Gesu INTEROFFICE 9 aitsrenes

To E. L. Ginzton Date January 7, 1960
Subject Propulsion by Ejection of From Edward W. Herold
Positive and Negative Ions of
Equal e/m

You showed me a proposal of Prof. J. Lederberg, dated March 23,
1959, in which he suggested use of certain compounds which, if dissoci-
ated into positive and negative ions, would have equal ratios of
charge to mass. Thus, any acceleration or guidance method which
depends on this ratio would act equally on both charges. You asked
for comments, and suggested that Bill McBride consider the possibilities
in this idea. Bill and I have talked this over, but we do not believe
the idea is enough of an improvement, nor is it readily put into
practice. My own comments are as follows.

The problem of space vehicle propulsion by ion injection is
a most important one, and in my opinion will be solved in the next
decade. However, many propulsion experts claim that the ion or plasma
ejection is a simple and straightforward matter compared to the problem
of a power supply, which is truly formidable. Although I believe that
a plasma of positive ions and electrons is the simplest and best material
to eject, one should always keep an open mind as to other possibilities.

Prof. Lederberg's proposal is interesting but incomplete since
the major problem of how to get negative ions ‘is not solved. It is
well known that such ions are extremely scarce, and for very obvious
reasons. If energy is added to a molecule, it always tends to lose an
electron, i.e. to go in a direction of positive ionization. A negative
ion, which requires an extra electron is fundamentally less likely and
less stable. As a result little practical use can be made of negative
ions, and the achievable currents are many orders of magnitude below
those which would probably be needed for propulsion.

A second point which might be made is that the requirement
for equal e/m, for both positive and negative particles, is not necessary.
For example, there are acceleration methods which impart the same velocity,
in the same direction, to positive and negative charged particles inde-
pendently of their e/m (a crossed electric and magnetic field does this).
Even if there is a vast difference between the e/m of the positive and
the negative particles, space charge forces keep them together, and the
plasma moves with an effective velocity depending on the geometric mean
of the two masses. Thus, any acceleration of either particle results
in acceleration of the oppositely-charged particle as well.
 

E. L. Ginzton ~2- 1/7/60

I am not sure I understand all of Prof. Lederberg's suggestions,
particularly on use of compounds. For example, he proposes GeCl, but
the normal stable compound of Ge and Cl is GeCl, and I'm not suré how
one could get the former material. Furthermore, if one could get it,
when it dissociates, it must go to doubly-ionized Ge, i.e. Ge'’, in
order to balance the e/m of a negative-ion chlorine. The more likely
compound, GeCl,, does not fulfil his requirement at all, unless two
electrons are added to the system. It is possible I have not correctly
understood his proposal at all, but the comments in the two paragraphs
above still apply.

I return Prof. Lederberg's letter and attachment herewith.

Curt

EWH: mb E. W. Herold

cc: Bill McBride