OUTSIDE, the freezing Connecticut winter has blanketed everything with several feet of snow. But in front of me a pinprick of atoms burns as hot as the Sun’s corona. I lean in towards the dazzling spot of light. Behind me, a couple of welders are getting fidgety. “Did he tell her not to touch that thing,” one of them whispers, pointing at the light. Above me an electrode sizzles, creating this droplet of purple plasma glowing at over 10,000 °C.
I’m standing in an industrial warehouse owned by Acceleron, a family-run welding company based in East Granby. Acceleron is part-funding Ady Hershcovitch, a physicist from the Brookhaven National Laboratory in New York State, to build a device that could revolutionise the construction of fighter jets, ships and luxury cars – anything, in fact, that requires metal to be welded as seamlessly as possible. Anyone who wants to make smooth, high-quality welds between two pieces of metal uses beams of high-energy electrons to melt and fuse the metal. But electron beam welding is expensive, and imposes restrictions on the designs that can be used. Hershcovitch plans to change all that.
Creating an electron beam involves applying a high voltage to an electron-emitting material such as tungsten. Electrons are emitted from the tungsten cathode and accelerated towards an anode with a hole in it. The process has to take place in a vacuum so that electrons don’t collide with air molecules and lose energy.
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But there’s a big problem: it is very difficult to get the beam out of the vacuum chamber and into the open air without it losing its energy. All materials strong enough to contain a vacuum are damaged by the electron beam and eventually break. Some welders employ a half-way solution. They fit the front of the electron gun with an open-ended pipe around a metre long, and then evacuate it using a “differential” pumping system. This removes air continuously from along the pipe’s length, so that there is a vacuum at the electron gun end and atmospheric pressure at the open end. But the beam still loses much of its energy before leaving the pipe, meaning its welding power is significantly reduced. As a result most electron beam welding takes place inside room-sized steel vacuum chambers big enough to house the chunks of metal to be welded. Evacuating these chambers takes hours, and the heavy pumps that do the job consume thousands of dollars a month in electricity, and often break down.
Now Hershcovitch believes he has finally solved the vacuum problem – though it wasn’t what he set out to do. In 1995 he approached some welding companies with a new idea to make electron beams. “They weren’t interested,” he says. But they did tell him about the vacuum problem. “They told me that if I could somehow get an electron beam from a vacuum to the atmosphere, without losing its energy, then I’d be onto something.” He quickly realised that plasma could be the key.
A plasma is a hot ionised gas of charged particles. Hershcovitch knew that confined plasma exerts pressure on its surroundings, just like any hot gas. So he reasoned that if you could find a way to hold a blob of plasma still, it should be possible to use it to hold back air. And, most importantly, a high-energy electron beam would pass right through it. Thus the plasma window was conceived.
The easy bit is creating the plasma. The “window” is a cylindrical hole about 3 centimetres deep and 5 millimetres wide, and embedded in the frame are an anode and a cathode (see Graphic). Hershcovitch sets up a few hundred volts across the electrodes, which strips electrons from the gas and accelerates them from cathode to anode, causing the ionised gas to heat up and form a thick plasma that fills the circular window. The plasma is contained at either end of the cylinder by magnetic fields, Hershcovitch keeps it topped up with fresh gas to make good any leakage. To maintain the vacuum, a small pump sucks out any gas that leaks past the plasma.
The difficult bit is keeping the plasma stable. That’s because the chain reaction that forms it is triggered by a random event: a single atom loses an electron and becomes a positively charged ion. As the electron and ion fly off towards oppositely charged electrodes, the electron crashes into other atoms, tearing off more electrons, cascading the ionisation.
This process tends to spill out of the region where the plasma is supposed to be confined. However, the cooler a plasma is, the denser and less conductive it becomes – and hence less likely to spread. Hershcovitch realised that it might be possible to control a hot core of plasma if he surrounded it with a ring of cooler plasma. And so he constructed a system of thin copper tubes that circulate tepid water around the window frame to maintain an outer zone of low-temperature plasma. This confines the hotter, more electrically conductive plasma to a stable cylinder in the centre of the window. Ceramic rings insulate the tubes to stop current flowing through the frame instead of through the centre of the plasma.
So how does it form a seal between the air and the vacuum? Although the electrons and ions in plasma are further apart than particles in a gas, at over 10,000 °C they move at tens of thousands of metres per second, so any low-energy gas molecule trying to get through is deflected. Hershcovitch’s experiments have shown that the plasma window can withstand a pressure difference of up to nine atmospheres without leaking. That would normally require several millimetres of steel.
The crucial part is that beams of highly energetic particles can get through the plasma, which means Hershcovitch can put his electron beam generator inside a small vacuum chamber fitted with a plasma window. The beam fires out through the window, so the metal to be welded doesn’t need to be in a vacuum chamber. And there’s an added bonus. The magnetic fields generated by the window help to focus the electron beam, and this could double the distance it can travel in air to as much as 5 centimetres.
The plasma window could allow electron guns to become as portable as the inferior laser welders. Electron beam welding could be done in situ on a fully assembled product, not just on parts. This means designers won’t have to think in terms of part sizes, and would be free to dream up machines made of continuous metal. Hershcovitch says that an aeroplane manufacturer is already looking into using his device to build its next generation of aircraft, although he won’t say which one.
There are still problems to be solved, however, not least of which is demonstrating that the plasma window device works. The first welding demonstration is scheduled to take place next month.
One person who is keen to see it succeed is Rory Montano, president of Acceleron, the Connecticut company whose welding plant I’m visiting, which specialises in electron beam welding. Although it hasn’t seen the device work, Acceleron has already bought a licence to the Brookhaven patent on the plasma window. “We could save about $16,000 dollars in electricity per month,” Montano says. “We could weld a part for $20 rather than $40 – our customers are going to love us.” The plasma window could also drastically increase the number of parts welded each day.
Allan Sanderson, an electron beam welding expert at the Welding Institute in Cambridge, England, is not so optimistic. He is not convinced that the welding equipment can be made small and light enough to be portable. “It’s an exciting idea,” he says. “But I’ve not seen any evidence to convince me that it’s a viable option.”
There is another problem with taking electron welding out of the vacuum chamber. When electron beams strike metal, X-rays are emitted, posing a serious radiation hazard to the welder. Hershcovitch concedes this is an issue, but says that shielding could mitigate the effects. “You could use lead glass, or even a solid lead shield,” he says.
But the first step is to make the window work for Acceleron. If it does, who knows where this technology could go? This snowbound warehouse could be host to the hottest thing in welding since the blowtorch.