Computational chemistry

With the advent of supercomputers, modern computational chemistry algorithms and codes, a powerful tool was created to help fill NASA's continuing need for information on the properties of matter in hostile or unusual environments. Computational resources provided under the National Aerodynamics Simulator (NAS) program were a cornerstone for recent advancements in this field. Properties of gases, materials, and their interactions can be determined from solutions of the governing equations. In the case of gases, for example, radiative transition probabilites per particle, bond-dissociation energies, and rates of simple chemical reactions can be determined computationally as reliably as from experiment. The data are proving to be quite valuable in providing inputs to real-gas flow simulation codes used to compute aerothermodynamic loads on NASA's aeroassist orbital transfer vehicles and a host of problems related to the National Aerospace Plane Program. Although more approximate, similar solutions can be obtained for ensembles of atoms simulating small particles of materials with and without the presence of gases. Computational chemistry has application in studying catalysis, properties of polymers, all of interest to various NASA missions, including those previously mentioned. In addition to discussing these applications of computational chemistry within NASA, the governing equations and the need for supercomputers for their solution is outlined

Evolution of reconnection along an arcade of magnetic loops

RHESSI observations of a solar flare showing continuous motions of double hard X-ray sources interpreted as footpoints of magnetic loops are presented. The temporal evolution shows many distinct emission peaks of duration of some tens of seconds ('elementary flare bursts'). Elementary flare bursts have been interpreted as instabilities or oscillations of the reconnection process leading to an unsteady release of magnetic energy. These interpretations based on two-dimensional concepts cannot explain these observations, showing that the flare elements are displaced in a third dimension along the arcade. Therefore, the observed flare elements are not a modulation of the reconnection process, but originate as this process progresses along an arcade of magnetic loops. Contrary to previous reports, we find no correlation between footpoint motion and hard X-ray flux. This flare apparently contradicts the predictions of the standard translation invariant 2.5D reconnection models.Comment: 4 pages, 3 figures, to be published in Astrophysical Journal Letter

Electrical conductivity and thermal dilepton rate from quenched lattice QCD

We report on a continuum extrapolation of the vector current correlation function for light valence quarks in the deconfined phase of quenched QCD. This is achieved by performing a systematic analysis of the influence of cut-off effects on light quark meson correlators at $T\simeq 1.45 T_c$ using clover improved Wilson fermions. We discuss resulting constraints on the electrical conductivity and the thermal dilepton rate in a quark gluon plasma. In addition new results at 1.2 and 3.0 $T_c$ will be presented.Comment: 4 pages, 6 eps figures, to appear in the proceedings of Quark Matter 2011, 23-28 May 2011, Annecy, Franc

The impact of supercomputers on experimentation: A view from a national laboratory

The relative roles of large scale scientific computers and physical experiments in several science and engineering disciplines are discussed. Increasing dependence on computers is shown to be motivated both by the rapid growth in computer speed and memory, which permits accurate numerical simulation of complex physical phenomena, and by the rapid reduction in the cost of performing a calculation, which makes computation an increasingly attractive complement to experimentation. Computer speed and memory requirements are presented for selected areas of such disciplines as fluid dynamics, aerodynamics, aerothermodynamics, chemistry, atmospheric sciences, astronomy, and astrophysics, together with some examples of the complementary nature of computation and experiment. Finally, the impact of the emerging role of computers in the technical disciplines is discussed in terms of both the requirements for experimentation and the attainment of previously inaccessible information on physical processes

Technologies for aerobraking

Aerobraking is one of the largest contributors to making both lunar and Mars missions affordable. The use of aerobraking/aeroassist over all propulsive approaches saves as much as 60 percent of the initial mass required in low earth orbit (LEO); thus, the number and size of earth to orbit launch vehicles is reduced. Lunar transfer vehicles (LTV), which will be used to transport personnel and materials from LEO to lunar outpost, will aerobrake into earth's atmosphere at approximately 11 km/sec on return from the lunar surface. Current plans for both manned and robotic missions to Mars use aerocapture during arrival at Mars and at return to Earth. At Mars, the entry velocities will range from about 6 to 9.5 km/sec, and at Earth the return velocity will be about 12.5 to 14 km/sec. These entry velocities depend on trajectories, flight dates, and mission scenarios and bound the range of velocities required for the current studies. In order to successfully design aerobrakes to withstand the aerodynamic forces and heating associated with these entry velocities, as well as to make them efficient, several critical technologies must be developed. These are vehicle concepts and configurations, aerothermodynamics, thermal protection system materials, and guidance, navigation, and control systems. The status of each of these technologies are described, and what must be accomplished in each area to meet the requirements of the Space Exploration Initiative is outlined

A computer program for a line-by-line calculation of spectra from diatomic molecules and atoms assuming a Voight line profile

Computer program predicts the spectra resulting from electronic transitions of diatomic molecules and atoms in local thermodynamic equilibrium. The program produces a spectrum by accounting for the contribution of each rotational and atomic line considered

A survey of laser lightning rod techniques

The work done to create a laser lightning rod (LLR) is discussed. Some ongoing research which has the potential for achieving an operational laser lightning rod for use in the protection of missile launch sites, launch vehicles, and other property is discussed. Because of the ease with which a laser beam can be steered into any cloud overhead, an LLR could be used to ascertain if there exists enough charge in the clouds to discharge to the ground as triggered lightning. This leads to the possibility of using LLRs to test clouds prior to launching missiles through the clouds or prior to flying aircraft through the clouds. LLRs could also be used to probe and discharge clouds before or during any hazardous ground operations. Thus, an operational LLR may be able to both detect such sub-critical electrical fields and effectively neutralize them

Collisional and viscous damping of MHD waves in partially ionized plasmas of the solar atmosphere

Magnetohydrodynamic (MHD) waves are widely considered as a possible source of heating for various parts of the outer solar atmosphere. Among the main energy dissipation mechanisms which convert the energy of damped MHD waves into thermal energy are collisional dissipation(resistivity) and viscosity. The presence of neutral atoms in the partially ionized plasmas of the solar photosphere, chromosphere and prominences enhances the efficiency of both these energy dissipation mechanisms. A comparative study of the efficiency of MHD wave damping in solar plasmas due to collisional and viscous energy dissipation mechanisms is presented here. The damping rates are taken from Braginskii 1965 and applied to the VAL C model of the quiet Sun (Vernazza et al. 1981). These estimations show which of the mechanisms are dominant in which regions. In general the correct description of MHD wave damping requires the consideration of all energy dissipation mechanisms via the inclusion of the appropriate terms in the generalized Ohm’s law, the momentum, energy and induction equations. Specific forms of the generalized Ohm’s Law and induction equation are presented that are suitable for regions of the solar atmosphere which are partially ionised

The Abelianization of QCD Plasma Instabilities

QCD plasma instabilities appear to play an important role in the equilibration of quark-gluon plasmas in heavy-ion collisions in the theoretical limit of weak coupling (i.e. asymptotically high energy). It is important to understand what non-linear physics eventually stops the exponential growth of unstable modes. It is already known that the initial growth of plasma instabilities in QCD closely parallels that in QED. However, once the unstable modes of the gauge-fields grow large enough for non-Abelian interactions between them to become important, one might guess that the dynamics of QCD plasma instabilities and QED plasma instabilities become very different. In this paper, we give suggestive arguments that non-Abelian self-interactions between the unstable modes are ineffective at stopping instability growth, and that the growing non-Abelian gauge fields become approximately Abelian after a certain stage in their growth. This in turn suggests that understanding the development of QCD plasma instabilities in the non-linear regime may have close parallels to similar processes in traditional plasma physics. We conjecture that the physics of collisionless plasma instabilities in SU(2) and SU(3) gauge theory becomes equivalent, respectively, to (i) traditional plasma physics, which is U(1) gauge theory, and (ii) plasma physics of U(1)x U(1) gauge theory.Comment: 36 pages; 15 figures [minor changes made to text, and new figure added, to reflect published version