Measurement of electron impact excitation cross sections for heliumlike titanium

Abstract

We have obtained the first measurement of electron impact excitation cross sections as a function of energy for a highly charged ion. Collisional excitation cross sections to four w-2 levels of He-like titanium were measured from threshold to 1.7 times threshold. The data conform well to established theory, especially for direct excitation; however, significant differences are found with predicted contributions from resonance excitation and in the treatment of radiative cascades. The latter changes plasma temperatures inferred from titanium Ka line intensities by as much as a factor of 2.PACS numbers: 34.80.Dp, 34.80.Kw The interaction of electrons with highly charged, high-Z ions plays a primary role in understanding the physics of hot plasmas. Electron impact excitation (IE) cross sections and excitation rates of highly stripped ions are fundamental in the understanding of x-ray emission from inertial confinement fusion plasmas [1], the study of solar phenomena [2], and the diagnosis of tokamak plasmas [3]. Therefore, not only is a comparison between theory and experiment of IE cross sections of highly charged, high-Z ions crucial to the development of the physics of electron-ion interactions, but also of great practical importance. Measurements of IE cross sections as a function of energy have been reported recently for low ion charge states [4]; these investigations considered excitation to the first excited level only. For highly charged ions, there is one measurement of IE cross sections for neonlike barium, but only at two electron energy points [5]. We present the first measurements of IE cross sections of a highly charged, high-Z ion over a wide range of electron energy, allowing us to assess both electron-ion interaction cross sections as well as excitation rate calculations. This was done for ground-state excitation to the n=2 levels in heliumlike titanium Ti 20+ , which result in characteristic Ka x-ray emission lines used for diagnostics of Maxwellian and non-Maxwellian magnetic fusion plasmas [6,7].The measurements were carried out on the Livermore electron-beam-ion trap (EBIT) [5,8]. In the apparatus, low charge state ions were injected into and trapped inside the space charge of an intense (100-150 mA, 65 //m diam) electron beam. The ions were successively ionized to heliumlike and excited by the beam, which has an energy resolution of 50 eV FWHM. X-ray spectra centered on the n = 2 to n = \ transitions were taken simultaneously at 90° to the beam by a low-resolution germanium detector and a high-resolution von Hamos geometry Bragg crystal spectrometer capable of resolving the intercombination lines , 5'o-3 / > 2 at 4734 eV and , 5o-3 / > i at 4727 eV, as well as the forbidden line l S 0 -3 S\ at 4702 eV, and the resonance line , S 0 -, / > i at 4750 eV [9]. Gabriel has designated these lines respectively as x, y, z, and w[10].

Electron impact excitation for He-like ions withZ= 20–42

“…Chantrenne et al (1992) obtained the measurement of electron impact excitation cross sections as a function of energy for w, x, y, and z lines of Ti 20+ . The experimental measurements have been extensively compared with theoretical values (Chantrenne et al 1992;Gorczyca et al 1995;Zhang & Pradhan 1995).…”

High–resolution measurements of the K-shell spectral lines of hydrogenlike and heliumlike xenon

“…In particular, the acquisition of well resolved spectral features is necessary for the experimental determination of electron-ion collision cross sections, e.g., the electron-impact excitation cross sections, which play an important role for an accurate interpretation and, hence, for reliable predictions of the atomic processes that take place in the x-ray emitting sources. In contrast to the energy-level studies, there is only a small pool of experimental crosssection values available for highly charged ions, especially, for the electron-impact excitation [8,9], which is certainly a drawback for the development of accurate electron-ion collision models. The qualitative and quantitative analysis of the spectral information obtained from the emission of high-Z ions in very high-temperature plasmas depends on these models.…”

Measurement of Emission Cross Sections for n = 3 → 2 Lines in Li-like Fe 23+

“…Normalization to RR has been used for the measurements of effective excitation cross sections (Chantrenne et al 1992;Wong, Beiersdorfer, & Vogel 1995) for K-shell ions, but the energy resolution (≥160 eV) of the broadband X-ray detectors used in these studies was not sufficient to determine the emission cross section of L-shell ions. This situation has changed with the advent of microcalorimeter detectors that have energy resolution better than 25 eV (Le Gros et al 1995;Kelley et al 1999;Stahle et al 1999).…”