How does SPI work?
An innovative device to prevent plasma from melting or damaging components of the reactor wall in JET has been used in experiments for the first time.
Have a look at the striking video: it’s the Shattered Pellet Injector or ‘SPI’, a technology that works by pre-empting a plasma disruption and releasing a frozen deuterium-neon pellet that significantly lowers the temperature of the plasma. And why is this important? Because excess plasma temperatures can damage the vessel and lowering excess plasma temperatures is a safe way of dissipating energy and minimising vessel damage.
In JET, the excess heat and energy from plasma has been removed by a technology known as the Massive Gas Injection. The SPI is much more efficient at this technology because the SPI adopts a ‘shot gun spray approach’ when distributing the frozen pellet. “Whereas the Massive Gas System interacts with the edge of the plasma, the SPI pellet, travel close to the centre of the plasma. This is much more efficient for cooling the plasma down,” says James Wilson, project manager for the SPI project at UKAEA.
The introduction of the SPI – which has been gradually installed on JET over the last few months – is the result of an international collaboration between EUROfusion, ITER, and Oak Ridge National Laboratory (ORNL) in the USA with the project being centrally managed by UKAEA.
Although scientists are still familiarising themselves with the mechanics of the SPI pellet system, between 20 and 30 have already been fired in the first days of experiments at the end of July. The SPI technology has previously been used on the DIII-D tokamak in San Diego, but using it on JET will mean that it offers a more complete picture of what to expect when deployed on ITER. When the deuterium pellet shoots out at a staggering 250 metres per second, it equates to nearly the same speed at which a jet airliner would be travelling while at full altitude.
Larry Baylor is a fusion scientist from the Fusion Energy Division of ORNL in Tennessee, USA. He is familiar with the SPI from when it was deployed on DIII-D. He says: “Technically the SPI on JET acts as a disruption itself, but in the future it would be used to pre-empt the disruption. If it detects a disruption coming, it would shoot a pellet (s) into the plasma to cool the plasma down very quickly. That way the disruption wouldn't cause any damage.”
“We are testing it on JET because we are trying to scale it to ITER. DIII-D is a much smaller device than JET, and JET is a much smaller one than ITER. So the purpose is in scaling in terms of plasma size and plasma energy to get closer to what ITER will be. I think one of the main things this new technology will perhaps tell us is the amount of SPI material that ITER will require, and whether it needs to be injected from more than one location in the vessel,” he adds.
“In using the SPI we are also interested in learning about how much neon is required to remove all the thermal energy and how symmetrically the energy is radiated around the torus. Other areas of study later down the line will include the dissipation of runaway electrons,” Larry explains. The SPI pellets mainly consist of deuterium with a small amount of neon. Each takes about 20 minutes to form using extremely cold helium gas before it can be fired and subsequently shattered. The next use of the SPI will be when the runaway electron experiments are performed.
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