The first experimental observation of a plasma hole structure in an ECR argon plasma is reported. The plasma hole is a cylindrical density cavity, which is formed spontaneously in the center of the plasma. The steep density gradient between the hole plasma and the ambient plasma is sustained by a thin interfacial layer, the width of which is a few ion Larmor radii. Supersonic rotation is found over the large cross-sectional area of Ar plasma hole. The axial flow can also exceed ion sound speed. Spectral measurements revealed that the neutral density profile exhibits a hole structure as well as that in ion density, the diameter of which is much shorter than the mean free path of neutral particles

Atsushi OKAMOTO

Masayoshi Y. TANAKA

OKAMOTO  Atsushi

Shinji YOSHIMURA

TANAKA  Masayoshi Y.

YOSHIMURA  Shinji

National Institute for Fusion Science (NIFS-Repository)

©2004 by The Japan Society of PlasmaScience and Nuclear Fusion Research1J. Plasma Fusion Res. SERIES, Vol. 6 (2004) 000-000Corresponding author’s e-mail: yshinji@nifs.ac.jpObservation of Plasma Hole in an ECR Ar PlasmaYOSHIMURA Shinji, OKAMOTO Atsushi1 and TANAKA Masayoshi Y.National Institute for Fusion Science, Toki, 509-5292, Japan1Nagoya University, Nagoya, 464-8602, Japan(Received: 9 December 2003 / Accepted: 11 March 2004)AbstractThe first experimental observation of a plasma hole structure in an ECR argon plasma is reported. The plasmahole is a cylindrical density cavity, which is formed spontaneously in the center of the plasma. The steep densitygradient between the hole plasma and the ambient plasma is sustained by a thin interfacial layer, the width of which isa few ion Larmor radii. Supersonic rotation is found over the large cross-sectional area of Ar plasma hole. The axialflow can also exceed ion sound speed. Spectral measurements revealed that the neutral density profile exhibits a holestructure as well as that in ion density, the diameter of which is much shorter than the mean free path of neutralparticles.Keywords:plasma hole, structure formation, macroscopic structure, vortex, supersonic ion flow, rotating plasma, ECR plasma, directional Langmuir probe1. IntroductionAn electric field in a magnetized plasma inevitablydrives rotating motion of ions by E B# drift, giving rise tothe formation of macroscopic vortical structures, which hasbeen a topic of interest in plasma physics because of therelation to self-organization and to transport phenomena.Various types of vortical structures have recently beeninvestigated in both nonneutral plasmas [1,2] and quasi-neutral laboratory plasmas [3-5]. One of the structuresobserved in electron cyclotron resonance (ECR) plasmasappears with sharp density depletion around the central axis,that is, a cylindrical density cavity is spontaneously formed inthe plasma. Since a nonluminous cavity region, which seemsto be a hole in the plasma, is clearly seen in the perspectiveimage taken by a CCD camera, this structure is referred to asplasma hole [4]. So far, the plasma hole has been observedonly in an ECR helium plasma (He plasma hole).In this paper, the first experimental observation of aplasma hole structure in an ECR argon plasma (Ar plasmahole) is reported, especially pertaining to the density profileand the ion flow velocity field. The existence of supersonicion flow over the large cross-sectional area is a noticeablecharacteristic of Ar plasma hole in comparison with Heplasma hole. A two-dimensional density profile of neutralparticles associated with the Ar plasma hole, which has beenmeasured with a spectroscopic method, is shown to examinethe formation mechanism of ion density depletion.2. Experimental setupThe experiments have been performed in the HighDensity Plasma Experiment (Hyper-I) device at NationalInstitute for Fusion Science (NIFS) [6]. Hyper-I is a lineardevice, which consists of ten magnetic coils and a cylindricalchamber with the inner diameter of 300 mm and axial lengthof 2000 mm. The plasma is produced by ECR heating with amicrowave of frequency 2.45 GHz. An argon gas has beenused at relatively low pressures .p 0 1Ar# mmTorr. The ionflow velocity field, the density and the electron temperatureprofiles have been measured with a directional Langmuirprobe (DLP) inserted from the radial ports. The details of theDLP and the method of flow-velocity measurement havebeen given in ref. [6]. A two-dimensional motor drive system with an opticalsystem is equipped at the end of the chamber to carry outspectroscopic measurements. The focal point of the opticalsystem is set to infinity to collect the visible light emittedparallel to the axis of the cylindrical plasma. We haveobserved the emission lines from the argon ions (488.0 nm)and the neutral argon (425.9 nm). A CCD camera is alsoinstalled to take the end view image of the plasma hole.3. Observation of Ar plasma holeA CCD image of the Ar plasma hole is shown in Fig. 1,evidently indicating a nonluminous hole region. The iondensity in the hole is about cm1010 3- , on the other hand, thedensity in the bright region (ambient plasma) is nearly oneorder of magnitude higher than that of the hole region. The610J. las a usion es. S I S, ol. 6 (2004) 610–613hole structure has been spontaneously formed in the center ofthe plasma under the operation condition with the pressurep 8 10Ar5#+ - Torr and an input power of 3 kW. Figure 2shows the ion density profiles measured at two different axialpositions, z 730= mm and z 1175= mm, where z denotesthe distance from the microwave injection window. It can beclearly seen that the ion density distribution has a good axialsymmetry and builds up a steep transition layer to theambient plasma. This shock-like density gradient is sustainedby the thin interfacial layer between the hole plasma and theambient plasma, the width of which is about 20 mm, i.e., afew ion Larmor radii. It should be noted that the size of theplasma hole is almost constant (60 mm) at two different axialpositions, indicating two-dimensional nature of the plasmahole structure. At z 1175= mm, the ion density reaches itsmaximum at the transition layer ( x mm40+ ), which issimilar to that of He plasma hole.4. Flow structure of Ar plasma holeThe profiles of azimuthal and axial ion-flow velocitymeasured at two different axial positions are shown in Fig. 3and 4, respectively. As seen in Fig. 3, the plasma rotatesazimuthally in the clockwise direction (direction of iondiamagnetic drift) when viewing from the end of thechamber. It is also confirmed from the plasma potentialmeasurement that the rotation is driven by the E B# drift.The hole plasma (core region) shows nearly rigid rotation at2Yoshimura S. et al., Observation of Plasma Hole in an ECR Ar PlasmaFig. 1  CCD image of Ar plama hole.Fig. 2 Density profiles of Ar plasma hole (z = 730 mm andz = 1175 mm).Fig. 3 Azimuthal velocity profiles of Ar plasma hole (z = 730mm and z = 1175 mm).Fig. 4 Axial velocity profiles of Ar plasma hole (z = 730 mmand z = 1175 mm).611z 730= mm, however, it weakly rotates at z 1175= mm. Itshould be noted that the ambient plasma rotates at asupersonic speed in both positions. The supersonic ion flowis also found in the axial direction at z 1175= mm, as shownin Fig. 4, where the flow direction is from the microwaveinjection point to the chamber end. Although the existence ofsupersonic ion flow over the large cross-sectional area is anoticeable and important characteristic of the Ar plasma holein comparison with the He plasma hole, quantitativetreatment for the supersonic flow field requires more reliableinterpretation of the DLP data or other diagnostic methodssuch as laser induced fluorescence (LIF) spectroscopy [7]. 5. DiscussionThe plasma hole is characterized by the cylindricaldensity cavity, the size of which is about 60 mm in diameter.The mean free path of neutral argon under our experimentalcondition is more than 400 mm, which is much longer thanthe diameter of the plasma hole. In order to examine whethera hole structure of which the scale is shorter than the neutralmean free path can be formed, the two-dimensionalspectroscopic measurement has been carried out. Theintensities of emitted spectral lines from neutrals and ionsare, respectively, given by I n nArI i n? and I niArII2? , wherethe quasi-neutrality condition and the constant electrontemperature are assumed. Figure 5 shows the electrontemperature profile of the Ar plasma hole at z 730= mm.Although the electron temperature slightly decreases withradial distance from 12 eV to 9 eV, neglecting the effect ofTe variation on the collision cross section is justified. Hencethe neutral density profile is obtained by the followingrelation:(1)Thanks to the two-dimensional nature of the plasma hole, theoptical measurement, which is a line-integrated quantityalong the axial direction, provides the radial distribution.( )( )( ).n rI rI rnArIIArI?3Yoshimura S. et al.,  Observation of Plasma Hole in an ECR Ar PlasmaFig. 5 Electron temperature profile of Ar plasma hole (z = 730mm).Fig. 6 Two-dimensional neutral density profile of Ar plasmahole.Fig. 7 Line intensity profile (ArI: 425.9 nm) of Ar plasma hole.The two-dimensional neutral density profile of the Arplasma hole is shown in Fig. 6. The remarkable point is thatthe neutrals do form a hole structure, the size of which isapproximately the same size of the ion density depletion andis much shorter than their mean free path. Here, we considerthe plasma production profile in connection with the densitystructure of Ar plasma hole. The number of particles ionizedin unit time and unit volume is given by (2)where the quantity iG Hvy , product of the ionization crosssection and electron thermal velocity averaged over velocitydistribution function, is a constant under constant electrontemperature. Hence, the profile of ( )I rArI is proportional tothe profile of ionization rate per unit volume. Figure 7 showsthe line intensity profile of ArI, indicating that thedistribution of the number of ionization events in unit timeand volume is also concave one. Thus we can conclude thatone of the causes of ion density depletion is the localizationof plasma production due to formation of neutral particlehole. As a possible explanation for neutral hole formation, itis suggested that rotating motion of the neutrals induced bycollisions with the rotating ions makes the apparent radialmean free path short, since the radial projection of arc lengthcorresponding to the ionization length is much shorter thanthat in a free streaming case.,n n Ii e n ArI?G Hvy6126. ConclusionA plasma hole structure in an ECR argon plasma wasfirst observed and examined experimentally. The Ar plasmahole is characterized by ion density depletion with the steeptransition layer and the existence of azimuthal and/or axialsupersonic ion flow. The two-dimensional neutral densityprofile was measured spectroscopically, revealing that theneutrals also form a hole structure as well.AcknowledgementsThis research was partially supported by the Ministry ofEducation, Culture, Sports, Science and Technology, Grant-in-Aid for Young Scientists (B), 15740332, 2003.References[1] K.S. Fine et al., Phys. Rev. Lett. 75, 3277 (1995). [2] Y. Kiwamoto et al., Phys. Rev. Lett. 85, 3173 (2000).[3] M. Kono and M.Y. Tanaka, Phys. Rev. Lett. 84, 4369(2000).[4] K. Nagaoka et al., Phys. Rev. Lett. 89, 075001 (2002).[5] A. Okamoto et al., Phys. Plasmas 10, 2211 (2003).[6] K. Nagaoka et al., J. Phys. Soc. Jpn. 70, 131 (2001).[7] R.A. Stern and J.A. Johnson III, Phys. Rev. Lett. 34,1548 (1975).4Yoshimura S. et al.,  Observation of Plasma Hole in an ECR Ar Plasma613

Observation of Plasma Hole in an ECR Ar Plasma

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