SPACE SOLAR POWER REVIEW Volume 4, Number 3, 1983 PERGAMON PRESS New York / Oxford / Toronto / Paris/ Frankfurt I Sydney
SPACE SOLAR POWER REVIEW Published under the auspices of the SUNSA T Energy Council Editor-in-Chief Dr. John W. Freeman Space Solar Power Research Program Rice University, P.O. Box 1892 Houston, TX 77251, USA Associate Editors Dr. Eleanor A. Blakely Lawrence Berkeley Laboratory Colonel Gerald P. Carr Bovay Engineers, Inc. Dr. M. Claverie Centre National de la Recherche Scientifique Dr. David Criswell California Space Institute Mr. Leonard David PRC Energy Analysis Company Mr. Hubert P. Davis Eagle Engineering Professor Alex J. Dessler Rice University Mr. Gerald W. Driggers L-5 Society Mr. Arthur M. Dula Attorney; Houston, Texas Editorial Assistant: Jean S. McHenry Editorial Office: John W. Freeman, Editor-in-Chief, Space Solar Power Research Program, Rice University P O Box 1892, Houston, TX 77251, USA. Professor Arthur A. Few Rice University Mr. I.V. Franklin British Aerospace, Dynamics Group Dr. Owen K. Garriott National Aeronautics and Space Administration Professor Norman E. Gary University of California, Davis Dr. Peter E. Glaser Arthur D. Little Inc. Professor Chad Gordon Rice University Dean William E. Gordon Rice University Dr. Arthur Kantrowitz Dartmouth College Mr. Richard L. Kline Grumman Aerospace Corporation Dr. Harold Liemohn Boeing Aerospace Company Dr. James W. Moyer Southern California Edison Company Professor Gerard K. O’Neill Princeton University Dr. Eckehard F. Schmidt AEG—Telef unken Dr. Klaus Schroeder Rockwell International Professor George L. Siscoe University of California, Los Angeles Professor Harlan J. Smith University of Texas Mr. Gordon R. Woodcock Boeing Aerospace Company Dr. John Zinn Los Alamos Scientific Laboratories
0191-9067/83 $3.00 + .00 Copyright ® 1984 SUNSAT Energy Council EDITORIAL The next issue of Space Solar Power Review will introduce a new feature to appear regularly: listings of generic or related papers or research projects associated with SPS. The objective is to allow readers to monitor generic research and SPS related work in the U.S. and other countries. The listings will be grouped according to subject, eg, solar cells, economics etc. New listings will appear in each issue, however, the complete listings will be available by writing to: Space Solar Power Review P.O. Box 1892 Rice University Houston, Texas 77251 We solicit from our readers papers or projects that should be included in these lists that we may not be aware of. We hope that our readers will find this new feature useful. John W. Freeman Editor-in-Chief
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0191-9067/83 $3.00 + .00 Copyright ® 1984 SUNSAT Energy Council KINETIC MODELING OF AN IBr SOLAR PUMPED LASER W. L. HARRIES Old Dominion University Norfolk, Virginia 23508, USA W. E. MEADOR NASA/Langley Research Center Hampton, Virginia 23665, USA Abstract — The possibility of using an IBr laser as a solar energy converter is examined theoretically, and reasons for its choice are given. Broadband absorption results in dissociation with the formation of excited Br* atoms, some of which then lase to the ground state Br. The ground state is depopulated by three-body recombination and, more importantly, by exchange reactions which more than compensate for the high quenching in heteronuclear halogen systems. Kinetic modeling indicates lasing is possible in the pulsed mode and possibly in the steady state with a cooled gas flow system. Temperature effects are discussed. The efficiency of the laser approaches 1.2% at optical thicknesses large enough for complete absorption of the photons. I. INTRODUCTION The concept of collecting solar radiation in large mirrors on orbiting space stations, and then transmitting the energy via laser beams has been considered previously (1-3). The efficiency of the system is expected to be highest if the laser could be directly pumped by the solar radiation. The criteria for an efficient solar pumped laser are as follows: • There must be broadband absorption; • Peak absorption should occur near the peak of the solar spectrum; • High quantum yield into a long-lived (metastable) state which serves as the upper laser level; • In general, quenching of the excited state should be small, but, as will be pointed out later, this condition is alleviated if • The lower level is rapidly depopulated to maintain inversion; • The upper and lower levels must be sufficiently separated to yield a reasonable quantum efficiency; • The process must be reversible; if not the components must be reconstituted by flow methods. Gas lasers are advantageous because of uniformity of medium and because size is not a limitation. Two classes of laser can be considered: [1] where the absorbing medium is distinct from the lasant, and [2] where one material performs both functions. The first solar pumped laser examined theoretically was type [1] above, namely a
Br2-CO2-He mixture (2-3). The Br2 acted as a broadband absorber, the CO2 was the lasing medium, and the He acted as a coolant. This laser was an electronic-to- vibrational energy transfer laser, and the low “transfer efficiency” as well as absorption efficiency resulted in an overall efficiency of less than 0.13% Higher overall efficiency might be attained if the transfer efficiency could be eliminated, as in lasers of type [2] above. An example would be photodissociation of a molecule to yield an excited atom, which then lases to the ground state. The population of the lower level might be removed by chemical processes. High pressure working should be possible which would enable more efficient absorption. The objectives of this paper are to study theoretically the IBr solar pumped laser as an example of class [2], to understand the essential lasing features by determining the dominating reactions, and to estimate the efficiency for power conversion. The criteria that have been listed for the selection of candidate lasants will also be assessed. In particular the question of whether steady-state lasing is possible will be discussed. As the solar radiance must be concentrated many times, and since the IBr absorption is broadband near the peak of the solar spectrum, the laser will quickly heat up and lasing inhibited (by mechanisms to be discussed), unless cooling is provided. In the theoretical study it is therefore assumed that the lasant is maintained around room temperature, the purpose being to examine the potential for lasing under favorable operating conditions. Methods for accomplishing cooling are proposed in Sec. IX. While this study was in progress, an experimental investigation of an IBr laser pumped by a xenon lamp was performed by L. Zapata (4). The objective of that experiment was to demonstrate lasing, and not necessarily to provide definitive quantitative data. Hence, only limited comparisons could be made, but the experimental results were useful in assessing the effects of excessive heating in the actual experimental environment. II. CHOICE OF IBr Broadband absorption is essential for high solar efficiency and there are many compounds which can be photodissociated to yield excited atoms X*. Here only halogens are considered for A. They can be divided into three types: diatomic homo- nuclear molecules A2, diatomic heteronuclear molecules YX, and complex molecules of the form RX, where R is a molecular radical. Solar pumped lasing has already been demonstrated for type RX, using a xenon arc to radiate perfluoropropyliodide C3F7I (5-7). Absorption occurred in the ultraviolet (230-320 nm) so that the fraction of the solar radiation absorbed (solar efficiency) was small. (Chemical recycling was also necessary for continuous working.) Types X2 and YX on the other hand can absorb near the peak of the solar spectrum when both X and Y are halogen atoms. The excited atoms F*, Cl*, Br*, and I* have energies of about 0.1,0.2, 0.44, and 1 eV, respectively, above ground; only Br* and I* have values high enough to give acceptable quantum efficiencies. Lasing characteristics depend on the competition between the rate of reduction of the excited species A* by quenching and the depopulation of the lower level A by the exchange reaction A -I- XY —» A2 + Y. Exchange reactions are possible only for heteronuclear molecules. Since the photodissociation of XY always seems to leave the lighter atom in the metastable excited state, and since the quantum efficiency increases with the atomic number of the lasing atom, it
appears that IBr should be the best laser candidate of type [2]. It absorbs near the solar peak with a high probability of dissociating into I + Br*. III. PHYSICAL MECHANISMS The processes occurring in IBr are assumed to be photodissociation with the formation of excited and ground state atoms, quenching of the excited atoms, recombination and exchange reactions. The species are assumed to be eight in all, namely, IBr, I, Br, I2, Br2, Br*, and I*, and photons created by spontaneous and stimulated emission. Vibrational excitations are neglected at room temperature because of very rapid relaxation. The reactions and rate constants are summarized in Table 1. The absorption of photons results in reactions 1 through 6. The photodissociation rates 5 are equal to C<t>(X)AXcr„(X)GV), where C is the number of times the solar radiation is concentrated, <F(X)AX is the number of photons arriving per unit area per second (8), cr„(X) the absorption cross-section in question, and (N) the number density of absorbers. As the ratio Br*/Br initially produced by the photodissociation of IBr is critical to attaining an inverted population, care must be taken in evaluating 5] and 54 in Table 1. The potential curves for the ACS*), A3nH and B3H0+ levels for IBr were plotted by computer on the energy diagram (Fig. 1) using data from Huber and Herzberg (9). The B'3H+ is included, and is drawn in approximately. The lowest vibrational level (v = 0) and the Franck-Condon transitions are shown. The horizontal dashed lines B and C are based on the transitions from the lower to the upper A3Hj and B3n(l levels and indicate the peaks and widths of the absorption curves. The absorption cross-section for IBr as a function of wavelength has been measured (10-11), and the function can be represented by three Gaussians whose peaks are at 268, 477, and 507 nm, respectively. These are plotted on the left. The peak of Gaussian F coincides almost exactly with the line C, but the peak of the Gaussian G is displaced from B. We believe the Gaussians to be accurate as well as the asymptotes A and D. The integral is a measure of what fraction of absorption events are caused by transitions to the level in question and au(\) is the respective Gaussian. Thus Gaussian H produces Br, but can be neglected as </>(X) is small here (8). The part of Gaussian F above asymptote A produces Br*; the part below A does not produce dissociation. The B'3H^ and B3IV curves cross and possibly absorption into the latter could result in Br. However, the translational energy at the crossing is sufficiently high and the nature of the crossing is such that the probability is near unity that all parts of the absorption curve above A in Fig. 1 correspond to dissociation into I + Br*, as confirmed experimentally (13). The Gaussian G, corresponding to absorption into the A3I1, level produces 1 -I- Br. Performing the integration of <(>(x)cr„(x) dX then yielded the fractions of absorption events resulting in the production of Br*, Br, and I (Table 1). The total absorption rate for IBr was found to be G X 1.25 X 1015 (IBr),
where C is the number of times the solar radiation is concentrated, of which a fraction 0.704 resulted in Br*, 0.259 in Br and 0.963 in I. Similar estimates for I2 and Br2 are also included. For all the initial pressures of IBr considered, it was assumed that 4% by number density of /2 and Br2 were present at 300°K, according to the law of mass action. Thereafter, all the densities varied with time. The absorption rate is proportional to IBr pressure for low pressures (and absorption lengths of 1 cm), until at about 50 torr essentially all the photons are absorbed. The degeneracy factor for the upper laser level Br* is 2, while that of the lower laser level Br(2P3/2) is 4. The population inversion AN for lasing is then TABLE 1 LIST OF REACTIONS
Fig. 1. Energy level diagram for IBr. The potential curves ATS?), /t3n,, B3n„+ are Morse functions generated from data of Huber and Herzberg. The repulsive curve B'3n„+ is approximate. The Gaussian absorption curves are plotted from data of Seery and Britton. The degeneracy factors in this instance help to reduce the threshold for lasing. The quenching of Br* and I* is given by reactions 7-14 in Table 1; the quenching of Br* and I* by Br* and I* was neglected as well as the quenching of Br* by Br and I* by I and Br. Computer runs in which these latter coefficients were arbitrarily assigned values equal to Q4, instead of zero, showed no great difference in the results. The large rate coefficient for the quenching of Br* by I is due to the electronic to translational energy transfer resulting from the “crossing" of (IBr)* potential energy curves (see Fig. 1). The process has been called the inverse predissociation mechanism (14). The two body recombination of I2 and Br2 (item 15) is the only reversible reaction included. At room temperature, by the law of mass action (assuming no photodissociation), the concentration of I2 and Br2 in IBr is 0.04. It then follows that if the forward reaction 2IBr -* I2 + Br2 has a rate coefficient K7, then the reverse coefficient K, is (0.04)2 x K7. Three-body recombinations are listed and it is seen that the reactions involving I* and Br* are less likely than those involving I and Br because of the difficulty of
Fig. 2. Comparison of (a) measured laser light output vs time compared with (b) calculated values. The pressure of IBr was 3 torr, with 4% Br2, I2. In the calculations C = 5000. The pumping light intensity is inverted. getting rid of the excitation energy. Some of the values are uncertain and are based on similar reactions in I2. The exchange reactions that depopulate the lower laser level and make lasing possible are listed as items 25 and 26. Experimental measurements of reaction 25 have been reported by Clyne and Cruze, (27) who deduce a high rate coefficient Ex = 3.5 x 1011 cm3 s-1. We have been unable to find a rate coefficient for reaction 26, but computer runs for E2 = 0 and E2 = E, did not differ greatly because the concentration of I2 is much less than that of IBr. IV. RATE EQUATIONS In the following equations the terms in parenthesis represent the densities of atomic and molecular species; n is the density of photons. The quantity A is the
Einstein coefficient for spontaneous emission, while T is the rate of stimulated emission. The rate equations for the eight species present are The quantity G in Eq. 9 is the geometrical factor 2rf/L2, where rb is the radius of the laser beam and L the length of the laser; for isotropic spontaneous emission only this fraction of photons is confined within the laser cavity. The last term represents the loss through the end mirrors of reflectivites and r2, and Tc *s the lifetime of an average stimulated emission photon travelling parallel to the axis;
where c is the velocity of light. Here, tc = 5 x 10-8 s. The quantity T, the stimulated emission rate, is equal to n<jecAN where ae is the stimulated emission cross-section. Then where Xe is the emitted wavelength, A (~1) is the Einstein coefficient for spontaneous emission from the upper laser level, and AXe is the emission bandwidth. With AXe taken as the bandwidth for Doppler broadening at 300 °K, then ae = 1.6 x 10~17 cm2. Equations 2 through 9 were solved by computer, and are compared with the experimental results of Zapata in Figs. 2a and b. In the experiment, the light intensity from the xenon lamp was measured with a photodetector, and varied approximately as IV. COMPARISON WITH EXPERIMENT The idealized model differed from the experiment in a number of ways. The theory assumed absorption occurred one-dimensionally in a thickness d. Experimentally the pumping source was a xenon lamp placed parallel to the laser tube. Its light was focussed onto the laser tube axis by a reflecting cylinder of elliptical cross-section. The illumination increased towards the axis, but as the source was distributed and the reflector was not perfectly elliptical, the maximum value of the pumping photon flux density was uncertain. The pumping density was equated to C times the solar radiance. The values of the equivalent C in the experiments were probably between 2000 and 40,000 for the various runs. There were also differences in the spectral distribution of the xenon lamp and the solar spectrum assumed. Figure 2 compares the measured light output from a 1-m long IBr laser pumped by a xenon discharge lamp powered from a capacitor bank, with the computer solution of Eqs. 2 through 9. The IBr pressure was 3 torr and C was about 5000 in the experiment, which are the values assumed in the calculations. The overall shape of the experimental and calculated laser output are similar. Both start about the same time, and consist of a sharp spike followed by a tail. The ratio of spike to tail is higher in the calculated version, and the oscillations in the theoretical wave shape are not evident in the experiment. Estimates of the time constant of the InAs photodetector circuit showed it was high enough to reduce the high frequency components of the signal. The duration of the experimental pulse for this and all other runs was always shorter than the calculated values by a factor of about two. We believe this was due to the lasant being heated near the axis. Rough calculations of the IBr gas temperature, assuming C = 5000, indicated a temperature rise of several hundred °K in 50 /zs. The temperature rise would cause dissociation of IBr and deplete the lasant. This view is consistent with an experimental measurement of IBr density on the laser axis during a pulse, which showed a drop of about 50%, a value too high to have been
caused by photodissociation (it would require C 105). High temperatures coqld also increase the ratio of reverse to forward rates of the exchange reactions, thus tending to neutralize the mechanisms necessary to depopulate the lower laser level (30). Also, the quenching reaction Br* + I —> Br + I is enhanced in the heated gas because of the greater number of I atoms. The calculations assumed no heating. On changing the pressure p both experiment and theory showed the output laser pulse duration was roughly independent ofp for pumping pulses of 200 p.s duration. Experimentally the output light signal amplitude increased with p up to about 20 torr, where it reached a maximum and then decreased; as p increased, absorption in the outer layers decreased the pump power on axis. The theory showed the amplitude proportional to p because constant pumping energy density was assumed throughout the gas. Both experiment and theory showed that increasing C increased the laser amplitude proportionally. There was also agreement between the measured and calculated output power of the laser. In an experiment with a tube length of 1 m and 2.22 cm radius, and an output mirror of 95% transmittance, the measured output was approximately 2 kW for an IBr pressure of 4 torr, with C ~ IO4 (4). The calculated peak output power P is V. INTERPRETATION By varying the parameters in the program one at a time, it was established that the dominant mechanisms were quenching of Br* by IBr and I and the exchange reaction Br + IBr = I + Br2. In particular, the program indicated lasing would not occur if the exchange reaction was removed. Three-body recombinations made little difference to the shape of the light output for pumping pulses of 200 p.s duration. Fortunately the rate coefficients most accurately known turned out to be dominant ones. However, for computer runs of longer duration pulses, it was found essential to include all the terms. Evidently the exchange reactions can overcome quenching, and high quenching cross-sections per se may not be a valid selection criterion for rejecting otherwise promising gases. VI. THRESHOLD CONDITION The threshold condition is rp2 exp(2aL) = 1, where r^2 are the reflectivities of the laser mirrors, a = a^N is the gain per unit length, and L is the length of the laser (29). The threshold condition is contained in Eq. 9, and occurs when Our results showed n = 1.2 x 1012 cm 13 for C = 1 X 104, corresponding to 1.7 kW which is in reasonable agreement.
Fig. 3. Plot of lowest C for lasing vs laser length L. As AN cc C it follows that the threshold value for C is proportional to ML. Computer runs were performed for successively lower values of C for L = 400,100, and 25 cm, and the threshold values of C estimated. The values are compared with experimental results in Fig. 3. The absolute values of the latter were difficult to estimate as the pumping light was focused on a cylinder, but both theory and experiment show C ^L~'. Thus C can be reduced by increasing L. VII. EFFICIENCY OF THE LASER Estimates of the efficiency can be obtained from solutions of the rate equations. For a laser of length L, collecting width w and of 1 cm absorption depth, the total solar power into the laser is 0.14CLiv W. The power output is 0.5cnhvrw. If the transmission factor r is 0.05, the efficiency is 17 = 3.93 x 10~10n/CL, where n (the photon density) is in cm-3, and L is in cm. Computer runs at various values of L, C, and gas pressures were made and the maximum n obtained to give 17 (Fig. 4). For L > 5 m, 17 is roughly independent of L and C, and at 5 torr, 17 = 5 x 10~4. At these low pressures -q p and it can be seen that higher pressures increase 17, provided overheating can be avoided.
Fig. 4. Efficiency of IBr solar pumped laser plotted vs length I. at different pressures, and concentration factors C. The reflectivities are r, = 1, r2 = 0.95, respectively. An understanding of why the efficiency is low can be obtained if the overall efficiency is considered to be the product of four efficiencies (3): r, = VsVaVkVo- Here ■qs, the solar efficiency depends on the absorption bandwidth of the total solar radiance, and its value for IBr is 0.12. The absorption efficiency r)a depends on the IBr gas pressure and depth of absorption J; for low pressures = o-„(IBr)J = 10-2pt/, where p is the pressure in torr, d the depth in cm. The expression is valid up to pd = 50; for higher pd most of the photons are absorbed and 1. The kinetic efficiency pk depends on the number of absorption events producing Br* and also on the competition between stimulated emission and quenching. Approximately 0.7 Br* atoms are produced for every absorbed photon (Table 1). The fraction of Br* atoms which yield a stimulated photon can be obtained by examining Eq. 6, and comparing the value of T with the rest of the loss processes, as all eight variables vs time were known. The ratio T/(all loss processes) was 0.82, when p was a maximum, the major competitors with T being quenching by IBr and I. The quantum efficiency is rja = 0.44 eV/2.5 eV = 0.18. Hence forpd < 50 torr-cm, p = 6 x 10-4 at 5 torr, d = 1 cm, agreeing with 5 x 10-4 in Fig. 4. With pd > 50 torr-cm, r)a —» 1 and the maximum efficiency would be about 1 2 x 10~2. Pulsed working would further reduce the efficiency because of the duty cycle, unless there was adequate storage of energy when the laser was not emitting. VIII. POSSIBILITY OF CONTINUOUS LASING The computer runs showed that IBr was depleted, while the quenchers I2, Br2, and
I grew with time. For 200 pumping pulses, the photon density n had a maximum value just prior to the peak input. The input light pulse was then assumed to be proportional to sin2(27rt/10-4) up to 50 /is and thereafter to be constant. For p = 3 torr the calculations indicated a pulse of 350 p.s, and for p = 13 torr the duration was of order 10 ms with C = 5000. Plots of the other variables indicated a gradual depletion of IBr and a growth of I. The long time scale suggests that a gas flow system would be feasible for steady-state working provided the gas temperature was kept within limits. IX. TEMPERATURE EFFECTS As the absorbed photons have an average energy of several eV, and as the heat conduction of IBr is low, very high temperatures can occur. If the pressure/? of IBr is greater than 50 torr in a vessel 1 cm deep, then the temperature of the lasing gas can be roughly estimated assuming all the photon energy is deposited in the gas; ifp <50 torr, then roughly a fraction 10-2p is deposited. The heat produced travels an average distance x to the walls of the containing vessel by gaseous conduction. The walls are assumed at a temperature Tw and are thermally connected to a radiator into space. Above about 1000 °K, the gas would also act as a blackbody radiator. Estimates show that ifx = 0.5 cm, then for C = 2000 and p = 5 torr the gas temperature would be of order 1000 °K. If the pressure is increased (a requirement for high absorption efficiency) still higher temperatures are possible, as heat conduction in gases is independent of pressure. Cooling can be enhanced by reducing x by inserting fingers in the gas, or by admitting a noble gas with a high heat conductivity and a low quenching cross section. Assuming perfect conduction from the walls to a radiator the same size as the collector, then the radiator temperature would be somewhat less than 300 °K. High temperatures can have large effects on quenching, recombination and dissociation rates, and on exchange reactions. To our knowledge experimental data on the effect of temperature on the quenching of Br* by IBr or I2 are not available. The possible analogous case of quenching of I* by I2 was reported by Kartazaef et al. (31), whose measurements showed the quenching rates at 1000 °K were 20 times lower than at 300 °K. If a similar dependence occurred for the quenching coefficient of Br* by IBr or I2, the high temperatures would prove advantageous in this respect. Turner and Rapagnani (26) have reported measurements of the recombination rates for I + I + I2—» 2I2, where A = 1.1 x io10? 3 884. The coefficient is 1000 times smaller at 1000 °K than at 300 °K and is analogous to C6 used above where I recombined with Br and the third body was IBr. High temperatures cause thermal dissociation of IBr with the formation of I, Br, and Br2, which have large cross sections for the quenching of Br*. The depletion of the lower level by exchange reactions would also be reduced as the increase in I would enhance the reverse exchange reaction. On balance then, heating would seem to be very detrimental to IBr solar laser operation. X. CONCLUSIONS The time varying solutions of the species rate Eqs. 2 through 9 show that inversion
is possible near the start of a pumping pulse, but continuous lasing may not occur. The reason is that early in the pulse the Br created from Br* by quenching and stimulated emission is sufficiently removed by exchange reactions to maintain inversion. Eventually, however, the IBr decreases and the Br density increases to cut off the lasing. The dominating mechanisms are the source terms, quenching by IBr and I, two body exchange reactions and stimulated emission. With 200 /zs pumping pulses the behavior of the light output can be represented using only these reactions. However, if long pulses are to be simulated, then all the reactions must be included. Our computer results showed that for 3 torr of IBr, steady depletion occurred and the pulse lasted for about 350 with C = 5000, L =1 m. At 13 torr the pulses were many tens of ms for the same conditions. The exchange reactions can overcome the effect of high quenching cross sections, and high quenching need not necessarily indicate that a material is unsuitable for solar pumped lasing. Heating will reduce quenching. It will also cause dissociation and depletion of IBr with the formation of I, I2, Br and Br2 all of which quench Br*. The exchange reaction rates will be reduced causing the population inversion to disappear, thus killing the laser. If the path length L is small, requiring the solar concentration C to be high for threshold to be reached, then the pressure has to be kept low to prevent heating (<50 torr). The overall efficiency r/ is then 10~4/? (torr) or 0.1% at 10 torr. Such a 10 kW laser would require a collector of 85 x 85 m2. If L is large, so that overheating does not occur, and p is high enough to absorb all the photons, then tq could reach 1.2% — a 10 kW laser collector would be 30 x 30 m2. The above efficiencies can be compared to the calculated efficiencies of a Br2-CO2-He laser and a C3F7I laser, each of which was around 0.1%. The effective length L can be increased by multiple passes in a flat “box type” laser. IfL = 10 m, then lasing should be possible withC = 300. The temperature rise would then be 700 °K even if p = 50 torr, with a depth of 0.5 cm, Tw = 300 and no admixture of a cooling gas. The receiving area of the laser would be 400/300 = 1.4 m2 and an effective length L of 10 m could be achieved in about 8 passes. The possibility of steady-state working seems feasible if a continuous flow system is used. Acknowledgements — The authors acknowledge useful discussions with Drs. J.W. Wilson, S. Raju, and L. Zapata. The work was partially supported by grant NSG 1568 from the National Aeronautics and Space Administration/Langley Research Center, Hampton, Virginia. REFERENCES 1. J.F. Coneybear, The Use of Lasers for the Transmission of Power. Radiation Energy Conversion in Space, K.H. Billman, ed., Program Astronaut. Aeronaut. 61, 279, 1979. 2. B.F. Gyordiets, L.I. Gudzenko, and V. Ya Pachenko, Pis’ ma Zh Eksp Tear. Fiz. 26, 163, 1977. 3. W.L. Harries and J.W. Wilson, Space Solar Power Rev. 2, 367, 1981. 4. L. Zapata, Private communication. 5. J.H. Lee and W.R. Weaver, Appl. Phys. Lett. 39, 137, 1981. 6. W.R. Weaver and J.H. Lee, Proceedings of the 16th Intersociety Energy Conversion Engineering Conference, Atlanta, Georgia, p. 84, 1981. 7. J.W. Wilson and J.H. Lee, Proc. Virginia Acad. Sci. 31, 34, 1980. 8. J.R. Carter and H.Y. Tada, Solar Cell Radiation Handbook. Jet Propulsion Laboratory, California Institute of Technology, Report No. 21945-6001, RV OU 1973, Table 2.1, p. 2-2.
9. K.P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure-Constants of Diatomic Molecules, Van Nostrand, NY, 1978. 10. D.J. Seery and D. Britton, J. Phys. Chem. 68, 2263, 1964. 11. A.A. Passchier, J.D. Christian, and N.W. Gregory, J. Phys. Chem. 71, 1937, 1967. 12. M.S. DeVries, N.J.A. Van Teen, T. Baller, and A.E. De Vries, Chem. Phys. Lett. 75, 27, 1980. 13. A.B. Petersen and W.M. Smith, Chem. Phys. 30, 407, 1978. 14. M.B. Faist and R.B. Bernstein, J. Chem. Phys. 64, 2971, 1976. 15. K. Hohla and K.L. Kompa, Handbook of Chemical Lasers, R.W.F. Lyon and J.F. Bolt, eds., ch. 12, John Wiley, NY, 1976. 16. D.H. Burde, R.A. McFarlane, and J.R. Wiesenfeld, Phys. Rev. A., 1917, 1974. 17. J. Tellinghuisen, J. Chem. Phys. 58, 2821, 1973. 18. B. Wellegehausen, K.H. Stephan, D. Friede, and H. Welling, Optics Commun. 23, 157, 1977. 19. R.E. Beverly and M.C. Wong, Optics Commun. 20, 23, 1977. 20. A.B. Petersen and W.M. Smith, Chem. Phys. 30, 407, 1978. 21. H. Hofmann and S.R. Leone, Chem. Phys. 54, 314, 1978. 22. E.B. Gordon, A.I. Nadkhin. S.A. Sothichenko, and LA. Boriev, Chem. Phys. Lett. 86, 2, 1982. 23. H. Hofmann and S.R. Leone, J. Chem. Phys. 69, 641, 1978. 24. D.H. Burde and R.A. McFarlane, J. Chem. Phys. 64, 1850. 1976. 25. E.H. Appleman and M.A.A. Clyne, J. Chem. Soc. Farad Trans. 2 72, 191, 1976. 26. These numbers are based on similar reactions in L: C. Turner and N.L. Rapagnani, Laser Fusion Program Semi-Annual Report UCRL-50021-13-1, Lawrence Livermore Laboratory UCID-16935, 1973. 27. The values of C5 - C9are based on similar reactions, I + I + L and I* + 1 + I2: K. Hohla and K.L. Kompa, The Photochemical Iodine Laser, Handbook of Chemical Lasers. R.W.F. Lyons and J.F. Bolt, eds., ch. 12, John Wiley, NY, 1976. 28. M.A.A. Clyne and H.W. Cruze, J. Chem. Soc. Farad Irans. 2 68, 1377, 1972. 29. B.A. Lengyel, Lasers, 2nd Ed., p. 61, John Wiley, NY, 1971. 30. This suggestion was made by L. Zapata. 31. V.A. Kartazaef, N.P. Penkin, and Yu. A. Tolmachev, Sov. J. Quantum Electron. 7, 608, 1977.
0191-9067/83 $3.00 + .00 Copyright © 1984 SUNSAT Energy Council DIRECT BROADCAST SATELLITES AND ECOSPACE 25TH IISL COLLOQUIUM ON THE LAW OF OUTER SPACE IV: 33RD INTERNATIONAL ASTRONAUTICAL CONFERENCE Paris, France — October 1, 1982 HONORABLE EDWARD R. FINCH, JR. Member New York. District of Columbia and Florida Bars Member International Astronautical Academy Finch & Schaefler 36 West 44th Street New York, New York 10036, USA Abstract — Direct Broadcast Satellites present some of today's most controversial outer space law and science issues. It legally presents a mix of outer space, air space and Earth- bound problems, both national and international. The legal, space application and technological issues also touch individual human rights and the sovereignty of nations. Recently, UNISPACE 1982 has accelerated these issues. Is this beneficial for orderly progress of DBS? Shouldn't the United Nations COPUOS now refer DBS back to its outstanding Scientific-Technical Subcommittee? Has the time come when outer space application technology and the stringent economic costs of outer space demand careful scientific restudy on DBS legal considerations for a few years? Thus, the rapidly advancing international law of outer space could get back in proper timing step with the new outer space science and technology of DBS. It is submitted that ECOSPACE, the economics of outer space and rapid technological changes are now the prime factors for developed and lesser developed countries in DBS. The matter should now, for a few years, be left to the ITU and RARC 1983 and WARC 1985 and WARC 1987, so that the DBS geostationary problems can be technologically addressed as to radio spectrum and physical slot allocations before the international legal and policy positions for the benefit of all nations of the world are again reviewed in the United Nations; and consensus more readily reached in 1988. The author reminds that he speaks only in his individual capacity and solicits new law and science approaches and ideas to the resolution of the DBS and ECOSPACE problems. Also, the related remote sensing developments may light the path to DBS legal and policy solutions by 1988. INTRODUCTION The theme of this Congress is “Outer Space in the Year 2000.” Dr. Lubos Perek, President of IAF, predicts that “many of the space missions that will be operating then will be based on today’s science and technology. It is therefore quite timely to consider now the space activities, space vehicles and space environment that awaits us at the dawn of the third millenium” (1). One topic for this IISL Colloquium “Legal Aspects of Direct Broadcast Satellites” involves space activities of today and tomorrow on the Earth and in the geostationary orbit, so the topic was certainly most appropriately chosen by our President and our Chairman. In addition, it is most appropriate because it is probably the most controversial space activity for today and tomorrow and the years to come, not only in the United Nations COPUOS, but in every country of this planet Earth, apart from the issue of militarization of outer space, now before the United Nations Committee on Disarmament.
DBS IN UNISPACE 1982 As a United States Delegate and a Legal Advisor, I had the privilege of serving on the U.S. Delegation to UNISPACE 1982 and making a study of the United Nations Draft Report for UNISPACE 1982 (A/Conf. 101/3 dated 20 April 1982). It states in certain most pertinent parts as follows: Reception of satellite-relayed television programmes by specially augmented television receivers is now a well proven application. Though the U.S.S.R. is at present the only country with an operational system, other countries — including Canada, Japan, India, the United States and, in ajoint venture, the Federal Republic of Germany and France — have carried out experiments and many are now planning operational domestic systems. This is obviously an application with tremendous possibilities, especially for education — in the broadest sense of the term — and as a means of accelerating development. Operational use of the 12 GHz Broadcast Satellite Service is likely to be introduced soon. This, together with the future utilization of the 30/20 GHz band, will make it possible to use receivers having parabolic antennae of the order of one meter diameter and possibly even smaller. Advances in low-noise receivers have also made possible comparatively small (3-4 m antenna), low-cost systems for reception of television broadcasts in the 4 GHz band. These are widely used for television networking in North America. Radio and television have been used as educational tools for many years. Radio has the advantages of wide reach, low cost and can be used even in unelectrified locations; despite this, and notwithstanding the fairly extensive radio coverage even in developing countries, radio has not yet been fully exploited for educational purposes. However, it does have the constraint of being only an aural medium. Television, on the other hand, can be an extremely powerful instrument for spreading education. Until recently, its use was constrained by the fact that reception of the broadcasts was possible only within about 100 km of a transmitter. Thus, to broadcast television programmes to a given area, one had to set up a television station nearby, or set up a television transmitter and link it to a television studio or station. Now, however, space technology has made possible the reception of television programmes, even in very remote areas, without the need for a television station nearby or ground links. The technology of direct broadcast satellites has been demonstrated by Canada, Japan and the United States, and a large-scale educational television experiment using the ATS-6 satellite of the United States was carried out in India in 1975/76. Some other countries also have carried out experiments involving the reception of satellite-relayed television programmes by specially agumented television receivers. Canada now has an experimental DBS system, and many countries or groups of countries are planning operational DBS systems. A DBS system basically uses the principle of “complexity inversion,” so that simple and inexpensive receiving equipment on the ground is made possible by a powerful and complex satellite. Obviously, such a satellite is expensive, and. therefore, the economics of a DBS system are dependent to a large degree on the number of receiving stations on the ground. There are, of course, a large number of other variables — including, in particular, the frequency band used and the EIRP of a satellite — and the cost of the receiving equipment is largely determined by these. However, it seems clear that a DBS system becomes worthwhile only if it serves a fairly large number of receivers. Many systems for direct-to-home television broadcasting are being proposed and planned. Selection of an appropriate reception mode, either individual or community, will be determined by the specific characteristics of each country. For the developing countries, emphasis may be put more on the community mode of reception. At the same time, it would seem economically attractive and beneficial for small countries to share a satellite, especially on a regional basis. Another possibility is an internationally-owned satellite that provides direct broadcast service to countries. In both cases, there could be transponders dedicated to each particular country, or countries could share a transponder, depending upon their usage. Differing time-zones could also enable a sharing of transponders, if the use was limited to a few hours only and the countries were
widely separated along the east-west direction. Such a sharing of a satellite — which could be as large as necessary, within technological limits — would also contribute to reducing the pressure on the geostationary orbit (GSO). It is therefore suggested that: (a) Countries might examine the feasibility of using DBS to aid the spread of education, (b) Countries could explore the possibility of sharing the space segment of a DBS system, including the possibility of using any existing/planned satellites that might be suitable, (c) Studies should be undertaken to examine the feasibility of international or regional satellite system(s) for providing direct broadcast television service, (d) The United Nations and concerned specialized agencies should encourage and provide assistance — as appropriate and if requested — for the above, (e) Existing organizations such as INTELSAT may choose to consider developing broadcasting satellite systems which could be used for educational purposes. It should be noted that many countries which are now planning direct broadcast satellites intend to combine this service with telecommunications, with obvious economic advantages. Other countries — or an international satellite system — could examine the cost-effectiveness of such multimission platforms so as to increase the attractiveness of the system. Community reception will probably be — as noted earlier— the primary mode of receiving satellite television broadcasts in developing countries. Tens of thousands of receivers will be needed in each country and it is essential to reduce the cost of each installation as much as possible. Also, given the lack of rural electrification in most developing countries, it is necessary to think of power sources that are inexpensive, and preferably use renewable forms of energy rather than hydrocarbons. It is therefore desirable that strong encouragement — including financial and technical assistance, if necessary — be given to efforts aimed at developing low-cost community receivers for DBS and low-cost, preferably renewable, power sources to operate the system in unelectrified locations. Since the cost of the reception system for a given coverage area depends on the frequency and EIRP of the satellite, it is suggested that efforts at developing more powerful broadcasting satellites, within the constraints of international regulations, be pursued and encouraged. The question of frequency is a complex one, and the choice of an appropriate band involves many considerations and constraints. The ITU World Administrative Radio Conference (WARC) in 1977 agreed on an orbit/frequency assignment plan for countries in Africa, Asia and Europe for the 12 GHz Broadcast Satellite Service band. Because of existing services, space service allocations in the lower portion of the frequency spectrum are constrained by flux density limitations so as to cause no harmful interference. The absence of such constraints could have led to simpler and cheaper ground reception equipment. The importance of these bands (especially the 0.7, 2.5 and 4 GHz bands) is not merely due to the lesser technological complexities compared to higher bands, but also to the fact that they do not suffer the severe rain-attenuation that higher frequencies (e.g., 12 GHz) do — a point of particular importance to countries with high rainfall rates. It is therefore desirable that ITU study the feasibility of phasing out — in the long run — the terrestrial services in some of these bands, so that they can be assigned exclusively to satellite broadcasting and the flux density limits relaxed. In view of the existing investments and extensive spread of terrestrial networks that share these bands, any such changes would be difficult and do not seem feasible in the immediate future. Also, allowing higher radiated power from spacecraft and smaller terminals on the ground could result in decreasing the “capacity” of the GSO due to need for greater separation between satellites so as to avoid interference. However, should ITU recommend, and one of the World Administrative Radio Conferences adopt such steps, they would result in greatly reduced costs for DBS systems and provide a major stimulus to education via satellite. While considerations with regard to the hardware elements of a DBS are of obvious importance, the other “software” elements unfortunately tend to be neglected. Experience has, however, indicated that these aspects are crucial to the success of a DBS system for education. The main ones include: (a) system planning and integration; (b) organizational aspects including system management and co-ordination; (c) design and production of appropriate television programmes, relevant to the needs of the audience and in keeping with national priorities; (d) feedback and evaluation mechanisms, especially with regard to audience reac-
tions and impact; (e) organization of viewing arrangements and of post-programme follow-up (“utilization”), including ensuring of actual availability of recommended items, training of teachers for optimal use of the system, etc.; and (f) an efficient field maintenance system. Utilization of GSO cannot be considered in isolation: the associated issue of use of the radio frequency (RF) spectrum must be simultaneously looked at. The RF spectrum is also — like GSO — a limited (in practice) though nondepleting resource. While in theory it does extend indefinitely, practical constraints limit its present use to a comparatively small band. Hence, its optimal use also requires planning and/or arrangements. It is in the light of this that the members of ITU have been making concerted efforts to evolve systems for planning and regulating the use of the GSO and the RF spectrum since 1963. It is noted that the forthcoming ITU Conferences in 1985 and 1987, which will continue this process in the light of technical progress and in the light of the broad considerations outlined here, will decide, in accordance with resolution 3 of WARC 1979, which space services and frequency bands should be planned. GSO is getting increasingly crowded with objects that have outlived their utility. While the danger of collision or physical damage by these objects to active satellites is yet small, this is a problem that is likely to become more serious in the future. Therefore, ITU should examine the feasibility of incorporating in its future regulations a stipulation that a satellite owner is responsible for removing its satellites from GSO when they are no longer in use. In noting the phenomenal growth in the use of GSO — especially for communication satellites — and the consequent usage of RF spectrum, it may become necessary for each country or international organization to examine whether all the satellites it is operating are really required. Increasing numbers of satellites are being used for various purposes by different countries. To the extent that these systems use national resources, it is the concern of the country involved. However, these systems use increasing amounts of a limited resource that is for use by all States. It is therefore desirable that Member States, within the ITU continue to evolve some criteria for the most equitable and efficient usage of GSO and the RF spectrum and to develop planning methods and/or arrangements that are based on the genuine needs, both present and future, identified by each country. (Clearly, such a planning method should emphasize those uses which promote development, including education. It should also ensure equity, consider the needs of developing tropical and far-northern countries, and take into account the special interest of the equatorial countries.) For certain purposes and locations, it may not be essential to use GSO. Since increasing concerns have been expressed regarding the congestion of GSO, countries should also examine whether for their needs they could use a satellite in elliptical orbit rather than in GSO. Similarly, the feasibility and overall advantages of using elliptical orbits for international communication merit re-examination. It needs to be noted that the development efforts undertaken by the technologically advanced nations have resulted in new techniques that contribute to more efficient use of GSO and of the RF spectrum. These efforts must be encouraged and continued, for success in this could effectively increase the capacity of GSO and thereby alleviate possible pressures on its use. New developments in the field of fiber-optic technology are also likely to contribute positively, by directing high-capacity traffic on transcontinental and transoceanic routes to optical fibre systems. In conclusion, considering the long-term implications of the growing activities in GSO, any solution on the use of GSO should be both equitable and flexible and take into consideration the economic, technical and legal aspects (2). At UNISPACE 1982, the author witnessed very considerable consensus adjustments to the above language by some 121 nations and NGOs represented there. While much of it involved semantics, complicated by the use of several languages, there was ultimately substantive real consensus to the above, as so adjusted. The future of DBS was directly accelerated by the UNISPACE 1982 Conference. The final UNISPACE 1982 official United Nations Report on UNISPACE 1982 will
clearly reflect this acceleration and the final amended consensus language. UNISPACE 1982 made a great contribution to DBS and the future of outer space for all. DBS AND ECOSPACE ECOSPACE is an acronym for the economics of outer space. The word ECOSPACE was first used in 1975 at The American Bar Association’s Presidential Function in Montreal, Canada, and widely accepted and understood in several languages by the United States, the Soviet Union and other specialists there in formal and many informal meetings. Noting the previously mentioned above acceleration of the future of DBS and its great value to lesser-developed countries, a clear trend has begun to evolve. Cost factors of rapidly advancing DBS technology promotes regional cooperation between lesser-developed countries. The ECOSPACE of DBS now, and particularly for the future, is dictating this regionalism trend. It is also breaking down language barriers. However, DBS technology and economic necessity pushing expanded regionalism does not necessarily promote world freedom of information, as regional reception can be most easily converted to a control of the local dissemination of information for a regional country group. It is submitted that the ultimate solution to the basic DBS problem of freedom of information to and for all mankind vs country or regional control of information from DBS will be dictated by ECOSPACE, rather than by a presently ill-timed treaty or multilateral agreement. The United Nations unsuccessful proposal for a DBS treaty to conform to “international law and to the Charter of the United Nations” does not really solve any problems. Article III of The 1976 Outer Space Principles Treaty (3) now contains such provisions and is in force and the DBS legal problems still remain. ECOSPACE demands, combined with the progress of DBS technology with its constantly changing and increasingly demanding costs, will promote regionalism. This, by the year 2000, may well expand into conti- nentalism, and ultimately expand into true world DBS internationalism. Why? Consider the six basic underlying DBS ECOSPACE principles. These are as follow: Number 1: Outer Space peculiarly requires very long-range policy planning. A minimum 5-year program is essential in Outer Space planning. Otherwise, a tremendous waste of money and time for all countries is the result. Planning is tied to costs, clearly ECOSPACE. Number 2: Outer Space is inherently international in nature. International cooperation saves costs, clearly ECOSPACE. Number 3: Outer Space holds an important solution to the global resources conflict. Less costly resources are clearly ECOSPACE. Number 4: Outer Space is the key factor for world peace, world information, world trade, every nation’s economic development and every nation’s national security. The cost of Outer Space national security is clearly ECOSPACE. Number 5: The greater the number of nations participating in each Outer Space DBS project, the greater the assurance of nonthreat to any nation’s national security; and the greater its national popular support and thus the greater its contribution to world peace. Cooperation reduces national security threats and is clearly ECOSPACE. Number 6: Outer Space is so big and so vast in economic cost and expense, so rich in rewards, that it has a scope the same as the word “infinity” has to a scientist (4). ECOSPACE is a DBS key which will show a DBS policy and legal pathway by 1988, all of which is proven in today’s actual budgeted and planned telecommunication expenditures of various nations. For example, the world importance of ECO-
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