Reaching for the Stars - Antimatter Explored

"Our goal is to remove antimatter from the far-out realms of science fiction into the commercially exploitable realm for transportation and medical applications."

Beyond the Enterprise - Fusion Power
A step back from antimatter is fusion, the power source of the future for the last five decades. Controlled fusion - joining two lightweight nuclei to get a slightly heavier nucleus and a lot of energy - has been challenging. In their quest to exceed Q=1, the break-even point, scientists have moved from low energy yields of Q=0.0000000000001 in the late 1950s to Q=0.3 today, and developed a large body of engineering and scientific knowledge showing that it can be made practical.

"From the NASA perspective, the challenge is to adapt fusion for space propulsion," said Dr. Francis Thio, a principal research scientist in the Propulsion Research Centre. "Magnetised Target Fusion is one of the major approaches that we are studying." NASA/Marshall is working with Los Alamos National Laboratory and the Air Force Research Laboratory to adapt MTF for propulsion.

"MTF tries to operate in an intermediate regime between the conventional magnetic fusion, and inertial confinement using a laser," Thio explained. The problem with conventional magnetic confinement is it operates at very low density. To achieve sufficient power, the fusion reactor must be large, which translates to a high cost.

On the other hand, inertial confinement fusion uses a tiny plasma, 1,000 trillion times denser than in a magnetic confinement scheme. But that requires a driver - usually banks of intense, short-pulse lasers - that heat and compress the target in a short time. That also drives the cost up.

"MTF tries to operate at not too low or too high a density," Thio explained, "and achieve a reasonable rate of fusion activity with a density 10,000 to 100,000 times higher than magnetic confinement, and 10,000 to 100,000 times lower than laser fusion."

It's more economical and uses pulse-power drivers - powerful capacitor banks that drive electromagnetic implosion - that are available today at low cost. It does not have the implosion speed generated by a laser beam, but a magnetic field confines the target plasma and insulates the inertial wall that implodes to cause the fusion.

Can I get the compact model?
Even if fusion is achieved, current methods are too cumbersome to use in rockets.

"The mass is quite prohibitive," said Professor T. Kammash of the University of Michigan. "We want to make the physics work without using very large magnets." The mirror magnets for a fusion rocket would weigh about 401 tonnes (metric tons), about 16 times a single Space Shuttle payload. The heat radiators would add 240 tonnes.

Kammash's students are experimenting with a droplet radiator design that, using liquid lithium as a coolant, could reduce the radiator mass to 57 tonnes.

A rotating magnetic field could induce a magnetic field and electrical currents, "a clever way of fooling the plasma" into behaving as if it was in a conventional magnetic mirror system.