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Sound Engineering: The roaring HSX engines

Until fusion energy is widely available, the scientists running fusion experiments will need some hefty machinery to power their plasma adventures. In this edition of Sound Engineering, we visit the basement of Engineering Hall, where a group of large engines drums up the 11,000 watts of energy needed to run the Helically Symmetric Experiment, or HSX. You’ll hear graduate student student Gavin Weir explain how the HSX power plant works, and you’ll also hear just how incredibly loud the engine room can get. 


Scott Gordon: The University of Wisconsin-Madison is home to three major fusion experiments, but what does it take to actually power one of them? Gavin Weir, one of the graduate students working at the College of Engineering’s Helically Symmetric Experiment, or HSX, took me down to the project’s engine room in the basement of Engineering Hall. Weir’s going to explain how the massive power plant for the HSX works, and then you’re going to hear what it sounds like at full blast.

[Engine noise in background]

Gavin Weir: These are the motor-generators. We’ve got 18 motors running right now that each have a one-ton flywheel on them, and we’ll spin those up to about 1,800 RPMs, and we’ll flip a switch in a controls so that instead of using the motors to spin up the one-ton flywheels, they’ll start to suck energy from the rotation, and they’ll do that over a very fast time scale, so that the sudden braking power is turned into a large voltage. That’s where the 11,000 amps comes from. It’s similar to regenerative braking in a hybrid or regenerative braking in a train, or in an elevator, all those things use the same principle, but this just happens on a very time scale. So that fast time scale means a big spike in current, so it goes from zero to 11,000 amps in about a fifth of a second—very fast.

[Engine noise in background]

Gavin Weir: When we really get humming we’ll be at about 1,800 RPMs, and we’ll be shooting for 11,000 amps. When that happens, it gets incredibly loud in here and you can’t really talk to anyone, so it’s fortuitous that we aren’t spun all the way up right now.

Scott Gordon: Upstairs, near the actual device, research scientist Simon Anderson and graduate students Tom Dobbins and Jason Smoniewski are preparing to fire the HSX.

Gavin Weir: One of the physics questions that Jason’s currently working on is whether there’s a particular resonance with a biased electrical field. He’s got a targeted experiment that he’s working on and he’s altered our other diagnostics in order to answer his question. That’s what he’s working on right now. So we find a question and design an experiment to answer that question.

Scott Gordon: When the proper output level is reached, a set of contactors close, and that allows all those amps to pass through to the HSX device. That’s the loud click you’re about to hear.

[Loud engine sounds, clicking sound of contactors closing]

Scott Gordon: The one disadvantage of working so close to the HSX? Sometimes it’s hard for the graduate students to concentrate. 

Gavin Weir: The graduate student offices are right down that hall, so when we’re really roaring, we can hear the motors from our own offices. Yeah, it’s kind of rough.

Scott Gordon: For more information on the HSX, visit

[Loud engine noise.] 

Scott Gordon