PREDICTED BY GENERAL ELECTRIC
2. FUSION REACTORS
2.1 Scientific Feasibility
The prospects for demonstrating the scientific feasibility of man-made fusion power by the early 1980s are considered quite good. However, we must be careful as to how scientific feasibility is defined because it is a considerable step from that point to engineering feasibility. The criterion for this milestone depends on the method by which the thermonuclear burn is achieved, either by magnetic confinement of a low den- sity hot plasma, or by the inertial confinement of a high den- sity hot plasma inside an imploded pellet.
In the case of maqnetic confinement, we can use the Lawson criterion [111-1201. The appropriate energy balance is-given below for the D-T reaction, although it could be applied to other fuel cycles with proper modifications:
Electrical Energy Out = Electrical Energy In
where :
n ,nT are the deuterium and tritium fuel-ion densities, respectively D ;
<ovis the special averaged reaction cross- >
section for D and T at a fuel-ion temperature Ti ; T is the energy-confinement time ;
WDT is the energy released per D-T reaction ;
n
is the efficiency with which the thermal energy is converted to electrical energy ;k is the Boltzmann constant.
If we assume a 50:50 mixture of D and T , a 40 per cent conversion efficiency, 22.2 MeV<£ energy released per reaction,
<' v>
,
we find that the product and operation at the maximum-
T .of the fuel-ion density and confinement time must be approximately
1014 sec/cm3 at approximately 10keV. The corresponding num-
several larger experiments are planned in Europe, the Soviet
questionable whether such a system could be a commercial reactor.
In summary, the prospects for achieving a meaningful scientific demonstration of D-T fusion within the next five to seven years are quite good for TOKAMAK reactors. The situation is not so clear for the Mirror and Theta Pinch concepts, al- though work will still continue in those areas. Because of the optimism in the TOKAMAK area and since most of the advanced reactor design studies have been done in that area, we will narrow our scope of consideration to that concept for the rest of this report. This is only done because of the limited extent of this subsection; future breakthroughs in the other systems may make them more desirable than the TOKAMAK.
A similar treatment of scientific feasibility for laser fusion must take a slightly different approach. It is presently envisioned that pellets containing D-T would be imploded to high densities approaching p R 3 g/cm2, where R is the radius of the imploded fuel, and p its density. For reference purposes the density of solid D-T at one atmosphere is approximately 0.2 g/cm3. The burning process is initiated when a small central portion of the fuel pellet is compressed to roughly 0.3 g/cm3 raising the temperature to approximately 5 keV, thus igniting the fuel. The alpha particles deposit their heat in this region, and the burn front will propagate outward, causing about 30 per cent of the fuel to be "burned" before disassembly.
This would give a yield of 10" J/g. Such a reaction yields the following energy balance [III-1311:
Electrical Energy Out = Electrical Energy In
where :
G o = gain of the fuel -
- thermonuclear reaction energy released
.
internal energy of all the fuel if heated to ignition
EL = efficiency of laser -
- laser energy out electrical energy in
fR = fraction of electrical energy used to pump the laser
n
= conversion efficiency from thermal to electrical energy;fH = internal energy of imploded pellet energy required to heat imploded
pellet to a uniform temperature of 5 keV
E I = efficiency of implosion
internal energy - - of imploded fuel
laser energy in
Scientific feasibility could be defined as when the gain of the pellet
- - thermonuclear yield from pellet input energy from the laser
reaches a value of one. However, a more meaningful definition of scientific feasibility might be the point where all of the output energy is fed back into the lasers (fR = I), and a ther- mal conversion efficiency of approximately 4U per cent is assumed. The relationship between G p and E L is then given in Figure 111-12 [III-1311, along with an estimate of the effi- ciencies of glass and gas lasers. In the most optimistic case of a laser efficiency of ten per cent we see that a minimum value of G p required would be 25. The required pellet gain would rise to 500 to 1000 if glass lasers were developed to their estimated maximum-efficiency range. From these con- siderations it is clear why gas lasers have potential reactor applications, while glass lasers probably do not. However, there is a problem with the coupling of COz laser light
(10.6 micron) to pellets, and it is possible that inordinately high powered lasers will be required.
No simple picture can be given as to the state-of-the-art for laser fusion, but several recent reviews should be helpful to the reader [111-131 to 111-1351. Laser-induced implosions have already been demonstrated [111-134, and 111-1351 with
lo3
Jlasers; the next step is to increase the laser energy to l o 4 J to achieve pellet gains of approximately 0.1. Workers at the Lawrence Livermore Laboratory in the US project that this may
0.1 1 I
I I I 1 1 1 1 110
I I I100
I, loo - O
PRESENT DAY GLASS LASERPELLET
GAIN (Gp 1
Figure 111-12: Laser Efficiency and Yield Ratio Requirements for Inertially Confined Fusion Reactors occur in the 1975 to 1930 period through the construction of several progressively larger lasers, as shown in Figure 111-13.
The details of the plan can be found elsewhere [111-1361. Pellet gains of approximately 25 requiring 100 kJ laser systems are not anticipated to be achieved until 1981 to 1983.
In summary, the laser fusion program is clearly limited at this time by high-energy (100 kJ), short-pulsed (<I nsec), high-efficiency (5% to 10%) lasers. The rate of progress is significant, and projections are that scientific feasibility demonstrations should take place at about the same time as those for magnetic confinement, i.e., in the early 1980s.
2.2 Engineering Feasibility
In order to progress from scientific to engineering feasi- bility one must demonstrate that the release of fusion energy can generate net power on a reasonably reliable basis over long periods of time. This step will be a large one, perhaps as
large as the demonstration of scientific feasibility itself.
It is beyond the scope of this discussion to outline all of the
F i g u r e 111-13: L a s e r F u s i o n E n e r g y Y i e l d P r o j e c t i o n s
Other examples include fast energy switching for Theta
planning and implementation. (This approach would be aimed
F I S C A L YEARS
Figure 111-14: Summary of US ERDA Projected Costs t o a Fusion Demonstration Power Reactor via Different Logic Sequences
FERF/ T E T R