A laboratory plasma experiment for X-ray astronomy using a compact electron beam ion trap (EBIT)

TL;DR

This work introduces the JAXA-EBIT, a compact electron beam ion trap, and demonstrates its capability to produce highly charged ions for precise atomic data measurements in X-ray astronomy. It reports dielectronic recombination benchmarks showing $E/\Delta E \approx 1000$ at $2.3$ keV, and resonant photoexcitation spectroscopy at the SPring-8 BL17SU beamline, successfully measuring the $O^{6+}$ He-like $2p$–$1s$ and the $Fe^{16+}$ $3C$ transitions with sub-meV level calibration precision, while setting limits on weaker lines. The results validate the JAXA-EBIT as a robust platform for atomic data benchmarks and outline a path to comprehensive data across broader energy ranges by integrating multiple beamlines (e.g., NanoTerasu BL13U and hard X-ray facilities) with XRISM and future missions. The work aims to substantially reduce plasma-modeling uncertainties in high-resolution X-ray spectra by providing experimentally anchored transition energies, oscillator strengths, and decay probabilities for highly charged ions.

Abstract

We present the basic performance and experimental results of an electron beam ion trap (JAXA-EBIT), newly introduced to the Japanese astronomical community. Accurate atomic data are indispensable for the reliable interpretation of high-resolution X-ray spectra of astrophysical plasmas. The JAXA-EBIT generates highly charged ions under well-controlled laboratory conditions, providing experimental benchmarks for atomic data. The JAXA-EBIT shows performance comparable to the Heidelberg compact EBIT through dielectronic recombination measurements of highly charged Ar ions. Furthermore, we conducted resonant photoexcitation spectroscopy of highly charged ions using the soft X-ray beamline BL17SU at the synchrotron radiation facility SPring-8. As a result, we successfully detected resonance transitions of He-like O$^{6+}$ and Ne-like Fe$^{16+}$. These results demonstrate the capability of the JAXA-EBIT for precise measurement of atomic data and show that it serves as a powerful tool for advancing astrophysical research.

Figures (4)

• Figure 1: Photograph (upper panel) and schematic diagram (bottom panel) of the principal components of the JAXA-EBIT.
• Figure 2: Schematic diagram of the $KLL$ dielectronic recombination of a He-like ion. A free electron with kinetic energy $E_{\rm free}$ is captured into the $L$-shell with binding energy $E_{L}$, while a bound $K$-shell electron is simultaneously excited to the $L$-shell by the released energy $E_{\mathrm{free}} + E_{L}$. The DR resonance occurs when the condition $E_{K} - E_{L}= E_{\mathrm{free}} + E_{L}$ is satisfied. The resulting Li-like ion subsequently decays via K$\alpha$ X-ray emission.
• Figure 3: Results of dielectronic recombination measurements of highly charged Ar ions with the JAXA-EBIT. (a) Count maps around the resonance energies of different charge states: (a--1) He-like, (a--2) Li-like, (a--3) Be-like, and (a--4) B- and C-like. The horizontal axis corresponds to the electron beam energy, and the vertical axis to the pulse height of the X-ray detector. (b) Projected spectra obtained by selecting events within the regions enclosed by the black dashed lines in panel (a) and projecting them onto the electron beam energy axis.
• Figure 4: Results of photoexcitation spectroscopy at the soft X-ray beamline BL17SU of SPring-8. (a) The O^6+ He$\alpha$ resonance line, (b) the Fe^16+ 3C transition, and (c) the Fe^16+ 3G transition. The horizontal axis represents the incident photon energy of the synchrotron beam, and the vertical axis shows the count rate recorded by the silicon drift detector. The red solid lines indicate the best-fit models (Gaussian plus linear background). Detailed descriptions of the experimental method and analysis procedures are provided in hirata2025.