Duck-Hee Kwon

Laboratory spectroscopy of silicon plasmas photoionized by mimic astrophysical compact objects

Photoionized plasmas are encountered in astrophysics wherever low-temperature gas/plasma is bathed in a strong radiation field. X-ray line emissions in the several kiloelectronvolts spectral range were observed from accreting clouds of binary systems, such as CYGNUS X-3 and VELA X-1, in which high-intensity x-ray continua from compact objects (neutron stars, black holes or white dwarfs) irradiate the cold and rarefied clouds. X-ray continuum- induced line emission accurately describes the accreting clouds, but experimental verification of this photoionized plasma model is scarce. Here we report the generation of photoionized plasmas in the laboratory under well-characterized conditions using a high-power laser. A blackbody radiator at a temperature of 500 eV, corresponding to a compact object, was created by means of a laser-driven implosion. The emerging x-rays irradiate a low-density (n(e) < 10(20) cm(-3)) and low- temperature (T(e) < 30 eV) silicon plasma. Line emissions from lithium- and helium-like silicon ions were observed from a thermally cold silicon plasma in the 1.8-1.9 keV spectral region, far from equilibrium conditions. This result reveals the laboratory generation of a photoionizing plasma. Atomic kinetic calculations imply the importance of direct K-shell photoionization by incoming hard x-rays.

High-resolution spectroscopy of Sm I performed with an extended-cavity violet diode laser

We performed high-resolution spectroscopy of Sm I in the near-UV transitions by using an extended-cavity violet diode laser. By adopting traditional Doppler-free saturated-absorption spectroscopy, we made accurate measurements of the isotope shifts for Sm I. For an atomic excitation laser we used an extended-cavity violet diode laser that was operated in the wavelength range 398–400 nm. As for the experimental results, Doppler-free spectra of the 399.002- and 399.102-nm transition lines of Sm were obtained and the isotope shifts between the even-mass isotopes were measured for the first time to our knowledge. Additionally, we used the King-plot method to check the consistency of the measured isotope shifts and to analyze them as mass shifts and field shifts. For a more quantitative analysis, we performed an ab initio calculation by using the relativistic multiconfiguration Dirac–Fock method.

Stable Isotope Production of 168Yb and 176Yb for Industrial and Medical Applications

We have developed a laser isotope separation technology for the production of the 168Yb and 176Yb isotopes. 168Yb is very useful for the generation of a non destructive testing source, 169Yb. 176Yb can be used to produce 177Lu which is known to be a promising radioisotope for a medical application. For these applications, the abundances of 168Yb and 176Yb isotopes should be enriched to more than 15% and 97%, respectively. Our developed system consists of three dye lasers pumped by a diode-pumped solid-state laser, a Yb evaporator, and a photo-ion extractor. Up to now, we could enrich 168Yb to more than 31% with a productivity of 0.5 mg/h. Also, we succeeded in enriching 176Yb to more than 97% with a productivity of 27 mg/h.

Optical diagnostics with radiation trapping effect in low density and low temperature helium plasma

Low density (ne < 1011 cm−3) and low temperature (Te < 10 eV) helium plasma was generated by hot filament discharge. Electron temperature and density of neutral helium plasma were measured by Langmuir probe and were determined by line intensity ratio method using optical emission spectroscopy with population modelings. Simple corona model and collisional-radiative (CR) model without consideration for radiation trapping effect are applied. In addition, CR model taking into account the radiation trapping effect (RTE) is adopted. The change of single line intensity ratio as a function of electron temperature and density were investigated when the RTE is included and excluded. The changes of multi line intensity ratios as a function of electron temperature were scanned for various radiative-excitation rate coefficients from the ground state and the helium gas pressures related with the RTE. Our CR modeling with RTE results in fairly better agreement of the spectroscopic diagnostics for the plasma temperature ...

Theoretical photoionization spectra for Mg-isoelectronic Cl5+ and Ar6+ ions

Autoionizing resonant structures of Mg-isoelectronic Cl5+ and Ar6+ ions for the removal of a 3s or 3p electron from initial and states have been studied using the eigenchannel R-matrix approach connected with multichannel quantum-defect theory (MQDT) at the R-matrix boundary. The resonant states are verified and analyzed using eigenphase gradients to get resonance locations Er, effective quantum numbers n*, and widths . It is noted that there are no former theoretical and experimental studies for photoionization cross sections of Mg-isoelectronic Cl5+ to compare with ours. For the Mg-isoelectronic Ar6+ ion, our calculated ionization thresholds are in good agreement with NIST data and Opacity Project (OP) results. The current photoionization cross sections for the Mg-isoelectronic Ar6+ ion are in general agreement with the OP results, except for resonance energy shifts and structures of a few resonances.

Proton radiography of a laser-driven cylindrical implosion

A recent experiment was performed at the Rutherford Appleton Laboratory (UK) to study fast electron propagation in cylindrically compressed targets, a subject of interest for fast ignition. This experiment was performed in the framework of the experimental road map of the Hiper project (the European High Power laser Energy Research facility Project). In this experiment, protons accelerated by a pecosecond laser pulse have been used to radiograph a 220 μm‐diameter, 20 μm‐wall cylinder filled with 0.1 g/cc foam, imploded with ∼200 J of green laser light in 4 symmetrically incident beams of pulse length 1 ns. Point projection proton backlighting was used to measure the compression degree as well as the stagnation time. Results were compared to those from hard X‐ray radiography. Finally, Monte Carlo simulations of proton propagation in the cold and in the compressed targets allowed a detailed comparison with 2D numerical hydro simulations.

Theoretical study for photoionization of Mg-like S4+ ion

Photoionization of the Mg-like S4+ ion has been studied for the ground 1S and excited 3,1P initial states. Using a noniterative eigenchannel R-matrix method, we have investigated the partial photoionization cross sections leading to the final photo-ionic 3l states. The autoionizing resonance structures arising from the ground and excited states are identified and characterized. Our calculational results, which show excellent agreement between length and velocity gauges, are compared with the available previous Opacity Project results, and some significant discrepancies are found.

Effects of configuration interaction for dielectronic recombination of Na-like ions forming Mg-like ions

Theoreticaldielectronicrecombination(DR)ratecoefficientcalculationscanbesensitivetoconfigurationinteraction (CI) between resonances with different captured electron principle quantum numbers n. Here we explore the importance of this multi-n CI process for DR via 2l → 3lcore excitations and its effect on the total DR rate coefficient. Results are presented for selected Na-like ions from Ca 9+ to Zn 19+ . We find that including this multi-n CI can reduce the DR rate coefficient by up to ∼10% at temperatures where an ion is predicted to form in collisional ionization equilibrium and up to ∼15% at higher temperatures. To a first approximation, this will translate into a corresponding increase in the ion abundance. Charge state distributions calculation seeking to be accurate to better than 10% will thus need to take this effect into account. We also present simple fits to the calculated rate coefficients for ease of incorporation into plasma models.

X-ray Spectroscopy of Non-thermal Equilibrium Laboratory Photo-ionized Plasma

Photo-ionized silicon plasma was studied using 500-eV Planckian radiation from implosion core plasma generated with Gekko-XII laser to simulate a astrophysical photo-ionized plasma. Three major features were observed in an energy range from 1.80 to 1.88 keV and origins of the features were replicated with a time-dependent collisional-radiative model including photo-ionization processes: The line at 1.865 keV is the resonance line by inner-shell photo-ionization of Li-like Si ions and that at 1.840 keV is the sum of satellite lines attributed to the inner-shell photo-ionization of Be-like ions. Emission lines in the soft x-ray/extreme ultra-violet (EUV) region (i.e., 140 to 300 eV) from the low-temperature Si plasma in prior to the photo-ionization were also identified to extract the intial temperature and density of the pre-formed Si.

Effect of plasma profile on ion acceleration in the interaction of a short laser pulse with a thin overdense target

Energetic ion generation from the interaction of a short laser pulse with a thin overdense plasma accompanied by a preplasma and a rear side plasma gradient is investigated by particle-in-cell simulations. The dynamics of ion acceleration depending on the maximum density of the preplasma in front of the overdense plasma slab with a smooth density gradient at the rear side are presented and discussed by comparing a sharp rear side boundary case.