Super-H Mode allows tokamaks to achieve higher fusion performance than previously possible. In recent experiments operating in and near the Super H-mode regime, researchers have achieved record-breaking values of fusion gain for a device of DIII-D’s size. Fusion gain is the ratio of fusion power generated to heating power.
Super-H Mode works by increasing temperature and pressure in the outer region of the plasma, called the pedestal. The experiments showed – as the theory predicted – that proper tuning of the plasma cross-sectional shape and density leads to pedestal temperatures and pressures that are more than twice as high as those of typical pedestals.
Because plasma conditions in the core – where fusion takes place – are dependent on conditions at the edge, Super-H Mode enables as much as a four-fold increase in fusion performance.
“Super-H Mode promises to reduce the cost and scale of future fusion reactors, thereby bringing the realization of fusion power closer,” said Steven Cowley, director of the Princeton Plasma Physics Laboratory who was not involved in the research. “It couldn’t be more significant.”
A tokamak is a doughnut-shaped device with strong magnetic fields that confine matter at temperatures exceeding 100 million degrees. Inside the tokamak, matter transitions to a plasma state where electrons are stripped from their nuclei. The resulting electrically charged plasma can be shaped and controlled by the magnetic fields. Within a sufficiently hot plasma, atoms collide and fuse together, producing fusion energy in a manner similar to the sun.
The ‘Super H-Mode’ regime is predicted to enable pedestal height and fusion performance substantially higher than standard H-Mode operation. This regime exists due to a bifurcation of the pedestal pressure, as a function of density, that is predicted by the EPED model to occur in strongly shaped plasmas above a critical pedestal density. Experiments on Alcator C-Mod and DIII-D have achieved access to the Super H-Mode (and Near Super H) regime, and obtained very high pedestal pressure, including the highest achieved on a tokamak (p ped ~ 80 kPa) in C-Mod experiments operating near the ITER magnetic field. DIII-D Super H experiments have demonstrated strong performance, including the highest stored energy in the present configuration of DIII-D (W ~ 2.2–3.2 MJ), while utilizing only about half of the available heating power (P heat ~ 7–12 MW). These DIII-D experiments have obtained the highest value of peak fusion gain, Q DT,equiv ~ 0.5, achieved on a medium scale (R less than 2 meter) tokamak. Sustained high performance operation (β N ~ 2.9, H98 ~ 1.6) has been achieved utilizing n = 3 magnetic perturbations for density and impurity control. Pedestal and global confinement has been maintained in the presence of deuterium and nitrogen gas puffing, which enables a more radiative divertor condition. A pair of simple performance metrics is developed to assess and compare regimes. Super H-Mode access is predicted for ITER and expected, based on both theoretical prediction and observed normalized performance, to allow ITER to achieve its goals (Q = 10) at I p less than 15 MA, and to potentially enable more compact, cost-effective pilot plant and reactor designs.
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