Good morning everyone.
Today I want to talk about one of my very favourite machines on the planet, the Z machine at Sandia National laboratories. The Z machine is the most powerful laboratory x-ray source on the planet and is primarily used for fusion research and testing our nuclear weapons stockpile by using simulations of conditions during nuclear explosions. At full power, the Z machine uses 26 million amps to reach peak x-ray emissions of 350 terawatts, and can produce temperatures at the target of over 3,600,000,000oF for a few billionths of a second. It is a most impressive device.
The image above is the famous “arcs ‘n sparks” photograph that was taken around 1996. You typically see it with articles about the Z machine, and for good reason – it’s really cool. The lightning is called ‘flashover’ and is caused by the huge EMP generated by the enormous amount of electricity being used during a shot. This is in spite of the insulation provided by over 500,000 gallons of transformer oil in the outer ring, and over 600,000 gallons of DI water in the inner ring. Yes, that’s correct: the machine is operated while under water (and oil). Divers in SCUBA gear make last minute adjustments prior to a shot before scrambling to the safety of a copper-lined concrete bunker next to the main chamber when the warning lights come on. It must be an interesting job for an engineer. The machine went through an upgrade from 2004-2007 that increased its power and performance and improved its turnaround time. It also improved its insulation characteristics to where flashovers like the one above no longer happen. Dang.
The Z machine is a pulsed power machine that gets its name from the method it uses to generate x-rays, the Z-pinch. A Z-pinch is a type of plasma confinement system that uses an electrical current in the plasma to generate a magnetic field that compresses it. The Z-pinch uses the Lorentz force that we last saw being used in the Navy’s railgun to propel a projectile at incredible speeds.
The image above is of a technician (or an engineer) building a target array which when completed looks like the image below:
The wires are all made of tungsten and are about 10 microns in diameter and surround a hohlraum made of thin gold sheet and containing a BB sized sphere of a mixture of deuterium and tritium, which is the actual target.
During a shot, the massive electrical current is fed into one end of the wires, instantly vaporizing them into plasma. The plasma, being electrically charged, conducts the massive current and generates a powerful magnetic field. Because of the Lorentz force, the magnetic field contracts on the z axis very fast and very strongly – one could even say violently. This is the Z-pinch, a diagram of which is below:
Inertial Confinement Fusion
There are two main branches of fusion research; magnetic confinement fusion, where the hot fusion fuel is contained by very strong magnetic fields in a device like a tokamak, and inertial confinement fusion (ICF), where fusion is initiated by heating and compressing a fuel pellet using lasers or other means. The Z machine uses the ICF method, and falls into the ‘other means’ category.
We can see how ICF is produced by the Z machine by continuing from the Z-pinch above: the magnetic field from the plasma continues rapidly compressing the plasma to such a high density that it will compress no more and has to slow down. Do you remember what happens when you force a large amount of fast moving energy to slow down? You get bremsstrahlung radiation, or x-rays. In this case you get around 2.7 megajoules worth of x-rays which is quite a lot. The x-rays heat the target to about 1.9 million degrees C, causing the outer layer to explode outward and forcing the remnants of the inner layer to explode inwards with great force, compressing the fuel pellet. The explosion also creates shock waves which greatly compresses and heats the fuel further, to the point where it partially fuses into helium, neutrons, and quite a bit of energy, so you get a nice loud ‘bang’ and a small earthquake. This process is basically how hydrogen bombs work.
The Z machine typically takes about a day to set up for a shot, and can make around up to 400 shots a year since its recent upgrade. “The process starts with wall-current electricity, which Z uses to charge up large capacitors. The electricity is supplied by a local utility company, and in every shot the machine consumes only about as much energy as it would take to light 100 homes for a few minutes.”
The Z machine is based on pulsed power technology. A series of Marx generators, capacitors, switches, and transmission cables are configured in modules to compress electrical energy in space and time. The high currents are achieved by connecting in parallel 36 of these nearly identical pulsed power modules. The modules are arranged like spokes on a wheel, with the target chamber at the hub. Thirty six individually controllable lasers trigger gas switches on the modules to shape the pulse and fire the modules. Getting the timing on everything in synch down to a few nanoseconds must be a real live bastard. Blessed are those with sufficient computing power. The energy storage capacitors and gas switches are submerged in transformer oil, and the timing lasers and components are submerged in DI water. The lasers themselves are located in a room underneath the Z machine, and the beams are routed through the floor into sealed towers and directed at the gas switches. The videos below describe, to a degree, how all of the components synch up and fire better than I can, since you can’t see my hands waving as I talk. It will also save you from reading several pages of my nattering, as I’m just about out of space and there is so much more to tell about this wonderful machine.
The engineering of the Z machine never ceases to impress and amaze. The earthquakes are impressive, too.
Here is a short video about pulsed power at Sandia and the Z machine in particular:
This next video is actually about a device called the ‘Z Beamlet’, a 1.3 petawatt laser that is used to take x-ray images of the actual compression of the fuel pellet, I’ll let that sink in for a minute…[jeopardy theme plays], which is quite incredible in many ways. The video also shows how the Z machine fires in synch with the Z beamlet quite nicely.
This last video talks about the upgrade to the Z machine, which I include because I find it interesting:
Note: I’ve done my best to get all of the various numbers and dates correct, but it wasn’t always clear as to what happened when in all of the documents. If you find where I have made mistakes, please let me know.
All images are from Sandia National Laboratories except where noted.
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