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starship-design: Questions Regarding Relativistic Electric Thrusters

Hi all

A critical component of most of the interesting interstellar
propulsion concepts (e.g., fusion-powered rockets, antimatter-
powered rockets, beam-driven sails in the deceleration phase) is a
relativistic electric thruster.  This is not an "ion rocket,"
which typically has ion energies in the multi-kilovolt range and
ion currents in milliamps, but it is a high-energy particle
accelerator with an exhaust-particle energy in the range of multi-
megavolts to gigavolts (see the 1060 MeV proton energy requirement
for a thruster to decelerate a beam-driven sail, in my note of
9/20/96) and particle currents in the range of kiloamps to megamps
(2940 megawatts of beam power per kg of thrust, per my note of

I think it would be useful for us to start off by examining the
characteristics of these particle accelerators to help select
the most desirable configuration.  In this vein, I would like to
suggest a sample list of questions regarding these thrusters, to
draw out any further questions you all might think of.  Phrasing
the question right is a significant part of solving the problem.

The sample questions are as follows (I haven't tried to address
any of them, so some may not be well thought out):

1. What is the best exhaust particle?  Electrons, protons, alphas,
   etc.?  What is the best parameter to compare them by?
   a. What is the relative thrust/amp for the options, as a
      function of their energy in MeV?
   b. What is their corresponding exhaust velocity?
   c. What is their "Isp" (thrust per unit rest-mass flow rate)?

2. Is there an electron energy that gives the same performance as
   protons? (from the so-called "mass amplification" with
   increasing velocity)

3. Can we estimate relative thruster weight per unit thrust?
   Thruster size? (to compare the different options)

4. What disadvantages/advantages accompany motion of electric
   a. What is the effect on performance (e.g., exhaust velocity)
      of charging of the ship due to incomplete charge
   b. What is the effect on performance of charge neutralization
      effected by dumping oppositely charged particles (at zero
      velocity, say)? 
   c. Will the current of the exhaust beam generate a magnetic
      field strong enough to deflect interstellar protons (ionized
      by a stripping foil out in front of the ship) away from the
      ship, to save shielding?
   d. Could the electric charge from incomplete neutralization
      create an electric field, around a sharp point out in front
      of the ship, strong enough to deflect interstellar protons
      as in c, above?
   e. Could electrodes on the sides of the particle beam at the
      exit end of the accelerator be used to steer the "jet"
      (and hence the ship)?
   f. Is there a possibility of entrainment of interstellar
      protons by the particle beam to cause jet augmentation?
   g. What might be the damage caused by impingement of the
      very-high-powered particle beam on another object in space?

Please feel free to add to this list and/or criticize the
questions suggested above.  The list is not meant to be exhaustive
nor definitive, but to stimulate further thought and discussion.

Rex Finke  <DotarSojat@aol.com>