Adiabatic Acceleration Research Articles

Diffusive Compression Acceleration of Energetic Particles with an Application to Shock Acceleration near Injection Energies

The behavior of energetic charged particles accelerated by a continuous plasma compression profile is explored in the framework of diffusion. At high enough energies, the accelerated particles have a power-law spectrum with a slope generally steeper than a shock spectrum with the same compression ratio. The spectral slope depends on the ratio of the diffusion coefficient to the product of the upstream plasma speed and the thickness of the compression region. In the limit of large diffusion, the spectrum becomes identical to that of diffusive shock acceleration, and when diffusion is small enough, it is consistent with adiabatic acceleration by a single passage through the compression region. For particles with a diffusion coefficient that increases with energy, the spectral shape of the accelerated particles changes from an adiabatic compressional acceleration spectrum at low energies to a shock spectrum at high energies. The flux level of the high-energy asymptotic shock spectrum is generally lower than that from a calculation with diffusive shock acceleration. When this result is applied to particles accelerated to high energies by shocks, the particle injection energy and efficiency can be determined once the energy dependence of the diffusion coefficient and the spectrum of the source particles are known. A threshold criterion for particle injection to shock acceleration can be set from the injection efficiency calculation.

Adiabatic trapping and acceleration of the beam in the booster synchrotron of the ITEP-TWAC accelerating-storage facility

A system for adiabatic trapping of injected particles into the acceleration mode was developed and used to increase the intensity of the ion beam accelerated in the booster synchrotron of the ITEP-TWAC accelerating-storage facility at the Institute for Theoretical and Experimental Physics. Using numerical simulation, the parameters of the injected beam were optimized and the conditions for the absence of particle loss upon a changeover into the acceleration mode and subsequent acceleration to the maximum energy were determined. The effect that the characteristics of a system controlling the radio frequency of the accelerating field had on the dynamics of synchrotron motion and the particle loss was investigated. During development and adjustment of a complex of equipment that provided optimal conditions for adiabatic trapping and acceleration of C4+ ions with an initial energy of 1.3 MeV/amu, the trapping efficiency was as high as 80%, being limited only by the initial distribution of the momentum spread in the injected beam.

TU‐D‐BRA‐02: Compact Laser Acceleration of Paticles for Oncological Applications

The proton beam therapy of cancer has been practiced to yield good therapeutic results. However, it remains a costly treatment, primarily because of its huge accelerator facility needed to drive proton beams. The introduction of laser acceleration promises to greatly reduce the size and possible cost to accelerate and provide a medically needed system for therapy. Not only its compact acceleration section, but also its compactness of necessary radiation shield and beam handling section (a portion similar to the gantry) contribute to the compactness of the overall therapy machine size. A series of innovations such as the adiabatic acceleration, double‐layer target, the optimized target thickness, etc. constitutes to provide a new paradigm of laser‐driven compact therapy. We envision that the verification of dosage with the self‐auto‐activation by PET combined with pencil‐beam scanning characteristics amounts to a new feedback therapy of cancer.

Modeling of the Heliospheric Interface, Magnetic Field, and Cosmic‐Ray Transport

Magnetized flow and cosmic-ray transport in the local astrosphere are studied. A hybrid numerical model is used to calculate the heliospheric interface, the heliospheric magnetic field, and cosmic-ray modulation. Assuming that the transport parameters scale inversely proportional to the magnetic field, the amplification of the field in the inner heliosheath results in a sudden decrease of these parameters over the shock. This, together with our model calculations showing the compressed and heated solar wind flow is not divergence-free in the postshock region, results in effective adiabatic acceleration of cosmic-ray particles in the heliosheath. In particular, the peak of the computed anomalous particles is not at the shock but some distance into the inner heliosheath, where this region becomes populated with relatively high intensities of heated anomalous particles. However, this effect is largely dependent on the values of the transport parameters in the heliosheath. It is also shown that an improvement in the kinematically transported heliospheric magnetic field leads to a significantly different spatial distribution of cosmic rays compared to a Parker model.

Dynamics of a self‐trapped quasiparticle in a one‐dimensional molecular lattice with two phonon modes

AbstractIn this work we study numerically the dynamics of a self‐trapped quasiparticle (electron, hole or excitation) in a one‐dimensional molecular chain with two phonon modes, one acoustical and one optical. This model generalizes the model of Davydov solitons and Holstein polaron model: Davydov solitons are formed by a quasiparticle interacting with acoustical phonons, and Holstein polarons are formed due the interaction with optical phonons. Our model describes soliton‐like (large polaron) states in soft quasi‐one‐dimensional systems, such as conducting polymers, alpha‐helical polypeptide macromolecules, DNA etc. For the purpose of modelling self‐trapping of quasiparticles without any influence of chain boundaries, we consider a molecular chain with periodic boundary conditions. Then, to study the dynamics of quasiparticles in such states, we use the adiabatic acceleration method. We show that soliton velocity is an oscillating function of time. These oscillations are caused by the presence of optical vibrations and by the Peierls–Nabarro barrier formed by discrete lattice. We also show that self‐trapping of a quasiparticle takes place at random dynamical lattice potential induced by some random initial conditions for acoustical and optical phonons, provided the random range constant, i.e., the amplitude of random field, is not too big. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Adiabatic expansion acceleration process in spontaneous fast magnetic reconnection

MHD study for the adiabatic expansion acceleration process associated with the spontaneous fast magnetic reconnection is reported. When the fast reconnection process steadily generates a plasmoid in the downstream, the adiabatic expansion acceleration region appears between the reconnection jet and plasmoid. It is pointed out that the appearance of the acceleration region is required to steadily keep the reconnection process. The reconnection jet and plasmoid is generally high beta but the plasma pressure in the acceleration region is extremely low, when the reconnection jet is supersonic. This feature may become a signature to detect where the Petschek reconnection is steadily caused in the current sheet of the geomagnetotail.

Photocathodes as electron sources for high resolution merged beam experiments

An ultra cold electron source was developed for the storage ring TSR in Heidelberg to study electron-ion interactions with high energy resolution. The heart of the source is a GaAs photocathode which emits electrons with energy spreads below 10 meV. Photoemitted electrons are extracted in the space charge mode and then undergo adiabatic magnetic expansion and adiabatic acceleration to obtain an ultra cold electron beam which is overlapped with the stored ion beam in a straight section of the ring. In the first recombination measurements on HD+ unprecedented energy resolution for low-energy resonances was found, demonstrating a transverse and longitudinal temperature of the electron beam of about 0.5 meV and 0.02 meV, respectively.

The high-resolution electron–ion collision facility at TSR

A new electron beam facility at the storage ring TSR is presented which makes electron–ion collision experiments possible without using the existing electron cooler device as an electron target. A long adiabatic acceleration section combined with magnetic adiabatic expansion (factor up to ≈90) is used to reduce the longitudinal and transverse electron temperatures. The facility can be operated with either a thermionic or a photocathode. Results from two-dimensional electron beam profile measurements and longitudinal electron energy distribution measurements are discussed.

The main features of the ion spectra variations in the transition region from dipole to tailward streched field lines

The variations of the ion spectra in the transition region from dipole to tailward stretched field lines are analyzed on the base of INTERBALL/Tail satellite observations. The simultaneous measurements of the CORALL, DOK-2 and SKA-2 instruments were used for the obtaining the ion spectra in a wide energy range. The events of small spectra variations along the satellite trajectory are selected. Some possibilities to explain the observed feature are analysed including the satellite travelling along single field line, the compensation of the loses of particles due to drift effects by the adiabatic acceleration due to the regular earthward drift in the dawn-dusk electric field, and intense plasma mixing in the conditions of the nonconservation of the magnetic moments of particles. It is shown that the action of turbulent auroral electric fields is the most probable explanation of the observed effect.

A physics‐based study of the Dst‐AL relationship

Understanding the storm‐substorm relationship is an important problem in magnetospheric dynamics. In the past this question has been explored using statistical techniques and empirical relationships involving the Dst and AE/AL indices. In this research we have developed a relationship between AL and Dst based on the physics of the process. We argue that the magnetospheric electric field causes both the westward auroral electrojet and the particle injection into the ring current region. The adiabatic acceleration of ring current particles and the thermal properties of the ring current source have been included in the calculations. The injection function deduced is found to be a function of AL index and the solar wind dynamical pressure Pdyn. For AL> AL0 (∼−500 nT) the injection function is proportional to AL [Pdyn (t − td)]0.33with a time lag td≈ 45 min; while for large AL < AL0(∼−500 nT) the injection function is proportional to −|AL|1.7 [Pdyn(t − td)]0.33. This result is consistent with previous statistical results. We also find that the ring current decay time is a function of AL. Finally, we obtain an algorithm which predicts Dst from ALand the solar wind dynamical pressure Pdyn. The correlation coefficients between Dst predicted by the algorithm and the observed Dst index are 0.84 and 0.80 for the years 1998 and 1999, respectively.

Rapid enhancement of radiation belt electron fluxes due to substorm dipolarization of the geomagnetic field

The classical pure radial diffusion mechanism appears not to fully explain the frequently observed rapid enhancement in the timescales of minutes to hours in the radiation belt electron fluxes in the Earth's magnetosphere. We here consider other physical mechanisms, such as energization mechanisms associated with substorm processes, to account for these sudden increases. A three‐dimensional electron kinetic model is used to simulate the dynamics of the geomagnetically trapped population of radiation belt electrons during a substorm injection event. In the past this model has been extensively used to study dynamics of energetic ions in the ring current. This work, for the first time, constitutes the development of a combined convection and diffusion model to radiation belt electrons in the 0.04–4 MeV kinetic energy range. The Tsyganenko 89 geomagnetic field model is used to simulate the time‐varying terrestrial magnetosphere during the growth phase elongation and the expansion phase contraction. We find that inductive electric field associated with the magnetic reconfiguration process is needed in order to transport substorm electrons into the trapped particle region of the magnetosphere. The maximum enhancement in energetic electron fluxes is found to be located around the geosynchronous orbit location (L = 6.6), with up to 2 orders of magnitude enhancement in the total fluxes (0.04–4 MeV). Although this enhancement in the inner magnetosphere is very sensitive to the temperature and, to a less extent, density of the source population in the plasma sheet, we suggest that the substorm‐associated energization in the magnetotail and the subsequent adiabatic acceleration in the earthward region account for the enhanced MeV electrons (killer electrons) seen at the geosynchronous orbit during storms and substorms.

Turbulent adiabatic shock waves and diffusive particle acceleration

We investigate the properties of turbulent adiabatic shock waves resulting from the self-consistent inclusion of finite Alfvén-wave pressure in the shock balance equations. We demonstrate (i) the absence of a switch-on shock; (ii) the existence of two separated domains where the Mach number is greater than unity that have gas compression, and an associated domain where the gas compression ratio is less than unity (i.e. a rarefaction domain); (iii) the difference between the scattering-centre compression ratio and the gas compression ratio for all upstream wave cross-helicity values except for the isolated case of zero helicity; (iv) the presence of anomalous domains where the cosmic-ray particle spectral index can be negative. All of these results are brought about by including the upstream and downstream turbulent wave energies in the shock balance structure. Without the waves, none of the effects persists.

Adiabatic expansion acceleration mechanism of superfast jets in the spontaneous fast magnetic reconnection model

In contrast to the Petschek reconnection model, the plasma outflow jet in front of the plasmoid associated with the spontaneous fast reconnection model is found to exceed steadily the Alfvén velocity measured in the upstream magnetic field region. According to two-dimensional magnetohydrodynamic simulations, the final velocity of the plasma jet is observed to be superfast and can reach 1.4 times of the Alfvén velocity, which is maintained until the jet encounters a fast shock generated in front of the plasmoid. On the basis of the Rankine Hugoniot relation and the Bernoulli equation, it is theoretically found that the superfast plasma jet generated by slow shocks associated with the reconnection process is effectively accelerated beyond the Alfvén velocity by the adiabatic expansion of the plasma jet without any magnetic effect. In the plasma accelerations, the initial plasma acceleration caused in the slow shock is consistent with that of the Petschek reconnection model, but the subsequent plasma acceleration caused by the adiabatic expansion is not considered in his model. In association with the new acceleration mechanism, one pair of low-pressure regions emerges in the upstream magnetic field region. The generation of the low-pressure regions indicates that the significant adiabatic expansion results from the distortion of the surrounding magnetic field lines associated with the swelling plasmoid.

Numerical simulation of the adiabatic acceleration of electron beams

The method of adiabatic acceleration of electron beams is presented and the connection between the form of the accelerating potential and the length of the accelerator is investigated. Numerical simulations of the acceleration process have been performed using a molecular dynamics approach. The influence of the adiabaticity on the longitudinal temperature and the shape of the longitudinal velocity distribution are examined.

Electric-drift generated trajectories and particle acceleration in collisionless magnetic reconnection

Adiabatic acceleration of charged particles along magnetic field lines originates from the coupling between the electric drift and longitudinal motion in a nonunidirectional magnetic field. As a result, initially slow particles entering the reconnection site of an X-type magnetic geometry can leave the latter as substantially accelerated jets directed along the magnetic separatrices. The corresponding energy spectrum has a power-law form, with the spectral index depending on the angle between the separatrices.

Three-dimensional transport of cosmic rays in the heliosphere

The three-dimensional transport of cosmic rays in the heliosphere is reviewed. We report on results from our numerical simulations including convection, anisotropic diffusion, drifts, and both adiabatic energy loss and shock acceleration. Our model-heliosphere incorporates a wavy current sheet and Corotating Interaction Regions due to the increase of solar wind speed away from the current sheet. We are able to model the latitudinal as well as longitudinal variations, which latter appear as 26- and 13-day intensity waves. Our simulations for solar-wind speed, magnetic field, and cosmic-ray variations seem in general form quite similar to observational findings. Quantitative predictions depend on a number of physical parameters as the parallel and perpendicular diffusion coefficients, as well as on assumptions made on the structure of the heliospheric magnetic fields.

Low energy intense electron beams with extra-low energy spread

Maximum achievable intensity for low energy electron beams is a feature that is not very often compatible with low energy spread. We show that a proper choice of the source and the acceleration optics allows one to match them together. In this scheme, a GaAs photocathode excited by a single-mode infrared laser and adiabatic acceleration in fully magnetised optics enables the production of a low-energy-spread electron beam with relatively high intensity. The technological problems associated with the method are discussed together with its limitations.

On the interpretation of the correlated optical and radio outbursts in AO0235+164

A detailed analysis of the spectral properties of the correlated optical and radio outbursts occurred in the BL Lac object AO02235+164 is presented, taking the outburst in Oct.–Nov. 1975 as an example. The observed features can be interpreted reasonably in the frame of relativistic shock-jet models; (1) The background jet spectrum is determined and its structure is analysed on the basis of Königl's jet model. The distributions of electron density and magnetic field along the jet and other parameters are discussed. (2) With the background jet spectrum subtracted, the spectrum of the outburst is determined. It is shown that it can be represented by a uniform synchrotron radiation source with the optically thin spectral index α ∼ 0.25, the break frequency v b 3 × 10 12Hz and the high frequency cutoff v ̃ h ∼ 2 × 10 14 Hz . (3) The adiabatic acceleration of particles and amplification of magnetic field, and the parameters of the emitting regions (i.e. the post-shock regions) are discussed. (4) The simultaneity of the optical and radio outbursts is considered to be due to the optical thinness of the emitting region. The optically thin spectral index in radio band ( α ∼ 0.25) is smaller than that of the background jet spectrum ( α s2 ≅ 0.5) at similar frequencies (∼ 10 12 Hz >), and this can be used to explain the much larger amplitude of variations in optical band. (5) It is suggested that the difference between the spike profile of optical outburst and the broad profile of radio outburst (as an example, for the correlated optical radio outburst in 1979) could be explained by shock propagating through irregularities of energetic electrons and magnetic field along the jet, that is, the scale of the optical emission region along the jet might be much smaller than that of the radio emission region. In summary, the results given here show that the correlated optical-radio outbursts in AO0235+164 may be intrinsic to the source, not due to gravitational micro-lensing effects.

Quasi‐linear velocity space diffusion of heavy cometary pickup ions on bispherical diffusion characteristics

We consider an ion transport equation describing the source, convection, adiabatic acceleration and quasi‐linear velocity diffusion of cometary pickup ions. The equation is solved numerically along solar wind plasma flow lines for the environment of comet Halley, to obtain distributions at positions on the Giotto spacecraft trajectory which may be compared with observations. We use realistic input models for the observed spectrum of upstream and downstream propagating wave turbulence and for the ion source and mass loading of the solar wind. The ion resonance condition is not approximated in the diffusion term and we obtain full two‐dimensional (pitch angle and velocity) numerical distributions which show pitch angle scattering about the wave scattering centers at ± VA where VA is the Alfvén wave speed in the solar wind frame. Peak ion phase‐space densities approximately follow the bispherical shell geometry. The energy distributions F(υ) agree well with Giotto observations at lower υ but in the high‐energy tail there is evidence possibly for the first‐order Fermi acceleration mechanism taking place in the comet Halley foreshock region.

Relaxations in electron beams and adiabatic acceleration.

In order to produce electron beams with the minimum possible energy spread we have investigated the possibility of replacing the usual thermocathodes with a photoemissive source such as GaAs. A comparison carried out between these two sources indicated the latter as the best device to obtain a very-low-energy spread. Beam relaxation after emission also leads to an increase in the energy spread. Therefore an experimental study on the nature of relaxations occurring in electron beams, yielded both by a thermocathode and by a photocathode, has been performed. More specifically, we investigated the possibility of reducing the transverse-longitudinal and the longitudinal-longitudinal relaxations. With this aim, the features of adiabatic acceleration, which damp the pure longitudinal relaxation, have been examined. The experience gained during the measurement cycle demonstrated that an adiabatic structure, accelerating electrons emitted by a GaAs photocathode, leads to the best performances.