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This work describes a novel optical heterodyne detection scheme that significantly extends the frequency response of the phase contrast imaging method to detect electron density fluctuations in tens of megahertz frequency range. The system employs a variable frequency electro-optic modulator to allow operation at any frequency in the range 10–40 MHz without the need to realign the system. The frequency coverage of the system makes it suitable to measure the radial structure of the electron density component of ion cyclotron emission on devices having confining magnetic field of a few tesla, thus extending the purely temporal measurements provided so far bymore » magnetic probes.« less

An upgrade to the Phase Contrast Imaging (PCI) diagnostic on the DIII-D tokamak has successfully combined PCI with density interferometry to provide electron density fluctuation measurements across an unprecedented range in wavenumber. The combined diagnostic uses a single laser and a single beam path through the tokamak, minimizing machine resources in anticipation of future reactor-scale devices where port access will be at a premium. The PCI multichannel detector provides low-noise, wavenumber-resolved detection of moderate-wavenumber turbulence, while the interferometer channel is sensitive to long-wavelength turbulence, including the peak of ion-scale turbulence and MHD. The overlap in wavenumber range between the twomore » schemes allows for an in situ absolute calibration of the PCI.« less

In this work, a novel combined diagnostic capable of measuring multiscale density fluctuations that extend from magnetohydrodynamic (MHD) to the lower range of electron temperature gradient turbulence has been designed, installed, and operated at DIII-D. The combined diagnostic was constructed by adding a heterodyne interferometer to the pre-existing phase contrast imaging (PCI) system, both of which measure line-integrated electron density fluctuations. The port-space footprint is minimized via use of a single 10.6 μm probe beam. With temporal bandwidths in excess of 1 MHz, the PCI measures high-k (1.5 cm^-1 < |k_R| ≤ 25 cm^-1) fluctuations with sensitivity 3×10¹³ m^-2/$$\sqrt{kHz}$$, whilemore » the interferometer simultaneously measures low-k (|k_R| < 5 cm^-1) fluctuations with sensitivity 3×10¹⁴ m^-2/$$\sqrt{kHz}$$. The intentional mid-k overlap has been empirically verified with sound-wave calibrations and allows quantitative investigation of multiscale effects that are predicted to be significant in the reactor-relevant T_e ~ T_i regime. Lastly, via correlation with the primary DIII-D interferometer, the toroidal mode numbers of core-localized MHD can be measured.« less

Heterodyne interferometry and phase contrast imaging (PCI) are robust, mature techniques for measuring low-k and high-k electron density fluctuations, respectively. Here, we describe the first-ever implementation of a combined PCI-interferometer. The combined system uses a single 10:6 μm probe beam, two interference schemes, and two detectors to measure electron density uctuations at large spatiotemporal bandwidth (10 kHz < f < 5MHz and 0 cm^-1 ≤ k ≤ 20 cm^-1), allowing simultaneous measurement of ion- and electron-scale instabilities. Further, correlating our interferometer's measurements with those from DIII-D's pre-existing, toroidally separated interferometer allows core-localized, low-n MHD studies that may otherwise be inaccessiblemore » via external magnetic measurements. In the combined diagnostic's small port requirements and minimal access restrictions make it well-suited to the harsh neutron environments and limited port space expected in next-step devices.« less

Recent experiments on C-mod seeding nitrogen into ohmic plasmas with q₉₅ = 3.4 found that the seeding greatly reduced long-wavelength (ITG-scale) turbulence. The long-wavelength turbulence that was reduced by the nitrogen seeding was localized to the region of r/a ≈ 0.85, where the turbulence is well above marginal stability (as evidenced by Q_i/Q_GB $$\gg$$ 1). The nonlinear gyrokinetic code GYRO was used to simulate the expected turbulence in these plasmas, and the simulated turbulent density fluctuations and turbulent energy fluxes quantitatively agreed with the experimental measurements both before and after the nitrogen seeding. Unexpectedly, the intrinsic rotation of the plasmamore » was also found to be affected by the nitrogen seeding, in a manner apparently unrelated to a change in the electron-ion collisionality that was proposed by other experiments.« less

Main ion dilution has been predicted by gyrokinetic simulations to have a significant effect on ion thermal transport in C-Mod ohmic plasmas. This effect was verified experimentally with a specific set of experiments on C-Mod in which ohmic deuterium plasmas across the linear ohmic confinement (LOC) through the saturated ohmic confinement (SOC) regimes were diluted by seeding with nitrogen gas (Z=7) injection. The seeding was observed to increase the normalized ion temperature gradients (ITGs) by up to 30% without a corresponding increase in the gyrobohm normalized ion energy flux, indicating a change in either the stiffness or the critical ionmore » temperature gradient associated with ITG turbulence. The seeding also reversed the direction of the intrinsic toroidal rotation in plasmas slightly above the normal intrinsic rotation reversal critical density. GYRO simulations of the seeded and unseeded plasmas show that the seeding affected both the critical gradient and the stiffness. For plasmas in the LOC regime, the dilution primarily increased the critical gradient, while for plasmas in the SOC regime the dilution primarily decreased the stiffness. At r/a = 0.8, where the experimental fluxes were above marginal stability, local GYRO predicted and experimental energy fluxes agreed, except for Q_i in the SOC regime where GYRO under-predicted the experimental energy flux. At r/a = 0.6, where the experimental fluxes were close to marginally stable, local GYRO predicted ITG modes to be strongly unstable and are responsible for both Q_i and Q_e (with Q_i > Q_e), as opposed to the experiment where Q_i < Q_e. In contrast, global GYRO in this region predicted the ITG modes to be closer to marginal stability, and accurately predict the experimental Q_i when the T_i profile is modified within experimental uncertainties. Further, the fact that Q_e is always less than Q_i in the r/a = 0.6 simulations with k_θρ_s ≤1 indicates that high-k electron temperature gradient driven (ETG) modes must be included in future simulations and may be responsible for the electron energy transport in this case.« less