A possible route to small, flexible fusion units

Enecq Vol. 4. PP. 16L171-J @l Pergamon Press Ltd., 1979.

Printed in Great Britain

A POSSIBLE ROUTE TO SMALL, FLEXIBLE FUSION UNITS? G. H. MILEY

and J. G. GILLIGAN

Fusion Studies Laboratory, Nuclear Engineering Program, University of Illinois, Urbana, IL 61801,U.S.A. (Receioed

1

November 1978)

Abstract-Conceptual desigh studies of a small D’He field-reversed mirror fusion plant are described. If achieved, the basic unit, termed SAFFIRE, would produce a few megawatts of power. By “stacking” of units, however, plants could be built in the 10s of megawatt size. This would make relatively clean fusion power available for applications requiring small, decentralized, power plants.

INTRODUCTION

Most of the effort in the national fusion energy program has been directed at development of large central power stations. This approach has occurred for two reasons: (1) historical precedence set in the utility industry, and (2) the fact that plasma containment is simply easier in large units. At the University of Iiilnois Fusion Studies Laboratory, under sponsorship of the Electric Power Research Institute, we are studying the possibility of developing small (lo100 MWe) fusion units. This work has been motivated by the view that as petroleum and gas fuels become scarce and environmental restrictions become more crucial, the energy source pattern in the United States could shift to a decentralized scheme where medium-sized power units become basic “building blocks”. Large central plants have dominated the utility industry to date, largely because of the “economy of scale”. However, as the new pressures cited above gain in intensity, the economics of power production may well shift to favor such small, decentralized units. Small fusion devices represent one very important way of potentially meeting this need. To do so, however, small fusion plants must be developed with certain key characteristics: relative “cleanliness” with respect to both radioactivity and thermal wastes; de~ndability, perhaps through redundancy of systems; and ease of control and maintainability. It is not yet clear to what extent these goals are attainable. However, present studies at the Fusion Studies Laboratory that involve a D3He fueled Field-Reversed Mirror (ERM) concept, termed SAFFIRE, are encouraging. ADVANCED

FUSION FUELS AND SATELLITES

Being easiest to ignite, deuterium-tritium (D-T) fuel will surely be used in early fusion devices. Two characteristics of D-T are important to recall. First, 80% of the fusion energy is carried off by neutrons. Second, since tritium does not occur naturally, it must be bred by neutron reactions with lithium in the blanket. Thus, the long range goal of fusion is to utilize “advanced” fuels like D-D, D311e, and p-“B that reduce the neutron yield and also eliminate the need for tritium breeding. There are, in fact, over 20 fusion reactions between light elements that offer reasonable cross sections,* but only the most important are listed in Table I. The p-“B reaction is ideal in that it essentially produces ail charged particles and only involves plentiful, naturally occurring fuels. Unfo~unately, the high temperatures required and subsequent radiation losses make p-“B very difficult to achieve. Consequently, there is a strong motivation to seek D-D fusion in order to capitalize on the availability of deuterium in sea water. The energy yield per DD reaction is relatively low, but much of the product tritium and 3He will react with the deuterium in situ (or can be reinjected), and this adds significantly to the fusion energy release. Such operation is referred to as a “catalyzed” deuterium (Cat-D) reaction because the added energy release eases achievement of ignition while only deuterium is consumed. Since ‘He bums tThis paper was presented at the 1978Midwest

Energy

Conference,

163

Chicago, Illinois, 19-21 November, 1978.

164

G.

H.

MILEY and

J. G.

GILLIGAN

Table 1. Some key fusion reactions. Energy Released, Ec, ?leV

Fraction of Ec to Charged particles, fc

D+T+n+a

17.6

.2

Dt3He+a+p

18.3

1.0

8.7

1.0

Reactions

11 B + 3a

*43.1 !,IeVreleased per 4D reacted if credit is also taken for burning the T and 3He product as in Cat.-D operation.

He

Generotor Facility --

3He

Tritium Storage D-3He

3He (optional)

Sotellites

Fig. 1. The generator-satellite concept.

relatively slowly, another option, illustrated in Fig. 1, is to separate 3He from the D-D exhaust and ship it to a satellite D3He reactor.= (The satellite could, if desired, also obtain 3He from the decay of excess tritium bred either in the blanket of the deuterium reactor or from a high breeding-ratio blanket of a D-T plant.) The D3He reaction represents the “cleanest” of the deuterium reactions in terms of minimizing neutron production and tritium inventory. It produces all charged particles, although D-D reactions will still occur in the mixture, producing some neutrons and tritium. DEVELOPMENT

STRATEGY

The early development of D3He satellites would add greatly to the deployment of fusion power. Due to the reduced radioactivity and improved efficiency, the D3He satellites might be sited near to the user. Shipment of the inert ‘He fuel to the site should be simple and safe. Thus, the combination of generator and satellite would add desirable flexibility to fusion power systems.6 Such satellites could, in fact, be introduced almost simultaneously with D-T fusion. First experimental units would purchase ‘He from weapons-related installations such as Savannah River in the U.S.; later ‘He would come from the decay of excess tritium produced in early D-T plants and eventually from D-D generator units. Such a strategy would have the advantage of introducing the favorable features of satellite fusion without waiting for the technology necessary for burning other advanced fuels like p-“B. CONFINEMENT

APPROACHES

Due to the high temperature required to burn advanced fuels, a central problem is that increased radiation losses make it difficult to maintain a suitable plasma energy balance, e.g. achieve ignition in non-driven systems. [Indeed, if the plasma balance can be sustained, radiation may be an efficient way to couple energy out of a fusion plasma, e.g. several high-efficiency “radiation-boiler” energy conversion schemes have been proposed7*81.This places emphasis on the use of high+ (plasma/magnetic pressure ratio) confinement systems,

A possible route to small, flexible fusion units

165

Table 2. Some high+ approaches. Confinement Scheme

B range

Comwnt

Flux Conserving Tokamak4

-0.2-0.4

Concept not verified experimentally

Bumpy Torus'

-0.5

Demonstration of scale-up, high-frequency rf heating required

Tarmac"

-1.0

Stability and sheath effects in scale-up to be demonstrated

Field-Reversed Mirror"-'2

-1.0

Experimental confirmation required

Surmac13

-1.0

Technological problem of internal conductors. Surface gradient effects (e.g. cyclotron emission) uncertain.

Inertial (Pellets)14-15

equivalent to -1.0

Large driver energy and efficiency may only be possible by heavy-ion accelerator techniques

Migma16

equivalent to -1.0

Beam-beam approach. Space charge and stability may limit power.

and some representative candidates are indicated in Table 2. A high p is beneficial for two reasons: the reduced field insde the plasma results in less generation of cyclotron radiation, and that produced is more likely to be reabsorbed before escaping (or during subsequent transits if the walls are reflective). As seen from Table 2, a number of potential approaches offer higher @. [This listing is intended to be representative, not all inclusive.] However, only limited studies are available on the use of advanced-fuels in these various concepts. The present work has focused on the possible use of a field-reversed mirror because of all of the approaches indicated, it appears to be the only one that combines the desired features of relatively small size and high @. THE SAFFIRE

CONCEPT

Interest in Field-Reversed Mirrors (FRMs) has intensified with recent high-b results from the 2X-11 experiment at the Lawrence Livermore Laboratory.‘* While similar to Astron, the FRM uses internal plasma currents, rather than single gyroradius high-energy electron or ion beams, to maintain the magnetic field reversal (Fig. 2). This makes initiation of reversal by neutral-beam injection at sub-MeV energies feasible. The resulting closed field-line region

L

REVERSED MAGNETIC FIELD CONFIGURATION

Fig. 2. The FRM confinement scheme displaying both closed and open field line regions.

166

G. H. MILEYand J. G. GELIGAN

SAFFIRE Self - Sustained l

Cold fueling -

diamagnetic

-sustain

l

current

profile

for reversal

supplemented

by

heating

Cold plasma -

pressure

burn

Fusion product heating ouxillary

l

maintains

divertor

blanket provides action

-connection or plasma

to direct collectar “dump”

Fig. 4. Magnetic field model for a field-reversed mirror. The closed field region essentially has a toroidal shape and is em~dd~d in an external open-type mirror field.

Fig. 3.

provides improved con~nement, while the surrounding mirror field is compatible with electrostatic direct collection. This along with a naturally high-p makes the FRM quite attractive for burning advanced fuels. We have termed our concept SAFFIRE (Self-Sustained Advanced-Fuel-Fleld-REversed Mirror) to stress the fueling and heating techniques that have been incorporated in an attempt to achieve steady-state operation without the high-energy injection used in other FRM designs (Fig. 3). To facilitate our study, a plasma model based on the Hill’s vortexI field configuration (Fig. 4) has been developed. This model is similar to that of Condit et af.;” however, it provides a measure of self-consistency through use of the vortex field-equilibrium to represent the reversed-field configuration. This analogy allows an explicit check on the field-reversal requirement and, in addition, provides an equilibrium plasma density-profile which is then used to reduce the steady-state particle and energy balance equations to average O-dimensional form. This model is also used to classify injected-ion orbits,” which in turn allows an estimate of the contribution that an ion makes to the circulating current. This classification relies on an effective potential which can be derived from the vortex field. Then, as shown in Fig. 5, depending on the angular momentum PO and the energy of the ions, the trapping potential “well” changes, which alters the ion orbital motion. In addition to their contribution to the current causing reversal, these ions, with their large orbits, are responsible for stabilizing the FRM plasma. 1.5

I P,=-3116

V

1.0

Fig. 5. The effective potential “well” created by the Hill’s vortex for ions having PO = 3/16. these ions undergo a betatron-type motion. Ions with different Pd “see” a different potential well and undergo a variety of motions rang@ from “figure eights” to off-center circles.‘*

167

A possible route to small, flexible fusion units

The SAFFIRE concept rests on the intimate relation between three aspects of the FRM; namely, fueling, fusion-product heating, and stability. To obtain an attractive energy multiplication, neutral-beam injection must be held to a minimum and steady-state or long-pulse operation must be achieved. Indeed, calculations show that once reversal is attained, diamagnetic currents can supply most of the current required to maintain reversal provided that cross-field diffusion, i.e. the radial density profile, can be maintained. To do this, cold fueling during operation is necessary. Fueling is a difficult problem for toroidal devices (e.g. entirely satisfactory solutions have not yet been devised for tokamaks), but due to the small size and outward “pushing” of field lines in the FRM (recall Fig. 4), fueling is greatly simplified in the FRM.19 The injection profile required to maintain the density gradient in a Hill’s vortex is shown as a solid line in Fig. 6. Since ions trapped at any point will rapidly spread along magnetic field surfaces, it is only necessary to deposit fuel between the outboard side and the null point, a mere 6-cm distance in present design. This can be done using “state-of-the-art” neutral-beam injectors, operating at 7 1 amp and z 10 keV, aimed at an angle to the field surfaces to provide good matching with the desired profile (dashed line of Fig. 6). In addition to fueling, the plasma must be continuously heated to make up for energy losses and bring the cold fuel up to temperature. Hopefully, a bulk of the heating can be supplied by fusion products, supplemented by some auxiliary ion-cyclotron type heating. The basic problem, however, is the small size of the FRM plasma relative to the fusion product (fp) orbits. Thus, in present 60-kG magnetic field designs, the 3.5MeV a and 14.1-MeV proton from D-3He have 5.4- and lO.S-cm radius orbits, respectively, while the typical closed-field region has a 20-cm distance to the $ = 0 surface. ($ denotes surfaces of constant magnetic flux and ranges from zero at the boundary of the closed region to 0.125 at the null point.) Consequently, it might seem that little of the fp energy would be deposited in the plasma. Fortunately, as shown in Fig. 7, although many of the fp’s escape the closed-field volume, they are confined such that their orbits pass through both opened and closed regions. ” Thus, their energy can still be deposited in the plasma, i.e. a 20-cm plasma could still retain as much as 70% of the (Yenergy and 20% of the proton energy. Another feature of SAFFIRE is the use of a cold plasma that is introduced at one end so as to flow along the open-field lines. ” This plasma serves to shield the reversed-field region from wall impurities, limit fusion-product ash buildup, reduce charge exchange with background neutral gas, and couple the diverted plasma to a dump or direct collector. Indeed, based on a fairly detailed study, we have concluded that the cold plasma region can be sustained with fusion product energy, that this region is quite effective for the purposes already indicated, and

Depth

(cm)

Fig. 6.

oB.

(m-Tesla)x103

Fig. 7.

Fig. 6. Particle source-density as a function of depth into the FRM plasma as a result of neutral-beam injection. Fig. 7. Containment of fusion products in the FFM as a function of plasma radius (a) and vacuum magnetic field (&).

168

G. H. MILEY and J. G.

GILLIGAN

Pumpmg Chambers

Fig. 8. Diagram (not to scale) of a thermal dump to collect particles and recover energy from the divertor plasma flow. The dump length is approx. 3 m.

START -UP POSSIBLE l

Fast Injection -- Neutral-Beam -

l

l

APPROACHES

Intense

Magnetic -

Ion Source

Compression

Other - Field Reversed -

Vortex

8 Pinch

Ring Fig. 9

that a thermal dump can be designed for a near-term experiment which serves much like a conventional divertor collector to handle the exhaust (Fig. 8). A key question currently faced by experimentalists, is how to start-up a field-reversed mirror. Possible approaches listed in Fig. 9, include fast neutral-beam injection, magnetic compression, and use of auxiliary field-reversed targets. For the present conceptual design, however, we rely heavily on a proposed neutral-beam start-up scenario. The remaining crucial aspect is stability. Beyond the basic question of whether a stable region exists at all, is the question of whether sufficiently large sizes are permitted to be attractive, i.e. to allow good fusion product heating and reasonable total power. The basic configuration is MHD unstable, but stability is thought to arise from finite ion gyro-radius effects. A rough analogy would be a jelly (the background plasma) stabilized by stiff wires (the ion orbits). There is some experimental evidence for this from field-reversed &pinch experiments.22*23Theoretical studies24 have generally been restricted to improper geometry (long-thin layers, bicycle-tire plasmas, etc.), although our recent results appear applicable to spherical plasmas.25 All of this work thus far suggests that a stable region occurs for a S = L/pi (plasma radius/ion gyroradius) of about 5 or less. In practice, values of - 10 are desirable for good performance with IL3He, so precise limits become a crucial objective for future theory and experiment. These various factors are brought together in Table 3 where typical parameters are presented for SAFFIRE. Indeed the relatively small size combined with the larger power fraction going into charged particles represent unique features anticipated in earlier discussions. A conceptual layout for SAFFIRE is shown in Figs. 10. In this version, the deuterium and ‘He fueling injectors are housed in a common unit, while a separate start-up injector is included. The concept shown in Fig. 10, was devised for an early pilot plant designed to demonstrate fusion burns with D3He. Consequently, a plasma dump rather than a direct convertor is used

169

A possible route to small, flexible fusion units Table 3. Typical SAPFIRE reactor parameters. Vacuum Field

6T

Ion Temp Plasma

-80 keV

Vol.

cl00 liters

Fusion Power

1-5 MI.1

Power Outflow Split, % plasma

.39

fP

.37 (

radiation

.22

neutrons

.02

1 .76

Neutral Beam

Fig. 10. Sketch of the SAFFIRE systems including neutral beam injectors and thermal dump.

in order to reduce costs and simplify construction. A direct converter would be added in later plants to increase the efficiency, thereby reducing the rejected heat load. Clearly, FRM performance is extremely sensitive to the detailed physics and allowable operating regime. The fact that attractive results can be obtained for what is now thought to be a reasonable regime provides a strong motivation to undertake expanded studies. The achievement of a relatively clean U-3He plant in the IO-100 MW size with a good energy balance would have a tremendous impact on man’s future energy strategy. REFERENCES 1. G. H. Miley, Fusion Energy Conversion, Am. Nut. Society, 555 North Kensington Avenue, LaGrange Park, IL 60525 (1976). 2. C. Choi (Ed.), Proc. Review Mfg. on Advanced Fuel Fusion, EPRI ER-536~SR, Electric Power Research Institute, Palo Alto, California (Sept. 1977). 3. G. H. Miley er al., Advanced-fuel fusion systems-The D’He satellite approach (The ILB reactors). In Ref. 2, p. 39. 4. G. H. Miley, Advanced-fuel fusion concepts. Int. Workshop on Emerging Concepts in Adv. Nucl. Sys. Analysis, Graz, Austria, March, 1978. 5. G. H. Miley er a/., Concepts for D’He satellite concepts. Proc. 7th IEEE Symposium on Engineering Problems of Fusion Research, Knoxville, Tennessee (Oct. 1977). 6. C. Ashworth, A utility view of fusion. In Ref. 2, p. 23. 7. J. Fillo and J. Powell, Fusion blankets for catalyzed-D and D’He reactor. In Ref. 2, p. 69. 8. A. Hertzberg and R. Taussig, High thermal efficiency advanced fuel fusion reactors. In Ref. 2, p. 179. 9. G. Gerdin, F. Southworth, and R. Stark, Advanced-fuel bumpy tori. In Ref. 2, p. 241. Also see Trans. Am. Nucl. Sot. 27,43 (1977). IO. M. Levine, The use of a hot sheath tarmac for advanced fuels. In Ref. 2, p. 285. I I. G. H. Miley and D. Driemeyer, The D-‘He field-reversed mirror as a minimum size satellite. In Ref. 2, p. 317. Also see Trans. Am. Nucl. Sot. 26, 53 (1977).

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G. H. MILEYand J. G.

GILLICAN

12. W. C. Condit, G. A. Carlson, R. S. Devoto, J. N. Doggett, W. S. Neef, and J. D. Hanson, Preliminary design calculations for a field-reversed mirror reactor. Lawrence Livermore Lab. UCRL-42170(1976). 13. A. Wong er al., Surmacs. In Ref. 2, p. 435. Also see: J. M. Dawson, Multipoles as reactors. In Ref. 2, p. 381. 14. G. H. Miley etal., Proc. 2nd Int. Topical Conf. on High Power Electron and Ion Beam Research and Technology. Vol. I, p. 243 (Oct. 1977). 15. C. K. Choi et al., Third ANS Topical Meeting on Fusion, Santa Fe, New Mexico, May 1978. 16. B. Maglich, Migmacell-a low-gain “driven” fusion power amplifier as an interim energy source. In Ref. 2, p. 133. 17. E. Morse, High beta, low aspect ratio plasmas. COO-2218-41,Fusion Studies Lab., University of Illinois (1976). 18. M. Y. Wang and G. H. Miley, Particle orbits in field-reversed mirrors. COO-2218-508,Fusion Studies Lab., University of Illinois submitted to Nucl. Fusion. 19. G. H. Miley et al., Trans. Am. Nucl. Sot. 28, 42 (1978). 20. M. Y. Wang, G. H. Miley, and L. S. Wang, Truns. Am. Nucl. Sot. 27,93 (1977). 21. J. Gilligan et al., Third ANS Topical Meeting on Fusion, Santa Fe, New Mexico, May 1978. 22. W. C. Condit, T. K. Fowler, and R. F. Post, Status report on mirror alternatives. Lawrence Livermore Laboratory, UCRL-52008,1976. 23. R. Linford, LASL, private communication, March 1978. 24. R. N. Sudan and M. N. Rosenbluth. Phvs. Rev. Lett. 36.972 (1976). 25. E. Morse and G. H. MiIey, Stability of field-reversed’mirrors. COO-2218-108,Fusion Studies Lab., University of Illinois, March 1978).