Stopping Length Research Articles

Investigation on creeping discharges propagating over epoxy resin and glass insulators in the presence of different gases and mixtures

This paper deals with the experimental characterization of discharges propagating over insulators of epoxy and glass, immersed in a gas or a gaseous mixture, under lightning impulse voltages (1.2/50 μs), using a point-plane electrode arrangement. The gases and mixtures we considered are SF6, N2, CO2, SF6-N2 and SF6-CO2. The morphology of creeping discharges and their final lengths are investigated versus the kind of insulator material, the amplitude and polarity of the voltage, the type of the gas (resp. mixture) and its pressure. It is shown that the shape of discharges and their final (stopping) lengths Lf depend significantly on the solid insulator and the type of gas. For given solid and gas, Lf increases quasi-linearly with the voltage and decreases when the gas pressure increases. The discharges do not always present a radial structure as reported in the literature. For given voltage and pressure, Lf is longer when the point electrode is positive than when it is negative while the initiation voltage of discharges is higher with a negative point than with a positive one; and Lf is longer with glass than with epoxy. Lf is shorter in SF6 than in CO2 or N2. On the other hand, the increase of SF6 content in SF6-CO2 mixture leads to a significant decrease of Lf. Therefore, the addition of small concentration of SF6 in a given gas mixture improves the dielectric strength of insulating structure.

Measurements of activation reaction rates in transverse shielding concrete exposed to the secondary particle field produced by intermediate energy heavy ions on an iron target

Reaction rate distributions were measured inside a 60-cm thick concrete pile placed at the lateral position of a thick (stopping length) iron target that was bombarded with heavy ions, 400 MeV/u C and 800 MeV/u Si. Foils of aluminum and gold, as well as gold, tungsten and manganese covered with cadmium were inserted at various locations in the concrete pile to serve as activation detectors. Features of reaction rate distribution, such as the shape of the reaction rate profile, contribution of the neutrons from intra-nuclear cascade and that from evaporation to the activation reactions are determined by the analysis of measured reaction rates. The measured reaction rates were compared with those calculated with radiation transport simulation codes, FLUKA and PHITS, to verify their capability to predict induced activity. The simulated reaction rates agree with the experimental results within a factor of three in general. However, systematic discrepancies between simulated reaction rates and measured reaction rates attributed to the neutron source terms are observed.

Measurements of induced activity in concrete by secondary particles at forward direction produced by intermediate energy heavy ions on an Fe target

Spallation and neutron capture reaction rate distributions were measured using activation detectors inside a 90-cm thick ordinary concrete pile exposed to a field of secondary particles escaping a thick (stopping length) iron target bombarded with various intermediate energy ions, 230 MeV/u He, 400 MeV/u C, and 800 MeV/u Si. Activation detectors of aluminum, bismuth, gold, and gold covered with cadmium were inserted at various depths in the concrete pile. In addition, the distributions of activation reaction rate were simulated by FLUKA and PHITS Monte-Carlo codes. Generally, comparison of measured and calculated reaction rates show agreement within a factor of two. The experimental data will be useful for benchmarking Monte-Carlo radiation transport simulation code capabilities in estimating radioactivity induced in accelerator radiation shielding.

Benchmarking of activation reaction distribution in an intermediate energy neutron field

Neutron-induced reaction rate depth profiles inside concrete shield irradiated by intermediate energy neutron were calculated using a Monte-Carlo code and compared with an experiment. An irradiation field of intermediate neutron produced in the forward direction from a thick (stopping length) target bombarded by 400 MeV nucleon(-1) carbon ions was arranged at the heavy ion medical accelerator in Chiba. Ordinary concrete shield of 90 cm thickness was installed 50 cm downstream the iron target. Activation detectors of aluminum, gold and gold covered with cadmium were inserted at various depths. Irradiated samples were extracted after exposure and gamma-ray spectrometry was performed for each sample. Comparison of experimental and calculated shows good agreement for both low- and high-energy neutron-induced reaction except for (27)Al(n,X)(24)Na reaction at the surface.

Analysis of creeping discharges activity at solid/liquid interfaces subjected to ac voltage

This paper presents the experimental characterisation of discharges propagating over insulators immersed in mineral oil, under ac using a point-plane electrode arrangement. The influence of voltage magnitude and the nature of insulator on the cumulative number of discharge events and their locations during one voltage cycle are also investigated. It is shown that the nature and the thickness of insulator have a significant influence on the properties of discharges and especially their morphology and stopping length Lf . For a given thickness, Lf increases quasi-linearly with voltage; it decreases when the thickness is increased and/or the dielectric constant of insulator decreases, thus indicating the important role of capacitive effect in the propagation mechanism. The total number of discharges recorded during 500 cycles of applied voltage increases significantly with the voltage magnitude. The threshold voltage Ui of discharges depends on the kind of material; and the average number of discharges ( nmoy ) increases with the dielectric constant e r , of insulator. For instance, nmoy is the highest for Bakelite (e r =4.8) and the lowest for Polycarbonate (e r =2.9). nmoy of negative discharges seems to be slightly higher than that for positive ones irrespective of the solid sample and the amplitude of voltage.

Collective deceleration: Toward a compact beam dump

With the increasing development of laser electron accelerators, electron energies beyond a GeV have been reached and higher values are expected in the near future. A conventional beam dump based on ionization or radiation loss mechanisms is cumbersome and costly, not to mention the radiological hazards. We revisit the stopping power theory of high-energy charged particles in matter and discuss the associated problem of beam dumping from the point of view of collective deceleration. The collective stopping length in an ionized gas can be several orders of magnitude shorter than that described by the Bethe-Bloch formulas and associated with multiple electromagnetic cascades in solids. At the same time, the tenuous density of the gas makes the radioactivation negligible. Such a compact beam dump without radioactivation works well for short and dense bunches, as they are typically generated from a laser wakefield accelerator. In addition, the nonuniform transverse wakefield can induce microbunching of the electron bunch by betatron oscillation. The microstructure could serve as a prebunched source for coherent radiation or feeding a free electron laser.

Interpretation of the SiKαx-ray spectra accompanying the stopping of swift Ca ions in low-density SiO2aerogel

This article presents a detailed analysis of the $K\ensuremath{\alpha}$ x-ray spectra of Si induced by $11.4$ MeV/u Ca projectiles penetrating a low-density SiO${}_{2}$ aerogel target measured with high spectral and spatial resolution at the UNILAC accelerator at GSI-Darmstadt. The low-density material used in the experiment was crucial for the space-resolved studies of the Si x-ray radiation (for different energies of stopping Ca ions). The stopping length of the $11$ MeV/u Ca ions reaches up to $10$ mm in the low-density SiO${}_{2}$ aerogel, whereas in regular quartz solid targets it is about $100$ times shorter. The analysis of the x-ray spectra emitted by the stopping medium has shown a high level of the $L$-shell ionization, especially in the later considered phase (${E}_{p}~5$ MeV/u) of the stopping process. It has been further demonstrated that the population of the highly ionized states produced in the ion-atom collisions can be substantially reduced in the time between the collision and the x-ray emission due to the very intense rearrangement processes occurring in Si situated in the chemical environment of oxygen atoms. Moreover, comparison of the experimental values of the $K\ensuremath{\alpha}$ $L$-shell satellite energy shifts with the results of the multiconfiguration Dirac-Fock calculations allows us to find that Si valence electron configuration is enriched due to electron transfer from valence-electron-rich oxygen atoms into highly ionized silicon atoms. Our results indicate that the Coulomb explosion in a highly ionized track core is prevented by rapid neutralization in the femtosecond time scale.

A potential space- and power-effective muon sensor module for imaging a volcano

The application of muon radiography will be greatly enhanced by the use of two muon sensor modules that save electric power consumption and are easily transportable. Muon sensor modules used for a volcano observation must have a low electric power consumption requirement and be both waterproof and portable. In this article, we discuss two candidate sensor modules: (1) a portable muon sensor module with wavelength-shifting (WLS) fibers and a multi-anode photomultiplier tube (MAPMT), and (2) a regular scintillator telescope with PMT complemented by a low-power Cockcroft-Walton circuit (CWPMT). A realistic telescope system consisting of a muon sensor module with MAPMT has been tested and found to consume 76 W, most of which (72 W) is used by the redundant electronic circuit required for pulse shaping; this could be modified to drastically improve the power consumption. In comparison, a muon telescope system with a CWPMT was found to consume 7.57 W. We also calculated the muon stopping length in SiO2 by means of a Monte-Carlo simulation. This calculation provided the average density structure along the muon path in rock, where the muon path length was shorter than 1.5 km, with an accuracy of about 5% during a 90-day measurement period by assuming a 1-m2 muon detector with an angular resolution of 25 mrad.

Reaction-in-flight neutrons as a signature for shell mixing in National Ignition Facility capsules

Analytic calculations and results from computational simulations are presented that suggest that reaction-in-flight (RIF) neutrons can be used to diagnose mixing of the ablator shell material into the fuel in deuterium-tritium (DT) capsules designed for the National Ignition Facility (NIF) [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)]. Such mixing processes in NIF capsules are of fundamental physical interest and can have important effects on capsule performance, quenching the total thermonuclear yield. The sensitivity of RIF neutrons to hydrodynamical mixing arises through the dependence of RIF production on charged-particle stopping lengths in the mixture of DT fuel and ablator material. Since the stopping power in the plasma is a sensitive function of the electron temperature and density, it is also sensitive to mix. RIF production scales approximately inversely with the degree of mixing taking place, and the ratio of RIF to down-scattered neutrons provides a measure of the mix fraction and/or the mixing length. For sufficiently high-yield capsules, where spatially resolved RIF images may be possible, neutron imaging could be used to map RIF images into detailed mix images.

Streamer propagation and breakdown in natural ester at high voltage

This paper reports an experimental investigation about streamer propagation and breakdown in natural esters (rape-seed oils). Experiments are carried out over a large range of gap distances (from 2 to 20 cm) and voltage (up to 460 kV), in order to determine the properties of natural esters for high voltage insulation. A detailed description of positive and negative streamers is obtained by the measurement of propagation velocity, stopping length, transient currents, charge, streak photographs of the emitted light. As in previous studies carried out in mineral transformer oil, several propagation "modes" can be observed in rape-seed oil. They show the transition from "slow" streamers (velocity about 1 km/s), to fast streamers (velocity up to 200 km/s). Fast positive streamers propagate at much lower voltage in rape-seed oil compared to mineral oil. In turn, this induces lower breakdown voltages and shorter time to breakdown in this liquid. Measurements of currents and streak photographs bring the evidence of the stepped character of fast positive streamers and show the constancy of their velocity under very high voltage.

Reduction of Threshold for Laser Fusion Ignition by Nonlinear Force Driven Block Acceleration

Block ignition was proposed recently as a possible alternate approach to fast ignition for ICF fusion. This approach uses a modified petawatt-picosecond (PW-ps) laser pulse shape where the prepulse is strongly suppressed. This results in highly directed plasma blocks due to nonlinear (ponderomotive) force acceleration with space charge neutral ion current densities above 1011 Amp/cm2. This allows ignition of deuterium-tritium targets at densities somewhat above solid state density. However, a key issue has been the need to reduce the extremely high thresholds for the high energy flux densities of the blocks as pointed out in a related theory by Bobin and Chu in 1972. Here we show how the threshold can be reduced by a factor up to 20 by two effects. An important contribution comes from the inhibition factor for thermal conductivity due to electric double layers created in the block process. The second effect is the reduction to the stopping length, giving increased heating by the fusion product alpha due to collective interactions in the blocks. Results from including these effects in a hydrodynamic analysis are presented. The advantage of this approach for an ICF fusion reactor is the relaxed pre-compression requirement for high gain.

Laser fusion with nonlinear force driven plasma blocks: Thresholds and dielectric effects

AbstractAnomalous interaction of picosecond laser pulses of terawatt to petawatt power is due to suppression of relativistic self-focusing if prepulses are cut-off by a contrast ratio higher than 108, resulting in quasi-neutral directed plasma blocks with deuterium tritium ion current densities above 1011A/cm2. This is still not high enough for ignition of solid-state density deuterium tritium because the energy flux densityE* has to be higher than the threshold of 4 ×108J/cm2obtained within the theory of Chu (1972). A revision of this evaluation shows a reduction of this threshold by a factor 20 if the later discovered inhibition factors for thermal conduction because of double layer effects as well as the shorter stopping lengths of the alpha particles due to collective effects are taken into account. Under these relaxed conditions, the parameters of nonlinear force generated blocks of dielectrically increased thickness for deuterium tritium ignition with directed ions of energies near the 80 keV resonances are discussed.

Gluon energy loss in the gauge-string duality

We estimate the stopping length of an energetic gluon in a thermal plasma of strongly coupled N=4 super-Yang-Mills theory by representing the gluon as a doubled string rising up out of the horizon.

Neutron production by a 13C thick target irradiated by 20–90 MeV protons

Neutron production using an enriched 13C carbon converter has been measured during the design study of the italian RIB facility SPES. Energy and angular distributions of neutrons emitted by bombarding a 13C target of stopping length with protons in the range of 20 to 90MeV have been measured by time-of-flight and activation and compared with the prediction of a Monte Carlo code developed at Snezhinsk. At the proton energy of 100MeV, firstly envisaged for SPES, the gain with respect to a natural C target is less than a factor of two, while yields still compare well with those for 40MeV deuterons on natural carbon adopted by SPIRAL-II. At energies near 30MeV the 13C thick target is definitely more prolific than the target of natural carbon, but both yields with protons are clearly lower than the one with deuterons. At the energy of 20MeV envisaged for a first stage of SPES it might be more efficient to irradiate the uranium target with protons rather than using the two-stage method with converter.

Development of a quasi-monoenergetic neutron field using the 7Li(p,n)7Be reaction in the energy range from 250 to 390 MeV at RCNP

A quasi-monoenergetic neutron field using the (7)Li(p,n)(7)Be reaction has been developed at the ring cyclotron facility at the Research Center for Nuclear Physics (RCNP), Osaka University. Neutrons were generated from a 10-mm-thick Li target injected by 250, 350 and 392 MeV protons and neutrons produced at 0 degrees were extracted into the time-of-flight (TOF) room of 100-m length through the concrete collimator of 10 x 12 cm aperture and 150 cm thickness. The neutron energy spectra were measured by a 12.7-cm diam x 12.7-cm long NE213 organic liquid scintillator using the TOF method. The peak neutron fluence was 1.94 x 10(10), 1.07 x 10(10) and 1.50 x 10(10) n sr(-1) per muC of 250, 350 and 392 MeV protons, respectively. The neutron spectra generated from various thick (stopping length) targets of carbon, aluminium, iron and lead, bombarded by 250 and 350 MeV protons, were also measured with the TOF method. Although these measurements were performed to obtain thick target neutron yields, they are also used as a continuous energy neutron field. These neutron fields are very useful for characterising neutron detectors, measuring neutron cross sections, testing irradiation effects for various materials and performing neutron shielding experiments.

Propagation dynamics of a LaMnO3 laser ablation plume in an oxygen atmosphere

The effect of ambient gas on the expansion dynamics of the plasma plume generated by excimer laser ablation of a LaMnO3 target is investigated by using fast photography. The plume propagation in an oxygen environment is examined with pressure ranging from vacuum to a few millibars. Imaging analysis performed over a complete set of images of the plume emission, for each pressure, has allowed following the changes in the plume structure, the plume front dynamics and stopping length, as well as the variation of plume angular distribution as a function of time and pressure. Our experimental findings evidence a number of interesting effects occurring at various times of the expansion and at different pressure levels. At low pressure (<10−2mbar) the plume expands freely resembling the propagation in high vacuum condition. Then, at intermediate pressure levels (10−2–1mbar) spatial splitting of the plume, plume front oscillations, plume broadening and sharpening, and, finally, plume stopping are observed as a function of the expansion time. The plume stopping length and the asymptotic angular width of the plume as a function of the pressure are analyzed and discussed. The expansion dynamics of the plume front is examined by means of a theoretical description of plume evolution and shock-wave propagation, which considers distance-related pressure dimensionless variables, resulting in a fairly good agreement.

Resistive filamentation of laser-driven fast electron beams

Simulations of fast electron transport in imploded targets are presented. We found that beam filamentation in low density zones can prevent 1-2MeV electron propagation towards the high density core. To mitigate this effect, we have considered electrons with mean kinetic energies of 4-5 MeV. The large stopping length of these electrons is partially balanced by collective stopping, resulting in a good coupling of the beam with the imploded target.

The influence of grain rotation on the structure of dust aggregates

We study the effect of rotation during the collision between dust aggregates, in order to address a mismatch between previous model calculations of Brownian motion driven aggregation and experiments. We show that rotation during the collision does influence the shape and internal structure of the aggregates formed. The effect is limited in the ballistic regime when aggregates can be considered to move on straight lines during a collision. However, if the stopping length of an aggregate becomes smaller than its physical size, extremely elongated aggregates can be produced. We show that this effect may have played a role in the inner regions of the solar nebula where densities were high.

Shielding experiment of heavy-ion produced neutrons using a tissue-equivalent proportional counter.

A shielding experiment was performed at the HIMAC (Heavy Ion Medical Accelerator in Chiba), of National Institute of Radiological Sciences (NIRS), to measure neutron dose using a spherical TEPC (tissue-equivalent proportional counter) of 12.55 cm inner diameter. Neutrons are produced from a 5 cm thick stopping length Cu target bombarded by 400 MeV/nucleon C6+ ions and penetrate concrete or iron shields of various thicknesses at 0 degree to the beam direction. From this shielding experiment. y-distribution, mean lineal energy, absorbed dose, dose equivalent and mean-quality factor were obtained behind the shield as a function of shield thickness. The neutron dose attenuation lengths were also obtained as 126 g cm(-2) for concrete and 211 g cm(-2) for iron. The measured results were compared with the calculated results using the MARS Monte Carlo code.

Simulations of electron transport in laser hot spots

Simulations of electron transport are carried out by solving the Fokker–Planck equation in the diffusive approximation. The system of a single laser hot spot, with open boundary conditions, is systematically studied by performing a scan over a wide range of the two relevant parameters. (1) Ratio of the stopping length over the width of the hot spot. (2) Relative importance of the heating through inverse Bremsstrahlung compared to the thermalization through self-collisions. As for uniform illumination [J. P. Matte et al., Plasma Phys. Controlled Fusion 30, 1665 (1988)], the bulk of the velocity distribution functions (VDFs) present a super-Gaussian dependence. However, as a result of spatial transport, the tails are observed to be well represented by a Maxwellian. A similar dependence of the distributions is also found for multiple hot spot systems. For its relevance with respect to stimulated Raman scattering, the linear Landau damping of the electron plasma wave is estimated for such VDFs. Finally, the nonlinear Fokker–Planck simulations of the single laser hot spot system are also compared to the results obtained with the linear nonlocal hydrodynamic approach [A. V. Brantov et al., Phys. Plasmas 5, 2742 (1998)], thus providing a quantitative limit to the latter method: The hydrodynamic approach presents more than 10% inaccuracy in the presence of temperature variations of the order ΔT/T≳1%, and similar levels of deformation of the Gaussian shape of the Maxwellian background.