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starship-design: LINAC efficiency

Nels said these machines are usually quite inefficient, but did not say that
these machines must be constrained to be quite inefficient in the general
case. Of course, they are not, for their efficiency limitations are aspects
of their engineering rather than fundamental physical restrictions.

The efficiency problems connected with accelerators are engineering issues,
while laser efficiency is inherently limited by material characteristics. An
accelerator device has never been designed as a main mover, for all large
accelerators currently are built as instruments to investigate other aspects
of physics. An accelerator purpose built for a spaceship engine could
eliminate ohmic losses by use of superconductors throughout. Since it would
be made for continuous operation, the losses associated with the run up of
power levels would not be a factor. Since it would be made for operation at
a constant power level, and thus at a single excitation frequency, none of
the losses in inductive and capacitative elements need be tolerated, for
resonant cavities could be used as the frequency determining component.
Needless to say, the pump down cycle would not drain any power, for the best
vacuum you could ask for is all around it in space!

Of course, Van de Graaf and Wimshurst type high-voltage machines, which comb
static electrons off a moving wheel or belt, are not suitable for efficiency
comparisons. Frictional forces and wear involving moving parts should not be
a feature of spaceship drive components. In general, moving streams of
plasma manipulated only by electromagnetic forces, without any contact with
solid structural elements, offer extraordinary opportunities for engineering
optimizations. In contrast, the monochromatic light sources available to us
have no comparable possibilities, because they are explicitly bounded by the
physical characteristics of the material of which they are composed.
Efficiencies exceeding 50% are doubtful for solid state lasers, though
quantum discoveries continually surprise us.

With waveguides propagating a constant excitation frequency to alternating
linac segments, and the entire structure attuned to the fundamental
frequency, there would be every reason to hope for a 99%+ electrical
efficiency measured against ohmic losses. Waveguide and resonator surfaces
could universally be of superconducting material, built to tuned dimensions.
We could hope to make this machine firing off protons, among the most
efficient machines we will ever be able to built. (And a good thing, too,
since it might be among the biggest and costliest machines we'll build for a
long, long time...)

Johnny Thunderbird

Nels Lindgren wrote:
Ion accelerators (linacs, cyclotrons, betatrons, etc.) are usually
quite inefficient.  For example, last year i worked at a high current
vandegraff tandem that ran at 10's of MeV, and 10's of nA (so beam power
about .1 watt) - would anyone like to guess how much power it took to run
the thing?  And the accelerator is the size of a double-wide.