Role In Particle Acceleration Research Articles

Turbulent Magnetic Reconnection as an Acceleration Mechanism in Earth’s Magnetotail

Using electric and magnetic fields measured by the Magnetospheric Multiscale (MMS) mission, we construct a test-particle simulation of a turbulent magnetic reconnection region to investigate observed ion acceleration. We identify three types of energized ions: (1) ion jets, (2) Speiser-like energized ions—both of which carry significant energy but do not produce a strong energetic (>80 keV) tail in the ion distribution—and (3) a separate but sizable population of ions that are accelerated to significantly higher energies (>80 keV) by the turbulent fields. The majority of ions that undergo energization by the turbulent fields cross the magnetic null plane multiple times. By preferentially energizing these particles, the turbulence creates a separate population of ions that mostly exits in the dawn direction of the magnetotail and forms a high-energy power-law tail in the ion flux-energy distribution. We also find that the highest acceleration energies are limited by the size of the turbulent region (with respect to ion gyroradii). Since turbulence is widespread in astrophysical plasmas and has no a priori limit on scale size, the MMS observations suggest turbulence may have a significant role in particle acceleration.

Cold Spray Additive Manufacturing of Ti6Al4V: Special Nozzle Design Using Numerical Simulation and Experimental Validation

Cold spray additive manufacturing (CSAM) shows great potential in titanium-alloy production as it is a solid-state process. However, data published so far have demonstrated the difficulty of producing dense and high-strength Ti alloy parts. Our previous studies have shown that nozzle design together with high-cost helium propulsive gas plays a crucial role in particle acceleration. In this work, special nozzles for Ti alloy were designed and validated experimentally with commercially available Ti6Al4V powder. Simulation results show that particle impact temperature increases remarkably for a long convergent length, while particle kinetic energy slightly increases, which is validated by experiments. The relationship between the particle impact temperature and practice diameter shows the first increase and then decrease. The experimental results show that as the nozzle convergent section becomes longer, the edges of the single-pass deposits become smoother, and the width, density, deposition efficiency, and microhardness of the single-pass deposits increase.

Simulating the Ion-trapping Acceleration at Rippled Reconnection Fronts

Abstract Reconnection fronts (RFs) play a vital role in particle acceleration and energy transport in the terrestrial magnetosphere. It is widely believed that RFs have planar monotonic profiles that determine the particle dynamics. However, recent in situ studies have revealed that the front surface is not planar as expected but rather rippled. How the surface irregularities of RFs’ impact particle energization and transport is still an open issue. Using a particle-tracing technique, we traced the trajectories of ions near fronts with or without surface ripples at different scales to understand how ions are mediated by such rippled structures. We find that the ion relative energy gain increases considerably when the rippled surface of RFs appears. The main acceleration mechanism is ion-trapping acceleration, in which ions are confined at the RFs for a longer time by the rippled structure and are accelerated by the duskward electric field. Moreover, ions can be accelerated effectively when their gyroradius is comparable to the size of the ripple. Formulas of relative energy gain as a function of the ripple size are presented.

Three dimensional analytical model of dipolarizing flux bundles

In many plasma systems, magnetic reconnection plays a crucial role in particle acceleration. In Earth's magnetotail, reconnection releases a significant portion of the stored magnetic energy (which is primarily converted into ion enthalpy) behind intense electromagnetic boundaries called dipolarization fronts. Dipolarizing flux bundles behind these fronts accelerate ambient plasma and transport charged particles from the magnetotail to near-Earth space. Appropriate modeling of such flux bundle-particle interactions requires self-consistent models that describe both the bundle's electromagnetic fields and the magnetotail's static background magnetic field. We develop a 2D solution of the Vlasov-Maxwell equation for a magnetotail with an embedded dipolarization front and generalize this solution to a 3D system. Our 3D model describes both global (within the magnetotail) and local (around the front) electromagnetic field distributions and a consistent distribution of plasma. The magnetic field topology in our model agrees with that deduced from multispacecraft observations. Our model can be used for a test particle tracing and investigation of particle acceleration/transport by dipolarization fronts in planetary magnetotails.

Particle acceleration and radiation in pulsars: New insights from kinetic simulations

Pulsar magnetospheres are efficient particle accelerators, as evidenced by high-energy gamma-ray observations. Where and how particle acceleration occurs are difficult questions to answer because it results from a complex interplay between relativistic electrodynamics, pair creation and non-thermal radiation. The recent development of global particle-in-cell simulations allows, for the first time, to address this problem from first principles and self-consistently. Simulations indicate that relativistic reconnection in the equatorial current sheet plays a key role in particle acceleration and the emission of high-energy radiation. We discuss these results in the context of young gamma-ray pulsars.

Electromagnetic disturbances observed near the dip region ahead of dipolarization front

AbstractDipolarization front (DF) is a thin magnetic structure embedded in fast flows in the magnetotail, which plays an important role in particle acceleration, flow braking, wave excitation, and other related processes. Electromagnetic disturbances near the magnetic dip region in front of DFs are investigated using Time History of Events and Macroscale Interactions during Substorms probe observations in this paper. Strong magnetic field and electric field fluctuations, with several wave bands below and around the lower hybrid frequency, are found in an event on 21 March 2008. The properties of the wave are similar to that of magnetosonic wave. Detailed analyses show that the phase space density for ions in the perpendicular direction has a positive slope near the local Alfvén speed, which is a possible free‐energy source for the generation of the wave. This type of ion distribution could result from the earthward reflected ions ahead of DF, though other forming mechanism could not be fully ruled out.

Interplanetary shocks and foreshocks observed by STEREO during 2007–2010

AbstractInterplanetary shocks in the heliosphere modify the solar wind through which they pass. In particular, shocks play an important role in particle acceleration. During the extended solar minimum (2007–2010) STEREO observed 65 forward shocks driven by stream interactions (SI), with magnetosonic Mach numbers Mms ≈ 1.1–4.0 and shock normal angles ~ 20–87°. We analyze the waves associated with these shocks and find that the region upstream can be permeated by whistler waves (f ~ 1 Hz) and/or ultra low frequency (ULF) waves (f ~ 10−2–10−1 Hz). While whistlers appear to be generated at the shock, the origin of ULF waves is most probably associated with local kinetic ion instabilities. We find that when the Mach number (Mms) is low and the shock is quasi‐perpendicular ( > 45°) whistler waves remain close to the shock. As Mms increases, the shock profile changes and can develop a foot and overshoot associated with ion reflection and gyration. Whistler precursors can be superposed on the foot region, so that some quasi‐perpendicular shocks have characteristics of both subcritical and supercritical shocks. When the shock is quasi‐parallel ( < 45°) a large foreshock with suprathermal ions and waves can form. Upstream, there are whistler trains at higher frequencies whose characteristics can be slightly modified probably by reflected and/or leaked ions and by almost circularly polarized waves at lower frequencies that may be locally generated by ion instabilities. In contrast with planetary bow shocks, most of the upstream waves studied here are mainly transverse and no steepening occurs. Some quasi‐perpendicular shocks (45° < < 60°) are preceded by ULF waves and ion foreshocks. Fluctuations downstream of quasi‐parallel shocks tend to have larger amplitudes than waves in the sheath of quasi‐perpendicular shocks. We compare SI‐driven shock properties with those of shocks generated by interplanetary coronal mass ejections (ICMEs). During the same years, STEREO observed 20 ICME‐driven shocks with Mms ≈ 1.2–4.0 and ~ 38–85°. We find that shocks driven by ICMEs tend to have larger proton foreshocks (dr ~ 0.1 AU) than shocks driven by stream interactions (dr ≤ 0.05 AU). This difference of ion foreshock size should be linked to shock age: ICME‐driven shocks form at shorter distances to the Sun and therefore can energize particles for longer times as they propagate to 1 AU, while stream interaction shocks form closer to Earth's orbit and have been accelerating ions for a shorter interval of time.

Beam position monitor design for a third generation light source

The measurement of the beam orbit plays a very important role in particle accelerators. The button-type beam position monitor (BPM) was designed for the Shanghai Synchrotron Radiation Facility to reduce the impedances and to guarantee a high resolution of the measurement. Position resolution, beam impedance, higher-order mode, and impedance matching have been studied during the design based on the physical parameters of the storage ring at the Shanghai Synchrotron Radiation Facility. Meanwhile, an analytic formula of the BPM broadband impedance was derived based on a resistor-capacitor equivalent circuit. Thus, the impedance of the BPM could be analyzed quantitatively by simply measuring the capacitance of the electrode. This formula had been verified by comparing the results of the calculations of the formula and the simulations in MAFIA.

Numerical Study on the Effect of Substrate Angle on Particle Impact Velocity and Normal Velocity Component in Cold Gas Dynamic Spraying Based on CFD

Numerical study was conducted to investigate the effect of substrate angle on particle impact velocity and normal velocity component in cold gas dynamic spraying by using three-dimensional models based on computational fluid dynamics. It was found that the substrate angle has significant effect on particle impact velocity and normal velocity component. With increasing the substrate angle, the bow shock strength becomes increasingly weak, which results in a gradual rise in particle impact velocity. The distribution of the impact velocity presents a linearly increase along the substrate centerline due to the existence of the substrate angle and the growth rate rises gradually with increasing the substrate angle. Furthermore, the normal velocity component reduces steeply with the increase in substrate angle, which may result in a sharp decrease in deposition efficiency. In addition, the study on the influence of procedure parameters showed that gas pressure, temperature, type, and particle size also play an important role in particle acceleration.

Theory of energetic particle transport in the magnetosphere: A noncanonical approach

This paper presents a new transport theory suitable for the study of energetic particles in the magnetospheric environment. A Fokker‐Planck diffusion equation governing the variation of the particle distribution function, along with its isotropic and bounce average approximations, is established in the phase space of location and momentum, which is similar to how cosmic rays are treated by the heliospheric community. The equation includes essentially all the particle transport mechanisms: streaming, convection, drift, adiabatic energy change, acceleration by parallel electric field, focusing, and diffusions in location, momentum, and pitch angle. All the coefficients of the equation are directly linked to plasma and magnetic configurations including electromagnetic fluctuations. The theory can form a base for developing full‐scale numerical models of energetic particle transport including acceleration in the magnetosphere. Unlike the conventional theory using canonical variables which is only applicable to the radiation belt of the inner magnetosphere, this theory includes the effects of nonuniform magnetospheric convection and thus is valid for the entire magnetosphere. The derivation of the transport equation has identified some important particle transport mechanisms that have not received careful study by the magnetospheric community. It is found that the compression of magnetospheric plasma can play a significant role in particle acceleration and trapping. The compressional acceleration is the first‐order particle acceleration mechanism, very similar to particle acceleration at shock waves in other space plasma environments. This acceleration mechanism is operable to both electrons and ions. A model map of magnetospheric convection flow shows that the compressional particle acceleration is a large‐scale phenomenon and it is the strongest in the near‐Earth nightside plasma sheet. Magnetospheric compression can also drive particles toward the 90° equatorial pitch angle, which is ideal for trapping them in the geomagnetic field.

Heliospheric “FALTS”: Favored Acceleration Locations at the Termination Shock

The standard explanation for Anomalous Cosmic Rays requires strong energization at the heliospheric termination shock. However, the tightly wrapped magnetic spiral in the outer heliosphere produces a quasi‐perpendicular shock where it is difficult to accelerate low energy particles. We show that motion of footpoints between regions of fast and slow solar wind at the Sun, create non‐spiral configurations and large radial components of the magnetic field. These regions preferentially inject low energy particles into diffusive shock acceleration. Further, footpoints that move from slow to fast source regions, generate field lines with local polarity inversions that connect to the termination shock at multiple locations. As these regions convect out, they trap particles between what are effectively two converging shocks, producing additional first‐order Fermi acceleration. Heliospheric “FALTS” ‐ Favored Acceleration Locations at the Termination Shock ‐ should play an important role in particle acceleration in the outer heliosphere.

Adsorption and desorption phenomena and their role in particle accelerators and space simulators

Adsorption and desorption phenomena and their role in particle accelerators and space simulators

Adsorption and desorption phenomena and their role in particle accelerators and space simulators: D.A. Degras, 2nd European Vac. Symp., 5–7 June, 1963, 54–67, Rudolph A. Lang Verlag, Esch (Taunus)

Adsorption and desorption phenomena and their role in particle accelerators and space simulators: D.A. Degras, 2nd European Vac. Symp., 5–7 June, 1963, 54–67, Rudolph A. Lang Verlag, Esch (Taunus)

Adsorption and desorption phenomena and their role in particle accelerators and space simulators

Adsorption and desorption phenomena and their role in particle accelerators and space simulators