Minimizing plasma temperature for antimatter mixing experiments

TL;DR

This paper investigates the unexpectedly high plasma temperatures in antimatter mixing experiments, identifying microwave radiation coupling as a key heating mechanism affecting antihydrogen production.

Contribution

It reveals that the open trap geometry causes anomalous plasma heating through microwave coupling, beyond known noise and expansion effects.

Findings

Plasmas reach higher temperatures than expected from known heating sources.

Open trap geometry facilitates microwave coupling with external environment.

Anomalous heating impacts antihydrogen production efficiency.

Abstract

The ASACUSA collaboration produces a beam of antihydrogen atoms by mixing pure positron and antiproton plasmas in a strong magnetic field with a double cusp geometry. The positrons cool via cyclotron radiation inside the cryogenic trap. Low positron temperature is essential for increasing the fraction of antihydrogen atoms which reach the ground state prior to exiting the trap. Many experimental groups observe that such plasmas reach equilibrium at a temperature well above the temperature of the surrounding electrodes. This problem is typically attributed to electronic noise and plasma expansion, which heat the plasma. The present work reports anomalous heating far beyond what can be attributed to those two sources. The heating seems to be a result of the axially open trap geometry, which couples the plasma to the external (300 K) environment via microwave radiation.