Page 73 - High Power Laser Handbook

44 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Chemical Lasers 45

relaxation rates in order to achieve population inversions. Pimental, who
was a major early worker in the chemical laser field, identified several
reaction types of chemical lasers (see Table 3.1).
3
Noteworthy about the reactions in Table 3.1 is that most require
that one of the reactants be a free radical or that an energetic photon
(or electric discharge) be available to produce the reaction of interest.
In practice, many of these reactions are “electric” lasers augmented
chemically. For example, in the case of the photolytic iodine laser, the
required ultraviolet (UV) photon is considerably more energetic than
the resultant iodine atom–derived photon. In a few cases, it is possi-
ble to produce the radicals through purely chemical means, such as
combustion-driven thermal dissociation of fluorine (F ). It is also
2
sometimes practical to minimize the number of required radicals by
exploiting chemical chain reaction approaches.
More recently, one must also include net chemical reactions in
liquid phase, such as

–
2H O + 2KOH → 2HO + 2H O + 2K + (3.1)
2 2 2 2
Followed by
1
–
Cl + 2HO → O ( ∆) + H O + 2 Cl – (3.2)
2 2 2 2 2
This process produces the metastable excited oxygen electronic state,
or the singlet delta; this state is the basis of COIL devices, which
transfer energy from the singlet delta to iodine atoms. Other analo-
gous excited metastable species of interest include nitrogen chloride
(NCl) singlet delta and nitrogen fluoride (NF) singlet delta. Only the
COIL devices manage to directly produce the excited species energy
source, though in a sense even they involve a radical of sorts—the
–
O H in solution.
2
This chapter’s emphasis is on high-power CW mixing devices.
HF and DF devices are taken as a general example and treated in some
detail, and then COIL devices are considered. Finally, very brief discus-
sions of other types of chemical lasers are presented. Readers interested
in more details than the brief treatments in this chapter should consult
5
4
6
7
Gross and Bott, Stitch, Cheo, and Endo and Walter, all of which
contain more detailed discussions and comprehensive reference lists.
3.3 Hydrogen Fluoride and Deuterium Fluoride Lasers
The hydrogen halide laser (HF and DF) was one of the early chemical
lasers to be demonstrated and one of the few lasers of any type that
could be scaled to megawatt-class average powers. These molecular
lasers lase based on vibrational and rotational transitions. Figure 3.1
shows a specific example of a DF* laser hardware layout, including
where the particular reactions occur. The layout is shown to scale,
with an adjacent drawing of the laser mixing nozzles magnified by a