1 NSTX Electron Bernstein Wave Emission Measurements Gary Taylor NSTX Results Review September 20-21, 2004
2 Antennas on NSTX Measured EBW Emission via Conversion to X-Mode and O-Mode During 2004 Campaign "X-mode" EBW coupling requires steep density gradient at EBW conversion layer: need L n ~ 2-3 mm for ~100% EBW conversion on NSTX need limiter to maintain steep L n very sensitive to L n fluctuations measured very low (~%) EBW conversion local reflectometry showed plasma underdense in front of antenna New oblique viewing "O-mode" EBW antenna less sensitive to fluctuations in L n at EBW mode conversion layer B
3 Modeling Predicts Efficient EBW Coupling with Oblique, Circularly Polarized, “O-Mode” Launch EBW coupling at 14 GHz modeled with OPTIPOL/GLOSI [Mark Carter, ORNL] EBW Coupling Efficiency 80% 60% 40% 20%
4 New Obliquely Viewing Antenna Installed at Bay G on NSTX Measures “O-Mode” EBW Emission Two 8-18 GHz radiometers simultaneously measure orthogonal polarizations with quad-ridged antenna: compare emission results to OPTIPOL/GLOSI coupling predictions Focusing lens optimized for GHz EBW emission Antenna views along 35 degree B field pitch, suitable for NSTX plasmas with I p ~ 1 MA at B t (0) ~ 4.0 kG Adjustable Limiters
5 Radiometers Measuring Orthogonally Polarized Emission Detect Similar EBW T rad EBW Emission Frequency = 16.5 GHz B t (0) = 4 kG Reflectometer Interference
6 Ray Tracing Calculations Show 16.5 GHz EBW Emission is Generated Locally at r/a = 0.4 CompX EBW Emission Frequency = 16.5 GHz B t (0) = 4 kG GENRAY t = 325 ms Shot OPTIPOL/GLOSI L n = 1 cm, f = 14 GHz Poloidal Angle (deg.) Toroidal Angle (deg.) > 90% EBW Coupling 10% B Approx. Antenna Acceptance Angle GENRAY ray tracing uses EFIT equilibrium and T e (R) & n e (R) from Thomson scattering Antenna acceptance angle much larger than predicted 90% EBW conversion region
7 EBW T rad /T e ~ 70%, Consistent with Theory; Radiometer Signal Ratios Consistent with Near-Circular Polarization Freq. = 16.5 GHz Emission fluctuations due to fluctuation in L n at EBW conversion layer Radiometer signal ratio ~ 1.3 ± 0.3, consistent with near-circular polarization predicted by modeling: need additional measurements to confirm polarization
8 Obliquely Viewing GHz EBW Radiometer to be Installed on NSTX Next Year Larger vacuum window & higher frequency should allow much better collimation: current GHz antenna has ± 12 degree acceptance angle, GHz antenna should achieve less than ± 5 degrees Detailed 28 GHz coupling study using OPTIPOL/AORSA1D and realistic EBW launcher model planned for 2005: compare to 28 GHz emission measurements
9 Summary Initial emission results at GHz via EBW conversion to “O-mode” look promising & consistent with theory X-mode emission measurements with local limiter had very low (~%) EBW conversion; plasma underdense in front of antenna Next year measure GHz EBW emission via “O-mode” conversion & compare to theory