Rydberg Electromagnetically Induced Transparency Enables Electron Beam Position Mapping in Rubidium

Understanding the characteristics of electron beams is crucial for a range of scientific applications, from medical imaging to high-energy physics. Rob Behary, Kevin Su, and Nicolas DeStefano, all from William & Mary, alongside Jennifer Tsai, Todd Averett, also of William & Mary, and Alexandre Camsonne from the Thomas Jefferson National Accelerator Facility, have developed a novel technique for profiling these beams using the unique properties of rubidium vapor. Their research details an all-optical method that determines both the position and spatial profile of an electron beam by monitoring shifts in electromagnetically induced transparency. This approach offers a fast and non-invasive diagnostic tool, particularly valuable for real-time monitoring of charged particle beams within accelerator facilities, despite offering a different resolution to existing fluorescence techniques. By cleverly employing crossed laser beams and advanced signal processing, the team successfully reconstructed two-dimensional profiles of a 20 keV electron beam with currents between 25 and 100 microamperes.

The MITRE Corporation, McLean, VA 22102, USA. This work presents an all-optical detection approach designed to determine the position and spatial profile of an electron beam, leveraging the quantum properties of alkali metal atoms. The research focuses on measuring the electric field generated by an electron beam by exciting thermal rubidium atoms to a highly excited Rydberg state through a two-photon ladder transition. Stark shifts of these Rydberg states are then detected by monitoring the frequencies of corresponding electromagnetically induced transparency (EIT) transmission peaks.

Rydberg Atoms Map Electron Beam Profiles

Scientists achieved a breakthrough in electron beam diagnostics by developing an all-detection method to determine beam position and spatial profile using alkali metal atoms. The research team measured the electric field generated by an electron beam by exciting thermal rubidium atoms to highly excited Rydberg states, then detecting Stark shifts of these states through electromagnetically induced transparency (EIT) transmission peaks. To gain spatial information regarding the electron beam’s position and geometry, the work employed crossed laser beams, a configuration that allowed for detailed mapping of the electric field. Crucially, the researchers implemented a pulsed electron beam coupled with phase-sensitive optical detection to effectively isolate the genuine electric signature of the beam from parasitic electric fields generated by photoelectric charges accumulating on optical windows.

This innovative technique significantly improved the signal-to-noise ratio and accuracy of the measurements, further refined through the application of principal component analysis, a statistical method used to enhance signal quality and reduce dimensionality. Experiments were conducted to detect and reconstruct a two-dimensional profile of a 20 keV electron beam, with beam currents ranging from 25 to 100 µA. The experimental setup involved a low-density rubidium vapour surrounding the electron beam path, with the team crossing the beams at a small angle within the rubidium chamber to achieve spatial resolution. This approach, while potentially increasing Doppler broadening, enabled sub-millimetre precision in reconstructing the electron beam profile.

The technique relies on the principle that electric fields lift the degeneracy of Rydberg state sublevels, causing a shift in the EIT transmission spectra proportional to the field strength. By analysing these Stark-shifted EIT spectra, the study accurately mapped the electric field experienced by the rubidium atoms, providing a non-invasive diagnostic tool for charged particle beams at accelerator facilities. This method offers a valuable alternative to fluorescence-based measurements, particularly in scenarios with limited optical access. The study meticulously tested this method to detect and reconstruct a two-dimensional profile of a 20 keV electron beam, achieving current measurements ranging from 25 to 100 µA.

Recorded EIT spectra exhibited full width at half maximum (FWHM) values of approximately 100MHz, broadened by the non-zero angle between the intersecting laser beams. Despite the broadening, the team successfully demonstrated reshaping of the EIT spectrum in response to the electron beam, and further amplified this signal through 5kHz pulsed modulation and lock-in detection. Data shows a monotonic relationship between the maximum lock-in signal value and the electron beam current, establishing a calibration curve for precise current measurement. To map the electron beam profile, the team rastered the relative position of the beam and the sensing region, achieving a spatial resolution limited by the 1mm diameter of the electron beam. Heat maps of lock-in signals demonstrated a feature at ±0.5mm range around zero displacement, indicating sensitivity to vertical beam positioning. Further experiments involved translating the blue laser beam to induce horizontal displacement, resulting in a lock-in signal heat map revealing an electric field gradient and a strong feature near zero displacement, confirming sensitivity to motion in the x-direction.

Rydberg Atoms Map Electron Beam Profiles

This work demonstrates a novel technique for characterising electron beams using the properties of Rydberg atoms. Researchers successfully measured the position and spatial profile of a 20 keV electron beam, with currents between 25 and 100 µA, by exciting rubidium atoms to highly excited Rydberg states and monitoring Stark shifts via electromagnetically induced transparency. The method employs crossed laser beams and phase-sensitive detection to isolate the electron beam’s electric field from background interference, further enhanced by principal component analysis to improve signal quality. The significance of this achievement lies in providing a non-invasive diagnostic tool for charged particle beams, particularly valuable in environments where traditional fluorescence-based measurements are impractical.

While acknowledging a spatial resolution currently limited by laser beam overlap and electron beam size, the authors highlight the potential for extending the technique to three-dimensional profiling. Furthermore, the observed linear relationship between PCA amplitude and electron beam current suggests a pathway towards developing a non-invasive current monitor. The authors note the method’s adaptability extends beyond electron beams, offering a means to characterise any spatial distribution of electric fields, such as those found near plasma boundaries.