Localized measurement of short wavelength plasma fluctuations with the DIII-D phase contrast imaging diagnostic

Dorris, J.; Rost, J. C.; Porkoláb, M.

doi:10.1063/1.3065094

Cited by 30 publications

(45 citation statements)

References 24 publications

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“…A 5 cm diameter CO 2 laser beam at λ 0 = 10.6 µm is used to probe the plasma. 11 The laser passes through the plasma nearly vertically. The laser is imaged onto a 16 channel HgMnTe detector with circular elements.…”

Section: Response Of Phase Contrast Imagingmentioning

confidence: 99%

Development of a synthetic phase contrast imaging diagnostic

Rost¹,

Lin²,

Porkoláb³

2010

Physics of Plasmas

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A "synthetic diagnostic" has been developed to calculate the expected experimental response of phase contrast imaging (PCI), a scattering diagnostic used to measure density fluctuations in laboratory plasmas, to a tokamak discharge modeled with the gyro nonlinear gyrokinetic code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)]. The synthetic PCI includes the spatial response of the experimental diagnostic, primarily implemented as a line integral of plasma density along the beam path, and the minimum and maximum wavenumber response resulting from the detection scheme. The synthetic PCI can be used for comparisons between gyro and experiment as well as studies of the PCI response.

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Section: Response Of Phase Contrast Imagingmentioning

confidence: 99%

Development of a synthetic phase contrast imaging diagnostic

Rost¹,

Lin²,

Porkoláb³

2010

Physics of Plasmas

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“…The value of the safety factor on axis was generally allowed to decrease below unity as, on the Alcator C-Mod tokamak, sawteeth are generally observed to trigger L-I transitions [14]. The electron temperature was measured by the Thomson Scattering (TS) [20] as well as by the Electron Cyclotron Emission (ECE) diagnostics [21]; density was gauged by TS, CO 2 Interferometer [22] and microwave reflectometry [23]; ion temperature along with toroidal and poloidal velocities were obtained by the Charge Exchange Recombination diagnostic (CER) [24]; fluctuations were monitored by a number of diagnostics including Beam Emission Spectroscopy (BES) [25], Doppler Back Scattering (DBS) [26] along with the Phase Contrast Imaging (PCI) [27,28], whose results are the focus of this paper. The normalized radial coordinate used throughout this paper is defined as the squared root of the normalized toroidal flux.…”

Section: Experimental Set-upmentioning

confidence: 99%

Characterization of density fluctuations during the search for an I-mode regime on the DIII-D tokamak

et al. 2015

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Abstract. The I-mode regime, routinely observed on the Alcator C-Mod tokamak, is characterized by an edge energy transport barrier without an accompanying particle barrier and with broadband instabilities, known as Weakly Coherent Modes (WCM), believed to regulate particle transport at the edge. Recent experiments on the DIII-D tokamak exhibit I-mode characteristics in various physical quantities. These DIII-D plasmas evolve over long periods, lasting several energy confinement times, during which the edge electron temperature slowly evolves towards an H-modelike profile, while maintaining a typical L-mode edge density profile. During these periods, referred to as I-mode phases, the radial electric field at the edge also gradually reaches values typically observed in H-mode. Density fluctuations measured with the Phase Contrast Imaging diagnostic during Imode phases exhibit three features typically observed in H-mode on DIII-D, although they develop progressively with time and without a sharp transition: the intensity of the fluctuations is reduced; the frequency spectrum is broadened and becomes nonmonotonic; two dimensional space-time spectra appear to approach those in H-mode, showing phase velocities of density fluctuations at the edge increasing to about 10 km/s. However, in DIII-D there is no clear evidence of the WCM. Preliminary linear gyro-kinetic simulations are performed in the pedestal region with the GS2 code and its recently upgraded model collision operator that conserves particles, energy and momentum. The increased bootstrap current and flow shear generated by the temperature pedestal are shown to decrease growth rates, thus possibly generating a feedback mechanism that progressively stabilizes fluctuations.

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“…These fluctuation diagnostics include Beam Emission Spectroscopy (BES) to look at long wavelength (in the ITG range) turbulence [16], Doppler Backscattering (DBS) looking at intermediate wavelength modes by a technique closely related to reflectometry [17], and phase-contrast imaging (PCI), examining chord-averaged fluctuations in a broad range of wavenumbers and a wide frequency range [18]. The fluctuations in the NB-only case are compared with those observed in the EC+NB case, with each diagnostic.…”

Section: Effects On Observed Density Fluctuationsmentioning

confidence: 99%

Application of ECH to the Study of Transport in ITER Baseline Scenario-like Discharges in DIII-D

Pinsker

Austin

Ernst

et al. 2015

EPJ Web of Conferences

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Abstract. Recent DIII-D experiments in the ITER Baseline Scenario (IBS) have shown strong increases in fluctuations and correlated reduction of confinement associated with entering the electron-heating-dominated regime with strong electron cyclotron heating (ECH). The addition of 3.2 MW of 110 GHz EC power deposited at ~0.42 to IBS discharges with ~3 MW of neutral beam injection causes large increases in low-k and medium-k turbulent density fluctuations observed with Doppler backscatter (DBS), beam emission spectroscopy (BES) and phase-contrast imaging (PCI) diagnostics, correlated with decreases in the energy, particle, and momentum confinement times. Power balance calculations show the electron heat diffusivity e increases significantly in the mid-radius region 0.4< <0.8, which is roughly the same region where the DBS and BES diagnostics show the increases in turbulent density fluctuations. Confinement of angular momentum is also reduced during ECH. Studies with the TGYRO transport solver show that the model of turbulent transport embodied in the TGLF code quantitatively reproduces the measured transport in both the neutral beam (NB)-only and in the NB plus EC cases. A simple model of the decrease in toroidal rotation with EC power is set forth, which exhibits a bifurcation in the rotational state of the discharge. The ITER Baseline Scenario on DIII-DProperties of discharges in the baseline operating scenario for ITER [ITER-similar shape, N~2 , q 95~3 , ELMing H-mode with H 98 (y,2)~1] are the subject of ongoing experiments on DIII-D; recent work has extended the similarities to include ITER-relevant (low) torque and T e /T i~1 (dominant electron heating). The aim of this work is to study confinement in the low rotation, dominant electron heating regime (as from alphas in a burning D-T plasma in ITER). The advantage of electron cyclotron heating as a principal heating tool for these studies is the fine control of the power deposition profile that is possible and the fact that all of the power goes to the electrons allows detailed study of the transport properties of the discharge.In this paper, we compare ITER Baseline Scenario (IBS) discharges with neutral beam (NB) heating alone with otherwise identical discharges in which some of the NB power is replaced with electron cyclotron heating (ECH) (110 GHz X2, radial launch, deposition at ~0.45). We examine the global confinement properties in the two cases, and analyze the radial profiles of diffusivities in both the electron and ion channels. Next, we carry out linear microstability analysis with the GS2 model to evaluate stability of ion temperature gradient (ITG) (long wavelength) and electron temperature gradient (ETG)/trapped electron modes (TEMs) (shorter wavelength), and compare the growth rates of the long wavelength modes with the measured shearing rates to estimate the effect of the E r shear on the ITG modes. Using the TGYRO nonlinear transport solver we evaluate the turbulent transport model embodied in the TGLF code, and find that the measured profi...

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Localized measurement of short wavelength plasma fluctuations with the DIII-D phase contrast imaging diagnostic

Cited by 30 publications

References 24 publications

Development of a synthetic phase contrast imaging diagnostic

Development of a synthetic phase contrast imaging diagnostic

Characterization of density fluctuations during the search for an I-mode regime on the DIII-D tokamak

Application of ECH to the Study of Transport in ITER Baseline Scenario-like Discharges in DIII-D

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