Another step on the road to fusion power?

In 1982 scientist Fritz Wagner discovered a stable operation mode in plasma at the Max Planck Institute for Plasma Physics in Germany.

Princeton Plasma Physics Lab General Atomics superH mode

Red signifies instability while blue is the quiescent region. Plasma density needs to increase along the narrow blue channel to reach the Super H-mode state

Now called H-mode, it was stable and had twice the confinement of other modes. Applicable to tokamak (toroidal) plasma vessels, it is now the mode that nuclear fusion experiments are designed to operate in.

Now a special variant of H-mode, dubbed ‘super H-mode’, has been verified.

“This new state raises the pressure at the edge of the plasma beyond what previously had been thought possible, creating the potential to increase the power production of the hot core of the plasma,” said US Department of Energy’s Princeton Plasma Physics Laboratory (PPPL). “It could prove particularly promising for ITER, the international experiment under construction in France to demonstrate the feasibility of fusion energy.”

H-mode it an ‘edge mode’. Scientist Philip Snyder of General Atomics developed a computational model (called EPED) that predicted more than one type of edge region in tokamak plasmas, with the previously unknown Super H-mode among them.

Such regions, known as pedestals, serve as ledges in H-mode plasmas from which pressure drops off sharply. The higher and wider the pedestal the greater the density and pressure, which together act like thermoses to contain the plasma at more than 100 million degrees C, according to PPPL.

Now the mode has been reproduced experimentally on the US DIII-D National Fusion Facility.

The Super H-mode regime was reached by steadily increasing density in a quiescent state that naturally avoids pedestal collapses.

“The results caused the plasma to follow a narrow path to the Super H-mode, the physics equivalent of steering a boat through rocky shores,” said PPPL.

 

Image credit: PPPL, based on a diagram from General Atomics.
With apologies from Electronics Weekly: no axis information was provided with the diagram.


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