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Novel fusion energy conversion methods
Nuclear Instruments and Methods in Physics Research A271(1988)188-196 North-Holland, Amsterdam
2) d B.G. AN 2) G.H. L atory, PO Box 5511, L-644, Livermore, CA 94550, USA ~ mace Lic ore National ®1 Fusion Studies tory, Department of Nuclear Engineenng, University of Illinois, Urbwj, IL 61801, USA 1 ),
The potential importance of dint enc. ti. conversion to the long-term development of fusion power is discussed with emphasis on the possibility of alleviating waste heat problems. 's is envisioned to he important for any central power station in the 21st century and 'al for future space applications. Various novel conversion methods are briefly considered, including direct collection, tic expansion, synchrotron radiation conversion, and nonthermal neutron energy conversion methods. Due to the intimate connection between the type of fusion fuel, the confinement scheme, and the energy conversion technique, all three elements must be optimized simultaneously for high overall efficiency.
Fusion should assume a key role in our long-term energy picture, Among its advantages are plentiful, cheap fuel plus relative cleanliness and safety. Fusion asses other features, however, that make it unique. 'erhaps one of the most important of these is the tential to ploy a variety of novel energy conversion concepts to achieve improved plant efficiency. Since waste energy rejection may emerge as one of the most limiting considerations in the design and siting of future war stations, the potential for higher efficiencies must be viewed as an essential characteristic for y candidate energy sources [1-41 . Power plant siting is already greatly restricted by problems associated with waste heat rejection [2]. By the time fusion is introduced as a commercial power source, restrictions will undoubtedly be even more limiting. There appear to be only two ways to alleviate the problem ; namely, find alternate uses for low-grade waste heat, or/and obtain uch higher plant efficiencies . Both approaches will undoubtedly be necessary . Fusion energy is released in two basic forms: kinetic energy associated with neutrons and with charged particles (fig. 1). Neutrons escape the plasma and enter the reactor blanket. Most of the charged fusion products are confined within the plasma where their energy is rapidly redistributed among plasma ions and electrons. ately, a fraction of the energy transferred to electrons pas from the plasma as bremsstr ung and synchrotron radiation. All of these energy flows from e plasma can be the alized and utilized in a conventional thermal cycle. However, to achieve improved efficiency it ' be necessary to use nonthernmal techniques, to extract energy from one or more of the flow 0168No
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paths. Indeed, each energy form will probably require a unique and different type of conversion. In the present discussion we will briefly consider some of the possible approaches . Since the best known direct conversion techniques operate on charged particles [5,6], we will briefly review these possibilities first. Various possible techniques to improve the extraction of both radiation and neutron energy will also be considered. Direct conversion systems can be divided into two categories : "mainline" and "auxiliary" systems . Auxiliary convertors are involved in subsystems such as neutral-beam injectors [7] . Mainline conversion, on the other hand, deals with recovery of the fusion energy itself, and that is the subject we will concentrate on here.
Neutrons Fusion Energy
radiation Charged Particles
thermalized plasma
L
escaping high energy charged particles
Fig. l. Fusion energy flow.
L.J. Perkins et al. / Nawel fusion energy conversion methods
eo
i
conversion
Material constraints impose a limit on the maximum temperature available in thermal cycles (fig. 2). On the other hand, if direct conversion can "couple" directly to charged particles in the plasma, such limitations are bypassed. Consequently, it becomes possible, in effect, to utilize the high temperature (-109 K) of the plasma in the conversion cycle, radically raising the efficiency limits. Then, if ultrahigh-efficiency cycles (e.g., in the 80% range) become feasible, as illustrated in fig . 3, rejected heat can be reduced by a factor of 4 or 5 compared to current plants . We should stress that, at present, conventional concepts of DT fusion power reactors are barely competitive with other developed forms of energy, because the complex and expensive fusion power core is coupled to an equally complex and expensive thermal steam cycle with its inherent low efficiency. Accordingly, if the efficiency of energy conversion is improved, the physical size of the plant required to provide the desired total output power can also be reduced, cutting capital costs (provided that the direct convertor itself is not exces-
CONVENTIONAL STEAM CYCLES (,i =e0%)
,k\'--M6terial Limitations
HIGH-TEMPERATURE TOPPING/ BOTTOMING CYCLES (,9=60%)
Accessible Only by Direct Co9plieg
EQUIVALENT PLASMA TEMPERATURE RANGE 10
102 io3 io` 1CP i& io' ioe io9 io'o T (°K)
Fig. 2. Material limits on conversion cycle temperatures versus direct coupling to charged particles in ? fusion plasma (ref. [5ll.
sively expensive). ' could a key factor in ultimately obtaining competitive fusion power costs.
Direct conversion is intimately cone to the concept of "advanced fuel" and aneutronic fusion [1,5,8] . Direct conversion is best suited for use with a hrel that releases a larger fraction of en with the charged particles than does DT. This makes possible a variety of electrical techniques that can directly extract the energy carried by the charged particles . h fuels are discussed in detail in other papers in the Proceedings, so the reader is referred to those pa
Due to the higher temperatures required, a fraction of the energy from an advanced fuel plasma will be in the form of radiation. With careful selection of conditions, radiation can held to less than 15% of the output power flow in catalyzed-D and D-3He reac tors [8] . However, this fraction can purposely enhanced, or, with other higher-Z fuels operating at hi, electron temperatures, radiation can carry over % of the output flow. This, suggests the concept of a "radiation dominated" fusion reactor [8-12] . Effectively, in this limit, neutron production is replaced by radiation emission. Such a reactor might rely on thermal conversion techniques to handle the radiation . This still has the potential of improved efficiencies since unique thermal cycles appear possible [9-12] . For example, it has been suggested that a reasonable fraction of the bremsstrahlung radiation could be transmitted through a lowZ first wall and absorbed in a high-Z fluid, which would then enter an MHD channel or special heat engine convertor . Other unique methods to utilize the part of the radiation energy carried by synchrotron radiation are discussed in later sections. 5. Interrelations
CURRENT
ADVANCED THERMAL TOPPING DIRECT CONVERSION
0 20 40 60 80 100 71,
Fig. 3. Reduction of waste heát with increasing efficiency (ref. [5]). Here (1- 71)/ -q represents the rejected-to-output power ratio.
Direct collection is the only form of direct conversion that has been explored in some detail both theoretically and experimentally [5,6]. Electromagnetic coupling, in the form of compression-expansion cycles, has received some theoretical attention, but no experimentation [5,13]. While there are a number of other convertor possibilities, none has yet been examined in sufficient detail to allow Its evaluation [5]. As stressed in fig. 4, direct conversion techniques are intimately connected with the fuel, the energy split, and the energy extraction techniques. Fuel consideratiors
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Direct Conversion Technique(s)
Fuel Energy Split Energy Extraction Technique (s)
Fig. 4. Intimate relation of components involved in the energy
flow.
have already been noto3 . In following sections, we will consider, along with specific approaches, procedures involved in obtaining efficient energy extraction (coupling).
6. Direct collection
loss due to a finite number of collector plates (i.e ., discontinuous voltage steps), plus a number of small but important parasitic losses such as charge exchange, secondary-electron production, retrograde orbits, incomplete expansion, and space charge effects . While each of these inefficiencies may be small, the overall sum can become quite significant . Thus, convertor efficiencies
can easily drop below 70% . The potential for direct collection can be illustrated by noting that even with only one plate, the efficiency can approach 506 if the parasitic losses are carefully reduced . Two plates give a limit of 675 whereas maximum values over 906 are achieved with only ten plates. These remarks assume a typical Maxwellian spread in ion energies - monoenergetic beams as encountered in injectors have ideal efficiency limits approaching 1006 with only one plate. As in most energy convertors, a. compromise is necessary between cost and efficiency. Methods for improving efficiency such as adding collector plates, increas-
ing the expansion, etc., clearly involve additional structure and cost . Consequently, mirror reactor conceptual designs have used the lower cost venetian-blind collector despite its efficiency disadvantage compared to the periodic-focusing collector [6] .
6.1 . General considerations
6.2. Reactor integration considerations
Direct collection involves conversion of the kinetic energy of plasma particles to potential energy in the form of high voltage on a collector plate . This voltage can then drive a current through a load resistor . As illustrated in fig. 5 for a mirror reactor, five basic steps are involved in direct collection. First is extraction of the plasma. Second is expansion, which accomplishes two purposes : (1) conversion of the gyromotion of the charged particles into directed motion that can be recovered in an electrostatic field, and (2) reduction in density such that, during the next (third) step when electrons and ions are separated, space charge effects do nod cause excessive perturbation of trajectories. Next, collection of the particles can be carried out using various devices such as a periodic-focused convertor or a venetian-blind collector. The fifth and final step involves conditioning the output power for use . Since direct collection does not involve a tnerma..ïynamic cycle *, the "ideal" upper limit on efficiency is 100% . There are, however, a number of small losses associated with each step of the conversion cycle, e .g ., the loss due to a nonmonoenergetic energy spectrum,
Direct collection requires that the charged particles be guided ("coupled") out of the confinement region and into the energy convertor . The way this is best done strongly depends on the type of confinement system that is employed . To illustrate this, we consider four different concepts : the simple mirror, the tokamak, the migma, and the field-reversed mirror (FRM) . The sim-
This efficiency refers to the collector system alone - since other energy flows must be processed by other convertors and some energy must be recirculated, the overall plant efficiency will be lower .
ENLARGED SIDE VIEW (SEC . A-A) TYPICAL ION TRAJECTORY s0
so 70
METERS
s0
EXPANDER
TOP VIEW
0 20 40 60 80 100
METERS
Fig. 5 . Mirror fusion reactor using direct collection . In this example, a fan-type expander is employed to couple to a periodic-focused (PF) collector.
L.J. Perkins et al / Novelfusion energy conversion methods
Girder stiffener
Tak k MUM
Fig. 6. A bundle divertor for energy extraction from a tokamak allows expansion past the toroidal field coils . ple mirror provides a good illustration of the issues involved but has too low an energy multiplication (Q) to be of practical interest for advanced fuels. Several studies have considered using a tokamak for burning catalyzed-D or D_3He fuels, but its relatively low plasma ß is a serious limitation. The migma and FRM offer very high plasma P and hence -re of strong interest for a variety of advanced fuels, Athough here we continue to use D- 3He for illustrative purposes . (The FRM (see fig. 7) contains large orbit ions circulating in a background plasma. In some respects this represents a limiting case for migma. While the ideal migma would involve only high energy beams, space charge neutralization requirements may force a modest density plasma background. This issue is not yet settled . Consequently, the FRM is purposely included here to provide insight into issues that emerge as one approaches this limit .) In a conventional mirror, coupling is relatively straightforward (see fig. 5). The plasma naturally flows out through the loss cone, carrying with it the energy deposited by the thermalized fusion product . The ease of extraction carries with it the disadvantage that the conventional mirror has a relatively low Q-value (i.e., fusion energy multiplication) due to the rapid flow out through the loss cone. In contrast, a toroidal design offers better confinement, resulting in a higher Q-value . Then, however, it is necessary to devise a different
requirements and strong stresses in the coil supports. Superconducting coils would minimize power losses, but the separatrix area may not offer sufficient room for shielding. Consequently, in the study of fig. 6, cryogenic aluminum magnets were selected as a compromise. This reduced the power consumption and allowed less shielding . This approach resulted in a reduction in overall waste heat for catalyzed-deuterium and D-3He tokamak designs ranging from 25% to 50`x. The migma concept offers still different approaches to direct collection. Like a simple mirror, a migma operating with D_3He fuel could result in a large fraction of the energy being carried by the 14MeV proton and 3.5-MeV alpha product . These energetic ions would exit through well-defined loss cones and could subsequently be handled in a fashion analogous to that already described for the mirror. A fundamental issue is the energy spread of the escaping particles and the energy transferred to the low density background plasma used for space charge neutralization in the migma reaction volume . The energy spread and back-
nnemlina tPrhninvip invnlvina a AivPrtnr
There are three basic types of divertors - toroidal, poloidal, and bundle divertors. While the poloidal divertor has been favored in conceptual tokamak studies, it is difficult to couple to a cdrect collection unit since the diverted plasma must be expanded out through the toroidal field coils . Thus, one study [14], illustrated in fig. 6, employed a bundle divertor to obtain expansion out between the field coils . The bundle divertor, however, faces two severe problems. First, the need to divert the main toroidal field results in large magnet power
-REVERSED MAGNETIC FIELD CONFIGURATION
Fig . 7. Conceptual illustration of the FILM. The external open-field lines provide a "natural" divertor action . 111. NEW CONCEPTS
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ground transfer tend to reduce the conversion efficiency, and the problem is further complicated by the possible need to operate on groups of particles having three widely varying mean energies. As with the simple mirror, expansion is required to obtain a high degree of directed energy, so this approach leads to fairly large size structures . These various problems need careful study. Still, direct collection appears to be reasonably well suited to the migma concept. The field-reversed mirror (FRM) concept, illustrated in fig. 7, represents yet another type of confinement system that combines toroidal and mirror characteristics. Here the open-field lines foamed by the external mirror coils provide a "natural" divertor [15]. Thus, plasma leaking across fields in the closed region is swept out along the open-field lines and can be directed to a direct collection unit. Like migma, the FRM faces unique coupling problems [16]. Since it is typically quite small (20-30 cm minor radius), a significant fraction of the high-energy fusion products leave the closed-field region and escape along the open-field lanes. Thus, to incorporate direct collection, both the MeV-fusion products and the low energy (keV) background plasma particles must be handled. This forces use of collector plate voltages ranging from megavolts down to a few kV, posing a stiff test for high voltage technology. This problem would be avoided if the fusion products were thermalized before entering the collector (of the conventional mirror) . Two approaches can be envisioned . One is to enlarge the closed-field region in size and simultaneously increase the mirror field strength. This approach is severely limited, however, by stability considerations that require the ratio of the plasma size to the ion-gyroradius (the S-value) to be < 5-10. A second possibility is to use a dense, cold plasma flowing along the external field lines in order to thermalize the fusion products entering this region [17].
7. Electromagnetic coupling Compression-expansion cycles represent a practical way to utilize electromagnetic coupling. This concept, first discussed in some depth by Budker and by Bickerton and jukes in 1959, is considered in some detail in ref . [5] . In this approach, the hot fusion plasma expands against the confining magnetic field such that the internal energy of the plasma is directly converted to electric cu:-rent in the field coils. The efficiency of expansion cycles is proportional to the expansion ratio. However, the cost also increases with expansion ratio due to the larger blanket and coil sizes required . For practical purposes, then, this approach is restricted to situations where ignition is achieved by compression . In that case
TYPE
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5
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1.0 E- C B
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COMPRESSION RATIO,
1
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Fig. 8. Ideal cycle efficiency vs compression ratio (CB) for various types of compression-expansion cycles, e.g., types S and P (see refs . [5,13]) .
the expansion becomes a natural part of the cycle with little added cost. Cycle efficiencies for various compression-expansion cycles are summarized [5] in fig . 8. Note that an expansion ratio of approximately two can provide 60% cycle efficiency (in the type S limit) * . Larger expansion ratios can lead to efficiencies over 809 . It should be stressed, however, that these efficiencies are ideal limits based on the thermodynamic efficiency of the cycle. Thus, practical efficiencies must include losses due to ohmic heating in the coils and inefficiencies in switching. In practice, these losses can be quite significant, reducing the ideal cycle efficiency by a factor of up to 2 or more. Consequently, use of compression-expansion as a mainline energy extraction technique requires large expansion ratios so that vessel sizes and costs become a limiting factor. Partial energy extraction via a modest expansion during the bum, envisioned in theta-pinch reactor designs [18], is attractive. However, it becomes an auxiliary, rather than mainline, conversion system since a portion of the plasma energy must still be removed by other means. Inertial confinement naturally lends itself to a quite large expansion ratio since the initial compressed state, i.e., the pellet, is so small [5.191. Further, the imposed magnetic field could serve a dual pu,-)ose; namely, protection of the wall from the pellet dc. ris and direct conversion . Most inertial reactor designs have, however, * As discussed in ref. [5], the cycle efficiency represents an overall thermodynamic efficiency for the compression-expansion trajectory. Thus it is considerably less than the 100% quoted in some literature for the extraction efficiency alone.
LJ. Perkins et al. / Novel fusion energy conversion methods
favored liquid lithium jets or falls that serve both for first wall protection and for tritium breeding. However, magnetic protection could become more advantageous if advanced-fuel pellets, providing larger energy releases in charged particles, are developed. An example of an alternate type of electromagnetic coupling is illustrated in an earlier study of the reversed-field-pinch (RFP) reactor [20]. In the RFP, the poloidal field is roughly the same magnitude as the toroidal field . This is achieved by driving large pulsed currents in the plasma that are, in fact, sufficient to achieve ignition without auxiliary heating. Conceptual reactor designs envision operation in a pulsed mode with burn times of the order of 10-100 s. With such short pulses and large currents, it is essential to recover the magnetic field energy associated with the plasma current at the end of the burn. This can be done, in effect, by introducing an opposing electric field such that the plasma current is slowly decreased (see ret. [5]). In the process, the stored energy associated with the plasma current is inductively transferred to the circuit used to create the opposing field (the same circuit originally used to drive the plasma current). A key question involved in this approach is whether or not the prima configuration will remain stable as the current is decreased. This can become quite crucial to the efficiency of the plant. For example, if 90% of the stored field energy is recovered, energy multiplications as much as 4 or 5 times as large as those with 50% recovery become possible. 8. Novel neutron energy conversion schemes 8.1 . Introduction
The first generation of fusion power reactors will, in all probability, be those burning a deuterium-tritium fuel mixture with 14 MeV neutrons as the primary energetic product. Accordingly, it has been proposed that significant economic advantages can be obtained for DT reactors if the expensive and complex conventional steam-turbine power conversion zycle can be replaced with innovative fusion-specific neutron energy conversion schemes [21]. In this way, the inherently higher cost of the fusion nuclear island can be offset by the. low roost of thc~ considerably simplified balance-ofplant. The DT fusion reaction releases 80% of its energy in the form of a 14.1 MeV neutron . However, we have up to now considered that its energy can only be converted to heat. On the face of it, this implies an expensive and complex steam/turbine balance-of-plant with relatively low associated efficiencies. It should be noted that, on initial emission from the fusion reaction, the neutron can be considered a very low entropy system (remember
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that the effective temperature of a 14 MeV neutron is 1 .7 x 10 11 K) because its energy is directed in only one degree of freedom . However, if we allow the neutron to slow down in a conventional neutron absorbing blanket, its energy becomes distributed over any degrees of freedom in the bulk heating of the blanket medium. Because of the increased entropy associated with the conversion of neutron energy to heat in the blanket, the efficiency of a "conventional" fusion blanket and associated steam-turbine cycle is Carnotlimited and dependent on the maximum blanket temperature attainable . Neutrons generate heat in an absorbing medium by intermediate stages of primary charged particle production and secondary ionization and excitation resulting finally in bulk thermal motion of the medium. The primary charged particles arise from a number of neutron interaction channels : (a) charged particle nuclear reactions, e.g., (n, p), (n, a), etc . ; (b) knock-on recoil, e.g., n-p scattering, etc.; and (c) electron production from gamma rays via photoelectric, Compton, or pairproduction effects, the gamma rays resulting from neutron capture [(n, -y)] or neutron inelastic scattering [(n, ri) y ]. We can, in principle, intercept the energy flow at any stage to generate electrical potential energy. The final heat stage is distinguished from the intermediate charged particle stages by the fact that the maximum energy conversion efficiency of a heat cycle operating between a hot reservoir Thot and a cold reservoir Tcold is the Camot efficiency : rarnot -
Thot - Tc Id Thot
It is important to appreciate that if we wish to generate electricity without a heat cycle, we have to somehow intercept the energy flowing from the intermediate charged particles into heating of the background absorbing medium . Energy conversion directly from charged particles does not involve heat energy and is therefore not Carrot-limited . In principle, therefore, maximum theoretical efficiencies can be significantly higher than Carnot. However, practical conditions and requirements may considerably reduce these high theoretical values. In table 1, taken from ref. [21], we compare eight possible methods for neutron energy conversion. Of these_ five are Camot-limited (i_e._ involve an intermediate heat stage) while three are not. We have also denoted those methods that are fusion-specific, i.e., that exploit the unique properties of DT fusion. The adw,stages, disadvantages, and practicdities of these eight methods are discussed in ref. [21] where it is shown that, compared with the conventional (but complex and expensive) steam cycle, only two novel methods appear to possess sufficient potential to be considered further, namely : radiation catalyzed MHD conversion and exIII . NEW CONCEPTS
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Table 1 Methods for fusion neutron energy conversion to electricity . CamotFusionMethod specific? limited? yes no Conventional steam cycle yes no Thermoelectric conversion yes no Thermionic conversion (electrohydrodynamic) EHD yes no conversion no yes Ionization-electric conversion Primary charged-particle yes no direct conversion Radiation-catalyzed MHD yes a) yes conversion Excimer-channeled UVno electric conversion yes a)
May, however, have application for gas-cooled fission systems .
cimer-channeled UV conversion. We discuss these further below . 8.2. In situ radiation-catalyzed MHD conversion In this scheme, MHD electrical power generator modules would be placed in remotely maintainable modules between the magnet coils of the fusion reactor . Rankine cycles using cesium.-seeded metal vapors enable us to locate the MHD conversion loop entirely within the reactor ; only the waste heat rejection fluid is sent to an external reservoir . Synchrotron and bremsstrahlung radiation from the plasma act as a catalyst in that they are used to superheat the MHD vapor and enhance its conductivity through nonequilibrium ionization . In this simple in situ scheme, net cycle efficiencies of - 30-45% are estimated, so that steam bottoming cycles can be eliminated. Experimental tasks and facilities required to address the technical issues for this method would be : (1) an in-depth analysis of reactor potential and fusion applications; (2) a detailed design of an integrated MHD flow facility; (3) small-scale experiments to investigate MHD nonequilibrium stability, the microwave absorption rates, and the broadening cross sections; (4) experiments with the proposed working fluid to determine temperature and pressure characteristics, corrosion, and handling procedures. 8.3. Excimer-charneled UV conversion Here we require the conversion of pri"nary neutroninduced charged particle energy (e.g., knock-on recoils and/or X(n, x) reactions) to narrow bandwidth singleline UV radiation with high efficiency. Subsequent high efficiency conversion of UV photon energy to high
current, low voltage, electrical energy is achieved by UV photocells with appropriately selected semiconductor band gaps. Measurements with noble gas excimers (e.g., bound excited state of He2 , Are , Xe2 , etc., which undergo specific UV transitions to free single atoms, i.e., bound-free transitions) under electron and fission fragment excitation have shown high energy channeling efficiencies (-- 60-é5%) from charged particle energy to UV output (see the overview in ref. [22]). In addition, due to the narrow UV linewidth, it is possible to envisage specially tailored high efficiency (50-80`x) UV photocells with band gap energies selected for this specific frequency. High overall conversion efficiencies from charged particle energy to electrical output of 30-60% may therefore be achievable. This method is, at present, in a preliminary concept phase and there are many technical issues that must be addressed before any reasonable assessment of its final potential can be attempted . The most significant issues are : (1) detailed measurements of neutron-induced charged particle energy channeling to UV in appropriate excimers; physics of production process ; effect of excimer gas pressure on linewidth; effect of additional material. for tritium breeding; (2) investigation of UV photon transport in realistic geometries; UV windows and reflectors; investigation of optimum operating range (UV, VUV, or XUV); (3) investigation of appropriate materials for high efficiency UV photocell convertors ; combination of photocells in a bulk generation array. 9. Synchrotron radiation conversion In section 4 we noted that at the higher operational temperatures envisaged for advanced fuel fusion, a significant fraction of the fusion energy can appear as radiation . Logan, in ref. [23], has suggested that microwave synchrotron radiation can be a valuable output form of fusion energy when used for direct conversion either by rectification using VLSI rectenna arrays or by superheating vapor for in situ MHD conversion (see section 8.2). With sufficient confinement, the D-3 He fuel cycle can allow the maximum fraction (- -s) of fusion power output in the form of microwaves of all potential advanced fuels. In the synchrotron direct conversion scheme, the microwave power is coupled out of the fusion reactor cavity via low loss waveguides, thereby mitigating first wall and limiter heat loads . We should also note that microwave emission and subsequent reflection in a preferential direction relative to the magnetic field could effect self-drive of the plasma current required in some fusion devices such as the tokamak or reversed-field pinch [24]. In this way we can envisage a very attractive reactor extrapolation, e.g., a D_3 He tokamak reactor operating in steady state with Q ,.- oo
LJ. Perkins et al. / Novel fusion energy conversion methods
synchrotron current-drive and rectenna conversion of guided synchrotron microwaves. For the large microwave radiation fractions required in this scheme, the D-3 He plasma would require better confinement (i.e., reduced plasma losses) relative to DT. Fusion concepts with superior confinement at high Te would be favored - presently the tokamak, but possibly to be superseded by alternate concepts such as the mirror in the future. The efficient use of this technique requires microwave power fractions relative to the fusion power of greater than - 50% . In addition, extraction through waveguides with aperture area less than -- 20% of the first wall area requires the D-3 He plasma to be embedded in strong local fields (-- 7-10 T) for which only moderate betas (12-25%) are permitted in order to keep first wall heat fluxes below 1 MW m -2. After the synchrotron radiation enters the first wall waveguides, it is sorted by a waveguide filter system into frequency bands for direction to the appropriate rectenna array. The rectenna are half-wavelength antennas with integral diodes that rectify the wave to a direct current . Logan's scheme employs a field-emissive vacuum diode in which the cathode-anode distance is determined by the frequency of the wave to be received; for 1000-100 GHz operation this gap would be in the 0.1-1 hum range . A plurality of such rectennas would be created on a VLSI chip via X-ray lithography. Because the rectenna is made entirely of ceramic and metal, it is resistant to heat and radiation damage and therefore is much more durable than semiconductor diodes. Overall conversion efficiency is the product of a waveguide skin efficiency (-., 0 .98 at 2800 GHz with a waveguide wall of resistivity ten times that of copper) and a diode efficiency (- 0.83 with internal capacitance of 0.05 pF and a load resistance of -10 times that of the internal resistance of 0.12) and could, therefore, approach 0.8. As an example of the application of these microwave direct convertors, a D-3 He reactor with a hision power of 5700 MW and 2600 MW of synchrotron power converted at an efficiency of -0.8 (the remaining bremsstrahlung and transport losses used only for process heat) could exhibit plant gross/net outputs in the range of 2080 MWe/2000 MWe, respectively . Assuming passive or dielectric fluid convection cooling on the rectenna wafer backsides at a heat flux of 0.1 MW m-2, the total rectenna wafer area for conversion would be - 70 X 70 m. 10. Alternate approaches As suggested earlier, a variety of approaches to direct conversion can be envisioned that have not been discussed here (e.g., see ref . [5]). In principle, any method of heating a plasma can be considered for "reverse" operation as an energy recovery technique . However, to
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date none of these alternate approaches has been shown to offer both a good conversion efficiency and the ability to couple out a reasonable fraction of fusion energy. This is not to say that such technique(s) will not be found, but such possibilities must be viewed as highly speculative. Further, the present discussion only considers conversion to electrical energy output . The possibility of direct conversion to other energy forms, such as chemical production by radiation output from a fusion device, potentially represents another route to improved energy extraction [5,10] . Finally, we should note that space exploration will require high efficiency, low mass energy production methods with low waste heat rejection [25]. A fusion reactor with direct conversion could, perhaps, be the ultimate realization of this requirement. 11. Conclusion In summary, the potential for direct conversion, hence increased efficiency, is one of the key features of fusion that makes it attractive as a long-term energy source for both terrestrial and space applications . Indeed, restrictions on waste heat rejection may well force high efficiency designs in future energy applications, especially in space. While "novel" conversion has been the center of attention in the present discussion, certainly thermal recovery will always play a role in any fusion system. Hence, improved thermal efficiencies must also be sought . It should be stressed that the path to high efficiencies is not at all simple or straightforward . Every percent increase above the current 40% power plant value will be difficult to obtain. Part of the reason is the tight "circle" between the fuel, confinement system, and plasma coupling to the external system (fig. 4). The overall inefficiency is a product of factors involved in each component . Thus ultrahigh eL.üiencies (above 60%) will require almost perfect mating of the components . References [1] C. Choi (ed .), Proc. Review Mtg. Advanced-Fuel Fusion, EPRI ER-536-SR (Sept. 1977). [2] H.R. Drew, An Electric Utility View of Fusion, in ref. [1]. [3] C. Ashworth, A Utility View of Fusion, in ref. [1] . [4] J.P. Holdren, Science 200 (1978) 168. [5] G.H. Miley, Fusion Energy Conversion, ANS, Hinsdale, IL (1976) . [6] W.L. Barr and R.W . Moir, A Review of Direct Energy Conversion for Fusion Reactors, Proc. 2nd ANS Topical Conf. Fusion, Richland, WA (1976) . [7] W.L. Barr and R.W. Moir, Direct Conversion for Neutral Beam Injectors, Proc. 1976 IEEE Int . Conf. Plasma Science, Austin, TX (1976). III. NEW CONCEPTS
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[8] G.H . Miley et al ., Exploratory Studies of High-Efficiency Advanced-fuel Fusion Reactors, EPRI ER-581 (Nov. 1977). [9] A. Hertzberg and R. Taussig, High Thermal Efficiency Advanced Fuel Fusion Reactors, in ref. [1]. [10] G.H. Miley, Overview of Nonelectrical Applications of Fusion, Proc. 2nd Miami Int. Conf. on Alternative Energy Sources (Dec. 10-13,1979) Miami Beach, FL . [11] J. Fillo and J. Powell, Fusion Blankets for Catalyzed-D and D-3 He Reactors, in Ref. [1]. [12] J.M . Dawson, CTR Using the p-11 B Reaction, in ref. [1]. [13] G.H . Miley, Compression-Expansion Cycles for Toroidal Fusion Reactors, Proc . ANS Top. Tech. Controlled Nuclear Fusion, San Diego, CA (1974). [14] G. Swift and F. Southworth, Bundle Divertor Designs for the ILB Advanced Fuel Tokamaks, Proc. 7th Symp . on Engineering Problems of Fusion Research, Vol. II (1977) 1198. [15] W.C . Condit et al ., Preliminary Design Calculations for an FRM Reactor, Lawrence Livermore National Laboratory, UCRL-52170 (1976). [16] G.H . Miley and D. Driemeyer, Trans. ANS 26 (1977) 53 . D_3 He Fieldiii] .i . Gilligan et al ., Divertor Design for the Reversed Mirror (FRM), Proc. 3rd ANS Topical Mtg. on
Tech. Controlled Nucl. Fusion, Santa Fe, NM (May 1978) pp . 279-283. [18] T.A . Oliphant, F. Ribe and T .A. Coultas, Nucl. Fusion 13 (1973) 529. [19] M.P. Rao and G.H . Miley, Trans. ANS 15 (1973) 38 . [20] R. Nebel and G.H . Miley, Refueling and Control of RFP Burns, Proc. 3rd ANS Topical Mtg. on Tech. Controlled Nucl . Fusion, Santa Fe, NM (May 1978) pp . 279-283. [21] L.J. Perkins, B.G. Logan, C.D. Henning and G.H. Miley, A Rationale for Novel Neutron Energy Conversion Schemes for D-T Fusion Reactors, Lawrence Livermore National Laboratory, UCRL-93988 (1986). [22] M.A. Prelas, F.P. Boody, G.H . Miley and J.F. Kunze, Nuclear Driven Flashlamps, Laser and Particle Beams, Vol. 6, Part 1 (Feb . 1988). [23] J.P. Holdren et al., Report of the Senior Committee on Environmental, Safety, and Economic Aspects of Magnetic Fusion Energy, Lawrence Livermore National Laboratory, UCRL-53766 (1987) . [24] J.M . Dawson and P.K. Kaw, Phys. Rev. Lett. 48 (1982) 1730 . [25] B.G. Logan, Optimum Strategy for Magnetic Fusion Powering Interplanetary Spacecraft, Lawrence Livermore National Laboratory, UCRL-21257 (1987).
2) d B.G. AN 2) G.H. L atory, PO Box 5511, L-644, Livermore, CA 94550, USA ~ mace Lic ore National ®1 Fusion Studies tory, Department of Nuclear Engineenng, University of Illinois, Urbwj, IL 61801, USA 1 ),
The potential importance of dint enc. ti. conversion to the long-term development of fusion power is discussed with emphasis on the possibility of alleviating waste heat problems. 's is envisioned to he important for any central power station in the 21st century and 'al for future space applications. Various novel conversion methods are briefly considered, including direct collection, tic expansion, synchrotron radiation conversion, and nonthermal neutron energy conversion methods. Due to the intimate connection between the type of fusion fuel, the confinement scheme, and the energy conversion technique, all three elements must be optimized simultaneously for high overall efficiency.
Fusion should assume a key role in our long-term energy picture, Among its advantages are plentiful, cheap fuel plus relative cleanliness and safety. Fusion asses other features, however, that make it unique. 'erhaps one of the most important of these is the tential to ploy a variety of novel energy conversion concepts to achieve improved plant efficiency. Since waste energy rejection may emerge as one of the most limiting considerations in the design and siting of future war stations, the potential for higher efficiencies must be viewed as an essential characteristic for y candidate energy sources [1-41 . Power plant siting is already greatly restricted by problems associated with waste heat rejection [2]. By the time fusion is introduced as a commercial power source, restrictions will undoubtedly be even more limiting. There appear to be only two ways to alleviate the problem ; namely, find alternate uses for low-grade waste heat, or/and obtain uch higher plant efficiencies . Both approaches will undoubtedly be necessary . Fusion energy is released in two basic forms: kinetic energy associated with neutrons and with charged particles (fig. 1). Neutrons escape the plasma and enter the reactor blanket. Most of the charged fusion products are confined within the plasma where their energy is rapidly redistributed among plasma ions and electrons. ately, a fraction of the energy transferred to electrons pas from the plasma as bremsstr ung and synchrotron radiation. All of these energy flows from e plasma can be the alized and utilized in a conventional thermal cycle. However, to achieve improved efficiency it ' be necessary to use nonthernmal techniques, to extract energy from one or more of the flow 0168No
2/88/$03.50 © sevier Science Publishers o d Physics Publishing Division)
paths. Indeed, each energy form will probably require a unique and different type of conversion. In the present discussion we will briefly consider some of the possible approaches . Since the best known direct conversion techniques operate on charged particles [5,6], we will briefly review these possibilities first. Various possible techniques to improve the extraction of both radiation and neutron energy will also be considered. Direct conversion systems can be divided into two categories : "mainline" and "auxiliary" systems . Auxiliary convertors are involved in subsystems such as neutral-beam injectors [7] . Mainline conversion, on the other hand, deals with recovery of the fusion energy itself, and that is the subject we will concentrate on here.
Neutrons Fusion Energy
radiation Charged Particles
thermalized plasma
L
escaping high energy charged particles
Fig. l. Fusion energy flow.
L.J. Perkins et al. / Nawel fusion energy conversion methods
eo
i
conversion
Material constraints impose a limit on the maximum temperature available in thermal cycles (fig. 2). On the other hand, if direct conversion can "couple" directly to charged particles in the plasma, such limitations are bypassed. Consequently, it becomes possible, in effect, to utilize the high temperature (-109 K) of the plasma in the conversion cycle, radically raising the efficiency limits. Then, if ultrahigh-efficiency cycles (e.g., in the 80% range) become feasible, as illustrated in fig . 3, rejected heat can be reduced by a factor of 4 or 5 compared to current plants . We should stress that, at present, conventional concepts of DT fusion power reactors are barely competitive with other developed forms of energy, because the complex and expensive fusion power core is coupled to an equally complex and expensive thermal steam cycle with its inherent low efficiency. Accordingly, if the efficiency of energy conversion is improved, the physical size of the plant required to provide the desired total output power can also be reduced, cutting capital costs (provided that the direct convertor itself is not exces-
CONVENTIONAL STEAM CYCLES (,i =e0%)
,k\'--M6terial Limitations
HIGH-TEMPERATURE TOPPING/ BOTTOMING CYCLES (,9=60%)
Accessible Only by Direct Co9plieg
EQUIVALENT PLASMA TEMPERATURE RANGE 10
102 io3 io` 1CP i& io' ioe io9 io'o T (°K)
Fig. 2. Material limits on conversion cycle temperatures versus direct coupling to charged particles in ? fusion plasma (ref. [5ll.
sively expensive). ' could a key factor in ultimately obtaining competitive fusion power costs.
Direct conversion is intimately cone to the concept of "advanced fuel" and aneutronic fusion [1,5,8] . Direct conversion is best suited for use with a hrel that releases a larger fraction of en with the charged particles than does DT. This makes possible a variety of electrical techniques that can directly extract the energy carried by the charged particles . h fuels are discussed in detail in other papers in the Proceedings, so the reader is referred to those pa
Due to the higher temperatures required, a fraction of the energy from an advanced fuel plasma will be in the form of radiation. With careful selection of conditions, radiation can held to less than 15% of the output power flow in catalyzed-D and D-3He reac tors [8] . However, this fraction can purposely enhanced, or, with other higher-Z fuels operating at hi, electron temperatures, radiation can carry over % of the output flow. This, suggests the concept of a "radiation dominated" fusion reactor [8-12] . Effectively, in this limit, neutron production is replaced by radiation emission. Such a reactor might rely on thermal conversion techniques to handle the radiation . This still has the potential of improved efficiencies since unique thermal cycles appear possible [9-12] . For example, it has been suggested that a reasonable fraction of the bremsstrahlung radiation could be transmitted through a lowZ first wall and absorbed in a high-Z fluid, which would then enter an MHD channel or special heat engine convertor . Other unique methods to utilize the part of the radiation energy carried by synchrotron radiation are discussed in later sections. 5. Interrelations
CURRENT
ADVANCED THERMAL TOPPING DIRECT CONVERSION
0 20 40 60 80 100 71,
Fig. 3. Reduction of waste heát with increasing efficiency (ref. [5]). Here (1- 71)/ -q represents the rejected-to-output power ratio.
Direct collection is the only form of direct conversion that has been explored in some detail both theoretically and experimentally [5,6]. Electromagnetic coupling, in the form of compression-expansion cycles, has received some theoretical attention, but no experimentation [5,13]. While there are a number of other convertor possibilities, none has yet been examined in sufficient detail to allow Its evaluation [5]. As stressed in fig. 4, direct conversion techniques are intimately connected with the fuel, the energy split, and the energy extraction techniques. Fuel consideratiors
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Direct Conversion Technique(s)
Fuel Energy Split Energy Extraction Technique (s)
Fig. 4. Intimate relation of components involved in the energy
flow.
have already been noto3 . In following sections, we will consider, along with specific approaches, procedures involved in obtaining efficient energy extraction (coupling).
6. Direct collection
loss due to a finite number of collector plates (i.e ., discontinuous voltage steps), plus a number of small but important parasitic losses such as charge exchange, secondary-electron production, retrograde orbits, incomplete expansion, and space charge effects . While each of these inefficiencies may be small, the overall sum can become quite significant . Thus, convertor efficiencies
can easily drop below 70% . The potential for direct collection can be illustrated by noting that even with only one plate, the efficiency can approach 506 if the parasitic losses are carefully reduced . Two plates give a limit of 675 whereas maximum values over 906 are achieved with only ten plates. These remarks assume a typical Maxwellian spread in ion energies - monoenergetic beams as encountered in injectors have ideal efficiency limits approaching 1006 with only one plate. As in most energy convertors, a. compromise is necessary between cost and efficiency. Methods for improving efficiency such as adding collector plates, increas-
ing the expansion, etc., clearly involve additional structure and cost . Consequently, mirror reactor conceptual designs have used the lower cost venetian-blind collector despite its efficiency disadvantage compared to the periodic-focusing collector [6] .
6.1 . General considerations
6.2. Reactor integration considerations
Direct collection involves conversion of the kinetic energy of plasma particles to potential energy in the form of high voltage on a collector plate . This voltage can then drive a current through a load resistor . As illustrated in fig. 5 for a mirror reactor, five basic steps are involved in direct collection. First is extraction of the plasma. Second is expansion, which accomplishes two purposes : (1) conversion of the gyromotion of the charged particles into directed motion that can be recovered in an electrostatic field, and (2) reduction in density such that, during the next (third) step when electrons and ions are separated, space charge effects do nod cause excessive perturbation of trajectories. Next, collection of the particles can be carried out using various devices such as a periodic-focused convertor or a venetian-blind collector. The fifth and final step involves conditioning the output power for use . Since direct collection does not involve a tnerma..ïynamic cycle *, the "ideal" upper limit on efficiency is 100% . There are, however, a number of small losses associated with each step of the conversion cycle, e .g ., the loss due to a nonmonoenergetic energy spectrum,
Direct collection requires that the charged particles be guided ("coupled") out of the confinement region and into the energy convertor . The way this is best done strongly depends on the type of confinement system that is employed . To illustrate this, we consider four different concepts : the simple mirror, the tokamak, the migma, and the field-reversed mirror (FRM) . The sim-
This efficiency refers to the collector system alone - since other energy flows must be processed by other convertors and some energy must be recirculated, the overall plant efficiency will be lower .
ENLARGED SIDE VIEW (SEC . A-A) TYPICAL ION TRAJECTORY s0
so 70
METERS
s0
EXPANDER
TOP VIEW
0 20 40 60 80 100
METERS
Fig. 5 . Mirror fusion reactor using direct collection . In this example, a fan-type expander is employed to couple to a periodic-focused (PF) collector.
L.J. Perkins et al / Novelfusion energy conversion methods
Girder stiffener
Tak k MUM
Fig. 6. A bundle divertor for energy extraction from a tokamak allows expansion past the toroidal field coils . ple mirror provides a good illustration of the issues involved but has too low an energy multiplication (Q) to be of practical interest for advanced fuels. Several studies have considered using a tokamak for burning catalyzed-D or D_3He fuels, but its relatively low plasma ß is a serious limitation. The migma and FRM offer very high plasma P and hence -re of strong interest for a variety of advanced fuels, Athough here we continue to use D- 3He for illustrative purposes . (The FRM (see fig. 7) contains large orbit ions circulating in a background plasma. In some respects this represents a limiting case for migma. While the ideal migma would involve only high energy beams, space charge neutralization requirements may force a modest density plasma background. This issue is not yet settled . Consequently, the FRM is purposely included here to provide insight into issues that emerge as one approaches this limit .) In a conventional mirror, coupling is relatively straightforward (see fig. 5). The plasma naturally flows out through the loss cone, carrying with it the energy deposited by the thermalized fusion product . The ease of extraction carries with it the disadvantage that the conventional mirror has a relatively low Q-value (i.e., fusion energy multiplication) due to the rapid flow out through the loss cone. In contrast, a toroidal design offers better confinement, resulting in a higher Q-value . Then, however, it is necessary to devise a different
requirements and strong stresses in the coil supports. Superconducting coils would minimize power losses, but the separatrix area may not offer sufficient room for shielding. Consequently, in the study of fig. 6, cryogenic aluminum magnets were selected as a compromise. This reduced the power consumption and allowed less shielding . This approach resulted in a reduction in overall waste heat for catalyzed-deuterium and D-3He tokamak designs ranging from 25% to 50`x. The migma concept offers still different approaches to direct collection. Like a simple mirror, a migma operating with D_3He fuel could result in a large fraction of the energy being carried by the 14MeV proton and 3.5-MeV alpha product . These energetic ions would exit through well-defined loss cones and could subsequently be handled in a fashion analogous to that already described for the mirror. A fundamental issue is the energy spread of the escaping particles and the energy transferred to the low density background plasma used for space charge neutralization in the migma reaction volume . The energy spread and back-
nnemlina tPrhninvip invnlvina a AivPrtnr
There are three basic types of divertors - toroidal, poloidal, and bundle divertors. While the poloidal divertor has been favored in conceptual tokamak studies, it is difficult to couple to a cdrect collection unit since the diverted plasma must be expanded out through the toroidal field coils . Thus, one study [14], illustrated in fig. 6, employed a bundle divertor to obtain expansion out between the field coils . The bundle divertor, however, faces two severe problems. First, the need to divert the main toroidal field results in large magnet power
-REVERSED MAGNETIC FIELD CONFIGURATION
Fig . 7. Conceptual illustration of the FILM. The external open-field lines provide a "natural" divertor action . 111. NEW CONCEPTS
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ground transfer tend to reduce the conversion efficiency, and the problem is further complicated by the possible need to operate on groups of particles having three widely varying mean energies. As with the simple mirror, expansion is required to obtain a high degree of directed energy, so this approach leads to fairly large size structures . These various problems need careful study. Still, direct collection appears to be reasonably well suited to the migma concept. The field-reversed mirror (FRM) concept, illustrated in fig. 7, represents yet another type of confinement system that combines toroidal and mirror characteristics. Here the open-field lines foamed by the external mirror coils provide a "natural" divertor [15]. Thus, plasma leaking across fields in the closed region is swept out along the open-field lines and can be directed to a direct collection unit. Like migma, the FRM faces unique coupling problems [16]. Since it is typically quite small (20-30 cm minor radius), a significant fraction of the high-energy fusion products leave the closed-field region and escape along the open-field lanes. Thus, to incorporate direct collection, both the MeV-fusion products and the low energy (keV) background plasma particles must be handled. This forces use of collector plate voltages ranging from megavolts down to a few kV, posing a stiff test for high voltage technology. This problem would be avoided if the fusion products were thermalized before entering the collector (of the conventional mirror) . Two approaches can be envisioned . One is to enlarge the closed-field region in size and simultaneously increase the mirror field strength. This approach is severely limited, however, by stability considerations that require the ratio of the plasma size to the ion-gyroradius (the S-value) to be < 5-10. A second possibility is to use a dense, cold plasma flowing along the external field lines in order to thermalize the fusion products entering this region [17].
7. Electromagnetic coupling Compression-expansion cycles represent a practical way to utilize electromagnetic coupling. This concept, first discussed in some depth by Budker and by Bickerton and jukes in 1959, is considered in some detail in ref . [5] . In this approach, the hot fusion plasma expands against the confining magnetic field such that the internal energy of the plasma is directly converted to electric cu:-rent in the field coils. The efficiency of expansion cycles is proportional to the expansion ratio. However, the cost also increases with expansion ratio due to the larger blanket and coil sizes required . For practical purposes, then, this approach is restricted to situations where ignition is achieved by compression . In that case
TYPE
S
SHOCK AND FAST ADIABATIC Q-1.0, y32=10 Z
w
0.6
SLOW ADIABATIC
4/i-L0 i`^0
U
w w
0.4
I
U
Û 0.2
0.oó 00
1
10
CB'
0.2
0.4
0.6
0.8
1 .0 ~
5
2.5
1 .66
1.225
1.0 E- C B
1
1
1
COMPRESSION RATIO,
1
CB
i
Fig. 8. Ideal cycle efficiency vs compression ratio (CB) for various types of compression-expansion cycles, e.g., types S and P (see refs . [5,13]) .
the expansion becomes a natural part of the cycle with little added cost. Cycle efficiencies for various compression-expansion cycles are summarized [5] in fig . 8. Note that an expansion ratio of approximately two can provide 60% cycle efficiency (in the type S limit) * . Larger expansion ratios can lead to efficiencies over 809 . It should be stressed, however, that these efficiencies are ideal limits based on the thermodynamic efficiency of the cycle. Thus, practical efficiencies must include losses due to ohmic heating in the coils and inefficiencies in switching. In practice, these losses can be quite significant, reducing the ideal cycle efficiency by a factor of up to 2 or more. Consequently, use of compression-expansion as a mainline energy extraction technique requires large expansion ratios so that vessel sizes and costs become a limiting factor. Partial energy extraction via a modest expansion during the bum, envisioned in theta-pinch reactor designs [18], is attractive. However, it becomes an auxiliary, rather than mainline, conversion system since a portion of the plasma energy must still be removed by other means. Inertial confinement naturally lends itself to a quite large expansion ratio since the initial compressed state, i.e., the pellet, is so small [5.191. Further, the imposed magnetic field could serve a dual pu,-)ose; namely, protection of the wall from the pellet dc. ris and direct conversion . Most inertial reactor designs have, however, * As discussed in ref. [5], the cycle efficiency represents an overall thermodynamic efficiency for the compression-expansion trajectory. Thus it is considerably less than the 100% quoted in some literature for the extraction efficiency alone.
LJ. Perkins et al. / Novel fusion energy conversion methods
favored liquid lithium jets or falls that serve both for first wall protection and for tritium breeding. However, magnetic protection could become more advantageous if advanced-fuel pellets, providing larger energy releases in charged particles, are developed. An example of an alternate type of electromagnetic coupling is illustrated in an earlier study of the reversed-field-pinch (RFP) reactor [20]. In the RFP, the poloidal field is roughly the same magnitude as the toroidal field . This is achieved by driving large pulsed currents in the plasma that are, in fact, sufficient to achieve ignition without auxiliary heating. Conceptual reactor designs envision operation in a pulsed mode with burn times of the order of 10-100 s. With such short pulses and large currents, it is essential to recover the magnetic field energy associated with the plasma current at the end of the burn. This can be done, in effect, by introducing an opposing electric field such that the plasma current is slowly decreased (see ret. [5]). In the process, the stored energy associated with the plasma current is inductively transferred to the circuit used to create the opposing field (the same circuit originally used to drive the plasma current). A key question involved in this approach is whether or not the prima configuration will remain stable as the current is decreased. This can become quite crucial to the efficiency of the plant. For example, if 90% of the stored field energy is recovered, energy multiplications as much as 4 or 5 times as large as those with 50% recovery become possible. 8. Novel neutron energy conversion schemes 8.1 . Introduction
The first generation of fusion power reactors will, in all probability, be those burning a deuterium-tritium fuel mixture with 14 MeV neutrons as the primary energetic product. Accordingly, it has been proposed that significant economic advantages can be obtained for DT reactors if the expensive and complex conventional steam-turbine power conversion zycle can be replaced with innovative fusion-specific neutron energy conversion schemes [21]. In this way, the inherently higher cost of the fusion nuclear island can be offset by the. low roost of thc~ considerably simplified balance-ofplant. The DT fusion reaction releases 80% of its energy in the form of a 14.1 MeV neutron . However, we have up to now considered that its energy can only be converted to heat. On the face of it, this implies an expensive and complex steam/turbine balance-of-plant with relatively low associated efficiencies. It should be noted that, on initial emission from the fusion reaction, the neutron can be considered a very low entropy system (remember
193
that the effective temperature of a 14 MeV neutron is 1 .7 x 10 11 K) because its energy is directed in only one degree of freedom . However, if we allow the neutron to slow down in a conventional neutron absorbing blanket, its energy becomes distributed over any degrees of freedom in the bulk heating of the blanket medium. Because of the increased entropy associated with the conversion of neutron energy to heat in the blanket, the efficiency of a "conventional" fusion blanket and associated steam-turbine cycle is Carnotlimited and dependent on the maximum blanket temperature attainable . Neutrons generate heat in an absorbing medium by intermediate stages of primary charged particle production and secondary ionization and excitation resulting finally in bulk thermal motion of the medium. The primary charged particles arise from a number of neutron interaction channels : (a) charged particle nuclear reactions, e.g., (n, p), (n, a), etc . ; (b) knock-on recoil, e.g., n-p scattering, etc.; and (c) electron production from gamma rays via photoelectric, Compton, or pairproduction effects, the gamma rays resulting from neutron capture [(n, -y)] or neutron inelastic scattering [(n, ri) y ]. We can, in principle, intercept the energy flow at any stage to generate electrical potential energy. The final heat stage is distinguished from the intermediate charged particle stages by the fact that the maximum energy conversion efficiency of a heat cycle operating between a hot reservoir Thot and a cold reservoir Tcold is the Camot efficiency : rarnot -
Thot - Tc Id Thot
It is important to appreciate that if we wish to generate electricity without a heat cycle, we have to somehow intercept the energy flowing from the intermediate charged particles into heating of the background absorbing medium . Energy conversion directly from charged particles does not involve heat energy and is therefore not Carrot-limited . In principle, therefore, maximum theoretical efficiencies can be significantly higher than Carnot. However, practical conditions and requirements may considerably reduce these high theoretical values. In table 1, taken from ref. [21], we compare eight possible methods for neutron energy conversion. Of these_ five are Camot-limited (i_e._ involve an intermediate heat stage) while three are not. We have also denoted those methods that are fusion-specific, i.e., that exploit the unique properties of DT fusion. The adw,stages, disadvantages, and practicdities of these eight methods are discussed in ref. [21] where it is shown that, compared with the conventional (but complex and expensive) steam cycle, only two novel methods appear to possess sufficient potential to be considered further, namely : radiation catalyzed MHD conversion and exIII . NEW CONCEPTS
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Table 1 Methods for fusion neutron energy conversion to electricity . CamotFusionMethod specific? limited? yes no Conventional steam cycle yes no Thermoelectric conversion yes no Thermionic conversion (electrohydrodynamic) EHD yes no conversion no yes Ionization-electric conversion Primary charged-particle yes no direct conversion Radiation-catalyzed MHD yes a) yes conversion Excimer-channeled UVno electric conversion yes a)
May, however, have application for gas-cooled fission systems .
cimer-channeled UV conversion. We discuss these further below . 8.2. In situ radiation-catalyzed MHD conversion In this scheme, MHD electrical power generator modules would be placed in remotely maintainable modules between the magnet coils of the fusion reactor . Rankine cycles using cesium.-seeded metal vapors enable us to locate the MHD conversion loop entirely within the reactor ; only the waste heat rejection fluid is sent to an external reservoir . Synchrotron and bremsstrahlung radiation from the plasma act as a catalyst in that they are used to superheat the MHD vapor and enhance its conductivity through nonequilibrium ionization . In this simple in situ scheme, net cycle efficiencies of - 30-45% are estimated, so that steam bottoming cycles can be eliminated. Experimental tasks and facilities required to address the technical issues for this method would be : (1) an in-depth analysis of reactor potential and fusion applications; (2) a detailed design of an integrated MHD flow facility; (3) small-scale experiments to investigate MHD nonequilibrium stability, the microwave absorption rates, and the broadening cross sections; (4) experiments with the proposed working fluid to determine temperature and pressure characteristics, corrosion, and handling procedures. 8.3. Excimer-charneled UV conversion Here we require the conversion of pri"nary neutroninduced charged particle energy (e.g., knock-on recoils and/or X(n, x) reactions) to narrow bandwidth singleline UV radiation with high efficiency. Subsequent high efficiency conversion of UV photon energy to high
current, low voltage, electrical energy is achieved by UV photocells with appropriately selected semiconductor band gaps. Measurements with noble gas excimers (e.g., bound excited state of He2 , Are , Xe2 , etc., which undergo specific UV transitions to free single atoms, i.e., bound-free transitions) under electron and fission fragment excitation have shown high energy channeling efficiencies (-- 60-é5%) from charged particle energy to UV output (see the overview in ref. [22]). In addition, due to the narrow UV linewidth, it is possible to envisage specially tailored high efficiency (50-80`x) UV photocells with band gap energies selected for this specific frequency. High overall conversion efficiencies from charged particle energy to electrical output of 30-60% may therefore be achievable. This method is, at present, in a preliminary concept phase and there are many technical issues that must be addressed before any reasonable assessment of its final potential can be attempted . The most significant issues are : (1) detailed measurements of neutron-induced charged particle energy channeling to UV in appropriate excimers; physics of production process ; effect of excimer gas pressure on linewidth; effect of additional material. for tritium breeding; (2) investigation of UV photon transport in realistic geometries; UV windows and reflectors; investigation of optimum operating range (UV, VUV, or XUV); (3) investigation of appropriate materials for high efficiency UV photocell convertors ; combination of photocells in a bulk generation array. 9. Synchrotron radiation conversion In section 4 we noted that at the higher operational temperatures envisaged for advanced fuel fusion, a significant fraction of the fusion energy can appear as radiation . Logan, in ref. [23], has suggested that microwave synchrotron radiation can be a valuable output form of fusion energy when used for direct conversion either by rectification using VLSI rectenna arrays or by superheating vapor for in situ MHD conversion (see section 8.2). With sufficient confinement, the D-3 He fuel cycle can allow the maximum fraction (- -s) of fusion power output in the form of microwaves of all potential advanced fuels. In the synchrotron direct conversion scheme, the microwave power is coupled out of the fusion reactor cavity via low loss waveguides, thereby mitigating first wall and limiter heat loads . We should also note that microwave emission and subsequent reflection in a preferential direction relative to the magnetic field could effect self-drive of the plasma current required in some fusion devices such as the tokamak or reversed-field pinch [24]. In this way we can envisage a very attractive reactor extrapolation, e.g., a D_3 He tokamak reactor operating in steady state with Q ,.- oo
LJ. Perkins et al. / Novel fusion energy conversion methods
synchrotron current-drive and rectenna conversion of guided synchrotron microwaves. For the large microwave radiation fractions required in this scheme, the D-3 He plasma would require better confinement (i.e., reduced plasma losses) relative to DT. Fusion concepts with superior confinement at high Te would be favored - presently the tokamak, but possibly to be superseded by alternate concepts such as the mirror in the future. The efficient use of this technique requires microwave power fractions relative to the fusion power of greater than - 50% . In addition, extraction through waveguides with aperture area less than -- 20% of the first wall area requires the D-3 He plasma to be embedded in strong local fields (-- 7-10 T) for which only moderate betas (12-25%) are permitted in order to keep first wall heat fluxes below 1 MW m -2. After the synchrotron radiation enters the first wall waveguides, it is sorted by a waveguide filter system into frequency bands for direction to the appropriate rectenna array. The rectenna are half-wavelength antennas with integral diodes that rectify the wave to a direct current . Logan's scheme employs a field-emissive vacuum diode in which the cathode-anode distance is determined by the frequency of the wave to be received; for 1000-100 GHz operation this gap would be in the 0.1-1 hum range . A plurality of such rectennas would be created on a VLSI chip via X-ray lithography. Because the rectenna is made entirely of ceramic and metal, it is resistant to heat and radiation damage and therefore is much more durable than semiconductor diodes. Overall conversion efficiency is the product of a waveguide skin efficiency (-., 0 .98 at 2800 GHz with a waveguide wall of resistivity ten times that of copper) and a diode efficiency (- 0.83 with internal capacitance of 0.05 pF and a load resistance of -10 times that of the internal resistance of 0.12) and could, therefore, approach 0.8. As an example of the application of these microwave direct convertors, a D-3 He reactor with a hision power of 5700 MW and 2600 MW of synchrotron power converted at an efficiency of -0.8 (the remaining bremsstrahlung and transport losses used only for process heat) could exhibit plant gross/net outputs in the range of 2080 MWe/2000 MWe, respectively . Assuming passive or dielectric fluid convection cooling on the rectenna wafer backsides at a heat flux of 0.1 MW m-2, the total rectenna wafer area for conversion would be - 70 X 70 m. 10. Alternate approaches As suggested earlier, a variety of approaches to direct conversion can be envisioned that have not been discussed here (e.g., see ref . [5]). In principle, any method of heating a plasma can be considered for "reverse" operation as an energy recovery technique . However, to
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date none of these alternate approaches has been shown to offer both a good conversion efficiency and the ability to couple out a reasonable fraction of fusion energy. This is not to say that such technique(s) will not be found, but such possibilities must be viewed as highly speculative. Further, the present discussion only considers conversion to electrical energy output . The possibility of direct conversion to other energy forms, such as chemical production by radiation output from a fusion device, potentially represents another route to improved energy extraction [5,10] . Finally, we should note that space exploration will require high efficiency, low mass energy production methods with low waste heat rejection [25]. A fusion reactor with direct conversion could, perhaps, be the ultimate realization of this requirement. 11. Conclusion In summary, the potential for direct conversion, hence increased efficiency, is one of the key features of fusion that makes it attractive as a long-term energy source for both terrestrial and space applications . Indeed, restrictions on waste heat rejection may well force high efficiency designs in future energy applications, especially in space. While "novel" conversion has been the center of attention in the present discussion, certainly thermal recovery will always play a role in any fusion system. Hence, improved thermal efficiencies must also be sought . It should be stressed that the path to high efficiencies is not at all simple or straightforward . Every percent increase above the current 40% power plant value will be difficult to obtain. Part of the reason is the tight "circle" between the fuel, confinement system, and plasma coupling to the external system (fig. 4). The overall inefficiency is a product of factors involved in each component . Thus ultrahigh eL.üiencies (above 60%) will require almost perfect mating of the components . References [1] C. Choi (ed .), Proc. Review Mtg. Advanced-Fuel Fusion, EPRI ER-536-SR (Sept. 1977). [2] H.R. Drew, An Electric Utility View of Fusion, in ref. [1]. [3] C. Ashworth, A Utility View of Fusion, in ref. [1] . [4] J.P. Holdren, Science 200 (1978) 168. [5] G.H. Miley, Fusion Energy Conversion, ANS, Hinsdale, IL (1976) . [6] W.L. Barr and R.W . Moir, A Review of Direct Energy Conversion for Fusion Reactors, Proc. 2nd ANS Topical Conf. Fusion, Richland, WA (1976) . [7] W.L. Barr and R.W. Moir, Direct Conversion for Neutral Beam Injectors, Proc. 1976 IEEE Int . Conf. Plasma Science, Austin, TX (1976). III. NEW CONCEPTS
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[8] G.H . Miley et al ., Exploratory Studies of High-Efficiency Advanced-fuel Fusion Reactors, EPRI ER-581 (Nov. 1977). [9] A. Hertzberg and R. Taussig, High Thermal Efficiency Advanced Fuel Fusion Reactors, in ref. [1]. [10] G.H. Miley, Overview of Nonelectrical Applications of Fusion, Proc. 2nd Miami Int. Conf. on Alternative Energy Sources (Dec. 10-13,1979) Miami Beach, FL . [11] J. Fillo and J. Powell, Fusion Blankets for Catalyzed-D and D-3 He Reactors, in Ref. [1]. [12] J.M . Dawson, CTR Using the p-11 B Reaction, in ref. [1]. [13] G.H . Miley, Compression-Expansion Cycles for Toroidal Fusion Reactors, Proc . ANS Top. Tech. Controlled Nuclear Fusion, San Diego, CA (1974). [14] G. Swift and F. Southworth, Bundle Divertor Designs for the ILB Advanced Fuel Tokamaks, Proc. 7th Symp . on Engineering Problems of Fusion Research, Vol. II (1977) 1198. [15] W.C . Condit et al ., Preliminary Design Calculations for an FRM Reactor, Lawrence Livermore National Laboratory, UCRL-52170 (1976). [16] G.H . Miley and D. Driemeyer, Trans. ANS 26 (1977) 53 . D_3 He Fieldiii] .i . Gilligan et al ., Divertor Design for the Reversed Mirror (FRM), Proc. 3rd ANS Topical Mtg. on
Tech. Controlled Nucl. Fusion, Santa Fe, NM (May 1978) pp . 279-283. [18] T.A . Oliphant, F. Ribe and T .A. Coultas, Nucl. Fusion 13 (1973) 529. [19] M.P. Rao and G.H . Miley, Trans. ANS 15 (1973) 38 . [20] R. Nebel and G.H . Miley, Refueling and Control of RFP Burns, Proc. 3rd ANS Topical Mtg. on Tech. Controlled Nucl . Fusion, Santa Fe, NM (May 1978) pp . 279-283. [21] L.J. Perkins, B.G. Logan, C.D. Henning and G.H. Miley, A Rationale for Novel Neutron Energy Conversion Schemes for D-T Fusion Reactors, Lawrence Livermore National Laboratory, UCRL-93988 (1986). [22] M.A. Prelas, F.P. Boody, G.H . Miley and J.F. Kunze, Nuclear Driven Flashlamps, Laser and Particle Beams, Vol. 6, Part 1 (Feb . 1988). [23] J.P. Holdren et al., Report of the Senior Committee on Environmental, Safety, and Economic Aspects of Magnetic Fusion Energy, Lawrence Livermore National Laboratory, UCRL-53766 (1987) . [24] J.M . Dawson and P.K. Kaw, Phys. Rev. Lett. 48 (1982) 1730 . [25] B.G. Logan, Optimum Strategy for Magnetic Fusion Powering Interplanetary Spacecraft, Lawrence Livermore National Laboratory, UCRL-21257 (1987).