Laser Pwered Mhd

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NASATechnicalPaper26211986

National Aeronauticsand Space AdministrationSc ient i f i c and Techn i ca lI n f o r m a t i on B r anch

Laser-Powered MHDGenerators forSpace ApplicationN. W. JalufkaLangley Research CenterHampton, Virginia


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ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1M H D T h e o r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Laser-Plasma Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Plasma Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Plasma Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Candidate MHD Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Pulsed LSD Systems . , . . . . . . . . . . . . . . . . . . . . . . . . . . 3Plasma System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Liquid-Metal Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Brayton cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Rankine cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Summary and Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 7Appendix-Calculation of Overall System Efficiency . . . . . . . . . . . . . . . . 9References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

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IntroductionThe rapid development and utilization of space

will require a readily available and relatively inex-pensive source of abundant power. In th e past,spacecraft have relied on chemical, nuclear, or solar-generated power to provide onboard power, commu-nication, and propulsion. While these methods haveproven adequate for past missions, studies (refs. 1to 14) over the past several years have identifiedspace-power transmission as a potential solution tothe problem of on-site power generation in space.Lasers are being considered for space-power trans-mission because of the relatively small transmittingand receiving systems required when compared withthose required by longer wavelength systems. Otherapplications of the transmitted laser power includepropulsion systems for spacecraft and deep-spacecommunications. These various application studiesalso conclude that th e use of lasers in space can re-duce the cost of space power and propulsion.One concept for a space-power transmission sys-tem has received considerable attention: a large cen-tral power station in geosynchronous orbit whichwould beam power to remote users (ref. 1). The over-all efficiency of such a system would depend not onlyon the efficiency with which the laser beam could begenerated but also on the efficiency with which thebeam could be converted to a useful form of energy atth e receiver. Some research on laser-plasma interac-tion has been carried out and was primarily directedtoward mathematical model development to studythe absorption of laser radiation and the formation ofa laser-supported plasma for laser propulsion systems(refs. 9 to 11). An experimental study (ref. 15) hasbeen conducted on the coupling of a C0 2 laser beamw i t h hydrogen gas. Laser-to-electrical power conver-sion has received some attention and one can readilydefine the requirements for a space-based laser-to-electrical power converter:1. Energy conversion efficiency should be high.2 . Efficiency should be independent of the laser3. Power-to-weight ratio should be high.4. Operation should be reliable and maintenance5. System should not be excessively expensive.Ideally, the converter should be waveiength indepen-dent to avoid th e problems associated with matchingthe wavelength of the laser to tha t at which the con-verter is most efficient. High conversion efficiency is amajor (and obvious) requirement, since laser energynot converted to eiectricai power iiiizji be convertedto heat. One would then have the additional problem

wavelength.

free.

of rejecting heat in space which would require largeradiators, thereby increasing the system mass. Fourconcepts have been identified that may satisfy theserequirements:1. Optical rectification2. Reverse free-electron laser3. Laser-driven magnetohydrodynamics (MHD)4. Laser photovoltaicsAlthough optical rectification, th e reverse free-electron laser, and laser photovoltaics are potentiallyhigh-efficiency systems, this paper examines only thelaser-driven magnetohydrodynamic system.M H D Theory

MHD concepts have existed since the time ofFaraday, and considerable research and developmenthave been carried out in this area, resulting in sev-eral large systems in operation around the world(ref. 16). The concept of a laser-driven MHD gen-erator, however, is conceivable only in a space-basedlaser energy-conversion system, since one must con-sider both power transmission as well as conversion.The laser-driven MHD converter offers high systemefficiency, high power density, and closed-cycle oper-ation. Furthermore, such a closed-cycle system canbe built with few (or no) moving mechanical parts;thus it would be highly reliable and require littlemaintenance. A high overall system efficiency is ex-pected because of the high conversion efficiency ofthe MHD generator (approximately 70 percent) andthe high absorption (approximately 100 percent) ofth e laser beam in the working fluid (refs. 17 to 19).One may also adjust the working fluid characteris-tics (in some cases) of such a system to maximizeabsorption at a given wavelength so that the systemcould be designed for any transmitting laser wave-length. An MHD generator consists of a conductormoving through a fixed magnetic field. In these sys-tems th e conductor is a high-temperature ionized gas(or plasma) or a liquid metal. Electrical currentis extracted from the flowing plasma or liquid metalby electrodes. Figure 1 is a schematic of a simplifiedMHD generator.

Plasma system MHD generators are often classi-fied according to the method employed to producethe plasma. These methods include shock waves,combustion, arcs, chemical processes, and explosives.More receIiLly, the extensive research carried out inthe area of laser-driven thermonuclear fusion h a sshown that plasmas may be efficiently produced andheated by lasers. This suggests th at laser-producedand/or laser-heated plasmas or laser-heated liquidtern might efficiently convert laser energy t o electricalmcta!s ir , ceI?junctinn with m M H D genera-tor ays-


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To loadIFigure 1. MHD generator geometry.

power. The most advanced linear-type generator iscalled a Hall generator since it utilizes the Hall cur-rent component, which is in the gas flow direction.The power equation for a Hall generator is given by(ref. 20)

P =J + W K ( 1- K )1+Pwhere ,D is the ra tio of t he electron mean free pathto the Larmor radius, (T is the plasma conductivity,

is the flow velocity, 6 is the magnetic field, andK is the ratio of load to open circuit voltage. Ifthe plasma is in the Coulomb-dominated regime,then (T is proportional to T3I2 ref. 21), so thatincreasing the temperature (with a laser) increasesthe power output of the generator (for those systemsthat opera te with a plasma). If the laser energy isdeposited in a shor t time (i.e., pulsed mode) , therapidly heated gas also expands, increasing f and,consequently, the generator output.

Magnetohydrodynamic systems were first pro-posed for space-power applications as a means ofconverting nuclear energy (heat) to electrical power(ref. 22) . This first investigation was based on atwo-fluid magnetohydrodynamic cycle and concludedth at th e efficiency of such a system (based on th e ex-isting technology) would be less than that of a tur-boelectric or thermionic system of approximately thesame specific weight. The stiidy did point out two po-tential advantages of the two-fluid MHD conversionsystem:1. Long lifetime and high reliability, relative t o tur-

boelectric systems, afforded by the absence ofmoving parts

2 . Ease of development, relative to thermionic andgaseous MHD systems, permitted by the rela-tively low temperatures

2

Because of the low efficiency of the two-fluid MHDsystem, no effort was made to develop MHD as aspace-power system. However, research a t ArgonneNational Laboratory initiated in the 1960's wasaimed at minimizing the high energy loss of MHDsystems. This research resulted in the two-phase-generator liquid-metal MHD (LMMHD) system whichexhibits a much higher cycle efficiency. This high-efficiency LMMHD concept may now be consideredfor space application, which would utilize t he uniquecombination of high efficiency and high-temperaturecapability.Laser-Plasma Interaction

Some of the candidate systems involve laser-plasma interaction, which may be broken down intotwo categories:1. Plasma production by lasers2 . Plasma heating by lasers

Plasma ProductionProduction of a plasma (ionized gas) by laser

irradiation requires a minimum (threshold) intensitywhich depends on the type of gas, gas density, laserbeam size, laser pulse width, and laser wavelength(ref. 23) . This threshold is typically of the orderof 109W/cm2 for a variety of gases at atmosphericpressure.

Production of the plasma then proceeds in twosteps. The first step is initial ionization, which canbe accomplished in a gas by multi-photon absorp-tion. After free electrons are produced, they are fur-ther heated by inverse bremsstrahlung resulting ina cascade process in which the energetic electronsproduce further ionization by collision with the neu-tra l atoms and ions. Once this stage is reached,the laser intensity required to maintain the plasmadrops to a value equal to the loss rate from theplasma. This is typically of the order of a few kilo-watts (ref. 24). The absorption coefficient (in cm- l)for inverse bremsstrahlung is given by (ref. 25)

where Z is the ionic charge, ne is the electron densityin ~ m - ~ ,is the high-frequency screening parame-ter, Te s the electron temperature in eV, v is the laserfrequency, and v p is th e plasma frequency. Couplingof the laser energy into the plasma is most efficientif the electron density of the plasma is such that v pis close to v. The absorption depth (Le., the dis-tance th e laser radiation penetrates into the plasma)


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is given by a-'. Because of th e s trong dependence ofa on the electron density and electron temperature,the plasma parameters may be varied t o achieve max-imum absorption of any laser radiation in a fixed dis-tance. If the electron density of the plasma reachesth e critical density given by (ref. 26)

(3 )2nc = (1.24 x 10- )vthen the laser beam does not penetrate into theplasma but is reflected instead. This situation re-sults in a laser-supported detonation (LSD) wavepropagating from the plasma surface along the laserbeam toward t he laser. These waves move at super-sonic speeds and ionize and heat the medium throughwhich they are propagating.

Plasma HeatingThe energy absorbed by the free electrons can

be dissipated in several ways: (1) expansion of theplasma volume, (2 ) heating of the electrons, ions, andatoms, (3) inelastic collisions resulting in excitationand ionization of the ions and atoms, (4 ) diffusion ofelectrons out of the plasma, (5) attachment of elec-trons to atoms to form negative ions, and (6 ) ra-diative losses such as bremsstrahlung and radiativerecombination. Th e relative importance of severalof these processes depends on plasma parameters,and, in many cases, some processes are negligible.Expansion of the plasma volume and heating of theelectrons are t,he most, important processes for MHDpower generation, as the other mechanisms representenergy losses. At th e pressures and tempera tures ap-pliczhe to MHD power generation, th e dominatingprocesses appear to be expansion and heating, withradiative recombination being the major loss mecha-nism (ref. 27) . The increase in electron temperatureis given by (ref. 28)

x [1 - e x p (-- )] t L L d t (4)where ni is the ion density in ~ m - ~ ,fi is the free-free gaunt factor, T e is in eV, h is Planck's constant,q is the cross section of the laser beam in cm2, t L isthe iaser pulse iength in seconds, and L is the laserintensity. The integral

1 t L L d t! l ois in joules/cm2.

If the plasma exists for an adequate length oftime, the electrons transfer energy to the ions bycollision, and equilibrium is reached in a time given,in seconds, by (ref. 29)

where A is the atomic weight of the particle and thesubscript f refers t o the field (heavier) particle.

These various properties of the laser-plasma in-teraction phenomena provide several options in thedesign of an MHD generator to be powered by laserradiation.Candidate MHD Systems

Among the MHD systems considered for space-power application are1. Pulsed systems in which gas breakdown, heating,

and flow depend on th e initiation of an LSD wave2. Plasma systems (pulsed or continuous wave ( CW) )

where the laser radiation is used to heat an ex-isting plasma either prior to its introduction intothe MHD generator or during its flow through thegenerator

3. Liquid-metal systems (pulsed or C W ) in whichthe laser radiation heats the liquid metal or thecarrier medium (or both) prior to mixing andintroduction into the MHD generatorPulsed LSD SystemsMaxwell and Myrabo (ref. 30) have considered in

detail pulsed laser MHD systems based on LSD wavegeneration. Figure 2 is a schematic of a pulsed LSDMHD generator. Two modes of power generation arepossible in this configuration. In mode 1, power isgenerated as the LSD wave passes through the gener-ator at a high velocity. In mode 2, power is generatedwhen the high-temperature plasma remaining in theplenum chamber is vented through the generator.

Figure 3 shows the physical process of the powercycle for mode 1. The cycle is started with anLSD wave being ignited at the exhaust port of theMHD generator and propagating at a high velocityconductivity "slug" immediately behind the LSDwave provides the working fluid for MHD powergeneration. This mode is unique in laser MHDpower generation since electrical power is generatednear!ysimu!taneous!y with the depnsition of energyin the working fluid.

CL..lllough th e generator. Th e high-tcmpcrature, high-

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and focusing optics,-leiiiirn chamber 7 ,- - - - - - _ _ _ _- - - - _ _ _ __ _ - - - - __

Laser @T I,)Figure 2. Pulsed LSD MHD generator.

Laser

-- - -

- - - - - - - - -

Figure 3 . Mode 1 power cycle for pulsed LSD generator.The velocity of the LSD wave is given by (ref. 31)

where y is the ratio of the specific heats of the gas,Io is th e incident laser flux, and po is the ambient gasdensity in front of th e wave. The pressure behind th edetonation wave is given by (ref. 31)

(7 )For practical values of incident laser flux and ambi-ent gas pressure, LSD wave velocities greater than1 km/sec and pressures behind the LSD wave of sev-eral atmospheres are possible. For these conditions,the electron temperature of t he LSD wave can bedriven to very high values and the electrical con-ductivity of the medium may reach the Coulomb-dominated limit (in the range of lo3 mho/m orgreater). These conditions are suitable for very highelectrical power densities.

To achieve a high extraction efficiency in thismode requires a conducting region of reasonablelength (several centimeters). This has not beenachieved experimentally because the short thermal-ization times at the pressures and temperatures ex-pected cause the plasma to reach equilibrium a veryshort distance behind the LSD wave. Two possibili-ties might provide adequate slug thicknesses:

1. Deposit energy in the electron gas by ohmicheating in order to sustain nonequilibrium imme-diately behind the LSD wave absorption region.

2. Decrease the absorption coefficient so that thelaser energy is absorbed over a length equal tothe desired slug thickness.

Neither of these methods has been investigated ex-perimentally. If either of these possibilities can beproven to be adequate, then high conversion efficien-cies would be expected in this mode.

Figure 4 hows th e physical process of the mode 2power cycle. In thi s mode of operation, th e LSD waveserves only to process the plenum chamber workingfluid to high temperatures and pressures. Becauseof the short electron-ion thermalization times, thismode of operation must generate power under equi-librium conditions. The mode 2 cycle begins with theLSD wave being reflected at the optical window, thatis, at t he termination of the mode 1 cycle. Passageof this wave through the plenum chamber creates adense, high-temperature plasma. The reflected wavehelps to drive all of the working fluid through theMHD generator and acts much like a piston. Electri-cal power is generated as the high-pressure plasmaexpands through the generator. In this mode, thesystem is rather similar to a shock-driven MHD gen-erator, which to date has achieved a generation ex-traction efficiency near 25 percent (ref. 32).

A space-based laser converter system based onthe above concepts would use a noble gas (argonperhaps) as the working fluid with the addition ofa seed material having a low ionization potential(cesium is a good candidate). Such a system wouldhave t o be closed cycle to avoid th e problem ofreplenishing the gas supplies.

While little experimental data are available onthis type of system, the feasibility study by Maxwelland Myrabo (ref. 30) assumes that the present stateof the art has not reached its limit and that effi-ciencies approaching 50 percent might be achievable.This potentially high conversion efficiency as well asth e simplicity of th e system are two major advan-tages which make the system attractive. Th e disad-vantages of the system include1. Exposure to the LSD wave and high-temperature

plasma might damage optical surfaces.2. The system must operate at high power levels

because of the high laser intensity required toachieve breakdown.

3. The system operates at high pressures and isclosed cycle, so that a minor leak would renderthe system inoperable.

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4\ . R ,-Collec.ting andfocusing optics.:.:.................. vFlow- f :::ii:~J--J........................

/

LHeated gas in plenum acts likea piston to drive plasma backthrough M H D generator

Figure 4. Mode 2 power cycle for pulsed LSD generator.Plasma SystemA second option for space-based laser-driven

MHD generators is to use the incoming laser beamto heat an existing low-temperature plasma to highertemperatures before it flows into the MHD channel.In this system, the laser radiation is absorbed bythe free electrons through inverse bremsstrahlung.Electron-ion thermalization occurs rapidly, so thatthe plasma flowing through the generator is in equi-librium. The intensity of the laser beam must bemaintained below the breakdown threshold in orderto prevent LSD wave formation and propagation backalong the laser beam.

Figure 5 is a schematic of a laser-driven plasmaMHD system. Thi s is a closed-cycle system whichemploys a noble gas (argon) as the working mcdium.The working medium is seeded with a low-ionizationelement (cesium) to increase the electron density andthereby the electrical conductivity of the workingmedium. This procedure requires the separation ofthe two elements after passage through the MHDgenerator and before compression of the gas.

This system also offers the possibility of heatingthe plasma by absorption of laser radiation while theplasma is in the MHD channel. The length of thechannel would be such that the laser beam is com-pletely absorbed in the channel. This would keep theplasma electron temperature at a higher value andcould possibly improve the efficiency of the MHDgenerator. Plasma temperatures achieved in CWlaser-produced plasmas depend on the type of gasand pressure in the system and generally range from15000 K to 20 000 K. This temperature would droprapidly outside of the interaction region because ofradiative cooling and diffusion. This system has th eadvantage of having been studied extensively bothexperimentally and theoretically, so that the efficien-cies and characteristics of th e various components arewell known (ref. 2 1 ) . With an auxiliary means ofachieving plasma ignition, the system would have the

I ' ' IFigure 5 . Laser plasma-flow MHD system.

advantage of operation at lower power levels than ifignition by laser breakdown were required. A disad-vantage of this system would be the exposure of thecollecting and focusing optics to the plasma, whichcould result in long-term damage to this part of thesystem.

Performance of a Hall MHD generator has beencalculated by Choi (ref. 33), assuming an unsteadyturbulent flow in the channel. This model assumes an80-percent efficiency for the absorption of the laserbeam, which is somewhat conservative. Laboratorymeasurements of the absorption of C02 laser radia-tion by a shock-heated plasma show that absorptionclose to 100 percent is achievable (refs. 17 and 18).Based on the above assumptions, the model predictsa generator efficiency of greater than 40 percent,which agrees well with experimentally measured val-ues of about 50 percent for a simplified generatordesign (ref. 34).

Th e efficiencies of th e various system componentsare listed in table I for the plasma-flow system shownin figure 5. The efficiencies are th e best values avail-able in the literature (refs. 35 and 36). The overallsystem efficiency is defined as the electrical energyextracted minus the energy used to operate the sys-tem components divided by the laser power into thesystem. This expression is derived in the appendix.Using the efficiencies listed in table I, an overall sys-tem efficiency of 30.6 percent is obtained. This doesnot include th e energy consumed by the magnet (neg-ligible if a superconducting magnet is used), energylosses associated with the'gas flow through t he con-necting piping (small if flow rates are not excessive),or radiative energy losses. Consequently, this figureis considered ail upper l imit hi. he eff;;cizncyef thissystem.

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r M H D generator ~ - - -ratorTABLE I. EFFICIENCIES OF COMPONENTS OFPLASMA MHD SYSTEM (FIG. 5)[Values from refs. 35 and 361

Efficiency ofAbsorption of laser, . . . . . . . . . . . . . .80MHD generator, E~ . . . . . . . . . . . . . . 0.50Separator, . . . . . . . . . . . . . . . . . .85Compressor. . . . . . . . . . . . . . . . . 0.85Mixer, . . . . . . . . . . . . . . . . . . .90

Liquid-Metal SystemsThe two-phase-generator liquid-metal magneto-

hydrodynamic (LMMHD) concept was developed atArgonne National Laboratory in 1969-1970 (ref. 35).This concept demonstrated that LMMHD systemscan be made with rather att rac tive efficiencies (4 0 to90 percent) . Th e two-phase-generator LMMHDsystem-which uses two working fluids, a thermo-dynamic fluid (gas or vapor) and an electrodynamicfluid (liquid metal)-is more flexible in coupling todifferent heat-source temperatures than the plasmaMHD system. Unlike the plasma MHD system,which requires a high temperature, LMMHD sys-tems can efficiently utilize heat-source temperaturesas low as 450 K (ref. 35) because the liquid metalprovides the conductivity in the generator and ion-ization is not required. Another difference betweenthe LMMHD and the plasma MHD system is that theLMMHD system does not require the liquid metal tobe in the atomic state (i.e., existing in the flow assingle atoms). The liquid metal can be mixed intothe flow in the form of an aerosol. The liquid metalacts as a heat source for the gas phase so that theexpansion through the MHD generator takes place atpractically a constant temperature.

There are two LMMHD cycles of interest for spaceapplications:1. Brayton cycle2 . Rankine cycle

Brayton cycle. Figure 6 is a schematic of aBrayton cycle LMMHD system. At temperaturesabove approximately 800 K, this system becomes atl-tractive because of its efficiencies, inherent simplic-i ty , and low sensitivity of efficiency to design powerlevel. An inert gas (e.g., helium) is th e thermody-namic working fluid and a liquid metal (e.g., sodiumor lithium) is the electrodynamic fluid in the MHDgenerator. Th e actual working fluid and liquid metalused in a space-based system would be determinedby a number of factors. These include th e operat-ing temperature of the system and the maximum

----Mixer

L-+- Nozzle4DifFuserPower outLaser

Laser receiverLiquid-metal loop

Gas loop--Regeneratorompressor'RadaoHeat out

Figure 6. Brayton cycle LMMHD system.

power generated. At typical flow velocities of approx-imately 150 m/sec and temperatures greater than1000 K , the helium and sodium (or lithium) systemis adequate. At higher flow rates and temperatures,liquid-metal carryover with the gas from the separa-tor can become a problem. In a laser-powered appli-cation the laser energy is absorbed by the gas, theliquid metal, or both. The laser energy can be cou-pled into the gas in the same manner as in the plasmaMHD system, with t he heated gas collisionally trans-ferring energy to the liquid metal. Since these MHDsystems operate at pressures of several atmospheres,the absorption length would be of the order of a fewcentimeters. Heating of the liquid metal directly hasnot been studied in great detail, although absorptionof radiation by high-temperature metals has been ob-served. The gas and th e liquid metal are combined inthe mixer, and the two-phase mixture flows throughthe MHD generator. The gas phase in this mixture isabout 80 percent of the total volume. In the genera-tor the gas expands, drives the liquid across the mag-netic field, and generates electrical power. Since th eliquid has a high heat content, this expansion occursat almost constant temperature, and considerableavailable energy remains in t he gas exiting the MHDgenerator. Thi s energy may be used by transferringit to the carrier gas prior to mixing with the liquidmetal. During the expansion through th e genera-tor, thermal energy is continuously transferred fromthe liquid to the gas, so that most of the enthalpychange in the generator comes from the liquid. Thisalmost constant tempera ture expansion is the sourceof the potentially higher thermodynamic efficiencyof the two-phase LMMHD system. After exiting thegenerator, the mixture flows through a nozzle wherethe liquid is accelerated. The resulting high-speedflow is separated and the liquid pressure required to

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return the liquid through th e absorption cell and tothe mixer is obtained in the diffuser. The nozzle anddiffuser could be replaced by a liquid-metal pump.

Blumenberg (ref. 36) has conducted optimiza-tion studies of space-power systems consisting of aBrayton cycle MHD system powered by a nuclear re-actor. Th e mass-to-power ratio for this system variedfrom 9 kg/kWE for a 100 kWE system to 6 kg/kWEfor a 10 MWE system. Since th e system envisionedhere has an external power source (the laser), thesevalues should be reduced by 0.2 kg/kWE (specificmass of the reactor). This gives 8.8 kg/kWE for theapproximate specific mass of the 100 kWE MHD sys-tem.

Brayton cycle LMMHD systems have a well-established technology base, at least for terrestrialapplications, and for the most part this informationis directly applicable in space. One problem area thatmay arise in the conventional LMMHD cycle is op-eration of the separator. In terrestrial applications,gravity plays an important role in the operation ofthis device, so that in the zero-gravity environmentof space some other method of separating the liquidmetal from the gas may have to be found.

Another problem area for this system is the re-quirement of the optical system to receive the laserbeam and focus it to the proper spot size. This re-quires that a window transparent to the laser wave-length be sealed into the high-pressure gas-flow sys-tem. At the operational pressures envisioned for thissystem (several atmospheres), even small leaks couldbe detrimental. The possibility of damage to the win-dow also exists if it comes in contact with t he high-tempera ture liquid metal. This could be particularlydetrimental to optical coatings on the window surface(if they are required).

Efficiencies for the various components of theBrayton cycle LMMHD are listed in table I1 (refs.35 and 36) . These values are typical of those quotedin the literature. Using these values and the proce-dure of the appendix, an overall system efficiency of55.8 percent is obtained. Energy losses such as radi-ation loss, magnet power, and losses due to flow arenot included but are assumed to be small. The cal-culated efficiency is therefore an upper limit for theoverall system efficiency.

Rankine cyde. The Rankine cycle is shown infigure 7 . This cycle is of interest at temperatures be-low 800 K . This cycle differs from the Brayton cycleonly in the use of a condensable fluid as the ther-modynamic working fluid. Since the thermodynamicfluid is in a liquid phase for part of the cycle, the re-ject heat exchanger and the compressor of the Bray-ton cycle are replaced, respectively, by a condenser

TABLE 11. EFFICIENCIES OF COMPONENTS OFBRAYTON CYCLE LMMHD SYSTEM (FIG. 6)

[Values from refs. 35 and 361Efficiency of-

Absorption of laser, . . . . . . . . . . . . . .80Mixer, . . . . . . . . . . . . . . . . . . .90MHD generator, E~ . . . . . . . . . . . . . . .80Separator, E~ . . . . . . . . . . . . . . . . . .85Compressor, . . . . . . . . . . . . . . . . .85

L iqu id -meta l pump

- - - - -

Vapor

'$-CondenseHeat out

Figure 7 . Rankine cycle LMMHD system.and a pump. Since th e Rankine cycle is identical tothe Brayton cycle in several respects, such as operat-ing pressure, flow rate, absorption length, materials,windows, and laser coupling, the advantages of theBrayton cycle as well as its disadvantages apply tothis system. In addition, the use of a condensablefluid would add some difficulty in a space-based sys-tem because the liquid phase is more complicated tocontrol in zero gravity than the gas phase. The oper-ating temperatures of the Rankine cycle are consid-erably lower than those of the Brayton cycle and aremuch lower than those anticipated in a laser-poweredMHD converter system. Consequently, th e Rankinecycle is not t he best choice for a laser MHD convertersystem even though the overall system efficiency isabout equal for the two systems.Summary and Recommendations

Laser-driven magnetohydrodynamic (MHD) sys-tems for laser-to-electrical energy conversion haveseveral advantages for space-based operat ion, includ-in g closed-cycle operation, high-temperature opera-tion (which minimizes waste heat management prob-lems), high conversion efficiency, and simplicity ofdesign. Since these systems have few (if any) moving

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mechanical part s, they should exhibit high reliabilityand low maintenance.

The pulsed laser-supported detonation (LSD) sys-tem discussed in this report offers the potential ofthis system can be built with no moving mechanicalpar ts, high reliability should be assured. This systemwould probably have the best power-to-weight ratiobecause auxiliary equipment is not required. Unfor-tunately, there has been little experimental researchthat is applicable to this system, so that the full po-tential of the system cannot be accurately assessed.

The plasma MHD system also offers a reason-ably high conversion efficiency, closed-cycle opera-tion, and simplicity of design, although the require-ment of flowing th e carrier gas does increase the com-plexity of the system over that of the pulsed LSDsystem. Thus the overall system efficiency would belower than the efficiency of the pulsed LSD system.However, because of the large amount of researchthat has been conducted on this plasma MHD sys-which would reduce development cost.

Of the two liquid-metal magnetohydrodynamic(LMMHD) systems, the Brayton cycle appears to bethe most attractive for space applications becauseof its higher operating temperature. In either case,however, the overall system efficiencies are quitehigh and both offer closed-cycle operation as well assimplicity of design.

All of the systems discussed here would requirehigh-pressure operation (greater than 1 atm) in order

high conversion efficiency and simple design. Since

tem, a much broader in-depth technology base exists

to achieve high conversion efficiency. There are

some problems inherent in this requirement. Inparticular, the adequate sealing of optical windowsinto a high-pressure system so that no leakage occursmay present some technical difficulties, but these donot appear to be insurmountable. Of more concernis damage to the optical windows because of possiblecontact with the high-temperature plasma. Thisproblem could become particularly difficult if opticalcoatings are required on th e windows. Care must betaken in the design of the system to eliminate (or atleast minimize) this problem.

Since MHD systems meet many of the require-ments for a space-based laser-to-electrical energyconverter, the following recommendations are made:1. Additional research on pulsed LSD MHD gener-

ator systems is required to determine their fullpotential.2 . Additional research on LMMHD systems is de-sirable to determine the best method of couplinglaser energy into the system (i.e., into the liquidmetal, the carrier gas, or both).3. Research on laser-plasma interaction and systemdesign should continue in order to minimize prob-lem areas.

In addition, a more extensive and in-depth systemstudy should be conducted t o ensure the best systemdesigns for particular space applications.

NASA Langley Research CenterHampton, VA 23665-5225July 14, 1986


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AppendixCalculation of Overall System Efficiency

The energy input t o t he system is the energy inthe laser beam EO. If the absorption efficiency is E , ,then the energy available for conversion to electricityis

&Ea ( A l )The energy in the system after passing through themixer with efficiency E~ is

and the energy extracted by the MHD generatorhaving an efficiency Eg is

The energy remaining in the system,

is the energy input to the separator which has anefficiency E ~ . he energy remaining in the systemafter passage through th e separator is then

and the energy in the system after passing throughthe compressor (efficiency E , - ) would be

if the compressor were extracting energy from thesystem. The energy "extracted" would be the differ-ence of these last two expressions, that is,

This was assumed to be the energy added back intoth e system by the compressor. Th e amount of energyrequired by the compressor to add this amount ofenergy to the system is

The efficiency of the system is defined to be thenet electrical power generated (i.e., the total powerproduced by th e MHD generator, eq. (A3),minus th epower required to operate the compressor, eq. (A8))divided by the input energy (ref. 2 2 ) , that is,

Using th e efficiencies from table I for the plasma-flowsystem gives

q =30.6 percent (A10)For the Brayton cycle LMMHD system (using theefficiencies from table 11),

9


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,References1. Holloway. Paul F.; and Garret t, L. Bernard: Utility

of and Technology for a Space Central Power Station.AIAA-81-0449. Feb. 1981.Rather, John D. G.: Final Report-A Stud y To Recom-mend NASA High Power Laser Technology and ResearchPrograms. WJSA-76-17 (Contract no. NASW-2866),W. J. Schafer Assoc., Inc., July 1976. (Available as

3. Rather, John D. G.; Gerry, Edward T.; and Zeiders,Glen W.: Investigation of Possibilities f o r Solar PoweredHigh Energy Lasers in Space. NASA CR-153271, 1977.

4. Garrett. L. B.; and Hook, W. R.: Future LargeSpace Systems Opportunit ies+ A Case for Space-to-Space Power? Large Space Systems Technology, NASACP-2035, Volume I, 1978, pp. 507-531.

5. Kantrowitz, Arthur: The Relev'ance of Space. Astro-naut. B Aeronaut., vol. 9, no. 3, Mar. 1971, pp. 34-35.6. Coneybear , J . Frank: T he Use of Lasers for the Trans-

mission of Power. Radiation Energy Conversion i n Space,Kenneth W. Billman, ed., American Inst. Aeronaut. &Astronaut ., c.1978, pp. 279--310.

7. Future Orbital Power Systems Technology Requirements.8. Rather, John D . G.; Gerry, Edward T.; and Zeiders,

Glenn W.: A Study To Survey N A SA Laser Applicationsand Identify Suztable Lasers for Specific NASA Needs.WJSA 77-14 TR1 (Subcontract under Contract NAS7-l oo ) , W. J . Schafer Assoc., Inc., Feb. 1978.

9. Hertzberg, Abraham; and Sun, Kenneth: Laser AircraftPropulsion. Radiation Energy Conversion in Space, Ken-neth W. Billman, ed., American Inst. Aeronaut. & As-tronaut. , c.1978, pp. 243- 263.

10. Minovitch, M. A.: Performance Analysis of a Laser Pro-pelled Interorbital Transfer Vehicle. NASA CR-134966,1976.

11. Rather, J . D. G .; and Myrabo, L.: Laser PropulsionSupport Program. NASA CR-170708, 1980.12. W. J. Schafer Assoc., Inc.: NASA Technology Workshopon High Power Lasers. Purchase Order No. 709-78-1,

1979. (Available as NASA CR-168751.)13. Bain, Claud N.: Potential ofLaser f o r SPS Power Trans-mission. HCP/R-4024-07 (Contract No. EG-77-C-01-

4024), PR C Energy Analysis Co., Oct. 1978. (Available14. Jones, W. S.; Forsyth, J. B.; and Skratt, J. P.: LaserRocket System Analysis. NASA CR-159521, 1978.15. Kemp, N. H.; and Krech, R. H.: Laser-Heated Thruster-

Final Report. NASA CR-161666, 1980.16. Gmndy, Robert F., cd.: ,~~agnetoFLydrodyrlurnic nergyf o r Electric Power Generation. Noyes Data Corp., 1978.17. Jalufka, N. W. ; and Kloc, Barbara J. : Interaction of

CO2 Laser Radiation With a Shock-Heated HydrogenPlasma. Bull. American Phys. SOC.,vol. 30, no. 2,Feb. 1985, p. 138.

18. Lee, Ja H.; McFarland, Donald R.; and Hohl, Frank:Production of Dense Plasmas in a Hypocycloidal PinchApparatus. Phys. Fluids, vol. 20, no. 2, Feb. 1977,pp. 313 321.

2.

NASA CR-155398.)

NASA CP-2058, 1978.

as NASA CR-157432.)

19. Box, S. J. C.; John, P. K.; and Byszewski, W. W.:Interaction of a C02 Laser Beam With a Shock-TubePlasma. J. Appl. Phys., vol. 48, no. 5, May 1977,

20. Sutton, George W.; and Gloersen, Per: Magnetohydro-dynamic Power and Propulsion. Magnetohydrodynamics,Ali Bulent Cambel, Thomas P. Anderson, and MiltonM. Slawsky, eds., Northwestern liniv. Press (Evanston,Ill.), c.1962, pp. 243-268.

21. Sutton, George W.; and Sherman, Arthur: EngineeringMagnetohydrodynamics. McGraw-Hill Book Co., Inc.,c.1965.

22. Elliott, David G.: A Two-Fluid MagnetohydrodynamicCycle fo r Nuclear-Electric Power Conversion. Tech. Rep.No. 32-116 (Cont ract No. NASw-6), Jet Propulsion Lab.,California Inst. Technol., June 30, 1961.

23. Morgan, F.; Evans, L. R.; and Morgan, C. Grey:Laser Beam Induced Breakdown in Helium and Ar-gon. J . Phys. D: Appl. Phys., vol. 4, no. 2, Feb. 1971,

24. Smith, David C.; and Fowler, Michael C.: Ignition andMaintenance of a CW Plasma in Atmospheric-PressureAir With C02 Laser Radiation. Appl. Phys. Lett.,vol. 22, no. 10, May 15, 1973, pp. 500-502.

25. Johnston, Tudor Wyatt; and Dawson, John M.: CorrectValues for High-Frequency Power Absorption by InverseBremsstrahlung in Plasmas. Phys. Fluids, vol. 16, no. 5,May 1973, p. 722.

26. Key, M. H.: Interact ions of Intense Laser RadiationWith Plasma. Philos. Trans. R. SO C. ondon, vol. A300,no. 1456, Apr. 23, 1981, pp. 599-612.

27. DeCoste, R.; Englehardt, A. G.; Fuchs, V.; and Neufeld,C. R.: Transverse Heat ing of a Cold Dense HeliumPlasma by a Pulsed C02 Laser Beam. J. Appl. Phys.,vol. 45, no. 3, Mar. 1974, pp. 1127-1134.28. Kunze, H. J.: The Laser as a Tool for Plasma Diag-nostics. Plasma Diagnostics, W. Lochte-Holtgreven, ed. ,North-Holland Publ. Co., 1968, pp. 550-616 .

29. Spitzer, Lyman Jr.: Physics of Fully Ionized Gases,Second rev. ed. Interscience Publ., c.1962.

30. Maxwell, Craig D.; and Myrabo, Leik N.: Feasibilityof Laser-Driven Repetitively-Pulsed MHD Generators.AIAA-83-1442, June 1983.

31. Rgz er, Yu. P.: Subsonic Propagation of a LightSpark and Threshold Conditions for the Maintenanceof Plasma by Radiation. Soviet Phys.-JETP, vol. 31,no. 6, Dec. 1970, pp. 1148-1154.

32. Tate, E.; Marston, C. H.; and Zauderer, B.: Large En-thalpy Extraction Eqxr irrie nts an a Non-Equiiibrium Mag-netohydrodynamic Generator. DOC. 74SD226, GeneralElectric Co., July 29, 1974.

33. Choi, Sang H.: Space-Bused Laser-Driven MHD Generu-tor: Feasibility Study. Final Rep. 681104 (Purchase Or-der No. L28161B), Information and Control Systems,Inc., Nov. 1981. (Available as NASA CR-178184.)

34. Jalufka, N. W.: Laser Heating of Low TemperaturePlasmas With Application to Energy Conversion. Bull.American Phys, SOC., ol. 31, no. 2, Feb. 1986, p. 158.

pp. 1946-1952.

pp. 225-235.

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35. Pierson, E. S.; Branover, H. ; Fabris, G.; and Reed, 36. Blumenberg, Jurgen: Optimisation of Powerful EnergyC. B.: Solar-Powered Liquid-Metal MHD Power Sys- Supply Systems for Application in Space. IAF Paperterns. Mech. Eng., vol. 102, no. 10, Oct. 1980, pp. 32-37. 79-169, Sept. 1979.

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Standard Bibliographic Page1. Report No.

NASA TP-26212. Government Accession N o. 3. Recipients Catalog No.

L-1614410. Work Unit No.. Performing Organization Name a nd AddressNASA Langley Research Center

1. Title and SubtitleLaser-Powered MHD Generators for Space Application

~

7. Author(s)N. W . Jalufka

5. Report DateOctober 1986

6. Performing Organization Code506-41-41-018. Performing Organization Report No.

Hampton, VA-23665-5225

16. AbstractMagnetohydrodynamic ( M H D ) energy conversion systems of the pulsed laser-supported detonation (LSD)wave, plasma MHD, and liquid-metal MHD (LMMHD) types are assessed for their potential as space-based laser-to-electrical power converters. These systems offer several advantages as energy convertersrelative to the present chemical, nuclear, and solar devices, including high conversion efficiency, simpledesign, high-temperature operation, high power density, and high reliability. Of these systems, th e Braytoncycle liquid-metal MHD system appears t o be the most attractive. The LMMHD technology base is wellestablished for terrestrial applications, particularly with regard to the generator, mixer, and other systemcomponents. However, further research is required to extend this technology base to space applications andto establish the technology required to couple the laser energy into the system most efficiently. Continuedresearch on each of the three system types is recommended.

11. Contract or Grant No.

17 . Key Words (Suggested by Authors(s))Magnetohydrodynamics

12. Sponsoring Agency Name and AddressNational Aeronautics and Space AdministrationWashington, DC 20546-0001

Laser-plasma interactionSpace power transmission

13. Type of Report and Period CoveredTechnical Paper

14. Sponsoring Agency Code

18. Distribution St atementUnclassified-Unlimited

19. Security Classif.(of this report) 20. Security Classif.(of this page) 21. N o. of PagesUnclassified Unclassified 14

22. PriceA02

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