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Convective equilibrium and mixing-length theory for stellarator reactors

Description: In high ..beta.. stellarator and tokamak reactors, the plasma pressure gradient in some regions of the plasma may exceed the critical pressure gradient set by ballooning instabilities. In these regions, convective cells break out to enhance the transport. As a result, the pressure gradient can rise only slightly above the critical gradient and the plasma is in another state of equilibrium - ''convective equilibrium'' - in these regions. Although the convective transport cannot be calculated precisely, it is shown that the density and temperature profiles in the convective region can still be estimated. A simple mixing-length theory, similar to that used for convection in stellar interiors, is introduced in this paper to provide a qualitative description of the convective cells and to show that the convective transport is highly efficient. A numerical example for obtaining the density and temperature profiles in a stellarator reactor is given.
Date: September 1, 1985
Creator: Ho, D.D.M. & Kulsrud, R.M.
Partner: UNT Libraries Government Documents Department

Thermonuclear inverse magnetic pumping power cycle for stellarator reactors

Description: A novel power cycle for direct conversion of alpha-particle energy into electricity is proposed for an ignited plasma in a stellarator reactor. The plasma column is alternately compressed and expanded in minor radius by periodic variation of the toroidal magnetic field strength. As a result of the way a stellarator is expected to work, the plasma pressure during expansion is greater than the corresponding pressure during compression. Therefore, negative work is done on the plasma during a complete cycle. This work manifests itself as a back-voltage in the toroidal field coils, and direct electrical energy is obtained from this voltage. For a typical reactor, the average power obtained from this cycle (with a minor radius compression factor on the order of 50%) can be as much as 50% of the electrical power obtained from the thermonuclear neutrons without compressing the plasma. Thus, if it is feasible to vary the toroidal field strength, the power cycle provides an alternative scheme of energy conversion for a deuterium-tritium fueled reactor. The cycle may become an important method of energy conversion for advanced neutron-lean fueled reactors. By operating two or more reactors in tandem, the cycle can be made self-sustaining.
Date: September 1, 1985
Creator: Ho, D.D.M. & Kulsrud, R.M.
Partner: UNT Libraries Government Documents Department

Ion transport in stellarators

Description: Stellarator ion transport in the low-collisionality regime with a radial electric field is calculated by a systematic expansion of the drift-Boltzmann equation. The shape of the helical well is taken into account in this calculation. It is found that the barely trapped ions with three to four times the thermal energy give the dominant contribution to the diffusion. Expressions for the ion particle and energy fluxes are derived.
Date: September 1, 1985
Creator: Ho, D.D.M. & Kulsrud, R.M.
Partner: UNT Libraries Government Documents Department

Neoclassical transport in stellarators

Description: The stellarator neoclassical transport due to particles trapped in local helical wells is calculated in the low-collisionality regime using a systematic expansion. The behavior of electron transport is found to be the same over a wide range of energies, but the behavior of ion transport for low energy ions is found to be different than that for high energy ions. Furthermore, the electron fluxes do not vary with the change in the radial ambipolar electric field nearly as much as do the ion fluxes. Thus, the particle diffusion is controlled by the electrons. A nonradial ambipolar electric field is induced by ion drift. This electric field enhances the transport by about 15 to 20%. A convenient graphical method that allows one to determine the magnitude of the radial ambipolar field for machines with different parameters is presented. Numerical examples show that electron energy confinement time is comparable to the ion energy confinement time for all the different size stellarators studied. Although the neoclassical losses are large, it is shown that ignition can be achieved in a reasonably sized stellarator reactor. Finally, from the standpoint of reactor economics, the confinement scaling law shows that in order to increase n tau, it is better to increase the aspect ratio than the overall dimensions of the reactor.
Date: September 1, 1985
Creator: Ho, D.D.M. & Kulsrud, R.M.
Partner: UNT Libraries Government Documents Department

Study of heavy ion beams during longitudinal compression using particle simulation

Description: Particle simulations show that during longitudinal compression, there is little growth in beam longitudinal and transverse emittance. Both longitudinal and transverse temperatures follow adiabatic laws. The compressed beam has negligible longitudinal momentum spread and therefore can satisfy stringent requirements for final focusing. 4 refs., 3 figs.
Date: August 8, 1988
Creator: Ho, D.D.M. & Brandon, S.T.
Partner: UNT Libraries Government Documents Department

Frontier HED Science accessible on NIF

Description: With the advent of high-energy-density (HED) experimental facilities, such as high-energy lasers and fast Z-pinch pulsed-power facilities, millimeter-scale quantities of matter can be placed in extreme states of density, temperature, and/or velocity. With the commissioning of the NIF laser facility in the very near future, regimes experimentally accessible will be pushed to even higher densities and pressures. This is enabling the emergence of a new class of experimental science, wherein the properties of matter and the processes that occur under the most extreme physical conditions can be examined in the laboratory. Areas particularly suitable to laboratory astrophysics include the study of opacities relevant to stellar interiors, equations of state relevant to planetary interiors, strong shock-driven nonlinear hydrodynamics and radiative dynamics relevant to supernova explosions and subsequent evolution, protostellar jets and high Mach number flows, radiatively driven molecular clouds, nonlinear photoevaporation front dynamics, and photoionized plasmas relevant to accretion disks around compact objects such as black holes and neutron stars. In the area of materials science and condensed matter physics, material properties such as phase, elastic coefficients such as shear modulus, Peierls stress, and transport coefficients such as thermal diffusivity can be accessed at considerably higher densities and pressure than any existing data. In the field of nonlinear optical phenomena, NIF will be an unparalleled setting for studying the nonlinear interactions of a ''statistical ensemble'' of 100 high power beams in large volumes of plasma. In the area of nuclear physics, nuclear reaction rates in dense, highly screened plasmas and on ignition implosions, reactions from excited nuclear states via multi-hit reactions should be possible. A selection from this frontier HED science accessible on NIF will be presented.
Date: September 24, 2007
Creator: Remington, B A; Ho, D D & Ilinskij, A
Partner: UNT Libraries Government Documents Department

LASNEX modeling of target expansion in the ETA-II experiment

Description: We have used the hydrodynamics code LASNEX to model the hydro-expansion of the tantalum target for the ETA-II experiment. The electron beam has kinetic energy of 6 MeV and has a total energy ranges from 720 to 1440 J. The electron beam profile resembles that of a Bennett pinch. The radius for the full-width-at-half-maximum ranges from 1 to 3 mm. For all these parameters, simulations show that the electron beam is able to ablate the central portion of the target. The expansion velocity of the target ranges from about 10� to 5 x 105 cm/s. The target is hot enough so that the surrounding low-density air is ionized and is expanding at a considerably higher velocity than the target itself. Therefore, care must be taken during the experiment to ensure that the measurement is for the tantalum and not for the ionized air.
Date: May 20, 1998
Creator: Ho, D D-M
Partner: UNT Libraries Government Documents Department

Dynamics of heavy ion beams during longitudinal compression

Description: Heavy ion beams with initially uniform line charge density can be compressed longitudinally by an order of magnitude in such a way that the compressed beam has uniform line charge density and velocity-tilt profiles. There are no envelope mismatch oscillations during compression. Although the transverse temperature varies along the beam and also varies with time, no substantial longitudinal and transverse emittance growth has been observed. Scaling laws for beam radius and transport system parameters are given.
Date: March 13, 1987
Creator: Ho, D.D.M.; Bangerter, R.O.; Lee, E.P.; Brandon, S. & Mark, J.W.K.
Partner: UNT Libraries Government Documents Department

Octopole correction of geometric aberrations for high-current heavy-ion fusion beams

Description: The success of heavy-ion fusion depends critically on the ability to focus heavy-ion beams to millimeter-size spots. Third-order geometric aberrations caused by fringe fields of the final focusing quadrupoles can significantly distort the focal spot size calculated by first-order theory. We present a method to calculate the locations and strengths of the octopoles that are needed to correct these aberrations. Calculation indicates that the strengths of the octopoles are substantially less than that of the final focusing quadrupoles. 9 refs., 1 fig.
Date: March 17, 1989
Creator: Ho, D.D.M.; Haber, I.; Crandall, K.R. & Brandon, S.T.
Partner: UNT Libraries Government Documents Department

Longitudinal compression of heavy-ion beams with minimum requirements on final focus

Description: A method is developed to compress a heavy-ion beam longitudinally in such a way that the compressed pulse has a constant line-charge density profile and uniform longitudinal momentum. These conditions may be important from the standpoint of final focusing. By realizing the similarity of the equations that describe the 1-D charged-particle motion to the equations that describe 1-D ideal gas flow, the evolution of lambda and the velocity tilt can be calculated using the method of characteristics developed for unsteady supersonic gasdynamics. Particle simulations confirm the theory. Various schemes for pulse shaping have been investigated.
Date: May 27, 1986
Creator: Ho, D.D.M.; Bangerter, R.O.; Mark, J.W.K.; Brandon, S.T. & Lee, E.P.
Partner: UNT Libraries Government Documents Department

Ignition Capsules with Aerogel-Supported Liquid DT Fuel For The National Ignition Facility

Description: For high repetition-rate fusion power plant applications, capsules with aerogel-supported liquid DT fuel can have much reduced fill time compared to {beta}-layering a solid DT fuel layer. The melting point of liquid DT can be lowered once liquid DT is embedded in an aerogel matrix, and the DT vapor density is consequently closer to the desired density for optimal capsule design requirement. We present design for NIF-scale aerogel-filled capsules based on 1-D and 2-D simulations. An optimal configuration is obtained when the outer radius is increased until the clean fuel fraction is within 65-75% at peak velocity. A scan (in ablator and fuel thickness parameter space) is used to optimize the capsule configurations. The optimized aerogel-filled capsule has good low-mode robustness and acceptable high-mode mix.
Date: October 25, 2011
Creator: Ho, D D; Salmonson, J D; Clark, D S; Lindl, J D; Haan, S W; Amendt, P et al.
Partner: UNT Libraries Government Documents Department

Approaching maximal performance of longitudinal beam compression in induction accelerator drivers

Description: Longitudinal beam compression is an integral part of the US induction accelerator development effort for heavy ion fusion. Producing maximal performance for key accelerator components is an essential element of the effort to reduce driver costs. We outline here initial studies directed towards defining the limits of final beam compression including considerations such as: maximal available compression, effects of longitudinal dispersion and beam emittance, combining pulse-shaping with beam compression to reduce the total number of beam manipulations, etc. The use of higher ion charge state Z greater than or equal to 3 is likely to test the limits of the previously envisaged beam compression and final focus hardware. A more conservative approach is to use additional beamlets in final compression and focus. On the other end of the spectrum of choices, alternate approaches might consider new final focus with greater tolerances for systematic momentum and current variations. Development of such final focus concepts would also allow more compact (and hopefully cheaper) hardware packages where the previously separate processes of beam compression, pulse-shaping and final focus occur as partially combined and nearly concurrent beam manipulations.
Date: May 27, 1986
Creator: Mark, J.W.K.; Ho, D.D.M.; Brandon, S.T.; Chang, C.L.; Drobot, A.T.; Faltens, A. et al.
Partner: UNT Libraries Government Documents Department

Approaching maximal performance of longitudinal beam compression in induction accelerator drivers

Description: Longitudinal beam compression occurs before final focus and fusion chamber beam transport and is a key process determining initial conditions for final focus hardware. Determining the limits for maximal performance of key accelerator components is an essential element of the effort to reduce driver costs. Studies directed towards defining the limits of final beam compression including considerations such as maximal available compression, effects of longitudinal dispersion and beam emittance, combining pulse-shaping with beam compression to reduce the total number of beam manipulators, etc., are given. Several possible techniques are illustrated for utilizing the beam compression process to provide the pulse shapes required by a number of targets. Without such capabilities to shape the pulse, an additional factor of two or so of beam energy would be required by the targets.
Date: September 24, 1986
Creator: Mark, J.W.K.; Ho, D.D.M.; Brandon, S.T.; Chang, C.L.; Drobot, A.T.; Faltens, A. et al.
Partner: UNT Libraries Government Documents Department

DARHT2 X-ray converter target system comparison

Description: Four short current pulses with various pulse widths and spacing will be delivered to the x-ray converter target on the second-axis of the Dual-Axis Radiographic Hydrodynamic Test (DARHT-II) facility. To ensure that the DARHT-II multi-pulse target will provide enough target material for x-ray production for all four pulses, the target needs either to survive the strike of four electron pulses or to accommodate target replenishment. A distributed target may survive hitting of four electron pulses. For target replenishment, two types of target configurations are being considered: stationary target systems with beam repositioning and dynamic moving target systems. They compare these three target systems and their radiographic performance.
Date: March 24, 1999
Creator: Bergstrom, P. M.; Caporaso, G. J.; Chen, Y. J.; Ho, D. D.; McCarrick, J. F.; Pincosy, P. A. et al.
Partner: UNT Libraries Government Documents Department

Stratospheric ozone conservation by electron attachment to chlorine atoms: The negative-ion chemistry

Description: Creating low-energy electrons in the stratosphere by photoelectric emission has the beneficial effect of suppressing ozone destruction by Cl. This is because Cl is converted to Cl{sup {minus}}, which is less reactive. Critical to the success of this scheme is the ability to attach most of the electrons to Cl{sup {minus}} and its hydrates Cl{sup {minus}} (H{sub 2}O). We found that this attachment efficiency is rather high. This is remarkable given the fact that the electron affinity of Cl{sup {minus}} is less than that of NO{sub 3}{minus}. Photoddetachment of NO{sub 3}{minus} is the key factor that leads to this high efficiency. Computer calculations show that ozone increases with electron injection, and most of the electrons end up attaching to Cl{sup {minus}}(H{sub 2}O). We also point out that 40 km, the altitude at which most of the ozone destruction occurs, is also the optimum altitude for injecting photoelectric electrons. 12 refs., 6 figs.
Date: October 12, 1990
Creator: Ho, D.D.M. (Lawrence Livermore National Lab., CA (USA)); Tsang, K.T. (Science Applications International Corp., McLean, VA (USA)); Wong, A.Y. & Siverson, R.J. (California Univ., Los Angeles, CA (USA). Dept. of Physics)
Partner: UNT Libraries Government Documents Department