Cottf-fooizd- LA-UR- 9 5-2674 7/tfe: POSSIBILITIES FOR MAGNETIC CONTROL OF FISSION PLASMA PROPULSION f! CO «- & iji •*?? ea u, K 2 I ^5 •§ ~ S" ^3 « so - u c w o SI §?S "1 iff 6 *E St T3 *> Author(s): R. A. Gervrin, R. A. Nebel, Theoretical Division, T-15 3 — SS2lis S M S g 8 D. I. Poston, TSA-12 * a lii c Los Alamos National Laboratory 1 a' "8 E s 2 .!»• (-< CO O 3 C 09 f-i !i S .•8-8 u * h c 4> .3 *5 a i 8 S3 § 8"° n O 35 „ « G d> U V3 "8-S 1§ u o u « « t. Coo s C V i! *"S s Ui *^ if ° >, ° I sz •8"S 5 fi E Submitted to: 31ST AIAA/ASME/SAE/ASEE JOINT PROPULSION CONFERENCE ii *.s.s AND EXHIBIT, JULY 10-12, 1995/San Diego, CA £ E ff til DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED <g$ A ?!S ? 0 f{-' Los Alamos NATIONAL LABORATORY Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes. The Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. Form No. 836 R5 ST 2629 10/91 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. AM A A AIAA 95 - 2903 Possibilities for Magnetic Control of Fission Plasma Propulsion R. A. Gerwin, D. I. Poston, and R. A. Nebel Los Alamos National Laboratory Los Alamos, NM 31 st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit July 10-12,1995/San Diego, CA For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics 370 L'Enfant Promenade, S.W., Washington, D.C. 20024 Possibilities for Magnetic Control of Fission Plasma Propulsion R. A. Gerwin*, D. I. Poston**, and R. A. Nebel* Los Alamos National Laboratory, Los Alamos, New Mexico Abstract Magnetic fusion energy research suggests the use of some magnetoplasma configurations to address certain critical issues in the gas-core fission approach to nuclear-thermal propulsion. The general framework of such an investigation that was outlined in a previous paper is directed here at the spheromak configuration in greater detail. In some unoptimized examples, we explore the compatibility of gas-core fission reactor criticality conditions with the dynamo action needed to non-inductively sustain the spheromak. The Lundquist number S is identified as a figure of merit, and is estimated by modeling to be as large as 100 in near-critical uranium (233U) plasmas of several-meter dimensions diluted with lithium (7Li) when the spheromak power consumption is treated as a constraint; whereas S as small as 200 is observed to be still able to preserve MHD dynamo activity in 3D resistive MHD simulations. Further optimization studies are required to ascertain whether these two values can be made to coincide. Introduction The gas-core fission approach to nuclear thermal propulsion has long attracted interest because the absence of solid structures within the reactor (fuel rods and heat transfer walls) allows higher temperature operation, and concomitantly increased specific impulse from the heated propellant in an inherently high thrust device.1"4 The simplest version of this approach to nuclear-thermal propulsion is illustrated in Fig. 1-a. However, comprehensive self-consistent simulations1 indicated that ideally separate fuel and propellant volumes are subject to dynamical mixing, thereby impeding the smooth flow of propellant to the nozzle and diluting and cooling the fissionable fuel core. Moreover, even a slight rocket acceleration was found to cause the fuel to displace the propellant in the neighborhood of the nozzle, leading to a rapid loss of fuel and a global deterioration of the propellant flow field. To address these critical issues, a general framework has been outlined for exploring the feasibility of magnetic control of gas-core fission plasmas (the fuel) by means of the simplest steady state configurations, namely, magnetic mirrors and spheromaks.5 Theoretical indications were there provided to the effect that magnetic fields can stiffen the fuel-propellant interface against Kelvin - Helmholtz type instabilities, and that, for magnetic mirrors in compact systems, bad-curvature regions that drive Rayleigh - Taylor type instabilities need not occur. Magnetic pressure balance of the core plasma is not feasible for these highly resistive high-pressure plasmas, which must be contained by the propellant or the wall; nevertheless, active magnetic guiding of the plasma-fuel flow was shown to be feasible for moderate field strengths of order 0.1 T, even in highly resistive uranium plasmas of a few eV temperature and including the effect of rocket accelerations of order go. Magnetic guiding can be resistively effected by the cross-field-flow-induced generation of a magnetic body force in the direction opposite to that flow. Active magnetic guiding of deliberately generated parallel flow of the fuel provides a lever with which to mitigate un-controlled fuel- propellant dynamical mixing, for instance if each species were to have its own nozzle, one on axis and one annular; but then magnetic deflection would have to be invoked to recycle the fuel. Assuming that the fuel leaves through a nozzle with choked flow, an example was presented for which a nozzle area ratio to the main reactor chamber of 10'3 implied a magnetic field in the nozzle throat near one megagauss (much more than is needed for magnetic deflection but which is required by the magnetic mirror geometry), as well as the necessity to magnetically recycle several kg/s of fuel. Here, a reduction in the recycling rate by the utilization of thinner nozzles implies the occurrence of multi-megagauss fields in the mirror-nozzle throat. In contrast, a spheromak concept also was suggested there,5 which does not envision a primary outflow of fuel through a nozzle. Instead, it was pointed out that cross-field flow of plasma is subject to a resistive Presented as paper AIAA-95-2903 at the 31st AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 10-12, 1995, San Diego, CA. Copyright © 1995 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. * Staff Member, Theory Division ** Staff Member, Technology and Safety Assesment Division friction (as described above), leading to the idea of pellet injection of fuel into the toroidal core of the spheromak, with enhanced retention of ionized fuel in the core due to this resistive friction. A schematic diagram of this approach to nuclear-thermal propulsion is displayed in Fig. 1-b. A characteristic slowing- down time for the cross-field flow of resistive plasma was identified and estimated to be quite short and therefore presumably quite effective in retaining the plasma. Of course, gradual depletion of the core and recycling of the fuel ultimately would then have to be dealt with, but apparently at a much lower rate than that associated with the magnetic mirror approach. (This optimistic prospect must be subject to re• examination and possible concept modification in view of the dynamo activity that is needed to non- inductively sustain the spheromak, as discussed below.) A critical issue for the spheromak approach is its non-inductive sustainment under conditions required for fission-reactor criticality. This type of sustainment involves the injection of magnetic helicity along the open magnetic field lines, and is thought to be mediated by the excitation and saturation of global magnetohydrodynamic symmetry-breaking instabilities.6,7 The presence of plasma resistivity, no matter how small, allows the MHD instabilities to cause changes of magnetic field line topology necessary to access states of lower magnetic energy.8 However, resistivity also produces resistive diffusion of the MHD mode structures, weakening their activity; and resistivity is also responsible for unwanted evolution of the mean axisymmetric equilibrium state of the plasma. (Since there can be no steadily applied voltage along the toroidal magnetic axis of the spheromak, its pure axisymmetric embodiment would necessarily decay away in the absence of any other effects.) The latter is combatted by axisymmetric contributions to the Lorentz electric field from second-order products of non-symmetric MHD fluctuations, and this is known as sustainment by the dynamo effect.' The competition between the time scale for MHD activity (nominally a radial Alfven time, r/V^) and a nominal resistive diffusion time scale, ?/D, where D is the resistive diffusivity TJ / JJ.0 , is characterized by the ratio of the latter time to the former. This leads to the Lundquist number, S = rV/D, which is like a magnetic Reynolds number but with the plasma flow velocity replaced by the Alfven velocity. In magnetic fusion energy research, one generally is interested in very large values of 5, lOMO8 , and even higher values. In the case of interest here, however, our concern is that S will be so small as a result of the high plasma resistivity that MHD instabilities will be resistively damped, to the point that they cannot contribute to the dynamo activity needed to counter the rapid resistive decay of the spheromak configuration. In this paper, we shall estimate values of S obtainable from non-optimized examples of gas-core fission propulsion configurations, and shall compare them to the lowest S value observed to produce dynamo sustainment in a non-optimized three-dimensional resistive MHD simulation.