mach2 simulations of a micro laser ablation plasma thruster

Aerospace Science and Technology 11 (2007) 481–489

www.elsevier.com/locate/aescte

MACH2 simulations of a micro laser ablationplasma thruster

T. Moeller a, Young-Keun Chang b,∗

a University of Tennessee Space Institute, Tullahoma, TN 37388, USAb School of Aerospace and Mechanical Engineering, Hankuk Aviation University Hwajeon-Dong, Duckyang-Gu, Goyang City 412-791, Korea

Received 16 June 2006; accepted 15 February 2007

Available online 22 February 2007

Abstract

In this work the detailed physical processes occurring in the high density plasma that is ejected from the solid propellant surface in a small laserablation thruster are simulated using MACH2. Qualitative results of the laser ablation process that leads to propellant erupting from the surfaceand leaving behind a crater in a solid Teflon® propellant are presented. Simulations were conducted for a 0.5 µs laser pulse (FWHM) at 935 nmwith laser pulse energy ranging from 20 µJ to 2 mJ. Simulation results indicate that crater diameter and depth increase with pulse energy. Theimpulse bit also increases with pulse energy. Specific impulse follows the opposite trend and decreases with laser pulse energy. The simulatedimpulse bit for a 2 mJ, 0.5 µs laser pulse over-predicts that reported in the literature for a 2 mJ, 2 ms laser ablation thruster pulse by approximatelyone order of magnitude and under-predicts the specific impulse by approximately one order of magnitude.© 2007 Elsevier Masson SAS. All rights reserved.

Keywords: Laser ablation thruster; Laser thruster simulation; Laser ablation simulation

1. Introduction

In recent years, interest in the reduction of satellite sizeand mass has grown. The main benefit of ultra-small satellitesless than 100 kg is low-cost development and reduced launchcost [10]. The reduction of satellite size requires componentsminiaturization and compact electronic packaging. Generally,the sizing of actuators for satellite attitude and orbit control nor-mally scales with the size of the satellite. The micro thrusteris frequently implemented as an actuator for small satellites.As satellite mass is reduced, the mass and power budgets forthe thruster systems is lowered accordingly. The kilowatt powerlevel available on large satellites is being reduced to less than100 Watts for microsatellites (100 kg class) and nanosatellites(10 kg class). Since the mass and power requirements of flight-ready satellite propulsion systems have precluded their use onsmall satellites, micro thruster systems must be developed.

* Corresponding author. Tel.: +82 2 3158 2286; fax: +82 2 3158 2191.E-mail address: [email protected] (Y.-K. Chang).

1270-9638/$ – see front matter © 2007 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.ast.2007.02.002

Reduction of thruster system size is not trivial. As physicalsize is reduced, the efficiency of thruster devices tends to dropoff significantly [6,18]. Many organizations around the worldare developing microsatellites and nanosatellites [9] with de-velopment of miniature technologies. These satellites requiremicro propulsion systems. Even with advanced space propul-sion devices, such as electric propulsion thrusters, whose largerversions have propulsion efficiencies that far exceed those oftheir chemical counterparts, the efficiency of electric propul-sion (EP) devices diminish as size is reduced [6,18]. Minia-ture electric propulsion devices have been studied, includingHall thrusters [7], microwave heated thermal thrusters [18], andpulsed power thrusters [1]. Most of these EP devices operate inthe 40 to 100 W power range [1,2,7,18]. These systems requirepower processing units that have a significant mass penalty,and many operate with gaseous propellant that requires a tankfor propellant storage. Laser ablation plasma thrusters offer analternative that has a solid propellant and a diode laser thatdoesn’t require significant power processing [13].

In the present work, the laser absorption and ablation processin a laser ablation thruster is modeled with MACH2 in an effortto gain insight into the physical processes that occur in these de-

482 T. Moeller, Y.-K. Chang / Aerospace Science and Technology 11 (2007) 481–489

vices. Extensive modeling of a laser ablation thruster has beenperformed using the direct simulation Monte Carlo (DSMC)code MONACO [4,5]. The DSMC modeling focused on thelower density plasma in the expansion of the plume and pro-vided realistic plume expansion simulations. The present workfocuses on the detailed physical processes occurring in the highdensity plasma that is blown out of the solid propellant surfaceto form thrust.

2. Laser ablation

The interaction of a laser with the surface of a material de-pends strongly on the irradiance level at the surface. Irradiancelevels for laser ablation applications tend to be between 106 and1010 W cm−2 [21]. Four phases of interaction between laserlight and the ablatant can be distinguished and separated byphase transitions and plasma ignition [21]. The first phase of in-teraction is described by a thermal heating mechanism throughwhich the laser light elevates the target temperature. Energydeposited into the material is quickly dissipated through con-ductive losses. This phase occurs at lower irradiance levels. Thesecond phase of interaction occurs when irradiance is increasedto the point where the surface temperature reaches the melt-ing point or sublimation temperature. A time-dependent plumeabove the surface and a liquid/solid interface that penetrates be-low the target surface are generated. The plume is optically thinin this phase [21]. The third phase begins when irradiance ishigh enough to create a plume density that is too high for theplume to be neglected, and hydrodynamic effects in the plumeare important. When the plume becomes optically thick and ir-radiance exceeds the plasma ignition threshold, a plasma forms,and laser absorption in the plume via inverse Bremsstrahlungabsorption becomes significant [21]. The plasma in the fourthphase totally absorbs the incoming laser light, and the targetsurface is shadowed by the plasma in the plume. The transferof energy from the plasma to the target surface is dominatedby plasma radiation [21]. The phase of interaction between thelaser and the propellant surface in laser ablation thrusters isbelieved to be classifiable in any of the last three phases, de-pending on the irradiance.

3. Laser ablation thrusters

Laser ablation thrusters of various types have been reportedin the literature. Zaidi et al., have reported work on an abla-tive laser thruster that utilizes a magnetic nozzle created witha 4.5 Tesla field and a 300 mJ pulse focused onto an alu-minum or Teflon target [22]. The laser pulse energy used inZaidi’s work [22] is approximately two orders of magnitudelarger than the µ-Laser Ablation Thruster (µ-LPT) developedby Phipps [13].

Phipps’ microthruster [12–14] is operated by focusing fiber-coupled semiconductor diode lasers that are incorporated intothe thruster onto a target to produce a small jet of ablated ma-terial. The µ-LPT has two modes of operation, a “Transmis-sion Mode” and a “Reflection Mode” [13]. In the transmissionmode, the laser beam passes through a transparent substrate film

from the back side. A layer of absorbing material on the frontsurface ablates away from the laser. In the reflection mode, thelaser impinges incident on the target material, and ablated ma-terial reflects from the surface. In the reflection mode the opticscan become coated with material; operation in the transmissionmode alleviates this problem. In both the reflection and trans-mission modes, the material ablated by the laser pulse leaves acrater in the surface of the solid propellant and thermally ex-pands out from the crater to form thrust [13].

The µ-LPT was originally tested in the ms-pulse regime[12,13]. For a ms-pulse device, the propellant material is con-strained to low thermal conductivity materials, such as poly-mers [8]. Early ms-pulse tests utilized a wide array of ablatantmaterials for non-exothermic propellants. Of these, PVC wasone of the most practical and offered the best thruster perfor-mance [12]. The MACH2 simulations described in the presentwork have a similar geometry to that of the µ-LPT operating intransmission mode.

4. MACH2 simulations of a micro laser ablation thruster

4.1. Description of computational model

MACH2 [11] is a time-dependent MHD computer code thatcan be used to simulate axisymmetric systems. Complicatedgeometries can be modeled with quadrilateral cells in the com-putation domain that are divided into blocks. Each of theseblocks can be assigned a different material and conditions.MACH2 solves the continuity, energy, and two componentsof the momentum equations for density, the radial and axialcomponents of velocity, and the internal energy [16]. The sim-ulations presented in this work take place on Eulerian meshes.Equations of state for the materials are tabular, and a Spitzermodel for electron thermal conductivity is utilized.

In an ionized gas, a free electron can pass near an ion andinteract with its electric field. This event can produce a free-free radiation transmission. This form of radiation is calledBremsstrahlung, or breaking, radiation. The electron can un-dergo a photon absorption resulting in higher kinetic energy, orit can emit a photon and drop to a lower free energy [17]. In thelaser ablation process, as soon as a plasma is formed in frontof the target, it is typical that a large portion of the incidentlaser light is absorbed in the plasma via the Bremsstrahlungprocess [3]. In the laser absorption modeling in this work,transfer of energy to the propellant occurs via the inverseBremsstrahlung process. Details of this model are describedelsewhere [19].

4.2. Laser ablation thruster simulation

The simulation results presented in this paper are early re-sults in an effort to develop a MACH2 model that can predictthe physical behavior of the solid propellant during laser ab-lation. In the configuration studied, the laser passes through a127 µm thick transparent plastic substrate and focuses onto thebase of a 52 µm absorbing layer of plastic propellant (Fig. 1)with a laser spot diameter of 5.2 µm. The laser light is absorbed

T. Moeller, Y.-K. Chang / Aerospace Science and Technology 11 (2007) 481–489 483

Fig. 1. Schematic showing the geometry of the laser ablation thruster.

Fig. 2. Computation grid utilized in laser ablation thruster simulations.

on the bottom surface of this layer and forms a high temper-ature, high pressure pocket that erupts through the top surfaceof the propellant and expands into low-pressure (5 Torr) dry airto generate thrust. This geometry is based on that described inµ-LPT experiments described in the literature [13] in which thesubstrate was clear acetate and the propellant was PVC.

The computation domain for simulations is shown in Fig. 2,and details of the laser focal region are shown in Fig. 3. Thecomputation domain is comprised of eight blocks with a totalof 56 radial cells and 116 axial cells. The two blocks (1 and 2) atthe bottom represent the clear substrate and contain a transpar-

Fig. 3. Detail of Laser Focus Region of Computation grid.

ent plastic. The next two rows of blocks (3 through 6) representthe absorbing Teflon® propellant, and the two blocks (7 and 8)on top contain dry air at 5 Torr. The laser light propagates fromthe bottom left block towards the Teflon® propellant in block 3.Note that the angle of convergence in block 1 is representativeof f/2 focusing optics. Thrust is generated when propellantmaterial is ejected from the propellant surface into the blockscontaining the dry air.

PVC equation of state property tables that would allow themodeling of this material in the solid phase is not available.This precludes the use of PVC as the propellant in the simu-lations. However, extensive property data for solid Teflon® isavailable. Therefore, Teflon® was selected for the propellant inthis modeling effort.

Acetate is the optically clear substrate upon which the PVCpropellant is placed in the µ-LPT [13]. Again, Teflon® was se-lected for the substrate material in the MACH2 simulation. Itis important to note that Teflon® readily absorbs laser light at935 nm, the wavelength utilized in the µ-LPT experiment [13].The Teflon® in the simulated substrate layer was forced to beoptically transparent at 935 nm by forcing the number of freeelectrons per ion to a value at which the laser light experiencesinsignificant inverse Bremsstrahlung absorption.

In the µ-LPT experiments performed by Phipps [13], thebackground pressure was held below 0.3 mTorr. In the MACH2simulation, dry air at a pressure of 5 Torr was selected forthe background gas into which propellant expands because oflimited equation of sate property data for pressures below thisvalue.

5. Results and discussion

In the µ-LPT experiments [13], a 935 nm laser pulse 2 mslong with an average power of 1 W is presented. This corre-sponds to a total pulse energy of 2 mJ and an average laser spotirradiance of approximately 2.3 × 104 W cm−2 (for a laser spotdiameter of 5.2 µm). Because computation times for the simu-


Fig. 4. Density (kg m−3) contour plot of 20 µJ laser pulse case at 0.3 µs.


lation of a two millisecond pulse are too large to preclude theiruse in this study, a laser pulse with a Gaussian profile and a fullwidth at half maximum (FWHM) of 0.5 µs was utilized. Thepeak intensity of the pulse was set to occur at 0.75 µs, and sim-ulations were run for a minimum of 2 µs. Six simulations wererun with total laser pulse energy ranging from 20 µJ to 2 mJ.This corresponds to an average laser spot irradiance that rangesfrom approximately 1.9 × 108 W cm−2 to 1.9 × 1010 W cm−2.In the discussion that follows, a qualitative description of thereaction of the propellant and substrate to the incident laser willbe presented. That will be followed by a discussion of quantita-tive results.

When the laser light focuses on the base of the Teflon®propellant, the absorbed energy causes the temperature andpressure to rise rapidly. This causes a low density bubble toform at the interface between the propellant and the transpar-ent substrate. The size of this bubble continues to grow with



the continued addition of energy. As energy is absorbed andadditional Teflon® propellant is incorporated into the growingbubble, and the top surface of the propellant bulges. Eventu-ally the bulging surface of the Teflon® is blown out, and theTeflon® plasma within the bubble erupts into the low densitydry air above. This process is readily seen in Figs. 4 through 12which show representative simulation results (20 µJ case) forthe Teflon® density in 0.2 µs increments from 0.3 µs through1.9 µs, respectively. Plots of the laser energy absorbed in theTeflon® between 0.3 µs and 1.5 µs, the range of time in whichthe laser energy is significant, are shown in Figs. 13 through 18,respectively. It is interesting to note that the hot Teflon® propel-lant inside of the bubble heats the transparent substrate material,a portion of which melts and vaporizes to leave behind a craterin the substrate. Note that absorbing Teflon® enters this crater,and laser absorption in this region results. The resulting crater(Fig. 12) is similar in shape to that described by Phipps [13]




in µ-LPT testing. During eruption, the ejected material rapidlyexpands to form a plume, which is shown in Fig. 19 at a timeof 1.9 µs. This plume is similar to that presented for the µ-LPTresearch presented by Phipps [15]. These MACH2 simulationresults suggest that the laser ablation thruster is operating in oneof the last two phases of laser ablation discussed in Section 2.

For the same laser pulse length of 0.5 µs, simulation resultsindicate that crater diameter and depth both increase with laserpulse energy (Fig. 20). The total mass ejected from the crateris consistent with these trends (Fig. 21). The momentum asso-ciated with this ejected mass is equivalent to a single impulsebit associated with the laser pulse. As expected, the impulsebit trends higher with laser pulse energy (Fig. 22). It should benoted that the experimental data point shown in Fig. 22 is cal-culated from data corresponding to a 2 mJ, 2 ms pulse reportedby Phipps [13]. This experimental data point is approximatelyone order of magnitude below that realized in the 2 mJ simu-



lation. This over-prediction by the simulation is caused by thesimulated laser pulse being delivered in four one-thousandthsof the laser pulse time utilized in the experiment. The shorterpulse time in the simulation results in higher irradiance thatleads to higher pressures and temperatures than are expectedin the longer laser pulse. The physical property differences be-tween the simulated propellant (Teflon®) and that used in theexperiment (PVC) also likely contribute to the over-predictionin impulse bit for the 2 mJ case.

The specific impulses associated with the six different sim-ulated laser pulse energies are shown in Fig. 23. As total laserpulse energy increases, the specific impulse trends lower fromapproximately 80 s down to approximately 17 s. In the µ-LPTtesting [13] upon which the simulated thruster geometry isbased, specific impulses range from approximately 100 s to500 s. The simulated specific impulse under-predicts the ex-periment by roughly one order of magnitude. This discrepancy



Fig. 13. Contour plot of the absorbed laser energy (J) during the time step at0.3 µs for the 20 µJ laser pulse case.

is believed to be associated with the difference in laser pulselength (higher irradiance used in the simulations) and the use ofa different plastic propellant in the model.

6. Conclusions

In this work the detailed physical processes occurring in thehigh density plasma that is ejected from the solid propellant sur-face in a laser ablation thruster were simulated using MACH2.Qualitative results of the laser ablation process that results inpropellant erupting from the surface to leave behind a craterin the solid Teflon® propellant were presented. Plume shapeand general crater geometry are similar to those presented inthe literature [15,20]. The simulations give a realistic descrip-tion of the laser ablation process. This suggests that further



MACH2 simulations might provide reasonable information onthe conditions of the mass entering the plume, information thatis required input for DSMC codes often used for the modelingof plasma thrusters.

Simulations were conducted for a 0.5 µs laser pulse withlaser pulse energy ranging from 20 µJ to 2 mJ. Crater diameterand depth increased with pulse energy. The impulse bit also in-creased with pulse energy. Specific impulse followed the oppo-site trend and decreased with laser pulse energy. The simulatedimpulse bit for a 2 mJ, 0.5 µs laser pulse over-predicted that re-ported in the literature for a 2 mJ, 2 ms laser pulse in a laserablation thruster [13] by approximately one order of magnitudeand under-predicted the specific impulse by approximately oneorder of magnitude. These discrepancies are believed to result




from a simulated irradiance 4,000 times that realized in the ex-periment. Future MACH2 simulations with a 2 mJ, 2 ms pulseare expected to improve these results. The generation of PVCequation of state tables is required before direct quantitativecomparison with the Phipps experimental results [13] can bemade.

Acknowledgements

Dr. Trevor Moeller and his colleagues at the University ofTennessee Space Institute performed all MACH2 modeling andanalysis of results, and Dr. Young Chang of Hankuk Avia-tion University provided valuable input on microsatellites andpropulsion needs. Much appreciation must be extended to Dr.


Fig. 19. Contour plot of the Teflon® propellant mass fraction for the 20 µJ laserpulse case.

Dennis Keefer and Mr. Robert Rhodes for their invaluable in-put during the development of this work. This research has beenpartially supported by “National Research Laboratory (NRL)”contract awarded by Korean Ministry of Science and Technol-ogy (MOST). Authors would like to appreciate MOST for thefinancial support.


Fig. 20. Impact of total laser pulse energy on crater diameter (µm) and substratecrater depth (µm).

Fig. 21. Effects of total laser pulse energy on mass ejected from Teflon® pro-pellant and transparent substrate.

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mach2 simulations of a micro laser ablation plasma thruster

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