A. K. Bhatia

Scaling of collisionally pumped 3s‐3p lasers in the neon isoelectronic sequence

Population inversions between the 3p and 3s levels of neonlike ions with atomic numbers Z=14, 18, 22, 26, 32, and 36 have been calculated. The population inversions result from the preferential population of the 3p level by electron collisional excitation from the ground configuration of the ion and occur over a wide range of electron temperature and electron density (from 1017 cm−3 for Si V to 1022 cm−3 for Kr XXVII). For all ions that are studied, the maximum value of the population inversion (N3p/g3p−N3s/g3s) for the transition 3p 1S0−3s uP1 (where u represents the upper of the singlet and triplet levels) is found to be approximately equal to 4×10−3 NI, where NI is the total density of neonlike ions. With the exception of Si V, laser gains greater than 1 cm−1 are possible for all the ions that are considered, and the gain increases with atomic number Z. For Si V and lower‐Z ions, electron collisional mixing of the upper and lower laser levels restricts the electron density at which the population inver...

Short wavelength laser calculations for electron pumping in neon‐like krypton (Kr XXVII)

We present calculations of electron impact collision strengths and spontaneous radiative decay rates for neon‐like krypton (Kr XXVII) for the 2s 22p 6, 2s 22p 53s, 2s 22p 53p, and 2s 22p 53d configurations. From these atomic data the level populations as a function of the electron density were calculated at two temperatures (T e =1×107 K and T e =3×107 K). The analysis of level populations shows that a volume of krypton in which a significant number of the ions are in the Kr XXVII degree of ionization can produce a significant gain in transitions between the 2s 22p 53s and 2s 22p 53p configurations. At an electron density of 1×101 9 cm− 3 the plasma length has to be on the order of 1 m; at a density of 1×102 1 cm− 3 the length is reduced to about 0.5 cm; and at an electron density of 1×102 2 cm− 3 the length of the plasma is further reduced to about 1 mm.

Emission-Line Intensity Ratios in Fe XVII Observed with a Microcalorimeter on an Electron Beam Ion Trap

We report new observations of emission line intensity ratios of Fe XVII under controlled experimental conditions, using the National Institute of Standards and Technology electron beam ion trap (EBIT) with a microcalorimeter detector. We compare our observations with collisional-radiative models using atomic data computed in distorted wave and R-matrix approximations, which follow the transfer of the polarization of level populations through radiative cascades. Our results for the intensity ratio of the 2p6 1S0-2p53d 1P1 15.014 A line to the 2p6 1S0-2p53d 3D1 15.265 A line are 2.94 ± 0.18 and 2.50 ± 0.13 at beam energies of 900 and 1250 eV, respectively. These results are not consistent with collisional-radiative models and support conclusions from earlier EBIT work at the Lawrence Livermore National Laboratory that the degree of resonance scattering in the solar 15.014 A line has been overestimated in previous analyses. Further observations assess the intensity ratio of the three lines between the 2p6-2p53s configurations to the three lines between the 2p6-2p53d configurations. Both R-matrix and distorted wave approximations agree with each other and our experimental results much better than most solar and stellar observations, suggesting that other processes not present in our experiment must play a role in forming the Fe XVII spectrum in solar and astrophysical plasmas.

Fe XVII Soft X-Ray Lines: Theory and Data Comparisons

Fe XVII soft X-ray spectral emission lines are examined using data from the Flat Crystal Spectrometer (FCS) on the Solar Maximum Mission satellite. Results are compared with theoretical calculations and with other recent observational results. Disparate findings from different studies on the inferred opacity of the bright resonance line at 15.01 A and on its center-to-limb behavior are reviewed. Present limitations on the use of resonance scattering to infer coronal plasma densities and absolute elemental abundances are discussed. An analysis is made of the temperature-insensitive ratio of the 15.01 A line of Fe XVII to the optically thin 16.78 A line. This analysis shows that approximately half of the photons expected in the 15.01 A line are missing from the bright emission cores of quiescent active regions on the solar disk; the missing fraction increases at most by 50% near the solar limb. If the missing flux has been resonantly scattered out of the line of sight, then the equivalent optical depth at line center of the 15.01 A line is τ0~2 on the disk, based on a simple escape probability treatment for a slab geometry. This suggests that the effects of resonance scattering for other FCS lines, with the possible exception of the O VIII doublet at 18.97 A, should be negligible for quiescent active region conditions. This is consistent with the lack of systematic center-to-limb dependence found previously for FCS lines other than Fe XVII at 15.01 A. Yohkoh Soft X-ray Telescope measurements of the expected lines of sight for active regions as a function of location on the solar disk, and resonance scattering results from other soft X-ray active region data sets all support a trend of increased opacity at the limb compared to disk center.

The Optically Thick Fe XVII Spectrum: X-Ray, Extreme-Ultraviolet, and Forbidden Line Ratios

Level populations, X-ray line fluxes, and extreme-ultraviolet (EUV) line intensities are calculated for a 37 level model of the Fe XVII ion, as functions of the column density of Fe XVII, in slab geometry and cylindrical geometry. The resonance lines at 15.02 and 15.25 A are found to exhibit initial increases in fluxes relative to the forbidden line at 17.10 A and the usual reference resonance line at 16.78 A as optical thickness increases. Eleven EUV lines and the forbidden 2p53s(3P0-3P1) transition near 1154 A are also found to be relatively enhanced by opacity.

Spectroscopy and atomic physics of highly ionized Cr, Fe, and Ni for tokamak plasmas

We consider the spectroscopy and atomic physics for some highly ionized Cr, Fe, and Ni ions produced in tokamak plasmas. Specifically, we consider both allowed and forbidden transitions between levels of the 2s22pk, 2s2pk+1, and 2pk+2 configurations. Forbidden and intersystem wavelengths for Cr and Ni ions are extrapolated and interpolated using the known wavelengths for Fe lines identified in solar‐flare plasmas. We present tables of transition probabilities for the B I, C I, N I, O I, and F I isoelectronic sequences, and give collision strengths and transition probabilities for Cr, Fe, and Ni ions of the Be I sequence. Proton excitation rate coefficients for fine‐structure transitions in the Be I through F I sequences are tabulated, as well as populations of all levels of the 2s22pk, 2s2pk+1, and 2pk+2 configurations for Cr, Fe, and Ni for the Be I sequences, and for Cr and Ni for the B I‐F I sequences. We discuss similarities of tokamak and solar spectra, and show how the atomic data we have presented ...

Density sensitive x‐ray line ratios in the Be i, B i, and Ne i isoelectronic sequences

The intensities of x‐ray transitions in highly charged ions in the Be i, B i, and Ne i isoelectronic sequences have been calculated as functions of electron density. The intensities of the transitions from the 2sn 2pm 3p configuration, relative to the intensities from the 2sn 2pm 3s and 2sn pm 3d configurations, are strong functions of electron density in high‐density plasmas. The density sensitivity occurs at electron densities between 1016 cm−3 (for Si ions) and 1022 cm−3 (for Kr ions). Opacity is unimportant for plasma dimensions that are characteristic of dense laser‐produced plasmas. These x‐ray line ratios represent a promising new density diagnostic for high‐density plasmas.

Atomic data and level populations of highly ionized Ti for tokamak plasmas

We present calculations of electron impact collision strengths and spontaneous radiative decay rates for titanium ions of the Li I through F I isoelectronic sequences for transitions between levels of the 2s22pk, 2s2pk+1, and 2pk+2 configurations. From these atomic data, excitation‐rate coefficients are calculated as well as level populations for the above‐mentioned three configurations. The calculations of level populations include the effects of proton excitation, and are carried out at electron temperatures and densities typical of tokamak plasmas. Wavelengths of forbidden and intersystem lines are given, and a synthetic spectrum is presented for a typical temperature and density.

Short wavelength laser calculations for electron pumping in Be I and B I isoelectronic sequences (18≤Z≤36)

Level populations of the 2s3p and 2s23p configurations in ions of the Be I and B I isoelectronic sequences can be inverted, with respect to populations of the 2s3s and 2s23s configurations, by electron collisional pumping. In the case of B I ions, the process is analogous to the process for Ne‐like ions, which has lately received considerable attention. In the case of Be I ions, the inversion is a consequence of the slow radiative decay of the 2s3p 3P2 and 2s3p 3P0 levels. Level populations are calculated for Be‐ and B‐like ions with atomic number Z between 18 and 36. For each sequence, 20 levels are involved with principal quantum number n equal either to two or three. Using the level populations, gain coefficients are calculated for transitions of the type 2s3s−2s3p and 2s23s−2s23p. Finally, the opacity of the 2s2p−2s3s and 2s22p−2s23s transitions are discussed, for plasma parameters for which a reasonable gain can be achieved.

Resonance Scattering of Fe XVII X-Ray and Extreme-Ultraviolet Lines

Over the years, a number of calculations have been carried out to derive intensities of various X-ray and extreme-ultraviolet (EUV) lines in Fe XVII for comparing with observed spectra. The predicted intensities have not agreed with solar observations, particularly for the line at 15.02 A; resonance scattering has been suggested as the source for much of the disagreement. The atomic data calculated earlier by Bhatia & Doschek used seven configurations having n = 3 orbitals, and the scattering calculations were carried out only for incident energies above the threshold of the highest fine-structure level. These calculations have now been extended to thirteen configurations having n = 4 orbitals, and the scattering calculations are carried out below as well as above the threshold of the highest fine-structure level. These improved calculations of Fe XVII change the intensity ratios compared to those obtained earlier, bringing the optically thin F(15.02)/F(16.78) ratio and several other ratios closer to the observed values. However, some disagreement with the solar observations still persists, even though the agreement of the currently calculated optically thin F(15.02)/F(15.26) ratio with the experimental results of Brown and coworkers, and Laming and coworkers has improved. Some of the remaining discrepancy is still thought to be the effect of opacity, which is consistent with expected physical conditions for solar sources. EUV intensity ratios are also calculated and compared with observations. Level populations and intensity ratios are calculated as a function of column density of Fe XVII in the slab and cylindrical geometries. As found previously, the predicted intensities for the resonance lines at 15.02 and 15.26 A exhibit initial increases in flux relative to the forbidden line at 17.10 A and the resonance line at 16.78 A as optical thickness increases. The same behavior is predicted for the lines at 12.262 and 12.122 A. Predicted intensities for some of the allowed EUV lines are also affected by opacity.