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March 25, 2010

Dense Plasma Focus (DPF) Fusion Systems for Space Propulsion

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There were a studies published in 2006 and 2008 that looked at making a fusion rocket using dense plasma focus and air-breathing MHD propulsion. These system proposals would work even if Lawrenceville Plasma physics does not succeed with dense plasma focus fusion and does not include the weight improvements of better ultracapacitors that appears to be likely in the next five years.

Advancements in Dense Plasma Focus (DPF) for Space Propulsion

The development of a dense plasma focus (DPF) propulsion device using p-11B is described. A propulsion system of this type is attractive because of its high thrust-to-weight ratio capabilities at high specific impulses. From a fuel standpoint, p-11B is advantageous because of the aneutronic nature of the reaction, which is favorable for the production of thrust since the charged particles can be channeled by a magnetic field. Different fusion mechanisms are investigated and their implication to the p-11B reaction is explored. Three main requirements must be satisfied to reach breakeven for DPF fusion: a high Ti/Te ratio (~20), an order of magnitude higher pinch lifetime, and the reflection and absorption of at least 50% radiation. Moreover, a power re-circulation method with high efficiency must be available for the relatively low Q value of the DPF fusion reactor. A possible direct energy conversion scheme using magnetic field compression is discussed. DPF parameters are estimated for thrust levels of 1000 kN and 500 kN, and possible propulsion applications are discussed, along with developmental issues.

Filipov and Mather initially engineered the dense plasma focus (DPF) in separate endeavors. During the 1960’s the DPF was first investigated for use as a power reactor, but was ultimately set aside in favor of other concepts. Since then, the DPF has been used predominantly as a laboratory source of both x-rays and neutrons. The United States Air Force (USAF) is currently investigating the DPF for its potential use as a fusion rocket propulsion system burning advanced fuels. In both the Filipov and Mather geometries, the energy stored in a capacitor bank is discharged into a coaxial set of cylindrical electrodes housed in a chamber kept at a pressure of a few torr of gas mixture. The discharge is initiated along an insulator placed at the base of the electrodes, and the rising magnetic pressure in the space moves the current sheath formed in the discharge forward between the two electrodes. Part of the fill gas is ionized and compressed at the top of the center electrode to high temperatures and densities (pinch), as shown in Figure 1. The objective of this paper is to estimate the parameters of a DPF rocket and identify the critical research areas for its development.


ADVANCES IN DENSE PLASMA FOR FUSION POWER AND SPACE PROPULSION, with George Miley, Ph.D.



The required pulse power, energy, and voltage are 800 MW, 80 MJ, and 400 kV. The estimated DPF mass is 16 tons. An increase in Isp will heighten the payload capacity of a mission, making an increase in Isp desirable. For instance, to increase the specific impulse to 2000 s, the mass propellant mass flow rate would need to be decreased by 55% and the bank energy increased to 120 MJ. The corresponding DPF mass is approximated to be 24 tons. Similarly, for a 1000 kN thrust level, with an Isp of 2000 s, the DPF mass would be 48 tons. Lower thrust levels and higher specific impulses can be obtained by varying.

Ultracapacitors
A critical technology to enable the rocket is to have better capacitors with higher joules per kilogram. How much weight in capacitors or ultracapacitors to hold 120-240 megajoules ?

Volume production of new ultracapacitors with 100 kj/kg would reduce the capacitor bank to 1.2 to 2.4 tons.

Even non-controversial ultracapacitor technology would be over twice as good as the 20 kj/kg figure used in the 2006 fusion rocket paper. Electric double layer capacitors are claiming 47 kj/kg from a company Tartu Technologies using mineral-based carbon.

The synthesised nanostructured porous carbon, often called Carbide Derived Carbon (CDC), has a surface area of about 400 m²/g to 2000 m²/g with a specific capacitance of up to 100 F/mL (in organic electrolyte). As of 2006[update] they claimed a supercapacitor with a volume of 135 mL and 200 g weight having 1.6 kF capacitance. The energy density is more than 47 kJ/L at 2.85 V and power density of over 20 W/g

MHD and DPF space plane
Propulsion and Power Generation Capabilities of a Dense Plasma Focus (DPF) Fusion System for Future Military Aerospace Vehicles

The objective of this study was to perform a parametric evaluation of the performance and interface characteristics of a dense plasma focus (DPF) fusion system in support of a USAF advanced military aerospace vehicle concept study. This vehicle is an aerospace plane that combines clean “aneutronic” dense plasma focus (DPF) fusion power and propulsion technology, with advanced “waverider”-like airframe configurations utilizing air-breathing MHD propulsion and power technology within a reusable single-stage-to-orbit vehicle. The applied approach was to evaluate the fusion system details (geometry, power, T/W, system mass, etc.) of a baseline p-11B DPF propulsion device with Q = 3.0 and thruster efficiency, ηprop = 90% for a range of thrust, Isp and capacitor specific energy values. The baseline details were then kept constant and the values of Q and ηprop were varied to evaluate excess power generation for communication systems, pulsed-train plasmoid weapons, ultrahigh-power lasers, shielding/cloaking devices and gravity or time-distorting devices. Thrust values were varied between 100 kN and 1,000 kN with Isp of 1,500 s and 2,000 s, while capacitor specific energy was varied from 1 - 15 kJ/kg. Q was varied from 3.0 to 6.0, resulting in gigawatts of excess power. Thruster efficiency was varied from 0.9 to 1.0, resulting in hundreds of megawatts of excess power. Resulting system masses were on the order of 10’s to 100’s of metric tons with thrust-to-weight ratios ranging from 2.1 to 44.1, depending on capacitor specific energy. Such a high thrust/high Isp system with a high power generation capability would allow military versatility in sub-orbital space, as early as 2025, and beyond as early as 2050.

By holding the specific impulse (2,000 s or 1,500 s), fusion gain (Q = 3.0), and repetition rate (10 Hz) constant, the resulting Bremsstrahlung energy and DPF electrode dimensions could be calculated.
The conclusions that can be drawn from this study are that, if a DPF fusion space thruster were designed that had a Q value of 3.0 and a propulsive efficiency of 0.9 for thrust levels ranging from 100 kN to 1,000 kN with specific impulse values of 1,500 s and 2,000 s, the DPF system would have a total mass ranging from 11.33 metric tons to 480 metric tons and total system volume of 25.5 m3 to 72 m3 depending on the specific energy of the capacitors, which ranged from 1.0 kJ/kg to 15.0 kJ/kg. This was also assuming a mass density for the capacitors of 5.0 MJ/kg and that the DPF itself was one-half the size and mass of the capacitor banks. Thrust-to-weight ratios for the baseline design varied from 2.08 kN/MT to 44.12 kN/MT, depending on propulsion properties.

If all of the system parameters from the baseline design are held constant and the Q value of the reactor is increased, in this study ranging from 3.0 to 6.0, the power made available for electricity generation ranges from a minimum value of 425 MW for thrust = 500 kN, specific impulse = 1,500 s, Q = 3.5 to a maximum value of 7.2 GW for thrust = 1,000 kN, specific impulse = 2,000 s, Q = 6.0.

If the thruster efficiency of the DPF system is increased from 90%, in this study ranging from 90% to 100%, while maintaining a Q value of 3.0, the minimum value of power for electricity generation 88.82 MW for thrust = 500 kN, specific impulse = 1,500 s, ηprop = 92% to a maximum value of 1.09 GW for thrust = 1,000 kN, specific impulse = 2,000 s, ηprop = 100%.

This study has shown that a DPF fusion space propulsion system could be developed with thrust values between 100 kN and 1,000 kN with specific impulses of 1,500 seconds to 2,000 seconds with total system masses of 10’s to 100’s of metric tons with thrust-to-weight ratios ranging from 2.0 to nearly 50.0 depending on capacitor specific energies, which show promise to attain values of 15.0 kJ/kg in the next 20 years.

If the Q value of 3.0 is increased to 6.0 or the propulsive efficiency is increased from 90% to 100%, then it would be conceivable to expect additional power output that could be put towards electricity generation of up to 7.2 GW for Q values of 6.0 with thruster efficiency of 90%, or values of 1.09 GW for propulsive efficiencies of 100% with Q value of 3.0.


Related Research

Pulse power capability of high energy density capacitors based on a new dielectric material
A new dielectric composite consisting of a polymer coated onto a high-density metallized Kraft has been developed for application in pulse power capacitors. The polymer coating is custom formulated for high dielectric constant and strength with minimum dielectric losses. The composite can be wound and processed using conventional wound film capacitor manufacturing equipment. This new system has the potential to achieve 2 to 3 J/cm3 whole capacitor energy density at voltage levels above 3.0 kV, and can maintain its physical properties to temperatures above 175°C. The technical and manufacturing development of the composite material and fabrication into capacitors are summarized in this paper. Energy discharge testing, including capacitance and charge-discharge efficiency at normal and elevated temperatures, as well as DC life testing were performed on capacitors manufactured using this material. TPL (Albuquerque, NM) has developed the material and Aerovox (New Bedford, MA) has used the material to build and test model capacitors. The results of the testing will focus on pulse power applications specifically those found in electro-magnetic armor and guns, high power microwave sources and defibrillators.

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