Precision spectroscopy of highly charged ions (HCIs) plays a crucial role in plasma diagnostics, tests of quantum electrodynamics (QED), exploration of nuclear structure, and the development of optical atomic clocks. Electron beam ion traps (EBITs) are capable of producing and confining HCIs of any element under well-controlled laboratory conditions. By bombarding neutral atoms or low-charge-state ions with a high-energy electron beam, these devices utilize electromagnetic fields to keep the ions nearly at rest within the trap region, thereby suppressing Doppler broadening and serving as ideal platforms for precision spectroscopy. Here, we report on the design, commissioning, and first spectroscopic application of a newly developed high-temperature superconducting EBIT, designated as the Shanghai-Lanzhou Electron Beam Ion Trap (SL-EBIT).
The apparatus fundamentally comprises an electron gun, a three-section drift tube assembly, ion extraction electrodes, high-temperature superconducting coils, and a liquid nitrogen cryogenic system. A pair of high-temperature superconducting coils generates an axial magnetic field of up to 0.24 T, providing strong radial compression of the electron beam. The electron beam energy is continuously tunable from 80 to 8000 eV, with a beam radius better than 65 µm and a maximum beam current of 10 mA. The trap region is maintained at an ultra-high vacuum of ∼ 10^-10 Torr via differential pumping and liquid nitrogen cooling, which effectively suppresses charge exchange between the produced HCIs and residual background gases. For wavelength calibration, the device is equipped with an innovative conjugate optical calibration system that images a standard emission lamp directly onto the trap center; this architecture inherently eliminates systematic geometric errors caused by optical path misalignment, enabling high-precision wavelength determination of emission lines in the visible range. In addition, dedicated spectroscopic access ports for both X-ray and visible wavelengths allow for precision spectroscopy of HCIs over a broad spectral range, laying a solid foundation for future experimental extensions toward the low-energy X-ray region.
To characterize the performance of the device, we produced and confined fluorine-like Ar⁹⁺ ions via successive electron-impact ionization of injected argon gas, and performed high-resolution wavelength measurements of the ground-state magnetic-dipole (M1) transition (1s²2s²2p^{5 2}P_3/2 → ²P_1/2). The ion emission spectra were recorded using a 0.5 m spectrometer equipped with a 1200 lines/mm grating and a liquid-nitrogen-cooled CCD detector. The line centroid was extracted by a Gaussian profile fit to the line shape, and the absolute wavelength scale was calibrated against a standard krypton (Kr) lamp. The transition wavelength is determined to be 553.3274(54) nm, which is in excellent agreement with previous high-precision measurements of 553.3260(2) nm from the Heidelberg EBIT, yielding a residual deviation of less than 0.002 nm. The total uncertainty of 5.4 pm is dominated by the dispersion function fitting (3.4 pm) and the calibration system stability (1.8 pm). These commissioning results demonstrate that the SL-EBIT achieves a spectroscopic precision comparable to that of leading international facilities.
The successful operation of the SL-EBIT establishes a novel, compact platform for precision spectroscopy of HCIs in China. Its integrated ion extraction capability opens up distinct possibilities for future trap-external laser spectroscopy, charge-state-selective manipulation, hyperfine structure measurements, and investigations of clock-relevant forbidden transitions. The device effectively bridges the gap between conventional low-energy compact EBITs and large-scale heavy-ion storage ring facilities, offering a versatile tool for both in-trap spectroscopy and extracted-ion-beam experiments.