This is puzzling to me, since combustion-driven gas dynamic lasers have been
around since the early 1970s. When I was doing student labor at Purdue, the
guy who became my MSME major professor had me make his huge vacuum facility
operational so that he could finally get his GDL running (he never did, as far
as I know). But he had the nozzle array for it, built by Bell Aerospace. It
was about a foot long, and an inch and a half square. Its design power was
5,000 watts, CW. It would be no trick to build a 150 kW GDL for aircraft use,
burning Jet-A along with a little water and helium injection - and no cooling
issues at all. And it certainly wouldn't weigh as much as what they're talking
about here.
On October 24, 2020 at 10:28 PM, ken mason <laserpro1234@xxxxxxxxx> wrote:
This technology may very well be sequestered, I haven;t seen so much as a
diagram or illustration to better visualize what is going on.
This could be the breakthrough that turns lasers from fantasy/toy to a
practical device. An order of magnitude weight reduction is not trivial.
I suspect the cooling they talk about is inside the mirad of laser pump diodes.
Photon coupling efficiencies between the 808nm pump and
808nm absorption band in the fiber laser medium which has an extremely high
surface area to volume ratio easy to cool convectively and conductivity.
I've read articles about etching micro cooling passages in these laser diodes
increasing cooling efficiency and power density. I just don't see where the
rectivive index matching comes into the picture. How does the coolant get in
the lasing cavity, the resonator is usually directly attached to the crystal.
Keep looking for technical briefs coming out of some research lab. I'm excited.
K
On Sat, Oct 24, 2020 at 10:55 AM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
wrote:
Another interesting question: Is the solid lasing element fiber optic, or
something else?
On 10/24/2020 12:57 AM, ken mason wrote:
"Not clear if the coolant actually takes part in the lasing, or simply doesn't
interfere with it, though the latter seems implied."
If it were the former then it may be analogous to a dye laser so double your
pleasure, nothing wasted, sounds almost too good to be true.
I still need a diagram to see the configurations of all the components and
coolant paths.
K
On Fri, Oct 23, 2020 at 12:10 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
wrote:
https://spectrum.ieee.org/tech-talk/aerospace/military/fiber-lasers-face-a-challenger-in-laser-weapons
And now a new contender for high-power defense apps - "liquid lasers". General
Atomics is working on them. Per this article, these are apparently solid-state lasers
with liquid cooling passages through the lasing medium. Coolant is carefully matched for
refractive index with the solid medium, and coolant flow is managed to avoid turbulence.
Not clear if the coolant actually takes part in the lasing, or simply doesn't interfere
with it, though the latter seems implied.
Also mentioned that they're shooting for a ballpark couple hundred kilowatts output for
useful weapon applications, at lasing-device mass on the order of 5 kg/kw output. (A
DARPA goal of 150kw output at 750kg mass is mentioned for "a fighter jet",
presumably a F-35.)
Also mentioned that apparently they can usefully combine the output of multiple
individual devices, which implies very good phase control. Also implied that
the same is true of fiber optic lasers. With limits (in both cases?) on how
many devices can be usefully combined. Mike Griffin Congressional testimony is
mentioned in this regard.
They also mention "a high-density modular high-power lithium-ion battery system able
to store three megajoules of energy", presumably to allow periods of high-power
laser operation in between longer periods of charging from lower-power continuous
sources. These might actually have rocketry applications, for running pumps, depending.
Henry
On 10/15/2020 3:38 PM, Henry Vanderbilt wrote:
Bill,
I'm sure they're electrically-powered continuous-wave lasers that are being reported on
as in field-test in the tens of kilowatts power range. If I was sure they were diode
lasers however, I wouldn't have phrased that part "I presume". It's been a
while since I followed the technology closely.
I suspect Ken/Roxanna has it right; the technology involved is likely fiber
optic lasers. Which on hasty reading-up, can be pumped with diode emitters, so
at least I'm not totally off base.
My understanding is that Free Electron Lasers are pulsed, not continuous
output. And my impression is that while they're still being looked at for
defense apps, they're not the frontrunner these days. To the extent one can
trust an impression of an inherently sensitive subject gained purely from
reports in the open press, of course.
Henry
On 10/15/2020 3:06 PM, William Claybaugh wrote:
Henry:
You sure those are diodes? All the one’s I’ve seen were FEL’s.
Bill
On Thu, Oct 15, 2020 at 2:59 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
wrote:
Apply a potential difference to a conductive material, and you'll get a net migration of
electrons along the potential direction. "electric".
Oh, you wanted something more specific than that? :-) Current public reports
of tests and prototypes seem to cluster around what I presume are diode lasers
with continuous outputs in the tens of kilowatts range, mounted with
appropriate pointing and beam-forming on ships, aircraft, and large ground
vehicles. Some stories give the impression that the customers think another
order of magnitude of reliable power output (in the hundreds of kilowatts) will
give them a useful area defense against targets heavier than the small
rockets/shells and small drones commonly mentioned as current test targets.
Henry
On 10/15/2020 1:48 PM, roxanna Mason wrote:
electric lasers these days.
Elaborate on "electric"
Ken
On Thu, Oct 15, 2020 at 1:41 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
wrote:
Utility in evading missile defenses aside, one thing to keep in mind is, for
certain targets - among them ships - payload fraction is less important. A
hypersonic impact delivers TNT-equivalent energy for the entire vehicle mass
involved even with no actual warhead on board at around Mach 9, and the
equivalent energy of a 25%-of-vehicle mass TNT warhead at Mach ~4.5.
Given that the main penetrating-defenses advantages seem to be maneuvering
beyond an interceptor's ability to follow, and operating in atmosphere bands
where the interceptor loses effectiveness, effective anti-missile beam weapons
will likely negate much of those advantages and largely obsolete expensive
hypersonics. Which may well explain why USN (and USAF) seems very interested
in electric lasers these days.
Henry
On 10/15/2020 2:23 AM, George Herbert wrote:
OT...
It’s not much of a secret I do missiles and nuclear proliferation research and
analysis including threat systems analysis as a side job.
There are a few things you can do with hypersonic weapons. You can approach
targets in the inconvenient altitude and speed band where there’s too little
lift for normal fins and too much for unstreamlined exoatmospheric
interceptors. Midcourse defense gets very difficult... You can change
direction in hypersonic flight, allowing flightpaths avoiding defenses or
attacking simultaneously from different directions, complicating defenses.
Under some circumstances they can use trajectory changes in target approach to
maneuver out of interceptor engagement envelopes after interceptor burnout.
Against rapidly maneuvering targets there is a better target volume capability.
Under some circumstances the range is longer than a smaller ballistic
trajectory Maneuvering Reentry Vehicle can do.
They are much more expensive and harder to do than maneuvering ballistic RVs.
They can’t carry effective decoys, IR sensors see them thousands of kilometers
away, and avoiding defenses has to be done essentially blind because they can’t
credibly detect incoming interceptors with onboard sensors.
The payload fraction of ballistic RVs is about 0.5, of maneuvering RVs 0.3-0.4,
of hypersonic glide weapons 0.2 or less and Hypersonic cruise weapons 0.1 or
so. For the same payload they’re tremendously larger missiles to start with,
with corresponding ship, aircraft, submarine, truck, or silo capacity issues.
They make better sense as Chinese antiship Missiles than as US prompt
conventional strike missiles. The countries don’t have analogous needs.
-george
Sent from my iPhone
On Oct 14, 2020, at 10:19 PM, Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx> wrote:
attracted a lot of attention and hype but had no military value
whatsoever.
(For the sponsoring nation, that is -- it had considerable value to their
enemies,
since it pulled desperately-scarce resources away from things that *did*
have
real military value.) The recent hypersonics hype smells much the same to
me...
Henry
I guess the difference now is the "enemy/ies" are all on the same hypersonic
bandwagon.
Troy