Measurement of Fluctuation in Velocity Space for Verification of Entropy Cascade in Electrostatic Turbulence in Magnetized Plasma

  • 林 家玄

學生論文: Master's Thesis

摘要

This research aims at experimentally verifying an entropy cascade in two-dimensional (2D) electrostatic turbulence of laboratory magnetized plasmas by measurement of velocity space structure of the ions that is considered as universal nature of 2D electrostatic plasma turbulence Specifically evaluation of a scaling law E ?_g (p) ~ p^(-4/3) deduced from a dimensional analysis with the gyrokinetic equations [A A Schekochihin et al Plasma Phys Controlled Fusion Vol 50 124024 (2008) T Tatsuno et al Phys Rev Lett Vol 103 015003 (2009)] is set as a goal where E_g and p are the power spectrum of fluctuation of g(v_⊥) (g(v_⊥) : ring averaged distribution function of ions at a fixed guiding center position v_⊥: the velocity component perpendicular to the background magnetic field) and the wavenumber (mode number) in the velocity space respectively This scaling law is akin to the Kolmogorov scaling law in three-dimensional turbulence of neutral fluids which is well-known as an evidence indicating existence of universality in turbulence To this end we have developed a novel diagnostic tool of g(v_⊥) named as ring-averaged ion distribution function probe (RIDFP) which achieves momentum selection of incoming ions by selection of the ion Lamor radii Through the use of a numerical ion orbit calculation code we evaluated accuracy of the g(v_⊥) reconstruction by the developed RIDFP The calculation results indicate that if we can suppress the potential difference ?? between the body of RIDFP and the target plasma that is the sheath potential lower than 0 025V the velocity mode number p up to 4 can be identified with the error of 35% To nullify the sheath potential surrounding RIDFP we have developed the potential control system of the RIDFP body which consist of a voltage follower and an emissive probe through the use of a circuit simulation After the design of potential control system we constructed this system and test it in a bench test and laboratory test From the test results it is confirmed that the potential control system has a capability of maintaining the ?? lower than 0 015V This is an acceptable range comparing with the calculation results After the completion of the developments we applied the RIDFPs to magnetized plasma experiments in the MPX device The equilibrium g(v_?) measured by the RIDFPs was roughly in accordance with the Maxwellian distribution having T_i=0 4eV Perturbation components of g(v_?) in this experiment indicated low frequency fluctuations at 1 4kHz which was considered to stem from the drift waves excited in the MPX plasma A preliminary result of evaluation of the spectrum E ?_g (p) was obtained from the RIDFP measurements which showed a significant fluctuation at the high mode number p region unlike the theoretical prediction by Schekochihin et al The noise induced by an unexpected interaction between the current detection circuits of the RIDFP system and the power supplies for the background magnetic field considerably contaminated the measured spectra of g(v_⊥ ) Further improvement of the signal to noise ratio in the RIDFP measurement is necessary
獎項日期2016 十二月 19
原文English
監督員Eiichirou Kawamori (Supervisor)

引用此文

Measurement of Fluctuation in Velocity Space for Verification of Entropy Cascade in Electrostatic Turbulence in Magnetized Plasma
家玄, 林. (Author). 2016 十二月 19

學生論文: Master's Thesis