Fast Ion Beam Research Articles

Generation of collimated beams of relativistic ions in laser-plasma interactions

A method is proposed for generating collimated beams of fast ions in laser-plasma interactions. Two-dimensional and three-dimensional particle-in-cell simulations show that the ponderomotive force expels electrons from the plasma region irradiated by a laser pulse. The ions with unneutralized electric charge that remain in this region are accelerated by Coulomb repulsive forces. The ions are focused by tailoring the target and also as a result of pinching in the magnetic field produced by the electric current of fast ions.

Spread-frequency model of the fast beam-ion instability in an electron storage ring

When a gap in the electron bunch train prevents the trapping of ions, a transverse electron-ion instability may result from the ions created and lost during a single passage of the bunch train. A spread-frequency model is used to study this instability when the ions have a broad distribution of natural oscillation frequencies about the center of the electron beam. A growing disturbance saturates from Balakin-Novokhatsky-Smirnov damping at approximately the same time, and with the same total growth, as in the case without an ion frequency spread. At the tail of the bunch train, an unstable disturbance is amplified by a factor $\ensuremath{\sim}\mathrm{exp}({\ensuremath{\omega}}_{i}{\ensuremath{\tau}}_{b}\phantom{\rule{0ex}{0ex}})$ before saturation occurs, where ${\ensuremath{\omega}}_{i}$ is a typical ion oscillation frequency and ${\ensuremath{\tau}}_{b}$ is the duration of the bunch train. Initially, the instability displays exponential growth in time, unlike the case where the ion-frequency spread is neglected. For a broad distribution of ion frequencies, instability may be prevented by a betatron damping rate that exceeds the incoherent betatron frequency shift induced by ions at the tail of the bunch train.

Laser cooling of fast stored ions in barrier buckets

We apply novel bunch forms for efficient laser cooling of fast stored ion beams at the Heidelberg Test Storage Ring. 9 Be + ions are longitudinally confined in barrier buckets providing free motion of the cooled ions inside the bucket (quasi-coasting beam). The implementation of laser cooling in barrier buckets is described, and longitudinal dynamics of laser cooling is compared to electron cooling. With laser cooling, longitudinal temperatures far below 1 K are reached, corresponding to a momentum spread of Δ p/ p<10 −6. When the thermal energy becomes sufficiently small, significant modifications of the ion distribution due to space-charge effects are observed.

Aspects of fast-ion dosimetry.

A first step in the dosimetry of fast-ion beams is the determination of accurate Bragg (ionization) functions. Bragg functions for several substances have been measured and calculated for 3480 MeV carbon ions. In the measurements, the ions first traverse an absorber in which the energy is reduced to either 1900 or 1200 MeV, then a "range gauge" followed by a thin ionization chamber. Functions are calculated with an analytical method using convolutions of straggling functions. This approach gives results without the stochastic variations implicit in Monte Carlo methods. The comparison of measured and calculated functions shows how reliable the calculations are. An important part of the calculations is the determination of the total range of the ions. The range can be determined from the Bragg function. The measured range is given by the sum of the thickness of the absorber and the residual range measured with the range gauge. For water, the range is about 150 mm, and the precision of the measurements is +/-0.05 mm. Because the ion energy at the surface of the absorber fluctuates with time, measurements with water are used to define this energy. Thus the ranges (or average stopping powers) in absorbers are obtained relative to those in water. Measured ranges R(m) are compared with ranges R(0) calculated with a current version of the Bethe theory. For light absorbers (atomic number Z < 20), differences between R(m) and R(0) are less than +/-0.3 mm; for Z > 20 differences are between 0 and +/-0.6 mm. This agreement between calculated and measured ranges confirms the value I = 80 eV for water measured earlier for protons. The ionization by nuclear fragments is obtained from the difference between measured and calculated ionization functions, and has little influence on the ranges of the primary ions.

Thiosulfoxides (X 2SS) and disulfanes (XSSX): : first observation of organic thiosulfoxides

Using a combination of tandem mass spectrometry methodologies, collisional activation of fast (8 keV) or slow (20–30 eV) ion beams, neutralization–reionization and ion–molecule reactions, it is shown that thiosulfoxides X 2SS (X = H, CH 3, C 2H 5) are stable in the gas phase as radical cations as well as neutral molecules. The absence of isomerization into the more conventional disulfane structure, XSSX, is firmly demonstrated by ion-molecule reactions of the isomeric ions with methyl isocyanide and by collisional activation of “survivor” ions generated in neutralization–reionization experiments. All the mass spectral data have been recorded with a single hybrid mass spectrometer of sectors-quadrupole-sectors configuration. Molecular orbital calculations demonstrate that ionized thiosulfoxides and disulfanes are local minima on the MP2/6-31G∗ potential energy surface. The disulfane structures are found to be the most stable species by 91, 65 and 59 kJ/mol for X = H, CH 3 and C 2H 5, respectively. Heat of formation values of 1000, 810 and 760 kJ/mol are estimated for X 2SS ·+ ions (X = H, CH 3 and C 2H 5, respectively).

Anomalous behaviour of laser cooled fast ion beams

We have observed a sudden disappearance of intrabeam scattering in a laser cooled stored 9Be+ beam at the Heidelberg Test Storage Ring. The transition takes place at about 106 ions corresponding to a mean ion distance of ≃50c μm. The disappearance of IBS is accompanied by a decrease of the longitudinal temperature by more than two orders of magnitude. At the same time, the transverse width of the ion beam shows an increase which is limited to the diameter of the laser beam. Experimental signatures of this anomalous beam behaviour are described, and possible interpretations are discussed.

Nonlinear features of electrostatic ion cyclotron instability driven by counterstreaming ion beams in equatorial outer plasmasphere

A commonly observed feature of the field‐aligned flow of ions on closed geomagnetic field lines is that the pitch angle distributions are considerably broadened. Such pitch angle distributions facilitate in trapping ions and thus contribute to the process of plasmaspheric refilling. Since counterstreaming ion beams are a common feature of the early stage refilling as shown by models, we have investigated the role of ion‐beam‐driven instabilities in broadening the pitch angle distribution of the ion beams. The study is performed using a 2.5‐dimensional particle‐in‐cell code. It is shown that for sufficiently fast ion beams as expected in the outer region of the plasmasphere, the ion cyclotron modes, which appear at frequencies less than the ion‐cyclotron frequency Ωi, in the rest frame of the nondrifting electrons, are the dominant unstable wave modes. In the early stage of the instability a nearly purely growing mode dominates, which efficiently mediates in transferring a part of the parallel drift energy into the perpendicular energy, significantly broadening the ion pitch angle distribution. The transfer abruptly saturates when Jn (k⊥ρif) → 0, where Jn ( ) is the Bessel function of an appropriate order n, k⊥ is the perpendicular wave number of the dominant mode, and ρif,y is the maximum value of the Larmor radius of the beam ions in anomalous cyclotron resonance with the wave. This behavior of the dominant mode in conjunction with the conservation of ion energy allows a method for including such wave‐particle interaction effects in mesoscale models for the plasmaspheric refilling. The perpendicular acceleration of ions by this instability reduces the parallel drift only by a relatively small fraction, leaving the counterstreaming intact. However, the perpendicular acceleration produces a pitch angle distribution which is inducive for the trapping of the counterstreaming ions in the plasmaspheric flux tubes.

Forward scattering peaks of ion-induced Auger electrons

We have investigated availability of fast ion beams for observing the forward scattering effect of Auger electrons emitted from a crystal surface. The angle-resolved patterns of Si KLL–Auger electrons (∼1.6 keV) induced by 12 MeV He 2+ exhibited clear crystallographic symmetry including enhanced intensities in the 〈1 0 0〉 and 〈1 1 1〉 directions. The results demonstrate applicability of the angle-resolved spectroscopy of ion-induced Auger electrons to structural analysis of surfaces and interfaces.

TLD 300 Dosimetry in a 175 MeV Proton Beam

TLD-300 dosemeters have been successfully employed in high LET radiotherapeutic fields generated by fast neutron, negative pion and heavy ion beams. By proper peak height analysis of the two main peaks of CaF 2 :Tm glow curves after such irradiations it is possible to separate the high and low LET content of the radiation field. Therefore the distribution of the biological equivalent dose can be determined in a single irradiation. The method was extended to a pre-therapeutic proton field of 175 MeV maximum kinetic energy. It turned out that in such proton fields the sensitivity of the high temperature peak is constant; thus the dose can be determined to an accuracy of ±4%. The sensitivity of the low temperature peak decreases linearly to 50% as a function of y D such that the average lineal energy can be determined to an accuracy of ±5 keV.μm -1 .

A method for measurements of neutral fragments kinetic energies released to a specific dissociation threshold: optical translational spectroscopy

Abstract The optical translational spectroscopy technique, based on the principles of fast ion beam laser spectroscopy (FIBLAS) and translational spectroscopy, allows the kinetic energies study of neutral fragments released through free dissociation of a neutral molecule. This method presents interesting features such as near-threshold energy measurements and selection of a specific dissociation limit. The fragments resulting from free dissociation (not induced) of neutral molecules, produced by charge exchange processes with a fast ion beam, are probed by laser radiation. Monitoring of the laser-induced fluorescence allows high-resolution spectra due to the kinematic compression of the velocity spread. Measurements of kinetic energies released to the second limit of dissociation H(1s) + H(2l) of H 2 are put forth and compared with those obtained by means of off-axis translational spectroscopy.

Application of solid-state nuclear track detectors for studies of fast ion beams within PF-1000 and other plasma-focus facilities

The paper presents results of studies of the fast ion emission from two plasma facilities: PF-1000 and PF-II. The measurements were carried out mainly with solid-state nuclear track detectors (SSNTDs). To register ion mass- and energy-spectra, obtained when primary ion beams are deflected by magnetic and electric fields of a Thomson-type spectrometer, the SSNTDs plates were installed inside the analyzer. To measure ion angular distributions the SSNTDs were located within the main discharge chamber, at different angles to the electrode axis. The SSNTDs were also applied to register ion beam images formed within an ion pinhole camera, and to determine a spatial location of the fast ion sources.

A technique for lifetime measurements of long-lived atomic states utilising laser excitation of stored ion beams

A laser probing technique for atomic lifetime measurements of metastable states using an ion storage ring is presented. The method benefits from the high spectral resolution of conventional fast ion beam laser spectroscopy, and is applicable to metastable systems in lowly charged ions with lifetimes in the 10 ms–1 s regime. Due to strong probing mechanisms, the technique can be applied to ion beams with low metastable fractions. A comprehensive description of the laser probing technique is given, with examples from recent experiments on singly charged calcium, strontium and xenon, and future prospects are discussed.

Three-dimensional laser cooling method based on resonant linear coupling

A three-dimensional (3D) laser cooling method of fast stored ion beams based on a linear coupling mechanism is explored. We extensively study two approaches proposed in previous publications, i.e., the dispersive coupling scheme and the coupling-cavity scheme, confirming how much one can improve the transverse cooling efficiency. A possible design of a coupling cavity is presented. We employ the tracking code SAD and the molecular dynamics code SOLID to carry out reliable numerical experiments where realistic lattice structures of storage rings and particle Coulomb interactions are taken into account. Through systematic simulations, it is demonstrated that resonant coupling remarkably enhances transverse cooling rates for any initial beams, making it feasible to reach an equilibrium temperature far below the current achievable level. We further emphasize the crucial importance of avoiding the Mathieu instability. We also discuss the minimum cooling power required for beam crystallization as well as on an interpretation of past experimental results in the TSR and ASTRID storage rings.

Laser-ablation charge exchange for collinear resonance ionization spectroscopy

A charge-exchange cell based on pulsed-laser ablation has been developed and applied to collinear resonance ionization spectroscopy. A fast lithium ion beam has been neutralized in the charge-exchange cell and resonance ionization spectra of the 2 s 2 S 1/2−2 p 2 P 1/2,3/2 0 transitions of lithium have been successfully observed. The charge-exchange efficiency of the cell is discussed.

Fast particles and the edge transport barrier

The role of fast particles in edge transport barrier formation is discussed. Analysis of recent experiments on JET that have been carried out in hydrogen, deuterium and tritium plasmas, and in a DT mixture, with NBI and ICRF heating is presented which supports the idea that the width of the edge transport barrier is controlled by the banana width of the fast beam ions in the case of NBI heating and by the banana width of the thermal ions in the case of ICRF minority heating. A simple theoretical model which can account for this effect is analysed.

Electron vortexes in high-current plasma lens

High-current plasma lens (PL) instability of two-dimensional oscillations caused by unremovable radial gradients of the magnetic field and fast ion beams are considered. It is supposed that electron motion is nonlinear and ion motion is linear. The nonlinear differential equation of the electric potential in a hydrodynamical approximation is obtained and its solutions are found for different configurations of external magnetic fields. It is shown that these solutions may be represented in the form of stable vortex structures. Computer models for such kinds of vortices have been created. Creation of vortexes limits maximum values of space electrostatic fields in plasma optics. In the low current regime of the PL with strong ion beam space charge overcompensation, the equation obtained describes nonlinear motion of the electron fluid caused by diocotron instability.

Rapid adiabatic passage in laser cooling of fast stored ion beams

We present a simple and efficient method for the broadband laser excitation of ions in a fast beam by optical rapid adiabatic passage. The method relies on the frequency chirp affecting the ions in continuous-wave fixed-frequency laser light as a result of the Doppler effect when they are accelerated in local electrostatic fields. We have studied this excitation scheme at the Heidelberg Test Storage Ring and demonstrated its particular application to overcome collisional losses in laser cooling of fast stored ion beams.

Electron entrainment current in gases bombarded with beams of fast highly-charged ions

It is shown that asymmetry in the ejection of electrons in an elementary collision event between an atom and a fast highly-charged ion may lead to a macroscopic effect, the electron entrainment current, when a gas target is bombarded by a beam of fast highly-charged ions. The entrainment current is calculated for the bombardment of a helium target by a 25 MeV/u Mo40+ beam.

New observations of ion beams in the plasma sheet boundary layer

The Wind perigee passes covered tail distances from 6–24 RE. By use of bulk quantities and the parent distributions, we have found new features in the PSBL that had been missed previously. The PSBL consists of a unidirectional earthward streaming ion beam at the edge and another unidirectional beam inside this edge streaming in the tailward direction. Bidirectional beams are observed with higher densities, further inside the PSBL. The plasma in the region supporting the tailward streaming beams consists of the beam distribution plus an isotropic component, whereas the earthward streaming beams consists mainly of the beam distribution. These distributions yield fast flows (&gt;400 km/s) in the earthward direction and slower flows (≈ 150 km/s) in the tailward direction. Both regions support counter streaming electron beams superposed on an isotropic component. These new findings are substantially different from previous observations and the interpretation of fast flows and ion beams in terms of a neutral line model needs to be reexamined.

Solar EUV line identifications from delayed beam-foil spectra

A number of electric dipole transitions from low-lying long-lived levels in Si-, P- and S-like ions of Fe and neighbouring elements have been identified by their longevity in delayed EUV spectra after foil excitation of fast ion beams. Some of these new identifications pertain to previously unidentified lines in solar flare spectra.