A. A. Babaev

On the deflection of a positron beam by the miscut surface of an oriented crystal

Trajectories of relativistic positrons moving in the electric field of oriented crystal near its surface have been simulated. A positron beam enters the field of the crystal at a small angle to crystallographic planes. Thus, planar channeling conditions are satisfied. The miscut surface of a crystal has a specific shape of a step sequence of terraces. It has been shown that such a surface can deflect a noticeable fraction of the beam from the crystal surface by means of quasichanneling. The possibility of the experimental observation of this phenomenon has been analyzed.

Complex structure of resonant coherent excitation peaks for heavy relativistic hydrogen-like ions under planar channeling

The computer model for the resonant coherent excitation of heavy relativistic ions under planar channeling in crystals taking into account the fine structure of the energy levels of the orbital electron and the ion ionization from both the ground and first excited state is presented. The model has been used to explain the experiments carried out under planar channeling of 390 MeV/n 17+Ar ions. Reasonably good agreement for the calculated and experimental data has been obtained.

Theory of resonant coherent excitation of relativistic hydrogen-like heavy ions under planar channelling in a Si crystal

The theory of resonant coherent excitation of relativistic hydrogen-like heavy ions under planar channelling in a crystal is developed, which takes into account the fine structure of electronic energy levels of an ion, the Stark effect due to continuous planar potential, and ionization both from ground and excited states during ion penetration through a crystal. The theory is applied to explain the experimental data on resonant coherent excitation of 390 MeV/ uA r 17+ ions under (220) Si planar channelling, and reasonably good agreement of calculations and experimental data is achieved.

Resonant coherent excitation of hydrogen-like ions planar channeled in a crystal; Transition into the first excited state ☆

Abstract The presented program is designed to simulate the characteristics of resonant coherent excitation of hydrogen-like ions planar-channeled in a crystal. The program realizes the numerical algorithm to solve the Schrodinger equation for the ion-bound electron at a special resonance excitation condition. The calculated wave function of the bound electron defines probabilities for the ion to be in the either ground or first excited state, or to be ionized. Finally, in the outgoing beam the fractions of ions in the ground state, in the first excited state, and ionized by collisions with target electrons, are defined. The program code is written on C++ and is designed for multiprocessing systems (clusters). The output data are presented in the table. Program summary Program title: RCE_H-like_1 Catalogue identifier: AEKX_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEKX_v1_0.html Program obtainable from: CPC Program Library, Queenʼs University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 2813 No. of bytes in distributed program, including test data, etc.: 34 667 Distribution format: tar.gz Programming language: C++ (g++, icc compilers) Computer: Multiprocessor systems (clusters) Operating system: Any OS based on LINUX; program was tested under Novell SLES 10 Has the code been vectorized or parallelized?: Yes. Contains MPI directives RAM: Classification: 2.1, 2.6, 7.10 External routines: MPI library for GNU C++, Intel C++ compilers Nature of problem: When relativistic hydrogen-like ion moves in the crystal in the planar channeling regime, in the ion rest frame the time-periodic electric field acts on the bound electron. If the frequency of this field matches the transition frequency between electronic energy levels, the resonant coherent excitation can take place. Therefore, ions in the different states may be observed in the outgoing beam behind the crystal. To get the probabilities for the ion to be in the ground state or in the first excited state, or to be ionized, the Schrodinger equation is solved for the electron of ion. The numerical solving of the Schrodinger equation is carried out taking into account the fine structure of electronic energy levels, the Stark effect due to the influence of the crystal electric field on electronic energy levels and the ionization of ion due to the collisions with crystal electrons. Solution method: The wave function of the electron of ion is the superposition of the wave functions of stationary states with time-dependent coefficients. These stationary wave functions and corresponding energies are defined from the stationary Schrodinger equation. The equation is reduced to the problem of the eigen values and vectors of Hermitian matrix. The corresponding matrix equation is considered as the linear equation system. Then the time-dependent coefficients of the electron wave function are defined from the Schrodinger equation, with a time-periodic crystal field. The time-periodic field is responsible for the transitions between the stationary states. The final time-dependent Schrodinger equation represents the matrix equation which has been solved by means of the QR-algorithm. Restrictions: As expected the program gives the correct results for relativistic hydrogen-like ions with the kinetic energies up to 1 GeV/u and at the crystal thicknesses of 1–100 μm. The restrictions are: first, the program might give inadequate results, when the ion kinetic energy is too large (>10 GeV/u); second, the unaccounted physical factors may be significant at specific conditions. For example, the spontaneous emission by exited highly charged ions, as well as both energy and angular spread of the incident beam, could lead to additional broadening of the resonance. The medium polarization by the electric field of ion can influence the electronic energy levels of the ion in the non-relativistic case. The role of these factors was discussed in the references. Also, the large crystal thickness may require large computational time. Running time: In general, the running time depends on the number of processors. In our tests we used the crystal thickness up to 100 μm and the number of 2.66 GHz processors was up to 100. The running time was about 1 hour in these conditions.

Heating of a Thin Crystal Target at the Passage of High-Energy Short Electron Bunches

A theoretical model is developed for the heating and deformation of a thin target at the passage of short electron bunches through it at the energies of modern free electron lasers, and the corresponding simulation is carried out. It is demonstrated that under these conditions, the target can undergo overheating or be irreversibly deformed, which makes it difficult to use such targets for the diagnostics of free electron beams used in modern lasers.

Resonant coherent excitation of Ar17+ ions taking into account a fine structure of energy levels

The effect of a fine structure of the orbital electron energy levels of an Ar17+ ion on the resonant coherent excitation under planar channeling has been investigated by computer simulation technique. The obtained resonance curves are characterized by two closely situated peaks due to the transitions of an electron from the ground to excited states corresponding to the different components of a fine structure of the first excited state which differ in the value of the total electron momentum (1/2 or 3/2).