Central Hot Spot Research Articles

Shock ignition: an alternative scheme for HiPER

Two main paths are now under investigation that aim at thermonuclear ignition of hydrogen isotopes using lasers: central hot spot self-ignition and externally driven fast ignition of preassembled fuel. A third, intermediate, scheme is shock ignition, which combines the simplicity of self-ignition capsules to the hydrodynamic robustness of the fast ignition fuel assembly. This study addresses the potential of shock ignition for the HiPER project and provides a preliminary assessment of possible detrimental effects. Monodimensional simulations are performed to study the robustness of the ignition scheme in terms of shock launching time and laser power. Bidimensional simulations address the sensitivity of shock ignition to irradiation nonuniformity and to low mode asymmetries of the fuel assembly.

X-pinch x-ray sources driven by a 1μs capacitor discharge

X pinches formed from tungsten, molybdenum, titanium, aluminum, and alloys, such as constantan and nichrome with diameters of 13–25μm, have been imploded with a 300kA, 1μs quarter-period capacitor discharge and observed with an array of diagnostics including time-integrated optical photography, x-ray pinhole cameras, x-ray Si photodiodes, a flat crystal spectrograph, and a Nd:YAG laser interferometry/schlieren system. The results of these comprehensive observations are presented. When a single, central hot spot is formed with the right choice of wire and x-ray filter, it is observed to be highly localized, very bright and small enough that point projection radiography can be routinely demonstrated for a simple biological specimen. For an aluminum X pinch, spectroscopic measurements indicate that the hot spots have a plasma density and electron temperature consistent with ∼1020cm−3 and ∼320–350eV, respectively.

Shock Ignition of Thermonuclear Fuel with High Areal Density

A novel method by C. Zhou and R. Betti [Bull. Am. Phys. Soc. 50, 140 (2005)] to assemble and ignite thermonuclear fuel is presented. Massive cryogenic shells are first imploded by direct laser light with a low implosion velocity and on a low adiabat leading to fuel assemblies with large areal densities. The assembled fuel is ignited from a central hot spot heated by the collision of a spherically convergent ignitor shock and the return shock. The resulting fuel assembly features a hot-spot pressure greater than the surrounding dense fuel pressure. Such a nonisobaric assembly requires a lower energy threshold for ignition than the conventional isobaric one. The ignitor shock can be launched by a spike in the laser power or by particle beams. The thermonuclear gain can be significantly larger than in conventional isobaric ignition for equal driver energy.

Geographic patterns in the distribution of Palearctic songbirds

AbstractA database was created of digitized equal area distribution maps of 3,036 phylogenetic species of Palearctic songbirds. Biogeographic patterns are reported for two data sets: (1) including all passeriform bird species reported as breeding within the boundaries of our study map, (2) passeriform species restricted in their distribution to our study region, thus excluding the partly extra-limital taxa. With respect to the data set excluding partly extra-limital taxa, the average range size is 238 grid cells (grid cell area: 4,062 km2). Analysis of the geographic distribution of species richness for the full data set showed several hotspot regions, mostly located in mountainous areas. The index of range-size rarity identified similar hotspot regions as that for species richness, albeit that the range-size rarity de-emphasized the central Siberian hotspot. Range-size rarity hotspots that are not evident on the measure of species richness concern a great number of islands. Much more prominent on the index of range-size rarity are the Atlas Mountains of northern Africa, the Jabal al Akhdar region in NE Libya, and the eastern border of the Mediterranean. Restricting the analysis of geographic variation to the 25% of the species with smallest ranges resulted in a greatly simplified pattern of hotspots.

Cortical Involvement in the Generation of Essential Tremor

Conflicting results on the existence of tremor-related cortical activity in essential tremor (ET) have raised questions on the role of the cortex in tremor generation. Here we attempt to address these issues. We recorded 64 channel surface EEGs and EMGs from forearm muscles in 15 patients with definite ET. EEG and EMG power spectra, relative power of the rhythmic EMG activity, relative EEG power at the tremor frequency, and EEG-EMG and EEG-EEG coherence were calculated and their dynamics over time explored. Corticomuscular delay was studied using a new method for narrow-band coherent signals. Corticomuscular coherence in the contralateral central region at the tremor frequency was present in all patients in recordings with a relative tremor EMG power exceeding a certain level. However, the coherence was lost intermittently even with tremors far above this level. Physiological 15- to 30-Hz coherence was found consistently in 11 patients with significantly weaker EMG activity in this frequency range. A more frontal (mesial) hot spot was also intermittently coupled with the tremor and the central hot spot in five patients. Corticomuscular delays were compatible with transmission in fast corticospinal pathways and feedback of the tremor signal. Thus the tremor rhythm is intermittently relayed only in different cortical motor areas. We hypothesize that tremor oscillations build up in different subcortical and subcortico-cortical circuits only temporarily entraining each other.

Inertial fusion science in Europe

Europe has built significant laser facilities to study ICF since the beginning of this science. The goal is to understand the processes of ignition and propagation of thermonuclear combustion. Three routes toward fusion are pursued, each of which has advantages and difficulties. The conventional routes are using a central hot spot created by the same compression and heating laser beams, either with indirect or direct drive. A more recent route, fast ignition, has been actively studied since the 90's, increasing the need for PW high energy lasers to create the hot spot; some European lasers of this kind are already functioning, others are under construction or planned. Among European facilities, Laser Mega Joule (LMJ), which is under construction, will be the most powerful tool at the end of the decade, along with NIF in the USA, to study and obtain fusion. On all these facilities, numerous results are and will be obtained in the fields of High Energy Density Physics (HEDP) and Ultra High Intensity (UHI).

Inertial fusion with the LMJ

The progress of the construction of very large laser facilities LMJ and NIF enables the prediction of inertial fusion achievement. These facilities will open new fields for research: the high energy density physics. Pressures of several 100 Mbars and temperatures of several 100 eV will be reached. Measurements of material properties (EOS and opacities) which have been demonstrated on current or former facilities will be possible at these never reached conditions. Pure hydrodynamics (instabilities) and radiative hydrodynamics astrophysical issues will be addressed.However, ignition and gain as a first proof of Inertial Confinement Fusion is a primary goal. The indirect drive route to inertial fusion has been prepared for many years by CEA (Commissariat à l'Energie Atomique). The last ten years were imprinted by a close collaboration between CEA and US-DOE in both the areas of facilities R&D and ignition target physics.The scientific issues are well known: the propagation of laser light through the very long plasma created inside the hohlraum has to be understood and mastered to be sure that less than 10% of laser energy will be backscattered by parametric instabilities. On the other hand, the stability of the capsule implosion has to be matched with the fabrication surface finish so as to avoid shell destruction and extinction of the central hot spot. Recent advances at CEA have allowed a better confidence of reaching ignition using the facility previously specified. These works used the CEA computing capability combined with plasma experiments on existing lasers facilities. Ignition achievement also supposes the realization of suitable cryogenic targets.CEA began the construction of the Laser Megajoule (LMJ), a 240-beam laser facility, at the CEA Laboratory CESTA near Bordeaux. The LMJ is designed to deliver 2 MJ of 0.35 µm light to targets for high energy density physics experiments.Four beams were operated for plasma experiments on the Ligne d'Integration Laser (LIL) at CESTA, for the end of 2004, meeting the specifications for LMJ. The realization phase of the LMJ facility was initiated in March 2003 with the start of construction of the building and the target chamber.

Overview of inertial fusion and high-intensity laser plasma research in Europe

Inertial fusion science is driven by ‘the quest for ignition’. For many years, a ‘conventional’ route towards inertial fusion has been investigated using two different approaches: ignition by an inertial central hot spot could be obtained either through the direct or the indirect drive scheme. Both imply the use of a very large facility to operate the driver, which is a powerful laser in the current projects (LMJ in France as well as NIF in the US). The LMJ construction being on the way, a large amount of experimental and computational work is currently being done to deepen the understanding of ignition requirements.On the other hand, the so-called ‘fast ignition’ approach has led to an increasingly important amount of scientific work since it was proposed at the beginning of the 1990s. During the last several years (from the previous IFSA held in Kyoto in 2001), several PW-class high-intensity laser facilities have been built in Europe.In the meantime, a large number of interesting results related to fast electron and proton production have been obtained with the existing facilities. Observation of laser-irradiated solid targets has provided the first evidence of electron bunches separated by half the period of light. Nevertheless, target heating remains modest. On the other hand, multi-megaelectronvolt highly collimated electron beams have been produced by table-top lasers interacting with the low-density plasmas. They open the feasibility of a lot of applications: x-ray probe beams in plasma physics, biology, chemistry, injector for conventional accelerators, etc). Laser-produced proton beams is also a growing field, with a lot of promising applications: proton therapy, radio-isotope production, diagnostic for transient phenomena in laser–plasma interaction, etc.Inertial fusion research is fostered by a sustained effort of organization and coordination at the national level (the creation of an Institute for Lasers and Plasmas in France) as well as at the European level (within the 6th Framework Program).

Converging geometry Rayleigh–Taylor instability and central ignition of inertial confinement fusion targets

The Rayleigh–Taylor instability (RTI) of the inner surface of an inertial confinement fusion shell is studied through high-resolution two-dimensional numerical simulations. The instability is seeded by a mass displacement introduced in the simulations at the end of the implosion coasting stage. Analysis of single-mode, small-amplitude perturbations confirms that ablation caused by electron conduction and fusion alpha-particles causes significant growth reduction of all modes and stabilization of high-l modes. Different measures of the instability are discussed and compared with modified Takabe-like expressions. Large-amplitude multi-mode simulations are performed to study the effects of RTI on ignition and burn. RTI perturbations reduce the size of the central hot spot and delay ignition. For a few different perturbation spectra the dependence of fusion yield on the initial perturbation root mean square amplitude is studied.

Self-healing femtosecond light filaments.

A recent experiment [Appl. Phys. Lett. 83, 213 (2003)] indicated that filaments created in femtosecond high-power pulses propagating in air are surprisingly robust when interacting with microscopic water droplets. We present numerical modeling of the dynamics of the filament-droplet interaction. Our simulation results provide further insight into the interplay between the filament's core and the wide transverse pedestal of the pulse. It is shown that the robustness of the filament comes from the transverse low-intensity pedestal that controls the formation of the central hot spot. Implications for penetration of wide, high-power beams through obscurants are discussed.

X-ray snapshots of the maturation of an antibody response to a protein antigen.

The process whereby the immune system generates antibodies of higher affinities during a response to antigen (affinity maturation) is a prototypical example of molecular evolution. Earlier studies have been confined to antibodies specific for small molecules (haptens) rather than for proteins. We compare the structures of four antibodies bound to the same site on hen egg white lysozyme (HEL) at different stages of affinity maturation. These X-ray snapshots reveal that binding is enhanced, not through the formation of additional hydrogen bonds or van der Waals contacts or by an increase in total buried surface, but by burial of increasing amounts of apolar surface at the expense of polar surface, accompanied by improved shape complementarity. The increase in hydrophobic interactions results from highly correlated rearrangements in antibody residues at the interface periphery, adjacent to the central energetic hot spot. This first visualization of the maturation of antibodies to protein provides insights into the evolution of high affinity in other protein-protein interfaces.

Assessment of temperature gradients in multianvil assemblies using spinel layer growth kinetics

We present an empirical equation to link the thickness of a MgAl2O4 spinel layer growing at the interface between MgO and Al2O3 multianvil high‐pressure assembly pieces to pressure (P), temperature (T), and time (t), extending the recent study of Watson et al. [2002] to the pressure range 6–16 GPa and temperature range 1673–2273 K. For the spinel thickness ΔX we obtain urn:x-wiley:15252027:media:ggge349:ggge349-math-0001 This equation can be used to assess the thermal gradient in any multianvil assembly where MgO and Al2O3 filler pieces are in contact at P‐T conditions within the MgAl2O4 spinel stability field. As an illustration, we show that the central hot spot, in which temperatures can be considered constant, is close to 1 mm long in a common octahedral multianvil assembly with 8 mm edge length and rhenium furnace, while it extends to 3 mm long in an octahedron with 18 mm edge length using a straight graphite heating element. In addition, we present the results of a spinel growth experiment performed at 2273 K and 15 GPa with 18O enriched MgO, which shows that oxygen is mobile at a length scale exceeding that of the spinel layer. This finding raises the possibility that under some circumstances growth of spinel (and of reaction‐product layers in other oxide systems) might be accomplished by concurrent fluxes of oxygen and cations. This “mobile oxygen” model differs from the more conventional Mg‐Al exchange model proposed by Watson and Price [2002] for the lower‐pressure experiments and might explain the observed differences in systematics between the high‐ and low‐pressure data sets.

De novo mutations in sporadic deletional Duchenne muscular dystrophy (DMD) cases.

Dinucleotide repeat polymorphism based genetic analysis is a powerful approach to gain insight into rare genetic events like germline mosaicism and de novo mutations. The loss of heterozygosity of polymorphic dinucleotide loci at "deletional hotspot" of dystrophin gene can provide direct evidence of carrier status in female relatives of affected DMD patients with overlapped exonic deletions. We have used 4 STR loci of the central deletional hotspot of the dystrophin gene for genetic analysis in sporadic unrelated DMD families. Twenty-nine mothers of sporadic deletional cases were analysed and their carrier status was determined. Eighteen of them showed heterozygosity in the deleted loci suggesting the occurrence of de novo mutations. In 9 cases, the carrier status was indeterminate while 2 showed germline mosaicism. Our observations reiterated the importance of STR analysis in determining the status of mothers of sporadic deletional DMD cases in order to provide proper genetic counselling.

Design and optimisation of a high-temperature silicon micro-hotplate for nanoporous palladium pellistors

The conventional design of the heater in a silicon micro-hotplate employ a simple meandering resistive track to form a square element. We show that this heater structure produces an uneven thermal profile characterised by a central hot spot with a significant variation in temperature of some 50°C across the plate at an average temperature of 500°C. Four novel micro-heater designs are reported here and fabricated on hotplates with an active area that ranges from (200×200)μm2 to (570×570)μm2 in order to vary systematically the ratio of membrane to heater length from a value of 5.0–2.7, respectively. All the designs have been simulated using a 3D electro-thermo-mechanical finite element model and results agree well with thermal profiles taken using an infrared microscope. One of the designs, referred to here as ‘drive-wheel’ structure, performs best and reduces the lateral variation in temperature to only ±10°C. The different resistive micro-heaters have been calibrated with the lowest power consumption being 50mW at 500°C, which is well below the power consumption of any commercial pellistor; the maximum temperature before rupture being 870°C. The micro-hotplates were electrochemically coated with a 20nm thick mesoporous palladium catalyst and the pellistors' response tested to 2.5% methane in air. The micro-heaters were observed to be stable for a period of 1000h and should provide a good platform for exploitation in commercial catalytic pellistors.

Percolation Cooling of the Three Mile Island Unit 2 Lower Head by Way of Thermal Cracking and Gap Formation

Two partial models have been developed to elucidate the Three Mile Island Unit 2 lower head coolability by water percolation from above into the thermally cracking debris bed and into a gap between the debris and the wall. The bulk permeability of the cracked top crust is estimated based on simple fracture mechanics and application of Poiseuille’s law to the fractures. The gap is considered as an abstraction representing an initially rugged interface, which probably expanded by thermal deformation and cracking in connection with the water ingress. The coupled flow and heat conduction problem for the top crust is solved in slab geometry based on the two-phase Darcy equations together with quasi-steady mass and energy conservation equations. The resulting water penetration depth is in good agreement with the depth of the so-called loose debris bed. The lower-head and bottom-crust problem is treated analogously by a two-dimensional axisymmetric model. The notion of a gap is maintained as a useful concept in the flow analysis. Simulations show that a central hot spot with a peak wall temperature of 1075 to 1100°C can be obtained, but the quenching rates are not satisfactory. It is concluded that a three-dimensional model with an additional mechanism to explain the sudden water ingress to the hot spot center would be more appropriate.

ICF target ignition studies in planar, cylindrical, and spherical geometries

A model for nonlocal α particle transport is implemented in a one-dimensional radiation hydrodynamics code and applied for typical directly and indirectly driven ICF target simulations. Ignition criteria are compared for four configurations; spherical, cylindrical, planar with a central hot spot, and another planar configuration with a hot spot on one side. This last planar configuration is then used for fast ignitor studies, since it is able to simulate hot spot formation by direct laser heating and free expansion towards the incoming beam. Ignition requirements as a function of laser intensity and pulse length, taking into account the relationship between intensity, hot electron temperature, and range, are then determined.

Area Networks for Conserving Iberian Insects: A Case Study of Dung Beetles (col., Scarabaeoidea)

Possibilities to create an efficient network of areas for the conservation of Iberian dung-beetle biodiversity were investigated. Data were taken from ‘BANDASCA’, a 15,740 record dung-beetle database with information for 101,996 specimens. The greatest species richness and phylogenetic diversity are found in the Iberian Central System cordilleras, while those of range-size rarity are in the southernmost corner of the Iberian Peninsula and in the coastal plains of the Guadalquivir basin. Additionally, broad regional score trends of these variables reach a maximum in a smaller northeastern, and a larger southeastern region (Baetic and sub-Baetic mountains). Major Central and Southern Spain dung-beetle hotspots contain many of the UTM grid cells which delimit the near-minimum-set of areas hosting maximum dung-beetle biodiversity scores. They are viewed as the nucleus of a potential network of reserves. A UTM grid cell (30TXK1) where Onthophagus albarracinus, an endemic and demographically rare species inhabits the Sierra of Albarracin and ‘Montes Universales’, was detected as an irreplaceable area. Because of recent land use changes and rapid human-induced habitat transformation threatening dung-beetle fauna, caution should be taken not to recommend an in memoriam network of areas no longer hosting a fauna extinct today. The abandoning of traditional herding management in Europe threatens the preservation of grasslands where dung-feeding beetles play a major ecological role in maintaining pasture quality and livestock health. A return to more traditional herding methods would require stable dung-beetle populations. Any insect conservation planning in Spain would benefit from more biogeographical information for other insect groups.

A Broadband X‐Ray Study of Supernova Remnant 3C 397

We present a broadband imaging and spectral study of the radio-bright supernova remnant (SNR) 3C 397 with ROSAT, ASCA, and RXTE. A bright X-ray spot seen in the HRI image hints at the presence of a pulsar-powered component and gives this SNR a composite X-ray morphology. Combined ROSAT and ASCA imaging shows that the remnant is highly asymmetric, with its X-ray emission peaking at the western lobe. The hard-band images obtained with the ASCA Gas Imaging Spectrometer show that much of the hard X-ray emission arises from the western lobe, associated with the SNR shell, with little hard X-ray emission associated with the central hot spot. The spectrum from 3C 397 is heavily absorbed and dominated by thermal emission with emission lines evident from Mg, Si, S, Ar and Fe. Single-component models fail to describe the X-ray spectrum, and at least two components are required: a soft component characterized by a low temperature and a large ionization timescale, and a hard component required to account for the Fe-K emission line and characterized by a much lower ionization timescale. We use a set of nonequilibrium ionization (NEI) models (Borkowski et al., in preparation), and find that the fitted parameters are robust. The temperatures from the soft and hard components are ~0.2 keV and ~1.6 keV respectively. The corresponding ionization timescales n0t (n0 being the preshock hydrogen density) are ~6 × 1012 cm-3 s and ~6 × 1010 cm-3 s, respectively. The large n0t of the soft component suggests it is approaching ionization equilibrium; thus it can be fit equally well with a collisional equilibrium ionization model. The spectrum obtained with the Proportional Counter Array (PCA) of RXTE is contaminated by emission from the Galactic ridge, with only ~15% of the count rate originating from 3C 397 in the 5-15 keV range. The PCA spectrum allowed us to confirm the thermal nature of the hard X-ray emission. A third component originating from a pulsar-driven component is possible, but the contamination of the source signal by the Galactic ridge did not allow us to determine its parameters or find pulsations from any hidden pulsar. We discuss the X-ray spectrum in the light of two scenarios: a young ejecta-dominated remnant of a core-collapse SN, and a middle-aged SNR expanding in a dense ISM. In the first scenario, the hot component arises from the SNR shell, and the soft component from an ejecta-dominated component. 3C 397 would be a young SNR (a few thousand years old), but intermediate in dynamical age between the young historical shells (like Tycho or Kepler), and those that are well into the Sedov phase of evolution (like Vela). In the second scenario, the soft component represents the blast wave propagating in a dense medium, and the hard component is associated with hot gas encountering a fast shock, or arising from thermal conduction. In this latter scenario, the SNR would be ~twice as old, and transitioning into the radiative phase. The current picture we present in this paper is marginally consistent with this scenario, but it cannot be excluded. A spatially resolved spectroscopic study is needed to resolve the soft and hard components and differentiate between the two scenarios. Future Chandra and XMM data will also address the nature of the mysterious central (radio-quiet) X-ray spot.

Hydrodynamique de l'implosion d'une cible FCI

We consider the hydrodynamic behaviour of an imploding ICF target. After recalling the requirements for thermonuclear ignition, we analyse in detail the two phases of the implosion. First, the acceleration, with its two important ignition parameters, the implosion velocity and the entropy generated in the DT. Second, the slowing down which, through electronic conduction, allows the creation of a central hot spot of sufficient mass. The method of successive shocks allows the détermination of the laser pulse shape which, by insuring an isentropic acceleration, optimises the energy delivered to the DT. We obtain the implosion velocity as a function of the initial capsule parameters and the maximum incident radiation. We clarify the hydrodynamical reason which makes the ignition threshold sensitive to the entropy generated during the acceleration. These elements constitute the basis of a global indirect drive ICF model, allowing target design optimisation.

Structural and functional analysis of the 1:1 growth hormone:receptor complex reveals the molecular basis for receptor affinity

The designed G120R mutant of human growth hormone (hGH) is an antagonist and can bind only one molecule of the growth hormone receptor. We have determined the crystal structure of the 1:1 complex between this mutant and the receptor extracellular domain (hGHbp) at 2.6 Å resolution, and used it to guide a detailed survey of the structural and functional basis for hormone-receptor recognition. The overall structure of the complex is very similar to the equivalent portion of the 1:2 complex, showing that formation of the active complex does not involve major conformational changes. However, a segment involved in receptor-receptor interactions in the 1:2 complex is disordered in this structure, suggesting that its productive conformation is stabilized by receptor dimerization. The hormone binding site of the receptor comprises a central hydrophobic patch dominated by Trp104 and Trp169, surrounded by a hydrophilic periphery containing several well-ordered water molecules. Previous alanine scanning showed that the hydrophobic “hot spot” confers most of the binding energy. The new structural data, coupled with binding and kinetic analysis of further mutants, indicate that the hot spot is assembled cooperatively and that many residues contribute indirectly to binding. Several hydrophobic residues serve to orient the key tryptophan residues; kinetic analysis suggests that Pro106 locks the Trp104 main-chain into a required conformation. The electrostatic contacts of Arg43 to hGH are less important than the intramolecular packing of its alkyl chain with Trp169. The true functional epitope that directly contributes binding energy may therefore comprise as few as six side-chains, participating mostly in alkyl-aromatic stacking interactions. Outside the functional epitope, multiple mutation of residues to alanine resulted in non-additive increases in affinity: up to tenfold for a hepta-alanine mutant. Contacts in the epitope periphery can therefore attenuate the affinity of the central hot spot, perhaps reflecting a role in conferring specificity to the interaction.