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ORNL-TM-3783

Contract No. W-7^05-eng-26

Thermonuclear Division

PROSPECTS FOB ALTERNATE FUSION FUEL CYCLES AT ULTRA-HIGH TEMPERATURES

J. Rand McNally, Jr.

APRIL 1972

— NOTICE : — This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com- pleteness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee 37830 operated by UNION CARBIDE CORPORATION for the U. S. ATOMIC ENERGY COMMISSION

BlSTRiBUTi-r*' Of TP"S aOC'JraT UKL 1ITE0

'A 1

PROSPECTS FOR ALTERNATE FUSION FUEL CYCLES AT ULTRA-HIGH TEMPERATURES

J. Rand McNally, Jr.

ABSTRACT

Recent experiments and theory give support to the idea

of developing a closed "by the self-field of

a trapped ring current. Since the mirror reactor may he

able to operate at much higher burning temperatures than

toroidal reactors, the viability of charged particle fusion

chain reactions is thereby increased and may permit the chain

reaction burning of cheap Li or Be . Reaction

kinetics studies in partially closed mirrors show MeV energies

for the light fusion reaction products which can act as cata-

lysts or chain centers for the propagation of such reactions.

The MeV particles may also permit sustainment of both the

ring current configuration and the burning temperature.

Numerous problem areas associated with this approach are

tabulated and will require extensive research.

I. INTRODUCTION

Fusion chain reactions can occur with the elements, lithium

(Z = 3) at least through neon (Z = 10), with very light particles such o as , , deuterons, tritons, light (He) and alpha particles (A He\) acting as chain centers or catalysts to speed up the reaction. Examples of uncontrolled fusion chain reaction systems may

1 2 be the superbomb and supernovae. Of particular interest to the CTR 3 program are charged particle fusion chain reactions involving the chain centers p, d, t, ^ He, ^ He and fuels like 6 Li, 'Li,7/ and/or9 ^Be.

These energetic charged light products of the fusion reactions can propagate further reactions together with a net energy release and thus may sustain a controlled nuclear fire quite analogous to the k chemical chain reactior.s in controlled combustion processes, except for the energy scale of the system.

A simple example of a multiplying fusion chain reaction is

6 f) a_fas t, + Li a' + Li* - 2.2 MeV + a' + dfas- t. + a" - 1-5 MeV d + Li 2 or ^ + 22.k MeV fast

"fast + 2 6Li - 2 "slow + 2 "fast

Here, a multi-MeV alpha particle catalyzes the burning of two ^Li nuclei to give two multi-MeV alpha particles for a multiplication factor of 2 in two generations. Consult Reference 3 for more details.

There are several experimental and theoretical results which suggest the possible viability of such charged particle fusion chain reactions at very high temperatures for a CTR scientific feasibility experiment. They are:

1. The experimental demonstration of the onset of exponential

build-up in the PHOENIX II device at Culham Laboratory.

2. The theoretical development of scaling laws for scatter-

limited mirror build-up.

3. The experimental demonstration of an Astron-like E-layer •j (closed mirror) at Cornell University. 3

b. The Fokker-Planck reaction kinetics calculations of D*T and

D* He fusioning plasmas which reveal mean energies of ^ 1 MeV

for the trapped fusion reaction products, as evaluated at 8 9 Culham and Livermore. 6 7 5- The potentiality of alternate fusion fuels, Li, 'Li, and

Be.10'3 II. EXPONENTIAL BUILD-UP IN MIRROR SYSTEMS

5 Culham results indicate the verification of an important scaling law which can, under proper conditions, lead to a large scale expo- nential build-up of the hot ion density in mirror devices b£ several orders of magnitude. The IjbZ fission experiment of Fermi, et al11 demonstrated an exponential scaling law of a process rate which per- mitted the design of the graphite reactor at ORNL and the Hanford reactors. Mirror devices have large T and T but are presently limited to low density regimes. The exponential build-up of the density in magnetic mirrors can provide large TnT values which would demonstrate the scientific feasibility of CTR. Exponentiation can be obtained by attaining b s 0 in the build-up equation:

dn+V 2 —rrdt- = la + bn+, - cn+ where a is the trapping fraction of the injection current I due to l) targets, 2) background gas and 3) Lorentz trapping; n+ is the ion density, V the plasma volume, and c the coefficient for scattering losses. The coefficient b is

b = ICTL - NQ V k and is usually negative. The exponential build-up condition for the injection current becomes (b 2s 0):

n ov V „ I s ° cx V TCTTL

Thus, by obtaining long containment times, T = 1 , the required no cr vc x injection current can be grossly reduced. This can best be done by going to very high injection energy for which the charge exchange cross section (CT ) drops by a million-fold (see Fig. l). cx -15 2 The Culham experiment had IT ~ ^0 mA sec, a,p(exp.) = 2.8 x 10 cm

(for ionization of excited H° atoms), and plasma trapping an5d the onset of exponentiation were claimed in their Madison IAEA paper.

Two circumstances prevented any large scale exponential build-up of the plasma density in PHOENIX II: a) the exponentiatio/ o*\ n was based on the very low population of highly excited atoms (H ) which can not permit any large scale exponentiation (cascade effects were small) and b) the plasma became unstable because the maximum energy appears to be too low to provide adequate electron Landau damping

(threshold n a max E ). Exponentiation at MeV energies would occur primarily on the large population1 2o f ground state H^ ions and would

ensure a large It (^ 2000 mA sec ); however, CTt, the trapping cross

section, is somewhat lower 1/1*0) than in PHOENIX II. Programmed 12 electron cyclotron heating (ECH) at MeV energies would provide a

large maximum Eg and thus minimize plasma potential effects as well

as provide adequate electron Landau damping. The velocity spread of 12 gradually increased. the ions (necessary to minimize the negative mass instability) is also ensured because of appropriate stepping effects as ne and Te are 5

If PHOENIX II were provided with cw ECH, it might be possible to keep the plasma potential low by increasing the max E by a factor of e j. 80 (to the ion energy of 8 keV), thus permitting the exponentiation of

the plasma density from 3 x Kr9 cm ^ to ~ 2 x 101 3 cm , assuming

Landau damping continues to control the threshold density. However, 11 this would give an nT of only about 2 x 10 at 8 keV assuming T ~ 1 cx sec (compared to an instability driven loss time of only 0.2 sec, when

Landau damping is limited by ecp ~ kT ~ 100 eV). If partial burnout e occurs in PHOENIX II, T becomes > 1 sec and nT is then limited as ' cx in the next section to about 7 x 10'.1 1 ion-sec/cm3 .

III. SCALING LAWS FOR SCATTER-LIMITFD MIRRORS

Should exponentiation persist, without instabilities, the equi-

librium condition for the plasma throughput rate is

nV 12 — — n av VP T 2 sc or 2 nT ~ av P sc

where P is the mirror loss factor (2/P ~ 6 1°S10 R)• The multiple-small-

a anc one angle Coulomb scattering parameter

2 nT = C E^ log10 R .

The coefficient C has been evaluated in terms of Fokker-Planck calcu-

lations for plasma build-up in mirrors by including energy diffusion and slowing down as well as scattering into the loss cone and is of

IT order 10 "" when the injection energy EQ is in keV (for deuterons in a 2/l mirror for which log1Q R ~ 0.3).^ Thus, one obtains the following table (neglecting mirror closure effects and nuclear elastic scattering)

TABLE I. MAXIMUM nT IN SCATTER-LIMITED MIRRORS

* Eo nTma x 10 ^ 1 keV 3 x 10 ion-sec/cm

10 keV 9 x 1011

100 keV 3 x 1013

1 MeV 9 x 10

2 MeV 3 x 1015

* Independent of injection current provided exponentiation exists.

Thus, with stable exponential build-up, an injection-accumulation

method is capable of attaining a mirror-confined plasma

(nT > 10 ) only for energies above about 200 keV. The ORNL DCX-1

experiment used E ~ 300 keV, but exponentiation could not be attained H because of the negative mass Instability (the threshold density of

which is proportional to AE ). At MeV energies AE, the energy spread, 12 becomes very large (~ E) as a result of slowing down effects so that exponential build-up, once attained, should be sustained to much higher 12 densities."

IV. THE IMPORTANCE OF CLOSING THE MIRROR

Kammash has indicated that it may be essential to almost close

the magnetic mirror in order to make fusion reactors competitive with Ik non- plants. At the time of his writing no closed

magnetic mirror had been produced although the ELMO device had very 7

15 1 ft 17 high 0, ASTRON had a 15$ E-layer, and the 9-pinch had high p. '

Recently, at Cornell University, an ASTRON-like E-layer has been produced in which an actual field reversal occurs on axis due to the 7 relativistic electron ring current. The Cornell E-layer appears to be stable for a time (~ 0.5 ^sec) about 10 times the electron beam in- jection time UO nsec) and thus fast micro-instabilities are apparently non-disastrous. Christofilos ha~ claimed that in the ASTRON the nega- tive mass instability appears to be cured (provided 3 ^ 0.02) because of the positive gradient due to the combined self-field of the current ring and the applied magnetic field.

Closing the mirror is essential in order to permit the propagation of fusion chain reactions and to maintain the ion temperature suffi- ciently high. Futch at Lawrence Livermore Laboratory has shown that partial closure of the mirror enhances the ^ (Q = fusion power/injected power) from 0.27 (log10 R = l) to O.69 (log10 R = for a D'^He system.l 8 This high

mirror case (logir, R = 4) simulates the approach

to an I(ion)-layer (or E-layer). Futch's calculations did not

include the reaction products t' and ' (which would have increased

Q to ~ l); hence the Q may well have scaled as log-^Q R and exceeded unity. Also, the D« 3 He high-mirror-ratio case was almost a D*D inasmuch as the t and 3He ' production rate due to D*D reactions 3 almost equalled the He injection rate.

^Li'D fuel would be more likely to permit chain propagation than 6 6 LiH because: a) the

can undergo further reactions with the D or ^Li fuel ions provided 8 their energy is 1 MeV, and c) these latter reactions are much faster than the D»D reaction alone at high chain center temperatures. The next section discusses the high energies calculated for the charged reaction products in fusioning plasmas.

An additional advantage of a closed mirror configuration is the

reduction of the synchrotron in the high 3 region.

V. MeV ION ENERGIES IN FUSIONING PLASMAS

Q Kuo-Petravic, Petravic and Watson and Futch, Holdren, Killeen 9

and Mirin have studied the reaction kinetics of mirror-confined

(R » 3 - 1°) fusioning plasmas by Fokker-Planck methods. In both

instances the mean energies of the fusion reaction products equal or

exceed 1 MeV (a from the D«T reaction, p and a from the D* He reaction,

p from the D*D reaction; no account was taken of t' and He' from D-D

reactions, but these would enter the system at 1 MeV or more energy

since the center of mass energy and the AQ+ per particle exceed Q 1 MeV).

Kuo-Petravic, et al consider only low energy D and T injection

(100 keV) for which case the mirror losses are very substantial

(R = 3 and 10) and the electron temperature quite low,9 but the alpha particles stay at or above 1 MeV energy. Futch, et al consider the

100 keV D*T case and the 1 MeV D* He case, both for log1Q R = 1. In

the D*T case, Futch, et al obtain T ~ 17 keV and E =1.2 MeV (abundance = 1$), whereas in the D'-'He e case, TO ~ 13at5 keV, EO f ~ 1.9 MeV

(abundance = 0.51o)f and E^ ~ 3A MeV (abundance = 3$)* Partial closure

of the mirror (log R = ^ vs. l) doubles the abundance of the potential

18 chain centers. Although radiation losses are not included in these calculations, the tremendous loss of energetic particles out the leaky- mirror probably compensates in large part for serious radiation losses.

Inclusion of center of mass motion effects would increase the mean ion energies somewhat.

Such high mean energies for the fusion reaction products suggest that they may act efficient chain centers or catalysts to permit fusion chain reactions to occur to a high degree in a high temperature, 6 9 closed-mirror configuration. This would require Li*D or Be-K fueled reactors which have a much larger reactivity (ovQT ) than do D*D reactprs An ultimate fusion fuel economy would, of course, be dependent on a D«I) catalyzed or breeder reactor of the closed mirror or toroidal type (i.e., one ir, which the t' and ^He' burn efficiently). The closed mirror would have the advantages of being smaller in size and would appear to permit better fuel feed and ash exhaust than the toroidal reactor. Possible fusion reactor types are discussed in Table II, whereas the status of fusion reaction systems is tabulated in Table III. VI. PROBLEM AREAS AND RESEARCH NEEDS

High temperature burning of high Z fuels in a closed mirror reactor poses many difficult problems. A number of these are tabulated

and discussed briefly. It is grossly uncertain whether an MeV plasma may operate as an energy amplifier > self-s\istained,

be a sub-critical breeder for fissile fuels, or be none of three•

1. The I(ion)-layer (or proton E-layer) magnetic configuration

may be unstable for the long times (~ 100 sec) required for T!

closed mirror reactors. The Cornell E-layer was observed only 10

TABLE II. POSSIBLE FUSION REACTOR TYPES

6 9 Parameter DT DD LiD or yBeH

Temperature (°K) 108 109 1010

Density (ions/cm3) 10ll+ lO1^ ' 10lU

Containment Time (sec) 1 100 100

Magnetic Field (kG) 10 30 100 Q

Neutral Burnout (x n /n ) 10 10" 10" o' +'

6 9 Primordial Fuel LiD D2 ^LiD; BeH

Fuel Reserves () 106(1010 sea) > 1013 106(1010 sea);10^

1Q = 10l8 Btu ~ 1021 J ~ 10^ tons nuclear fuel

Present world consumption of energy ~ 0.2Q/yr

„ .T f would last 500 years at present rate Fossil reserves ~ 100Q j J would last 70 years at growth

*n /n, at E, = 10, 100, 1000 keV o + d TABLE III. PLASMA PARAMETERS

Minimum Super- Theta Laser Plasma Multi- Parameter Required Pinch Pellet DCX-1 Focus ELMO MeV

8 8 8 7 9 7 10 10 T(°K) 10 >10 10 10 10 (3X10 ). 10 (I0 )e 10

3 1 1 23 16 21 13 7 19 12 12 n( ions/cm ) lO * " «10 ? 10 10 ? 5xl0 . 10 10 10 10

-6 -5 -9 -2 -7 T(sec) 10 10 10 10 150 10 0.5 10-

22 25 ? 19 19 18 18 19 21 2 TnT 10 <10 ? 10 10 ? 5xl0 (5xl0 ) 10 (5xl0 ) 10 5

Scalable Increase Exponen- Close Ends Exponen- Feature Close Ends Energy Injection tiation Injection tiation

Termi- EIMO Pro- Status Achieved Scyllac ORMAK-II nated posed

Operation Pulsed Pulsed Pulsed Pulsed D.C. Pulsed D.C. D.C.

( ).: indicates not a true ion temperature

( ) : indicates hot electron plasma only; ions are < 100 eV. 12 to ~ 0.5 [isec duration and stability studies of this and the proposed ASTRON E-layer (T up to 12 msec possible?) should be pushed. MeV I-layers may be produced and studied for the much longer T'S, but would be quite expensive to build and investigate."*"^

There would be increased synchrotron radiation losses at the high electron temperatures required (T ^ 100 keV). This is e a major problem area, but it is in principle a reducible loss

Research should be pursued in three directions: a) further 19 development of the Grad-Burkhardt effect in high 0 plasmas

(this reduces the radiation from a volume effect to a quasi- surface radiation problem); b) investigation of the re- absorption of synchrotron radiation in a magnetic well and by the use of reflectors; c) investigation of depletion effects (non-Maxwellian distribution) on the cold electron population (v < v) which controls the energy drain from

— T the ions to the (electrostatic turbulence may 20 selectively drive the electrons out of the cold population )

Bremsstrahluno g from higher Z constituents woul2d1 be larger ^Pbrem a ' and relativistic effects and electron-

electron would become important. This is an

irreducible loss unless one goes through an inefficient heat

and energy conversion cycle to feed energy and/or particles

back into the reacting plasma. Self-sustainment studies

should be made to evaluate whether this single loss mechanism

can be compensated by reactor power or whether it prevents a 13

self-sustained plasma (P in charged particles/P injected > l).

More reaction kinetics studies of the Fokker-Planck type are

needed which correspond to very high mirror ratios and include

radiation losses. Radiation cooling effects on the electron

distribution function should be evaluated.

2 2/2 b. There will be greater Coulomb scattering losses (cj a z^ z^/e )

because of the high Z of the fuel ions; however, this may be

adequately compensated for2 b2 y the higher particle energy. Nuclear elastic scattering becomes increasingly important at

multi-MeV chain center energies and will provide a more effi-

cient feed of energy to fuel ions than to electrons. More

research should be done on these effects using realistic

plasmY a models. 5. The Be radioisotope (53-6d) poses an additional radiological 2*3 and afterheat problem (Be also poses a chemical toxicity

problem). The whole problem of afterheat and radiological Y 2b problems (T, Be, etc.) deserves a thorough study inasmuch

as no completely "clean" fusion reactor appears likely.

6. Higher magnetic fields (~ 100,000 gauss) appear to be necessary

to provide interesting power densities at MeV temperatures

(but then the DT reactor may have too high a power density);

thus, there is a need to develop high field, large super-

conducting magnets.

7. A MeV plasma would probably operate at a lower plasma density

than a DT reactor and neutral "burnout" may pose a problem;

however, the highly ionized, high Z nuclei may better scavenge Ik

any bound electrons (without being lost from the plasma) pc because of their very large capture cross sections 7 and

they would re-ionize rather rapidly. More atomic cross

sections (charge exchange, ionization, etc) are needed for

the energetic light ions in various simulated situations.

8. Nuclear reaction cross sections are poorly known, in general, 6 7 9 for Li, 'Li, and Be fuel nuclei. This could be a major undertaking of the community.

9. Although sputtering of metals by ene:getic light ions

decreases with increasing energy, little information is

available on sputtering at MeV energies.

10. Hazards Evaluation and Elimination: The chemical and nuclear

stored energy in a molten Li blanket of a DT fusion reactor

will be about 1012 (250 tons TNT equivalent) and lO1^

joules (2500 megatons TNT equivalent), respectively. Obviously

such potentials for disaster should be carefully scrutinized

and eliminated. "Reactor engineers have the responsibility to

assure that there is no imaginable way that reacting masses

or systems can get together."2^

The possibility of the nuclear burning of readily Ik accesible elements, like N from an air leak into an ultra- 27 high temperature plasma, must also be thoroughly investigated.

No nuclear fault possibilities should be available even by the

remotest chance. 15

VII. SUMMARY

There are several important developments which presage the potential application of c'.osed mirror reactors to the CTR problem.

I. N. Golovin of the USSR is now evaluating mirror fusion reactor 28 designs and some interesting developments may be forthcoming. The role of alternate fuel cycles and ultra-high temperature fusion reactors involves many unknowns and uncertainties, but it appears that more research and development studies are desirable. ACKNOWLEDGMENT

I am deeply indebted to William C. Gough of the U. S. Atomic

Energy Commission, Division of Controlled Thermonuclear Research for encouragement to prepare a position paper on alternate fuel cycles at ultra-high temperatures. 16

REFERENCES

1. U. Jetter, Physik. Blatter 6, 199 (1950)} available in English

translation as ORNL-tr-842.

2. J. R. McNally, Jr., Bull. Am. Phys. Soc. 16, 1269 (1971).

3. J. R. McNally, Jr., 11, 187, 189, 55^ (1971)-

B. Lewis and G. von Elbe, Combustion, Flames and of

Gases, Academic Press, N. Y. (1961). See also J. R. McNally, Jr.,

Fusion, Nuclear, to be published in Encyclopedia of Chemistry, 1972.

5. E. Thompson, J. G. Cordey and D. R. Sweetman, Paper CN-28/G-10,

Fourth IAEA Conference on Plasma Physics and Controlled Nuclear

Fusion Research, Madison, Wisconsin, June 17-23, 1971.

6. N. H. Lazar and G. R. Haste, Plasma Physics 13, 1*33 (1971).

7. M. L. Andrews, H. Davitian, H. H. Fleischmann, B. Kusse, R. E.

Kribel and J. A. Nation, Phys. Rev. Letters 1^28 (1971).

8. L. G. Kuo-Petravic, M. Petravic, and C. J. H. Watson, Paper 2.^,

B.N.E.S. Nuclear Fusion Reactor Conference, Culhara Laboratory,

England, Sept. 1969.

9. A. H. Futch, Jr., J. P. Holdren, J. Killeen, and A. A. Mirin,

UCRL-73226, May 1971. (To be published in Plasma Physics.)

10. V. S. Crocker, S. Blow, and C. J. H. Watson, B.N.E.S. Nuclear Data

for Reactors, IAEA, Vienna, 1970.

11. See S. Glasstone, Sourcebook on Atomic Energy, p. 386, D. Van

Nostrand Co., Inc., 1950.

12. J. Rand McNally, Jr., Nuclear Fusion 11, 191 (1971); 0RNL-TM-3207,

(1970). 17

13- H. Postma, J. L. Dunlap, R. A. Dory, G. R. Haste, and R. A. Young, Phys. Rev. Letters l6, 265 (l966).

Ih. T. Kammash, Nuclear Fusion 11, 575 (1971)-

15. R. A. Dandl, J. L. Dunlap, H. 0. Eason, P. H. Edmonds, A. C.

England, W. J. Hermann, and N. H. Lazar, p. ^35* Plasma Physics

and Controlled Nuclear Fusion Research, Novosibirsk, USSR, IAEA,

Vienna, 1969.

16. N. C. Christofilos, W. C. Condit, Jr., T. J. Fessenden, R. E.

Hester, S. Humphries, G. D. Porter, B. W. Stallard, and P. B.

Weiss, Paper CN-28/A-9, Fourth IAEA Conference on Plasma Physics

and Controlled Nuclear Fusion Research, Madison, Wisconsin,

June 17-23i 1971-

17* E. M. Little, A. A. Newton, W. E. Quinn, and F. L. Ribe, Third

IAEA Conference on Plasma Physics and Controlled Nuclear Fusion

Research, Novosibirsk, USSR, IAEA, Vienna, 1969.

18. A. H. Futch, Jr., private communication, 1971-

19. H. Grad, Plasma Physics and Thermonuclear Research, Vol. 2, I89

(1963) Pergamon Press; H. Burkhardt, Nuclear Fusion 2, 1 (1962).

20. H. Margenau, Phys. Rev. 69, 508 (19^6); J. R. McNally, Jr.,

unpublished (1972).

21. J. Stickworth, z. f. Phynik l6k, 1 (1961).

22. J. J. Devaney and M. L. Stein, LA-^3 (1970), Nuclear Science

and Engineering h6, 323 (1971).

23. J. Rand McNally, Jr., submitted to Science, 1972.

2k. See discussion of DT reactors by D. Steiner, New Scientist 52,

168 (1971)i and H. Postma, Nuclear News, p. 57> April (1971). 18

25- S. K. Allison, J. Cuevas, and M. Garcia-Munoz, Phys. Rev. 120,

1266 (1960); V. S. Mikolaev, I. S. Dmitriev, L. N. Fateeva, and

Ya. A. Teplova, Sov. Phys. JETP, 13, 695 (1961).

26. Paraphrase from H. Postma, Nuclear News, p. 57> April 1971*

27. Fusion chain reactions involving C, N, 0, and Ne in supernovae are

discussed by J. R. McNally, Jr., submitted to Astrophysical Journal

(1972).

28. W. C. Gough, USAEC, private communication, Feb. 1972. 19

FIGURE CAPTION

Figure 1. The stripping cross section of H° and the charge

H" exchange cross section (o^) of H in Hg gas as a function

of energy. Note the charge exchange cross section decreases + a million fold at 1 MeV compared to 10 keV H energies. + +

Cross sections for D and T can be obtained readily by

using the graph at E/2 and E/3, respectively. 20

OfWL-DWO W- 1S9R ENERGY (keV) 400 800 1500 2500 nT« 10 100

EXPERIMENTAL TOBUREN et al. STIER AND BARNETT<°> BARNETT AND REYNOLDS*" WELSH ef o/!c) SCHRYBER

THEORETICAL • MAPLET0N(,>

^V^CX H* - 5

8 12 16 20 24 (xiO8) VELOCITY (cm/sec) Fig. 1