Laser Assisted Pinched Field Fusion Reactor for Space Application

Abstract: a three stage fusion reactor intended for space applications is conceptually developed and compared to an older design determined infeasible.

The previous design included three stages: plasma initiation – confinement area one, a helical progressively confined plasma compression stage – area two, and third – the intended fusion zone. Magnetically, stage one was spherical with magnetic valves fore and aft. Plasma excitation performed by radio frequency saturation. Supply is liquid deuterium. Stage two was physically conical with apex aft and ‘base’ fore – infrared reflection toward stage one to enhance plasma excitation. Reflectors were conical wall configurations with focal range within stage one. Intended magnetic configuration of zone two was to progressively confine plasma to fusion threshold. But the conceptual error not identified initially is the plasma spillage along the length of zone two effectively nullifying any practicality. Another conceptual failure was plasma initiation in zone three – nothing was accounted for to produce the extreme pressure/density/temperature required for fusion to occur.

It is hoped this second conceptual design rectifies these errors. Stage two is corrected and made feasible. Stage three is corrected and made feasible. Included are some ideas concerning energy supply for navigation and life-support systems.

The purpose of this design is to provide both a primary variable thruster and primary power source for space applications that exceeds specifications for conventional chemical rocket motors and fuel cell technologies. Safety and reliability are primary concerns which will be discussed alongside energy for primary systems. As is typical of electro-mechanical systems, energy conversion is always and obstacle to be overcome – some suggestions are made in this direction.

Zone one is modified to be a prolate spheroid with elongation fore and aft. Zone two is modified – no longer helical but still conical with parabolic ring heat reflectors toward the axis of zone one. All wall material must be magnetically permeable. Progressively stronger superconducting electromagnets of ring shape, exterior to cone, force the plasma from fore to aft – progressively confining it along the axis of zone two. Of course, all valves and plasma progression via magnetic confinement are computer controlled. Visualize plasma progression similar to food in your esophagus: you swallow and muscles force the food down toward your stomach. The initial design calls for five superconducting electromagnets which are powered in sequence to force the plasma from fore to aft much like food in your esophagus with one exception: plasma is progressively pinched toward fusion threshold as it passes aft. Zone three is now redesigned to be the physical end of zone two: a parabolic infrared reflector with focus in zone one. A small physical opening with associated magnetic valve to control exhaust products and fusion zone contiguity exists at aft of zone two. Zone three was initially spherical in shape magnetically but now is likely oblate spheroid with axis collinear with gross axis.

Zone three now requires modification to include laser induced fusion after plasma is delivered. I suggest two rings of lasers pointing at the core of zone three: one ring points straight at the core perpendicular to the equatorial surface of the oblate spheroid. A second ring rests toward the fore and points back and centrally again at the core of zone three. The reasoning for no more than two rings of lasers is geometry: there simply isn’t room for more. The parabolic reflector aft of zone three prohibits laser placement there. The electromagnets surrounding zone two prohibit laser placement there. A note about timing: fusion initiation must coincide with introduction of a following pre-plasma ‘ball’ into zone one. This is to avoid constantly pumping energy into the rf-exciters. The purpose of the radio frequency plasma exciters is to drive deuterium gas into the plasma state initially when the engine is first powered up for a journey. Hopefully, if all goes well in design and implementation, thrust can be adjusted by rate of introduction of deuterium gas balls and rate of plasma-balls passing through zone two. Of course, the laser pulsing rate must correspondingly increase. I surmise the limiting factors (amounting to maximum thrust) will be duty-cycle of lasers, maximum cooling rate of sodium loops, and battery capacity. Research needs to be performed about the preferable laser frequency and frequency of the rf-exciters / antenna configuration. Please write to me at q2(at)unc.edu if you’re interested in participating in this project. We need to design and implement some simulations for modeling and demonstration purposes. We need to make plans about building a prototype. We need to consider other more efficient energy translation systems.

Now that we’ve discussed overall design, we can move on with other concerns. The primary products of nuclear reactions are heat and reaction products. The reaction products will be vented away as rocket exhaust. The primary question in any engineer’s mind worth his/her salts is: how to convert heat into electricity? My present solution is unpalatable but unfortunately – ‘the only horse in town’. Molten sodium is typically employed in fission reactors to extract heat toward steam turbine systems. We will employ this scenario as a temporary solution until more efficient energy translation systems are developed.

Two rings of molten sodium rest behind and around the parabolic reflector in zone three. These are connected to a heat-exchange system which employs conventional steam turbine technology: the molten sodium loop is interlocked with a liquid water loop which turns into high pressure steam which drives a turbine. That steam is collected near another heat-exchange subsystem which translates infrared energy to radiation fins exterior to the hull of the ship. The turbine is physically connected to an electrical generator. The collected liquid water is recycled into the interlocked subsystem. High efficiency molten sodium-sulfur batteries physically ring the exterior of the two cooling rings but are not chemically connected. The purpose of the battery ring is two-fold: to provide an energy-sink to stabilize the gross system (powering lasers, rf-exciters, and magnets) and to provide stable power for ship operations.

Safety consideration: nuclear fusion is unlike nuclear fission – a runaway fission reaction results in a melt-down of the core and severe radiation contamination locally. Nuclear fusion is typically extremely hard to induce. Even if the scenario above is technically feasible, the chances for a nuclear fusion reaction to progress from zone three – spreading to zone two and one are practically nil considering energy thresholds. There is more danger from the molten sodium cooling system than the fusion zone. That system needs careful engineering such that chances are small for subsystem failure causing any personnel harm. Functionality, reliability, and safety are coequally important concerns in this scenario. For instance, it may be determined that the engine must be run in an ‘idle mode’ similar to a conventional internal combustion engine idling – just to keep the sodium cooling loop molten and batteries functional.

This project and any associated subsystem is suitable for senior/graduate level research for electrical, mechanical, nuclear, material, and systems engineering majors. Please consult your advisor about getting university credit for participating in this project. Anyone who makes a significant and positive contribution to this project will be recognized for their work on these webpages. Participate in creating a space-faring civilization; seriously consider participating in this project. Of course, research faculty and practicing engineers are always welcome to participate. Please indicate your research area / area of interest / desired contribution area in any correspondence.