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Incoherent Thomson Scattering


Séminaire proposé par: Benjamin VINCENT

ICARE, CNRS Orléans

me 15 novembre 2017, 15h00
amphi 3, RC, LPGP, bat 210 rue Bequrel, UPSud, Orsay


Abstract:

Incoherent Thomson Scattering (ITS) involves the elastic scattering of electromagnetic radiation on randomly-distributed charged particles, and is used to recover information on electron properties in a wide range of plasmas. The scattered spectrum spectral shape gives access to the electron distribution function, the overall spectral shift from the incident probe wavelength provides the global drift velocity of the electrons (ve), while the integrated spectrum area over the wavelengths provides the electron density (ne) from a calibrated diagnostic. For Maxwellian plasmas the spectral distribution of the scattered spectrum is Gaussian, its width provides access to the electron temperature (Te).
This diagnostic approach has a long history of implementation in magnetic fusion devices, dating back to the 1960s and up to the present day. Over the past few decades, this technique has been also been increasingly used for the study of electron properties in a variety of low-temperature plasmas. It is non-perturbative, non-invasive and provides direct access to the EEDF without the requirement of intervening oversimplifications; these are all significant advantages over conventional Langmuir probe diagnostics.
In this work, we describe development efforts and preliminary results using the newly-developed THETIS (THomson scattering Experiments for low Temperature Ion Sources) platform. This platform is intended for measurements of electron properties in a diverse range of plasma environments for which such information has been lacking.
This development effort is driven by certain questions and requirements:

• The diagnostic is intended to provide measurements of electron properties in the Hall thruster exit plane region, where deviations from Maxwellian EEDFs are expected. This will be important to our understanding of basic processes, such as microturbulence-induced transport in thrusters, and electron confinement in alternative architectures such as the wall-less Hall thruster. Averaged and time-resolved measurements are envisaged. Coupled with the recently-developed coherent Thomson scattering diagnostic PRAXIS, the new platform is expected to provide access to valuable information on electron dynamics.
• The diagnostic has been designed for maximum compactness and flexibility of implementation on different sources (including magnetrons, electron cyclotron resonance ion sources and helicons).
• The diagnostic is designed for increased sensitivity with respect to existing diagnostics of this type, facilitating measurements in low-density plasma environments.
In this work, a hollow cathode is used as a test plasma source for the diagnostic optimization. Preliminary measurements in the vicinity of the cathode orifice provide evidence for electron temperatures on the order of 2 eV and below, and confirm an initial detection limit for the diagnostic approaching 10^16 m−3. These results are compared Langmuir probe measurements obtained with the same cathode under similar conditions.

 

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