Can a slanted magnetic field be created with this insulated stranded wire arrangement?

Question

I am considering building a small-scale experimental coil gun because I have an idea of how a magnetized projectile could be rotated while it is traveling through the coil(s), similar to how a bullet is rotated while it is traveling down a rifled gun barrel.

Before I start on this experimental project, I first want to find out from someone within the Electrical Engineering.SE community if creating a slanted magnetic field is even possible. I am neither an engineer nor a scientist, so I have a limited knowledge of Electromagnetism and of magnetic fields.

The idea I have is to create a slanted magnetic field within the coil(s) which I believe should rotate the magnetized cylindrical projectile (or a cylindrical permanent magnet) as it is traveling through the coil or through multiple coils.

I have created two drawings, shown below, which illustrate how I believe a slanted magnetic field should be created using this insulated stranded wire arrangement. I am planning to use insulated copper wire, which is enamel-covered 20 AWG 'magnet wire'.

I want to note that the first drawing shows just an exterior magnetic field, but there should also be an interior magnetic field.

Also, the hollow space between the wires is necessary because these wires will be wrapped around a PVC pipe and the projectile will be traveling through that PVC pipe. There will be a copper ring at each end of the coil and the ends of the wires will be soldered to those rings. A separate wire coming from each of these rings will be connected to a battery/DC power source.

I do not know if this slanted magnetic field will rotate the magnetized projectile as it travels through the coil(s), but it seems to me that the slanted magnetic field should create a circular torque upon the magnetic field of the magnetic projectile, and this circular torque should cause the projectile to rotate as it travels through the coil(s).

Can a slanted magnetic field be created with this insulated stranded wire arrangement?

EDIT

I thought about the issue brought up by user 'Andy aka' concerning the magnetic flux and I think this design change should resolve it. All the wires will be straightened out for a certain length at each end of the coil before they are soldered to the two copper rings.

Answer 1

One trick you can try to get the rifling you desire is to put "rotating" stages between the linear stages. Effectively, you would have a motor stator in between the linear stages (look up 3-phase motor stators on Google to get familiar with what I'm talking about). The basic rotator design would be 3 coils spaced 120 degrees apart with the magnetic flux pointing in the radial direction of the tube. You would then have a (modified) motor driver activating these coils in sequence. The projectile is essentially the motor rotor.

For clarity, the coils in the rotating stages are sequenced independently from the linear stages and, if power permits, would be left on before, during, and after the launch. In other words, you leave this static rotating field on the whole time independent of the linear stage sequencing.

However, all the rotating stages can be synchronized with each other and run from a common driver.

Answer 2

I first want to find out from someone within the Electrical Engineering.SE community if creating a slanted magnetic field is even possible. I am neither a engineer nor a scientist, so I have a limited knowledge of Electromagnetism and of magnetic fields.

No this won't work.

Your picture shows current in wires sloping upwards from bottom right to top left but, what you appear to have forgotten are the wires behind that slope from top right to bottom left that must form lines of flux that join to the lines of flux from the wires at the front. You can't create a helical shaped magnetic field without (for example) using ferrites forming a helix shape.

The idea I have is to create a slanted magnetic field within the coil(s) which I believe should rotate the magnetized cylindrical projectile (or a cylindrical permanent magnet) as it is traveling through the coil or through multiple coils.

This is the elephant in the room because you are expecting a magnetic field inside the wire arrangement. By virtue of Gauss's law, a tube that carries current (even if the tube is in fact a bunch of wires forming a circle) forms no magnetic field inside the tube. It's a well-known phenomena.

Here's an image I made some time ago: -

It shows a tube of current (in and out of the page). The current is nominally 1,000 amps but, that's not important. The important thing is the field colour within the tube; it's blue and indicates that the magnetic field is zero. You can get the student edition of QuickField for free if you want to try it yourself.

Answer 3

I first want to find out from someone within the Electrical Engineering.SE community if creating a slanted magnetic field is even possible.

1. Making a transient helical magnetic field is possible.

2. Even though possible, it may not help you to create spin in your projectile.

3. The mechanism you propose to create a helical magnetic field is not feasible.

A magnetic field with helical lines of force is a superposition of a field with circular lines of force with a field with straight lines of force axial to the circles.

As pointed out by TooTea, a magnetic field with circular lines of force has a non-zero curl. According to Ampere's Law of with Maxwell's correction,

$$\nabla \times \vec{B} = \mu_0\left(\vec{J} + \epsilon_0 \frac{\partial \vec{E}}{\partial t} \right)$$

\$\vec{J}\$ is the current density. Since presumably there will be no current through the bore of your coil gun, this will be zero. This leaves only option of creating a time-varying electric field. The fact that this electric field must be time varying is the reason why the circular field, and hence the helical field cannot be a steady state field.

I can think of at least three mechanisms for creating a time varying electric field which lies along the bore of the coil gun.

The first is to use a resistive solenoid. We usually want conductors to have low resistance. However, low resistance also implies low voltage drop. Since we want the voltage to drop across some length of the bore, using a resistive solenoid will create such an electric field. If the current in the solenoid is time varying, the electric field will also be time varying, and the circular component of the magnetic field will be created.

The second is to use two rings around the centerline of the bore of your coil gun. They form a capacitor. Have a circuit to charge this capacitor. While the capacitor is charging, a time-varying electric field will be created, and consequently the circular component of the helical magnetic field as well.

The third method is to use a toroidal coil (preferably without a magnetic core) such as a Rogowski coil. Feed it with a time varying current, and there will be a time varying electric field, and consequently the circular component of the helical magnetic field as well.

Now, we also need a magnetic field that is linear and axial to the bore of the rail gun. This is produced by a solenoid coil with an axis the same as the axis of the rail gun. Note that the twisted multiwire hollow core cable that is illustrated in the question is an example of a solenoid coil, although not a very efficient one. Contrary to Andy aka's answer, the center of that cable will have a magnetic field within it. However, the field will be weak because the pitch of the turns is very long, so there are few turns. However, the twisted insulated wires are NOT equivalent to the "tube" used in his magnetic field simulation. In the twisted wires case, the current does not flow only axially, but also circumferentially. In the case of the tube, the current only flows axially.

Once again, although these mechanisms can be used to create transient magnetic field with circular field lines, and consequently when combined with an axial field, to create a field with helical field lines, it is not clear how this will impart a spin to your projectile.

It is also not clear why you would need to have an axial component to your "rifling" section at all. If a helical magnetic field will impart spin, wouldn't a circular one impart spin just as well?

Answer 4

EDIT: As I seemed to understand the issue the same way as Andy, my statement that the answer is explained very well by Andy, the answer to your question is no. It's still no, the field would not be slanted - it would be coaxial to the coil - a poor solenoid. See answer from Math Keeps Me Busy.

The core of my answer is anyway not to detail the physics, but to provide links to the X of this XY problem - how to impart spin on a EM-gun projectile. And the comment I originally posted were:

But the core problem you're trying to solve have been worked on for many decades already. There are several patents to look into regarding electromagnetically imparting spin on the projectile of a rail or coil gun.

Electromagnetic projectile launcher with magnetic spin stabilization by Dan R. McAllister: patents.google.com/patent/US4449441A/en
Method and apparatus for spinning projectiles fired from a rail gun by Willaim F. Weldon: patents.google.com/patent/US5189244A/en Projectile for round bore electromagnetic launchers with spin produced or prevented by electromagnetic means by Robert M. DelVecchio and Emmanuel Aivalotis: patents.google.com/patent/US4741271A/en

Are some examples.