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LAN_403

Reaching

Sat May 06 2006, 10:32AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Okay, im totally newbe on this area so please forgive me my stupid questions

in the next few month i need a projectile accelerator with a very high velocity for experimenting and also for demonstrations.

since my coilgun attempts were in the lower velocity range of around 70m/s i want to give a railgun a try.

i have lots of high capacitynce electrolytics availiable 6x 12000yF 350volts and 3x 8200yF 350volts, so around 5,9kJ, ,

sure i dont want to use them all at first, maybe 2kj for beginning

what is the best arrange for the cap bank, parallel, or in series ?

anyone out there to help me with the rails? its very difficult here to get the right material for them, copper bars should be the first choice.
i buyed some 12x5mm copperbars on ebay a while ago, not very big i know, but usable for low power tests, say 2kj or something?
i have lots of HDPE lying around, a very strong material in every size and thickness up to 10mm.

so, but my main question is, how can i switch the amount of power. ok i have a few puk scrs with a current rating up to 10kA pulsed but i dont know if they can survive for long.

so i thought i should make the projectile out of 2 materials, say copper and iron so i can shoot the projectile into the railgun via a small coilgun,.

i hope my thoughts are not to stupid.

GODSFUSION

Sat May 06 2006, 11:07AM

Registered Member #157 Joined: Sun Feb 12 2006, 08:00PM
Location:
Posts: 76

There is no such thing as a stupid question, and always be assertive of yourself.

- wayne -

Cesiumsponge

Sat May 06 2006, 03:38PM

Registered Member #397 Joined: Wed Apr 19 2006, 12:56AM
Location: Western Washington
Posts: 125

I have been out of the HV loop for quite some time but I don't think things have changed too drastically in several years. If I am incorrect, please feel free to correct me.

Switching very high voltages at very high currents are best done through the a basic old-timer--the spark gap, or a modified form thereof. Search around and see what type of information you can obtain on trigatrons or the triggered spark gap. Most of these can be built reasonably close to shape and size as compared to commercial units, though usually less the fancier materials. Copper, brass, or tungsten carbide electrode materials should be fine and a PVC or lexan enclosure should work in a pinch, with additional safety shielding. Many of the commercial units also have the ability to plumb in gas like sulfur hexaflouride, or more easily obtainable nitrogen gas for more repeatable results. It really depends on how much effort and design you wish to put into it but ease of replacing and setting the electrodes is probably paramount if you plan on using it with regular duty.

They are obviously lossier than solid state switching, but they still reign supreme (and cheap) in the realm of ultra-high pulsed devices. Assuming even such solid state devices exist (I have seen several "super SCRs" in research papers for ultra-high pulsed power, but they were manufactured in-house), the chances are almost impossible for them to come into the surplus channels like Ebay...not for perhaps another decade. Many of the pulsed power capacitors and other esoteric high power engineering components collected by several large surplus companies come from places like LLNL when they dismantled their older laser facilities.

There is still plenty of work, information, or effort required on the amateur level in correctly obtaining a proper pulse shaping network in railguns as no matter what, it appears all current examples sofar have incredibly short pulses whereby the actual acceleration range of the armature takes place during the first few inches of the rails, resulting in exaggerated thermal loads and accelerated erosion where the projectile was accelerated. Most people short the capacitor bank to the rails and hope for the best. A shaped inductor is something that needs to be incorporated into the system to further tune and tinker with the idea. They are also present in all institutional and government accelerator installations.

For high density polyethylene, I would not recommend it as anything other than an insulative material. It isn't the most rigid thermoplastic out there but it's easily available. At lower power levels, it is probably sufficient enough but rigidity is of extreme importance when you start dealing with larger power levels. The "big boys" use laminar composite/metal and metallic sheathing for rigidity.

Plasma armatures are also probably your best bet for good amateur results. A sliding metal armature will need a tight fit throughout it's entire acceleration down the barrel and many forces are at work so it is unlikely that you'll get a good fit. Even with machining, a long, thin rail is hard to keep at proper tolerances and with so many bolts typical of such devices in assembly, torquing improperly can result in misalignment and areas where a metal armature can get pinched or arc needlessly. The addition of the additional static and kinetic friction present in such a concept is also detrimental to an amateur railgun as you don't have spare megajoules to begin with.

A plasma armature with a softer deformable plastic like the PE family sounds more ideal because the acceleration forces should compress the projectile axially upon acceleration, which of course spreads it out radially (in a concentric bore, as an example). I am not certain just how extensive this effect is in lower powered devices but it is also cheaper.

Self Defenestrate

Sat May 06 2006, 05:07PM

Registered Member #87 Joined: Thu Feb 09 2006, 01:36PM
Location: San Jose
Posts: 191

Don't worry about an scr, just use a pneumatic injector of sorts. Keep the power to the rails on and blow the projectile between them, it's its own switch. If you really want to use a coil gun for an injector, figure out a way for the iron to stay behind while the copper flys into the rails. You could build both the style injectors, incase one isn't working out too well

badastronaut

Sat May 06 2006, 07:24PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
Location:
Posts: 96

Hello, I would like to refer you to this thread

In it, you will find the answers to many of your questions, and along with the answers, it contains the reasons why the answers are what they are....

You will want to go with plasma armatures if you want speed and efficiency, though still probably less than 1%. If you do use a solid armature, you will probably have your rails about 2-4 inches long to stop friction from robbing all the KE.

Using plasma armatures at low energy levels is better because some of the thermal energy gets converted into kinetic energy.

You will want to use all your caps at once. HDPE is not good for plasma armatures due to the high peak pressures, but you can certainly try it.

I would not recommend using an inductor. The only reason one might use one is to reduce the peak current and or reduce the instantaneous di/dt so the switching equipment doesn't get damaged. Inductors are energy storage devices, they will not help with electromechanical energy conversion efficiency because they don't affect the amount of action. Real inductors might actually hurt performance because of the added series resistance.

Cesiumsponge

Sat May 06 2006, 07:24PM

Registered Member #397 Joined: Wed Apr 19 2006, 12:56AM
Location: Western Washington
Posts: 125

If you use a metal armature as the switching method, due to the nature of high voltage, it will form an arc from the rails to the front of the projectile before it even physically establishes contact with the rails. At lower â€œhigh voltagesâ€ (it being a relative term), plasma arc formation might not be so severe. Since you donâ€™t know what configuration to run your capacitor bank, I will just provide the unabridged version and you can pick and choose what applies.

Why this is bad:

a) Arc flash. Whereby a plasma channel forms, displaces, and heats up surrounding air around it very rapidly, along with converting solids like copper into an obviously much more volumetric gaseous form. There are plenty of studies done by OSHA and other similar agencies under the headings of â€œflash arcingâ€ or similar. Peak currents in amateur railguns easily match or exceed currents current found in industrial power equipment. This is all enclosed in a bore so you donâ€™t have to worry about having your clothes spontaneously combust, but the effects of this occurring in front of the projectile in fact decelerates your projectile or armature as you establish this arc at, or in front of the projectile. Ideally you want this occurring behind your projectile as a secondary source of acceleration. This explosive force has shattered and blown apart many a homebrew spark gap switch. That is a lot of force, and you honestly donâ€™t want this force working in front of your projectile.

a) It requires energy to establish this plasma arcing and thereby reduces efficiency. You arenâ€™t supposed to form arcing using a metal armature at all, ideally. The best designs have eliminated these altogether using projectiles under compression against the contact faces. Using the rails and armature as the primary switching method makes this impossible to avoid as a significantly high voltage source will establish an arc before it makes metal-to-metal contact in most any scenario. Likewise you are now forcing the railgun to accelerate a much less massive plasma front, and the more massive metal armature/projectile which only really has kinetic energy remaining from itâ€™s initial injector push. It is easier to magnetically accelerate something with the mass of a gas than a solid.

You donâ€™t have to take all these factors into consideration if you want to build a railgun for the sole purpose of bragging rights and accomplishing something technically awesome. A railgun, can in fact, be as simple as shooting a piece of metal across electrically live rails hooked to a capacitor bank. However, you can also simplify a magnetic motor to itâ€™s core constituents by wrapping a length of magnet wire around your thumb, hooking it up to a battery, and placing it over a ceramic magnet. There are a lot of refinements that can overall add to a more efficient device, if that is your goal. The inclusion of a spark gap switch can be done for next to no money by a frugal hobbyist and any component can be as elaborate as your wallet or imagination wants it to be.

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