A physics team from The University of Alabama in Huntsville’s Department of Mechanical and Aerospace Engineering soon will take delivery of a specialized system to see if they can “Z-pinch” a tiny bit of that salt into the heart of a star.
“We are trying to develop a small, lightweight pulsed nuclear fusion system for deep space missions,” explained Dr. Jason Cassibry, an associate professor of engineering at UAHuntsville. “If this works we could reach Mars in six to eight weeks instead of six to eight months.”
In hockey, a slapshot digs the head of the hockey stick into the ice to bend the shaft, like an archer’s bow, storing energy for a sharper snap against the puck and drive it down the ice rink. Cassibry and his team will attempt to drive a hollowed-out puck in on itself, fusing lithium and hydrogen atoms and turning a little of their mass into pure energy.
The “pucks” are approximately two inches wide and an inch thick, smaller than a regulation three-inch puck. They are made of lithium deuteride (LH 2), the lightest metal combined with the middle-weight form of the lightest element.
In May, 2012, big flatbed trucks were unloading combine-sized pieces of equipment at the lab deep on Redstone. When assembled, they will make a Decade Module Two (DM2) pulsed power generator. It was originally designed for the Department of Defense for weapons testing in the 1990s and is coming to Huntsville from Tullahoma, Tenn.
The DM2 consists of banks of capacitors that store an electrical charge for release on command. The analogy is a photographer’s flash. That electrical charge will slam lithium and hydrogen atoms into each other and turn their mass into a tiny burst of pure energy.
The reactions of the lithium deuteride to the energy pulse will tell Cassibry and the team if their theory is correct and their fusion propulsion model is valid. If it is, they will begin crunching the numbers to see how to “scale up” a tiny project into something that might power a rocket.
Z-Pinch Engine: Physical nozzle shown for illustrative purposes only.
Z-pinch and Dense Plasma Focus (DPF) are two promising techniques for bringing fusion power to the field of in-space propulsion. A design team comprising of engineers and scientists from UA Huntsville, NASA’s George C. Marshall Space Flight Center and the University of Wisconsin developed concept vehicles for a crewed round trip mission to Mars and an interstellar precursor mission. Outlined in this paper are vehicle concepts, complete with conceptual analysis of the mission profile, operations, structural and thermal analysis and power/avionics design. Additionally engineering design of the thruster itself is included. The design efforts adds greatly to the fidelity of estimates for power density (alpha) and overall performance for these thruster concepts.
The approach investigated in this study involves the use of a confinement scheme known as a Z-Pinch, which falls under the MIF regime. The premise of a Z-Pinch is to run very large currents (Megampere scale) through a plasma over short timescales (200 ns). The magnetic field resulting from the large current then compresses the plasma to fusion conditions. This plasma formation is widely used in the field of Nuclear Weapons Effects (NWE) testing in the defense industry, as well as fusion energy research. Facilities of note include the Z Machine at Sandia National Laboratories (SNL) and MAGPIE at Imperial College, London. For a fusion propulsion system, the Z-Pinch is formed using annular nozzles with Deuterium-Tritium (D-T) fuel in the innermost nozzle and a Lithium mixture containing Lithium-6/7 in the outermost nozzle. The configuration would be focused in a conical manner so the D-T fuel and Lithium-6/7 mixture meet at a specific point that acts as a cathode so that the lithium mixture can serve as a current return path to complete the circuit.
In addition to serving as a current return path, the lithium liner also serves as a radiation shield. The advantage to this configuration is the reaction between neutrons and Lithium-6 resulting in the production of Tritium, thus adding further fuel to the fusion reaction, and boosting the energy output. In utilizing this method of fusion for propulsion, high thrusts and/or specific impulse can be produced.
Fusion Ignition Chamber
For this study, we selected 38,120 N and a mass flow rate of 0.2 kg/sec resulting in 19,400 seconds of specific impulse, consistent with one of the calculated performance points in the model.
A vehicle of this magnitude represents a substantial investment. In order to justify its development, it must be reusable and suitable for a wide variety of missions. It is possible that with some refurbishment between missions, the vehicle could have a very long service life (much like Naval vessels). New (full) propellant tanks could replace the spent ones, and components that incur significant wear and/or damage (Z-Pinch electrical leads, Lithium nozzles, Z-Pinch diodes and switches, SP-100 Reactor, etc) could be replaced while the vehicle is at dock at L1.