A new interferometric/polarimetric setup for plasma density measurements in compact microwave-based Ion Sources

Abstract

A: A K-band (18.5 ÷ 26.5 GHz) microwave interferometry/polarimetry setup, based on the Frequency-Modulated Continuous-Wave (FMCW) method, has been developed at INFN-LNS under the VESPRI project. The interferometer has been proven to provide reliable measurements of the plasma density even in the extreme unfavorable conditions λ p L p L c , being λ p , L p and L c the probing signal wavelength, the plasma dimension and the plasma chamber length respectively. The VESPRI setup has been therefore upgraded with a rotating polarimetric system based on waveguide OMTs (OrthoModeTransducers) for the measurement of the magnetoplasma-induced Faraday rotation. An analysis method has been developed on purpose in order to discriminate the polarization plane rotation due to the plasma only, excluding the effects of the cavity resonator which represents the primary error source on phase angle measurement. Results about the first collected data, showing a significative agreement of the plasma-induced polarization plane rotation with the well-known λ 2 law, are hereby presented. The developed method will be a powerful tool for probing plasmas in very compact magnetic traps such as Electron Cyclotron Resonance Ion Sources.

The first measurement of plasma density by means of an interfero-polarimetric setup in a compact ECR-plasma trap

“…Hereby we show the data coming from polarimetric measurements (preliminary dataset are already commented in the paper [7]) that have allowed the first line-integrated measurement of plasma density via Faraday-rotation in ECRISs.…”

Electromagnetic inverse profiling for plasma diagnostics in compact microwave-based ion sources

“…Moreover, in order to deal with expected plasma profile having physical meaning, two further constraints are added in the solution of the convex problem: the first one is the positivity constraint about the retrievable value of the plasma angular frequency, while the second one is a prior information about the estimated/expected support of the plasma density profile over the chamber axis. This latter, can be inferred by a priori information from experimental measurements [1,2] or performing a preliminary reconstruction via standard L 2 -norm approaches, with the aim to retrieve only the location wherein the plasma slab is different from zero [10].…”

“…Non-invasive measurements of electron density and temperature in magnetoplasmas are crucial for improving plasma-heating efficiency in ECRIS (Electron Cyclotron Resonance Ion Sources) and MDIS (Microwave Discharge Ion Sources). Recently, non-invasive diagnostic methods based on microwave interferometry and polarimetry [1,2], X-ray and optical emission spectroscopy [3,4], have been developed to measure the line-of-sight-averaged electron density in compact microwavebased plasma ion sources. These plasma diagnostics tools are even more important for the study and development of heating schemes alternative to ECR employing Electrostatic Bernstein Waves in overdense plasma [5].…”

Electromagnetic diagnostics of ECR-Ion Sources plasmas: optical/X-ray imaging and spectroscopy

“…The set of diagnostics developed in the last years at INFN-LNS is now suitable for multi-component analysis of a magnetoplasma of an ECRIS-type plasma trap. At the present time, an interferopolarimetric setup named VESPRI [14] is going to be completed, and first measurements of lineintegrated plasma densities have been carried out [15]. For the requests of the forthcoming projects, however, such as PANDORA [16] (Plasmas for Astrophysics, Nuclear Decays Observation and Radiation for Archaeometry), even more advanced diagnostics systems will be needed.…”