What's this small source of glowing blue plasma next to Psyche's Hall effect thruster for? What does it do and why does the plasma curve and connect?

Question

PV Magazine's NASA’s 1.5 billion-mile Psyche asteroid mission will be solar-powered includes the image below which is PIA24030 and the relevent part of the NASA caption is:

The photo on the left captures an operating electric Hall thruster identical to those that will propel NASA's Psyche spacecraft, which is set to launch in August 2022 and travel to the main asteroid belt between Mars and Jupiter. The xenon plasma emits a blue glow as the thruster operates. The photo on the right shows a similar non-operating Hall thruster. The photo on the left was taken at NASA's Jet Propulsion Laboratory in Southern California; the photo on the right was taken at NASA's Glenn Research Center.

The right image shows the business end of a large Hall effect thruster with an annular opening from which the bright glow of plasma can be seen in the left image.

Immediately above the thruster is a small cylindrical device with a small central opening, and the image on the left shows a similar but more purplish glow from it. If we follow the glow out of this small device it suddenly takes a 90 degree turn towards the ion engine's "exhaust plume" and joins the big glow from the engine, with a color gradient from purplish to blue.

At least that's how it appears to me.

Question:: What's this small source of glowing blue plasma next to Psyche's Hall effect thruster for? What does it do and why does the plasma take a right-angle turn and connect to the main engine's glowing plasma "exhaust plume"?

A solar-powered Hall thruster uses xenon gas as propellant. Image: NASA

Original image linked above.

Answer 1

It is the hollow electron emitting cathode used to produce the plasma. The electrons are moving within an electric and a magnetic field so they are moving on a circular path. The electrons are hitting atoms emitting the blue light. So the path of the electrons is visible.

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Psyche will actually use Hall thrusters, not ion thrusters, but I started with ion thrusters because Hall thrusters are a lot harder to explain. Hall thrusters don’t use high voltage grids, but do use an electron discharge to produce the plasma. In this case, the hollow cathode is outside the thruster and the electron collector (remember it’s called the anode?) is inside. The electrons are very light and fast, and all the power we apply would just go into the electron current flowing to the anode and blue glow if that was all we did. Instead, we apply a magnetic field to force the electrons to go in circular orbits around the center of the thruster. The magnetic field impedes the electron motion to the anode, which allows the ions that are generated in this discharge to be accelerated by the field between anode and cathode. The ions are too massive to be affected by the weak magnetic field, and are shot out of the channel to form the beam and make thrust. This circular electron motion is why Hall thrusters have that annular channel full of glowing plasma you see in all the photos of these thrusters. Since the acceleration happens within the plasma, the ions can drag enough electrons from the discharge with them to automatically neutralize the charge build up that I described above for ion thrusters.

A Hall thruster uses an electron discharge to create a plasma—a quasi-neutral collection of positive ions and electrons—not unlike what goes on in a fluorescent lamp.

The thruster includes a hollow cathode (negative electrode), placed outside the thruster body, and an anode (positive electrode) positioned inside a ring-shaped discharge chamber. If these electrodes were all there was, the power applied to the thruster would just go into making a current of electrons flowing from cathode to anode, emitting some blue glow along the way. Instead, a Hall thruster applies a radially directed magnetic field across its discharge channel.

The electrons emitted by the cathode are very light and fast. So this magnetic field impedes the flow of electrons to the anode, forcing them instead to go in circular orbits around the center line of the thruster. The positive xenon ions that are generated inside the discharge chamber accelerate toward the cloud of circling electrons, but these ions are too massive to be affected by the weak magnetic field. So they shoot straight out in a beam, sweeping up electrons along the way. The ejection of that material at high speed creates thrust. It’s not much thrust—equal to about the weight of a few quarters—but applied steadily for months on end, it’s enough to get the spacecraft zooming.

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