# EMP Halbach Array

Let's say I have a horizontal facing halbach array with equal north and south poles aligned one after the other, and a large circular gap in between them. If I fire an EMP gun through it, wouldn't the waves going through the array cause a repulsive force?

I'm speaking in terms of the Laplace Force Jumping Wire demonstration for my hypothesis, thinking that if the wires could have a replusive force against the magnets, then shouldn't an opposing magnetic field do the same?

I'm not a scientist or anything, so please go easy on me. It's just an open question, I'm just trying to learn. I don't understand calculations, so please just refer back to me in plain and concise English.

## 1 Answer

Let's say I have a horizontal facing halbach array with equal north and south poles aligned one after the other, and a large circular gap in between them

It is very unclear from your description what you even mean. All halbach arrays have an equal distribution of north and south poles on either side, but they enhance the field strength on the strong side and cancel it on the weak side. There is nothing special or particularly advanced or mysterious about them. They're common every-day objects that are ubiquitous in nearly every household, you probably have several on your fridge right now. Flip one of the thin flexible style fridge magnets so the other side is facing the fridge. It will not stick. This is because it is a halbach array.

Regardless, it is unclear what you mean between 'a large circular gap between them'. Between what? The poles? Then it is no longer a halbach array, as the poles must be continuously distributed to have their flux enhancing and canceling effects. Gaps between the poles would render it no longer a halbach array.

The poles also can't he aligned with each other, as a halbach array requires each magnet (imagine cube shaped magnets) be have their fields oriented 90 degrees from the previous magnet in the chain. And they must always rotate 90 degrees in the same direction, either always clockwise or counterclockwise. Like this:

The arrows indicate the orientation magnetic field/poles. You can't have the north and south poles aligned with each other, that's just a stack of magnets.

I almost think you're trying to describe a wiggler/undulator, but you didn't say halbach arrays, as in more than one, so again, it is unclear what you are imagining and what gap or poles you are talking of that can even be unique to something that can still be called a halbach array.

Maybe you mean there is a hole drilled down the chain, lengthwise?

If I fire an EMP gun through it

Why does it have to be an EMP gun? If it 'fires' something in a direction, then it must be an EMP made up entirely of electromagnetic radiation (vs one primarily in the form of an electrostatic or magnetic field, which are near field and do not radiate). Are you imagining one of those anti-quadrotor/drone rifles? Those are just weaponized radio transmitters. And so would any EMP gun that can be called a gun (can radiate the pulse somewhat directionally). Regardless, electromagnetic waves are electromagnetic waves. Being high amplitude in a short duration (aka, in a pulse) makes no difference here. They behave the same whether in a pulse or not. Anything is a 'pulse' if you're using long enough time scales.

wouldn't the waves going through the array cause a repulsive force?

No, it wouldn't.

I'm speaking in terms of the Laplace Force Jumping Wire demonstration for my hypothesis, thinking that if the wires could have a repulsive force against the magnets

The wires don't have a repulsive force against the magnets, the magnetic field created by the current flowing through the wire does. It is repelled by the magnetic field because the wire has an opposing magnetic field.

then shouldn't an opposing magnetic field do the same?

That is why the wire is repelled. And yes, opposing magnetic fields repel, as demonstrated by any pair of permanent magnets or the very demo you're describing.

As for why the electromagnetic pulse will not create a repulsive force, it doesn't matter if you have a halbach arraoy, or several, or any magnets arranged or shaped in any which way, or have the pulse moving through or in or beside them in any combination you want to imagine.

This is because electromagnetic waves do not interact with magnetic fields or electric fields. If they did, then magnets would interact with light, which is an electromagnetic wave. They do not. At least classically, electromagnetic waves are the vacuum solution to electromagnetism, which means that they are simply superimposed on any other solution. So if there are permanent magnets or magnetic fields (or electric fields for that matter) present, they have no effect on electromagnetic waves nearby, as those waves are simply added (superimposed) on top without interacting with the electric or magnetic fields. Electromagnetic waves don't even interact with each other. For this same reason, light doesn't interact with other light, it can only can be superimposed (interfere constructively or destructively) with other light waves And light is just very high frequency electromagnetic radiation. Put another way, radio is just a color of light far far below red.

The wire jumping demo is between a magnetic field and charge carriers, specifically moving ones, in this case, electrons.

It is important to not get confused here:

1. Electric fields are only created by electric charge, and electric charge is only carried by charge carriers, particles like electrons, protons, or ionized atoms (atoms missing or possessing more electrons than they have protons in their nucleus - but this is just being carried by electrons and protons with extra steps). Only charge carriers can interact with electric fields. As these fields are both produced by, and only interact with electric charge.

2. Magnetic fields are created by moving charge carriers. Like electric current moving through a wire, or the oscillating spherical harmonics/atomic orbitals of electrons surrounding the nuclei of atoms (which is ultimately responsible for ferromagnetism). Or put another way, there is an inertial reference frame in which any magnetic field will become purely an electric field, and like wise all frames but that one, that same electric field will look like a magnetic field. Only electric charge that is moving relative to a magnetic field will experience a force from that field.

3. Electromagnetic radiation is another name for electromagnetic waves, and these do not carry any electric charge and thus do not interact with electric or magnetic fields. Electromagnetic waves are produced not by electric charge, nor even electric charge moving, but by electric charge accelerating. A constant DC current through a wire does not radiate any electromagnetic waves (with the exception of re-emission of incident ones like light, or black body radiation, neither of which depend on electric current and occur with or without it) except briefly when the current is turned on and again when it turns off, as these are when the electrons are actually being accelerated. Radio antennas radiate electromagnetic waves by wiggling (accelerating) electrons back and forth. So do those anti-drone rifles. The EMP created by a nuclear bomb is because it really accelerates a lot of charged particles very fast.

So:

Electric fields exert a force on static (non-moving) charge, and are created by static charge.

Magnetic fields exert a force on moving charge, and are created by moving charge.

Electromagnetic waves don't exert a force at all, and are created by accelerating charge.

While Maxwell's equations forbid any interaction between electromagnetic waves and magnetic fields, Maxwells's equations are merely the classical limit to quantum electrodynamics, and there are some non-classical interactions that have been predicted and observed between electromagnetic waves and magnetic/electric fields, but none of them produce a force nor do they happen at field strengths practically achievable.

The one interaction between electromagnetic waves and magnetic fields only occurs when said magnetic fields are of sufficiently high strength. By 'high', I mean very very high - above $$\ 10^{9}\$$ Teslas. Fields this strong really only exist near magnetars, a special class of neutron star with magnetic fields between $$\ 10^{9} \$$ and $$\ 10^{11} \$$ Teslas. In such extreme fields, the vacuum itself becomes birefringent. This does not produce a force, repulsive or otherwise, however.

• Thank you for your fantastic answer. I always come back to reread your answer every now and again, and I've learned so much from you. Commented May 9, 2022 at 6:01