Biloiu, Ioana A.

Scime, Earl E.

English

The Research Repository @ WVU (West Virginia University)

Faculty Scholarship2009Temporal evolution of double layers in pulsedhelicon plasmasIoana A. BiloiuEarl E. ScimeFollow this and additional works at: https://researchrepository.wvu.edu/faculty_publicationsThis Article is brought to you for free and open access by The Research Repository @ WVU. It has been accepted for inclusion in Faculty Scholarshipby an authorized administrator of The Research Repository @ WVU. For more information, please contact ian.harmon@mail.wvu.edu.Digital Commons CitationBiloiu, Ioana A. and Scime, Earl E., "Temporal evolution of double layers in pulsed helicon plasmas" (2009). Faculty Scholarship. 363.https://researchrepository.wvu.edu/faculty_publications/363Temporal evolution of double layers in pulsed helicon plasmasIoana A. Biloiu1,2,a and Earl E. Scime11Department of Physics, West Virginia University, Morgantown, West Virginia 26506, USA2Faculty of Physics, University of Bucharest, P.O. Box MG-11, Bucharest 077125, RomaniaReceived 6 July 2009; accepted 20 July 2009; published online 7 August 2009Delays of the order of tens of milliseconds in the appearance of the fast argon ion population in theexpansion region of a pulsed helicon plasma are observed in time-resolved, laser inducedfluorescence measurements. The fast ion population is a proxy for the presence of a double layer.The magnitude of the time delay depends strongly on the length of the interval between plasmapulses; the shorter the time between pulses, the shorter the time delay. The time delay approacheszero for inter-pulse intervals smaller than 30 ms. The double layer strength is not affected by plasmasource modulation frequency. © 2009 American Institute of Physics. DOI: 10.1063/1.3204014The energetic ion beam generated by the current-freeelectric double layer EDL which forms spontaneously inthe magnetic expansion region of low pressure heliconplasma sources1 could provide the thrust needed for extrater-restrial spacecraft propulsion. The EDL forms with the highpotential side toward the helicon source and the low potentialside toward the expansion region. The resultant ion accelera-tion to supersonic speeds and the characteristic high ion den-sities of helicon discharges motivated researchers from theAustralian National University ANU to propose the helicondouble layer thruster HDLT concept.2 Although the specificimpulse and thrust of the HDTL are low compared to con-ventional thrusters, the possibility of electrically steering theejected ion beam,3 stable operation at very low pressure in avariety of gases,4 and the possibility of high energyefficiency5 are sufficiently intriguing that HDLT optimizationstudies have continued. For example, recent studies haveshown that the overall energy efficiency can be significantlyimproved by replacing the heavy solenoids used to generatethe axial magnetic fields with smaller and lighter permanentmagnets.6,7 The dependence of the ion beam velocity onthe source driving frequency has also been recentlydemonstrated.8 Pulsing the helicon discharge might solve re-maining important HDLT thruster issues such as plasma de-tachment, turbulent cross-field diffusion, and antenna heat-ing. Therefore, a detailed understanding of the temporalevolution of the EDL is required to identify the optimal op-erational parameters duty cycle, pulse length, input power,driving frequency, etc. of a pulsed helicon source thruster.In this letter, we present measurements of the time delay,typically a few milliseconds to a few tens of milliseconds,between plasma ignition and the appearance of an acceler-ated supersonic ion population. The measurements of theargon ion velocity distribution function ivdf in the expan-sion region of a helicon plasma source are obtained by timeresolved laser induced fluorescence LIF.The experiments were performed in the HELIX-LEIAHL helicon source-diffusion chamber system described indetail elsewhere.9 Briefly, the helicon source HELIX con-sists of two coaxial tubes of a total length of 1.5 m. ThePyrex source tube is 10 cm in diameter and is surrounded bya 19 cm long, half wave, m=+1, helical antenna that couplesthe rf power into the plasma. The rf antenna is 37 cm fromthe closed end of the source tube. The second, 15 cm diam-eter, stainless-steel tube connects the plasma generation re-gion with plasma expansion region, a 4.5 m long, 2 m diam-eter aluminum diffusion chamber LEIA. The axialmagnetic field of 0–1.2 kG in HELIX and 0–150 G in LEIAis created by external solenoids. Previous investigations haveshown that the EDL is localized to the region of strong axialmagnetic field gradient which, for the HL system, is a maxi-mum close to steel tube-aluminum chamber junction. To in-vestigate the temporal evolution of the ivdf, the dischargewas pulsed at different frequencies and duty cycles; twofunction generators, one to provide the 9.5 MHz driving fre-quency and one to amplitude modulate the rf wave were usedin conjunction with a wide bandwidth rf amplifier. The elec-tronic rise and decay times in the case of the square wavepulses used for these investigations were 70 ns, muchshorter than the intervals between plasma pulses, which weretens to hundreds of milliseconds.For Ar+ LIF, we used the classic three-level LIF schemein which the metastable 3d 2G9/2 state is optically pumped by611.6616 nm vacuum wavelength radiation to the short liv-ing 4p 2F7/20 state, which then decays to the 4s 2D5/2 state byemitting a photon at 461.09 nm vacuum wavelength. De-tails of the laser used and the time resolved LIF diagnostictechnique are given elsewhere.10 The injected laser light ismodulated with an acousto-optic modulator AOM at 10kHz and then transported with a multimode, nonpolarizationpreserving, optical fiber to the injection optics. A high-frequency lock-in amplifier provided the reference modula-tion signal for the AOM driver. Adequate signal-to-noise ra-tios of 10:1 were obtained by averaging the LIF signal ateach laser wavelength over 400 plasma pulses. To obtain adata record long enough to cover the entire plasma pulse, theLIF signals were sampled at a digitization rate of 10 kHz.Thus, a time resolution of 100 s is theoretically possible.However, for the lock-in LIF detection scheme, the time res-olution is limited to 1 ms by the 1 ms integration time of thelock-in amplifier; needed to obtain sufficient photon countingstatistics ten on/off chopping cycles of the AOM. To deter-mine the parallel ivdf, laser light was injected at an angle of52° with respect to the direction of flow and the fluores-cence signal detected by an integrated collection optics-photomultiplier tube mounted on a scanning probe.11 Due toaElectronic mail: iabiloiu@yahoo.com.APPLIED PHYSICS LETTERS 95, 051504 20090003-6951/2009/955/051504/3/$25.00 © 2009 American Institute of Physics95, 051504-1Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspan error in our previous publications,12,13 the reported valuesof the parallel ion speeds were uncorrected for the laser in-jection angle, i.e., underestimated by a factor of 1.6. Be-cause the plasma conditions were chosen to foster EDL for-mation, the detected ivdf is bimodal, with peaks separated by10 GHz. For reliable velocity and temperature resolution,the ivdf was sampled at 71 equally spaced laser frequenciesover a range of 20 GHz see Fig. 1.The evolution of the parallel argon ivdf at 19 cm down-stream of helicon source-diffusion chamber junction for a50% duty cycle, 10 Hz pulse frequency is shown in Fig. 2.Since previous investigations showed oscillations of the LIFsignal with a characteristic frequency of 1 kHz when thelower hybrid frequency in the source was comparable to thedriving frequency,11 the source was operated at magneticfield strengths of 600 G in the source to avoid the lower-hybrid frequency and 35 G in the diffusion chamber. Neutralpressure in the helicon source was 2 mTorr, sufficient pres-sure for a good LIF signal, but low enough to allow forma-tion of the EDL. The pressure in the expansion chamber was0.24 mTorr. To determine the absolute flow of each distribu-tion component, at each time step the background light levelswere subtracted from the measured ivdf, normalized to thepeak signal value. The processed ivdf was then fit withGauss and LogNormal functions for the slow and fast ionpopulations, respectively. The LogNormal function was usedto include the tail of the fast ion ivdf toward slower speedsthat arises from elastic scattering and/or charge-exchangecollisions with the background neutrals and ions.12 AlthoughLIF only provides detailed information about the populationof a certain excitated state—in our case the metastableAr+3d 2G9/2 state—the large level degeneracy ratio 17:1of the metastable ion relative to the ground state ion,14 whichmakes the main channel for the formation of this state thetwo-step, electron impact process of ionization 15.8 eV fol-lowed by excitation 19.1 eV, as well as our previous stud-ies that demonstrated that the LIF signal ne2Te1/2, leads usto expect that the measured ivdfs accurately represent theivdfs of the entire ion population.Consistent with measurements in steady-state heliconplasmas, the slow ion population exhibits a bulk speed of0.5 km /s and the fast ion population a bulk flow speed of10 km /s.15 These observations are consistent with differ-ent origins for the two ion populations: a the slow ions area background population created by local ionization; theirparallel flow speed is slightly slower than the neutral gasflow speed 600 m /s estimated at HELIX-LEIA junctionbased on the mass flow rate, the pressure difference, and gasconductance under the assumption of molecular flow; b thefast ions are created upstream in the plasma source and ac-celerated by the EDL and magnetic expansion on their wayfrom the helicon source toward diffusion chamber.1 Forslightly different steady-state plasma parameters, electro-static probe measurements indicated a potential drop of18 V from plasma source to expansion chamber,16 corre-sponding to an EDL strength of 3kBTe /e—sufficient to ac-celerate the fast ions to the measured speeds. The differentdurations of the ivdf components during pulsed operationalso point toward different origins for each component. Theslow ion population appears at the start of the rf pulse andpersists for 30 ms after the rf pulse terminates. The exis-tence of an argon afterglow plasma is typically attributed tothe long lifetime of the 3d 2G9/2 state 6.1 s in low collision-ality systems. Given that the quenching cross-section for col-lisions of the 3d 2G9/2 state with ground state neutral argon is110−14 cm2,17 at the LIF detection location in the expan-sion region the quenching mean free path mfp is quite long12.5 cm. However, the resulting 2.4103 s−1 quenchingfrequency is still too large to explain the observed 30 msafterglow plasma. Close inspection of the ivdf temporal evo-lution shows that after pulse termination, the LIF signal ofthe slow ion population does not immediately decay but ac-tually increases for about 20 ms and then decays. This over-shoot of LIF signal might be related to continued excitationinto the 3d 2G9/2 by low energy electron impact from wellpopulated lower ionic metastable states.18The LIF signal for the fast ion population appears19 ms after the start of the plasma pulse and has a shortafterglow 7 ms. The 7 ms decay time is consistent withrapid disappearance of the fast ion population convolvedwith the 1 ms lock-in integration time which artificiallystretches out any rapidly terminating phenomena over a fewmilliseconds.19 The rapid disappearance of the fast ionpopulation suggests the rapid termination of the mechanismthat generates, accelerates and then injects the fast ions in thediffusion chamber. Given that the EDL forms 4–5 cm insidethe source, where the pressure is roughly one order of mag-FIG. 1. Color online Parallel ivdf at t=25 ms into the pulse for 10 Hzmodulation frequency: raw data points; fits solid lines of the slow andfast ion distributions.FIG. 2. Color online The evolution of the argon ion parallel ivdf duringtwo cycles of a 50 ms plasma pulse: top—surface plot showing fast10 km /s and a slow 0.5 km /s ion populations; bottom—a contourprojection showing the time lag 19 ms in the appearance of the fastion population.051504-2 I. A. Biloiu and E. E. Scime Appl. Phys. Lett. 95, 051504 2009Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspnitude higher than in the expansion region, it follows that inthe absence of ongoing ionization rapid quenching of the3d 2G9/2 metastable state in the source occurs; the 1.5 cmquenching mfp and 10 km /s ion drift velocity yield a15 s decay time. Similar fast population decay times of10 s were observed for current-free EDLs appearingin a two-electron population plasma when the discharge wasswitched off.20 The most significant feature of the fastpopulation time evolution is the 19 ms time lag in its ap-pearance after the plasma pulse begins. Such a time lag istoo long to result from the plasma relaxation time—thetime needed to establish steady-state conditions which isroughly equal to the transit time across the system of an iontraveling at acoustic speed.21 For an electron temperature of6 eV from probe measurements, the acoustic ion speed cscs= gkBTe /m1/2, where m is the ion mass and  is assumed1 for isothermal expansion of 3.8 km /s yields a transittime of 0.5 ms. The time lag is also unlikely to result fromthe propagation of the EDL, such as those observed in colli-sionless expanding plasmas, because the motion of thoseEDLs stagnated after only 0.2–0.3 ms.22 Two other possibleexplanations are that time is needed to build up a sufficientpopulation of the 3d 2G9/2 ionic state to allow detection byLIF or that the EDL requires some many ms to form. SinceLIF signal from the slow population, observed in the same3d 2G9/2 ionic state, appears immediately at the onset of theplasma pulse, it seems most likely that the time lag arisesfrom a delay in formation of the EDL. Qualitatively, the timelag can be interpreted as the time needed after the breakdownof the gas to generate the downstream plasma, establish theequilibrium between the two different plasmas, and then toallow for charge separation to occur at their interface.To investigate the effects of pulsing conditions on theobserved time lag for fast ion creation in the LIF data, ivdfswere measured for the same duty cycle and different modu-lation frequencies 3.33, 5, and 10 Hz. As shown in Fig.3a, the higher the modulation frequency the shorter theobserved time lags. We hypothesize that long lived neutraland ionic metastable states survive during “plasma off” time,making the plasma ignition and apparently ion beam forma-tion faster for shorter time off intervals. Having longer“plasma off” times leads to complete extinguishment of ex-cited states between pulses, followed by a completely “new”breakdown and a new EDL formation. That the plasma offtime controls the time lag in the appearance of the fast ionpopulation was confirmed by holding the modulation fre-quency fixed at 5 Hz and varying the duty cycle from 50% to80%.14 Decreasing the “plasma off” time from 100 to 40 msresulted in a decreased 28 versus 7 ms time lag in theappearance of the fast ion population. A linear extrapolationto zero time lag predicts that no delay in the appearance ofthe fast ion population will occur for modulation frequencieslarger than 17 Hz 30 ms plasma off time.These observations suggest a resolution for an outstand-ing discrepancy between previous, time-resolved, LIF andresolved retarding field energy analyzer RFEA measure-ments in the ANU helicon source23 and in an identical heli-con source constructed at the L’Ecole Polytechnique ParisLPTP.24 The time resolved RFEA measurements indicatedthe presence of a small, but finite population of fast ionpopulation within 150–200 s of the beginning of the rfpulse. Besides the mechanical differences between HELIX-LEIA and Chi-Kung helicon sources, those RFEA measure-ments were obtained for considerably shorter plasma “off”intervals in a pulsed system; plasma on /off=2 /10 ms in theANU experiments and 750 /500 s for the LPTP measure-ments.Note that the slow and fast ion population parallel ve-locities do not vary with “plasma on” time: 0.5 and10 km /s for the parallel flow speed of the slow and fastion populations see Fig. 3b. Since the parallel velocity offast ion population is independent of the length of the plasmapulse, the energy of the ejected ion beam appears to be fixedfor a given system geometry and/or magnetic field configu-ration. Since the relative fast ion population density ob-tained from the integration of LIF signals depends onlymodestly on the modulation frequency from 0.6 at 3.33 Hzto 0.52 at 10 Hz, the overall thrust generated by a HDLTshould be invariant under changes in pulsing frequency.1C. Charles, Plasma Sources Sci. Technol. 16, R1 2007.2C. Charles, R. W. Boswell, P. Alexander, C. Costa, and O. Sutherland,Proceedings of 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Confer-ence, Sacramento, CA, 2006.3C. Charles, R. W. Boswell, W. Cox, R. Laine, and P. MacLellan, Appl.Phys. Lett. 93, 201501 2008.4C. Charles, R. W. Boswell, R. Laine, and P. MacLellan, J. Phys. D: Appl.Phys. 41, 175213 2008.5C. Charles and R. W. Boswell, IEEE Trans. Plasma Sci. 36, 2141 2008.6F. F. Chen, IEEE Trans. Plasma Sci. 36, 2095 2008.7K. Takahashi and T. Fujiwara, Appl. Phys. Lett. 94, 061502 2009.8S. Chakraborty Thakur, Z. Harvey, I. A. Biloiu, A. Hansen, R. A. Hardin,W. S. Przybysz, and E. E. Scime, Phys. Rev. Lett. 102, 035004 2009.9C. Biloiu, X. Sun, E. Choueri, F. Doss, E. Scime, R. Spektor, J. Heard, andD. Ventura, Plasma Sources Sci. Technol. 14, 766 2005.10I. A. Biloiu, X. Sun, and E. E. Scime, Rev. Sci. Instrum. 77, 10F3012006.11I. A. Biloiu, E. E. Scime, and C. Biloiu, Plasma Sources Sci. Technol. 18,025012 2009.12I. A. Biloiu and E. E. Scime, IEEE Trans. Plasma Sci. 36, 1376 2008.13I. A. Biloiu, Ph.D. thesis, West Virginia University, 2009.14K. P. Giapis, N. Sadeghi, J. Margot, R. A. Gottscho, and T. C. John Lee,J. Appl. Phys. 73, 7188 1993.15I. A. Biloiu, E. E. Scime, and C. Biloiu, Appl. Phys. Lett. 92, 1915022008.16X. Sun, A. M. Keesee, C. Biloiu, E. E. Scime, A. Meige, C. Charles, andR. W. Boswell, Phys. Rev. Lett. 95, 025004 2005.17F. Skiff, G. Bachet, and F. Doveil, Phys. Plasmas 8, 3139 2001.18B. Pelissier and N. Sadeghi, Rev. Sci. Instrum. 67, 3405 1996.19E. Scime, C. Biloiu, C. Compton, F. Doss, D. Ventura, J. Heard, E.Choueiri, and R. Spektor, Rev. Sci. Instrum. 76, 026107 2005.20G. Hairapetian and R. L. Stenzel, Phys. Rev. Lett. 65, 175 1990.21A. B. Sefkow and S. A. Cohen, Phys. Plasmas 16, 053501 2009.22G. Hairapetian and R. L. Stenzel, Phys. Fluids B 3, 899 1991.23C. Charles and R. W. Boswell, Phys. Plasmas 11, 3808 2004.24N. Plihon, P. Chabert, and C. S. Corr, Phys. Plasmas 14, 013506 2007.FIG. 3. Color online a Dependence of the delay on modulation frequencyand a linear extrapolation to zero delay. b Dependence of the slow downtriangles and fast up triangles ion parallel velocities and fast ion popula-tion fraction circles on modulation frequency. For all three frequencies theduty cycle was 50%.051504-3 I. A. Biloiu and E. E. Scime Appl. Phys. Lett. 95, 051504 2009Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Temporal evolution of double layers in pulsed helicon plasmas

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Temporal evolution of double layers in pulsed helicon plasmas

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