8 Measurement of Ion Density Electron Temperature JAXA RR 8 JAXA Research Development Report JAXA-RR-1-1E Electron Eensity (1 7 cm -3 ) Fig. 1 Fig. 3

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1 7 Measurement of Ion Density Electron Temperature by Double-Probe Method to Study Critical Phenomena in Kazuo TAKAHASHI *1, Satoshi ADACHI *, Hiroo TOTSUJI *3 Abstract: A dusty plasma research was performed to investigate critical phenomena in JAXA, participating in missions of a joint Russian/German Scientific project with cooperation of PK-3 plus flight module on International Space Station. In research, it was necessary to obtain plasma parameters such as densities temperatures of electron ion for analyzing a state of dusty plasmas expressed by parameters in a phase diagram of Coulomb coupling parameter ratio of inter-particle distance to Debye length. This work was dedicated to obtain ion density electron temperature by using a double-probe method. The ion density was measured to be in order of cm 3. The electron temperature was observed to be enhanced by injecting dust particles to plasmas. Keywords: Dusty Plasma, Fine Particle Plasma, Complex Plasma, PK-3 Plus, Microgravity, ISS 1. Introduction This research was motivated in experiments on International Space Station (ISS) of dusty (complex or fine particle) plasmas, which had been going on with collaboration between Max-Planck-Institute for Extraterrestrial Physics (MPE, Germany) Joint Institute for HighTemperatures(JIHT, Russia) for several years. Plasmas including dust particles (typically, micrometer-sized), so-called dusty plasmas, have attracted considerable scientific interest in recent decades. The dust particles are charged by fluxes of electron ion in plasmas. The charge of dust particles can be in order of a few thouss of elementary charge in typical laboratory plasmas. The charged dust particles are regarded as a strongly coupled Coulomb system. In system, one can observe many physical phenomena found in solid or liquid state, such as crystallization, phase transition, wave propagation, so on. Complex plasma experiments have been done in microgravity conditions with apparatuses boarding on parabolic flight, sounding rocket ISS for recent years. Several physical phenomena, e.g., wave propagation so on, have been reported by MPE JIHT in experiments on ISS. The utility for dusty plasmas on ISS was replaced an improved apparatus denoted by PK-3 plus set in Russian module at end of 1). Several scientists in Japan have joined to mission of PK-3 plus since July 9, for demonstrating a critical phenomenon in dusty plasmas predicted in calculation by one of authors ). Plasmas of high density were required to approach to critical point. Referring a previous work for diagnostics in PK-3 plus, high power high pressure conditions were employed to obtain plasmas of high density 3). In previous diagnostics, electron densities were measured by hairpin resonator, which was relatively large antenna compared with size of chamber possibly affected plasmas. In present research, a double-probe method was used for measuring ion density electron temperature to reduce disturbance in plasmas examined results from hairpin resonator.. Diagnostics in.1 Measurement of electron density by hairpin resonator The previous work was done for measurements of electron density with a hairpin resonator in PK-3 plus apparatus, equipped with parallel plate electrodes *1 Department of Electronics, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto -88, Japan * Institute of Space Astronautical Science, Japan Aerospace Exploration Agency, -1-1 Sengen, Tsukuba, Ibaraki 3-8, Japan *3 Okayama University, Tsushimanaka, Kitaku, Okayama 7-83, Japan ( takahash@kit.jp)

2 8 Measurement of Ion Density Electron Temperature JAXA RR 8 JAXA Research Development Report JAXA-RR-1-1E Electron Eensity (1 7 cm -3 ) Fig. 1 Fig Top Bottom The spatial distribution profiles of electron density between top bottom electrodes at Pa, measured with changing rf power from to 8 mw. (a) (b) The spatial distribution profiles of electron density in a Ne plasma calculated by a PIC/MCC code, (a) a pristine plasma, (b) plasma with antenna of hairpin. surrounded by grounded guard rings in a chamber 3). The electrodes at top bottom sides were separated 3 mm. The diameter of electrodes was mm. Figure 1 shows spatial distribution profiles of electron density between top bottom electrodes in Ar plasmas at Pa, measured with changing rf power from to 8 mw. Two peaks are found in profiles of electron density at distance of 8 mm from each electrode. Conversely, profile was simulated for a Ne plasma by a particle-in-cell Monte Carlo code (PIC/MCC) ). The electron density is highest at axial radial center in pristine plasma without an antenna of hairpin resonator (Fig. (a)). Introducing hairpin resonator to center of chamber, electrons are missed around hairpin resonator, density of electron is reduced all over chamber (Fig. (b)). The plasma is affected by hairpin resonator electrons are lost on its surface. Therefore it is reasonable to think that profile of electron density obtained by hairpin resonator misses highest around center of plasma electron density is estimated under that of pristine plasma.. Measurement of ion density electron temperature by double-probe method Hindering loss of electron on an equipment for diagnostic, a traditional double-probe method was employed, which did not take electrons ions as currents in an electrical circuit for method,). Ion density electron temperature were measured with tips of.3 mm in diameter, 8 mm in length separated 7 mm each or, which were made of tungsten wire (Fig. 3). The tips were connected to a voltage source with floating on ground of plasmas. Figure shows pictures of tips introducing to a dust cloud illuminated by a laser. Here dust particles of. µm in diameter were injected with extremely high density, instability wave were excited in dust cloud. The tips were set at 8 mm high from bottom electrode surrounded by sheath, where corresponds to a dust-free region 7,8). In figure, right tip is initially applied -3 V negative to left one. The voltage was swept to +3 V with sampling current in an electrical circuit insulated from that for generating plasmas. When tip has negative potential to or, a sheath of corrected ions around tip exps to thickness determined by Bohm criterion. The dust particles reach to an equilibrium position near

3 Measurement of Ion Density Electron Temperature JAXA RR 9 Measurement of Ion Density Electron Temperature by Double-Probe Method to Study Critical Phenomena in 9 Probe Current, Ip (A) h= φ φ -x1 - - Probe Voltage, Vp (V) Fig. Electric characteristics in double-probe method. The probe currents (I p ) were plotted as functions of voltage applied between tips (V p ). The curves were obtained with changing height of tips, denoted by "h", from to 1 mm. Fig. 3 Fig. Schematics of PK-3 plus chamber tips for double-probe method. -3 V - V -1 V V +3 V + V +1 V Pictures of tips for double-probe method introduced to dust cloud in plasma. The tips were set at 8 mm high from bottom electrode. The dust particles were illuminated by a laser observed to distribute from. to 13 mm high. The right tip was negatively biased at -3 V to left one, initially. The bias voltage was changed to positive side reached to +3 V. sheath edge 9). Therefore spatial distribution of dust cloud would be a rough stard to show sheath. Each tip was surrounded by each sheath separated through plasma dust cloud, which was clearly shown in pictures. Here tips were regarded to work for measuring ion density electron temperature in dusty plasma. Figure shows electric characteristics of circuit for double-probe method, probe current (I p ) plotted as a function of probe voltage (V p ), with variation of tip height from bottom electrode in a plasma generated at Pa of Ar mw of rf power. Ion currents linearly increasing with biasing were observed in regimes of highly negative positive voltage, denoted by I i I i+, respectively. The lines of ion current define parameters of slops, S i S i+, crosssections on V p =, I isat I isat+ as ion saturation currents. The ion saturation currents tended to increase with tip closing to center of plasma. The slop of ion current is redefined to be S S i S i+. The linear part around V p = indicates a current from tip di mainly correcting electrons, whose slope, p Vp dv, is p = integrated by an electron energy distribution function in a plasma. It is noted that slope is enhanced by that of ion current coming from sheath exping with biasing tips. Hence electron temperature, T e is expressed by formula with following manner

4 1 Measurement of Ion Density Electronwhere Temperature m i AJAXA RR correspond to mass of ion surface 1 JAXA Research Development Report JAXA-RR-1-1E area of tip, respectively. 1.x1 9 Ion Density (cm -3 ) Fig Electron Temperature (ev) 3 w/o Particles with Particles (a) (b) 3 with Particles w/o Particles 3 Spatial distribution profiles of (a) ion density (b) electron temperature measured by using double-probe method in Ar plasma at Pa mw. Open solid circles indicate parameters in case without with dust particles, respectively. which suppresses slope of ion current from that of electron current 1), T e = e I isat + I isat+ ( k ), (1) B di p Vp dv.8s p = where e k B are elementary charge Boltzmann constant, respectively. Ion density, n i, is calculated from equation, I isat I isat+ =.1n i e k B T e m i A, () where m i A correspond to mass of ion surface area of tip, respectively. 3. State of dusty plasmas Figures (a) (b) show ion densities electron temperatures, respectively, measured by using 3. State of dusty plasmas double-probe Figures (a) method (b) inshow Ar ionplasma densities at Pa electron mw. temperatures, The spatial respectively, distributionmeasured profile ofby ion using density around double-probe center method should in be identical Ar plasma toat that of Paelec- tron mw. density. TheThis spatial profile distribution corresponds profile to that of ion of pristine density plasma around derived center byshould PIC/MCC be identical code. to The that electron of electron density. expected This profile from corresponds ion densities to that measured of pristine by densities plasma double-probe derived bymethod PIC/MCC are higher code. thanthe thoselectron by hairpin densitiesresonator. expected from Therefore ionit densities is concluded measured that by hairpin double-probe resonator can method affect are ionization higher than in volume those by of chamber hairpin resonator. extinguish Therefore plasma, it is concluded resulting inthat reducing hairpin electron resonator density can affect making ionization its spatial in volume profileof with two chamber peaks. extinguish plasma, resulting in reducinginelectron is notedensity that electron making temperature its spatialis profile enhanced with throughout two peaks. plasma by injecting dust particles. Density In is noted of that dustelectron particle temperature was hard to be is enhanced precisely measured throughoutdue to plasma instability, by injectinghowever, dust might particles. be reached Density to of 1 cm dust 3 particle. The total wasarea hardontosurface be precisely of measured dust particle due into cloud instability, can be a however, few tens mm might. The be reached plasma should to 1 be cmlost 3. on The total surface areaby onrecombination surface of dust between particle electrons in cloud ions can as be seen a few in tens measurement mm. The of plasma hairpin shouldresonator. be lost Hence surface ionization by recombination rate should between kept toelectrons sustain plasma, ions seen encountering measurement loss by recombination. of hairpin resonator. This leads Hence to ionization electron temperature rate should enhanced be kept to 11,1) sustain. In Fig. plasma,, too many encountering dust particles loss bywere injected recombination. just before Thisextinguishing leads to electron plasma. temperature The ion densities enhancedin 11,1) case. Inwithout Fig., too many dust particles dust particles couldwere not be injected maintained, just before although extinguishing electron temperature plasma. Thewas ion enhanced densities in (Fig. case (a)). without dust particles could not be maintained, although electron temperature was enhanced (Fig. (a)). Particle Charge, Particle Qp/e Charge, Qp/e I isat I isat+ =.1n i e Ni Qp/e Ni Ne Qp/e N i =.X1 8 cm -3 T e =. ev T i =.3 ev Ne ND p =. µm i =.X1 8 cm -3 T e =. ev T i =.3 ev 1 3 D 1 p =. µm 1 1 Particle Density (cm -3 ), Np k B T e m i A, () Electron Electron Ion Densities Ion Densities (cm -3 ), Ne, (cm Ni -3 ), Ne, Ni Variations 1 3 of 1 electron 1 density 1 charge of Particle Density (cm dust particle calculated -3 ), Np from parameters obtained by double-probe method Variations of electron density charge of in changing dust particle density, plotted dust particle calculated from parameters obtained by double-probe method with ion density. in changing dust particle density, plotted with ion density. Var ete in c wit

5 Measurement of Ion Density Electron Temperature JAXA RR Measurement of Ion Density Electron Temperature by Double-Probe Method to Study Critical Phenomena in Two parameters of ion density electron temperature enable to calculate or parameters in dusty plasmas. Electron density charge of dust particle are calculated in orbit-motion-limited (OML) ory thors 7) Klindworth, (K.T.) thanks M., Prof. Piel, A., Noriyoshi Melzer, Sato A., (Prof. Konopka, EmeritusRormel, Tohoku University) H., Tarantik, K., Prof. Morfill, YukioG. Watanabe E., Phys. U., (Prof. Rev. Emeritus Lett., Kyushu 93, (), University) 19. for valuable discussion. 8) Thomas, E., Jr., Avinash, K., Merlino, R. L., Phys. 13). In, assuming parameters of. 1 8 Plasmas, 11, (), pp cm 3 9) Barnes, M. S., Keller, for ion density,. ev for electron temperature, References J. H., Forster, J. C., O Neill, J. A., Coultas, D. K., Phys. Rev. Lett, 8, (199), pp. room temperature for ion temperature, one can estimate 1) Thomas, H. M., Morfill, G. E., Fortov, V. E. et al., New charge of dust particles to be 1 1 with varying density of dust particle. The electron densities should be conserved by charge neutrality in plasmas. 1) J. Dote, T., Jpn. 1, J. Appl. (8), , (198), 9. 11) ) Totsuji, Carlile, R. H., N., J. Geha, Phys. S., A: O Hanlon, Math. Theor., J. J.,, Stewart, (9), J., 1. Appl. Phys. Lett., 9, (1991), pp ) 3) Takahashi, Carlile, R. N. K., Hayashi, Geha, S. Y., S., J. Appl. Adachi, S., J. Appl. 73, (1993), pp. 11, (11), Conclusion 13) ) Heidemann, Mott-Smith, H. R., M. Takahashi, Langmuir, K., Chaudhur, I., Phys. M. Rev., et al., unpublished. 8, (19), pp The double-probe method seemed to be more appropriate ) Johnson, E. O. Malter, L., Phys. Rev., 7, for diagnostics in PK-3 plus than hair- pin resonator. The spatial distribution profiles of ion (199), pp ) Johnson, E. O. Malter, L., Phys. Rev., 8, density obtained by double-probe method was reasonable compared with result from PIC/MCC (19), pp ) Klindworth, M., Piel, A., Melzer, A., Konopka, U., Rormel, H., Tarantik, K., Morfill, G. E., Phys. code. In PK-3 plus, electron ion densities Rev. Lett., 93, (), 19. could reached to 1 9 cm 3 at several Watts of rf power. Injecting dust particles, electron temperature 8) Thomas, E., Jr., Avinash, K., Merlino, R. L., Phys. Plasmas, 11, (), pp should be enhanced. This may lead to complicate 9) Barnes, M. S., Keller, J. H., Forster, J. C., O Neill, J. A., prediction for critical phenomena. The double-probe method, however, gives several measures in ion density electron temperature for analyzing phenomena with helps of or ories, e.g., OML. Acknowledgements Coultas, D. K., Phys. Rev. Lett, 8, (199), pp ) Dote, T., Jpn. J. Appl. 7, (198), 9. 11) Carlile, R. N., Geha, S., O Hanlon, J. J., Stewart, J., Appl. Phys. Lett., 9, (1991), pp ) Carlile, R. N. Geha, S. S., J. Appl. 73, (1993), pp The authors would like to thank members of PK- 3 plus scientific team. This work was supported by 13) Mott-Smith, H. M. Langmuir, I., Phys. Rev., 8, (19), pp ISS Science Project Office of JAXA. One of au- thors (K.T.) thanks Prof. Noriyoshi Sato (Prof. Emeritus Tohoku University) Prof. Yukio Watanabe (Prof. Emeritus Kyushu University) for valuable discussion. References 1) Thomas, H. M., Morfill, G. E., Fortov, V. E. et al., New J. 1, (8), 333. ) Totsuji, H., J. Phys. A: Math. Theor.,, (9), 1. 3) Takahashi, K., Hayashi, Y., Adachi, S., J. Appl. 11, (11), ) Heidemann, R., Takahashi, K., Chaudhur, M. et al., unpublished. ) Johnson, E. O. Malter, L., Phys. Rev., 7, (199), pp ) Johnson, E. O. Malter, L., Phys. Rev., 8, (19), pp ) Klindworth, M., Piel, A., Melzer, A., Konopka, U.,