CHARGING OF DUST IN A NEGATIVE ION PLASMA (APS DPP-06 Poster LP )

Transcription

1 1 CHARGING OF DUST IN A NEGATIVE ION PLASMA (APS DPP-06 Poter LP ) Robert Merlino, Ro Fiher, Su Hyun Kim And Nathan Quarderer Department of Phyic and Atronomy, The Univerity of Iowa Abtract. We invetigate experimentally the charging of dut particle in a plama coniting of poitive ion, negative ion and electron. In typical laboratory plama containing electron and poitive ion, dut grain acquire a negative charge. In negative ion plama, charging due to the negative ion, in addition to poitive ion and electron, mut be taken into account. Calculation how that if a ignificant fraction of the electron are attached to negative ion, the magnitude of the charge on the dut particle i reduced. If the ratio ε = n e /n of the electron denity to poitive ion denity i ufficiently mall and the poitive ion are lighter than the negative ion, then the dut charge can be poitive. Thi poibility i invetigated in Q machine plama operating with potaium ion, and in which the highly electronegative ga SF 6 i added which attache low energy electron to produce the SF negative ion. The relatively cold electron 6 in the Q machine plama (T e = 0.2 ev) enhance the attachment probability allowing value of ε < 10 3 to be attained. 1. Introduction. Dut grain immered in a typical electron/poitive ion plama will acquire a negative charge due to the preferential attachment of the more mobile electron [1, 2]. The dut grain charge to a negative potential relative to the plama o that electron are repelled and poitive ion are attracted to the particle. The floating potential of the dut grain i determined by the balance of electron and ion current that are collected. If energetic electron are preent in the plama, the effect of econdary emiion may alo have to be taken into account [3]. Photoelectric emiion from dut in the preence of UV light can reult in dut acquiring a poitive charge [4]. Charging of dut in pace and atrophyical environment i typically dominated by thi photoelectron emiion. A detailed dicuion of dut charging mechanim, including reference to primary ource, i preented in Chapter 2 of the monograph of Shukla and Mamun [5].

2 2 In thi paper we invetigate the charging of dut in a plama coniting of poitive ion, electron and negative ion (for implicity we refer to thi a a negative ion plama). Mamun and Shukla [6] conidered the charging of dut grain in a plama with negative ion and howed that the negative ion ignificantly decreae the magnitude of the dut grain charge. D Angelo [7] dicued theoretically the excitation of ion-acoutic and dut acoutic wave in a plama with poitive dut grain produced in a negative ion plama. Annaratone and Allen computed the floating potential of a dut particle in an electronegative plama uing orbital motion limited theory [8]. They howed alo that under certain condition, poitively charged dut could be obtained. In Sec. 2 we compute the charge on a dut grain in a negative ion plama and how that a poitive charge i poible if the poitive ion are lighter than the negative ion and the relative concentration of free electron i ufficiently mall. In Sec. 3 the device for tudying the charging of dut in a negative ion plama, and the method of producing a negative ion plama i decribed. The experimental method ued to infer the ign of the dut charge in the negative ion plama i dicued in Sec. 4. Experimental evidence of the production of poitively charged dut baed on thi meaurement i preented in Sec. 5. A final ummary i given in Sec The charge on a dut particle in a negative ion plama. Conider an iolated pherical dut grain of radiu a introduced into a plama coniting of electron of denity n e, ingly charged poitive ion of denity n, and ingly charged negative ion of denity n. Define n ε [1] n

3 3 a the fraction of negative ion relative to poitive ion. Uing the charge neutrality condition n = n e n [2] we have that n e n = 1 ε [3] The temperature of the poitive ion, electron and negative ion are T, T e and T -, repectively. a. Current to a dut grain in a negative ion plama. The electron, negative ion and poitive ion current to the dut grain of radiu a are given by : I e = I eo 1 ev ev e kt e kt e V V > 0 < 0 [4] I = I o 1 ev ev e kt kt V V > 0 < 0 [5] I = I o ev e 1 ev kt kt V V > 0 < 0 [6] 1/ 2 j 2 where I = q n 4πa. [7] jo j kt j m j V S i the potential of the dut grain relative to the plama. The grain urface potential i then obtained by requiring I I I = 0 [8] e

4 4 We give an example to how under what condition one could obtain poitively charged grain in uch a plama. For implicity, conider the cae where all pecie are at the ame temperature = T, and define the normalized urface potential ψ = ev / kt [9] Combining Eq. [1] Eq. [9] we obtain (1 ε ) m m e ε m m = ψ e 1 ψ 1 ψ ψ e ψ ψ > 0 < 0 [10] Thi equation can be olved numerically for ψ S. A plot of ψ S v. the parameter n e /n for the cae in which the poitive ion i potaium K (ma = 39) and the negative ion i SF (ma = 146) i hown in 6 Fig. 1. Notice that the poitive ion i the lighter pecie. Thu in the preence of a heavy (compared to the ion) negative ion, the charge on the dut i reduced, and for n e /n < the dut urface potential (and charge) can be poitive. 0.5 ψ n e /n Fig. 1. The normalized dut urface potential v. the fractional concentration of electron in the plama. T = T e = T -

5 5 Once the dut urface potential i determined, the charge on the dut i computed uing where a i the radiu of the dut particle. Q= [11] 4πε o av S b. Effect of the temperature ratio. Fig. 2 how a plot imilar to Fig. 1 but for two value of the negative ion temperature, T - = ev and 0.2 ev. A expected, the tranition to poitively charged dut occur at a higher value of n e /n for T - = ev T = T = 0.2 ev e T_ = ev ψ S T_ = 0.2 ev n e /n Fig. 2. The normalized dut urface potential v. the fractional electron concentration, for two value of the negative ion temperature. c. Effect of poitive ion ma. Fig. 3 how a plot of the normalized dut potential, ψ S, for ingly charged argon, helium and xenon ion. The effect of variou poitive ion mae i ummarized in Fig. 4 which how a plot of the quantity ε(0), the value of ε = n e /n for which ψ S = 0, veru the poitive ion ma

6 6 number. With the negative ion (SF 6 ) -, poitive dut i more eaily achieved with a lighter ion ma. 1 0 ψ S ψ S [Ar, 2 ev] ψ S [He, 2 ev] ψ S [Xe, 2 ev] n n /n Fig. 3. The normalized dut urface potential v. the fractional electron concentration for argon, helium and xenon poitive ion H He Ar Xe (SF 6 ) _ ε(0) δ(0) Poitive Ion Ma Number Fig. 4. The quantity ε(0), the value of the fractional electron concentration for which the dut urface potential = 0, v. the poitive ion ma number.

7 7 3. Experimental device and method. In thi ection we decribe the device ued to produce a duty plama (duty Q machine) and the method of obtaining a negative ion plama. a. The duty plama device. A chematic of the experimental device ued to produce a duty plama i hown in Fig. 5.The baic plama ource i a Q machine which produce a fully ionized K / e - plama of approximately 6 cm diameter and ~1 m length by the urface ionization of potaium atom from an atomic beam oven on a hot (~2500 K) tantalum plate. The ion and electron have approximately equal temperature T T e 0.2 ev and denitie up to ~ cm -3. The plama i confined radially by a uniform axial magnetic field of 0.3 T. The method of dipering dut into the plama i eentially the ame a that ued in our previou experiment and decribed in Xu et al. [9] The dipener conit of a rotating cylinder which urround the plama column of a length of 30 cm. The inner urface of the cylinder i lined with aluminum wool which i embedded with fine dut particle that are initially loaded into the bottom of the cylinder. A the cylinder rotate the dut particle gently fall through the plama where they become charged by electron and ion collection. A decribed in our earlier work, Potaium Oven Tantalum Hot Plate Langmuir Probe K / SF - 6 Plama Cold End Plate SF 6 Rotating Dut Dipener Solenoid Magnet Coil Fig. 5. Schematic of the duty plama device.

8 8 the dut in thi cae acquire a negative charge. The dut particle were hollow gla microphere that had a large ize ditribution ranging from a few micron uo to about 100 micron, with the majority of particle (50%) approximately 35 micron in diameter. b. Production of negative ion plama. A negative ion plama i formed by attachment of electron on the highly electronegative ulfur hexafluoride SF 6 molecule by the reaction e SF SF 6 6 The ulfur hexafluoride ga i admitted into the vacuum chamber through a variable leak valve. The attachment efficiency depend on the electron energy and i mot pronounced for electron with energie in the range of a few tenth of an ev, which coincide quite well with the electron in the Q machine plama. At higher electron energie, diociation of the SF 6 molecule become increaingly likely, leading to the formation of additional negative ion pecie uch a SF and 5 F [10]. For thi reaon the Q machine i an ideal device in which to form negative ion plama. In fact it i poible to produce a negative ion plama in which the electron concentration relative to the poitive ion, n e /n, i o mall that we have eentially a poitive ion/negative ion plama. The effect of negative ion production can be oberved uing a Langmuir probe. Fig. 6 how a erie of Langmuir probe current v voltage characteritic a the partial preure of SF 6 i increaed. Poitive current in thee plot correpond to the collection of negative particle (electron and SF ). The uppermot plot 6 correpond to the cae in which no SF 6 ga ha been added. A the SF 6 preure i increaed, more electron become attached to form SF 6 ion and there i a correponding reduction in the negative current due to the fact that the SF ion are 6 coniderably le mobile than the electron. The effectivene of electron

9 9 5 K_SF6-_dat_ I[P(SF6=0) I[P(SF6=6e-6) I[P(SF6=8e-6) I[P(SF6=1e-5) I[P(SF6=1.5e-5) I[P(SF6=2e-5) I[P(SF6=5e-5) I[P(SF6=1e-4) I[P(SF6=2e-4) I[P(SF6=4e-4) Probe Voltage [V] Fig. 6. Langmuir probe characteritic for variou value of the SF 6 partial preure. Poitive current correpond to the collection of electron and negative ion. attachment i evident from the fact that at an SF 6 partial preure of only Torr, the negative probe current i reduced by about a factor of 2. The Langmuir probe characteritic correponding to the highet SF 6 partial preure i hown on an expanded cale in Fig. 7. Thi nearly ymmetric probe characteritic (comparable poitive and negative current) i an indication that a nearly electron-free plama have been formed. An etimate for the quantity ε = n e /n, the fraction of free electron remaining in the plama can be made from the meaurement of the ratio of negative probe aturation current to poitive probe aturation current. Taking the poitive aturation current a I = en v,th A and the negative aturation current a I - = en e v e,th A en - v -,th A, where v j,th = (kt j /m j ) 1/2 i the

10 Probe Voltage [V] Fig. 7. Replot of the Langmuir probe characteritic in Fig. 6 for the SF 6 partial preure of Torr. thermal peed of pecie j, and A i the probe area, and uing n = n e n -, the quantity ε = R m m T T m m T T, where R = I e e e e - /I. For the characteritic hown in Fig. 7 with R 1we find that ε 10-3 for a plama with K poitive ion SF negative ion and electron, with T T 2T. 6 e The probe meaurement provide an order of magnitude etimate of ε. A more accurate determination can be made uing the reult of work of Sato [11] and Ihikawa et al [12], who howed that ε can be determined by meaurement of the propagation characteritic of variou electrotatic plama wave in a negative ion plama. A plot of ε v the SF 6 partial preure obtained under eentially identical condition to our etup (a ingle ended Q machine operating with potaium at about the ame magnetic field trength) i hown in Fig. 8. The higher SF 6 preure ued in our experiment correpond to an ε ~ Thu we ee that by operating at ufficiently high SF 6 preure, it i poible to produce a plama in which the electron are removed to about one part in 10,000. We how in

11 11 the next ection how thi plama can be ued to produce a duty plama having poitively charged dut n e /n SF 6 Preure (Torr) Fig. 8. The fractional electron concentration in a Q machine negative ion plama with K poitive ion v. the SF 6 partial preure, From ref. [11]. 4. Experimental evidence for a duty plama with poitively charged dut. The experimental procedure conit of firt introducing SF 6 into the plama to produce the negative ion and then dipering dut into thi plama. The main diagnotic tool of the duty plama wa the Langmuir probe. In our earlier work [1] on the charging of dut grain in an ordinary electron/poitive ion plama, the Langmuir probe wa ued to determine how the negative charge in the plama wa divided between free electron and negatively charged dut grain. Langmuir probe characteritic were obtained under identical condition except for the anence or preence of dut. When the dut wa preent, the electron aturation current to the poitively biaed probe wa reduced compared to the current meaured without

12 12 dut. Thi wa due to the fact that electron which attach to the dut grain of extremely low mobility are not collected by the probe. A typical Langmuir probe characteritic illutrating thi i hown in Fig. 9. From the reduction in electron aturation current and applying the condition for charge neutrality, the quantity Qn d /e, the number of negative elementary charged per unit volume on dut grain could be inferred. Dut off Dut on Dut off Fig. 9. Langmuir probe characteritic taken in an electron/k plama, taken from ref. [1], howing the reduction in the electron current when dut particle are dipered into the plama. A very imilar method wa applied in the preent work to infer the charge (ign) of the dut introduced into the negative ion plama. For ufficiently low value of ε = n e /n we aume that electron collection by the dut i mall compared to collection of negative ion. Thi i jutified if n e v e,th << n - v -,th. Thi

13 13 condition can be expreed a ε << kt m kt m. For T e e - (0.5-1)T e thi correpond to an ε << ( ) Now at an SF 6 preure ~ T, we have that ε ~ 10-4, jutifying thi aumption. In uch a poitive ion negative ion plama, in the preence of charged dut, the condition of charge neutrality read n (Q/e)n d = n -. Before the dut i added, the probe aturation current (per m 2 ) can be written a I o = en o v,th and I -o = en -o v -,th, where n -o n o = n o. When the dut i preent, the probe current are I = en v,th and I - = en - v -,th. We aume, a in our previou work that there the contribution to the probe current from charged dut i negligible. Combining the equation of charge neutrality with the probe current we have Q I I n = Zn = n d d o e I I o o [11] Thu, meaurement of the relative change in aturation current can be ued to determine the ign of Z. Thu a Z > 0 (poitive dut) i indicated by the condition I I > I I, or the fractional reduction in the poitive current mut be greater o o than the fractional reduction in negative current. The Langmuir probe characteritic taken with dut introduced into the poitive ion/negative ion plama i hown in Fig. 10. The blue curve i one taken before dut wa added and the red on wa taken with the dut preent. The yellow curve are portion of a trace taken immediately after the dut wa turned off to enure that dut contamination of the probe wa not ignificant. Meaurement of the aturation current from Fig. 10 indicate that I I 0.7, while I I 0.6, o o or that Z > 0. According to the dut charging theory of Sec. 2, at ε ~ 10-4, the normalized dut urface potential hould be in thi cae ev /kt ~ 0.3. Although one example ha been hown, we have oberved repeatedly, that when a ufficient

14 14 number of negative ion i preent in the plama, the probe characteritic meaurement indicate the preence of poitively charged dut. Before Dut ON 0.1 ON OFF Dut On Probe Bia (V) Probe Voltage [V] Fig. 10. Upper plot-langmuir probe characteritic obtained in a poitive ion/negative ion plama befroe during and after the diperal of dut into the plama. Bottom plot- expanded verion of upper plot before dut (black) and in the preence of dut (red).

15 15 6. Summary and concluion. A method for producing poitive charged duty plama ha been decribed. Thi method relie on attaching the relatively mobile electron to negative ion having a ma greater than the ma of the poitive ion pecie. The preence of the negative ion can reduce the charge on the dut (decharging) and even allow the more mobile poitive ion to charge the dut poitively. Acknowledgement. Thi work wa upported by the US Department of Energy. We thank M. Miller for hi technical aitance and N. D Angelo for ueful dicuion. We alo thank Chuck Crepi of Emeron Cuming for providing free ample of their gla microphere. Reference. 1. A. Barkan, N. D Angelo and R. L. Merlino, Charging of dut grain in a plama, Phy. Rev Lett. 73, 3093 (1994). 2. B. Walch, M. Horanyi, and S. Roberton, Meaurement of the charging of individual dut grain in a plama, IEEE Tran, Plama Sci. 22, 97 (1994). 3. B. Walch, M. Horanyi, and S. Roberton, Charging of dut grain in a plama with energetic electron, Phy. rev. Lett. 75, 838 (1995). 4. A. A. Sickafooe, J. E. Colwell, M. Horanyi, and S. Roberton, Photoelectric charging of dut particle in vacuum, Phy. Rev. Lett. 84, 6034 (2000). 5. P. K. Shukla and A. A. Mamun, Introduction to Duty Plama Phyic, Britol: Intitute of Phyic, A. A. Mamun and P. K. Shukla, Charging of dut grain in a plama with negative ion, Phy. Plama 10, 1518 (2003). 7. N. D Angelo, Low-frequency wave in colliional poitive duty plama, J. Phy. D: Appl. Phy. 37, 860 (2004).

16 16 8. B. M, Annaratone and J. E. Allen, A note on the potential acquired by a dut particle in an electronegative plama, J. Phy. D: Appl. Phy. 38, 26 (2005). 9. W. Xu, B. Song, R. L. Merlino, and N. D Angelo, A duty plama device for producing extended, teady tate, magnetized duty plama column, Rev. Sci. Intrum. 63, 5266 (1992). 10. R. K. Aundi and J. D. Cragg, Electron capture and ionization phenomena in SF 6 and C 7 F 14, Proc. Phy. Soc. 83, 611 (1964). 11. N. Sato, Negative ion plama, A Variety of Plama, p , Proc Int. Conf. on Plama Phyic, ed. A. Sen and P. K. Kaw, Indian Academy of Science, Bangalore, 1989; N. Sato, Production of negative ion plama in a Q machine, Plama Source Sci. Technol. 3, 395 (1994). 12. I. Ihikawa, S. Iizuka, R. Hatakeyama, and N. Sato, Probe meaurement in a negative ion plama, J. Phy. Soc. Japan 67, 158 (1998).