Mm-wave polarimeter and profilometry design study for retrieving plasma density in the PANDORA experiment

In the recent past, the possibility to use a superconducting trap confining a hot and dense plasma as a tool to investigate radioactivity in astrophysical scenarios has been proposed. Making possible these kind of unprecedented measurements is the main aim of the PANDORA (Plasmas for Astrophysics Nuclear Decays Observation and Radiation for Archaeometry) project. In this context, it is planned to build a compact and flexible magnetic plasma trap where plasma reaches an electron density n (e) & SIM; 10(11)-10(13) cm(-3), and an electron temperature, in units of kT, kT (e) & SIM; 0.1-30 keV. The setup is conceived to be able to measure, for the first time, nuclear beta-decay rates in stellar-like conditions in terms of ionization states. In this paper, the design study of a mm-wave polarimeter for the PANDORA plasma line-integrated electron density measurement is presented. The paper highlights the method of this type of measurements for the first time proposed for a magneto-plasma trap which represents an intermediate case between the ultra-compact plasma ion sources and the large-size thermonuclear fusion devices. Preliminary measurements at scaled microwave frequencies have carried out both on a free-space setup by using a wire-grid polarizer and a rotable Ka-band OMT + horn antennas system, and on a compact trap (called Flexible Plasma Trap) installed at INFN-LNS and used as PANDORA down-sized testbench are described. The polarimeter technique will support beta-decay investigation by simultaneous measurements of the total plasma density, which is crucial to carefully evaluate the decay-constant and to extrapolate the laboratory observed data to the astrophysical scenarios. Moreover, this work proposes to adopt an electromagnetic inverse-scattering-based technique-based method to retrieve the electron density profile along the probing antennas line-of-sight. Numerical results of this so-called inverse profilometry are also shown.

Automated summary

This paper presents the design study of a mm-wave polarimeter for measuring the line-integrated electron density in the PANDORA project, which aims to investigate nuclear beta-decay rates in stellar-like conditions. The paper introduces a method for measurements in a magnetic plasma trap that is intermediate between compact plasma ion sources and large thermonuclear fusion devices. Preliminary measurements at scaled microwave frequencies and on a compact trap are described. The paper also proposes an electromagnetic inverse-scattering-based method for retrieving the electron density profile.