[Paper Review] Storage-ring ionization and recombination experiments with multiply charged ions relevant to astrophysical and fusion plasmas

S. Schippers|arXiv (Cornell University)|Aug 4, 2011

Atomic and Molecular Physics23 references3 citations

TL;DR

This paper presents high-precision storage-ring measurements of electron-impact ionization and dielectronic recombination (DR) cross sections and rate coefficients for multiply charged iron and tungsten ions, using merged-beams techniques at the TSR facility. The key finding is that DR rate coefficients for W²⁰⁺ are up to 4.3 times higher than those in the ADAS database, significantly revising expectations for tungsten behavior in fusion plasmas and improving astrophysical modeling accuracy.

ABSTRACT

Past and ongoing research activities at the Heidelberg heavy-ion storage-ring TSR are reviewed which aim at providing accurate absolute rate coefficients and cross sections of atomic collision processes for applications in astrophysics and magnetically confined fusion. In particular, dielectronic recombination and electron impact ionization of iron ions are discussed as well as dielectronic recombination of tungsten ions.

Motivation & Objective

• To provide accurate experimental rate coefficients and cross sections for electron-impact ionization (EII) and dielectronic recombination (DR) of multiply charged ions relevant to astrophysical and fusion plasmas.
• To benchmark theoretical calculations of atomic collision processes by comparing them with absolute measurements from a heavy-ion storage ring.
• To resolve discrepancies in existing plasma modeling data, particularly for tungsten ions in fusion environments, where current data are largely theoretical and unverified.
• To improve the reliability of plasma diagnostics and modeling by providing experimental data free from metastable contamination, using stored ground-state ion beams.
• To investigate the role of unresolved DR resonances in influencing plasma-level recombination rates, especially at low energies.

Proposed method

• Utilizes the heavy-ion storage ring TSR at the Max-Planck-Institute for Nuclear Physics, where well-defined, stored ground-state ion beams are overlapped with a co-propagating electron beam over a 1.5 m interaction region.
• Measures absolute rate coefficients via normalization of detected product ion count rate (R) to stored ion current (Iᵢ) and electron density (nₑ), using the merged-beams formula: αₘᵦ(Eᵣₑₗ) = R × e × vᵢ / [(1 - βᵢβₑ) × Iᵢ × nₑ × L × η].
• Converts merged-beams rate coefficients to apparent cross sections σ(Eᵣₑₗ) = αₘᵦ(Eᵣₑₗ) / vᵣₑₗ for energy-dependent analysis.
• Derives plasma rate coefficients α(Tₑ) by convolving cross sections with a Maxwellian electron energy distribution, using the integral formula: α(Tₑ) = 1/(kBT)³ᐟ² × √(8/πmₑ) × ∫σ(Eᵣₑₗ) × Eᵣₑₗ × exp(-Eᵣₑₗ/kBTₑ) dEᵣₑₗ.
• Employs single-particle detectors with near-100% efficiency (η ≈ 1) to minimize detection losses and reduce systematic uncertainty.
• Leverages the storage ring’s ability to purify ion beams by storing them long enough to decay metastable states, ensuring measurements reflect only ground-state ions.

Experimental results

Research questions

• RQ1 How do experimentally measured dielectronic recombination (DR) rate coefficients for W²⁰⁺ compare to theoretical predictions and existing data in the ADAS database?
• RQ2 To what extent do unresolved DR resonances in W²⁰⁺ significantly alter the plasma-level recombination rate coefficient at fusion-relevant temperatures?
• RQ3 How do the electron-impact ionization (EII) cross sections for Fe¹¹⁺ and Fe¹²⁺ ions compare with theoretical calculations and previous single-pass experiments?
• RQ4 What is the impact of high-precision, metastable-free measurements on the accuracy of astrophysical plasma models, particularly for iron K-line emission in X-ray astronomy?
• RQ5 Why do DR resonances in W²⁰⁺ exhibit broad, unresolved structures with magnitudes orders of magnitude above non-resonant radiative recombination (RR) predictions?

Key findings

• The experimentally derived DR rate coefficient for W²⁰⁺ at 160 eV plasma temperature is 4.3 times higher than the value in the ADAS data base, indicating a major discrepancy in current fusion plasma modeling.
• At low electron-ion collision energies (≤12 eV), the measured DR rate coefficient for W²⁰⁺ exceeds the hydrogenic non-resonant RR prediction by up to three orders of magnitude, indicating strong resonant enhancement.
• Broad, unresolved resonance structures in the W²⁰⁺ DR cross section are attributed to blends of many individual DR resonances due to the complex 4f¹⁰ configuration, with widths exceeding the 1 meV experimental energy resolution.
• The EII cross sections for Fe¹¹⁺ and Fe¹²⁺ ions show good agreement with theoretical calculations by Pindzola et al. and Dere, particularly in the threshold and excitation-autoionization regions.
• The measured DR rate coefficient for Fe ions is consistent with recent astrophysical models, suggesting that photoionized gas in AGNs may be less highly ionized than previously thought, due to more efficient recombination.
• Systematic uncertainties in the measurements are estimated at 10–15% (one-sigma), primarily from background subtraction, ion current, and electron density determination, with counting statistics being the dominant source.