What are the relative roles of heating and cooling in generating solar wind temperature anisotropies?

Temperature anisotropy in the solar wind results from a combination of mechanisms of anisotropic heating (e.g., cyclotron-resonant heating and dissipation of kinetic Alfvén waves) and cooling (e.g., Chew-Goldberger-Low double-adiabatic expansion). In contrast, anisotropy-driven instabilities such as the cyclotron, mirror, and firehose instabilities limit the allowable departure of the plasma from isotropy. This study used data from the Faraday cups on the Wind spacecraft to examine scalar temperature and temperature components of protons. Plasma unstable to the mirror or firehose instability was found to be about 3-4 times hotter than stable plasma. Since anisotropy-driven instabilities are not understood to heat the plasma, these results suggest that heating processes are more effective than cooling processes at creating and maintaining proton temperature anisotropy in the solar wind.     

Scenario for output pulse shortening in microwave generators drivenby relativistic electron beams

At the present time, microwave generators driven by high current relativistic electron beams are not baked and sealed, so their inner surfaces are densely covered with molecules of gas and oil. This allows the production of microwave pulses of 10^-8 s to 10^-7 s duration, but not longer. A microwave pulse termination scenario is speculated as follows: (1) Electrons oscillating in the strong RF field near the metallic surfaces multiply owing to the secondary emission (the multipactor effect); (2) the multipactor electron bombardment stimulates desorption of gas molecules from the metallic surfaces; (3) the gas undergoes avalanche RF breakdown; and (4) the resultant plasma stops microwave generation and, since electron-ion recombination is slow, does not allow the RF field to revive. At the gigawatt power level, the characteristic time of such a scenario is much shorter than that of the cathode and collector plasma expansion and electron beam instabilities. The energy output parameters of relativistic electron microwave generators can be (and usually are) improved at high pulse repetition rates. A more radical improvement is possible using the technology typical for high vacuum tubes, i.e., baking and sealing     

Preserving monotonicity in anisotropic diffusion

We show that standard algorithms for anisotropic diffusion based on centered differencing (including the recent symmetric algorithm) do not preserve monotonicity. In the context of anisotropic thermal conduction, this can lead to the violation of the entropy constraints of the second law of thermodynamics, causing heat to flow from regions of lower temperature to higher temperature. In regions of large temperature variations, this can cause the temperature to become negative. Test cases to illustrate this for centered asymmetric and symmetric differencing are presented. Algorithms based on slope limiters, analogous to those used in second order schemes for hyperbolic equations, are proposed to fix these problems. While centered algorithms may be good for many cases, the main advantage of limited methods is that they are guaranteed to avoid negative temperature (which can cause numerical instabilities) in the presence of large temperature gradients. In particular, limited methods will be useful to simulate hot, dilute astrophysical plasmas where conduction is anisotropic and the temperature gradients are enormous, e.g., collisionless shocks and disk-corona interface.     

Clean HMBC: suppression of strong-coupling induced artifacts in HMBC spectra

A new experiment, clean HMBC, is introduced for suppression of strong-coupling induced artifacts in HMBC spectra. The culprits of these artifacts are an inherent shortcoming of low-pass J filters in the presence of strong coupling and the (1)H pi pulse in the middle of the evolution period aimed at suppressing evolution under heteronuclear J couplings and (1)H chemical shifts. A pi pulse causes coherence transfer in strongly coupled spin systems and, as is well known in e.g., homonuclear J spectra, this leads to peaks that would not be there in the absence of strong coupling. Similar artifacts occur in HMBC spectra, but they have apparently been overlooked, presumably because they have been assigned to inefficiency of low-pass J filters or not noticed because of a coarse digital resolution in the spectra. Clean HMBC is the HMBC technique of choice for molecules notorious for strong coupling among protons, such as carbohydrates, and the new technique is demonstrated on D-mannose. Finally, a fundamental difference between HMBC and H2BC explains why strong-coupling artifacts are much less of a problem in the latter type of spectra.     

Fundamental study towards a better understanding of low pressure radio-frequency plasmas for industrial applications

Two classic radio-frequency (RF) plasmas, i.e., the capacitively and the inductively coupled plasmas (CCP and ICP), are widely employed in material processing, e.g., etching and thin film deposition, etc. Since RF plasmas are usually operated in particular circumstances, e.g., low pressures (mTorr-Torr), high-frequency electric field (13.56 MHz-200 MHz), reactive feedstock gases, diverse reactor configurations, etc., a variety of physical phenomena, e.g., electron resonance heating, discharge mode transitions, striated structures, standing wave effects, etc., arise. These physical effects could significantly influence plasma-based material processing. Therefore, understanding the fundamental processes of RF plasma is not only of fundamental interest, but also of practical significance for the improvement of the performance of the plasma sources. In this article, we review the major progresses that have been achieved in the fundamental study on the RF plasmas, and the topics include 1) electron heating mechanism, 2) plasma operation mode, 3) pulse modulated plasma, and 4) electromagnetic effects. These topics cover the typical issues in RF plasma field, ranging from fundamental to application.     

Nonlinear structures and thermodynamic instabilities in a one-dimensional lattice system

The equilibrium states of the discrete Peyrard-Bishop Hamiltonian with one end fixed are computed exactly from the two-dimensional nonlinear Morse map. These exact nonlinear structures are interpreted as domain walls, interpolating between bound and unbound segments of the chain. Their free energy is calculated to leading order beyond the Gaussian approximation. Thermodynamic instabilities (e.g., DNA unzipping and/or thermal denaturation) can be understood in terms of domain wall formation.     

Linear response in the strong field domain

We show that, while it is well-known that first-order perturbation theory leads to linear response (of, e.g., a material system to an external field), the reverse is not true: linear response does not necessarily imply the validity of first-order perturbation theory, nor does it follow from it that the external perturbation is weak. We do so by analyzing the intensity dependence in the photoexcitation followed by dissociation or isomerization of a bound molecular system by a shaped broadband laser pulse. We show that, in certain cases where strong field effects are definitely present, the observed photoexcitation yield as a function of intensity may exhibit linear dependence over a wide range of intensities. The behavior is shown to coexist with a rather extensive range of coherent control over the branching ratios, an effect that was shown in the past to be impossible in the single precursor state (e.g., in the first-order perturbation theory) domain. For example, we demonstrate computationally that when (flat continuum-mediated) Raman transitions are present, appropriate pulse shaping can lead to a linear yield with intensity over a wide range of intensities, while coherent control over the branching ratio is significant. Thus, it is not necessary to invoke external bath effects (as is currently being done) to explain present-day experiments where coherent control is observed in the linear response regime.     

“Slow” active control of combustion instabilities by modification of liquid fuel spray properties

This paper describes an experimental investigation of the feasibility of using “slow” active control approaches, which “instantaneously” change liquid fuel spray properties, to suppress combustion instabilities. The objective of this control approach was to break up the feedback between the combustion process heat release and combustor pressure oscillations that drive the instability by changing the characteristics of the combustion process (e.g., the characteristic combustion time). To demonstrate the feasibility of such control, this study used a proprietary fuel injector (Nanomiser^TM), which can vary its fuel spray properties, to investigate the dependence of acoustics–combustion process coupling, i.e., the driving of combustion instabilities, upon the fuel spray properties. This study showed that by changing the spray characteristics it is possible to significantly damp combustion instabilities. Furthermore, using combustion zone chemiluminescence distributions, which were obtained by Abel’s deconvolution synchronized with measured acoustic data, it has been shown that the instabilities were mostly driven midway between the combustor centerline and wall, a short distance downstream from the flame holder, where the mean axial flow velocity is approximately zero in the vortex near the flame holder. The results of this study strongly suggest that a “slow” active control system that employs controllable fuel injectors could be effectively used to prevent the onset of detrimental combustion instabilities.     

The refocused INADEQUATE MAS NMR experiment in multiple spin-systems: interpreting observed correlation peaks and optimising lineshapes

The robustness of the refocused INADEQUATE MAS NMR pulse sequence for probing through-bond connectivities has been demonstrated in a large range of solid-state applications. This pulse sequence nevertheless suffers from artifacts when applied to multispin systems, e.g. uniformly labeled (13)C solids, which distort the lineshapes and can potentially result in misleading correlation peaks. In this paper, we present a detailed account that combines product-operator analysis, numerical simulations and experiments of the behavior of a three-spin system during the refocused INADEQUATE pulse sequence. The origin of undesired anti-phase contributions to the spectral lineshapes are described, and we show that they do not interfere with the observation of long-range correlations (e.g. two-bond (13)C-(13)C correlations). The suppression of undesired contributions to the refocused INADEQUATE spectra is shown to require the removal of zero-quantum coherences within a z-filter. A method is proposed to eliminate zero-quantum coherences through dephasing by heteronuclear dipolar couplings, which leads to pure in-phase spectra.     

Chaos-induced pulse trains in the ionization of hydrogen

We predict that a hydrogen atom in parallel electric and magnetic fields, excited by a short laser pulse to an energy above the classical saddle, ionizes via a train of electron pulses. These pulses are a consequence of classical chaos induced by the magnetic field. We connect the structure of this pulse train (e.g., pulse size and spacing) to fractal structure in the classical dynamics. This structure displays a weak self-similarity, which we call "epistrophic self-similarity." We demonstrate how this self-similarity is reflected in the pulse train.     

Proposed beatwave experiment at RAL with the Vulcan CPA laser

A chirped pulse amplification (CPA) laser configuration capable of driving a plasma beat wave into saturation before modulation instabilities can grow is reported. The proposal is based on generating a single sub-ps, broad bandwidth pulse (~16 nm) and stretching and filtering to select two wavelength components (separation ~7 nm). The two spectral components are temporally stretched (to >100 ps) and separated (by ~1 ns). The pulses are then amplified sequentially in a single Nd:glass chain to greater than 15 J per pulse. Using a single-pass reflective grating compressor, the pulses are compressed (from 2 to 5 ps) and automatically synchronized. The system is capable of introducing a chirp, such that the compressed pulses can compensate for relativistic detuning of the plasma wave. The optimum laser amplification and recombination configuration for generating a saturated laser-driven beatwave is presented, and options for future work are discussed     

New methods for calibration of the RF field strength for indirectly observed nuclei

Two novel pulse sequences, CALIS-1 and CALIS-2, for accurate calibration of the RF field strength for an indirectly observed spin are introduced. CALIS-2 is intended for calibration of e.g., (13)C or (15)N pulses on natural abundance samples whilst CALIS-1 is recommended primarily for enriched samples. Both experiments can be performed without prior knowledge or guess of the RF field strength and no delays in the pulse sequences are critically dependent on coupling constants.     

SOA-based loop mirror for tunable pulse-width generation

We propose and experimentally demonstrate a system for the generation of pulses of tunable pulse-width as those required in high spectral efficiency optically routed networks. Pulse narrowing of 500 ps pulses by 90% is accomplished through a SOA based non-linear loop mirror. Optical switching through the SOA loop mirror is used to shape and carve these large pulses (e.g., 500 ps) generated by non-expensive low-frequency optoelectronic components to narrow pulses (e.g., 50 ps). We also calculate the minimum loop size and optimum repetition rate of the original pulse train for the generation of the shorter pulse-width pulse train.     

3D imaging: wave front sensing utilizing a birefringent crystal

Utilizing a rotatable thin birefringent crystal and two polarizers, the three-dimensional (3D) wave front of any light wave can be measured conveniently. In first experiments phase-front distortions as small as 15 &mgr;m are detected with a dynamic range of 3 mm and a spatial resolution of 50 &mgr;m. Such a dynamic range and spatial resolution exceed the performance of conventional wave front sensors of, e.g., the Shack-Hartmann type, significantly. Furthermore, the new system is rather insensitive against mechanical instabilities in opposition to interferometric and holographic techniques.     

Solitons in anharmonic chains with ultra-long-range interatomic interactions

We study the influence of long-range interatomic interactions on the properties of supersonic pulse solitons in anharmonic chains. We show that in the case of ultra-long-range (e.g., screened Coulomb) interactions three different types of pulse solitons coexist in a certain velocity interval: one type is unstable but the two others are stable. The high-energy stable soliton is broad and can be described in the quasicontinuum approximation. But the low-energy stable soliton consists of two components, short-range and long-range ones, and can be considered as a bound state of these components.     

Instabilities Propagated Along the External Magnetic Field in Magnetized Active Plasmas

The conditions for spontaneous onset of instabilities propagated along the external (stationary or quasi-stationary) magnetic field in cold and magnetized active plasmas are considered in collisionless approximation. Attention is given to non-resonant cases, i.e. to situations in which the modal frequencies of the wave significantly differ from the frequency of coherent electromagnetic radiation arising due to the active particles. It is shown that instabilities are also possible in these non-resonant cases and that, under certain specified conditions, they develop only in a very narrow wavelength band. The pertaining amplitude increments are evaluated.     

Instabilities of current distribution in smeared transition zones between two electrical conducting phases

In the transition zone between phases of different conduction types, the electrical current is not only unevenly distributed, but also, in many cases, local or temporal instabilities occur. Most of these effects have already been observed, but until now, no theoretical treatment has been published. As a straightforward extension of the linear mathematical treatment of the current distribution in porous electrodes¹, an explanation of two classes of purely electrical instabilities can be given. Beyond these purely electrical instabilities, a number of more complicated cases has to be considered. These are local instabilities caused by overlapping singularities, e.g. spots of minor density or conductivity, local and temporal instabilities caused by thermal breakthroughs and instabilities caused by hydrodynamical eddies. Moreover the current distribution shows local and temporal fluctuations as the result of the granular structure of the transition zone.     

Propagation instabilities of high-intensity laser-produced electron beams

Measurements of energetic electron beams generated from ultrahigh intensity laser interactions (I>10(19) W/cm(2)) with dense plasmas are discussed. These interactions have been shown to produce very directional beams, although with a broad energy spectrum. In the regime where the beam density approaches the density of the background plasma, we show that these beams are unstable to filamentation and "hosing" instabilities. Particle-in-cell simulations also indicate the development of such instabilities. This is a regime of particular interest for inertial confinement fusion applications of these beams (i.e., "fast ignition").     

Identification of walked-upon materials in auditory, kinesthetic, haptic, and audio-haptic conditions

Locomotion generates multisensory information about walked-upon objects. How perceptual systems use such information to get to know the environment remains unexplored. The ability to identify solid (e.g., marble) and aggregate (e.g., gravel) walked-upon materials was investigated in auditory, haptic or audio-haptic conditions, and in a kinesthetic condition where tactile information was perturbed with a vibromechanical noise. Overall, identification performance was better than chance in all experimental conditions and for both solids and the better identified aggregates. Despite large mechanical differences between the response of solids and aggregates to locomotion, for both material categories discrimination was at its worst in the auditory and kinesthetic conditions and at its best in the haptic and audio-haptic conditions. An analysis of the dominance of sensory information in the audio-haptic context supported a focus on the most accurate modality, haptics, but only for the identification of solid materials. When identifying aggregates, response biases appeared to produce a focus on the least accurate modality--kinesthesia. When walking on loose materials such as gravel, individuals do not perceive surfaces by focusing on the most accurate modality, but by focusing on the modality that would most promptly signal postural instabilities.