Posted on: 6th February 2012
Science is like playing lotto.
Thirty years ago this week, the team at the ASDEX tokamak, in Garching, on the northern outskirts of Munich were conducting an apparently routine set of experiments on beam-heated plasmas. Despite the mundane start, the results would change the history of fusion physics: today they would observe H-mode for the first time.
In charge of the physics programme that day at ASDEX was Fritz Wagner, who was still considered a newcomer, even though he’d been in the field for seven years: “This was an extremely boring programme because the L-mode is so horribly reproduceable. But then suddenly we saw this transition!” ASDEX’s sensors were picking up intense bursts of pink light from the bottom part of the vessel, the pressure was higher than normal, and there seemed to be more impurities in the plasma.
“Looking at the traces on the recorder we saw something new, and right away started discussing it.” says Professor Wagner, “I think it was clear to me that this was something exceptional.” However, the more experienced physicists did not agree, writing off the observations as transient effects. Undeterred, he spent the weekend analysing the data and the following week presented evidence that it was indeed something new and significant.
What they had stumbled upon was H-mode – high confinement mode – in which a barrier layer spontaneously forms around the plasma and confines the particles within the hot centre for longer. As high confinement is one of the primary requirements for successful fusion, H-mode has become a cornerstone of tokamak operation.
For the next two months Wagner worked hard to understand the physics of H-mode, as the news spread of the discovery. He relates that, at the time, he feared a major mistake would come to light in his work, and he would be discredited. “This luckily did not happen!” he says. “The evidence was fairly clear, although in those days the diagnostics were totally different, we did not have diagnostics close to the plasma edge, and it is at the very edge of the plasma where the music plays.”
He is particularly proud of the analysis he carried out followed the discovery, which explored the transport barrier at the edge of the plasma. Yet he is modest about the discovery itself, which he saw more as a responsibility. “Science is like playing lotto,” he says. “Many physics observations come out of nothing. So I felt it was my responsibility to analyse these things and point out that we had something new. It was a tremendous feeling!”