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current to toroidal magnetic
field strength does
that trick. Again, the field
line lengths did not matter.
These data are part of a
growing body of evidence
that self-regulatory heat
transport mechanisms
are at play, which tend
to clamp the width of the
heat flux profiles at a critical
scale-length value.
The physics of these
processes is a subject
of further investigation.
High-power microwave tube sources, which provide power to drive steady-state
plasma current in Alcator C-Mod.
Alcator C-Mod have uncovered important clues,
including behaviors that appear counter-intuitive
at first. One prevailing model holds that, as the
length of the magnetic field line becomes longer,
the footprint should get wider; the heat has more
time to spread out.
To test this model, Alcator C-Mod prepared two
plasmas: one with the usual footprints and one
with an additional set of footprints, each having
half the field line length. No significant change
in footprint shape was seen. In other plasmas, field
line lengths were varied by changing how tightly they
twist around the torus – changing the ratio of plasma
Steady-state operation
is generally considered
to be an essential requirement for a practical fusion
reactor. However, the confining magnetic
field in most tokamaks such as Alcator C-Mod is
produced in part by a toroidal current generated
by magnetic induction (the ”poloidal” magnetic
field component). This means that such tokamaks
are inherently pulsed plasma confinement systems
and are most likely unsuitable for an economical
reactor application. In addition, non-inductive
methods of driving the toroidal current in a tokamak
are available and include injecting RF
waves (or microwaves) and/or neutral beams.
In addition, only a fraction of the total current
needs to be supplied by external sources, since
substantial current is
also produced naturally
by the plasma pressure
gradient (the so called
“bootstrap” current). Developing
the scientific
basis for steady-state tokamak
operation using
the injection of microwaves
to supplement
the self-generated current
is a high priority
within the Alcator C-Mod
research program.
Important progress toward
the steady-state
goal was made this year
using the lower hybrid
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