RASC White Paper

 

Fusion Propulsion for Spacecraft

 

by

 

Gerald Walberg

 

 

 

FUSION BASICS

 

Nuclear fusion occurs when two atoms collide with sufficient energy to overcome the coulomb barriers about their nuclei and yield product atoms and particles with energies higher than those of the reactant atoms. From the standpoint of power production, the most promising reactions are,

 

1.      D + D ®   T(1.01) + p(3.02)         Ignition Temperature = 300 x 106 C

3He(0.82) + n(2.45)

                        2.   D + T ®    4He(3.5) + n(14.1)       Ignition Temperature = 50 x 106 C

                        3.   D + 3He ®  4He(3.6) + p(14.7)      Ignition Temperature = 500 x 106 C

 

[where D = deuterium, T = tritium, p = proton, n = neutron, nHe = Helium isotope of mass number n and the values in parentheses are the energies (in MeV) carried by the various particles]

 

Because of its’ order-of-magnitude lower ignition temperature, the D-T reaction is the primary candidate for Earth-based fusion power production 1,2. For propulsion applications, however, the D-3He reaction is often chosen because the majority of the reaction-product energy is carried by charged particles (protons) that can be converted into directional thrust by a magnetic nozzle. The D-3He reaction has serious drawbacks, however. Naturally occurring 3He is essentially non-existent on Earth so it must be obtained from breeder reactors, the surface of the Moon or the atmospheres of the giant planets. Also, as shown in equation 1, it has an ignition temperature that is an order-of-magnitude higher than that of the D-T reaction.

 

In order for a fusion reaction to proceed without external power input (i.e. produce a “burning plasma”), the temperature must exceed the ignition temperature and the Lawson criteria,

 

            nt >  1020 (at T = 10 keV)

[where n = number density and t = lifetime or energy confinement time]

 

must be satisfied. For the D-T reaction, this criterion is often stated as the triple product, ntT > 2x1021 sec-keV/m3 where T is expressed in keV 5.

 

Hence, the basic problem in producing fusion power is to bring the fuel mixture to ignition temperature and confine it long enough for the triple product criteria to be exceeded. In the fusion literature the symbol, Q, is often used to represent the ratio of the energy produced by a fusion reactor to the input energy. Hence, Q = 1 is the break-even condition above which a self-sustaining burn can be maintained.

While fusion reactions have been produced for many years, Q = 1 has never been obtained in a controlled fusion experiment. As the ignition condition is approached, energy losses due to bremsstrahlung and synchrotron radiation, magnetohydrodynamic turbulence and diffusion of the plasma to the cold walls of the reaction vessel increase rapidly requiring input energy requirements to become huge. The highest Q values obtained to date are on the order of 0.3.

 

 The two leading approaches to doing this are (1) magnetic confinement 1,2 where a relatively low density plasma is confined for long times by magnetic fields and (2) inertial confinement 3,4 where a fuel pellet is imploded by laser or ion beam radiation to produce very high densities and temperatures for short times. Recently, an intermediate approach called magnetized target fusion 10 has been proposed. In this approach, a field-reversed configuration (or plasmoid – ball lightening is a plasmoid) is created and then compressed by an imploding liner to reach the triple product criterion. The densities and times are intermediate to those for magnetic or inertial confinement.

 

 

 

 

HISTORY AND PRESENT STATUS OF FUSION RESEARCH

 

 

 

Magnetic confinement fusion

 

 

 Research on magnetic confinement fusion as a possible power source began in 1950’s shortly after the development of the Hydrogen bomb. There was initial optimism that controlled fusion could be obtained quickly, but plasma instabilities proved difficult to handle and by the 1960’s general pessimism pervaded program. In the late 60’s and early 70’s major breakthroughs occurred in understanding configurations that would be stable to gross plasma instabilities and the tokamak emerged as the leading magnetic containment concept.

 

 The containment devices that received the most study in the early stages of the fusion program were the so-called mirror machines 1,16. In the mirror machine, illustrated schematically in Fig. 1 where B represents magnetic field strength, plasma confinement is obtained by strong axial (x direction in Fig. 1) magnetic fields that compress the plasma and keep it away from the cold walls of the cylindrical containment vessel. At either end of the containment region the magnetic fields vary rapidly in the z direction and produce a “magnetic stopper” that prevents the plasma from escaping. Because of its’ axial geometry the mirror machine lends itself to propulsion applications where it is joined to a magnetic nozzle.

 

Fig. 1 Mirror machine schematic

 

 The most successful mirror machines employ tandem magnetic mirrors and exhibit good stability at moderate densities. However, producing the ntT values required for Q > 1 has proved to be very difficult because of heat loss to the walls. For this reason, the major emphasis of the fusion program was shifted from mirror machines to tokamaks.

 

By the mid 1970’s success in achieving fusion with multiple approaches (but with power input much greater than power output) led to strong government support for a broad program. However, budget constraints became important in the 1980’s and most support was focused on tokamaks. The tokamak is a large, doughnut-shaped facility wherein the plasma is confined to an annular region by externally applied and self-generated magnetic fields. At present, tokamak research has reached the point where tens of megawatts of fusion power have been produced in the laboratory and the next major step is to build an ignition facility to produce and study self-sustaining, burning plasmas (i.e. plasmas where the power generated by fusion reactions is equal to the input power). This, however, has turned out to be prohibitively expensive 1,2. The basic tokamak geometry is illustrated in Fig. 2. where the toroidal direction is indicated by j and the poloidal direction is indicated by q.

 

 

Fig. 2 Toroidal geometry

 

 

 

 

 

 

 In the mid 1990’s, the DOE fusion program was restructured to put more emphasis on basic plasma physics and less on developing a prototype power plant. It is now U.S. policy to participate meaningfully in an international program to develop a burning-plasma facility (ITER, International Thermonuclear Experimental Reactor) rather than going it alone. Japan and Europe both have significantly larger programs in magnetic confinement fusion than the U.S. Their respective budgets are approximately 1.5 and 3 times that of the U.S.

An initial design for the ITER has been developed but, because of its’ projected cost (>$11 billion), no detailed funding mechanism or start date have been decided upon 9. A drawing of the proposed ITER is presented in Fig. 3. It is a very large facility whose scale can be judged by the size of the human figure at the lower right-hand corner.

Fig.3 International Thermonuclear Experimental Reactor

 

 

 

 

 

 In the basic plasma physics program, approaches other than the tokamak are receiving renewed study. In particular, there is renewed research on mirror machines, stellarators, fast z-pinches, reversed-field pinches and compact toroids (also called field-reversed configurations) 2.

 The stellerator is a toroidal confinement facilitiy that has three-dimensional magnetic fields, exhibits superior stability characteristics and is an intrinsically steady-state device. Fast z-pinches are open-ended configurations where high currents in many parallel thin fibers (in the z direction) produce magnetic fields that wind poloidally about the z direction and, for short periods of time, maintain a uniform pinch and very high levels of density and temperature.

The reversed-field pinch forms a toroidal pinch like the tokamak but most of the toroidal magnetic field is produced by current flowing in the plasma.

The compact toroid (field-reversed configuration) is a toroidal configuration wherein the  toroidal radius is so small that the hole in the middle of the “doughnut” has dissaperaed. This produces a “plasmoid” within which the poloidal magnetic fields reverse direction between the center and the outer edge.

Each of these concepts has significant potential advantages and, with further research, may be capable of producing burning plasmas 1,2.

It appears to be the view of most experts in the field that plasma ignition will be demonstrated within the next ten to twenty years but that a commercially viable fusion power plant may be as much as fifty years away 1,2,3,8,9.

 

 

 

 

 

Inertial confinement fusion

 

 Research on this approach to fusion power began in the 1960’s shortly after the invention of the laser. As shown in Fig. 4, a pellet containing the fuel (deuterium-tritium in Fig. 4) is bombarded by drive energy (usually laser radiation). Ablation of the pellet causes it to implode, producing very high density and temperature for a short time.

 

Fig. 4 Inertial confinement fusion

 

 

 The program was completely classified until 1973 and is still carried out under DOEs’ defense program. Substantial progress has been made using both lasers and ion beams as drivers. Like magnetically confined fusion, inertial confinement fusion has proved to be much more difficult to achieve than was initially believed. While multi-megawatt fusion power outputs have been achieved, it now appears that the achievement of ignition conditions will require an extremely high level of driver and pellet uniformity 3,4. The problem seems to get more difficult as ignition conditions are approached.

 

 A National Ignition Facility, shown in Fig. 5, is under construction at Lawrence Livermore National Laboratory and there is high confidence that ignition-level performance will be achieved. The facility is scheduled for completion in 2005 4, however it is a very challenging project and has experienced significant delays and cost overruns.

 

 The National Ignition Facility will utilize glass laser drivers because they are the only presently available drivers that can deliver the required energy with the necessary uniformity. The glass laser is not a good candidate for a power plant because of its low efficiency of transferring energy to the pellet. The most promising driver for an inertial confinement power plant is a heavy ion accelerator. However, major technology advances requiring 10 to 20 years are required before a heavy ion driver will be ready for application 4,5.

 

Fig. 5 National Ignition Facility

 

Since inertial confinement fusion is essentially a series of explosions, the power output depends on the achievable duty cycle (i.e. the number of explosions per minute). Also, since these explosions produce strong neutron fluxes, there are serious issues related to long  term survivability of the combustion chamber and associated power generation equipment.

 

It is the consensus of experts in the community that ignition will very likely be demonstrated within the next 10 years but that a viable power plant may be much further in the future 8,9. The scale of the National Ignition Facility can be inferred from the tanker truck shown at the lower right in Fig. 5. The reaction chamber with many lasers pointing radially inward at the pellet is on the right-hand end of the facility. The rest of the building contains laser power supplies and power conditioning equipment.

 

From figs. 3 and 5 it can be seen that both the ITER and the National Ignition Facility are so large as to be unusable as propulsion systems. In fact, one of the major challenges in fusion propulsion for spacecraft is reducing the size of the system. This usually requires the use of advanced technologies that have not been demonstrated in the terrestrial fusion power program.

 

 

Magnetized target Fusion

 

This relatively new concept (the earliest reference I have found is dated 1984) constitutes a middle ground between magnetic and inertial confinement. In this scheme a field-reversed configuration (plasmoid) is produced in a theta pinch and transported to a reaction chamber where it is compressed to high density and temperature by an imploding liner10.

 

Fig 6. Magnetized Target Fusion

 

 The densities required are lower than those typical for inertial confinement because the confinement lifetimes are longer. The MHD stability problems associated with magnetic confinement are avoided. The term, magnetized-target fusion derives from the fact that the magnetic fields within the field-reversed configuration reverse direction between the middle and outer edge of the plasmoid in a manner similar to that of a bar magnet1. The presence of the magnetic fields within the target plasma greatly reduces conduction losses hence producing significantly increased confinement lifetimes and allowing relatively slow and efficient compression to be used. In theory, this reduces the power and precision required to compress and heat the plasma to fusion conditions compared to conventional inertial confinement10 and greatly reduces the size of the fusion reactor. While this is approach has many attractive features, it is quite immature compared to both magnetic and inertial confinement fusion. It has reached the point where a proof-of-principle experiment was proposed in 199810.

 

 

Muon-Catalyzed Fusion

 

In 1989, Drs. Stanley Pons and Martin Fleischmsn announced in a press conference that they had achieved “cold fusion” by passing an electrical current between Platinum electrodes in a beaker of Deuterium water. About the same time (actually earlier, in 19867), Dr. Stanley Jones was publishing papers reporting another type of low-temperature fusion, muon-catalyzed fusion. While most physicists believe the “cold fusion” of Pons and Fleishman was the result of faulty experimental technique and interpretation, muon-catalyzed fusion is very real.

 

The muon is an elementary particle that has the same negative charge as an electron but a mass that is 207 times that of an electron. Muons can be generated in accelerator facilities that first produce antiprotons by collisions of intense proton beams with metal targets. When these antiprotons annihilate on matter they produce pions, which upon decay produce muons.

 

Muons introduced into cold, dense deuterium-tritium mixtures can replace the atomic electrons and produce muonic molecules that participate readily in nuclear fusion reactions. The overall process of muon-catalyzed fusion is illustrated in Fig. 7.

 

Fig. 7 Muon-catalyzed fusion

 

Catalysis yields of 150 fusion reactions per muon have been achieved leading to interest in muon-catalyzed fusion as a possible source of energy6.

 

There is a major problem, however. The amount of energy required to produce a muon by shooting deuterons at Carbon is approximately three times larger than that gained by fusion catalyzed by that muon6.

 

 

The energy balance can be made positive by employing the muon-catalyzed hybrid reactor scheme6 whereby a beam of deuterons is directed to a Be or Li target and 30% of the beam energy is used to create pions and 70% is used to fission 238U or to breed 239Pu. The pions decay to muons and are guided to a synthesizer filled with a D-T mixture where muon-catalyzed fusion takes place. Neutrons generated in the synthesizer hit a 6Li-238U blanket and tritium and plutonium are bred. The tritium is used to replace that used in the fusion process and the 238U is used to generate energy in a conventional fission reactor. This provides a four-to one energy gain but the potential environmental impact together with the very ambitious technology make it unlikely that this scheme will become reality in the near future6.

 

With regard to propulsion, a major problem is the production or storage of muons onboard the spacecraft The use of accelerators in space vehicles is not practical, so other means must be found to produce muons. It has been proposed that antihydrogen  (made up of antiprotons and positrons) could be produced and used as a fuel for spacecraft. Annihilation of the antihydrogen on deuterium-tritium  would produce pions, which on decay would produce muons that would participate in muon-catalyzed fusion reactions.

 A number of programs are under way to develop improved antimatter storage facilities. The present state-of-the-art is the Penning trap, which can store 108 muons for short times (weeks) 24. These capabilities will have to be increased by many orders of magnitude to support space missions. Even if the storage problem is solved there is still the problem of generating the antimatter. At present, the world production of antimatter is on the order of nanograms per year at prohibitively large cost. It has been described as the world’s most expensive substance.

 

Accordingly, muon-catalyzed fusion does not appear to be feasible for spacecraft propulsion in the RASC time frame.  

 

 

APPLICATIONS OF FUSION TO SPACECRAFT PROPULSION

 

Many studies of spacecraft propulsion systems based on nuclear fusion have been carried out over the past 35 years and propulsion systems employing many types of plasma confinement have been proposed. The proposed propulsion concepts can be divided into two broad categories: (1) those that are based on extrapolations from the experimental database established in the fusion power program and (2) those for which an experimental database has not yet been established. The first category contains concepts based on inertial confinement fusion, toriodal, takamak-like reactors and mirror machines. The second category contains more recently proposed concepts such as magnetized target fusion and antimatter-driven inertial fusion. Representative designs typifying each of these categories are discussed below.

 

Catagory 1- Extrapolations From the Current Fusion Database

 

Inertial confinement fusion

 

 A design concept that has received considerable attention involves a series of small nuclear explosions that are produced behind the vehicle base with thrust being produced by the aftward acceleration of the ionized explosion products by a magnetic nozzle. The fusion explosions are most often produced by the implosion of inertial confinement fusion pellets by high power lasers or ion beams.

 

In 1979, Anthony Martin and Alan Bond published a historical review of nuclear pulse propulsion 13.The earliest studies reported by Martin and Bond were carried out from 1946 to 1966 for vehicles that used relatively large (~ 1 kTon yield) fission bombs and produced thrust by the reflection of blast products off a “pusher plate” on the vehicle base. Studies of this type of vehicle culminated in Project Orion, a $10M investigation carried out from 1957 to 1965 by the General Atomic Division of General Dynamics Corporation and administered in turn by The Advanced Research Projects Agency (ARPA), the U.S. Air Force and NASA.

 

In 1966 Freeman Dyson proposed replacing the fission explosions with fusion explosions in order to produce a vehicle with interplanetary (and interstellar) capabilities.

 

In the late 1960’s and early 1970’s, studies indicated the possibility of replacing the relatively large explosions of the Orion –type systems with small deuterium-tritium pellets heated to fusion temperature by intense laser beams. Magnetic nozzles were also introduced to produce higher exhaust velocities and higher thrust efficiencies.

 

From 1973 to 1978, the British Interplanetary Society coordinated Project Daedalus, a detailed study of a fusion-propelled, two-stage spacecraft designed to carry out a fly-by of a nearby star. The fuel used was D-3He because it produces neutron-free exhaust products. Relativistic electron beams ignited the fuel pellets and a magnetic nozzle was used. The levels of technology assumed were those expected within the next 50 years so the design intentionally stretched the technological limits. For instance, the

3He was assumed to be obtained from the atmosphere of Jupiter. In spite of this very optimistic approach, the study was done in considerable detail and convinced many of the participants that such a vehicle could be developed in the not-to-distant future.

 

In the late 1970’s and early 1980’s, however, experiments aimed at producing inertial confinement fusion showed that pellet implosion required extremely uniform pellet surfaces and much higher power levels than had been projected. In 1982, Reupke 14 reviewed the status of inertial fusion research as it applied to fusion rocket studies. He reported that, since the Daedalus studies, the estimates of the energy required to produce D-T pellet fusion had increased by two to three orders of magnitude. This, of course, implies much larger, more massive fusion-propelled space vehicles.

None-the-less, studies of this type of fusion-propelled spacecraft have continued. A recent, rather detailed design is reported by Orth, et al 15,18. This study is noteworthy because it is based in large part on recent experimental results from the inertial fusion research program and D-T fuel rather than D-3He was assumed. In order to achieve acceptable vehicle performance, however, some very optimistic assumptions were required. For instance, target (pellet) gains (fusion energy produced / incident energy) of 1500 were assumed, whereas, in 1982, Reupke estimated the maximum practically achievable gains to be on the order of 200. The achievement of such high gains would require successful implementation of an approach such as the “fast igniter” concept 4 whereby an ultra intense laser beam is used to burrow through the corona surrounding the imploding pellet and deliver energy in the form of energetic electrons to an igniter region interior to the pellet. At present, the fast igniter concept is considered to be very speculative 4. Also, a firing rate of 30 implosions per second is assumed whereas most studies of Earth-based fusion power plants assume pulse rates of 1 Hz or less. Even with these optimistic assumptions, the spacecraft is very large (see Fig. 8) and has an initial mass in Earth orbit of 6000 metric tonnes. An interesting aspect of the propulsion system design is that large quantities (4124 tonnes) of H2 expellant are mixed with the fusion products to produce higher exhaust mass flows and hence acceptably high thrust levels.

 

 

Fig. 8 Spacecraft powered by inertial confinement fusion 15,18

 

Magnetic confinement fusion – toroidal reactor

 

While many investigators consider the magnetic topology of toroidal reactors to be unsuitable for propulsion applications because of the problems associated with extracting the reaction products from the toroidal confinement region 16, there have been have been a number of such concepts proposed. An admirably detailed analysis of such a vehicle (shown in Fig. 9) was recently presented by Williams et al 19. Again, very optimistic assumptions are required to produce a viable design. In this case a low aspect ratio, spherical torus reactor burning D-3He fuel was assumed.

 In toroidal reactor terminology the aspect ratio is the ratio of the minor radius (r in Fig. 2) to the major radius (R0 in Fig.2). If  r/R0 = 1, the torus becomes a sphere with no hole in the middle. Such configurations have been studied and are predicted to be much smaller (they are called compact toroids or spheromaks 1) and to require much lower powers than tokamaks. However, the development of such facilities is still in its’ early stages and the experimental database is meager compared to that for tokamaks.

 In reference 19, an aspect ratio of 2, midway between tokamaks  and spheromaks, was assumed. This produced a relatively small, low mass reactor that could be designed by using scaling laws developed for tokamaks.  The confinement region had an elliptical rather than circular cross section yielding an overall reactor shape that was approximately spherical. Also, it was assumed that that the spin vector of the D-3He fuel was maintained parallel to the magnetic field, yielding a 50% increase in fusion reactivity cross section and a large increase in output power. This is based on existing theory and experiment but is still quite speculative.

The 3He fuel was assumed to be obtained from the atmospheres of the outer planets. As mentioned previously, the advantage of D-3He is that very small quantities of neutrons are produced as compared to D-T. Since neutrons are neutral particles they cannot be controlled by magnetic fields. They essentially constitute “waste heat” that must be absorbed and radiated away. (The conical configuration of the D-T inertial fusion spacecraft shown in Fig.8 was chosen so that most of the neutrons produced in the fusion reaction would miss the spacecraft structure and be radiated to space.) Even with D-3He fuel, a significant amount of waste heat is generated by secondary nuclear reactions and the spacecraft power system. The magnitude of the waste heat problem can be seen from Fig. 9 where the four square panels are radiators 150 m on a side. The overall vehicle length is 260 m.

 As with the previously described inertial fusion spacecraft, a large quantity of slush H2 is carried for mixing with the fusion products to produce higher thrust levels. The vehicle shown in Fig. 9 has an initial mass in Earth orbit of 2941 metric tonnes including 1292 tonnes of H2. Both this vehicle and the previously discussed inertial confinement vehicle (Fig. 8) are assembled in a high Earth orbit (outside the Van Allen belts) from major components transported from Earth by Saturn 5+ class heavy lift launch vehicles and low thrust (probably nuclear electric) orbit raising stages.

 

Fig. 9 Toroidal confinement vehicle concept

 

Magnetic confinement fusion– mirror machine

 

The mirror machine (illustrared in Fig. 1) would appear to be well suited to propulsion applications. Its linear geometry is well suited to packaging in a spacecraft and, by making the “magnetic stopper” at one end of the device only partially effective; a thrust-producing nozzle can be created. Kammash17 has carried through a rough conceptual design for such a vehicle. He designed the vehicle to support a 165-day round-trip crewed mission to Mars. Two fuel combinations, D-T and D-3He, were considered and the D-T fuel was found to be preferable because it’s lower ignition temperature allowed for a much smaller and lighter vehicle. Even so, the D-T vehicle is very large by present-day standards, having an IMLEO of 7210 metric tonnes. A major design issue with such a mirror machine vehicle is the tendency to incur large energy losses from the fusion plasma to the walls of the cylindrical confinement vessel. It is because of these large losses (which have never been successfully dealt with in practice) that the main thrust in fusion-reactor research was shifted from mirror machines to tokamaks.

 

 

 

Comparison of fusion vehicles

 

Comparative mass summaries these three fusion vehicles are presented in Table. 1.

 

Table 1

 

 

 

Note that all three vehicles are very massive by present day standards. At first glance the tokamak vehicle appears to be significantly lighter than the other two, but it is designed for an unusual one-way delivery mission. Were it designed for a round-trip mission, it would appear that it would have a comparable IMLEO. All three vehicles are also very large. The inertial-confinement vehicle has a diameter of approximately 175 m, the tokamak vehicle has an overall length of 260 m. The design of the mirror-machine vehicle did not really address vehicle components other than the propulsion system, but the mirror machine itself was approximately 70 m long. All of these vehicles would require very large on-orbit assembly operations.

 

 

 

Catagory 2 – Concepts Beyond the Current Fusion Database

 

Magnetically Insulated Inertial Confinement Fusion (MCIF)

 

Kammash 16,20,21,22,26 has proposed a variant on the inertial confinement approach in which driver energy is beamed through a hole in the pellet and initiates a fusion reaction on the pellet inner wall as illustrated in Fig. 10.

 

 

Fig. 10 Magnetically insulated inertial confinement fusion

 

This concept is similar to the “fast igniter” in that energy sufficient to initiate the fusion reaction is deposited inside the pellet. The resulting fusion reaction creates a magnetic field within the pellet that insulates the hot plasma from the Tungsten pellet walls, produces a longer reaction time and, in theory, produces much higher pellet yields. Kammash has analyzed this concept for a number of drivers including lasers 20,21, particle beams and antimatter 16,22,26. In the case of antimatter, his analysis shows that the resulting fusion reaction is actually muon-catalyzed fusion with the muons being produced by the annihilation reaction of antihydrogen with the D-T fuel inside the fuel pellet. The problem with this concept is that the muon catalyzed fusion reaction proceeds rapidly only at relatively low temperatures (<5000K) and the fusion reactions near the pellet wall will slow down significantly when higher temperatures are encountered. The net result could be that the effective reaction time will be unacceptably short.

 

 

 

 

Concepts Proposed by the RASC GRC/LaRC/MSFC Design Team 28 

 

In February, 2002 the RASC Propulsion Design Team proposed four fusion propulsion concepts, (1) Magnetokinetic Compression of FRC Fusion Rocket (Fig.11), Magnetized Target Fusion Rocket (Fig.12), Gasdynamic Trap (GDT) Fusion Rocket (Fig.13) and Steady-State or Quasi-Steady-State FRC Fusion Rocket (Fig.14). Three of these concepts represent different applications of magnetized target fusion.

 

 In the Magnetokinetic Compression of FRC Fusion Rocket (Fig. 11), a plasmoid is formed and accelerated to high speed by an inductive magnetized accelerator (IMPAC). The high energy plasmoid is then decelerated in a burn chamber where a large part of its directed kinetic energy is converted to internal energy, producing temperatures and densities sufficient to produce fusion reactions. The fusion products are then expanded through a magnetic nozzle to produce thrust.

 

In the Magnetized Target Fusion (MTF) Rocket (Fig. 12), The plasmoid is imploded by a surrounding array of plasmajets. Fusion conditions are produced and the fusion products are expanded through a magnetic nozzle.

 

In the Steady-State or Quasi-Steady-State Fusion Rocket (Fig. 14), a plasmoid is stabilized and held stationary within the reaction chamber by magnetic fields. The plasmoid is then pumped by ion beams until fusion conditions are acheived and the fusion products are expanded through a magnetic nozzle.

 

These three concepts differ primarily in the way in which energy is added to the field reversed configuration (FRG). The first two are pulsed concepts. The concept shown in Fig. 14 may have the potential for steady-state operation. All three are interesting but all lack an established database. Hence they are considerably more speculative than those based on extrapolations from the fusion power program.

 

The concept shown Fig. 13 appears to be a variation on the mirror machine, but the information on the figure does not provide a complete description and no references are given.

 

 

 

 

Fig. 11 Magnetokinetic compression FRC fusion rocket

 

 

Fig. 12 Magnetized target fusion (MTF) rocket

 

Fig. 13 Gasdynamic Trap (GDT) fusion rocket

 

Fig 14 Steady-state or Quasi-steady-state FRC fusion rocket

SUMMARY

 

The terrestrial fusion power program has produced an extensive experimental and analytical database that can be applied to the design of fusion propulsion systems for spacecraft. The Earth-based reactors are too large and massive to be applied directly to spacecraft. Accordingly, most recent designs of fusion-propelled spacecraft are based on extrapolations and optimistic (but defensible) assumptions. Even when these optimistic assumptions are made, the resulting spacecraft are extremely large and massive by present-day standards and invariably involve extensive on-orbit assembly operations supported by many launches of Saturn 5 + class launch vehicles and advanced technology (e.g. nuclear electric) orbital transfer vehicles. In order to produce relatively small, lightweight fusion-propelled vehicles, it is necessary to invoke advanced fusion concepts, such as antimatter-initiated fusion or magnetized target fusion, for which key technologies (such as antimatter storage) or the requisite databases have not yet been established.

 

 

REFERENCES

           

 

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