Monday, November 14, 2011
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This schematic shows the DIII-D tokamak and layout of the UCLA Doppler Backscattering “radar gun” diagnostic. Image: Lothar Schmitz (UCLA) and M.R. Wade (General Atomics)
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Recent
experiments carried out at the DIII-D tokamak in San Diego have allowed
scientists to observe how fusion plasmas spontaneously turn off the
plasma turbulence responsible for most of the heat loss in plasmas
confined by toroidal magnetic fields. Using a new microwave instrument
based on the same principles as police radar guns, researchers from UCLA
observed the complex interplay between plasma turbulence and plasma
flows occurring on the surface of tokamak plasmas.
"We
found that the turbulent eddies on the surface of the plasma produced
surface flows that eventually grow large enough to shred the eddies,
turning off the turbulence," said Dr. Lothar Schmitz, who made the
measurements with a microwave instrument designed and built by the
Plasma Diagnostics Group at UCLA. "Much like the population of predators
and prey find a balance in the wild, we find that the plasma flow and
the plasma turbulence reach an equilibrium in the tokamak plasma."
The
finding is important for fusion research because scientists have been
seeking to understand how it is that, in a tokamak (a doughnut-shaped
vacuum chamber linked by a toroidal magnetic field), the surface plasma
turbulence suddenly switches off as the heating power increases. The
reduction in turbulence improves the thermal insulation provided by the
magnetic field so that much less power is required to achieve
temperatures required for fusion (100 million degrees). Until new
measurements were obtained, researchers were not able to observe the
very rapid change in edge turbulence which occurs in less than a
millisecond over a zone less than 1 cm thick.
Dr.
Schmitz and his coworkers observed the connection between the flow and
the turbulence when looking at tokamak plasmas that jumped back and
forth from having low thermal insulation to high thermal insulation many
times over a few hundredths of a second before finally settling down to
the high insulation state (called H-mode by fusion scientists to
distinguish it from the low insulation L-mode state). The H mode was
discovered in 1982, but the trigger mechanism of the H mode transition
has so far been elusive.
The
UCLA group designed their new Microwave Doppler Backscattering
Diagnostic tool (operating in a way similar to a radar gun), for use on
DIII-D to measure the speed at which turbulent eddies propagate in the
plasma, as well as the strength of the turbulence. By aiming an array of
microwave "radar guns" at the plasma, the time evolution of plasma flow
and turbulence intensity can be followed across an extended radial
layer in the plasma boundary.
The
microwave measurements reveal the predator-prey oscillations between
the plasma flow (predator) and density turbulence (prey) by their
relative timing. Like the abundance of prey feeds the population growth
of predators, high turbulence near the plasma edge is found to drive
high flow velocities which, in turn, shred the turbulent eddies and turn
off the turbulence, causing the flows to die away. The predator-prey
cycle then repeats itself: the Zonal Flow dies away once the turbulence
has calmed, thus allowing turbulence to grow again (red yellow zones),
which restarts the flow. Now that they've seen the process up close,
Schmitz hopes to use the improved understanding to figure out ways to
make it easier to achieve and maintain high thermal insulation in future
fusion experiments such as the ITER experiment now under construction
in France.
SOURCE
