Anti-matter engines

As i mentioned before Star Trek is the closest thing i have to knowledge about the solar system. It really caught my interests when i learned of ideas and theories regarding Anti-matter for engines. A scientest by the name of Iddo Genuth, working for positronic research LLC, is researching with NASA the idea of a positronic engine using positrons as a propulsion system. positrons are the anti-particle of an electron. 10 mg of this has the energe of 428 tons of TNT. There are currently 3 ideas for a positronic engine system.

1. Solid core - Energy is transferred to a propellant in tungsten metal matrix heated by annihilation gamma rays.

Advantages - Well understood technology.
Disadvantages - Performance limited by melting temperature of tungsten.

2. Gas core - Energy is transferred to liquid/gas propellant directly heated by annihilation gamma rays.

Advantages - Improvement over solid core, not limited by melting temperature.
Disadvantages - Flowing multi-fluid is unstable at boundaries, may ionize and create plasma.

3. Solid Ablation - Energy is transferred to a material that ablates off surface of a pusher plate.

Advantages - Simplicity in design, no obvious technology limits.
Disadvantages - Half of the gamma rays do not strike the pusher plate, maximum efficiency 50%. ( found in an article [33] by the future of things)

These ideas seem very ambitious but look to be a great oppurtunity for Intergalatic travel. Positrons are created by a colider and currently to get 10 mg ( estimated amount for mars mission) it would take 5 years after builiding a $1.5 billion acceleration plant. no matter how you swing it, intergalactic travel isn't going to be cheap. holding this anti-matter is another problem that has already been solved by the research team. to hold the anti-matter, they have proposed a neutrel charged positronium after being stabelized in magnetic fields. IN other words a big tank using a magnetic field and other stablized antimatter to sustain the energy of the positrons.