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starship-design: Dirtside Fusion Energy

I was thinking on why I find fusion (in-system) drives so much easier to
invent, than fusion power reactors. Fusion drives are simpler machines.
Harnessing their power, for conversion to usable electricity, is a much
harder problem. My approach to fusion is to use the technique which has
demonstrated nuclear fusion in the laboratory since the 30's, hitting a
target with a beam of fast ions. The principles I adopted were to use
accelerators to produce fusion, and to avoid neutron production in the
primary reaction, and to suppress secondary reactions. Since the beam is so
fast, the target can be in any physical state, any temperature, or any
degree of ionization. All that matters is what is in its nucleus, which is a
sitting duck for the ion beam, so you will have fusion. By selecting the
appropriate species for the ion beam and the target, you can keep neutrons
from being produced, and you can have clean fusion, which is easier to live
with. The proton-boron and proton-lithium reactions are clean. The reaction
is linear in shape, with a gaseous target, which progressively absorbs the
ion beam in fusion reactions down the beam.

A tube of flowing hydrogen, terminated by a divergent region in a high
transverse magnetic field, which divergent region is bounded by electrodes
orthogonal to the field, forms a magnetohydrodynamics (MHD) generator. Thus
if any events happen in that tube which produce ionization of the hydrogen,
from energy being added to it, that energy will largely be recovered in the
MHD generator region as direct current. If the tube carrying the hydrogen is
surrounded by a boiler consisting of an annular vessel of water, any events
that happen which add heat to the hydrogen, from energy added to it, that
energy will largely be recovered as heat within the annular boiler. This
energy can be recovered by flash evaporation of the superheated water
against the blades of a turbine, which turns a generator.

If you add energy to the hydrogen by means of a beam of lithium ions, much
of the energy it took to accelerate the ions of lithium can be recovered
from either the MHD generator, or the steam power plant. Power plants which
use recondensing, triple expansion steam turbines achieve typical Carnot
heat efficiencies approaching 30%. The single large-scale MHD power plant I
know of, in Russia, operates at about 40% thermal efficiency. (Your car is
perhaps 15% efficient.) So about a third of the energy introduced by the
beam of lithium nuclei can be recycled. Except the lithium produces nuclear
fusion in the hydrogen, which requires the calculation to be modified.

The Earthbound fusion power reactor is thus characterized by a long, linear
shape to absorb its power. The hydrogen effluent from the pipe, still no
doubt too hot for turbines, has its energy scavenged
magnetohydrodynamically, by flowing through a transverse magnetic field
which diverts its charged components to electrode plates. It is then
condensed in a liquifaction plant, fractionally distilled off the helium
which is the sole fusion product, and recycled. Heat absorbed along the
length of the pipe is removed by supercritical water, which is flash
evaporated into steam to turn its own turbine. The electrical output from
the generator is combined with the MHD power.

Fusion does not require extremely fast collisions, nor exotic accelerators.
Since simple electrostatic potential difference will boost ions to the speed
needed to produce fusion, and since such potential difference can be
generated with near perfect efficiency, accelerator design to generate
nuclear fusion can be optimized. The Cockcroft-Walton rectifier stack is a
device which generates high voltage electronically with high efficiency,
already. Accelerators are presumed to be very lossy machines, but they don't
have to be. As electrical devices, they are subject to rules which are very
well understood. Since they do not involve thermal processes which have
inherent loss, there is opportunity for optimization at every point of
energy dissipation, so their engineering can work toward achieving
efficiency levels above 99%. No process using heat can meet any such goal.
Electrical motors and generators of large scale routinely exceed 95%
efficiency in continuous operation. These figures include copper losses,
iron losses and friction losses, none of which need be involved in
accelerator operation. We know how to work with electrical machines in
intimate detail, so the prospect of an extremely efficient accelerator is
quite feasible.

Fusion power in a pipe may be easier to handle. If the fusion reaction is
extended linearly, the heat intensity at any point decreases. A long, skinny
fusion reaction may be attenuated in heat intensity, to the point at which
it can be held in a pipe. For a given ion beam intensity, the amount of heat
any given section of the pipe must absorb, can be controlled by changing the
flow speed of the target gas. Higher speed has the effect of lengthening the
reaction zone, thus cooling the pipe. If this post isn't considered to be
strictly on topic, consider it a test rig for a space drive, or actually a
starship power plant.

Johnny Thunderbird