If you keep twisting a straight elastic string, at some moment it starts kinking
in a wild way. Something similar occurs when one increases the electrical current
flowing in a magnetized plasma doughnut: it takes on a wild helical shape, which
spoils its performance. This phenomenon concerns scientists exploring fusion
power, who use powerful magnetic fields to confine plasma during their experiments.
But a discovery made in the RFX-mod Reversed Field Pinch (RFP) fusion device
demonstrated that if one keeps on increasing the current, above 1.5 million
amperes, the helix becomes very smooth, the trapping of the plasma in the magnetic
fields improves and the plasma gets hotter. Moreover, the helical state appears
to be the preferred one at high current and is spontaneously chosen, or self-organized,
by the plasma. RFX is a large RFP device (ten cubic meter plasma) located in
Padova, Italy.
In the past, RFP experiments suffered from poor stability, which reduced confinement
performance due to magnetic turbulence. But as a result of increased order in
the self-organized helical state, magnetic confinement improves and a broad
zone of the plasma becomes hot. This renews the fusion prospects of the RFP
device, a magnetic confinement configuration germane to the tokamak (which uses
a magnetic torus for plasma confinement) because a current flows in the plasma
doughnut, and to the stellarator (which uses a magnetic helix) because the plasma
takes on a helical shape.
An attractive feature of the RFP fusion device is that most of its magnetic
field is produced by the current flowing in the plasma. This eliminates many
of the costs and technical difficulties associated with producing strong magnetic
fields using high-tech superconducting coils, such as those used in tokamak
and stellarator fusion containment designs. The helical shape of the plasma
comes with an additional bonus: the current lines are also helical. This greatly
increases the length of the electrical circuit with respect to the tokamak.
This could make reaching thermonuclear temperatures possible with only the electric
power dissipated in the plasma. In principle, no additional heating is necessary—an
additional positive economic and technical feature.
The remarkable helical shape of the plasma doughnut is the signature of self-organization
dynamics. Similar helical structures may be found in astrophysical objects as
well as living things, where self-organization provides stability and resilience
against external perturbations. This should lead the RFP to be free of the disruptions
that threaten tokamak discharges, where the plasma ring suddenly becomes a wild
helix that damages the confinement vessel with heat fluxes and electromagnetic
forces.
To confirm their relevance for thermonuclear fusion, RFX helical plasmas should
be further improved, especially in two important respects. First, their density
should be increased; this is presently being sought using fuel pellet injection.
Second, the helical shape should be further optimized to improve confinement.
Progress in magnetic control, a further increase of plasma current, and use
of modern wall-conditioning techniques might accomplish this.
Source: American Physical Society
Posted November 2nd, 2009
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