Division of Plasma Physics


A Star on Earth

The hordes of normal, glowing stars like the sun are a much more common sight in the sky than such extreme displays. In the stars' central cores, the plasma is so hot that light nuclei within it can overcome their mutual repulsion and fuse into heavier nuclei, emitting energetic particles in the process and ultimately driving the stars' luminous output. Since the fuel for such reactions is plentiful on Earth, plasma physicists have labored for decades to stably confine hot thermonuclear plasmas in laboratory devices, with the goal of using them as large power plants to replace those based on nonrenewable fossil fuels. This power production method would produce no greenhouse gases and its radioactive by-products would have much shorter life times than those created in fission reactors. On the downside, because of a hot plasma's furious and infuriating ability to squirm through any cage that is put around it, no net-power-generating plasma has yet been demonstrated. Thus the feasibility of a practical fusion power plant is still to be proved.

Still, many plasma physicists remain optimistic. They point out that the overall amounts of power produced by real fusion devices have increased by a factor of a trillion over the past two and a half decades. Researchers have scored a number of recent successes in donut-shaped devices called tokamaks, which use twisting magnetic field lines to confine hot plasmas. In laboratories in the United States and around the world, researchers believe they may have gained crucial insights into how to confine such plasmas without inducing the deleterious instabilities arising from spontaneously generated waves that usually spring leaks in the confinement. The potential payoff of such research is enormous.

Yet another concept for making fusion power plants ignores magnetic fields for confining the plasma and instead zaps a pellet of fuel from all directions with intense laser or ion beams. This compresses and heats the pellet, briefly creating the proper conditions for fusion to take place. While the potential of this method as a practical energy source will likely be the subject of research for some time to come, such research enjoys strong governmental support because of its utility for weapons research. The conditions within a compressed fusion pellet are similar to those in a nuclear bomb. Therefore these studies can be used to ensure the safety and efficacy of nuclear weapons without actually detonating them.

figure 17

figure 17b

COPIOUS AMOUNTS OF FUSION POWER has been observed in the Tokamak Fusion Test Reactor (TFTR). The toroidal region in which the plasma is contained is shown in (a). On the outside, shown in (b), the toroidal structure is surrounded by magnetic field coils, neutral beam injectors, and the vast array of diagnostics necessary to study ultra-hot plasmas. [Courtesy of Princeton Plasma Physics Laboratory]

figure 19

INERTIAL CONFINEMENT FUSION has four stages. (a) Intense laser or ion beam illumination rapidly heats the surface of the fuel capsule. (b) The fuel is compressed by the rocket-like inward push of the hot surface material. (c) The final stage of the implosion allows the core to reach 20 times the density of lead and ignite at a temperature of 100,000,000 degrees centigrade. (d) The thermonuclear burn spreads rapidly through the compressed fuel, producing a burst of useful energy.


LASER INDUCED plasma dynamics are often similar to important astrophysical phenomena.

(a) Emission from molecular hydrogen shocked by winds from young stars in the Orion star-forming region. [Courtesy of Mark McCaughrean and Mordecai-Mark Mac Low, Max Planck Institute for Astronomy];

(b) Blast wave produced in a laser-plasma experiment [Courtesy of Jacob Grun, Naval Research Laboratory]. These two phenomena exhibit bullet-like structures that are caused by hydrodynamic instabilities.

emission

A

blast wave

B

fusion confinement

PROGRESS IN FUSION confinement is approaching practical fusion energy producing regimes. [Courtesy of Contempory Physics Education Project]

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