Safdar Ali

Bio: Safdar Ali is an academic researcher from University of Electro-Communications. The author has contributed to research in topics: Electron beam ion trap & Extreme ultraviolet. The author has an hindex of 4, co-authored 13 publications receiving 45 citations.

Measurements of density dependent intensity ratios of extreme ultraviolet line emission from Fe X, XI, and XII

Abstract: We report high-resolution density dependent intensity ratio measurements for middle charge states of iron in the extreme ultraviolet (EUV) spectral wavelength range of 160−200 A. The measurements were performed at the Tokyo EBIT laboratory by employing a flat-field grazing incidence spectrometer installed on a low energy compact electron beam ion trap. The intensity ratios for several line pairs stemming from Fe X, Fe XI and Fe XII were extracted from spectra collected at the electron beam energies of 340 and 400 eV by varying the beam current between 7.5 and 12 mA at each energy. In addition, the effective electron densities were obtained experimentally by imaging the electron beam profile and ion cloud size with a pinhole camera and visible spectrometer, respectively. In this paper, the experimental results are compared with previous data from the literature and with the present calculations performed using a collisional-radiative model. Our experimental results show a rather good agreement with the calculations and previous reported results.

Resonant Electron Impact Excitation of 3d levels in Fe$^{14+}$ and Fe$^{15+}$

Abstract: We present laboratory spectra of the $3p$--$3d$ transitions in Fe$^{14+}$ and Fe$^{15+}$ excited with a mono-energetic electron beam. In the energy dependent spectra obtained by sweeping the electron energy, resonant excitation is confirmed as an intensity enhancement at specific electron energies. The experimental results are compared with theoretical cross sections calculated based on fully relativistic wave functions and the distorted-wave approximation. Comparisons between the experimental and theoretical results show good agreement for the resonance strength. A significant discrepancy is, however, found for the non-resonant cross section in Fe$^{14+}$. %, which can be considered as a fundamental cause of the line intensity ratio problem that has often been found in both observatory and laboratory measurements. This discrepancy is considered to be the fundamental cause of the previously reported inconsistency of the model with the observed intensity ratio between the $^3\!P_2$ -- $^3\!D_3$ and $^1\!P_1$ -- $^1\!D_2$ transitions.

Resonant Electron Impact Excitation of 3d Levels in Fe14+ and Fe15+

Abstract: We present laboratory spectra of the 3p–3d transitions in Fe14+ and Fe15+ excited with a mono-energetic electron beam. In the energy-dependent spectra obtained by sweeping the electron energy, resonant excitation is confirmed as an intensity enhancement at specific electron energies. The experimental results are compared with theoretical cross sections calculated based on fully relativistic wave functions and the distorted wave approximation. Comparisons between the experimental and theoretical results show good agreement for the resonance strength. A significant discrepancy is, however, found for the non-resonant cross section in Fe14+. This discrepancy is considered to be the fundamental cause of the previously reported inconsistency of the model with the observed intensity ratio between the and transitions.

Resonant excitation of Fe 14+ observed with a compact electron beam ion trap

Abstract: We present extreme ultraviolet spectra of highly charged Fe ions observed with a compact electron beam ion trap as a function of monoenergetic electron beam energy. For 3 s 3 p – 3 s 3 d lines in Fe XV, strong intensity enhancement at a specific electron energy is confirmed. The enhancement is assigned as the resonant excitation via dielectronic capture followed by autoionization. The resonance contribution to line intensity ratio, which is important for the diagnostics of astrophysical plasmas, is discussed.

High resolution EUV spectroscopy of xenon ions with a compact electron beam ion trap

Abstract: We performed high resolution extreme ultraviolet (EUV) spectroscopy measurements of highly charged xenon ions with a compact electron beam ion trap. The spectra were recorded with a flat-field grazing incidence spectrometer while varying the electron beam energy between 200 and 890 eV. We measured the wavelengths for several lines of Rh-like Xe9+ - Cd-like Xe6+ and Cu-like Xe25+- Se-like Xe20+ in the range of 150–200 A with an uncertainty of 0.05 A. Previously, most of these lines have been reported from EBITs with a wavelength uncertainty of 0.2 A. Additionally, based on the electron beam energy dependence of the observed spectra we tentatively identified three new lines, which were reported as unidentified lines in the previous studies.

Coronal Plasma Motions near Footpoints of Active Region Loops Revealed from Spectroscopic Observations with {it Hinode} EIS

Abstract: The solar active region 10938 has been observed from the disk center to the west limb with the Hinode EUV Imaging Spectrometer. In the disk-center observation, subsonic upflow motions of tens of km s -->−1 and enhanced nonthermal velocities have been found near the footpoints of the active region loops assuming a single Gaussian approximation for the emission-line profiles. When the same part of the active region is observed near the limb, both upflows and enhanced nonthermal velocities essentially decrease. There is a strong correlation between Doppler velocity and nonthermal velocity. Significant deviations from a single Gaussian profile are found in the blue wing of the line profiles for the upflows. These suggest that there are unresolved high-speed upflows. We discuss the implications for coronal heating mechanisms.

Experimental and theoretical evaluation of density sensitive N VII, Ar XIV and Fe XXII line ratios

Abstract: The line ratios of the 2p-3d transitions in the B-like spectra Ar XIV and Fe XXII have been measured using the electron beam ion traps at Livermore. Radiative-collisional model calculations show these line ratios to be sensitive to the electron density in the ranges ne = 10{sup 10} to 10{sup 12} cm{sup -3} and ne = 10{sup 13} to 10{sup 15} cm{sup -3}, respectively. In our experiment, the electron beam density of about 10{sup 11} cm{sup -3} was varied by about a factor of 5. Our data show a density effect for the line doublet in Ar XIV, and good agreement with theory is found. The relative intensity of the Fe XXII doublet shows good agreement with our predicted low density limit. The N VI K-shell spectrum was used to infer the actual electron density in the overlap region of ion cloud and electron beam, and systematic measurements and calculations of this spectrum are presented as well. The Ar XIV and Fe XXII spectra promise to be reliable density diagnostics for stellar coronae, complementing the K-shell diagnostics of helium-like ions.

Highly Charged Ions in Magnetic Fusion Plasmas: Research Opportunities and Diagnostic Necessities

Abstract: Highly charged ions play a crucial role in magnetic fusion plasmas. These plasmas are excellent sources for producing highly charged ions and copious amounts of radiation for studying their atomic properties. These studies include calibration of density diagnostics, x-ray production by charge exchange, line identifications and accurate wavelength measurements, and benchmark data for ionization balance calculations. Studies of magnetic fusion plasmas also consume a large amount of atomic data, especially in order to develop new spectral diagnostics. Examples we give are the need for highly accurate wavelengths as references for measurements of bulk plasma motion, the need for accurate line excitation rates that encompass both electron-impact excitation and indirect line formation processes, for accurate position and resonance strength information of dielectronic recombination satellite lines that may broaden or shift diagnostic lines or that may provide electron temperature information, and the need for accurate ionization balance calculations. We show that the highly charged ions of several elements are of special current interest to magnetic fusion, notably highly charged ions of argon, iron, krypton, xenon, and foremost of tungsten. The electron temperatures thought to be achievable in the near future may produce W70+ ions and possibly ions with even higher charge states. This means that all but a few of the most highly charged ions are of potential interest as plasma diagnostics or are available for basic research.

Laboratory Astrophysics Survey Of Key X-Ray Diagnostic Lines Using A Microcalorimeter On An Electron Beam Ion Trap

Abstract: Cosmic plasma conditions created in an electron beam ion trap (EBIT) make it possible to simulate the dependencies of key diagnostic X-ray lines on density, temperature, and excitation conditions that exist in astrophysical sources. We used a microcalorimeter for such laboratory astrophysics studies because it has a resolving power ≈1000, quantum efficiency approaching 100%, and a bandwidth that spans the X-ray energies from 0.2 keV to 10 keV. Our microcalorimeter, coupled with an X-ray optic to increase the effective solid angle, provides a significant new capability for laboratory astrophysics measurements. Broadband spectra obtained from the National Institute of Standards and Technology EBIT with an energy resolution approaching that of a Bragg crystal spectrometer are presented for nitrogen, oxygen, neon, argon, and krypton in various stages of ionization. We have compared the measured line intensities to theoretical predictions for an EBIT plasma.

Roadmap on photonic, electronic and atomic collision physics: II. Electron and antimatter interactions

Abstract: We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. In Roadmap II we focus on electron and antimatter interactions. Modern theoretical and experimental approaches provide detailed insight into the many body quantum dynamics of leptonic collisions with targets of varying complexity ranging from neutral and charged atoms to large biomolecules and clusters. These developments have been driven by technological progress and by the needs of adjacent areas of science such as astrophysics, plasma physics and radiation biophysics. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting contributions from eighteen leading groups from the field.