Z-Pinch: Powering Up Fusion in Huntsville

by Paul Gilster on June 26, 2012

The road to fusion is a long slog, a fact that began to become apparent as early as the 1950s. It was then that the ZETA — Zero-Energy Toroidal (or Thermonuclear) Assembly — had pride of place as the fusion machine of the future, or so scientists working on the device in the UK thought. A design based on a confinement technique called Z-pinch (about which more in a moment), ZETA began operations in 1957 and began producing bursts of neutrons, thought to flag fusion reactions in an apparent sign that the UK had taken the lead over fusion efforts in the US.

This was major news in its day and it invigorated a world looking for newer, cheaper sources of power, but sadly, the results proved bogus, the neutrons being byproducts of instabilities in the system and not the result of fusion at all. Fusion has had public relations problems ever since, always the power source of the future and always just a decade or two away from realization. But of course, we learn from such errors, and refined pinch concepts — in which electric current induced into a plasma causes (via the Lorenz force) the plasma to pinch in on itself, compressing it to fusion conditions — continue to be explored.

Image: The ZETA device at Harwell. The toroidal confinement tube is roughly centered, surrounded by a series of stabilizing magnets (silver rings). The much larger peanut shaped device is the magnet used to induce the pinch current in the tube. Credit: Wikimedia Commons.

In today’s Z-pinch work, current to the plasma is provided by a large bank of capacitors, creating the magnetic field that causes the plasma to be pinched into a smaller cylinder to reach fusion conditions (the axis of current flow is called the z axis, hence the name). Here we can think of two major facilities used in nuclear weapons effects testing in the defense industry as well as in fusion energy research: The Z Machine located at Sandia National Laboratories in New Mexico and the MAGPIE pulsed power generator at Imperial College, London. And lately a Z-pinch machine called a Decade Module Two has made the news.

As explained by an article in The Huntsville Times back in May, researchers at Marshall Space Flight Center and their colleagues at the University of Alabama at Huntsville will have a Decade Module Two (DM2) at their disposal once they finish the process of unloading and assembling the house-sized installation at Redstone Arsenal. Originally designed as one of several modules to be used in radiation testing, the DM2 was the prototype for a larger machine, but even this smaller version required quite an effort to move it, as can be seen in this Request for Proposal issued back in August of 2011, which laid out the dismantling and move from L-3 Communications Titan Corporation in San Leandro, CA.

The DM2 got a brief spike in the press as news of the acquisition became available, but note that the 60-foot machine is not expected to be powered up any earlier than 2013 and will not reach break-even fusion when it does. But this pulsed-power facility should be quite useful as engineers test Z-pinch fusion techniques and nozzle systems that would allow the resulting plasma explosions to be directed into a flow of thrust to propel a spacecraft. Z-pinch and related ignition methods have been under study for half a decade, but Huntsville’s DM2 may move us a step closer to learning whether pulsed fusion propulsion will be practical.

Image: Z-pinch engine concept. Credit: NASA.

The fusion power effort includes not just MSFC and UAH but Boeing as well. Jason Cassibry (UAH) talks about its ultimate goal as being a lightweight pulsed fusion system that could cut the travel time to Mars down to six to eight weeks, and he’s quoted in this story in Popular Mechanics as saying that “The time is perfect to reevaluate fusion for space propulsion.” Working the kinks out of Z-pinch would be a major contribution to that effort, and we can hope the DM2 installation may lead to the design and testing of actual thrusters, though right now we’re still early in the process.

The Popular Mechanics article goes on to explain Z-pinch’s propulsion possibilities this way:

Cassibry imagines attaching a large reactor on the back of a human transport vessel. Similar to how the piston of a car compresses fuel and air in the engine, the reactor would use electrical and magnetic currents to compress hydrogen gas. That compression raises temperatures within the reactor up to 100 million degrees C—hot enough to strip the electrons off of hydrogen atoms, create a plasma, and fuse two hydrogen nuclei together. In the process of fusing, the atoms release more energy, which keeps the reactor hot and causes more hydrogen to fuse and release more energy. (These reactions occur about 10 times per second, which is why it’s “pulsed.”) A nozzle in the reactor would allow some of the plasma to rush outward and propel the spacecraft forward.

There aren’t many machines that can produce the power demanded by pulsed fusion experiments and Centauri Dreams wishes the DM2 team success not only at the tricky business of Z-pinch fusion experimentation but the even trickier challenge of funding deep space propulsion research. None of that will be easy, but if we can get to the stage of testing a magnetic nozzle for a pulsed fusion engine using the DM2 we’ll be making serious headway. Right now the work proceeds one step at a time, the first being to get the DM2 up and running.

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Greg June 26, 2012 at 11:25

Whats missing is how much input energy does it take to even get an equivalent amount out? I’m only taking a conservative guess, but I’m sure something akin to many megawatts input are needed. Something not doable in space without a nuclear reactor.

Mike Lockmoore June 26, 2012 at 12:05

Eric Lerner’s startup (http://www.lawrencevilleplasmaphysics.com/) is working on developing a fairly small z-pinch machine for electic power generation, hoping to ultimately fuse boron and protons from hyrdrogen, which should produce no neutron radiation and lots of alpha particles directly convertable to electric current. They seem to be working on a shoestring budget, but periodically post some encouraging news.

A. A. Jackson June 26, 2012 at 15:45

Hope it’s not off topic.
Who first used fusion for propulsion in prose science fiction?
After 1945 , maybe before, I don’t have it at hand, someone must of had a story.
After the war L.R. Shepherd was was looking at fission and then fusion on the pages of the JBIS. There was ground work , not in public, laid at Los Alamos that led to an actual fission motor. (Not to forget the secret Orion.)
Saenger was thinking hard about it tho not publishing until very early 50′s.
Can always remember Rocket Ship Galileo, that Boy’s Life story that led to Heinlein’s so-called ‘juvies’. Rocket Ship Galileo was worked and reworked into Destination Moon. Heinlein favored fission propulsion, I remember it in Space Cadet.
Can’t for the life of me remember who first used fusion propulsion in SF.

Bob June 26, 2012 at 15:59

Greg,

For deep space propulsion, I do not think you would need to get energy equivalence. Energy for the sun would be converted to electrical energy to run the pulsed fusion engine creating far more thrust than using the same solar energy in say , a solar sail of some kind and using far far less mass than any known chemical engine.

Correct me if I a wrong but such an engine might get one to Mars in a few days or at least weeks.

Paul Gilster June 26, 2012 at 16:22

A. A. Jackson writes:

Can’t for the life of me remember who first used fusion propulsion in SF.

Great question, Al, and I can’t recall it either. But I’m digging around in 1930s SF these days looking for connections just like this, so maybe I’ll find something soon. My guess, though, is that it’s post WWII. Bethe was working on fusion by the late 1930s, but how widely that work was known outside the scientific community is unknown to me. Some of the readers may know.

Alex Tolley June 26, 2012 at 17:20

@Bob
I would like to see some numbers on this. What would be the benefit of inducing fusion with solar power, rather than a more direct conversion? To surpass an ion drive, the fusion drive would have to be very low mass.

The second problem is that solar power rapidly diminishes with distance, so a solar powered fusion drive would quickly run out of juice for deep space missions. Maybe this could be overcome with beamed power.

A. A. Jackson June 26, 2012 at 17:55

Paul,
Hans Bethe was such almost a magician as a genius physics.
It’s amazing , he really figured out how fusion ran the sun.*
Fusion was understood during the war and Teller occupied himself with it, which led to we know what.
So just wondering.

*It’s kind of goofy that Bethe did not get a Nobel till 1967 , never figured that out.

tom baty June 26, 2012 at 18:40

Do not know for sure but my guess ( yes, guess) for the first SF story to use fusion might be “Time for the Stars” by R.H.Heinlein in 1956. Now I admit that the author does not refer to his “Torchships” as fusion powered, but indicates that in their operation and capacity they were more then fission power. By inference I would say that he had some sort of fusion process in mind. My $.02 adjusted for inflation.

Tom

Jim Early June 26, 2012 at 23:02

I’m trying to remember stuff from a decade ago so this may not be correct. I thought that one of the “problems” with the Sandia Z-pinch concept was that the architecture drove one to large pulse energy targets. The power reactor system would have a low pulse rate with large pulse energies and probably had to be at least 10GW. This was a major problem for a reactor design, but it is not a problem for a propulsion application.
In laser fusion targets the alpha particles supply the energy to drive the target burn and the neutrons escape to the walls. In a large Z target the neutrons might deposit their energy within the target and raise the target temperature enough for D-D reactions. You would still need a D-T core for ignition. This may be the best technology for fusion propulsion.

Joy June 27, 2012 at 1:08

Never mind deep space applications, which would be a remote application at best. I would be happy if Z pinch could ever work even as a cost effective terrestrial power plant. Affordable electric power would be useful for laser launch systems, not to mention keeping the lights on down here without exceeding the Eocene thermal maximum. Time will tell.

Christopher Doll June 27, 2012 at 2:33

Solar-generated power for such a drive would open up inner-planet travel, to the point where you could fabricate nuclear power plants off-world. Getting just one of these into orbit will be an expensive proposition, but the potential is exciting.

kzb June 27, 2012 at 7:25

tom baty: the torchships had a “mass converter”. Also, they accelerated at 1.5g to highly relativistic velocities. So I think Heinlein must have been assuming total conversion of mass to energy, not simply fusion.

Funny thing is, must be over forty years since I read that book, but details like that have stayed with me. Must’ve made a big impression on me.

ljk June 27, 2012 at 9:05

Would using hydrogen bombs to push an Orion space vessel count as fusion powered?

Bob June 27, 2012 at 13:39

Alex,

I did not mean that solar power is the best source of the energy to kick start the reaction. I only meant it as a counter to the idea that the energy out has to be equal to the energy in to be worthwhile. We don’t care about energy efficiency as much as conversion to thrust.

Ideally, one would want a low mass engine capable of igniting, containing and directing the pulsed fusion energy into thrust and to be able to recycle enough of that energy to ignite the next pulse.

I would be disappointed if a pulsed fusion engine could not get one to Mars in a week or at least a few weeks because it should be capable of continuous thrust.

Greg June 27, 2012 at 15:18

@Bob,

Ok let’s look at the physics of using Solar as the power source. Using the ISS as the largest deployed in orbit solar panels we have so far that I can tell, Power Generation: 8 solar arrays = 84 kilowatts (from http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html)
I couldn’t find just how much power the z-machine uses per pulse, but I backward engineered it, where they use 20 million Amp pulses through tungsten wire, tungsten has a 5.65 micro-Ohm’s per cm of resistivity. Just using 1 cm of wire and Ohm’s law gives 100 volts to produce 20 million amps at that resistance, the power for that pulse is 2 gigawatts for one second input (100v x 20 million Amps). These are all estimates as there is not much information available and so pardon the assumptions made, but based on this type of power source you probably get 1 pulse per 1 hour. Not sure if that would get us useful acceleration.

Greg June 27, 2012 at 15:29

Checked my calculations I was off on my resistivity of Tungsten, the voltage comes out to 1130 v which means the watts per sec is 22.6 Gw per 1 second pulse. So it just emphasizes the point that solar just doesn’t seem to have energy density needed.

Bob June 27, 2012 at 16:11

Greg,

I am not suggesting that solar power is the way to kick start pulsed fusion engines but if it were to be used, neither the current design limitations of the ISS or the Z machine would be relevant to a future design unless it showed no possible future improvement.

However, just for fun, I did a quick calculation and it seems the energy collected in an array of 160m per side would collect 2 billion joules every 6o seconds , an array about 400m per side would give that energy every ten seconds and and array of a 1200m per side (yes, that is big) would provide that energy every one second not counting losses and such and the fact that you want to discharge that energy in one second to get 2 GW.

So, there is a lot of energy available. One would just have to design the system to use it.

Bob June 28, 2012 at 15:11

Greg,

The sun has all the energy one would need. It just needs to be engineered.
Also, there would be no point in using pulsed fusion if you had to supply all the energy to make the reaction go. Might as well make a really big ion engine instead.

ljk June 28, 2012 at 16:45

Bob said on June 28, 2012 at 15:11:

“The sun has all the energy one would need. It just needs to be engineered.
Also, there would be no point in using pulsed fusion if you had to supply all the energy to make the reaction go. Might as well make a really big ion engine instead.”

It’s funny that you mention an ion engine for interstellar propulsion, because Ted Merkle, the guy who was the director of Project Pluto during the Cold War (the nuclear-powered nuclear missile concept), was also interested in using ion to get around the galay – and I quote from here:

http://www.merkle.com/pluto/side.html

“In fact, Merkle’s impatience was legendary. Testifying before Congress on spaceships of the future, Merkle dubbed a nuclear rocket then under development at the rival Los Alamos lab “Old Pokey.” He said he wanted to explore the cosmos in a near-light-speed, ion-propelled rocket.”

Jason June 29, 2012 at 16:38

Greg, your nuclear reactor comment is spot on. The z-pinch concept we are investigating is pulsed, unlike the original 1957 reactor concept. The instantaneous power during the pulse is actually better measured in Terawatts. DM2, which we are calling the Charger Facility (Charger being the UAH mascot), is capable of 3 TW for about 200 to 500 ns. A breakeven experiment would require about 50 MJ or so, about 100 times our current capability. A fusion propulsion system will need a separate nuclear fission reactor for startups, and continuous fusion reactor power would be in the Gigawatts (time averaged).

Greg July 2, 2012 at 14:57

Jason,

Thanks for filling in the blanks! Very interesting, if you need a 50 MJ for breakeven, I can’t help notice that’s a bit more than what the Navy’s rail-gun that provided 32MJ but a bit less than the 64MJ rail-gun of 2020. I would be curious if either technology could benefit from each other?!

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