Direct Drive Fusion Propulsion with Centrifugal Plasma Confinement

Author: Ray Sedwick, Keystone Professor, Aerospace Engineering, University of Maryland

Abstract: The proposed direct-drive fusion concept merges several technologies: Gas Dynamic Mirror (GDM); Centrifugal Confinement; Advanced Magnetic Nozzle Flows, HTS Magnets and Power Management.  The GDM has been researched for many years as a direct-drive fusion device, leveraging the unavoidable plasma loss as a mechanism to produce thrust.  However, plasma stability issues dictate that the aspect ratio of a simple mirror device must be prohibitively large for practical purposes.   A novel modification of the GDM that addresses both confinement and stability is Centrifugal Confinement.  Starting from a low aspect ratio GDM, an electrode is inserted along the axis and biased with respect to the platform.  This drives an azimuthal ExB rotation of the plasma.  The centrifugal acceleration of the rotation forces ion guiding centers along the B-field toward the mid-plane.  When the rotation is sufficiently supersonic, the centrifugal acceleration can contain the pressure and the ions are completely confined, even under collisions.  In addition, the usual MHD interchange instability is stabilized by the supersonic velocity shearing rate.  The electrons are electrostatically held in due to deeply confined ions.  While electron heat loss can occur along the field, it can be shown that at Mach 6 the Lawson Criterion for net fusion energy under electron heat loss can be achieved, as a result of the well-known Pastukhov factor that arises from the deeply confined ions.  The escaping plasma is necessary for direct-drive propulsion, but is generally too energetic for optimal performance.  To address this, we introduce a bypass flow to be entrained within the highly energetic fusion plasma flow, reducing the specific impulse and increasing the thrust at (ideally) constant power.  This method, proposed in early fusion propulsion concepts, has not been fully investigated and must be incorporated into exhaust flow simulations.  This provides the possibility of using a more prevalent propellant – like water – with other advantages, such as a high storage density, safe handling, and natural neutron moderation.  An overview of the concept and preliminary performance estimates will be presented.