| Presentation Title: | Study on non-Maxwellian Electron Velocity Distribution in Laboratory Plasmas via high-resolution spectroscopy and spectropolarimetry |
| Author(s): | T. Kawate, M. Goto, T. Oishi, Y. Kawamoto, I. Yamada, H. Funaba, H. Takahashi |
Abstract:
The electron velocity distribution function (EVDF) represents the nature of energy transfer and dissipation processes in solar, space, and thermonuclear fusion plasmas. Non-Maxwellian/anisotropic electrons often interact with electromagnetic waves and excite kinetic instabilities. In fusion plasmas, non-Maxwellian EVDF is expected during electron cyclotron resonance heating (ECRH), lower hybrid heating, and DC heating operations. Such heating processes create anisotropic EVDF as well. Thus, high-temperature plasmas created and maintained by these heating methods are good for studying the role of non-thermal electrons in high-energy astrophysical plasmas.
One method to measure EVDF is population kinetics of ions, i.e., line spectra. X-ray lines emitted from helium-like ions and their satellite lines have been used as temperature diagnostics, and intensities of lines that are formed by inner-shell excitation and ionization are affected by high-energy electrons as reported from observations by Hinotori satellite (Kato & Masai 1992). In addition, ions colliding with anisotropic electrons are excited with population alignment and emit polarized light when de-excitation. X-ray crystal spectrometers behave as polarizers and are potentially used for the diagnostics of anisotropic EVDF (Akita et al. 1983). Therefore, X-ray line spectroscopy with crystal spectrometers can be a good diagnostics of nonthermal electrons.
We performed X-ray measurements of ECRH plasmas in the Large Helical Device at the National Institute for Fusion Science. Thomson scattering and electron cyclotron emission are also used to measure electron temperatures. Argon gas was injected for the x-ray diagnostics of high-temperature plasmas. In the low electron density conditions, Ar XII and the satellite lines at around 3.95-4.00Å show some signs of suprathermal/anisotropic electrons. In the presentation, we introduce the results of the experiment of nonthermal electrons and ongoing projects on the interaction between nonthermal electrons and bulk plasmas via spectroscopy/spectropolarimetry.