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
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
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
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
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 ) 2 2 2 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)
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
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
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.
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 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 3.5e+19 3.0e+19 2.5e+19 2.0e+19 1.5e+19 1.0e+19 0.5e+19 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 X [m] Frequency [GHz] PLASMA 80 70 60 50 40 30 20 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 X [m]
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
10 GHz oblique injection with = 70 o and 30 o 10 GHz, = 70 o E x /7 10 5 [V/m] 10 GHz, = 30 o E x /1.00 10 6 [V/m] E y /3 10 5 [V/m] E y /0.77 10 6 [V/m] E z /1 10 6 [V/m] E z /0.81 10 6 [V/m]
20 GHz oblique injection with = 70 o and 50 o 20 GHz, = 70 o E x /1 10 6 [V/m] 20 GHz, = 50 o E x /1 10 6 [V/m] E y /4 10 5 [V/m] E y /7 10 5 [V/m] E z /1 10 6 [V/m] E z /1 10 6 [V/m]
Total electric field at plasma edge 1.4e+12 10 GHz 1.2e+12 20 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+11 0 0 0.2 0.4 0.6 0.8 1 0 0 0.2 0.4 0.6 0.8 1
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