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Zeus

Wed Oct 28 2009, 12:17AM

Registered Member #2316 Joined: Tue Aug 25 2009, 03:04AM
Location: Bendigo, Australia
Posts: 107

I have been reading about laser ablation accceleration where a large part of a projectile is
vaporised with a high power laser. The projectile is contained in a barrel with an enclosed and
transparent end. As the projectile is ablated the material that has been disintergrated
expands and propelled along the barrel.
I am interested if anyone has attemped this.
Also if anyone has access to a high power pulse laser and wishes to try thsi I have a fair amount
of infomation on this topic.
I am not confident that this thread so if a moderator would like to let me know I
would appreciate it.

Z28Fistergod

Wed Oct 28 2009, 03:34AM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

That sounds similar to this:
http://science.howstuffworks.com/light-propulsion1.htm

The NIF also uses a similar concept to compress a fuel pellet in hopes of initiating fusion.

I have a 60J ND:Glass laser that is almost complete. If it weren't for a certain BX-24 micro processor being so crappy, I'd be done by now. I'll have to add this to the to-do list.

Dr. Slack

Wed Oct 28 2009, 01:24PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Why?

I can understand why ablation acceleration might be used to accelerate stuff for various sorts of fusion ignition (NIF, bombs and the like), but if you want the back end of a projectile to vaporise and expand at M or GWatts, then making it out of cordite or similar and just burning it real fast is about 10 orders of magnitude cheaper, lighter and smaller than packing a laser for the purpose.

Unless it's just a "because it's there" sort of project.

Zeus

Wed Oct 28 2009, 11:18PM

Registered Member #2316 Joined: Tue Aug 25 2009, 03:04AM
Location: Bendigo, Australia
Posts: 107

Cordite is nothing compared to this. Also it is not pratical nor is it simple.
So called fast bullets travel at 500 to 1000 meters per second, with laser ablation
acceleration 140 kilometers per second is possible.
Also I like the sound of a 60j Nd:Glass pulse laser. I can PM info if anyone
would like.

Wed Oct 28 2009, 11:47PM

Registered Member #2372 Joined:
Location:
Posts: 62

Where did the 140 km/s come from? The fastest things that I know of are flyer plates driven by ultrahigh current pulsed power systems and those are around 40 km/s.

Z28Fistergod

Thu Oct 29 2009, 02:14AM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

Dr. Slack wrote ...

Why?

Unless it's just a "because it's there" sort of project.

I don't think I would start a laser project just for this purpose, but as long as I've got the laser, the tube with a window seems pretty simple so why not try it.

Zeus

Thu Oct 29 2009, 02:25AM

Registered Member #2316 Joined: Tue Aug 25 2009, 03:04AM
Location: Bendigo, Australia
Posts: 107

dugg wrote ...

Where did the 140 km/s come from? The fastest things that I know of are flyer plates driven by ultrahigh current pulsed power systems and those are around 40 km/s.

That is a computer model. I don't have any lasers (not counting half a dozen diodes
lasers) so that is why I am posting this, so someone can have a go.

DYI

Wed Nov 11 2009, 12:11AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
Location:
Posts: 62

The LASER required to achieve something like this, even for a very lightweight projectile, would be absolutely mind-boggling. I don't have enough knowledge on the optics side of things to give any useful numbers, but a decent pulse LASER might get 10% efficiency from capacitors to light. Even if your ablation is very efficient and achieves 50% propulsive efficiency, you're still stuck at 5%.

Now at anything under about 1000m/s, you're just wasting money by using unconventional propulsion methods. Considering that at least 2km/s is quite feasible (and at 30%+ overall efficiency) with ETGs, using laser ablation below that point is questionable. Combustion light gas guns (burning high pressure/liquid oxygen/hydrogen mixes) have reached up to 4km/s. If railguns and LGGs aren't your cup of tee, you can start thinking about LASER ablation at this point. At this point, you need to sink 8kj of KE into each gram of mass you accelerate, which translates into at least 160kJ from you capacitor bank to accelerate a single gram using LASER ablation. It's a valid concept, but only useful when you want something to go REALLY fast. And of course, you'd need a new barrel every shot, but that's pretty standard for current high speed accelerators, and easier than the "reloading" procedures for hypervelocity railguns (which may require new rails installed) or LGGs, which require you to machine the mangled remains of the piston out of the barrel after every shot.

Of course, there's always the "because it's there" rationale, and I'll not discourage anyone from trying.

Wed Nov 11 2009, 02:15AM

Registered Member #2372 Joined:
Location:
Posts: 62

Ok, I heard some new info about this at a conference. So they shoot a very thin target that is like 1mm in diameter with a big laser (Nike at NRL) and they get it to 700km/s. These things are really small that they are shooting and the distance they travel is less than 1mm. They are trying to use them for shock compression fast ignition (its an inertial confinement fusion thing). The research is mostly being done in Japan at Gekko XII and at NRL on Nike.

here is the abstract

klugesmith

Wed Nov 11 2009, 05:17AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

[edit] Since the topic is "acceleration" -- y'all might like to know that in Dugg's reference, and mine below, new machines are using laser ablation to accelerate bulk matter at about 10^13 g's. That's a bit more than the surface gravity of neutron stars.

[/edit]

I have seen similar numbers in a lecture and some papers about NIF, the facility mentioned in this thread by Z28Fistergod.

NIF is designed to radially implode 2mm hollow spheres lined with frozen D-T mixture.
From page 25 of

,the implosion velocity needs to reach 300 to 500 km/s.
The shell accelerates to that speed in about 5 ns. Fusion ignition is supposed to occur when the radius is about 1/30 of its initial value. [Density factor 27000, obviously requiring an almost perfectly symmetrical implosion. Indirect heating by X-ray flux confined in a tiny box is key.]

A great success would be 10 MJ of fusion yield (later I'll figure out how much DT must burn) from 1.8 MJ of 350 nm laser energy on target. That takes 4 MJ of 1050 nm light from the amplifiers, which are pumped by flashlamps with over 300 MJ from capacitor banks. So much for "break-even". See

for more info.

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