X-Message-Number: 13934
From: 
Date: Tue, 13 Jun 2000 15:16:30 EDT
Subject: Shock pumped laser for QND biological reader

Back on line for some weeks, I add some technical informations
about the laser system used in a Quantum NonDemolition
brain (or more broadly, biological organ) reader.

Energy source for the chemical laser.

I have said before that the chemical laser, the first element
in the three stage laser train, will be shock pumped metal
vapor desing. I have pointed out the analogy between the
shock generator and a very big rocket motor in the 32 000
tons thrust range. Here I must explain briefly how a rocket
motor works: Some propergols are burned in a combustion
chamber at high pressure and subsonic speed. At the
chamber exit, the presure drops by a factor near 1.8 and
the flow become both, sonic and turbulent. This is the
throat nozzle domain. Beyond that, the bell shape of the
nozzle allows a rapid expansion then near the nozzle exit
the flow is redirected in the motor axis to get the most
useful thrust.

The first expansion generates a depressurisation shock
and then the near exit redirection produces a
compression shock. Rocket designers try to play with
shapes and local sonic velocity so that both shocks
forms at the same place and cancel each other. If they
fall, to do that, there is first a decompression shock and
then a reconpression shock nearby down the gas flow.

In a laser the objective is to get an inverted "bad rocket"
with fist a commpression shock to pump atoms in an
excited state and then decompression zone where the 
asing action takes place. Such nozzles are first conical,
then near cylindrical on a short distance to form the
compression shock and then comes the fan out section
requested to form the deconpression shock. No surprise,
this is very bad for thrust, indeed if we want to extract
gas energy in a laser process, it can't be recovered in
the mechanical form of a thrust.

Another desing starts from a classical bell nozzle and
cut the redirection section. A device in inserted in the
gas flow just beyond the nozzle throat so that a
compression shock is formed here. By varying the
position, shape and diameter of the obstructing body
many configuration can be tested. This is attractive
for 3 reasons:

- Simple multiple testings in different conditions may
overcome a weak hydrodynamical computation capability.

- The compression shock forms at higher pressure
and this is best for laser pumping.

- The lasing domain is smaller and so needs smaller
mirrors and optical systems.

Yvan Bozzonetti.

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