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.
As seen in the striking video (below), the Shattered Pellet Injector (or ‘SPI’) pre-empts an abrupt termination of the plasma – known as a ‘disruption’ – by releasing a frozen deuterium-neon pellet designed to bring the temperature of the plasma down significantly. This acts as a safe way of dissipating energy and minimising damage to the vessel from the disruption.
It is also a much quicker way of removing excess heat and energy than the pre-existing Massive Gas Injection. This is because the SPI adopts a ‘shot gun spray approach’ when distributing the frozen pellet.
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 ITER, EUROfusion, and Oak Ridge National Laboratory (ORNL) in the USA with the project being centrally managed by Culham Centre for Fusion Energy.
Although scientists are still familiarising themselves with the mechanics of the SPI pellet system, between 20 and 30 were 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.
Scaling to ITER
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 said: “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 into the plasma to cool it 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.”
Further experiments could see a second SPI opposite the current one. This would tell scientists more information about the effect of the pellets when entering the plasma from different locations.
But for now, Larry added, there are other key things to find out. “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.”
“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.”
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.
James Wilson, project manager for the SPI project at CCFE, said: “The Shattered Pellet Injector is one of the most complex systems we use on JET. It encompasses a lot of different elements including cryogenics, vacuum, gas manipulation and computing. Although not quite there yet the system will soon be fully automated.”