High Voltage Determination and Stabilization for Collinear Laser Spectroscopy Applications

TL;DR

The paper tackles the dominant systematic in fast-beam collinear laser spectroscopy—the accuracy of the electrostatic acceleration potential—by deploying custom high-voltage dividers and a DAC-based feedback loop to achieve ppm-level voltage stabilization. It compares two facilities (MSU and TU Darmstadt) and demonstrates how field-penetration effects in the laser-ion interaction region influence rest-frame frequencies and isotope shifts, providing methods to quantify and correct these effects. The work shows that HV stabilization and careful field-penetration mapping enable precise determinations of ν0, hyperfine splittings, and isotope shifts, with practical uncertainties at the 100 ppm–40 ppm level, thereby enhancing the reliability of nuclear ground-state measurements in exotic systems. Overall, the approach improves measurement precision and offers a framework for addressing HV drifts and field penetration in collinear laser spectroscopy setups.

Abstract

Fast beam collinear laser spectroscopy is the established method to investigate nuclear ground state properties such as the spin, the electromagnetic moments, and the charge radius of exotic nuclei. These are extracted with high precision from atomic observables, i.e., the hyperfine splitting and its the isotope shift, which becomes possible due to a large reduction of the Doppler broadening by compressing the velocity width of the ion beam through electrostatic acceleration. With the advancement of the experimental methods and applied devices, e.g., to measure and stabilize the laser frequency, the acceleration potential became the dominant systematic uncertainty contribution. To overcome this, we present a custom-built high-voltage divider, which was developed and tested at the German metrology institute (PTB), and a feedback loop that enabled collinear laser spectroscopy to be performed at a 100-kHz level. Furthermore, we describe the impact of field penetration into the laser-ion-interaction region. This strongly affects the determined isotope shifts and hyperfine splittings, if Doppler tuning is applied, i.e., the ion beam energy is altered instead of scanning the laser frequency. Using different laser frequencies that were referenced to a frequency comb, the field penetration was extracted laser spectroscopically. This allowed us to define an effective scanning potential to still apply the faster and easier Doppler tuning without introducing systematic deviations.

Figures (8)

• Figure 1: Voltage stabilization schemes at (a) MSU and (b,c) TUDa in combination with different ion sources. In all approaches, the applied high voltage is measured with a custom-made voltage divider and a digital voltmeter. A control loop is used to correct the high voltage with a 0--5 V DAC unit. At MSU (a) the high-voltage power supply can be floated on this correction voltage, which is not possible at TUDa. Here (b,c), the DAC unit is placed on a high-voltage platform. Electrical connections to the platform are shown in red and communication with the DAC is realized via optical fiber (yellow). Using the surface ionization source (b) also requires a power supply that generates the heating current on the high-voltage platform, which is supplied with line voltage using an isolation transformer. The operation of the EBIS (c) does not require additional devices and the DAC can be supplied from a 5-V battery, which removes the voltage noise induced by the isolation transformer. To avoid malfunctions caused by high-voltage sparks, the DAC is protected with a varistor and a transient-voltage suppressor diode (d) in all cases.
• Figure 2: Cross section of the high voltage divider, illustrating the air flow in the temperature stabilized housing.
• Figure 3: Scale factor of TUDa divider for different high voltages measured with the MT100 reference from PTB.
• Figure 4: Relative deviation of the TUDa divider scale factor over time measured against the MT100 reference from PTB.
• Figure 5: 30-kV high-voltage measurement at BECOLA using a commercial high-voltage divider (black) and the present custom build divider (red) read out by 6.5 digits digital voltmeters. While the present divider showed a realistic slow drift of the high voltage, the commercial divider caused significant fluctuations of the voltage reading. Using the present divider and a feed-back loop, the slow, most-likely thermal drift of the power supply was compensated.