Experimental Verification of Entropy Cascade in 2-D GyroKinetic Turbulence by Velocity-Space measurement

  • 陳 勁廷

Student thesis: Master's Thesis


Two-dimensional (2D) electrostatic turbulence in weakly-collisional magnetized plasma at ion gyro scales can be illustrated by cascades of entropy through phase space dynamics according to a theory and gyrokinetic simulations [A A Schekochihin et al Plasma Phys Controlled Fusion Vol 50 124024 (2008) T Tatsuno et al Phys Rev Lett Vol 103 015003 (2009)] The scaling law of wave number spectrum of electrostatic potential fluctuations E_? (k_? ) ? k_?^(-10/3) deduce by the theory was verified in a subsequent laboratory experiment with the use of the magnetized plasma experiment (MPX) device at NCKU [E Kawamori Phys Rev Lett Vol 110 095001 (2013)] where E_? and k_? are the power spectrum of potential fluctuations and wave number respectively These theoretical numerical and experimental results have revealed Kolmogorov-like universal property of 2D magnetized plasma gyrokinetic turbulences Although the observations in MPX were consistent with those in the theory there have been no experimental evidence to support the Kolmogorov type cascade in velocity space in 2-D electrostatic gyrokinetic turbulences which is the central part of the theory One of the eventual goals of this research (thesis) is to experimentally verify the entropy cascade in 2D electrostatic turbulence of laboratory magnetized plasmas by means of velocity space structure measurement of ions Specifically we attempt to experimentally examine a scaling law of the velocity space structure predicted by the gyrokinetic entropy cascade theory E ?_g (p) ~ p^(-4/3) in 2-D electrostatic turbulences in MPX where E ?_g (p) and p are power spectrum of fluctuation of g(v) (g(v) : the ion distribution function at a fixed guiding center) and wavenumber in the velocity space To this end we have developed a novel diagnostic instrument of g(v) which is named as ring ion distribution function probe (RIDF probe) RIDF probe is an ion velocity analyzer which achieves momentum selection of incoming ions by selection of the ion Lamor radii Another key idea applied to the RIDF probe is nullification of a potential drop (sheath) between the body of RIDF probe and a target plasma which causes deviation of ion orbits from the pure ions gyro orbit To that end we employ an external control of potential of the chassis of the RIDF probe with a feedback system using a space potential measurement with an emissive probe The RIDF probe consist of three parts a sensor head the potential control system and current detection circuits The sensor head of RIDF probe was designed through particle orbit calculations with the numerical code we have developed from scratch Specifically by particle orbit calculations (a) dimensions of the sensor heads (b) the number of velocity channels and (c) dimensions of the ion collectors were determined under some physical constraints Also we optimized the other details such as detailed positions of the collectors and so on from the results of the calculations The final design of sensor head has ability of reconstructing ion velocity distribution function with an error of 10%~16% if the potential difference between the probe body and the space potential of the target plasma is controlled within 0 1V The typical ion current detected by RIDF probe ranges between 10pA~100pA We constructed two sensor heads based on the numerical calculation a multichannel small current detection system and the potential control system After the completion of the RIDF probe development we applied the RIDF probe to MPX to measure fluctuation of ring averaged ion distribution We have confirmed that the amplitude of ion current signals in the RIDF probe measurement was consistent with our particle orbit calculation results As a preliminary result we observed fluctuation in the ion current in RIDF probe measurement whose frequency~ 2 kHz was consistent with the drift wave frequency range of the MPX plasma Although we have not yet initiated a full scale measurement of velocity structures of ions in MPX it can be said that a powerful diagnostic tool of the 2D gyrokinetic turbulence is almost ready for investigation of entropy cascade in gyrokinetic turbulences
Date of Award2016 Feb 2
Original languageEnglish
SupervisorEiichirou Kawamori (Supervisor)

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