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Jordan aka Axiom

Fri Apr 16 2010, 07:52PM

Registered Member #2317 Joined: Thu Aug 27 2009, 01:45AM
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Posts: 27

Pulsed Solenoidal Accelerator - One

Below is a parallel projection view from the side of the barrel, two coil stages and the projectile. The system will have a couple of distinct attributes. It has a very low coil/projectile ratio and there is very little space between the inside of the coil and the outside of the projectile. This should translate to a excellent coupling.

Each stage will have one (1) 450V (500V Surge) 6800 uF Hitachi electrolytic capacitor with 25 mO ESR. Each stage will be switched by a 1200V 9900A (peak) SCR that will be triggered by an optical gate, including the first stage, as I intend to create a pre-accelerator most likely using a powerful spring.

The resistance of each coil is 26 milliohms, but I believe with liquid nitrogen cooling, that can be lowered to 7.8 milliohms. With an air core, the inductance of each coil is 3 microhenries but with the projectile mostly inserted, I believe it will be around 9 microhenries.

Given these data, the pulse will be 7,076 amperes and have a width of 864 microseconds. Each coil has 75 turns, so this gives us 530,700 ampere-turns per stage.

Below is a general perspective view of the system.

Finally, we have a cross-sectional view of the coil, barrel and projectile. You can see that the barrel is very thin relative to the projectile and there is very little space between the projectile and barrel. The ID of the barrel is 0.503" and the OD of the projectile is 0.500" with a -0.003" tolerance. So there will be between 0.0015" and 0.003" space between the projectile and barrel. The coil is a single layer of 0.054-inch-thick AWG 16 magnet wire.

I was hoping to get some input from those of you who are experienced in this field. My hypothesis is that I can get the most efficiency by creating a pulse width as short as possible (with the current up to, but not past the respective point of saturation of the projectile) such that I can fire the pulse when the projectile is as close to center as possible, since that is where the energy will be used most efficiently.

Traditional thinking includes having longer pulses at first and short pulses for later stages. This sounds reasonable but not necessary. Whatever the pulse width is for the last stage in your system, should be the pulse width for every stage. The difference, however, is that the pulse will be triggered progressively earlier with regards to the position of the projectile for the later stages. This is, at least, my hypothesis for this system.

The greatest unknown variable to me right now, is how strong of a magnetic field I can create without saturating the projectile. If someone could help me figure out how close I am to saturation of the projectile, my appreciation would know no bounds!

Thanks for your time.

klugesmith

Sat Apr 17 2010, 01:31AM

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

>>The greatest unknown variable to me right now, is how strong of a magnetic field I can create without saturating the projectile.

A first step:
From basic solenoid formula, what is the magnetic field at center of coil when there is NO projectile?
If more than 2T your projectile will surely be saturated when centered in the coil.
If much less than 1T you will be fine.

I think eddy current in the projectile will be a significant source of loss in your configuration.
But it opposes the coil current and tends to keep you out of saturation.

What is the pulse current rating of the capacitor?

Jordan aka Axiom

Sat Apr 17 2010, 03:20AM

Registered Member #2317 Joined: Thu Aug 27 2009, 01:45AM
Location:
Posts: 27

Ok, so here is the FEMM result. It looks like the field strength inside the coil is 6.4 to 6.7 Teslas with an air core. In other words, I'm way over-saturating the projectile, right? Also, here is the datasheet for my capacitors. Mine are the 450V 6800uF models.

It seems that I have collected the notion that for a given voltage, capacitance and resistance, the attractive force will be greater as the projectile is closer to the center of the coil. The only way I can see this being true is because of the fact that the projectile causes the inductance to increase as it enters the coil.

On the contrary, it seems that as more of the projectile goes past the center-point of the coil, the net forward force on the projectile will decline because of the part of the projectile that is being pulled back. Should the current ideally be shut off as the projectile is centered with the coil OR when the end of the projectile reaches the center point of the coil, such that NONE of the projectile goes past the center point of the projectile while the current pulse is on?

klugesmith

Sat Apr 17 2010, 06:56AM

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

1. Thanks for sharing your calculated numbers from the beginning. You've done more upfront analysis than many visitors here. But could benefit from practicing Internet information skills. Scan this forum for a solid hour, or use the search function. Archives from the early years have better information density -- fewer repetitions of silly basic questions and answers. Here is one thread pointing to a highly technical analysis:

2. You may want to double check your calculations, or explicitly invite review. Your FEMM result agrees with simple long solenoid formula B=u0*I*N/L. But a couple of formulas/calculators give L of about 10 uH (not 3 uH) for your air core coil. Effects of core saturation, mitigation of eddy currents, etc. have been discussed dozens of times on this forum.

3. Re. location for maximum force: sketch a chart of inductance vs. axial position of projectile center.
Axial force (for a given DC current) is proportional to the slope of that curve -- obviously zero when projectile is centered and inductance is maximum (by symmetry).
I think force will be maximum when displacement is -roughly- half the length of projectile and coil. You could measure the DC force curve in lab by standing a projectile on a weighing scale and holding a DC-energized test coil around it. Slowly lift the coil and chart the reduction of apparent weight. Test coil could be wound in same volume with much finer wire, to get measurable force with much^2 less current.

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