large coil gun / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

large coil gun

first previous 2 3 4 5 next last

Move Thread

LAN_403

Uspring

Mon Feb 23 2015, 09:59AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

DerAlbi wrote:

Who here actually could do some advanced FEM or even knows what the cross correlation of Coilcurrent and potential coil force means physically.

A number of us. The statement about cross correlation is somewhat opaque. Cross correlations are usually used in statistics. Integrals over products of functions in the context here should be named differently.

Signification

Tue Feb 24 2015, 08:39PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

DerAlbi, This is my first attempt at applying the concept of cross correlation with coilgun parameters (the two you specify), but I will try to mathematically connect the two:
We are concerned with these two given cross correlation coilgun functions:

Function 1: "Coil current"
Let's call it I(t) this could be a couple of things. Of the two in mind, I will assume it means STEADY current, i. Not a pulse flowing through the "coil".

Function 2: "potential coil force"
This one's a bit more complex. First let's assume this energized coil creates a steady magnetic FORCE field (B-field), and add the constraint that the only significant B-field is that which occupies the inner core (barrel) and protrudes beyond each end for about 1/2 the length of the barrel. This is what we will consider the VOLUME of your "potential coil force" field. Since we must primarily consider FORCE, how is force mathematically related to the B-field? Consider the -magnetic field energy density- of this B-field volume--call it 'E'. From physics, E is proportional to the square of the field strength B; specifically:
*** E = [1/(2*uo)] * (B^2) {joules per cubic meter} ***
We'll just say "E is proportional to the square of the B-field intensity" To get at the desired force, F, requires a directional derivative. Because we are only considering the direction along the barrel axis (say, the x-axis), this becomes a single variable derivative. Mathematically, the force may now be written as the rate of change of the square of B, or,

F[b] = (k) * d/dx(B^2) Newtons; where k is a constant.

=============================

Finally, The cross correlation functions are: I(t)= i (Amps) and F[b] = (k) * d/dx(B^2) Newtons.

If a specific case defining the variables of these functions are given, we could plot the force exerted on a "particle" (of negligible effect on B) by your "potential coil force" as it Lags along the coil axis in the 'force field' created by your "coil current".

DerAlbi

Wed Feb 25 2015, 02:06AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

There is no analytical solution to this problem. Inventing Force-Field is more something for StarTreck

Coil force behaves like this
F(x):= -a*x^(1+n1) * exp[ b * |x|^(3+n2) ] with
a: depends on the current you have while you make a projectile position (=x) sweep.
b: depends on your coil gemometry
n1,n2: correction factors, depending also on coilgeometry. should be close to 0.
If you normalize this function to a maximum of 1 you get the percentage of force that a coil can do. You basically get a graph where the coil is most efficient because the same current gives you the most pullforce. a,b,n can be determined by LeastSquares. of a FEM simulation.

The goal is now to find a realizable (Current-)function that (when quared and normalized also to 1) maximizes the cross correlation at zero. Then you got the highest energy transfer you can have. Of yourse you get a y-mirrored Stepfunction (going from 1 to 0 at x=0) for the current, that cant be realized. So one can throw in a resistive loss term (proportional to IÂ³) and maximize efficiency too.
Of course then the current will be 0. So what you do is actually maximizing Efficieny^n * Force.CurrentÂ²Integral. with n > 1..3 whatever priority you give to efficiency.

But seriously: with SCR designs... you have virtually no way for current pulse shaping. So its not even worth thinking about it. if you get 2% eff or 4%... who cares. Youtube cares only for design, technical stuff doesnt matter as long one destrys stuff that is fragile anyway

BigBad

Wed Feb 25 2015, 12:31PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

You would have a point except... multistage. Even some suckback isn't directly lossy, only to the extent it indirectly creates coil and other resistance losses, but resistive losses, as a percentage, go down as speed increases.

basically with coilguns multistage > all other tricks

DerAlbi

Wed Feb 25 2015, 05:55PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

You are totally right

Actually multistage is kind of the only thing where this is advantageous usable.. multistage halfbridges, awwww <3

Uspring

Thu Feb 26 2015, 09:40AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Probably a faq: Couldn't a high efficiency coil gun be made with a transmission line creating a traveling magnetic pulse? Would require a lot of caps, but electronics would be simple.

DerAlbi

Thu Feb 26 2015, 04:32PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Here we go. Have you done any Thinking before proposing this? (if so, why dont you at least try to answer it yourself and present your thoughts?)
First of all a transmission line propagates electromagnetic waves. Thats a magnetic field entangeled with an electric field.
-> So what exactly do you think would be the propellant force ? At least pushed or pulled? Magnetic or electric?
-> how do you think to jump on a wave that travels near light speed? (You really think thats like suring on water??)
- have you thought about which frequencies yould you use ? At multiple THz... such as light - you can kick electrons and apply a small force on reflecting surfaces. Since you know that there infact are standong towers of steel and iron which are used to transmit medium wave radio without beeing crushed.......

......
- even more interesting: what makes you think this would be specially high efficient??? If you have this intuition, why you hold back the scientific basis for this in your post? let us know what you think and why you think.

Thats exactly what i was raging above -.-
If thats a faq, then we should include "how to open dors".

Shrad

Thu Feb 26 2015, 06:33PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

Uspring wrote ...

Probably a faq: Couldn't a high efficiency coil gun be made with a transmission line creating a traveling magnetic pulse? Would require a lot of caps, but electronics would be simple.

that's my suggestion of magnetic pulse compression with a hybrid coilgun/railgun where the solenoid would be the rail and the projectile would short the solenoid to compress the magnetic pulse

you could also use a coaxial deionized water pulse capacitor using the breakdown voltage of the capacitor itself as a spark gap for the thing

the difficulty would be to machine the solenoid rail with turns spaced closely enough to achieve a dense flux

EDIT:

for reference

having talked with the guy (who would actually love this forum) I think the idea is not that bad and if someone would be able to do it he would be in the lot

DerAlbi

Thu Feb 26 2015, 06:40PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Shrad, what you describe is could be interesting - can you draw a pricture just to make sure there is no missunderstanding?
And no, thats not Usprings version

What you describe is in no way a transmission line for EM waves.

What i dont see in a Rail-/Coil-gun hybrid, how the projectile would look like. Railguns (commonly) dont use ferromagnetic projectiles while Coilguns depend on it.

Shrad

Thu Feb 26 2015, 07:16PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

this https://books.google.be/books?id=zmYrAAAAYAAJ&pg=PA157&lpg=PA157&dq=coaxial+water+capacitor&source=bl&ots=dYeRP9k-Oi&sig=l4Xdv27NyYk_9W7xqrreZCaOST4&hl=fr&sa=X&ei=P2fvVP-oGtDQ7AbwgoHIAg&ved=0CGAQ6AEwDQ#v=onepage&q=coaxial%20water%20capacitor&f=false states 1,7cm of transmission line length per nanosecond of discharge

due to the small value of L and high dI/dt it seems feasible to discharge megawatts in L and generate enough magnetic field during the discharge time

my setup would consist of two spiral coils in parallel, separated by an insulating material, with a coaxial capacitor surrounding it

connection of the outer capacitor plate to the red solenoid is done at the exit side, connection of the green solenoid is central plate body

it could be practical to be able to screw the red solenoid and insulator material in the grooved barrel for easy replacement

a preliminary coilgun of small power could be used to trigger the discharge without using the capacitor breakdown voltage, but water is self healing so it would be convenient

don't use that near your TV

first previous 2 3 4 5 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob