Parameters of an ion beam and characteristic features of its slowing-down in a plasma during fast ignition of an inertial fusion target

The physics of the heating of an inertial fusion target by a high-energy ion beam under the conditions of fast ignition of fusion reactions is studied theoretically. The characteristic features of the formation of the spatial distribution of the energy transferred to the plasma from a beam of ions with different initial energies, masses, and charges under fast ignition conditions are determined. The notion of the Bragg peak is extended with respect to the spatial distribution of the temperature of the ion-beam-heated medium. The parameters of the ion beams are determined with which to initiate different regimes of fast ignition of a thermonuclear fuel precompressed to a density of 300–500 g/cm³—the edge regime, in which the ignition region is formed at the outer boundary of the target, and the internal regime, in which the ignition region is formed within the target and, in particular, in its central parts.     

Fast ignition of inertial confinement fusion targets

Results of studies on fast ignition of inertial confinement fusion (ICF) targets are reviewed. The aspects of the fast ignition concept, which consists in the separation of the processes of target ignition and compression due to the synchronized action of different energy drivers, are considered. Criteria for the compression ratio and heating rate of a fast ignition target, the energy balance, and the thermonuclear gain are discussed. The results of experimental and theoretical studies of the heating of a compressed target by various types of igniting drivers, namely, beams of fast electrons and light ions produced under the action of a petawatt laser pulse on the target, a heavy-ion beam generated in the accelerator, an X-ray pulse, and a hydrodynamic flow of laser-accelerated matter, are analyzed. Requirements to the igniting-driver parameters that depend on the fast ignition criteria under the conditions of specific target heating mechanisms, as well as possibilities of practical implementation of these requirements, are discussed. The experimental programs of various laboratories and the prospects of practical implementation of fast ignition of ICF targets are reviewed. To date, fast ignition is the most promising method for decreasing the ignition energy and increasing the thermonuclear gain of an ICF plasma. A large number of publications have been devoted to investigations of this method and adjacent problems of the physics of igniting drivers and their interaction with plasma. This review presents results of only some of these studies that, in the author’s opinion, allow one to discuss in detail the main physical aspects of the fast ignition concept and understand the current state and prospects of studies in this direction.     

Interaction of an ion beam with the target in electron microscope applications

When based on the power-potential law of Lindhard et al. (Mat. Fys. Dan. Vid. Selsk, 33: 1–42, 1963) for ionic impact phenomena on the surfaces of a target, the universal curves of nuclear and electronic energy loss-energy, their resulting yield-energy relationships of sputtering and secondary electron emission yield-energy and range-energy have consistently been derived.According to the results obtained from the above experimental data, a diffusion model of an ion beam penetrating a target is proposed, which takes place throughout a hemisphere with its centre located at half the diffusion depth, and which is found to agree well with the empirical data of ion beam penetration, energy-dissipation profiles and the backscattering coefficient as a function of the reduced depth.Owing to the diffusion model's data, the total secondary electron emission yield due to both primary and backscattering ions is obtained. More importantly, radiation damage in ion beam applications is consistently evaluated as a function of the reduced energy ratio.     

Magneto-inertial fusion with laser compression of a magnetized spherical target

The paper is devoted to the principles of magneto-inertial fusion and laser-plasma methods of generation of a Megagauss field during spherical implosion of a magnetized target. A model based on a magnetic confinement system, namely, a cusp configuration with inertial compression of the target by a laser driver, is developed. The dynamics of plasma in a cusp compressed under the effect of laser beams is precalculated. Analytical and numerical estimates of the particle number and magnetic field intensity during magneto-inertial plasma compression are obtained. The problems of irradiation of a spherically closed volume by a high-energy laser pulse are discussed.     

Low-frequency instability of an ion beam in the field of a high-current REB

A study is made of the linear and nonlinear stages of the low-frequency instability of an ensemble of ions that execute radial oscillations in the electric field of the space charge of an unneutralized high-current relativistic electron beam. Nonlinear mechanisms for stabilizing the low-frequency ion instability are considered. It is shown, in particular, that, under certain conditions, the development of the low-frequency instability can lead to the ejection of ions onto the walls of the drift chamber.     

Experimental study of the evaporation and expansion of a solid pellet in a plasma heated by an electron beam

Results are presented from experiments on the injection of solid pellets into a plasma heated by an electron beam in the GOL-3 device. For this purpose, two pellet injectors were installed in the device. The target plasma with a density of ～10¹⁵ cm^?3 was produced in a solenoid with a field of 4.8 T and was heated by a highpower electron beam with an electron energy of ～1 MeV, a duration of ～7 s, and a total energy of 120–150 kJ. Before heating, the pellet was injected into the center of the plasma column transversely to the magnetic field. The injection point was located at a distance of 6.5 or 2 m from the input magnetic mirror. Polyethylene pellets with a mass of 0.1–1 mg and lithium-deuteride pellets with a mass of 0.02–0.5 mg were used. A few microseconds after the electron beam starts to be injected into the plasma, a dense plasma bunch is formed. In the initial stage of expansion, the plasma bunch remains spherically symmetric. The plasma at the periphery of the bunch is then heated and becomes magnetized. Next, the dense plasma expands along the magnetic field with a velocity on the order of 300 km/s. A comparison of the measured parameters with calculations by a hydrodynamic model shows that, in order to provide such a high expansion velocity, the total energy density deposited in the pellet must be ～1 kJ/cm². This value substantially exceeds the energy density yielded by the target plasma; i.e., the energy is concentrated across the magnetic field onto a dense plasma bunch produced from the evaporated particle.     

Improvement of astaxanthin production by a newly isolated Phaffia rhodozyma mutant with low-energy ion beam implantation

Aims: Isolation, characterization and identification of Phaffia sp. ZJB 00010, and improvement of astaxanthin production with low‐energy ion beam implantation. Methods and Results: A strain of ZJB 00010, capable of producing astaxanthin, was isolated and identified as Phaffia rhodozyma, based on its physiological and biochemical characteristics as well as its internal transcribed spacer (ITS) rDNA gene sequence analysis. With low‐energy ion beam implantation, this wild‐type strain was bred for improving the yield of astaxanthin. After ion beam implantation, the best mutant, E5042, was obtained. The production of astaxanthin in E5042 was 2512 μg g^?1 (dry cell weight, DCW), while the wild‐type strain was about 1114 μg g^?1 (DCW), an increase of 125·5%. Moreover, the fermentation conditions of mutant E5042 for producing astaxanthin were optimized. The astaxanthin production under the optimized conditions was upscaled and studied in a 50‐l fermentor. Conclusions: A genetically stable mutant strain with high yield of astaxanthin was obtained using low‐energy ion beam implantation. This mutant may be a suitable candidate for the industrial‐scale production of astaxanthin. Significance and Impact of the Study: Astaxanthin production in Phaffia rhodozyma could be fficiently improved by low‐energy ion beam implantation, which is a new technology in the mutant breeding of micro‐organisms. The mutant obtained in this work could potentially be utilized in industrial production of astaxanthin.     

Fast ignition upon the implosion of a thin shell onto a precompressed deuterium-tritium ball

Fast ignition of a precompressed inertial confinement fusion (ICF) target by a hydrodynamic material flux is investigated. A model system of hydrodynamic objects consisting of a central deuterium-tritium (DT) ball and a concentric two-layer shell separated by a vacuum gap is analyzed. The outer layer of the shell is an ablator, while the inner layer consists of DT ice. The igniting hydrodynamic flux forms as a result of laser-driven acceleration and compression of the shell toward the system center. A series of one-dimensional numerical simulations of the shell implosion, the collision of the shell with the DT ball, and the generation and propagation of thermonuclear burn waves in both parts of the system are performed. Analytic models are developed that describe the implosion of a thin shell onto a central homogeneous ball of arbitrary radius and density and the initiation and propagation of a thermonuclear burn wave induced by such an implosion. Application of the solution of a model problem to analyzing the implosion of a segment of a spherical shell in a conical channel indicates the possibility of fast ignition of a spherical ICF target from a conical target driven by a laser pulse with an energy of 500?C700 kJ.     

Longitudinal ion source with the current self-compensation of the focused ion beam

Utilization of ballistic focusing in the longitudinal Hall-type ion source is described. It allows transformation of the ion beam shape from an “ellipsoidal” one to a linear one, as well as increasing the ion beam current density per the operating surface. Both the influence of transverse magnetic field and edge effects on the beam shape are investigated. The ion beam is charge compensated by a plasma neutralizer designed on the basis of a supplementary semi-self-maintained magnetron-type discharge and hollow cathode effect.     

The exocytotic fusion pore of small granules has a conductance similar to an ion channel

We measured capacitance changes in cell attached patches of human neutrophils using a high frequency lock-in method. With this technique the noise level is reduced to 0.025 fF such that capacitance steps of 0.1 fF are clearly detected corresponding to exo- and endocytosis of single 60 nm vesicles. It is thus possible to detect almost all known exocytotic and endocytotic processes including exocytosis of small neurotransmitter containing vesicles in most cell types as well as endocytosis of coated and uncoated pits. In neutrophils we demonstrate a stepwise capacitance decrease generated by 60-165 nm vesicles as expected for endocytosis of coated and non-coated pits. Following ionomycin stimulation a stepwise capacitance increase is observed consisting of 0.1-5 fF steps corresponding to the different granule types of human neutrophils from secretory vesicles to azurophil granules. The opening of individual fusion pores is resolved during exocytosis of 200 nm vesicles. The initial conductance has a mean value of 150 pS and can be as low as 35 pS which is similar to the conductance of many ion channels suggesting that the initial fusion pore is formed by a protein complex.     

Radiobiological effects of a low-energy ion beam on wheat

The radiobiological effects of a low-energy nitrogen ion (N⁺) beam on wheat were studied, particularly with regard to the induction of chromosome aberrations. The results demonstrated that the three test varieties showed different sensitivities to ion implantation, and a higher dose of ion implantation had a marked effect on the germination and survival rate of the seeds exposed. The germination rate and survival rate curve basically followed a similar trend in the same variety. Cytological analysis indicated that ion beams were effective in producing chromosome aberrations. The frequencies of mitotic or meiotic cells with chromosome aberrations increased linearly with increasing doses. The aberration types included, for example, acentric fragments, chromosome deletions, lagging chromosomes, chromosome bridges and micronuclei. In the root tip cells, aberrations chiefly consisted of acentric fragments and deletions. Chromosome bridges and lagging chromosomes were the main aberration phenomena observed in the pollen mother cells. The highest frequencies of root tip cells and pollen mother cells with chromosome aberrations were 15.2% and 39.8%, respectively. Changes in morphology and mutant were also observed in the plants derived from exposed seeds. Received: 10 April 2000 / Accepted: 10 October 2000     

Mutants of Ficus pumila produced by ion beam irradiation with an improved ability to uptake and assimilate atmospheric nitrogen dioxide

Production of novel mutants with a high ability to mitigate pollutants is important for phytoremediation. We investigated the use of ion beam irradiation to produce mutants of Ficus pumila L. with an improved ability to mitigate atmospheric nitrogen dioxide (NO2). More than 25,000 shoot explants were irradiated with an ion beam (12C5+, 12C6+, or 4He2+), from which 263 independent plant lines were obtained. The plants were analyzed for NO2 uptake by fumigation with 1 ppm 15N-labeled NO2 for 8 h in light, followed by mass spectrometric analysis. The mean NO2 uptake values of each of the 263 lines differed over a 110-fold range. Propagation was attempted using cuttings from 44 lines showing the greatest NO2 uptake; in total, 15 lines were propagated. Two of the 15 lines showed a mean NO2 uptake 1.7- to 1.8-fold greater than that of the wild-type. This increase in NO2 uptake was heritable in both lines; their progenies showed a significantly greater ability to take up and assimilate NO2 than did the wild-type. RAPD analysis demonstrated DNA variation between the progeny plants and the wild type, suggesting that the progeny were true mutants. These mutants of F. pumila may prove useful in mitigating atmospheric NO2.     

Linear theory of the interaction of an electron beam with electromagnetic waves in the ion focus regime in a pasotron

A study is made of the interaction of an electron beam focused by a positive ion background with electromagnetic waves in a pasotron—a modern plasma microwave electronics device with long-term interaction in the ion focus regime.     

Molecular characterization of microbial mutations induced by ion beam irradiation

A positive selection system for gene disruption using a sucrose-sensitive transgenic rhizobium was established and used for the molecular characterization of mutations induced by ion beam irradiations. Single nucleotide substitutions, insertions, and deletions were found to occur in the sucrose sensitivity gene, sacB, when the reporter line was irradiated with highly accelerated carbon and iron ion beams. In all of the insertion lines, fragments of essentially the same sequence and of approximately 1188bp in size were identified in the sacB regions. In the deletion lines, iron ions showed a tendency to induce larger deletions than carbon ions, suggesting that higher LET beams cause larger deletions. We found also that ion beams, particularly "heavier" ion beams, can produce single gene disruptions and may present an effective alternative to transgenic approaches.     

Protein-induced fusion can be modulated by target membrane lipids through a structural switch at the level of the fusion peptide

Regulatory features of protein-induced membrane fusion are largely unclear, particularly at the level of the fusion peptide. Fusion peptides being part of larger protein complexes, such investigations are met with technical limitations. Here, we show that the fusion activity of influenza virus or Golgi membranes is strongly inhibited by minor amounts of (lyso)lipids when present in the target membrane but not when inserted into the viral or Golgi membrane itself. To investigate the underlying mechanism, we employ a membrane-anchored peptide system and show that fusion is similarly regulated by these lipids when inserted into the target but not when present in the peptide-containing membrane. Peptide-induced fusion is regulated by a reversible switch of secondary structure from a fusion-permissive alpha-helix to a nonfusogenic beta-sheet. The "on/off" activation of this switch is governed by minor amounts of (lyso)-phospholipids in targets, causing a drop in alpha-helix and a dramatic increase in beta-sheet contents. Concomitantly, fusion is inhibited, due to impaired peptide insertion into the target membrane. Our observations in biological fusion systems together with the model studies suggest that distinct lipids in target membranes provide a means for regulating membrane fusion by causing a reversible secondary structure switch of the fusion peptides.     

Calcium-permeable acid-sensing ion channel is a molecular target of the neurotoxic metal ion lead

Acid-sensing ion channels (ASICs) are emerging as fundamental players in the regulation of neural plasticity and in pathological conditions. Here we showed that lead (Pb2+), a well known neurotoxic metal ion, reversibly and concentration-dependently inhibited ASIC currents in the acutely dissociated spinal dorsal horn and hippocampal CA1 neurons of rats. In vitro expression of ASIC subunits in combination demonstrated that both ASIC1 and -3 subunits were sensitive to Pb2+. Mechanistically, Pb2+ reduced the pH sensitivity of ASICs independent of membrane voltage change. Moreover, Pb2+ inhibited the ASIC-mediated membrane depolarization and the elevation of intracellular Ca2+ concentration. In addition, we compared the effect of Pb2+ with that of Ca2+ or amiloride to explore the possible interactions of Pb2+ and Ca2+ in regulating ASICs, and we found that Pb2+ inhibited ASIC currents independent of the amiloride/Ca2+ blockade. Because ASIC1b and -3 subunits are mainly expressed in peripheral neurons, our data identified ASIC1a-containing Ca2+-permeable ASIC as a novel central target of Pb2+ action, which may contribute to Pb2+ neurotoxicity.     

Fast subcellular localization by cascaded fusion of signal-based and homology-based methods

Background

The functions of proteins are closely related to their subcellular locations. In the post-genomics era, the amount of gene and protein data grows exponentially, which necessitates the prediction of subcellular localization by computational means.

Results

This paper proposes mitigating the computation burden of alignment-based approaches to subcellular localization prediction by a cascaded fusion of cleavage site prediction and profile alignment. Specifically, the informative segments of protein sequences are identified by a cleavage site predictor using the information in their N-terminal shorting signals. Then, the sequences are truncated at the cleavage site positions, and the shortened sequences are passed to PSI-BLAST for computing their profiles. Subcellular localization are subsequently predicted by a profile-to-profile alignment support-vector-machine (SVM) classifier. To further reduce the training and recognition time of the classifier, the SVM classifier is replaced by a new kernel method based on the perturbational discriminant analysis (PDA).

Conclusions

Experimental results on a new dataset based on Swiss-Prot Release 57.5 show that the method can make use of the best property of signal- and homology-based approaches and can attain an accuracy comparable to that achieved by using full-length sequences. Analysis of profile-alignment score matrices suggest that both profile creation time and profile alignment time can be reduced without significant reduction in subcellular localization accuracy. It was found that PDA enjoys a short training time as compared to the conventional SVM. We advocate that the method will be important for biologists to conduct large-scale protein annotation or for bioinformaticians to perform preliminary investigations on new algorithms that involve pairwise alignments.