Reproducible Control of Laser-Cooled Pure-Ion Plasma Parameters Using the SDREVC Technique

The strong drive regime evaporative cooling technique (SDREVC) was developed at the ALPHA experiment to control the parameters of single-species lepton plasmas consisting of either positrons or electrons [1]. The development of this technique was a critical milestone, since the primary goal of ALPHA is to make precision measurements of antihydrogen, the creation of which relies on reproducible control of the properties of the plasmas that are used to form antihydrogen.

The SDREVC technique is a combination of two methods that are used extensively in Penning trap experiments. The first method is the strong drive regime rotating wall, which employs a rotating electric field to control the radial size of the plasma column, and the second method is evaporative cooling, which sets the final number of particles in the plasma. Underpinning the SDREVC technique is the requirement of a strong cooling mechanism that is necessary to counteract significant heating from the rotating wall. For leptons, the cooling is passively provided by cyclotron radiation due to the large axial magnetic field typical of Penning traps.

In this work, we present the extension of the SDREVC technique to a pure-ion plasma consisting of laser-cooled beryllium ions. Application of the SDREVC technique to ions is more challenging due to the added complexity of using laser-cooling to keep the plasma temperature low enough for effective coupling to the rotating wall. Utilizing the SDREVC technique, we can control the radial size and total number of particles in the beryllium ion plasma to a variation of less 10% for a wide range of initial conditions. The extension of the SDREVC technique to beryllium ion plasmas was a critical stepping stone in the development of beryllium-assisted antihydrogen production, which led to an eight-fold increase in the amount of antihydrogen produced at ALPHA. The application of the SDREVC technique to laser-cooled ion plasmas has wider applicability to Penning trap experiments where precise and reproducible control of plasma parameters is required.

[1] M. Ahmadi et al., Phys. Rev. Lett. 120, 025001 (2018)