Tokamak edge pedestal, ELMs and heat exhaust

5 problems

• What sets the energy lost per ELM at low collisionality

  Open

  Each ELM throws out a fraction of the edge layer's stored energy, and that fraction grows as the edge becomes hotter and less collisional, as it will be in ITER. The mechanism behind this trend, and hence the size of ITER's ELMs, is not established.

• Is any ELM-free or small-ELM regime compatible with reactor conditions

  Open

  Several tokamak operating modes keep good confinement without large ELM bursts, but each has been found only under conditions that a reactor may not reach. Whether any of them survives at reactor density, low rotation and a cool divertor is unknown.

• Scrape-off-layer heat flux width in ITER and reactors

  Contested

  Exhaust heat leaves a tokamak in a very thin layer along the boundary field lines and lands on a narrow stripe of the divertor plate. Its width decides whether the plate survives, and theory and data disagree on how wide it will be in ITER.

• Predicting pedestal pressure height and width in reactor conditions

  Partially resolved

  The pressure reached at the top of the edge layer largely decides how much fusion power a tokamak makes. Models that predict it work in many present machines but fail in some, such as those with metal walls and heavy gas fueling.

• Will resonant magnetic perturbations suppress ELMs in ITER

  Open

  ELMs would erode ITER's wall too fast if left alone. Small helical magnetic perturbations from extra coils can suppress them in present tokamaks, but only in narrow conditions.