Full-wave feasibility study of magnetic diagnostic based on O-X mode conversion and oblique reflectometry imaging

Transcription

1 Full-wave feasibility study of magnetic diagnostic based on O-X mode conversion and oblique reflectometry imaging 20 th topical conference on radio frequency power in plasmas Orso Meneghini, M. Choi #, F. Volpe? Oak Ridge Associated Universities, Oak Ridge, TN USA # IMSOL-X, San Diego, CA USA? Columbia University, New York, NY USA June 24 th 2013

2 Motivation: pitch-angle measurements are of paramount importance for numerous fusion applications Stability q profile (BS and non-inductive currents) Resistive Wall Modes (RWM) Neoclassical Tearing Modes (NTM) Error fields penetration Edge Harmonic Oscillation (EHO) in Quiescent H-modes (QH) Transport Internal transport barriers H-mode transition Magnetic field fluctuations for EM turbulence and understanding of anomalous electron energy transport Pedestal and pasma boundary Resonant Magnetic Perturbations (RMP) Edge current density for peeling-balooning stability Advanced and hybrid scenarios Distruption prediction ELM suppression by RMP Mostly needed at the edge, with high temporal resolution, 1D/2D/3D

3 Physical principle: idea of anti-radar magnetic diagnostic 1 O-mode beam of millimeter waves (33-75GHz) is obliquely injected in the magnetized plasma 2 Angular dependent mode conversion at the O-mode cutoff partly reflected as O-mode partly transmitted to X-mode 3 Angular-dependent mode conversion affects reflected beam pattern, that will exhibit a hole when plotted as a function of the horizontal and vertical view angles 4 Reflected beam pattern provides info on the magnetic pitch angle at the cutoff, in the pedestal region

4 Benefits of the anti-radar technique Internal local magnetic measurement Temporally and spatially resolved (reflectometer-like) Measure magnetic fluctuations and 2D/3D magnetic structures Frequency-resolved measurements! radially resolved measurements of the pitch angle Same applicability of reflectometry (broader applicability that EBE, which is limited to overdense EBW emitter plasmas) Active diagnostic: Stronger signals with strong source Noise subtraction by modulation Control over injected frequency, for probing different radii Only depends on XO conversion (no BXO), less complications and fewer degradation mechanisms

5 Fundamental equations O-mode to X-mode transmission efficiency at O-mode cutoff: ( r Y C =exp k 0 L 2(1 + Y )(Nz,opt N z ) 2 + N 2 ) y 2 O/X-mode dispersion relation for oblique propagation n 2? =1 1 X h i (Y sin ) 2 2(1 X) ± (Y sin ) 4 1/2 n 4(1 X) +(Y cos ) z where k 0 is the vacuum wavelength L is the local density scalelength Y = e /! N z,opt = p Y/(Y + 1) H. P. Laqua, Phys. Rev. Lett 78 (1997)

6 Fundamental equations Elliptical polarization required for pure pure O-mode oblique injection: 2 3 E p = 4 2i sin q p 5 E p Y cos 2 p + (Y cos 2 p ) 2 +4sin 2 p PLASMA J. L. Doane, Manual of Polarizer Miter Bend Fabricated by General Atomics

7 EBE imaging EBW emission for! >! UH OXB conversion requires the pasma to be overdense! n e <! pe ( = 0) Signal amplitude depends on temperature complicated EBW trajectories degradation mechanisms: collisional losses at UHR back-conversion to fast X-mode conversion effeciency degraged at O-mode cutoff due to density fluctuations

8 Full wave modeling of anti-radar diagnostic with COMSOL To demonstrate this as well as to assess the diagnostic capabilities and limitations, we modeled the wave scattering and mode-conversion processes by means of the finite-element COMSOL Multiphysics code in two dimensions (2D) Initial sensitivity studies for mock-up DIII-D plasmas injection angle frequency Simulations confirmed the presence of a minimum in reflectivity of an externally injected O-mode beam, and confirmed that this minimum depends on the magnetic field at the cutoff.

9 Simplified slab model to mimic DIII-D plasma parameters Density [m -3 ] Magnetic field [T] Slab cold magnetized plasma Radially varying n e, B z Oblique O-mode injection in vacuum e e e e e e e X [m] Frequency [GHz] PLASMA X [m]

10 10 GHz oblique injection with = 70 o and 30 o 20 GHz oblique injection with = 70 o and 50 o 10 GHz, = 70 o 10 GHz, = 30 o plasma edge plasma edge plasma edge 20 GHz, = 70 o plasma edge 20 GHz, = 50 o

11 10 GHz oblique injection with = 70 o and 30 o 10 GHz, = 70 o E x / [V/m] 10 GHz, = 30 o E x / [V/m] E y / [V/m] E y / [V/m] E z / [V/m] E z / [V/m]

12 20 GHz oblique injection with = 70 o and 50 o 20 GHz, = 70 o E x / [V/m] 20 GHz, = 50 o E x / [V/m] E y / [V/m] E y / [V/m] E z / [V/m] E z / [V/m]

13 Total electric field at plasma edge 1.4e GHz 1.2e GHz 1.2e+12 1e+12 1e+12 8e+11 8e+11 Injected wave Reflected wave 6e+11 Injected wave Reflected wave 6e+11 4e+11 4e+11 2e+11 2e

14 Conclusions and future work This study gives confidence in the feasibility of the diagnostic and provides a basis to interpret future experimental data. Presence of L-mode cutoff complicates original intuitive picture, and underlines importance of fullwave modeling Future work Increase operating frequency in the 33 to 75 GHz band Inclusion of n e and B fluctuation Inclusion of toroidal ripple effects Extensions to full 3D model