I urval från Stockholms universitets publikationsdatabas

• Design of an optical trap for storing femtosecond laser pulses

  2018. Abdullah Shehata (et al.). Applied Optics 57 (5), 1212-1217

  Artikel

  An optical trap for storing femtosecond laser pulses to enhance the interaction effectiveness with optically thin targets is being proposed and investigated. Presently, we studied the trapping of 10-200 fs laser pulses of wavelength 800 nm, 1 mu J energy per pulse, and 10 Hz repetition rate. To compensate the optical losses in the trap, a Ti: Sapphire crystal as an amplifying medium is being considered, which should be synchronously pumped by the second harmonic of the Nd: YAG laser. Due to the propagation of the short pulses through optical trap components, group velocity dispersion introduces a significant broadening in pulse duration. To compensate for this broadening, a chirped mirror with suitable parameters is being proposed. An increase of the average power of the laser pulse in the optical trap that includes an amplifying medium (Ti: Sapphire crystal) by a factor of 805 compared to a single passage of the laser pulse was derived. It should be possible to store the laser pulse in the optical trap for >4 mu s with constant power and with a repetition rate of up to 250 MHz.

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• Transmission of electrons through the conical glass capillary with the grounded conducting outer surface

  2017. Li-Bing Qian (et al.). Wuli xuebao 66 (12)

  Artikel

  The transmission of 1.5 keV-electrons through a conical glass capillary is reported. This study aims to understand the so-called guiding effect for the negatively charged particles (e.g. electrons). The guiding mechanism is understood quite well with positively charged particles in particular highly charged ions, but not clear with electrons, i. e., even the basic scheme mediated by the existence of negative charge patches to guide the electrons is still somewhat controversial.. The study of the charging-up dynamics causing the electrons transport inside the capillary will shed light on this issue. In order to perform this, a data acquisition system has been setup to follow the time evolution of the two-dimensional angular distribution of the transmitted electrons. The electrons are detected by the multi-channel plate (MCP) detector with a phosphor screen. The image from the phosphor screen is recorded by a charge-coupled device camera. The timing signals for the detected events are extracted from the back stack of the MCP detector and recorded by the data acquisition system, synchronized with the acquired images. The electron beam has a size of 0.5 mm x 0.5 mm and a divergence of less than 0.35.. The inner diameter of the straight part of the capillary is 1.2 mm and the exit diameter is 225 mu m. A small conducting aperture of 0.3 mm in diameter is placed at the entrance of the capillary. Two-dimensional angular distribution of the transmitted electrons through conical glass capillary and its time evolution are measured. The results show that the transmission rate decreases and reaches to a constant value for the completely discharged glass capillary with time going by. The centroid of the angular distribution moves to an asymptotic value while the width remains unchanged. These transmission characteristics are different from those indicated in our previous work (2016 Acta Phys: Si n: 65 204103). The difference originates from the different manipulations of the capillary outer surface. A conducting layer is coated on the outer surface of the capillary and grounded in this work. This isolates various discharge/charge channels and forms a new stable discharge channel. The transmission rate as a function of the tilt angle shows that the allowed transmission occurs at the tilt angle limited by the geometrical factors, i. e., the geometrical opening angle given by the aspect ratio as well as the beam divergence. The transmission characteristics suggest that most likely there are formed no negative patches to facilitate the electron transmission through the glass capillary at this selected beam energy. It is different from that of highly charged ions, where the formation of the charge patches prohibits the close collisions between the following ions and guides them out of the capillary.

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• Transmission profiles of ions through nano-capillaries of rectangular cross-section in mica

  2017. Hongqiang Zhang (et al.). Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 406, 421-424

  Artikel

  We investigated transmission characteristics of Ne7+ ions through nanocapillaries of rectangular cross-section. The capillaries were produced by chemical etching of ion tracks in phlogopite mica. The two dimensional transmission profiles are rhombic when the capillaries are tilted at angles smaller than the geometrical opening angle given by the aspect ratio of the capillaries. For the angles larger than the geometrical opening angle with respect to the beam direction, the rhombic profile is degrading. Possible reasons for the degrading of the shapes by the deposited charge inside the capillaries are investigated and discussed.

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• Water fragmentation induced by ion impact

  2017. N. D. Cariatore (et al.). Nuclear Instruments and Methods in Physics Research Section B 408, 198-202

  Artikel

  This work focuses on the energy distributions of positive water ionic fragments produced by ion impact at MeV impact energies. An improved Coulomb explosion model coupled to a classical trajectory Monte Carlo simulation is used to provide energy centroids of the fragments for the dissociation channels resulting from the removal of two to five electrons from the water molecule. This model explicitly includes the post-collisional interaction of the projectile with the resulting ionic fragments affecting their kinetic energy release spectra especially at low impact energies. Theoretical data are benchmarked against recent data collected for 220 keV Xe22+ + H2O collisions which corresponds to a large Zp/v collision parameter. To extend our tests to the low Zp/v regime, fragment species as a function of emission energy and time-of-flight were recorded in 3 MeV Li3+ collisions by using an electrostatic spectrometer and a time-of-flight mass spectrometer, respectively. Present experimental data reveals the existence of multiple-ionization processes leading to charge state up to 4+.

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• Dynamics of slow electrons transmitting through straight glass capillary and tapered glass capillary

  2016. Cheng-Liang Wan (et al.). Wuli xuebao 65 (20)

  Artikel

  It has been found that the transmission rate of the electrons through insulating capillaries as a function of time/incident charge is not the same as that of the ions. The question arises that by using the electrons, if the negative charge patches can be formed to facilitate the transmission of the following electrons, thereby substantiating that the so-called guiding effect works also for electrons. This study aims to observe the time evolutions of the transmission of electrons through a straight glass tube and a tapered glass capillary. This will reveal the details of how and (or) if the negative charge patches can be formed when the electrons transport through them. In this work, a set of MCP/phosphor two-dimensional detection system based on Labview platform is developed to obtain the time evolution of the angular distribution of the transmitted electrons. The pulsed electron beams are obtained to test our detection system. The time evolution of the angular profile of 1.5 keV electrons transmitting through the glass tube/capillary is observed. The transmitted electrons are observed on the detector for a very short time and disappear for a time and then appear again for both the glass tube and tapered glass capillary, leading to an oscillation. The positive charge patches are formed in the insulating glass tube and tapered glass capillary since the secondary electron emission coefficient for the incident energy is larger than 1. It is due to the fact that fast discharge of the deposited charge leads to the increase of the transmission rate, while the fast blocking of the incident electrons due to the deposited positive charge leads to the decrease of the transmission rate. The geometrical configuration of the taper glass capillary tends to make the secondary electrons deposited at the exit part to form the negative patches that facilitate the transmission of electrons. This suggests that if the stable transmission needs to be reached for producing the electron micro-beam by using tapered glass capillaries, the steps must be taken to have the proper grounding and shielding of the glass capillaries and tubes. Our results show a difference in transmission through the insulating capillary between electrons and highly charged ions.

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• Guiding and scattering of ions in transmission through mica nanocapillaries

  2016. Hongqiang Zhang, Nadeem Akram, Reinhold Schuch. Physical review A 94 (3)

  Artikel

  The transition from guiding to scattering in the transmission of 70-keVNe(7+) through mica nanocapillaries of rhombic cross section is studied. Transmitted ions and neutrals are separated and their angular distributions are measured for various tilt angles of the capillaries with respect to the beam direction. The ions and neutrals have different angular profiles and different transmission dependences on tilt angle. The profiles of the transmitted ions are of rectangular shape while bananalike shapes appear for the neutrals. The time evolution measurements during charging up show a shift of the center of the ion angular distribution while that of the neutrals remains fixed. Trajectory simulations are performed by taking the image force and the Coulomb repulsive force from the deposited charge, as well as scattering from capillary walls into account. These show good agreement with the data and how the deposited or image charge deflects and shapes the ionic portion of the beam differently from the neutral part. The experimental separation of the ions from neutrals and their very different behaviors together with simulations gives us further insight into the mechanisms of guiding and scattering in transmission through nanocapillaries.

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• Physics book

  2016. M. Lestinsky (et al.). The European Physical Journal Special Topics 225 (5), 797-882

  Artikel

  The exploration of the unique properties of stored and cooled beams of highly-charged ions as provided by heavy-ion storage rings has opened novel and fascinating research opportunities in the realm of atomic and nuclear physics research. Since the late 1980s, pioneering work has been performed at the CRYRING at Stockholm (Abrahamsson et al. 1993) and at the Test Storage Ring (TSR) at Heidelberg (Baumann et al. 1988). For the heaviest ions in the highest charge-states, a real quantum jump was achieved in the early 1990s by the commissioning of the Experimental Storage Ring (ESR) at GSI Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt (Franzke 1987) where challenging experiments on the electron dynamics in the strong field regime as well as nuclear physics studies on exotic nuclei and at the borderline to atomic physics were performed. Meanwhile also at Lanzhou a heavy-ion storage ring has been taken in operation, exploiting the unique research opportunities in particular for medium-heavy ions and exotic nuclei (Xia et al. 2002).

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• Water fragmentation by bare and dressed light ions with MeV energies

  2016. W. Wolff (et al.). Physical Review A 94 (2)

  Artikel

  The energy and time-of-flight distributions of water ionic fragments produced by impact of fast atoms and bare and dressed ions; namely, H+, Li0-3+, C1+, and C2+ are reported in this work. Fragment species as a function of emission energy and time-of-flight were recorded by using an electrostatic spectrometer and a time-of-flight mass spectrometer, respectively. An improved Coulomb explosion model coupled to a classical trajectory Monte Carlo (CTMC) simulation gave the energy centroids of the fragments for the dissociation channels resulting from the removal of two to five electrons from the water molecule. For the energy distribution ranging up to 50 eV, both the experiment and model reveal an isotropic production of multiple charged oxygen ions, as well as hydrogen ions. From the ion energy distribution, relative yields of the dissociation resulting from multiple ionization were obtained as a function of the charge state, as well as for several projectile energies. Multiple-ionization yields with charge state up to 4+, were extracted from the measurements of the time-of-flight spectra, focused on the production of single and multiple charged oxygen ions. The measurements were compared to ion-water collision experiments investigated at the keV energy range available in the literature, revealing differences and similarities in the fragment-ion energy distribution.

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• A linear optical trap with active medium for experiments with high power laser pulses

  2015. Tarek Mohamed, Guillermo Andler, Reinhold Schuch. Review of Scientific Instruments 86 (2)

  Artikel

  A linear optical trap for circulating high power laser pulses and tuning these pulses to high repetition frequency of several tens of MHz has been developed. A ns excimer pumped dye laser pulse has been injected with help of a Wollaston prism and a synchronized Pockels cell into an optical trap formed by two highly reflecting mirrors in a linear configuration. The test was done at lambda = 580 nm, but the optical trap can be used without limitations in a broad band of optical wavelengths (400-700 nm). Power considerations give an increase of the efficiency of the optical trap of about 7 times compared to single passage of the laser pulse through the experimental section. The time structure of the trapped laser pulses can be controlled by changing the distance between the two high reflecting mirrors. The efficiency of the optical trap strongly depends upon optical losses. To compensate the optical losses, an amplifying cell was introduced, and the efficiency was about 60 times higher than that by single passage of the laser pulse through the experimental section. (C) 2015 AIP Publishing LLC.

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• A seeded dye laser cavity for intracavity experiments

  2015. Tarek Mohamed, Guillermo Andler, Reinhold Schuch. Laser physics 25 (9)

  Artikel

  A seeded dye laser cavity, synchronously pumped by the 2nd harmonic of the Nd:YAG laser has been designed and experimentally tested. The used seed signal was the well defined narrow linewidth output laser signal (Delta lambda = 0.013 nm) from the excimer-dye laser system. Energy considerations showed that the intracavity laser energy, that can be used for an experimental section inside the cavity, can reach an efficiency of 20% of the pumping energy. The wavelength and linewidth are fully controlled by the wavelength and linewidth of the seeding laser.

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• APPA at FAIR

  2015. Th. Stoehlker (et al.). Nuclear Instruments and Methods in Physics Research Section B 235, 680-685

  Artikel

  FAIR with its intense beams of ions and antiprotons provides outstanding and worldwide unique experimental conditions for extreme matter research in atomic and plasma physics and for application oriented research in biophysics, medical physics and materials science. The associated research programs comprise interaction of matter with highest electromagnetic fields, properties of plasmas and of solid matter under extreme pressure, density, and temperature conditions, simulation of galactic cosmic radiation, research in nanoscience and charged particle radiotherapy. A broad variety of APPA-dedicated facilities including experimental stations, storage rings, and traps, equipped with most sophisticated instrumentation will allow the APPA community to tackle new challenges. The worldwide most intense source of slow antiprotons will expand the scope of APPA related research to the exciting field of antimatter.

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• Evaporative cooling of hydrogen molecular ions in a Penning trap

  2014. Ke Yao, Reinhold Schuch. Hyperfine interactions, 191-196

  Konferens

  Evaporative cooling of singly charged ions in a Penning trap is studied. The ions are created by in-situ electron bombardment of hydrogen molecules and trapped in a cylindrical Penning trap. Cooling of the ions is observed by their axial motion after trapping of a few hundred milliseconds. The ions temperature decreases by a factor of more than 6 in 800 ms, while the bunch density of the coldest ions increases by up to a factor of 10. By studying the time constants of the dependence of ion loss and axial temperature on the magnetic field strength, we exclude the effects of ions loss through the cyclotron motion on the axial ion temperature.

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• SPARC collaboration

  2014. Thomas Stoehlker (et al.). Hyperfine interactions, 45-53

  Konferens

  SPARC collaboration at FAIR pursues the worldwide unique research program by utilizing storage ring and trapping facilities for highly-charged heavy ions. The main focus is laid on the exploration of the physics at strong, ultra-short electromagnetic fields including the fundamental interactions between electrons and heavy nuclei as well as on the experiments at the border between nuclear and atomic physics. Very recently SPARC worked out a realization scheme for experiments with highly-charged heavy-ions at relativistic energies in the High-Energy Storage Ring HESR and at very low-energies at the CRYRING coupled to the present ESR. Both facilities provide unprecedented physics opportunities already at the very early stage of FAIR operation. The installation of CRYRING, dedicated Low-energy Storage Ring (LSR) for FLAIR, may even enable a much earlier realisation of the physics program of FLAIR with slow anti-protons.

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• Transmission of highly charged ions through nanocapillaries of noncircular cross sections

  2014. H. Q. Zhang (et al.). Journal of Physics, Conference Series 488, 012035

  Artikel

  We report on effects from the geometrical shape of the guiding channels on the ion transmission profile. We find that capillaries of rhombic cross section produce rectangular shaped ion transmission profiles and, vice versa, capillaries of rectangular geometry give a rhombic beam shape. Our trajectory simulations for the incidence of 14-keV Ne7+ ions give clear evidence that the observed effect is due to the image forces experienced by the transmitting ions. They gain transverse energy due the image charge attraction towards the inner surfaces of the capillary. This leads to a defocusing of the ions leaving the capillaries. Due to the blocking of large deflection angles at the exit of the capillary, the transmitted ion beam is tailored into certain geometrical patterns.

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• Transmission of slow highly charged ions through rectangular nanocapillaries

  2014. Nadeem Akram (et al.). Journal of Physics, Conference Series 488, 132043

  Artikel

  Transmission of slow highly charged ions through rectangular nanocapillaries in phlogopite mica is studied. The transmission profiles have rhombic pattern at tilt angles within the geometrical opening angle of the capillaries. The time evolution of ion transmission reveals certain features contributing to the tailored transmission profiles.

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• RECOMBINATION RATE COEFFICIENTS OF BORON-LIKE Ne

  2013. Sultan Mahmood (et al.). Astrophysical Journal 771 (2), 78

  Artikel

  Recombination of Ne5+ was measured in a merged-beam type experiment at the heavy-ion storage ring CRYRING. In the collision energy range 0-110 eV resonances due to 2s(2)2p -> 2s2p(2) (Delta n=0) and 2s(2)2p -> 2s(2)3l (Delta n=1), core excitations were observed. The experimentally derived rate coefficients agree well with the calculations obtained using AUTOSTRUCTURE. At low energies, recombination is dominated by resonances belonging to the spin-forbidden 2s2p(2)(P-4(J))nl series. The energy-dependent rate coefficients were convoluted with a Maxwell-Boltzmann electron energy distribution to obtain plasma recombination rate coefficients. The data from the literature deviate from the measured results at low temperature.

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• SPARC experiments at the high-energy storage ring

  2013. Thomas Stoehlker (et al.). Physica Scripta T156

  Artikel

  The physics program of the SPARC collaboration at the Facility for Antiproton and Ion Research (FAIR) focuses on the study of collision phenomena in strong and even extreme electromagnetic fields and on the fundamental interactions between electrons and heavy nuclei up to bare uranium. Here we give a short overview on the challenging physics opportunities of the high-energy storage ring at FAIR for future experiments with heavy-ion beams at relativistic energies with particular emphasis on the basic beam properties to be expected.

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• Electron-ion recombination rate coefficients for C II forming C I

  2012. Safdar Ali (et al.). Astrophysical Journal 753 (2), 132

  Artikel

  We have determined absolute dielectronic recombination rate coefficients for C II, using the CRYRING heavy-ions storage ring. The resonances due to 2s-2p (∆n = 0) core excitations are detected in the center-of-mass energy range of 0-15 eV. The experimental results are compared with intermediate coupling AUTOSTRUCTURE calculations. Plasma rate coefficients are obtained from the DR spectrum by convoluting it with a Maxwell-Boltzmann energy distribution for temperatures in the range of 10³-10⁶ K. The derived temperature dependent plasma recombination rate coefficients are presented graphically to compare with the theoretical data available in literature and parameterized by using a fit formula for convenient use in plasma modelling codes. In the temperature range of 10³-2×10⁴ K, our experimental results show that previous calculations severely underestimate the plasma rate coefficients and also our AUTOSTRUCTURE calculation does not reproduce the experimental plasma rate coefficients well. Above 2×10⁴ K the agreement between the experimental and theoretical rate coefficients is much better, and the deviations are smaller than the estimated uncertainties.

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• Recombination and electron impact excitation rate coefficients for S XV AND S XVI

  2012. Sultan Mahmood (et al.). Astrophysical Journal 754 (2), 86

  Artikel

  Recombination and electron impact excitation of S¹⁴⁺ and S¹⁵⁺ ions was measured at the Stockholm refrigerated electron beam ion trap. The collision energy range was 1.4-3 keV, where we covered the KLL, KLM, KLN, and KLO dielectronic recombination resonances resulting in S¹³⁺ and S¹⁴⁺ ions. The recombination rates were obtained by detecting the charge state distribution with a newly developed time-of-flight technique. Resonance energies and cross sections calculated within the relativistic many-body perturbation theory for S¹⁵⁺ agree well with the experimental data. The temperature dependent rate coefficients have been extracted from the measured rates and compared with calculations from literature used for studies of collisionally ionized astrophysical plasmas. A good agreement for S15+ was obtained, while the plasma rates for S¹⁴⁺ were 23% lower than the so far published values. In addition to the time-of-flight spectra, the x-ray spectra, produced mainly by photo-recombination and excitation, have been also collected. The combination of these two measurements allowed us to separate the photo-recombination and the excitation spectra, and the excitation rate coefficients for summed intensities with known fractions of S¹⁴⁺ and S¹⁵⁺ ions were extracted.

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• Tailoring of keV-Ion Beams by Image Charge when Transmitting through Rhombic and Rectangular Shaped Nanocapillaries

  2012. Hongqiang Q. Zhang (et al.). Physical Review Letters 108 (19), 193202

  Artikel

  We report on an unexpected effect of tailoring transmission profiles of Ne7+ ions through nanocapillaries of rhombic and rectangular cross sections in mica. We find that capillaries of rhombic cross sections produce rectangular shaped ion transmission profiles and, vice versa, that capillaries of rectangular geometry give a rhombic beam shape. This shaping effect only occurs for transmitted ions and is absent for the small fraction of neutralized particles. The experimental findings and simulations of the projectile trajectories give clear evidence that the observed effect is due to the image forces experienced by the transmitting ions. This novel beam shaping mechanism suggests applications for the guiding, focusing, and shaping of ion beams.

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