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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,675 records · Page 93
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Detecting chaos in particle accelerators through the frequency map analysis method

The motion of beams in particle accelerators is dominated by a plethora of non-linear effects, which can enhance chaotic motion and limit their performance. The application of advanced non-linear dynamics methods for detecting and correcting these effects and thereby increasing the region of beam stability plays an essential role during the accelerator design phase but also their operation. After describing the nature of non-linear effects and their impact on performance parameters of different particle accelerator categories, the theory of non-linear particle motion is outlined. The recent developments on the methods employed for the analysis of chaotic beam motion are detailed. In particular, the ability of the frequency map analysis method to detect chaotic motion and guide the correction of non-linear effects is demonstrated in particle tracking simulations but also experimental data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

On the role of terahertz field acceleration and beaming of surface plasmon generated ultrashort electron pulses

A mechanism for control of the energy and pitch angle of surface plasmon accelerated electron pulses is proposed. Electrons generated via multi-photon absorption in a silver film on a glass prism are ponderomotively accelerated in the surface plasmon field excited by a 30 fs, 800 nm optical pulse. Through introduction of a single-cycle terahertz (THz) pulse, the energy spectrum and trajectory of the generated electron pulse can be controlled via the THz field strength. Generated electron pulses achieve peak kinetic energies up to 1.56 keV, while utilizing an incident optical field strength five times less than comparable plasmon accelerated electron pulses. These results demonstrate that THz pulses can be utilized to achieve tunable, high energy, trajectory controlled electron pulses necessary for various applications that require ultrafast electron pulse manipulation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

2D electron density profile measurement in tokamak by laser-accelerated ion-beam probe

A new concept of Heavy Ion Beam Probe (HIBP) diagnostic has been proposed, of which the key is to replace the electrostatic accelerator of traditional HIBP by a laser-driven ion accelerator. Due to the large energy spread of ions, the laser-accelerated HIBP can measure the two-dimensional (2D) electron density profile of tokamak plasma. In a preliminary simulation, a 2D density profile was reconstructed with a spatial resolution of about 2 cm, and with the error below 15% in the core region. Diagnostics of 2D density fluctuation is also discussed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

A hole accelerator for InGaN/GaN light-emitting diodes

The quantum efficiency of InGaN/GaN light-emitting diodes (LEDs) has been significantly limited by the insufficient hole injection, and this is caused by the inefficient p-type doping and the low hole mobility. The low hole mobility makes the holes less energetic, which hinders the hole injection into the multiple quantum wells (MQWs) especially when a p-type AlGaN electron blocking layer (EBL) is adopted. In this work, we report a hole accelerator to accelerate the holes so that the holes can obtain adequate kinetic energy, travel across the p-type EBL, and then enter the MQWs more efficiently and smoothly. In addition to the numerical study, the effectiveness of the hole accelerator is experimentally shown through achieving improved optical output power and reduced efficiency droop for the proposed InGaN/GaN LED.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Generation of quasi-monoenergetic carbon ions accelerated parallel to the plane of a sandwich target

A new ion acceleration scheme, namely, target parallel Coulomb acceleration, is proposed in which a carbon plate sandwiched between gold layers is irradiated with intense linearly polarized laser pulses. The high electrostatic field generated by the gold ions efficiently accelerates the embedded carbon ions parallel to the plane of the target. The ion beam is found to be collimated by the concave-shaped Coulomb potential. As a result, a quasi-monoenergetic and collimated C{sup 6+}-ion beam with an energy exceeding 10 MeV/nucleon is produced at a laser intensity of 5 × 10{sup 19} W/cm{sup 2}.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Refluxed electrons direct laser acceleration in ultrahigh laser and relativistic critical density plasma interaction

Refluxed electrons direct laser acceleration is proposed so as to generate a high-charge energetic electron beam. When a laser pulse is incident on a relativistic critical density target, the rising edge of the pulse heats the target and the sheath fields on the both sides of the target reflux some electrons inside the expanding target. These electrons can be trapped and accelerated due to the self-transparency and the negative longitudinal electrostatic field in the expanding target. Some of the electrons can be accelerated to energies exceeding the ponderomotive limit 1/2a{sub 0}{sup 2}mc{sup 2}. Effective temperature significantly above the ponderomotive scaling is observed. Furthermore, due to the limited expanding length, the laser propagating instabilities are suppressed in the interaction. Thus, high collimated beams with tens of μC charge can be generated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Enhanced laser-radiation-pressure-driven proton acceleration by moving focusing electric-fields in a foil-in-cone target

A foil-in-cone target is proposed to enhance stable laser-radiation-pressure-driven proton acceleration by avoiding the beam degradation in whole stage of acceleration. Two and three-dimensional particle-in-cell simulations demonstrate that the guiding cone can substantially improve the spectral and spatial properties of the ion beam and lead to better preservation of the beam quality. This can be attributed to the focusing effect of the radial sheath electric fields formed on the inner walls of the cone, which co-move with the accelerated foil and effectively suppress the undesirable transverse explosion of the foil. It is shown that, by using a transversely Gaussian laser pulse with intensity of ∼2.74 × 10{sup 22 }W∕cm{sup 2}, a quasi-monoenergetic proton beam with a peak energy of ∼1.5 GeV/u, density ∼10n{sub c}, and transverse size ∼1λ{sub 0} can be obtained.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Effect of electromagnetic pulse transverse inhomogeneity on ion acceleration by radiation pressure

During ion acceleration by radiation pressure, a transverse inhomogeneity of an electromagnetic pulse leads to an off-axis displacement of the irradiated target, limiting the achievable ion energy. This effect is analytically described within the framework of a thin foil target model and with particle-in-cell simulations showing that the maximum energy of the accelerated ions decreases as the displacement from the axis of the target's initial position increases. The results obtained can be applied to the optimization of ion acceleration by the laser radiation pressure with mass-limited targets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Enhancement of electron energy during vacuum laser acceleration in an inhomogeneous magnetic field

In this paper, the effect of a stationary inhomogeneous magnetic field on the electron acceleration by a high intensity Gaussian laser pulse is investigated. A focused TEM (0,0) laser mode with linear polarization in the transverse x-direction that propagates along the z-axis is considered. The magnetic field is assumed to be stationary in time, but varies longitudinally in space. A linear spatial profile for the magnetic field is adopted. In other words, the axial magnetic field increases linearly in the z-direction up to an optimum point z{sub m} and then becomes constant with magnitude equal to that at z{sub m}. Three-dimensional single-particle simulations are performed to find the energy and trajectory of the electron. The electron rotates around and stays near the z-axis. It is shown that with a proper choice of the magnetic field parameters, the electron will be trapped at the focus of the laser pulse. Because of the cyclotron resonance, the electron receives enough energy from the laser fields to be accelerated to relativistic energies. Using numerical simulations, the criteria for optimum regime of the acceleration mechanism is found. With the optimized parameters, an electron initially at rest located at the origin achieves final energy of γ=802. The dynamics of a distribution of off-axis electrons are also investigated in which shows that high energy electrons with small energy and spatial spread can be obtained.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

A multi-beamlet analysis of the MITICA accelerator

The thermo-mechanical analysis and the mechanical design of the accelerator of MITICA (i.e. the full size prototype of the ITER neutral beam injector under construction at RFX [1]) are based on the calculation of the power deposition induced by particle impacts. This calculation is performed by EAMCC [2], a relativistic particle tracking code based on the Monte-Carlo method for describing collisions inside the accelerator, under prescribed electric and magnetic fields. The magnetic field maps are produced by 3D codes, while the electric field maps come from the 2D axi-symmetric code SLACCAD [3]. In order to perform a multi-beamlet analysis, which allows to take into account the beamlet-beamlet repulsion, and to consider other effects neglected under the hypothesis of axi-symmetric beam, a fully 3D version of the code is required. In this paper, a modified version of EAMCC, fully 3D, capable of modifying the mesh of the 3D maps and of dealing with uneven meshes is presented. A finer mesh is used just in the regions where a more detailed description of the fields is required, for a more realistic simulation. A comparison between the original code and the modified version is presented at first, as a validation of the modifications introduced in the latter. Subsequently, the main results of a single-beamlet analysis performed with the two versions of the code are shown and the differences between the 2D and the 3D simulations discussed. The last part is dedicated to the multi-beamlet simulation of the accelerator.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Linear vs. nonlinear acceleration in plasma turbulence. II. Hall–finite-Larmor-radius magnetohydrodynamics

The local k-space ratio of linear and nonlinear accelerations associated with a variety of initial conditions undergoing steady relaxation is investigated for the Hall–finite-Larmor-radius magnetohydrodynamics (MHD) system in the presence of a mean magnetic field. Building on a related study (Paper I) where it was shown that discrepancies exist between describing the global and local characterizations of the pure MHD system with mean magnetic field, we find regions of the Fourier space that are consistently dominated by linear acceleration and other regions that are consistently dominated by nonlinear acceleration, independent of the overall system's description as linear, weakly nonlinear, or turbulent. In general, dynamics within a certain angular range of the mean magnetic field direction are predominantly linear, while dynamics adjacent the Hall scales along the field-parallel direction and dynamics adjacent the finite Larmor radius scales in the field-perpendicular direction can become strongly nonlinear. The nonlinear influences are particularly significant as the plasma beta increases from unity to higher values.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Analysis and design of nonlocal spin devices with electric-field-induced spin-transport acceleration

We apply electric-field-induced acceleration for spin transport to a four-terminal nonlocal device and theoretically analyze its Hanle-effect signals. The effect of the ferromagnetic contact widths of the spin injector and detector on the signals is carefully discussed. Although Hanle-effect signals are randomized owing to the effect of the contact widths, this can be excluded by selecting an appropriate electric field for acceleration of spin transport. Spin lifetime can be correctly extracted by nonlocal devices with electric-field acceleration even using the spin injector and detector with finite contact widths.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Simple scalings for various regimes of electron acceleration in surface plasma waves

Different electron acceleration regimes in the evanescent field of a surface plasma wave are studied by considering the interaction of a test electron with the high-frequency electromagnetic field of a surface wave. The non-relativistic and relativistic limits are investigated. Simple scalings are found demonstrating the possibility to achieve an efficient conversion of the surface wave field energy into electron kinetic energy. This mechanism of electron acceleration can provide a high-frequency pulsed source of relativistic electrons with a well defined energy. In the relativistic limit, the most energetic electrons are obtained in the so-called electromagnetic regime for surface waves. In this regime, the particles are accelerated to velocities larger than the wave phase velocity, mainly in the direction parallel to the plasma-vacuum interface.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Guiding and collimation of laser-accelerated proton beams using thin foils followed with a hollow plasma channel

It is proposed that guided and collimated proton acceleration by intense lasers can be achieved using an advanced target—a thin foil followed by a hollow plasma channel. For the advanced target, the laser-accelerated hot electrons can be confined in the hollow channel at the foil rear side, which leads to the formation of transversely localized, Gaussian-distributed sheath electric field and resultantly guiding of proton acceleration. Further, due to the hot electron flow along the channel wall, a strong focusing transverse electric field is induced, taking the place of the original defocusing one driven by hot electron pressure in the case of a purely thin foil target, which results in collimation of proton beams. Two-dimensional particle-in-cell simulations show that collimated proton beams with energy about 20 MeV and nearly half-reduced divergence of 26° are produced at laser intensities 10{sup 20 }W/cm{sup 2} by using the advanced target.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Compact RF ion source for industrial electrostatic ion accelerator

Korea Multi-purpose Accelerator Complex is developing a single-ended electrostatic ion accelerator to irradiate gaseous ions, such as hydrogen and nitrogen, on materials for industrial applications. ELV type high voltage power supply has been selected. Because of the limited space, electrical power, and robust operation, a 200 MHz RF ion source has been developed. In this paper, the accelerator system, test stand of the ion source, and its test results are described.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Simulation of diatomic gas-wall interaction and accommodation coefficients for negative ion sources and accelerators

Particle-wall interactions determine in different ways the operating conditions of plasma sources, ion accelerators, and beams operating in vacuum. For instance, a contribution to gas heating is given by ion neutralization at walls; beam losses and stray particle production—detrimental for high current negative ion systems such as beam sources for fusion—are caused by collisional processes with residual gas, with the gas density profile that is determined by the scattering of neutral particles at the walls. This paper shows that Molecular Dynamics (MD) studies at the nano-scale can provide accommodation parameters for gas-wall interactions, such as the momentum accommodation coefficient and energy accommodation coefficient: in non-isothermal flows (such as the neutral gas in the accelerator, coming from the plasma source), these affect the gas density gradients and influence efficiency and losses in particular of negative ion accelerators. For ideal surfaces, the computation also provides the angular distribution of scattered particles. Classical MD method has been applied to the case of diatomic hydrogen molecules. Single collision events, against a frozen wall or a fully thermal lattice, have been simulated by using probe molecules. Different modelling approximations are compared.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Investigation on target normal sheath acceleration through measurements of ions energy distribution

An experimental campaign aiming at investigating the ion acceleration mechanisms through laser-matter interaction in femtosecond domain has been carried out at the Intense Laser Irradiation Laboratory facility with a laser intensity of up to 2 × 10{sup 19} W/cm{sup 2}. A Thomson parabola spectrometer was used to obtain the spectra of the ions of the different species accelerated. Here, we show the energy spectra of light-ions and we discuss their dependence on structural characteristics of the target and the role of surface and target bulk in the acceleration process.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Review of R&D at DESY on ingot niobium for accelerators

An R&D program at DESY exploring the potential of the production of 1.3 GHz TESLA shape cavities from large grain or ingot niobium material has been carried out as a feasibility study for using ingot niobium material for the European X-ray Free-Electron Laser project. The most important issues like feasibility of the fabrication of niobium discs from ingots, material properties, cavity fabrication, treatment, and finally the cryogenic radiofrequency (RF) performance of the cavity in the accelerator modules are presented and discussed. At least 25% higher intrinsic quality factor comparing to conventional fine-grain niobium and high accelerating gradient of up to 45 MV/m have been demonstrated during the cryogenic RF tests both in the vertical cryostats and in a completely assembled accelerating cryo-module.

43 PARTICLE ACCELERATORS↗