Helium Particles Research Articles

Optimization of Stellarator Reactor Parameters

Four quasi-axisymmetric compact stellarator plasma and coil configurations are analyzed for their potential as reactors. A 0-D (volume-average) approach for optimizing the main reactor parameters allows study of the relationship between global parameters and the compatibility of different constraints for a given power output including plasma-coil spacing, coil-coil spacing, maximum field and coil current density, neutron wall loading, plasma beta value, etc. The result is reactor candidates with average major radii <R> in the 6-7 m range, a factor of two smaller than those of previous studies. A 1-D power balance code is used to study the ignited operating point and the effect of different plasma and confinement assumptions including density and temperature profiles, alpha particle losses, and helium particle confinement time for the different plasma and coil configurations.

Bystander signaling between glioma cells and fibroblasts targeted with counted particles

Radiation-induced bystander effects may play an important role in cancer risks associated with environmental, occupational and medical exposures and they may also present a therapeutic opportunity to modulate the efficacy of radiotherapy. However, the mechanisms underpinning these responses between tumor and normal cells are poorly understood. Using a microbeam, we investigated interactions between T98G malignant glioma cells and AG01522 normal fibroblasts by targeting cells through their nuclei in one population, then detecting cellular responses in the other co-cultured non-irradiated population. It was found that when a fraction of cells was individually irradiated with exactly 1 or 5 helium particles ((3)He(2+)), the yield of micronuclei (MN) in the non-irradiated population was significantly increased. This increase was not related to the fraction of cells targeted or the number of particles delivered to those cells. Even when one cell was targeted with a single (3)He(2+), the induction of MN in the bystander non-irradiated population could be increased by 79% for AG01522 and 28% for T98G. Furthermore, studies showed that nitric oxide (NO) and reactive oxygen species (ROS) were involved in these bystander responses. Following nuclear irradiation in only 1% of cells, the NO level in the T98G population was increased by 31% and the ROS level in the AG0 population was increased by 18%. Treatment of cultures with 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (c-PTIO), an NO scavenger, abolished the bystander MN induction in non-irradiated AG01522 cells but only partially in non-irradiated T98G cells, and this could be eliminated by treatment with either DMSO or antioxidants. Our findings indicate that differential mechanisms involving NO and ROS signaling factors play a role in bystander responses generated from targeted T98G glioma and AG0 fibroblasts, respectively. These bystander interactions suggest that a mechanistic control of the bystander effect could be of benefit to radiotherapy.

New data on the ternary fission of 252Cf from the Gammasphere facility

Ternary fission of 252Cf was studied at Gammasphere using eight ΔE×E particle telescopes. Helium, beryllium, boron, and carbon light charged particles (LCPs) emitted with kinetic energy more than 9, 21, 26, and 32 MeV, respectively, were identified. The 3368-keV γ transition from the first 2+ excited state in 10Be was found and the population probability ratio N(2+)/N(0+) = 0.160 ± 0.025 was estimated. No evidence was found for 3368-keV γ rays emitted from a triple molecular state. For the first time, charge distributions are obtained for ternary fission fragments emitted with helium, beryllium, and carbon LCPs.

Hydrogen particles in liquid helium

Numerical investigations of phase changes during the generation of solid hydrogen particles in cryogenic helium, which include vaporization and re-condensation of helium around a hydrogen droplet and solidification of the droplet in helium, are conducted to unravel the basic mechanisms underlying the phase change phenomena. In a single-fluid, two-phase mixture framework, the simulation of vaporization/condensation involves a vapor–liquid exchange model that accounts for mass and energy transfers due to phase changes between liquid and vapor helium. The enthalpy-porosity, solidification–melting technique is employed to investigate the hydrogen solidification. The volume fraction of each phase in the solidification process is determined using a volume of fluid (VOF) two-phase model, along with a piecewise-linear approach to trace the liquid/solid interface. The results throw some light on the helium flow field resulting from the phase change and the motion of the droplet. They provide some insight into the evolution of liquid–vapor phase-exchange processes as well as the solidification evolution of hydrogen particles of various diameters.

Beta scaling of transport on the DIII-D Tokamak: Is transport electrostatic or electromagnetic?

Determining the scaling of transport with beta (β), the ratio of the plasma kinetic pressure to the magnetic pressure, helps to differentiate between various proposed theories of turbulent transport since mechanisms that are primarily electrostatic show little change in transport with increasing β, while primarily electromagnetic mechanisms generally have a strong unfavorable β scaling. Experiments on the DIII-D tokamak [J.L. Luxon, Nucl. Fusion 42, 614 (2002)] have measured the β scaling of heat transport with all of the other dimensionless parameters held constant in high confinement mode (H-mode) plasmas with edge localized modes. A four point scan varied β from 30% to 85% of the ideal ballooning stability limit (normalized beta from 1.0 to 2.8) and found no change in the normalized confinement time, i.e., Bτth∝β−0.01±0.09. The measured thermal diffusivities, normalized to the Bohm diffusion coefficient, also did not vary during the β scan to within the experimental uncertainties, whereas the normalized helium particle transport decreased with increasing β. The H-mode pedestal β varied in concert with the core β and showed no signs of saturation. This weak, possibly nonexistent, β scaling of transport favors primarily electrostatic mechanisms such as E×B transport, and is in marked disagreement with the strong unfavorable β dependence contained in empirical scaling relations derived from multimachine H-mode confinement databases.

Measurements of the Nuclear Spin Relaxation Times for Small Grains of Solid Hydrogen Suspended in Liquid Helium

Measurements of the nuclear spin-lattice relaxation times (T 1 ) and the nuclear spin-spin relaxation times (T 2 ) are reported for suspensions of small grains of solid hydrogen in liquid helium. The measurements have been carried out for temperatures 1.8 < T < 4.9 K and for a nuclear Larmor frequency of 4.0 MHz. The samples were prepared by rapid condensation of hydrogen'and helium gas in a precooled cell. The hydrogen pressure was maintained below the triple point to prepare a slush of hydrogen particles in liquid helium. The characteristic relaxation times are much shorter, typically by an order of magnitude for T 2 , than those observed and reported previously for bulk samples of solid hydrogen for the same ortho-hydrogen concentrations and applied magnetic fields. Possible origins for this difference between the relaxation in bulk and micro-geometries are discussed.

Experiments on selective removal of helium by an application of radio frequency field

Proof of principle experiments on preferential removal of helium (He) particles using radio frequency (RF) ponderomotive force have been conducted in linear device (NPX) and tokamak (TXTOR-94). In both experiments, the reduction of the helium ion flux was obtained by applying the RF field at the entrance of the scoop in the pump limiter configuration. The enhancement of helium pumping was observed in the frequency ω range of ω CHe< ω<2 ω CHe where ω CHe is the angular ion cyclotron frequency of singly ionized helium ion.

Helium and neon enrichment studies in the JET Mark IIAP and Mark IIGB divertors

Adequate helium enrichment and exhaust have beenachieved in reactor relevant ELMy H mode plasmas in JETperformed in the Mark II divertor geometry. These quantities,describing the retention of impurities in divertors, have beenexperimentally inferred from CXRS measurements in the coreplasma, and from a spectroscopic analysis of a Penning gaugedischarge in the exhaust gas. The retention of helium was foundto be sufficient with respect to the requirements in a next stepdevice, with helium enrichment factors exceeding 0.1 in highdensity ELMy H mode discharges. With increasing core plasmadensity the helium partial pressure in the exhaust channelincreases. While in L mode plasmas the helium enrichmentdecreases with increasing core plasma density, it remains almostconstant in ELMy H mode plasma. The noble gas neon is betterenriched in the divertor at high core plasma densities in bothconfinement regimes. These experimental results can be explainedby the significant differences between the penetration depths ofthe impurity neutrals and by their subsequent different impuritytransport mechanisms. Analytical and numerical analyses of theseplasmas using the impurity code package DIVIMP/NIMBUS supportthe proposition that, owing to their much longer ionization meanfree path, helium particles can escape from the divertor chamberas neutrals, while neon escapes by means of ion leakage.Consequently, the divertor plasma conditions strongly influencethe noble gas compression and enrichment. Variations of thedivertor plasma configuration and modifications to the divertorgeometry have enhanced the pumping capabilities of the tokamak,but have been found not to affect the helium enrichment.

Estafette of drift resonances, stochasticity and control of particle motion in a toroidal magnetic trap

A new method of particle motion control in toroidal magnetic traps with a rotational transform using an estafette of drift resonances and stochasticity of particle trajectories is proposed. The use of the word estafette here means that particles pass through a set of resonances in consecutive order from one to another during their motion. The overlapping of adjacent resonances can be moved radially from the centre to the edge of the plasma by switching on the corresponding perturbations at times in accordance with a particular rule. In this way, particles (e.g., cold alpha particles) can be removed from the centre of the confinement volume to the plasma periphery. For an analytical treatment of the stochastic behaviour of particle motion, the equation for the drift surface function Ψ* (r,ϑ,φ,V∥,V⊥,t) = const is reduced to a diffusion type equation in the case when some adjacent resonances take place and the stochastic diffusion coefficients Dr,r, Dr,ϑ and Dϑ,ϑ in the real space variables are introduced. This new approach is demonstrated by numerical computations of test helium particle trajectories in the LHD toroidal trap.

Computational studies of gas–carbon nanotube collision dynamics

Molecular dynamics simulations of collisions between gas particles and carbon nanotubes at elevated temperatures show that low frequency thermal vibrations of the nanotube dominate the scattering dynamics and that particle–tube energy transfer in the tube radial direction is far more efficient than that in the axial direction. The dominant role of the tube's thermal fluctuations reduces the importance of other collision parameters such as collider mass and collider–tube interaction strengths, and xenon, neon, helium and hydrogen particles all have similar collision dynamics and high scattering probabilities. At 1300 K, which is the temperature used in catalytic chemical vapour deposition single-walled nanotube growth, hydrogen molecules scatter from the tube while carbon atoms are efficiently trapped on the tube surface.

Study of the chemical transformations induced at the surface of bulk iron by protons and α beams

In this work the chemical transformations induced by protons and α beams (10 17 ion cm −2) at the surface of polycrystalline iron are analysed by X-ray Photoemission Spectroscopy (XPS). Spectroscopic changes for both incident particles are globally different. We show that helium particles contribute to etch away the oxygen leaving the iron surface in a metallic state. In contrast, hydrogen ions do not remove, at least completely, the iron oxidised species present at the surface, but change its oxidation state by reducing Fe 2O 3 and Fe 3O 4 to FeO.

Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy

Energetic neutral particles from neutral hydrogenic beam heating systems are widely used for active spectroscopic measurements of key plasma parameters in fusion experiments. Helium beams are used in dedicated diagnostic beamlines offering deeper penetration and resonant double charge exchange with alpha particles. Neutral beam systems using pure helium either require specialised helium gas pumping with a pumping speed in excess of 1000 m 3/h or are restricted to short pulses (normally less than 1 s). A doped hydrogen/helium beam combines the requirements for plasma heating and diagnostics without the need for sophisticated helium pumping. A small flow of helium gas is injected into the plasma source for the time helium particles are required. The helium current is typically 10% of the total extracted current. The reduction in heating power of the doped beam can be kept below 5%. Doped deuterium/helium beams have been successfully tested and routinely used at JET. HeI beam emission spectra obtained with a doped deuterium/helium beam offer sufficiently strong visible lines for spectroscopic applications.

Observation of the interstellar helium cone by the NOZOMI spacecraft

‘The Japanese Mars probe, NOZOMI, is staying in the interplanetary space (1–1.5 AU) until its Mars’ orbit insertion scheduled in early 2004. Every 16 months on this interplanetary orbit the spacecraft crosses around 1 AU the ‘gravitational focusing cone’ of the interstellar helium, which are penetrating into the inner heliosphere under the solar gravity. During the first crossing of the cone in the season of March–May 2000, we observed these helium particles after the solar wind pickup process with an E/q type ion detector aboard NOZOMI. We have estimated the original temperature of the interstellar helium as 11 000 K.

A new model for the effective thermal conductivity of packed beds of solid spheroids: alumina in helium between 100 and 500°C

An analytical model is given for the thermal conductivity of a bed of solid spheroidal particles in static gas, when the conductivity of the solid is substantially greater than that of the gas. It has two fitting parameters, the width and average radius of the narrow gaps that exist between the irregularly shaped particles and which contribute significantly to the thermal conductivity. Since both parameters are physically measurable, the model holds the potential for calculating the thermal conductivity without any adjustable parameters. Agreement is excellent with measurements on alumina particles in helium at 100–500°C up to 100 kPa pressure.

Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy

Energetic neutral particles from neutral beam heating systems are widely used for active spectroscopic measurements of key plasma parameters in fusion experiments. Both the plasma discharges and the neutral beam systems are normally operated with hydrogen or deuterium. Helium beams are used in dedicated diagnostic beam lines as they offer deeper penetration and are subject to less background radiation and enable resonant double charge exchange with alpha particles. Neutral beam systems using pure helium either require specialized helium gas pumping with a pumping speed in excess of 1000 m3/h or are restricted to short pulses (normally less than 1 s). A doped hydrogen/helium beam combines the requirements for plasma heating and diagnostics without the need for sophisticated helium pumping. A small flow of helium gas is injected into the plasma source for the time helium particles are required. The helium current is typically 10% of the total extracted current. The reduction in heating power of the doped beam can be kept below 5%. The small amount of helium gas does not cause an excessive pressure rise along the beam line and does not reduce the reliability of the beam heating system. Doped deuterium/helium beams have been successfully tested and routinely used at JET. The Hel beam emission spectra obtained with a doped deuterium/helium beam produce sufficiently strong visible lines for spectroscopic applications. Furthermore, the simultaneous availability of helium and hydrogenic particles in the beam allows us to extend spectroscopic measurements to another atomic system and hence cross-check results from helium beams with those from hydrogenic beams. The only investment required is an additional helium gas inlet system into the ion source.

Sputtering efficiency of LiF surfaces on impact of low energy neutral helium (20–80 eV): Calibration of the interstellar neutral helium instrument on ULYSSES

For the detection of interstellar neutral helium particles entering the inner solar system, a novel detection technique has been developed using sputtering of secondary ions from lithium-fluoride (LiF) surfaces on the impact of He particles. In order to be able to determine the interstellar He fluxes, the detection efficiency had to be determined in an energy range Ekin≈20 to 200 eV, an energy range for which data were not available in literature. Therefore, a calibration had to be performed in a dedicated accelerator system providing a neutral beam in the required energy range. While precise values could be obtained for energies above about 50 eV, significant uncertainties in the efficiency remained for lower energies, due to the difficulty in determining the absolute flux of the primary beam after acceleration, as elastic scattering of particles out of the beam geometry during the charge exchange process becomes more important. Corrections have been applied using a theoretical simulation of this process. These uncertainties have subsequently been eliminated, using the inflight measurements of neutral He during the ULYSSES mission. Due to the orbital parameters of this spacecraft, the relative energy of the interstellar He particles in the instrument’s reference system varies in the range 20–80 eV. Assuming that the interstellar He flow is stationary, i.e., it shows only negligible time variations, constant count rates would be expected over this energy range. The energy dependence in the actually observed count rates has been used to refine the efficiency calibration at low energies (⩽40 eV).

Angular Momentum Sharing in Dissipative Collisions

Light charged particles emitted by the projectile-like fragment were measured in the direct and reverse collision of $^{93}$Nb and $^{116}$Sn at 25 AMeV. The experimental multiplicities of Hydrogen and Helium particles as a function of the primary mass of the emitting fragment show evidence for a correlation with net mass transfer. The ratio of Hydrogen and Helium multiplicities points to a dependence of the angular momentum sharing on the net mass transfer.

Double ionisation of helium and lithium by ion impact using independent event models

Double ionisation is considered within the context of the impact parameter version of the continuum distorted wave eikonal initial state approximation. Starting from the independent event model, the correlation of the ejected electrons is accounted for by explicitly including the Coulomb density of states factor for the electron–electron interaction in the calculation. Total cross sections for proton, alpha particle and Li 3+ impact on helium, and proton and alpha particle impact on lithium, over the energy range 50–2000 keV/amu are presented. It is concluded that the inclusion of electron–electron correlation in the exit channel is crucial if an adequate account of the double ionisation process is to be obtained.

Interaction of plasma facing materials for fusion devices with low energy hydrogen and helium particles

Radiation damage formed in metal specimens exposed to long pulse tokamak plasmas of TRIAM-1M was examined by transmission electron microscopy. By comparing these results with those of low energy hydrogen ion irradiation it was concluded that the charge exchange energetic neutrals of hydrogen emitted from the core plasma caused remarkable displacement damage. The flux of the neutrals in the energy range of 0.5–3 keV which were responsible for displacement damage, was estimated to be about 1.5–3 × 1018 H/m2/s. These energetic neutrals cause not only material degradation at the sub-surface region but also change bulk properties of plasma facing components in a plasma confinement device. Effect of helium plasma was also discussed with emphasis on very strong effects on damage accumulation. Damage by He is a serious issue of plasma facing materials.

Separation method of heavy-ion particle image from gamma-ray mixed images using an imaging plate

Abstract We have developed a separation method of alpha-ray and gamma-ray images using the imaging plate (IP). The IP from which the first image was read out by an image reader was annealed at 50°C for 2 h in a drying oven and the second image was read out by the image reader. It was found out that an annealing ratio, k , which is defined as a ratio of the photo-stimulated luminescence (PSL) density at the first measurement to that at the second measurement, was different for alpha rays and gamma rays. By subtracting the second image multiplied by a factor of k from the first image, the alpha-ray image was separated from the alpha and gamma-ray mixed images. This method was applied to identify the images of helium, carbon and neon particles of high energies using the heavy-ion medical accelerator, HIMAC.