different psitins. The results are shwn in Fig. 4, in which Fig. 4(a) is fr 2O cm frm the micrwave launching psitin, and Fig. 4(b) fr 110 cm. The density prfile in Fig. a(b) is identical t that measured alng the hrizntal crd passing thrugh the center f vrtices (y = 0) in Fig. 3. The density prfiles in Fig. 4 have cmmnly tw maxima 4-5 times higher than the backgrund density. The distance between the peaks is abut 9 cm fr Fig. 4(a), and 10.5 cm fr Fig. 4(b), which is attributable t rhe weak diverging magnetic field. Accrding t these results, it is cncluded that the vrtex pair is axially extends ver 90 cm. E 't Fig. 3 Density cntur f vrtex pair. The nrmalized by the maximum density..= g =d L IE E C.! tr5 ti L G Q tr (a) (b) u,z density is 100 Fig. 4 Density prfile at tw different axial psitins: (a) 20 cm, (b) 110 cm frm the micrwave injectin windw. The density prfile is nrmalized by the maximum densitv. 365

4 The azimuthal in velcity assciated with the vrtex pair is shwn in Fig. 5. The velcity measurements have been dne using a directinal Langmuir prbe [6], alng the hrizntal crd passing thrugh the center f vrtices. The directin f the external magnetic field is perpendicular t this figure' and the directin f magnetic field vectr is indicated in the upper right f the figure. The hatched regins in Fig. 5 crrespnd t the full width at half maximum f the density humps, the dashed lines indicate the psitins f the maximum density. The in velcity is nrmalized by the in sund velcity Cs. As seen in Fig.5, the azimuthal in flw velcity is prprtinal t the distance frm the center f vrtices, exhibiting a rigidlike rtatin. The tw vrtices rtate in the same directin (cunterclckwise rtatin). The electrn flw velcity, which cannt be accurately determined by the directinal Langmuir prbe, is fund t be in the same directin f in flw. When the magnetic field directin is inverted, n remarkable change has been bserved in the vrtical pattern except fr a change in the azimuthal psitin (tilting f the hrizntal crd with respect t chamber axis). Figure 6 shws the azimuthal in velcity fr the inverted magnetic field case. The asymmetric prfile in the azimuthal in velcity is attributable t the tilting f vrtex pattern. It is easily fund that the rtatin f in flw changes its directin in this case. This result implies that the in rtatin is due t F x B drift with a frce F acting in the radial and inward directin. The ptential prfile measurement revealed that the electric field in the vrtex is radially utward (< 1 V/cm) and that the rtatin directin is ppsite t that f E x B drift. It is wrth nthing that E x B rtatin velcity with a electric fielde- I V/cm is I x 105 cm/s = 0.3 C5. The experimental result shws that there exists an inward frce, which dminates an electric field f the rder f 1 V/cm. The rigin f inward frce is nt fully understd yet, and the experiments are nw in prgress. The axial in flw prfile is shwn in Fig. 7, which is independent f the directin f the magnetic field. In the cre regin f each vrtex (hatched regin), the directin f axial flw is the same as that f 1t grad-b frce, where p is the magnetic mment. On the ther hand, in the peripheral regins, the flw directin is ppsite t that f the cre regins, exhibiting the existence f strng shear in axial velcity at the edge regin. The vrtex pair is cnsidered t be cnfined within this shear layer. Okamt A. et al., Multiple Vrtices in an ECR Plasma.2 at C) (J Fig. 5 Azimuthal velcity Prfile x lmml BO B8 Fig. 6 Azimuthal velcity prfile with reversed magnetic field..2 \ 0.0 N -0.2 x lmml Fig. 7 Axial velcity prfile 366

5 Okamt A. et al., Multiple Vrtices in an ECR Plasma 4. Summary We have bserved spntaneus frmatin f multiple vrtices in an ECR plasma. The vrtex pattem is statinary during the whle perid f discharge. The number f ples decreases with increasing the external magnetic field, and the vrtex pair remain as the simplest structure. It is fund that tw vrtices have the same plarity in vrticity and the directin f in rtatin is ppsite t that fe x B drift. The existence f strng shear layer in the axial velcity seems t be imprtant fr the cnfinement and arrangement f vrtices. References tll K.S. Fine et a/., Phys. Rev. Lett. 67, 588 (1991). t2l Y. Kiwamt et al., J. Phys. Sc. Jpn. 68, 3766 (1999\. [3] M.Y. Tanaka et al., Prceedings f 1996 Internatinal Cnference n Plasma Physics 1650 (1996). t4l tsl 6l M. Kn and M.Y. Tanaka, Phys. Rev. Lett. 84, 4369 (2000). M. Tanaka et al., J. Phys. Sc. Jpn. 60, 1600 (199r). K. Nagaka et al., J. Phys. Sc. Jpn. 70, 131 (2001). 367