Plasma Density Fluctuation Driven by High Harmonic Fast Wave in NSTX

Abstract

Significant progress in understanding the RF wave physics has been done using numerical simulations and it is possible to predict the electric field distribution using RF wave simulation codes, but it is crucial to verify the validity and accuracy of the theoretical calculation by comparing the results with the experimental measurement. Though it is not easy to measure the RF electric field directly, it may be possible to compare the experimental measurement of density fluctuation and the calculated RF electric field since RF electric field drives local density fluctuation. The target of this study is to calculate the RF-driven density fluctuation from electric field solutions of full wave code and compare the calculated results with experimental observation. In this thesis, 3D distribution of density fluctuation driven by 30 MHz high harmonic fast wave (HHFW) on National Spherical Torus Experiment (NSTX) was calculated from TORIC solution using linearized fluid equations and the signal strength which is expected to be measured by FIR tangential interferometry/polarimetry (FIReTIP) was calculated by square-integrating it along the FIReTIP chords. TORIC is a full wave ICRF code which solves Maxwell’s wave equation for a single frequency in axisymmetric toroidal geometry. In order to obtain three dimensional (3D) electric field distributions, all the present modes with different toroidal mode number were added together with appropriate weight factors. 3D distribution of electric field shows the wave behavior such as toroidal and poloidal rotation, and single pass absorption as in ray tracing simulation. FIReTIP was used for the measurement of the density fluctuation driven by HHFW. FIR laser beam is scattered by a density fluctuation in a plasma. Laser frequency will be shifted by the frequency of the density fluctuation and the scattered intensity is proportional to the square of the fluctuation amplitude. Prototype of RF detection circuit was added to FIReTIP detection circuit while retaining the ability to measure the equilibrium density at the same time. Since RF detector circuit detected only the RF pickup by the circuitry, the detector need to be improved by enhancing the gain and noise figure.