Non-neutral plasma manipulation techniques in development of a high-capacity positron trap

Abstract

Preliminary experiments have been performed toward the development of a multi-cell Penning–Malmberg trap for the storage of large numbers of positrons (≥1010 e+). We introduce the master-cell test trap and the diagnostic tools for first experiments with electrons. The usage of a phosphor screen to measure the z-integrated plasma distribution and the number of confined particles is demonstrated, as well as the trap alignment to the magnetic field ( B = 3.1 T) using the m = 1 diocotron mode. The plasma parameters and expansion are described along with the autoresonant excitation of the diocotron mode using rotating dipole fields and frequency chirped sinusoidal drive signals. We analyze the reproducibility of the excitation and use these findings to settle on the path for the next generation multi-cell test device.

Multi-cell trap developments towards the accumulation and confinement of large quantities of positrons

“…The experimental cycles are controlled with a pulsed-pattern generator, based on a field-programmable gate array (FPGA) (Ziegler et al 2012;Singer et al 2021). They comprise fill, hold and manipulate and dump periods.…”

“…When measuring the two-dimensional (2-D) plasma distribution, the screen is biased to +5 kV. A super-Gaussian fit is applied to the measured signal to determine the central density n 0 , plasma radius r P and its position (x, y) (Singer et al 2021). The latter two need the factor κ for the conversion from pixel to mm.…”

“…The latter two need the factor κ for the conversion from pixel to mm. It is obtained by autoresonantly exciting the m = 1 diocotron mode in the master cell, using a rotating, sinusoidal, frequency-chirped dipole field (Singer et al 2021). Due to the changing position of the screen during the alignment (topic of the next section), the calibration is usually performed afterwards.…”

“…To align the trap to the magnetic field, as it is necessary to achieve long time confinement (Witteman et al 2023), a technique is adapted (Aoki et al 2004;Singer et al 2021) that uses the residual m = 1 diocotron mode, which is a remnant from the filling procedure. Repeated measurements of the mode's amplitude at different z-positions provide a fast and direct measure of the misalignment and its direction.…”

“…Section 2 introduces the second prototype MCT and its diagnostics, and § 3 describes calibration and commissioning. Adaption of the master-cell alignment routine (Singer et al 2021) to the new geometry is addressed in § 4. Section 5 describes electron plasma creation with a LaB 6 emitter, followed by a discussion of confinement limits in § 6.…”