Experiments on a Propellant-less Electric Propulsion Using Photon Pressure

Kyoichiro Toki1, Norihiro Asakura2, Tomohiro Ohtsuka3 and Taku Akazawa4 Tokyo University of Agriculture and Technology, Toky, Japan

E-mail: toki@cc.tuat.ac.jp

A Nobel prize laureate of 1984 Carlo Rubbia who predicted the existence of weak boson says that the blackbody radiation can produce usable thrust density even in the moderate range of cavity temperature around 3,000 K. In this paper a few experiments were performed to evaluate the thrust produced by photon pressure. One is the thrust from 300-400 W incandescent tungsten filament placed on the focal point of a parabolic Winston mirror with the exit diameter of 12 cm. Another is the thrust from a CW semiconductor laser of 100 W-class. The experimental results showed that the thrust from the tungsten filament of 400 W input power produced 0.2 μN corresponding to the thrust efficiency of 15% and the semiconductor laser produced 0.14 μN at the total input power of 270 W also corresponding to the thrust efficiency of 15%.

Nomenclature c = speed of light d = distance E = energy F = thrust g0 = gravitational acceleration at sea level h = Planck’s constant p = momentum of photon P = power S = area V = voltage ε = emissivity ε 0 = permittivity at vacuum η = thrust efficiency ν = frequency of light

I. Introduction HE first sequence of a newly launched satellite / spacecraft is to deploy its solar array to provide electrical power for the onboard devices. The solar radiation density reaches 1,370 W/m2 facing normal to the sun at an

altitude of 1,000 km from the Earth. This value is called Solar Constant. Recently, a triple-junction solar cell demonstrates 30 % or higher conversion efficiency, however, even in this value, the deploying area of the solar panel amounts to 2.5 m2/kW. Modern satellites usually require several kW, and hence huge solar panels must be deployed in space. A certain kind of satellite deploys a huge antenna having a diameter of 10 m or larger, so the projected area of the satellite is surely increasing. When the solar radiation pressure acts on this huge area, it gives a good deal of disturbance against the attitude and/or station keeping. Actually, as shown in Fig. 1, the solar radiation

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1 Professor, Mechanical Systems Engineering, 2-24-165 Naka-cho, Koganei-city, Tokyo 184-8588, Member AIAA. 2 Graduate student, ditto, Non-member. 3 Graduate student, ditto, Non-member. 4 Undergraduate student, ditto, Non-member.

This work is supported by JSPS Grant-in-Aid for exploratory research.

44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit21 - 23 July 2008, Hartford, CT

AIAA 2008-4819

Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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pressure is 46 mN per 100 m2 normal to the sun at altitudes higher than 1,200 km. This photon pressure is comparable to the air-drag encountered at the upper atmosphere. The well-known disturbances affecting the spacecraft are air-drag, solar and lunar gravitational forces, and gravity gradient torque from the Earth. Besides these disturbances, a satellite consumes kilograms of propellant every year for the compensation of the above photon pressures. Recently the averaged satellite lifetime is extended to 10 years but it is not determined by device longevity but the depletion of propellant for attitude control / station keeping.

When one hears the words “radiation from the sun”, one can easily imagine “the solar wind” consisting of ultra high energy particles, but in fact the solar photon pressure is larger than the solar wind by 3 orders of magnitude. So, the reversal concept is necessary. If the photon has momentum and affects the spacecraft attitude and orbit, we would rather take advantage of it as the useful thrust. The solar sail is one of the methods, however, the solar radiation vanishes inversely proportional to the distance squared from the sun. Another solution is a concept of energy direct conversion to thrust. We propose that the thrust production from an intense light emission source on the spacecraft. As long as the thrust production relies on the light emission, this system requires no propellant but only power source for photon emission. Thus, the spacecraft lifetime is no longer determined by depletion of the propellant. The major objective in this paper is a feasibility study of the propulsion with energy direct conversion. We have started with a measurement system development for the evaluation of thrust efficiency in a thruster having a grey-body radiation source as the photon pressure propulsion.

Fig. 1 Solar photon pressure vs. orbit altitude.

II. Procedure for Experiment A. Grey-body Radiation Source

In the far future, if a very high density and ultra-lightweight energy source becomes available, it enables a space propulsion where the photon pressure is directly converted into thrust. This idea was already proposed by a physicist Carlo Rubbia who was the Nobel prize laureate of 1984. Unexpectedly he suggested the utilization of a blackbody radiation shown in Fig. 2 instead of coherent laser light.1 As shown in this figure, even a 1 cm2 blackbody radiation of 2,700 K is able to produce 1 μN if a 100 % conversion efficiency is assumed. Since this is an ideal case of perfect blackbody radiation, actually a great deal of input power is lost due to grey-body radiation with the emissivity less than unity or other reasons. B. Experimental Set-up

First of all, we prepared a tungsten filament as a grey-body radiation source which is made from a 10 mm diam. coil of tungsten wire with a diam. of 0.65 mm and 30 cm in length extended. Calculated from the tungsten characteristics of electrical resistivity–temperature relationship, the ideal 1 μN thrust will be generated at 2,400 K with electrical input of 22 A×14 V. At this point the tungsten evaporation rate is 4.36×10-10 g/cm2s corresponding to the lifetime of 2×107 s. This value is unsatisfactory for practical application, but satisfactory for laboratory experiments.2-3 Figure 4 shows a parabolic mirror like a “Winston Cone” with an exit diameter of 12 cm and in the vicinity of its focal point, the tungsten filament is placed. This mirror is made of SUS-303 of which interior was machined by NC lathe of the Tokyo University of Agriculture and Technology.

Fig 2. Thrust density expected by an ideal blackbody radiation.

C. Semiconductor Laser Light Source Besides the parabolic Winston mirror with a tungsten

filament, we also prepared a semiconductor laser as another light source. A CW semiconductor laser was assembled using a laser head nLight CASCADESTM C1-808-100, an electrical power source VueMetrix Vue-HV 5.0V-120A, a water cooling system ORION RKS400F-VS 0.89kW and an exclusive ventilated clean box installation as shown in Fig. 4. This laser source emits an laser beam apature of 100 μm × 1 cm with the divergence angles of 10ο in slow-axis and 2ο in fast-axis. The maximum output power is 100 W at the center wavelength of 808 nm with a conversion efficiency of about 48%. When we use the semiconductor laser as the light source, we removed the parabolic Winston mirror and introduced the laser light from outside through a vacuum chamber window made of fused silica. The reflected laser beam on the aluminum foil target was dumped into the vacuum chamber by random reflection.

Fig. 3 The tungsten filament (upper) and the parabolic mirror machined (lower).

D. Experimental Procedure

The electrical power source provides 40 A- 150 Vdcmax to the tungsten filament. The current value is determined by the estimated electrical resistivity at the equilibrium temperature. Because the expected thrust production is sub-μN, we had to design and assemble a torsion-balance precision thrust stand as depicted in Fig. 5. A 12 μm thickness of 200 mm square aluminum foil receives photon pressure. The thin aluminum foil behaves as if it is a rigid body against the thrust of μN order. This is stapled to 1 mm diam. copper rod and suspended by 10 mm apart parallel copper wires of 0.1 mm diam. from a vacuum chamber ceiling. It is electrically insulated by an acrylic plate. The arm length from this fulcrum to the center of the aluminum foil is 100 mm. All these parts must be assembled by metallic parts because the thrust calibration is done by voltage application via these thin copper wires. The opposite end of the copper rod stapled

Fig. 5 Schematic of experimental setup of torsion balance type thrust measurement device with a parabolic Winston mirror.Fig. 4 A 100 W-class CW semiconductor laser assembly

used (λ=808 nm).

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Fig 7. Temperature of tungsten filament vs. electrical input power. Fig 6. Electrostatic force exerted between capacitive

plates vs. displacement of aluminum foil.

with the aluminum foil has a beam reflection target for a laser displacement sensor KEYENCE LK-080/2000 having the minimum resolution of 3 μm from outside the vacuum chamber. This system was analyzed by the bending-torsion moment theory of material mechanics and estimated to have a sensitivity of 20 μm displacement of aluminum foil at a 0.1 μN thrust input. This value is easily detected by the above laser displacement sensor. The reflectivity of aluminum is about 90 % from visible to near-infrared wavelength. We must also be careful to the incident angle of the light source onto the aluminum foil, therefore the measured thrust becomes square root times of the actual value. The exit of the parabolic mirror is covered by quartz glass plate. The thrust stand is calibrated by known electrostatic forces acting between a pair of capacitive plates with the area of S m2, one of which is the aluminum foil and the other is a 140 mm diam. copper plate covered by thin insulator film so as to avoid electrical contact to each other. When the copper plate is applied voltages ranging from 0 to 60 Vdc based on the grounded aluminum foil of 0 Vdc, they attract each other by electrostatic force. This force F [N] is very weak as long as the applied voltage is low, and expressed as follows with the vacuum permittivity [F/m].

2

2

021

dVSF ε= (1)

III. Experimental Results and Discussion A. Precautions of Experiment

The obtained calibration results exhibited very good linearity and Fig. 6 shows the plotted data. But a relatively large discrepancy of the sensitivity was found about twice the value than estimated from the material mechanics of 20μm / 0.1μN. This may be due to the deviation from the designed rigidity of 2 parallel copper wires of 0.1 mm diam. apart The manipulation of these wires easily changes the rigidity of rotation and in turn the proportional constant of the torsion-balance thrust stand.

large discrepancy of the sensitivity was found about twice the value than estimated from the material mechanics of 20μm / 0.1μN. This may be due to the deviation from the designed rigidity of 2 parallel copper wires of 0.1 mm diam. apart The manipulation of these wires easily changes the rigidity of rotation and in turn the proportional constant of the torsion-balance thrust stand.

On the other hand, the temperature of tungsten filament was measured by a radiation thermometer. As shown in Fig. 7, the emissivity of ε =0.4 provided the best fitted curve to the law of T4 Stefan-Boltzmann plot.

On the other hand, the temperature of tungsten filament was measured by a radiation thermometer. As shown in Fig. 7, the emissivity of ε =0.4 provided the best fitted curve to the law of T4 Stefan-Boltzmann plot.

Before starting the experiment, we expected that we could easily extract only photon pressure thrust, but it was an illusion. We had to carefully remove several unexpected disturbances from the net photon pressure thrust. Firstly, the electronic charging of the aluminum foil in vacuum condition was completely released by grounding the entire thrust stand to the vacuum chamber wall during thrust measurement. Secondly, due to high temperature tungsten filament, the electrical insulator of boron nitride of which maximum operating temperature was 2,000 K was slightly evaporated and this outgases made a convection flow inside the vacuum chamber to affect the thrust stand output. We had to cover the exit of the parabolic mirror by a quartz glass plate transparent to almost all the wavelength concerned. Thirdly, a photoelectric emission of electrons from the aluminum foil are to be considered, however, the quartz glass plate covering the exit of light emission source can scarcely transmit such ultra-violet wavelength where the photoelectric emission prevails. Furthermore, both the aluminum foil and other parts of the thrust stand were

Before starting the experiment, we expected that we could easily extract only photon pressure thrust, but it was an illusion. We had to carefully remove several unexpected disturbances from the net photon pressure thrust. Firstly, the electronic charging of the aluminum foil in vacuum condition was completely released by grounding the entire thrust stand to the vacuum chamber wall during thrust measurement. Secondly, due to high temperature tungsten filament, the electrical insulator of boron nitride of which maximum operating temperature was 2,000 K was slightly evaporated and this outgases made a convection flow inside the vacuum chamber to affect the thrust stand output. We had to cover the exit of the parabolic mirror by a quartz glass plate transparent to almost all the wavelength concerned. Thirdly, a photoelectric emission of electrons from the aluminum foil are to be considered, however, the quartz glass plate covering the exit of light emission source can scarcely transmit such ultra-violet wavelength where the photoelectric emission prevails. Furthermore, both the aluminum foil and other parts of the thrust stand were

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grounded to the chamber wall, therefore any kind of continuous electron emissions were not allowed. Thus, the only one disturbance was remained unavoidable as described later.

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B. Experimental Results

Figure 8 displays the photon pressure thruster with a parabolic mirror working inside the vacuum chamber. In this case, the typical thrust stand responses superimposed by peculiar oscillation are saved as raw signal data in Fig. 9. It is understood from the waveform that immediately after the complete turn-on of the tungsten filament, the thrust was generated and then gradually seemed to migrate downward (direction of thrust increase). At the turn-off of the tungsten filament, it is also found that an abrupt change of the thrust took place and was followed by a gradual relaxation. Judging from these responses of the thrust stand, the net thrust generated by photon pressure was evaluated at the stepwise change of the signal of the turn-on phase. At the turn-off phase, it is obscure because the peculiar oscillation is largely superimposed at random manner. The gradual changes of the thrust stand signal after the turn-on and turn-off are considered to be a kind of thermal drift, because after a long time from the turn-off,the thrust stand signal returned to zero position. The right end of Fig. 9, the signal disappeared from the oscilloscope frame to avoid overwriting the datum. We are to prepare a shutter mechanism to mitigate the effect of thermal drift and replacing boron nitride insulator with much refractory ceramic material.

Fig. 8 Photon pressure thruster operation in a vacuum chamber with a quartz glass rid.

Figure 10 gives the time-averaged plot of the thrust stand outputs for various input power to the tungsten filament where the peculiar oscillation is filtered. After one read out only the net photon pressure thrust from these curves and plot them against the input power, one can finally obtain a result of proportional relationship between them as shown in Fig. 11. In this case the measured photon pressure thrust was 0.24 μN at the input power of 300 W.

Fig. 9 Response of thrust stand (raw data 100μm/div, 5 sec/div).

C. Discussion about Result

One of the most important objective in this primitive experiment is to evaluate the thrust efficiency of the photon pressure thruster like other electric propulsions. The following equations are to be defined using c as the speed of light.

Photon Energy νhE = (2) Photon Momentum (3)

chp ν

=

In this system, we propose that the definition of the thrust efficiency (= power conversion efficiency) of photon pressure thruster should be as follows:

Fig 10. Thrust stand output profile for various input power. Thrust Efficiency (4) P

cF=η

When applying this definition to the result of Fig. 11, we can obtain a thrust efficiency of about 24%. This value is apart from 100%, because the light source is not a perfect blackbody, the mirror reflectivity is not 100%, the solid angle from the light source is not fully covered by the mirror, and the filament itself has a finite dimension rather than a focal point. Unfortunately this value was modified downward in the next step of the more accurate experiment.

On the other hand, the measurement technique is yet imperfect as already mentioned in the thrust stand response. The output involves a drift besides the net thrust of the light source. This is perhaps due to the thermal input rather than the light input. The absorbed heat may cause the deformation of the aluminum foil resulting in an imaginary thrust additional to the net photon pressure thrust. Presently, these true and false thrusts are discriminated by their time-constant difference to reach equilibrium, however, as the next step in order to prevent the thermal drift a shutter mechanism should be employed, which closes after the light source build-up, for a short time enough before heat absorption.

Fig. 11 Relationship between photon pressure thrust vs. input power.

D. Further Experiment Using Mechanical Shutter

To prevent thermal drift of the torsion-balance precision thrust stand, we newly constructed a light shielding shutter just in front of the aluminum foil target as shown in Fig. 12. This shield consists of dual layer aluminum plates.

As shown in Fig. 13, the effect of thermal drift was dramatically improved. The trapezoidal waveform shows the light input, however the thrust stand response is almost zero or only slightly minus by virtue of the closed shutter. Then, the shutter was opened to measure the thrust for only 5 s at the end of light input profile. In this case the thrust stand response was filtered by an appropriate shifting average, but just after the shutter was opened, the thrust profile went down a little (minus thrust) then rapidly went up toward the plus thrust, dropped down to the minus thrust again to a fair extent and finally recovered to zero or so. This implies that the shutter opening period is only 5 s, very short time compared with the former experiments in Figs. 9-10, however, even 5 s could effect the thrust stand response as the thermal drift to some extent.

Shut

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After another step of eliminating the external disturbance to the thrust stand response, which was a technical problem of start/stop the shutter mechanism generating unfavorable vibration inside the vacuum chamber, we finally obtained the better data of photon pressure thrust in Fig. 14.

E. Experimental Result of Semiconductor Laser Source

Using the semiconductor laser, the produced thrust was measured by the torsion-balance thrust stand as shown in Fig. 15, however the zero drift by thermal effects appeared again to some extent. The laser was empowered by 270

Double shading shutter mechanism.

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Wmax electrical input power (110 A × 2.5 V including cable loss) to emit 100 W light intensity for 5 s period. The response of thrust stand was plotted by moving average. This response was calibrated by a 5 s electrostatic force applied between the aluminum foil and the calibration plate as the known impulse. Figure 16 shows the final results of thrust measured vs. electrical input power. The obtained thrust was about 0.14 μN at an power input of 270 Wmax and the thrust efficiency was calculated as 16%. This value is only slightly higher than that obtained in a tungsten filament with parabolic Winston mirror, therefore some improvements will be necessary for the cable loss, laser beam divergence loss and reflection loss, etc.

F. Future Scope

The usage of tungsten filament in the present experiment implies non-zero evaporative mass loss during the operation. In the future application for long-time flight, instead of the incandescent filament, an electrodeless plasma light emitting source or a laser light source are suitable. For this purpose, a helicon plasma source with RF antenna or a semiconductor laser with high conversion efficiency will be expected.

However, the most annoying drawback for this application of photon pressure thruster is the power

source problem, exactly speaking, the specific mass of the power source. If the acceleration level of electric propulsion is required as a reasonable time of acceleration period, that will be in the order of 10μg0 where g0 is gravitational acceleration at sea level. When the thrust-to-power ratio is improved to 0.3 μN / 300 W =1 μN/kW, a little bit greater than that of our present experiment, we need at least a very excellent specific mass of power source, the target value of about 100 kg / MW. Until this value is attained by a space nuclear power source, we would charge the battery and use it in space. Although this is a modest use and very different from the original photon rocket proposed by E. Sänger in 1953, it may be possible to use it right now and also may be a stepping-stone toward interstellar flights.4

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Fig. 15 Thrust stand response in case of 5 s semiconductor laser irradiation (blue line is raw data and pink line is moving average).

Fig. 16 Delivered thrust vs. Laser input power.

IV. Conclusion

A parabolic mirror like a “Winston Cone” with a tungsten filament at the focal point is manufactured and demonstrated to generate photon pressure thrust inside a vacuum chamber. A precision torsion-type thrust stand having a resolution of sub 0.1 μN was also developed and dedicated to the thrust measurement experiment. This grey-body radiation thruster generated a thrust of 0.2 μN at 400 W input and resulted in 15 % thrust conversion efficiency.

A 100 W-class semiconductor laser of 808 nm was assembled to evaluate the irradiation thrust. The delivered thrust was recognized as 0.14 μN for the electrical input power to the semiconductor laser of 300 W. This implies that the thrust efficiency is 15 % comparable to the result of parabolic Winston mirror with filament type thruster.

For the next step, a thermal drift of the thrust stand must be further improved by an advanced shutter mechanism. Also for the future task besides laser application, non-evaporating light sources such as the array of light emitting diodes or an RF helicon plasma source are considered. However, it must be noted that the specific mass of 1 kg/kW for the electrical power source is required to this photon pressure propulsion for actual space applications.

References 1Rubbia, C., “Nuclear Space Propulsion with a Pure Electro-Magnetic Thrust”, CERN-SL-2002-006 (ECT), Super Proton

Synchrotron and LEP Division (Energy and Cavity Technology Group), 2002. 2Toki, K., Asakura, N. and Seto, T., “Primitive Experiments on Photon Pressure Space Propulsion – Energy Direct

Conversion to Thrust”, AIAA-2007-5311, 43rd AIAA/ASME/SAE/ASEE Joint Propulsion conference & Exhibit, 8-11 July 2007, Cincinnati, OH, USA.

3Toki, K., Asakura, N. and Ohtsuka, T., “Energy Direct conversion to Thrust by Photon Pressure Propulsion”, ISTS 2008-b-12, 26th International Symposium on Space Technology and Science, Hamamatsu, Japan, June 2008.

4Derosa, L. and Maccone, C., “Propulsion Tradeoffs for a Mission to Alpha Centauri”, Acta Astronautica, Vol. 60, Issues 8-9, April-May 2007, pp. 711-718.