Measurements of Mode Converted ICRF Waves with Phase Contrast Imaging in Alcator C-Mod N. Tsujii, M. Porkolab, E.M. Edlund, L. Lin, Y. Lin, J.C. Wright, S.J. Wukitch MIT Plasma Science and Fusion Center *Work supported by US DOE under DE-FG02-94-ER54235 and DE-FC02-99-ER54512
Introduction The Phase Contrast Imaging(PCI) system in Alcator C-Mod is used to measure MHD modes, turbulence and RF waves The system was absolutely calibrated for the heterodyne measurement of the RF waves Mode converted ion cyclotron waves(icw) were observed by PCI around the ion-ion hybrid layer in a D- 3 He plasma The PCI measurements were compared with the full wave simulation code TORIC
Wave Propagation in an Axisymmetric Plasma The local dispersion relation takes the form, : wavevector parallel to the magnetic field : wavevector perpendicular to the magnetic field The toroidal mode number n is conserved, : wavevector in the toroidal direction In terms of wavevector in the poloidal direction,
3 He-D Mode Conversion Scenario(1) Mode conversion from fast magnetosonic wave to hot plasma waves occurs around the ion-ion hybrid layer The mode conversion occurs around a narrow region, and the inhomogeneity can be considered 1D, mainly from the variation of the toroidal magnetic field The local dispersion relation can be solved in terms of wavevector in the direction of inhomogeneity, k R
3 He-D Mode Conversion Scenario(2) k R [cm -1 ] Ion species concentration, D 78%, 3 He 9%, H 4% I p = 0.83 MA, R 0 = 0.683 m, B 0 = 4.9 T, n e0 = 1.5 * 10 20 m -3, T e0 = 2.1 kev Dispersion curves at and above the midplane, for n = 8 10 5 0-5 -10 IBW MC 0.64 0.66 0.68 0.7 R[m] z = 0.01 m FW 3 He Fund. k R [cm -1 ] 8 6 4 2 0-2 -4-6 MC ICW 3 He Fund. -8 0.62 0.64 0.66 0.68 0.7 R[m] z = 0.17 m
TORIC simulation(1) TORIC Full wave solver in axisymmetric geometry Electric field solution (n = 8, for MW of power absorbed) ICW IBW FW
TORIC simulation(2) Power deposition profile (n = 8, for MW of power absorbed) Power absorbed by 3 He at fundamental cyclotron resonance Power absorbed by electrons
Phase Contrast Imaging Laser beam propagating through the plasma acquires phase delay proportional to the line-integrated electron density : PCI laser wavelength By introducing π/2 phase delay to the unscattered component of the beam, phase variation is converted to beam intensity variation
PCI System in C-Mod(1) The laser beam is focused on the phase plate with an off-axis parabolic mirror π/2 phase delay is introduced by a reflective phase plate with a λ/8 groove at the center λ/8 groove phase plate Line integrated density fluctuation is imaged onto the 32 channel photodiode detector array
PCI System in C-Mod(2) Laser: 60 W CO 2 CW λ = 10.6 μm TEM 00 Wave number: k R = 0.5 55 cm -1 Frequency: 2 khz 5 MHz Heterodyne frequency: 40 80 MHz Localization: R = 60 79 cm
Absolute Calibration Sound wave pattern of the calibration source was measured with a calibrated microphone The source is used as a reference for calibrating PCI density fluctuation level 15 khz loudspeaker
Optical Heterodyne System Laser beam amplitude is modulated with RF frequency using acousto-optic shifters The mixing of the RF fluctuation with this modulation reduces the frequency to ~1 MHz No need for fast detector array Less RF pick up compared to electronic heterodyne
Heterodyned RF Signal on PCI RF frequency 50 MHz PCI beam intensity modulation 48.21 MHz signal at 1.79 MHz ch22 signal (R = 0.66[m]) Coherent fluctuation is observed at the expected frequency, which shows the excitation of mode converted ion cyclotron wave Strong flow drive is observed in this shot [Y. Lin. BI2.00006] RF power[mw] 3 2 1 0 0.8 1 1.2 1.4 time[s]
Observed RF Fluctuation Profile Density fluctuation profile at 1790 khz Radial structure Wavenumber structure
Simulation of PCI Signal with Particle number conservation, TORIC Major contributions to the electron velocity are, where Z is the Fried-Conte plasma dispersion function
P abs [MW/kA 2 ] 15 10 5 Toroidal Mode Number 0-20 -10 0 10 20 n Dependence Phasing +90 Total power coupled 1 MW Simulated wave amplitude is normalized to coupled power real wave amplitude Power absorption efficiency P abs [MW] Antenna spectrum antenna current 0.35 0.3 0.25 0.2 0.15 0.1 0.05 1.5 x 10-3 1 0.5 0-20 -10 0 10 20 n 0-20 -10 0 10 20 n Power absorbed
Comparison of TORIC simulation with PCI measurement density fluctuation[(10 16 m -2 ) 2 ] 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.62 0.64 0.66 0.68 0.7 0.72 R[m] -10-5 0 5 10 : Line-integrated density fluctuation from TORIC * : PCI measurement The simulation and experiment are within factors of 2-3 density fluctuation[(10 16 m -2 ) 2 /cm -1 ] 3.5 x 10-3 3 2.5 2 1.5 1 0.5 0 k R [cm -1 ]
Future Work Expected PCI signal for different 3 He concentrations density fluctuation[(10 16 m -2 ) 2 ] 2 1.5 1 0.5 9 % 20 % 30 % 0 0.6 0.65 0.7 R[m] Future experimental plans include scanning the minority concentration and comparing the experimental measurements with these predictions
Summary PCI signal in the mode conversion experiment has been simulated with TORIC The simulation and the experiment are within factors of 2-3 Comparisons will be made for different minority concentrations For a more detailed comparison, PCI channel response has to be calibrated more accurately Simulation of PCI signal with AORSA is planned