High-energy Electron Accelerators Research Articles

Laser accelerator developments for future high-energy accelerators

Laser-driven accelerators will be realized as the next generation particle accelerators in the near future. Their development has been accelerated by success of high-energy gain electron acceleration by means of a laser wakefield excited via interaction of intense ultrashort laser pulses with underdense plasmas. On the basis of achievements of laser wakefield acceleration (LWFA) experiments, a design of LWFA for the second-generation study is considered from the point of view of application to high-energy accelerators, such as future linear colliders.

Time-dependent chemical compositions of 13N and 15O induced in air by the operation of a high energy electron accelerator.

Time-dependent chemical compositions for 13N and 15O induced in the air atmosphere of a high energy electron accelerator room have been studied using a computer simulation method. A radiation chemistry model was developed to describe the chemical reactions of 13N and 15O species with the air molecules and their radiolytic products. By assuming several chemical forms of 13N and 15O generated by the (gamma, n) reaction, the variations of the concentrations of 13N and 15O species were simulated under a radiation field. From the comparison between the simulations and experiment in a 100 MeV electron linear accelerator (linac) facility, the following conclusions were obtained: (1) Just after the (gamma, n) reaction, 25-50% of 13N and 15O are present as atoms (13N, 15O) and/or their ions (13N+, 15O+) and the remainder as nitrogen and oxygen molecules (13NN, 15OO) and/or their ions (13NN+, 15OO+); (2) Neutralization of 13N+ and 15O+ ions into 13N and 15O atoms occurs instantaneously and the same is the case with the neutralization of 13NN+ and 15OO+ ions to 13NN and 15OO molecules; (3) The neutralized 13N and 15O atoms react with the air molecules and the radiolytic products to form nitrogen oxide compounds and ozone, while 13NN and 15OO remain as these molecules. Factors that control the chemical reactions of 13N and 15O are discussed.

Detection of TeV Gamma Rays from SN1006

In spite of the recent progress of high energy gamma-ray astronomy, there still remains quite unclear and important problem about the origin of cosmic rays. Supernova remnants (SNRs) are the favoured site for cosmic rays up to 1016 eV, as they satisfy the requirements such as an energy input rate. But direct supporting evidence is sparse. Recently intense non-thermal X-ray emission from the rims of the Type Ia SNR SN1006 (G327.6+14.6) has been observed by ASCA (Koyama et al. 1995)and ROSAT (Willingale et al. 1996), which is considered, by attributing the emission to synchrotron radiation, to be strong evidence of shock acceleration of high energy electrons up to ~100 TeV. If so, TeV gamma rays would also be expected from inverse Compton scattering (IC) of low energy photons (mostly attributable to the 2.7 K cosmic background photons) by these electrons. By assuming the magnetic field strength (B) in the emission region of the SNR, several theorists (Pohl 1996; Mastichiadis 1996; Mastichiadis & de Jager 1996; Yoshida & Yanagita 1997) calculated the expected spectra of TeV gamma rays using the observed radio/X-ray spectra. Observation of TeV gamma rays would thus provide not only the further direct evidence of the existence of very high energy electrons but also the another important information such as the strength of the magnetic field and diffusion coefficient of the shock acceleration. With this motivation, SN1006 was observed by the CANGAROO imaging air Cerenkov telescope in 1996 March and June, also 1997 March and April.

Measurements of Neutron Spectra at the Stanford Linear Accelerator Center

The purpose of the present measurements was to determine neutron spectra outside the shielding of high energy electron accelerators, with a special emphasis on the high energy (>20 MeV) portion of the spectra. The spectrometer system used consists of a standard set of Bonner spheres augmented by a custom made 30.48 cm (12) sphere with a 1 cm inner layer of lead, which increases its response in the high energy region. The system was further complemented by a plastic scintillator used for the measurement of 11 C activation. A pair of rem meters was also used: one standard Anderson-Braun, and one with an inner layer of lead which has an extended high energy response. Measured spectra and relevant dosimetric quantities outside shielding at three different electron beam energies are presented.

加速器施設の放射線防護の諸問題 高エネルギー加速器について

加速器施設の放射線防護の諸問題 高エネルギー加速器について

低エネルギー電子加速器による天然ゴムラテックスの放射線加硫

Radiation vulcanization of natural rubber latex (RVNRL) with low energy electron accelerator seems to be more economic than those with Co-60 and high energy electron accelerator because low energy electron accelerator does not need special bio-shielding. The drawback of the low energy electron accelerator is low penetration capacity of the electron beams into the latex. Two irradiation systems, vessel type irradiator with a stirrer installed from the bottom of the vessel and a drum type irradiator were tested for RVNRL with low energy electron accelerator to overcome the disadvantage. Both irradiating system are workable for the vulcanization of latex. Significant effects of stirrer speed and weak dose rate effects were found in the case of vessel type irradiator. The optimum stirring condition was achieved at the 150rpm of stirrer speed with Reynolds number of around 1, 200. Drum type irradiator favorable to irradiate latex continuously in the form of thin layer.

Evidence for shock acceleration of high-energy electrons in the supernova remnant SN1006

HIGH-ENERGY cosmic rays (relativistic heavy nuclei) play an important role in heating interstellar matter in the Milky Way1,2, and they affect chemical abundances through collisions with atoms in the interstellar gas2. Although it has long been thought that these cosmic rays arise from supernovae3,4, direct evidence for such an association has been lacking. Here we report X-ray observations of the remnant of supernova 1006, made by the ASCA satellite, which indicate that emission from the edges of the remnant shell is dominated by radiation from electrons accelerated to energies of ~ 100 TeV within the shock front. Ions in the shell are likely to have been accelerated to similar energies, thus giving rise to very-high-energy cosmic rays.

Perspectives of Personnel External Dosimetry at Stanford Linear Accelerator Centre

The need for providing personnel external monitoring for exposure to low energy photon (<150 keV) and beta radiation at the Stanford Linear Accelerator Center (SLAC) has been studied. At SLAC, a high energy electron accelerator facility, the major sources of low energy photons and betas (electrons and positrons) are from the induced activities in accelerator parts and components. Shower theory and measurements show that these induced activities are generally distributed deep inside the materials. Therefore, the low energy photons and betas will be significantly absorbed within the materials. Calculations based on the characteristics of the radionuclides produced by photonuclear reactions in several common materials have indicated that the beta doses on the material surface are small fractions of the photon doses. Field measurements at SLAC were made using Kodak type 2 films, Panasonic UD810 TLDs and a Victoreen 450 survey meter to determine the doses from low energy photons and betas, and then compared them with the doses from high energy photons. The X rays from the klystrons and the synchrotron radiation are also discussed. The results verify that the doses are small enough that, from the technical point of view, no personnel monitoring is needed for low energy photons and betas.

Electron trapping in evolving coronal structures during a large gradual hard X-ray/radio burst

Gradual hard X-ray/radio bursts are characterized by their long duration, smooth time profile, time delays between peaks at different hard X-ray energies and microwaves, and radiation from extended sources in the low and middle corona. Their characteristic properties have been ascribed to the dynamic evolution of the accelerated electrons in coronal magnetic traps or to the separate acceleration of high-energy electrons in a ‘second step’ process. The information available so far was drawn from qualitative considerations of time profiles or even only from the common occurrence of emissions in different spectral ranges. This paper presents model computations of the temporal evolution of hard X-ray and microwave spectra, together with a qualitative discussion of radio lightcurves over a wide spectral range, and metric imaging observations. The basic hypothesis investigated is that the peculiar ‘gradual’ features can be related to the dynamical evolution of electrons injected over an extended time interval in a coronal trap, with electrons up to relativistic energies being injected simultaneously. The analyzed event (26 April, 1981) is particularly challenging to this hypothesis because of the long time delays between peaks at different X-ray energies and microwave frequencies. The observations are shown to be consistent with the hypothesis, provided that the electrons lose their energy by Coulomb collisions and possibly betatron deceleration. The access of the electrons to different coronal structures varies in the course of the event. The evolution and likely destabilisation of part of the coronal plasma-magnetic field configuration is of crucial influence in determining the access to these structures and possibly the dynamical evolution of the trapped electrons through betatron deceleration in the late phase of the event.

A 5MV 30mA EB/X-ray processing system

We developed a 5MV 30mA electron processing system to meet the increasing need for a high energy electron accelerator for applications such as sterilization of medical goods, bulk plastic material and/or animal feed. The developed is the world-record electron accelerator using Cockcroft-Walton D.C. circuit and is used not only for the electron irradiation but also for X-ray irradiation by using X-ray conversion target. We expect this machine to replace disinfecting chemicals such as ETO and radioactive Co-60 or Cs-137 which are presently used for sterilization or bulk material irradiation.

An investigation of the influence of applied voltage on ionization due to electrons in free-air ionization-chambers

Secondary electrons, produced in a free-air ionization-chamber, gain or lose energy (i.e., accelerate or decelerate) due to the electric field and, hence, cause an enhancement or reduction in ionization. In order to estimate this effect computer calculations were performed. These calculations showed that when exposure is measured, for example for X-rays generated at a tube voltage of 300 kV and at an electric field strength of 20 kV / m, the output charge of the chamber is increased by 0.8% due to the gain in energy of the secondary electrons. At the same time, the output charge is decreased by a similar amount due to the loss in energy of the secondary electrons. The net increase in the output signal charge is estimated to be 0.05%. However, the effect of acceleration and/or deceleration of high-energy secondary electrons emitted by X-rays, is relatively more significant than that of the gas multiplication at low electric field strengths.

High-energy x-ray beam characterization using photonuclear reaction.

Spectral changes in the primary photon beam of a high-energy medical electron accelerator have been monitored using photonuclear reactions. The beam across the radiation field was measured using induced activity in Teflon cylinder and copper foil samples at several off-axis angles, at different SSDs in air and in a solid water phantom. The induced activities of the positron emitters were measured using a coincidence detection system. The experimental results are in agreement with Monte Carlo calculations and Schiff's theory.

Cumulative beam breakup in linear accelerators with periodic beam current.

An analytic formalism of cumulative beam breakup in linear accelerators is developed. This formalism is applied to both low-velocity ion accelerators and high-energy electron accelerators. It includes arbitrary velocity, acceleration, focusing, initial conditions, beam-cavity resonances, finite bunch length, and arbitrary charge distribution within the bunches, and variable cavity geometry and spacing along the accelerator. For both direct-current beams and beams comprised of \ensuremath{\delta}-function bunches, both the steady-state and transient displacements of the beam are calculated, and scaling laws are determined for the transient beam breakup. The steady-state transverse displacement of particles between bunches is also calculated since, if allowed to impinge on the accelerating structures, these particles could cause activation over long periods of continuous-wave operation. The formalism is then applied to high-current ion accelerators by studying the effects of finite bunch length and arbitrary charge distribution within the bunches. The role of focusing in controlling cumulative beam breakup is quantified in each of these cases. Additionally, the effects of random initial conditions and a distribution of deflecting-mode frequencies in the cavities are also quantified.

Net Inverse Bremsstrahlung Acceleration in Plasma Waves

It is shown that the acceleration by the net inverse bremsstrahlung of laser light is a classical phenomenon in which high-energy electrons, whose momentum and energy are modulated by the laser light, strictly obey the momentum and energy conservation laws and the time-reversal invariance of the transition probability. Net inverse bremsstrahlung acceleration of high-energy electrons traveling in strong laser light and plasma waves is investigated using the relativistic quantum kinetics based on the Dirac equation. The quantum kinetics treats the classical phenomenon exhibited by high-energy electrons through the correspondence principle. The dc force in the laser light direction by the net inverse bremsstrahlung is independent of the spread of the electron beam energy, and increases with the electron beam energy. The dc force also increases with the laser intensity if the potential amplitude of the laser wave is smaller than the electron rest energy.

Theory of free-electron-laser heating and current drive in magnetized plasmas

The introduction of a powerful new microwave source, the free-electron laser, provides new opportunities for novel heating and current-drive schemes to be used in toroidal fusion devices. This high-power, pulsed source has a number of technical advantages for these applications, and its use is predicted to lead to improved current-drive efficiencies and opacities in reactor-grade fusion plasmas in specific cases. The Microwave Tokamak Experiment at the Lawrence Livermore National Laboratory will provide a test for some of these new heating and current-drive schemes. Although the motivation for much of this research has derived from the application of a free-electron laser to the heating of a tokamak plasma at a frequency near the electron cyclotron frequency, the underlying physics, i.e., the highly nonlinear interaction of an intense, pulsed, coherent electromagnetic wave with an electron in a magnetized plasma including relativistic effects, is of general interest. Other relevant applications include ionospheric modification by radio-frequency waves, high-energy electron accelerators, and the propagation of intense, pulsed electromagnetic waves in space and astrophysical plasmas. This review reports recent theoretical progress in the analysis and computer simulation of the absorption and current drive produced by intense pulses, and of the possible complications that may arise, e.g., parametric instabilities, nonlinear self-focusing, trapped-particle sideband instability, and instabilities of the heated plasma.

Progress and open problems in hypernuclear structure

A hypernuclei exist as particle-stable systems with spectroscopies analogous to those of ordinary nuclei. The analysis of Brookhaven data on (π+,k+) reactions, for selected targets up to A~ 90, has yielded information on Λ single-particle energies and thus the Λ-nucleus mean field. Better energy resolution and higher intensity beams are needed to carry out coincidence experiments and detailed spectroscopic studies. The prospects for investigating hypernuclear structure at high-energy electron accelerator facilities via the (e, e'K) or (γ,K) reactions, which are kinematically similar to the (π+,k+) reaction, but which favor spin-flip transitions, are also considered.

A HIGH POWER LNA LASER FOR APPLICATION TO A NEW POLARIZED ELECTRON SOURCE

The recent development of high energy electron accelerators has generated a renewed interest in high current, high polarization electron sources. We have investigated several modifications to a method based on a pumped helium afterglow from which we expect improvements over the performances. These ones include the development of a high power, tunable LNA laser and the application of a new optical pumping scheme to the metastable helium atoms.

A high-speed shock wave in the impulsive phase of 1984 April 24 flare

An unusually high speed (10,000 km/s) type II radio burst was observed in the rising phase of a strong gamma-ray burst which started at 2359:58.7 UT on April 24, 1984. The high-speed type II burst decelerated suddenly to 3600 km/s and, after that, activated an advancing front moving type IV radio burst and an H-alpha Moreton wave. Comparisons of the type II burst with X-ray and gamma-ray emissions suggest that shock acceleration did not play a major role in acceleration of high-energy electrons and ions which were responsible for production of the strong X-ray and gamma-ray emissions in the impulsive phase of the April 24 flare. 13 refs.

Applications of EGS4 to industrial irradiator design and use

Historically, it has been difficult to use analytic models to accurately and completely predict radiation interaction with matter when designing high energy electron accelerators and their utilization for radiation processing. One possible solution to this problem is the EGS4 computer code system. It is a general purpose Monte-Carlo type package to simulate the transport of electrons, positrons and photons with energies from a few keV to well over 50 MeV in arbitrary, user-defined, complex geometries made up of any element or material. EGS4 has been used extensively by the high-energy particle accelerator community, medical physicists and health physicists the world over. This paper will describe the application of EGS4 as a design tool in the radiation processing field, for both accelerator design and end-use applications. Specific examples for simulations of electron energy deposition and x-ray production will be given. Included in the examples will be samples of graphics output directly from EGS4 to a graphics terminal using Unified Graphics. This capability allows the designer to “see” radiation interactions, such as electron scattering within and from the surface of a x-ray target. Also, graphical output of numerical data in the form of convenient histograms, spectral distributions and isodose contours may be generated.

電磁波による電子の加速

It is found that an electron can be efficiently accelerated by a simple system which is composed of a plus plane electromagnetic (EM) wave and a astatic magnetic field. In this system the direction of the magnetic component of the EM wave is pa rallel to that of the static fields. Initialy an electron is positioned in front of the pluse plane EM wave and the electron speed is less than the speed of light(c). The pulse plane EM wave travels across the static magnetic field with the speed of fight(c). Therefore, the pluse EM wave catches up with the electron and leaves it behind. Without the static magnetic field in the system, the electron would not be able to absorb the wave energy because of the symmetry of the EM wave. However, because the static magnetic field breaks the symmetry of the EM wave in the system, the electron can absorb the wave energy and be efficently accelerated in the direction of the electic component of the of the EM wave. We have obtained the optimal magnitude for the static magnetic field and the final γ by a simple analysis.The single particle analyses and the simulations also show that this system ones well for a high-energy electron acceleration and the simulation results agree with the analytical results.