Thermomagnetic burn control for magnetic fusion reactor

Apparatus is provided for controlling the plasma energy production rate of a magnetic-confinement fusion reactor, by controlling the magnetic field ripple. The apparatus includes a group of shield sectors (30a, 30b, etc.) formed of ferromagnetic material which has a temperature-dependent saturation magnetization, with each shield lying between the plasma (12) and a toroidal field coil (18). A mechanism (60) for controlling the temperature of the magnetic shields, as by controlling the flow of cooling water therethrough, thereby controls the saturation magnetization of the shields and therefore the amount of ripple in the magnetic field that confines the plasma, to thereby control the amount of heat loss from the plasma. This heat loss in turn determines the plasma state and thus the rate of energy production.

A tokomak magnetic confinement fusion reactor confines a plasma within a 
toroidal plasma region by the use of a group of field coils that each 
encircle the plasma region. When the plasma, which may consist of 
deuterium and tritium, is "ignited," the plasma generates heat by nuclear 
fusion. Some of the heat escapes the plasma region and may be used to 
generate electricity. Successful operation of the reactor requires that 
sufficient heat escape to prevent such a high plasma temperature that the 
plasma becomes magnetohydrodynamically unstable and causes complete loss 
of plasma. At the same time, excessive heat should not escape from the 
plasma that could quench the fusion reaction. A mechanism is required to 
control heat loss from the plasma to maintain it at a desired operation 
point. 
A major source of heat loss from the plasma arises from ion heat conduction 
due to ripple in the toroidal magnetic field that confines the plasma. 
Toroidal field ripple is the amount of variation of the toroidal magnetic 
field as measured along a circular path extending along the toroidal 
plasma region. The principal contribution to field ripple is the 
geometrical arrangement of the field coils which encircle the toroidal 
plasma region, and arises because of the spacing between of the outer legs 
of the field coils from one another. It is generally desirable to enable 
operation of the reactor with minimal field ripple to minimize heat losses 
during starting up of the reactor. A mechanism which enables controllable 
variations of magnetic field ripple, and which does not require large 
amounts of additional space within the already-filled space of typical 
magnetic confinement reactors, would be of considerable value. 
OBJECTS AND SUMMARY OF THE INVENTION 
One object of the present invention is to provide an apparatus for 
controlling the energy state of the plasma in a magnetic confinement 
fusion reactor. 
Another object is to provide an apparatus for controlling magnetic field 
ripple in a tokamak type of fusion reactor. 
Another object is to provide an apparatus for controlling the heat flow out 
of magnetic confinement fusion reactors. 
Another object is to decrease the ripple in a magnetic containment fusion 
reactor during startup, and then to increase the ripple to enhance heat 
propagation from the plasma during normal operation. 
Another object is to provide a method for controlling the energy state of a 
magnetic confinement fusion reactor. 
In accordance with one embodiment of the present invention, an apparatus is 
provided for controlling the plasma energy state in a tokamak-type fusion 
reactor, which requires minimal additional space within the reactor. The 
apparatus includes a magnetic shield structure lying radially between the 
plasma region and the toroidal field coils of the reactor, with the 
shields constructed of a magnetic material which has a 
temperature-dependent saturation magnetization. The apparatus also 
includes a mechanism for controlling the temperature of the shields, to 
thereby vary their influence on the magnetic field ripple in the reactor 
and therefore the heat loss from the plasma. 
The shield can include primary shield sectors lying directly between the 
outer leg of each toroidal field coil and the plasma, so that the magnetic 
ripple produced by the primary sectors counters the ripple produced by the 
spaced toroidal field coils. The range of values of ripple achievable can 
be increased by means of additional secondary magnetic shield sectors 
placed in the spaces between the primary shield sectors. An entire shield 
structure which includes the primary and secondary shields, is a largely 
self supporting keystoned shell. In optimal designs, the attraction 
between adjacent magnetic shield sectors overcomes the outward force due 
to radially adjacent toroidal field coils, so that a relatively modest net 
inward force is created which keeps the sectors under compression. 
Temperature control of the shields can be produced by heat transfer 
fluids, such as cold water for rapidly cooling the shields and steam or 
pressurized hot water for heating them. 
The novel features of the invention are set forth with particularity in the 
appended claims. The invention will best be understood from the following 
description read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The figures show a tokamak fusion reactor 10 which includes a plasma 12 
lying in a toroidal plasma region 14 which extends around a main toroidal 
axis 16 of the reactor. A group of toroidal field coils 18 each encircle 
the plasma region 14 to produce an intense toroidal magnetic field that 
confines the plasma to the plasma region. As indicated in FIG. 3, the 
reactor may have a limited number of toroidal field coils 18a, 18b, 18c, 
etc. such as ten of such coils. The inner leg 20 of each coil lies 
adjacent to the inner leg of other coils, but the other legs 22 of the 
coils are spaced from one another about the outside of the reactor. Each 
of the coils may be constructed of superconducting material and maintained 
at a superconducting temperature, and may carry large currents to produce 
a very intense magnetic field that confines the plasma within the plasma 
region. The plasma 12, which may consist of deuterium and tritium, is 
assumed to be at a sufficiently high temperature and density to produce a 
self-sustaining fusion reaction, so that heat can be withdrawn from the 
plasma for generating electric power or other purposes. 
The strength of the magnetic field produced by the toroidal field coils 18 
deviates from axisymmetry, or in other words, varies in strength at 
positions angularly spaced about the main axis 16 of the toroid. The 
magnetic flux line 24 shown in FIG. 3 with exaggerated waviness, 
represents the flux due solely to the toroidal field coils 18. It can be 
seen that the field coil flux lines 24 tend to bulge inwardly between the 
angular positions of the field coils 18. Toroidal field ripple is a 
measure of the amount of variation of the magnetic field along a path 
which is at a constant distance R from the main toroidal axis 16 and which 
is at a constant height Z (FIG. 2) relative to the midplane of the torous 
on which the toroid centerline 24 lies. Specifically, the ripple, as a 
function of R and Z is given by 
##EQU1## 
where .DELTA. is the field ripple at a particular distance R from the main 
axis of the toroid and a particular distance Z from the midplane of the 
toroid, and B.sub.max and B.sub.min are the maximum and minimum values of 
the toroidal field along this path. 
The toroidal magnetic field ripple can be reduced by the use of a shield 
structure 29 which includes primary magnetic shield sectors 30 that lie 
between the plasma region 14 and the toroidal field coils 18. (as well as 
other shield sectors 44, which will be discussed hereinafter). The 
magnetic shield sectors such as 30a,30b lie inside the outer legs of the 
corresponding toroidal field coils such as 18a,18b. The effect of the 
magnetic shield sectors 30, which are formed of ferromagnetic material, is 
to add a nonuniformity of the character shown by the flux lines 32 in FIG. 
3, wherein there is an outward bulge at each coil 18. By employing primary 
shield sectors 30 of proper strength and shape, their effect can be to 
largely counter the ripple produced by the main field coils 18. While 
quiescent primary shields or shield sectors, such as 30a, 30b, etc. can 
minimize the ripple in a reactor, which is useful particularly during 
start-up when minimal heat losses from the plasma are desired, the simple 
quiescent shields are not especially useful in increasing ripple in a 
controlled manner so as to increase energy losses from the plasma to 
maintain the plasma at a desired operating level. 
In accordance with the present invention, the magnetic shield sectors 30 
are utilized not only to minimize ripple in the magnetic field when this 
is desired, but are utilized to control the amount of magnetic field 
ripple and therefore of heat losses from the plasma. This is accomplished 
essentially by constructing the shield sectors 30 of a material having a 
significant temperature-dependent saturation magnetization, and by 
utilizing a mechanism to control the temperature of the shields. 
The saturation magnetization of a ferromagnetic material (material which 
permits a large magnetic flux density for a specific magnetizing force) is 
the maximum flux density or magnetic induction produced in the material 
when the magnetizing force increases virtually without limit. A typical 
ferromagnetic material such as iron has an intrinsic saturation of about 
1.5 tesla (14 kilogauss). However, the saturation magnetization normally 
decreases as the temperature of the material increases, and at the Curie 
temperature the ferromagnetic material reverts to a paramagnetic state and 
thereby produces only a small magnetic flux for a given magnetizing force. 
The high magnetic flux density necessary to confine the plasma will 
saturate any magnetic material, so the saturation magnetization level of 
the magnetic shield sectors determines their effect on the field ripple. 
Thus, by controlling the temperature of the magnetic shield sectors 30 of 
FIG. 3, the amount of ripple reduction that they produce can be 
controlled, to thereby control the net field ripple and therefore the heat 
losses from the plasma. If the shield sectors 30 are heated to more than 
their Curie temperature, they have no effect in reducing ripple, and the 
ripple is of the original magnitude as shown at 24. 
The temperature of the shield sectors can be rapidly adjusted in a 
hydraulic manner, by utilizing fluids to heat and/or cool the shields 
rapidly, and by utilizing valves or the like to control the flow of such 
fluids. FIG. 1 shows a conduit 36 for carrying a heat transfer medium such 
as water to the shield 30, so that the fluid can pass through passages 38 
in the shield sector and flow out through another conduit 40. The 
temperature of the shield sector can be rapidly adjusted by operating a 
valve 42 to control the amount of fluid passing through the shield. The 
shield sector can be rapidly cooled by passing cold water through the 
conduits, and can be rapidly heated by passing superheated steam or 
pressurized hot water through the conduits. 
The primary shield sectors 30 are useful in reducing magnetic field ripple 
from a moderate amount to perhaps 1/10th as much. When the primary shield 
sectors are heated to beyond the Curie temperature, the original ripple 
and corresponding heat losses occur. However, such ripple and heat loss 
may not be sufficient in the control of the reactor. Enhanced field ripple 
and therefore cooling of the plasma can be produced by utilizing a group 
of secondary shield sectors 44 which are located inbetween the angular 
positions of the main field coils 18. The secondary shields 44a, 44b, etc. 
are constructed of ferromagnetic material, and their location causes a 
ripple or bulging of the magnetic field at the same locations as for the 
toroidal field shown at 24. However, if low ripple is desired the 
additional bulging caused by the secondary shield sectors 44 can be 
counteracted by utilizing larger primary shield sectors 30, or by 
disabling the secondary shield sectors 44, for example, by heating them. 
As in the case of the main shield sectors 30, the temperature of the 
secondary shield sectors 44 are closely maintained, as by the use of heat 
transfer fluids to cool and/or heat them. As shown in FIG. 1, valves 46 
can be placed along conduits 48 leading to the secondary shield sectors to 
control the flow of such heat transfer fluids to thereby control the 
temperature and therefore the magnetization saturation of the secondary 
shield sectors. 
In operation of the reactor with primary shield sectors 30 lying inside the 
toroidal field coils 18 and secondary shield sectors 44 lying in the space 
between the field coils, the primary shield sectors 30 may be maintained 
at a relatively low temperature during start up of the reactor to minimize 
field ripple. At the same time, the secondary shield sectors 44 may be 
maintained at an elevated temperature, which may exceed the Curie 
temperature of the material of the secondary shields, to avoid an increase 
the field ripple. As the desired energy level of the plasma is approached, 
the temperature of the set of primary shield sectors 30 may be raised 
and/or the temperature of the set of secondary shield sectors 44 
decreased, to increase field ripple and therefore increase heat losses 
from the plasma to prevent further temperature rise of the plasma. If the 
plasma temperature begins to rapidly rise or fall, heat losses from the 
plasma may be rapidly increased or decreased by rapidly increasing the 
temperature of one set of shield sectors while decreasing the temperature 
of the other set of shield sectors. The provision of secondary shield 
sectors 44 enables the production of much greater field ripple than could 
be achieved by only varying the temperature of the primary shield sector 
30, to thereby permit greater control of plasma temperature. 
The secondary shield sectors 44 are useful not only to enhance control of 
field ripple and therefore plasma energy state, but also to facilitate 
support of the primary shield sectors 30. The primary shield sectors 30 
are subjected to large magnetic forces tending to draw them radially 
inwardly in an optimized design where the outward force is 
overcompensated. By the use of a substantially continuous ring of 
alternating primary and secondary magnetic shield sectors, which are 
preferably fastened together, the shields support one another against 
radially inward movement. This is useful in minimizing the amount of 
additional structural supports within the reactor, where the space is 
already very crowded. 
The shield 29 as a whole, which consists of the primary and secondary 
shield sectors, not only helps control the energy state of the plasma, but 
is also useful as a radiation shield that prevents the escape of high 
energy neutrons along the outer portion of the plasma and also serves to 
minimize heating of the toroidal field coils 18. As shown in FIG. 2, the 
magnetic portions of the shield such as primary shield sector 30, extend 
about halfway around the plasma. The complete radiation shield 50 of which 
the magnetic shield sectors are a part, will normally be required to 
extend along at least the top and bottom of the reactor to also minimize 
the escape of neutrons that are dangerous to people. Since it is 
envisioned that the magnetic shield sectors will be composed of materials 
that are suitable for the radiation shielding function, the ripple 
reduction function of magnetic shield sectors 30, 44 can be achieved 
without occupying additional space. It also may be noted that a heat 
transfer blanket 52 may be utilized immediately around the plasma region 
14 to transfer most of the high-energy heat produced by the plasma which 
can be utilized for generating electricity and other purposes. By 
utilizing magnetic field ripple control to control the loss of heat from 
the plasma 12, the shield sectors 30, 44 avoid the need for additional 
penetrations of the heat transfer blanket 42 to maintain such control. 
FIG. 1 shows a simplified control system for controlling the amount of 
magnetic field ripple in the plasma 12 by controlling the temperatures of 
the primary and secondary shield sectors 30, 44. A controller 60 monitors 
the energy state or temperature of the plasma 12 by the use of a sensor 
62. The sensor 62 can be a device such as a proton-recoil neutron detector 
which measures neutron flux from the plasma to indicate the energy state 
thereof, and delivers its output to an input 64 of the controller. The 
temperatures of the shield sectors 30 and 44 are monitored by 
thermocouples such as 66 which deliver signals to inputs 68 and 70 of the 
controller to indicate the temperatures of the shields. When the 
temperature of the plasma increases beyond a desired operating point, the 
controller can deliver signals over outputs 72 and 74 to the valves 42, 46 
that control the flow of heat transfer fluids to the shields. For example, 
if it is assumed that the plasma creates sufficient heat to heat the 
primary and secondary shields above their Curie temperatures, and if 
minimal additional heat losses from the plasma are desired, then coolant 
may flow through only valve 42 to cool the primary shield sectors 30, 
while the valves 46 may be kept closed to avoid cooling the secondary 
shield sectors. If the temperature of the plasma begins to increase to 
above the desired level, then the valve 46 may be quickly opened to permit 
cooling water to flow therethrough and cool the secondary shield sectors 
44 so as to increase ripple and therefore heat loss from the plamsa. At 
the same time, the valves 42 may be closed to permit the primary shield 
sectors to increase in temperature. More rapid response can be provided by 
connecting a source of superheated steam or pressurized hot water to flow 
through the valves 42, to more rapidly heat the primary shield sectors 
than they would be heated merely by heat generated by the plasma. Cooling 
of the plasma can be accomplished in a corresponding manner by opening the 
valves 42 to the flow through of cooling water to rapidly cool the primary 
shield sectors, while flowing steam or pressurized hot water through the 
valves 46 to rapidly heat the secondary shield sectors. Such control is 
useful in terminating plasma burn without sudden loss of plasma. 
One design of a reactor in accordance with the invention utilizes Carpenter 
"32" magnetic material for the primary and secondary shield sectors 30, 
44. This material has a Curie temperature of 200.degree. C. and a room 
temperature value of saturation magnetization of 1.06 Tesla. The shield 
sectors may be formed of laminations, to minimize eddy current losses from 
rapidly changing magnetic field, although the high magnetic flux density 
necessary to confine the plasma will normally saturate the magnetic 
material. 
The reactor design utilized a ten coil tokamak with a plasma center located 
a distance C (FIG. 2) of 5.2 meters from the main axis 16 of the tokamak. 
The plasma outer edge was located a distance D of 6.5 meters from the axis 
16, while the center of the return leg of the field coil was located at a 
distance E of 10.1 meters from the axis. Both the return leg 22 of the 
coil and the middle of the magnetic shield sector 30 had a thickness of 
one meter. The inside of each shield sector 30 was located a distance F of 
7.6 meters from the toroidal axis. The following table shows the variation 
in magnetic field ripple at a location 80 which is 5.85 meters from the 
axis 16, or in other words, halfway between the center and outer edge of 
the plasma. 
TABLE I 
______________________________________ 
Temperature of 
Saturation Magnetization 
Ripple 
Primary Shields .degree.C. 
Of Shield Material Tesla 
% 
______________________________________ 
0 1.1 0.22 
20 1.06 0.24 
60 0.94 0.32 
100 0.74 0.42 
150 0.44 0.52 
200 0 0.71 
______________________________________ 
The table shows that a temperature excursion of the primary shields from 
room temperature (20.degree. C.) to the Curie temperature (200.degree. C.) 
increases the ripple by a factor of about 3. 
Thus, the invention provides an apparatus for use with a magnetic 
confinement fusion reactor, and especially of the tokamak design, which 
facilitates control of the energy state of the plasma. This is 
accomplished by utilizing a shield structure formed of magnetic material 
for controlling the amount of ripple of the magnetic field in the plasma, 
and by controlling the magnetic effect of the shield. A shield structure 
formed of ferromagnetic material which has a considerable temperature 
dependence of the saturation magnetization can be utilized together with a 
means for controlling the temperature of the shield. Primary shield 
sectors can be utilized which are located between the plasma and the outer 
legs of the main field coils, to reduce magnetic field ripple produced by 
the spacing of the outer legs of the field coils from one another. 
Fluid-flow cooling and/or heating of the magnetic material can be 
employed, to enable rapid change in field ripple in a relatively simple 
manner. In addition to the primary shield sectors, secondary shield 
sectors can be utilized between adjacent primary shield sectors, with the 
secondary shield sectors controlled in temperature to enable even more 
extensive controllable variation in magnetic field ripple. The primary and 
magnetic shield sectors are preferably fastened to one another to form a 
ring that enables the shield sectors to support one another against 
movement by the large magnetic forces produced by the field coils. The 
magnetic shield can serve not only for ripple control, but can also serve 
as a portion of the necessary radiation shield of the reactor. The shield 
enables plasma control in a relatively simple manner without taking up 
large additional amounts of space within the reactor. The ripple control 
is useful not only in a fully operating reactor, but also in presetting 
the operating point of a research magnetic fusion device, even one without 
reacting fuel. 
Although particular embodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and variations may 
readily occur to those skilled in the art and consequently, it is intended 
that the claims be interpreted to cover such modifications and equivalents 
.