2017 Asia-Pacific Engineering and Technology Conference (APETC 2017) ISBN: 978-1-60595-443-1
Special Working Media for Laser Plasma Generators Loktionov Egor and Protasov Yuriy
ABSTRACT For condensed matter, laser ablation has been mainly considered for solids since high energy density can be reached in a surface layer, and plasma plume consists of light particles with a high speed. Although applied research has revealed numerous problems with reliable, long-term, exchangeable solid medium supply in industrial laser plasma generators prototypes. Despite feeding systems numerous advantages, poor laser ablation performance has been discovered for liquids even after special efforts. We have suggested and experimentally evaluated possible benefits for making laser plasma generators working media UV curable and magneto rheological – being supplied in liquid and irradiated in solid form. Depending on parameter (specific mass flow rate, momentum coupling, specific impulse, energy efficiency), performance improvement varied from 10-s of percents to 3 orders of magnitude at corresponding dopants concentrations down to percents. Use of special dopants considered could help to resolve main media supply problems for laser plasma generators.1 INTRODUCTION Laser plasma generation has been considered mainly for solids rather than liquids [1], since high energy density can be reached in a surface layer of the former, and plasma plume containing mainly light particles with a high speed. Early works declared simple solids supply systems (rotating disk or cylinder) as an advantage for laser thrusters, this was right for lab modeling. Applied research has revealed numerous problems with reliable, long-term, exchangeable solid medium supply in industrial laser plasma generators prototypes though. According to laser ablation specific energy, medium dosing should be available down to 10-5 g/J, output 10-6 g/s to 1 g/s with stability better than 5%, and virtually no capacity limits. Life time possibly close to lasers source (104 hrs for diode lasers), here resistance to laser impact would be crucial. Energy efficiency, low dead weight and volume, possibility for auxiliary impact (electric, magnetic, etc.), and medium exchangeability would be also demanded for a number of applications. Liquid media were considered because of easy desired contents preparation (properties variation) [2], well developed precise dosing (down to 10-15 L and 0.1% LOKTIONOV Egor1,a and PROTASOV Yuriy1,a 1 Bauman Moscow State Technical University, 5-1, 2nd Baumanskaya Str., Moscow 105005, Russia a
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stability [3]) and feeding systems. The main problems at laser impact on those were energy dissipation [4] and droplets formation due to splashing [5], leading to poor performance [6] even after application of special tricks like viscosity increase [7], thin films [8], cavities or droplets [9] formation. To the best of our knowledge, this trade-off has never been considered before in terms of in situ solidification of a supplied liquid [10]. Solidification can be induced by several means, the fastest and the most temporally and spatially controllable ones are light and electron beam curing [11]. Curing time is down to 10-2 s and layer can be varied 10-5 to 10-2 m. At high laser power irradiation with certain wavelengths curing may take place at impact itself, i.e. no pre-pulse is needed [12] since curing speed is proportional to light intensity square root. Similar to photo condensation of supersaturated vapor, supersaturated solutions of numerous substances can undergo light-induced crystallization [13]. This process typically takes tens of seconds, limited not to the impact zone only, but also throughout the entire available volume eventually. Medium supply, dosing and positioning are now well developed in paper (inkjet, laserjet, sublimation, UV) and 3D-printing (extrusion, light polymerization, powder bed, laminated, wire) [14]. However, supply system reliability is generally defined by number of moving parts and mechanisms complexity. Some physical effects could reduce need in moving parts for liquids: capillary effect, ultrasound (cavity resonance, capillary effect enhancement) [15], electro osmosis, electro spray [16], magneto hydrodynamic (MHD) [17], piezoelectric [18, 19] and magneto elastic [20, 21] seem to be most applicable. For bulk solids dosing is not needed, and supply could be substituted by surface laser scanning system, which would be great for asteroid or used spacecraft fed missions. Although electro optical [22] and linear drives [23] need high voltage and current sources respectively, that is not always possible, e.g. due to electromagnetic compatibility issues. Ferro fluids are known for their ability to move and form itself without applying mechanical force [24], just following the magnetic field that could be easily arranged with electromagnets, viscosity and virtual hydrophobicity could be also adjusted proportional to the magnetic field strength and magnetic particles size [25] (electrorheological effect leads to viscosity increase by 5 orders of magnitude in milliseconds, but at 10 kV/cm electric field strength [26, 27]). Moreover, magnetic field stops and retrieves droplets formed at laser impact [28] along non-magnetic nozzle walls. This would reduce mass loss and make plume more homogenous. The amount of stock materials making a liquid light-curable or magneto-rheological is of 10% vol. order [29]. To conclude, among variety of solid and liquid supply systems developed there’s still no really good solution for laser plasma generators. Main shortcomings are related to exact medium and system performance restrictions or to laser plasma production poor performance. The aim of our research was to find out possible benefits of using special media, ability to add special properties to usual working media, and to combine several special properties for working media and supply systems choice trade-offs resolving.
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EXPERIMENTAL LAYOUT Laser impact was performed on acrylic UV printing ink (SPC-0659Y, Mimaki) in liquid and cured states, it and 3D printing ink (UVresin, 3D Ink) were mixed with a Ferro fluid (EFH1, Ferrotec) to find out special properties combination possibility. The liquids were deposited on a coverslip substrate in droplets of ~0.5 L with a dosing pipette (Finnpipette Digital). These targets were weighed using analytical scales (CAS CAUW-120D) with an accuracy of 10 g at all stages of the investigation. The targets were placed horizontally, attached to a calibrated PVDF-film force sensor (PZ-02, Images SI) connected to an OEM signal conditioner, and then to digital oscilloscope (2024B, Tektronix). The target was cured by exposing it for ca. 2 s to a 405 nm diode laser (LSR405NL, Lasever) ~0.8 W/cm2 radiation, an optimal wavelength for studied ink curing is 36515 nm [30]. For laser ablation we used five harmonics (1064 nm, 532 nm, 355 nm, 266 nm, and 213 nm) of nanosecond (18 and 12 ns) Nd:YAG lasers (LS-2147, Lotis TII; LQ929, Solar). The focal spot size (~0.02 mm2) was measured using an optical microscope. Optical characteristics were studied using spectrophotometer (SF-2000, OKB Spectr) in the interval 190–1100 nm. The Raman spectra were measured using R-3000 complex (Raman Systems; 785 nm, 200–2700 cm–1). For supply system durability tests piezo micro pump (7616, Burkert) driven by a pulse generator (DG645, Stanford Research Systems) with OEM current amplifier was used for non-magnetic liquids and OEM solenoid on a glass capillary – for magnetic ones. A ZrO2 ceramic ferrule for optical fibers (230 m capillary – CF230, Thorlabs) was used to produce droplets at its tip. To create magnetic field we used commercial cubic Nd-magnets, field strength at droplet upper layer was measured by a calibrated magnetometer (ATE-8072, Aktakom) and could be controlled by magnet size and it’s distance to the target substrate. Cylindrical (9 mm dia. x 90 mm) non-magnetic (glass) nozzle was place between the target and force sensor (plume interacted with transparent to laser radiation cover slip attached to the sensor’s tip). EXPERIMENTAL RESULTS Solidification dynamics of UV ink was investigated in a number of applied studies [31]. The methods of diagnostics used in those studies were based either on light scattering by polymer chains or on IR absorption measurement. Our analysis of Raman spectra revealed peaks similar to the ones known for polymethylmethacrylate (PMMA). The luminescence quantum yields at some wavelengths were also different for the liquid and solidified states. And, of course, simple mechanical tests could evidence curing. For mixtures containing a Ferro fluid, stop of reaction to magnetic field could also be used. In contrast to the case of water and ice [32], momentum coupling coefficient for liquid UV ink is higher not due to an amplitude, but due to a longer impact. Same to other liquids, laser irradiation resulted in a number of droplets with a characteristic size of ~0.2 mm, this substantially increased the mass flow rate and reduced the mass averaged velocity. At 1064 nm irradiation, either glass substrate or even underlying PVDF force sensor (not UV ink) were damaged due to low spectral absorption. For the second 2117
harmonic, efficiency was the lowest of obtained. At 266 nm irradiation solid and liquid phases of investigated UV-inks have demonstrated about similar characteristics relation known for water and ice [32]: liquid phase had greater ablation threshold (4.67 vs. 1.02 J/cm2 – the lowest of obtained), mass flow rate (6.77 vs. 0.35 mg/J) and momentum coupling coefficient (0.933 vs. 0.167 mN/W), solid phase had greater specific impulse (476 vs. 138 s), and energy efficiency was about equal, data obtained for cured ink match well those known for PMMA [33]. At 213 nm an unexpected ablation threshold increase was observed, that could be explained by intensive luminescence energy losses – it’s 5 times more intensive for liquid ink and results in about the same ratio of ablation thresholds of liquid to cured states the highest of obtained. The ablation rate of plain Ferro fluid laser ablation was proportional to the viscosity increase induced by magnetic field: 2.1*10-1 g/J without field, and 1.6*10-1 g/J at 140 mT. Once a glass nozzle was added, mass loss has been reduced by 3 orders of magnitude – down to 2*10-4 g/J at 480 mT, that is about equal to solid polymers. We performed a special study of the curing of UV ink mixtures with ethanol, isopropanol, and nitromethane, which revealed that photo polymerization of thin layers occurred when ink concentration was down to at least ~10 % vol. Same results were obtained for Ferro fluid, and for adding both Ferro fluid and UV ink. However, there should be certain mixing limitations due to liquids solubility, reactivity and so on. DISCUSSION The most interesting results were obtained at 355 nm irradiation that matches curing optimum specified for our UV ink. Data on specific ablated mass and momentum coupling show that high-power pulsed radiation induces curing (its rate is proportional to square root of radiation intensity) at nanosecond scale, so irradiation performance became about equal for initially solid and cured medium for fluency over 40 J/cm2. Maintaining a balance between the light cured and ablated layers thicknesses for a long time is a challenging problem because of the ferrule clogging with the cured ink. No capillary clogging was observed for irradiation at 355 nm (without initial curing) and 1064 nm, since the cured layer thickness is less than the ablated layer. As the UV ink concentration decreases, so does the photo polymerization rate, thereby reducing the probability of capillary clogging. Reactive solvents are known to reduce the polymerization rate and polymer molecular weight, whereas inactive ones do increase those. So, varying mixture contents and curing irradiation proper cured state could be adjusted. Despite significant reduction of mass loss of Ferro fluid in the magnetic field with a non-magnetic nozzle, momentum coupling coefficient and energy efficiency were improved only by 30% and 50% – to 4*10-3 N*s/J and 3% respectively – at levels characteristic to liquids. This happens because ablation rate is still usual for liquids, but magnetic droplets are stopped by the field and then retrieved back to the target along nozzle walls resulting in low mass loss after all. This effect could be used when mass consumption, not power expense is an issue.
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We suggest that best performance could be reached by using high-power laser resistant intermediate surfaces for liquids regenerative deposit to avoid negative multiple impact effects. Medium residuals could be removed mechanically, chemically (cured UV ink become resoluble after irradiation to 250 nm), by magnetic or electrostatic field. CONCLUSIONS At the moment, there are no practical limits for laser generators supply systems elements specific performance parameters, but there are multiple trade-offs for combination of those. Liquids and solids advantages could be combined at light curing of liquids; curing rate may be adjusted by liquid contents and irradiation conditions for better actual performance (e.g. Cm/Isp ratio adjustment). Use of Ferro fluids could benefit to laser plasma generators supply systems by high output and reliability MHD pumping; minimum evaporation to vacuum; storage system simplification by keeping in one bubble or spread along magnetized walls even at 0-gravity; secondary use for cooling due to magnetic field sustained convection. Combination of light curing and magneto rheological abilities discovered to be possible by adding 10% vol. of corresponding stock liquids to the ‘main’ one. There’s still room for other special dopants permanent or temporary injection. ACKNOWLEDGEMENT Our experiments were performed at the “Beam-M” facility, following the governmental task and grant 14.Z56.16.5407-MK by the Russian Ministry of Education and Science, and supported by the Russian Foundation for Basic Research (grants 14-08-01087 and 15-38-20890). REFERENCES [1] C.R. Phipps, ed. Laser Ablation and its Applications, Springer Science, New York, 2007. [2] T. Lippert, L. Urech, R. Fardel, M. Nagel, C.R. Phipps, A. Wokaun. Materials for laser propulsion: "liquid" polymers, Proc. SPIE 7005 (2008) 700512-10. [3] D. Li, ed. Encyclopedia of Microfluidics and Nanofluidics, Springer, New York, 2015. [4] J.E. Sinko, A.V. Pakhomov. Laser Propulsion with Liquid Propellants Part I: an Overview, Proc. AIP 997 (2008) 195-208. [5] R. Fardel, L. Urech, T. Lippert, C. Phipps, J. Fitz-Gerald, A. Wokaun, Laser ablation of energetic polymer solutions: effect of viscosity and fluence on the splashing behavior, Appl. Phys. A 94 (2009) 657-665. [6] J. Sinko, Time resolved force and imaging study on the laser ablation of liquids, Huntsville, 2005. [7] S. Choi, T.-h. Han, A. Gojani, J. Yoh, Thrust enhancement via gel-type liquid confinement of laser ablation of solid metal propellant, Appl. Phys. A 98 (2010) 147-151. [8] B. Wang, L. Li, Z.-p. Tang, J. Cai, Experimental Investigation of Liquid-propellant Laser Propulsion with a Horizontal Momentum Measuring Lever, Proc. AIP 1230 (2010) 243-253. [9] X.-q. Li, Y.-j. Hong, G.-q. He, Reviews of the propulsive characteristics study on liquid propellants for laser propulsion, J. Propulsion Technol. 31 (2010) 105-110. [10] E.Y. Loktionov, Study of laser ablation efficiency for an acrylic-based photopolymerizing composition, J. Appl. Spectroscopy 81 (2014) 305-308. [11] C. Decker, C. Bianchi, Ultrafast hardening of a modelling paste by UV-curing of a polyamide filled acrylic resin, J. Materials Science 40 (2005) 5491-5497.
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