Electron Orbit at the Null Point (DC sputter coating)

Question

This situation pertains to 'thin film sputter coating.'

Suppose I have some magnets in between a high voltage anode and cathode in a suitable vacuum. I understand that the electrons get trapped in the magnetic field lines and will orbit around them as they bounce back and forth between the magnets. I drew my artists rendition below for two cases. One with weaker magnets (left) and one with stronger magnets (right):

Question 1: What happens to the electron at the null point? This is where the magnetic field lines begin to change direction. What does the electrons orbit look like in this case?

my guess: as drawn below, the radius of this orbit should drastically increase, correct?

Question 2: When sputtering to apply thin coats, it looks like stronger magnetic fields produce a narrower wear on the target material. Why is this?

my guess: If my guess to 1 is correct (the electrons orbit radius near the null point is increased) then the same charged electrons that are following the magnetic field lines push the electron orbiting the null point from both sides. Since the field lines are longer than for a weaker magnet, this allows for more time to align the electron orbiting the null point with the center between the two magnets. This causes the distribution of electron/ion bombardment to be centered between the two magnets with less variance (narrower distribution).

Answer 1

In general you would use magnetic fields to steer an electron beam, not to accelerate it. Typically, sputtering does not need magnets. It uses heat to boil atoms off a source. The atoms fly everywhere and some hit the target.

A plasma sputtering system uses charged ions. Magnetic and electric fields are used to confine the plasma.

Without the electric field, your picture is more appropriate for Aurora Borealis. Or perhaps a particle accelerator, though they are not designed to produce a spiral path.

To make a moving electron go faster, there must be an acceleration with a component in the direction of motion. To make it go slower, there must be a component against the direction of motion. Any sideways component will change the direction without speeding or slowing the electron.

$$F = qv \times B$$

The cross product tells you that $F$ is perpendicular to $v$ and $B$. So the electron will follow a curved path without speeding or slowing.

Nothing special happens at the null point. The entire path is curved. This is just another point.

I might not have this right, but this is what I remember from an E&M course. A plasma has a surprising amount of shear strength. It rotates something like a rigid object. I cannot justify this.

You might expect the plasma to be shaped like a curved bar connecting the magnets. The entire plasma rotates, including the region around the null point.