Download or read book Fast Motional Stark Effect Diagnostic Measurements of Hybrid like Magnetic Flux pumping and Neoclassical Tearing Modes in the DIII D Tokamak written by Joshua Daniel King and published by . This book was released on 2011 with total page 105 pages. Available in PDF, EPUB and Kindle. Book excerpt: MSE measurements reveal hybrid-like flux-pumping associated with 2/1 NTM-ELM coupling. Analysis of MSE signals using digital lock-in amplifiers shows the strength of the flux-pumping is more than twice that of typical hybrid discharges. This flux-pumping maintains the minimum safety factor above unity, thereby avoiding sawteeth. The strength of the flux-pumping and ELM-NTM coupling have a clear upward dependence on normalized beta and NTM-pedestal proximity. The size of the island does not appear to effect flux-pumping, except that the mode must be present, suggesting the island chain serves as a radial pivot surface around which poloidal flux is pumped from the core to the edge. This result implies that higher normalized fusion performance (lower q95 and higher beta) may be achieved in hybrid discharges that contain a partially suppressed 2/1 NTM. ELM-NTM coupling consists of an Alfvénic timescale drop in the island width followed by a resistive recovery. The recovery phase is successfully modeled using the modified Rutherford equation. The depth of the drop in island width increases as the size of the ELM increases. To aid in the design of a highly resolved MSE pedestal measurement, full spectral analysis was preformed on existing edge channels. This analysis has revealed that coherent core MHD oscillations cause interference with present dual PEM polarimeters. Avoiding this interference requires a dedicated pedestal polarimeter with second harmonic frequencies greater than those of MHD fluctuations.

Download or read book Development of the B Stark Motional Stark Effect Diagnostic for Measurements of the Internal Magnetic Field in the DIII D Tokamak written by Novimir Antoniuk Pablant and published by . This book was released on 2010 with total page 414 pages. Available in PDF, EPUB and Kindle. Book excerpt: A new diagnostic, B-Stark, has been developed at the DIII-D tokamak for measurements of the magnitude and direction of the internal magnetic field. The B-Stark system is a version of a motional Stark effect (MSE) diagnostic based on the Stark split D/[alpha] emission from injected neutral beams. This diagnostic uses the spacing of the Stark lines to measure the magnitude of the magnetic field, and the intensities of the [pi]3 and [sigma]1 lines to measure the magnetic pitch angle. These lines originate from the same upper level, and are therefore not dependent on the n=3 level populations. The measurement of the magnetic pitch angle requires a specific viewing geometry with respect to the neutral beams, which is provided by the B-Stark diagnostic installation. The B-Stark technique may have advantages over MSE polarimetry diagnostics in future devices with high densities and temperatures, such as ITER. Under these conditions coatings on the plasma facing mirrors are expected, which can cause changes in the polarization state of the reflected light. The B-Stark technique is insensitive to the polarization direction, and can calibrate for polarization dependent transmission by using an in-situ beam-into-gas calibration. This dissertation describes the development and characterization of the B-Stark diagnostic. The hardware design and spectral fitting techniques are discussed in detail. Calibration procedures are described including the in-situ determination of the beam emission line profiles, viewing geometry and properties of the collection optics. The performance of the system is evaluated over the range of plasma conditions accessible at DIII-D. Measurements of the magnetic field have been made with toroidal fields in the range 1.2 - 2.1Tesla, plasma currents in the range 0.5 - 2.0MA, densities between 1.7 - 9.0 x 1019m−3, and neutral beam voltages between 50 - 81keV. These results are compared to values found from plasma equilibrium reconstructions (EFIT) and the MSE polarimetry system on DIII-D. The B-Stark system has been shown to provide measurements with a random errors as low as 0.2-0.3° in the magnetic pitch angle and 0.001-0.002T in [B]. Finally, proposed future improvements for the B-Stark diagnostic are presented.

Download or read book Motional Stark Effect Diagnostic Expansion for the DIII D Tokamak written by and published by . This book was released on 2005 with total page 29 pages. Available in PDF, EPUB and Kindle. Book excerpt: A repositioning of a heating neutral beam on the DIII-D tokamak provides an opportunity to expand and improve the Motional Stark Effect diagnostic (MSE) used to constrain the current profile. D[alpha] emission from the neutral beam is split into components parallel ([pi]) and perpendicular ([sigma]) to the total electric field ETotal = vxB + Eplasma. The MSE diagnostic measures the polarization of the [sigma] component to determine the local magnetic field pitch angle Bz/B[phi] and the local radial plasma electric field ER. This is typically done using the EFIT current profile reconstruction code. Two independent measurements of the pitch angle [gamma] at each radius are required to differentiate the contributions from the Stark and plasma electric fields. Presently, three MSE diagnostics provide multiple views of a single neutral beam. Our ability to accurately differentiate Bz and ER is limited because these views do not overlap with sufficient radial resolution in some locations, and this limits the accuracy of the current profile reconstructions. The beam rearrangement allows us to build a fourth MSE view of a second beam injected counter to the plasma current. The combination of the new view with the old will improve radial resolution about a factor of 3, reduce ER uncertainty by a factor of 2 in the core and 5-6 in the edge, and reduce Bz uncertainty by 20-30%. The design of the new system is presented in this paper, focusing on the mechanical and optical details at the tokamak port on which it will be installed.

Download or read book Computational Modeling of Neoclassical and Resistive MHD Tearing Modes in Tokamaks written by Thomas A. Gianakon and published by . This book was released on 1996 with total page 316 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book An Overview Of The Motional Stark Effect Diagnostic On DIII D And Design Work For An ITER MSE written by and published by . This book was released on 2007 with total page 6 pages. Available in PDF, EPUB and Kindle. Book excerpt: The advanced tokamak research program at DIII-D relies critically on the measurement of the current density profile. This was made possible by the development of a Motional Stark Effect (MSE) polarimeter that was first installed in 1992. Three major upgrades have since occurred, and improvements in our understanding of critical performance issues and calibration techniques are ongoing. In parallel with these improvements, we have drawn on our DIII-D experience to begin studies and design work for MSE on burning plasmas and ITER. This paper first reviews how Motional Stark Effect polarimetry (MSE) is used to determine the tokamak current profile. It uses the DIII-D MSE system as an example, and shows results from the latest upgrade that incorporates an array of channels from a new counter-Ip injected neutral beam. The various calibration techniques presently used are reviewed. High-leverage or unresolved issues affecting MSE performance and reliability in ITER are discussed. Next, we show a four-mirror collection optics design for the two ITER MSE views. Finally, we discuss measurements of the polarization properties of a few candidate mirrors for the ITER MSE.

Download or read book Neoclassical Tearing Modes in Tokamak Fusion Test Reactor Experiments Part 1 Measurements of Magnetic Islands and Delta Prime written by and published by . This book was released on 1997 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Tearing-type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from the electron cyclotron emission (ECE) diagnostic, and an experiment where the plasma major radius is rapidly shifted ("Jog" experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing-mode eigenfunction mapped from the jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory.

Download or read book OBSERVATION OF MHD INSTABILITY AND DIRECT MEASUREMENT OF LOCAL PERTURBED MAGNETIC FIELD USING MOTIONAL STARK EFFECT DIAGNOSTIC written by and published by . This book was released on 2003 with total page 21 pages. Available in PDF, EPUB and Kindle. Book excerpt: OAK-B135 The local oscillating component of the poloidal magnetic field in plasma associated with MHD instabilities has been measured using the motional Stark effect (MSE) diagnostic on the DIII-D tokamak. The magnetic field perturbations associated with a resistive wall mode (RWM) rotated by internal coils at 20 Hz was measured using the conventional MSE operation mode. These first observations of perturbations due to a MHD mode were obtained on multiple MSE channels covering a significant portion of the plasma and the radial profile o the amplitude of the perturbed field oscillations was obtained. The measured profile is similar to the profile of the amplitude of the electron temperature oscillation measured by electron cyclotron emission (ECE) measurements. In a new mode of measurement, the amplitude of a tearing mode rotating at a high frequency ((almost equal to) 7 kHz) was observed using the spectral analysis of high frequency MSE data on one channel. The spectrum consists of the harmonics of the light modulation employed in the MSE diagnostics, their mutual beat frequencies and their beat frequencies with the rotation frequency of the tearing mode. The value and time variation of the frequency of the observed perturbations is in good agreement with that measured by Mirnov probes and ECE. The paper demonstrates that the MSE diagnostic can be used for observing low and high frequency phenomena such as MHD instabilities and electromagnetic turbulence.

Download or read book Neoclassical Tearing Modes in Tokamak Fusion Test Reactor Experiments Part I Measurements of Magnetic Islands and Delta Prime written by and published by . This book was released on 1997 with total page 33 pages. Available in PDF, EPUB and Kindle. Book excerpt: Tearing type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from electron cyclotron emission (ECE) diagnostic and an experiment where the plasma major radius is rapidly shifted ('Jog' experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing mode eigenfunction mapped from the Jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory.

Download or read book The Multichannel Motional Stark Effect Diagnostic on TFTR written by and published by . This book was released on 1992 with total page 17 pages. Available in PDF, EPUB and Kindle. Book excerpt: Although the q profile plays a key role in theories of instabilities and plasma equilibrium, it has been quite difficult to measure until the recent development of the motional Stark effect (MSE) diagnostic. A multichannel motional Stark effect polarimeter system has recently been installed on the Tokamak Fusion Test Reactor (TFTR). The diagnostic can measure the magnetic field pitch angle (?{sub p} = tan−1 (B{sub T})/(B{sup p})) at ten radial locations. The doppler shifted D{sub alpha} radiation from a TFTR heating beam is viewed near tangential to the toroidal magnetic field via a re-entrant front surface reflecting mirror. The field of view covers from inboard of the magnetic axis to near the outboard edge of the plasma with a radial spatial resolution of 3--5 cm. A high throughput f/2 optics system results in an uncertainty for?{sub p} of -0.1°--0.2° with a time resolution of -5--10 ms. Initial pinch angle profiles from TFTR have been obtained. The MSE data is consistent with the estimated magnetic axis position from external magnetic measurements and the q=1 radius is in good agreement with the inversion radius from the electron cyclotron emission temperature measurements.

Download or read book DIII D Disruption Experiment Results written by and published by . This book was released on 1995 with total page 4 pages. Available in PDF, EPUB and Kindle. Book excerpt: Tokamak discharges are frequently ended by disruptions, an instability leading to a very sudden termination of the plasma. Although numerous studies of disruptions have been made, critical data is still needed to develop theoretical models for fundamental aspects of the disruption. This paper describes simultaneous profile measurements of the current, temperature, and density during the disruptive phase of a beam heated single-null divertor discharge in the DIII-D tokamak. The stored energy is lost early in the first phase of the disruption (thermal quench) after which the central temperature is a few hundred eV, and following the start of the current decay (current quench) the temperature has fallen to 100 eV. The current profile measured using the Motional Stark Effect (MSE) diagnostic is observed to flatten rapidly (

Download or read book Velocity space Resolved Fast ion Measurements in the DIII D Tokamak written by Christopher Michael Muscatello and published by . This book was released on 2012 with total page 150 pages. Available in PDF, EPUB and Kindle. Book excerpt: Superthermal ions in tokamak plasmas play a critical role in heating and current drive, and their confinement within the core of the plasma is crucial for obtaining ignition and sustaining burn in future reactors. At the DIII-D tokamak, a suite of fast-ion measurements is available to diagnose various properties of the superthermal population. This thesis work involves a contribution to DIII-D's fast-ion diagnostic collection: the 2nd generation fast-ion deuterium alpha (2G FIDA) detector. FIDA works on the principle of measuring the light that is emitted from neutralized fast ions that undergo charge exchange events with injected neutral atoms. 2G FIDA complements the other FIDA installations on DIII-D with its unique velocity-space sampling volume. Output from a synthetic diagnostic code (FIDAsim) that predicts FIDA emission levels is compared with measurements from 2G FIDA. We find that, while the predicted and measured shapes of the FIDA spectra agree well, the absolute magnitude of the spectral amplitudes are inconsistent. Results from various FIDAsim trials are presented adjusting several parameters, and it is hypothesized that mischaracterization of the diagnostic neutral beams is a major source of error. Instabilities in tokamaks can cause fast-ion transport. The sawtooth instability is particularly important because the crash phase has been observed to cause reductions up to 50% in the central fast-ion density. Passing ions of all energies are redistributed, but only low energy trapped ions suffer redistribution. The observations are consistent with transport by flux-attachment. Comparisons with theory suggest that the intensity of sawtooth-induced transport depends on the magnitude of toroidal drift. Instabilities characterized by toroidal and poloidal mode numbers and real frequency can coherently interact with energetic particles through mode-particle resonances. During a sawtooth crash, even fast ions whose energies are above the threshold for flux-attachment can experience transport if their orbits satisfy the bounce-precessional resonance condition. On DIII-D, a spatially localized population of beam ions accelerated above the injection energy by ion-cyclotron radio frequency (ICRF) heating is diminished at a sawtooth crash. Furthermore, fast-ion losses concurrent with sawtooth crashes are observed. Calculations show that mode-particle resonances could be responsible. Transport of energetic particles by resonant interactions pertains to many types of instabilities; other examples besides sawteeth will also be presented. Analysis shows that large amplitude modes cause significant resonant transport of fast particles. Even small amplitude modes can resonantly drive transport if multiple harmonics exist.

Download or read book Neoclassical Tearing Modes in Tokamak Fusion Test Reactor Experiments Part I Measurements of Magnetic Islands and Delta written by Zuoyang Chang and published by . This book was released on 1997 with total page 33 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The multichannel Motional Stark Effect diagnostic in the JFT 2M tokamak written by Kensaku Kamiya and published by . This book was released on 2001 with total page 17 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Studies of Turbulence and Flows in the DIII D Tokamak written by Jon Clark Hillesheim and published by . This book was released on 2012 with total page 307 pages. Available in PDF, EPUB and Kindle. Book excerpt: Understanding the turbulent transport of particles, momentum, and heat continues to be an important goal for magnetic confinement fusion energy research. The turbulence in tokamaks and other magnetic confinement devices is widely thought to arise due to linearly unstable gyroradius-scale modes. A long predicted characteristic of these linear instabilities is a critical gradient, where the modes are stable below a critical value related to the gradient providing free energy for the instability and unstable above it. In this dissertation, a critical gradient threshold for long wavelength ($k_{\theta} \rho_s \lesssim 0.4$) electron temperature fluctuations is reported, where the temperature fluctuations do not change, within uncertainties, below a threshold value in $L_{T_e}^{-1}=\nabla T_e / T_e$ and steadily increase above it. This principal result, the direct observation of a critical gradient for electron temperature fluctuations, is also the first observation of critical gradient behavior for \textit{any} locally measured turbulent quantity in the core of a high temperature plasma in a systematic experiment. The critical gradient was found to be $L_{T_e}^{-1}_{crit}=2.8 \pm 0.4 \ \mathrm{m}^{-1}$. The experimental value for the critical gradient quantitatively disagrees with analytical predictions for its value. In the experiment, the local value of $L_{T_e}^{-1}$ was systematically varied by changing the deposition location of electron cyclotron heating gyrotrons in the DIII-D tokamak. The temperature fluctuation measurements were acquired with a correlation electron cyclotron emission radiometer. The dimensionless parameter $\eta_e=L_{n_e}/L_{T_e}$ is found to describe both the temperature fluctuation threshold and a threshold observed in linear gyrofluid growth rate calculations over the measured wave numbers, where a rapid increase at $\eta_e \approx 2$ is observed in both. Doppler backscattering (DBS) measurements of intermediate-scale density fluctuations also show a frequency-localized increase on the electron diamagnetic side of the measured spectrum that increases with $L_{T_e}^{-1}$. Measurements of the crossphase angle between long wavelength electron density and temperature fluctuations, as well as measurements of long wavelength density fluctuation levels were also acquired. Multiple aspects of the fluctuation measurements and calculations are individually consistent with the attribution of the critical gradient to the $\nabla T_e$-driven trapped electron mode. The accumulated evidence strongly enforces this conclusion. The threshold value for the temperature fluctuation measurements was also within uncertainties of a critical gradient for the electron thermal diffusivity found through heat pulse analysis, above which the electron heat flux and electron temperature profile stiffness rapidly increased. Toroidal rotation was also systematically varied with neutral beam injection, which had little effect on the temperature fluctuation measurements. The crossphase measurements indicated the presence of different instabilities below the critical gradient depending on the neutral beam configuration, which is supported by linear gyrofluid calculations. In a second set of results reported in this dissertation, the geodesic acoustic mode is investigated in detail. Geodesic acoustic modes (GAMs) and zonal flows are nonlinearly driven, axisymmetric ($m=0,\ n=0$ potential) $E \times B$ flows, which are thought to play an important role in establishing the saturated level of turbulence in tokamaks. Zonal flows are linearly stable, but are driven to finite amplitude through nonlinear interaction with the turbulence. They are then thought to either shear apart the turbulent eddies or act as a catalyst to transfer energy to damped modes. Results are presented showing the GAM's observed spatial scales, temporal scales, and nonlinear interaction characteristics, which may have implications for the assumptions underpinning turbulence models towards the tokamak edge ($r/a \gtrsim 0.75$). Measurements in the DIII-D tokamak have been made with multichannel Doppler backscattering systems at toroidal locations separated by $180^{\circ}$; analysis reveals that the GAM is highly coherent between the toroidally separated systems ($\gamma> 0.8$) and that measurements are consistent with the expected $m=0,\ n=0$ structure. Observations show that the GAM in L-mode plasmas with $\sim 2.5-4.5$ MW auxiliary heating occurs as a radially coherent eigenmode, rather than as a continuum of frequencies as occurs in lower temperature discharges; this is consistent with theoretical expectations when finite ion Larmor radius effects are included. The intermittency of the GAM has been quantified, revealing that its autocorrelation time is fairly short, ranging from about 4 to about 15 GAM periods in cases examined, a difference that is accompanied by a modification to the probability distribution function of the $E \times B$ velocity at the GAM frequency. Conditionally-averaged bispectral analysis shows the strength of the nonlinear interaction of the GAM with broadband turbulence can vary with the magnitude of the GAM. Data also indicates a wave number dependence to the GAM's interaction with turbulence. Measurements also showed the existence of additional low frequency zonal flows (LFZF) at a few kilohertz in the core of DIII-D plasmas. These LFZF also correlated toroidally. The amplitude of both the GAM and LFZF were observed to depend on toroidal rotation, with both types of flows barely detectable in counter-injected plasmas. In a third set of results the development of diagnostic hardware, techniques used to acquire the above data, and related work is described. A novel multichannel Doppler backscattering system was developed. The five channel system operates in V-band (50-75 GHz) and has an array of 5 frequencies, separated by 350 MHz, which is tunable as a group. Laboratory tests of the hardware are presented. Doppler backscattering is a diagnostic technique for the radially localized measurement of intermediate-scale ($k_{\theta} \rho_s \sim 1$) density fluctuations and the laboratory frame propagation velocity of turbulent structures. Ray tracing, with experimental profiles and equilibria for inputs, is used to determine the scattering wave number and location. Full wave modeling, also with experimental inputs, is used for a synthetic Doppler backscattering diagnostic for nonlinear turbulence simulations. A number of non-ideal processes for DBS are also investigated; their impact on measurements in DIII-D are found, for the most part, to be small.

Download or read book Physics and Control of Locked Modes in the DIII D Tokamak written by and published by . This book was released on 2017 with total page 12 pages. Available in PDF, EPUB and Kindle. Book excerpt: This Final Technical Report summarizes an investigation, carried out under the auspices of the DOE Early Career Award, of the physics and control of non-rotating magnetic islands ("locked modes") in tokamak plasmas. Locked modes are one of the main causes of disruptions in present tokamaks, and could be an even bigger concern in ITER, due to its relatively high beta (favoring the formation of Neoclassical Tearing Mode islands) and low rotation (favoring locking). For these reasons, this research had the goal of studying and learning how to control locked modes in the DIII-D National Fusion Facility under ITER-relevant conditions of high pressure and low rotation. Major results included: the first full suppression of locked modes and avoidance of the associated disruptions; the demonstration of error field detection from the interaction between locked modes, applied rotating fields and intrinsic errors; the analysis of a vast database of disruptive locked modes, which led to criteria for disruption prediction and avoidance.

Download or read book Tearing Mode Activity in the Phaedrus T Tokamak written by Binsheng Cui and published by . This book was released on 1993 with total page 316 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Investigation of Magnetic Reconnection During a Sawtooth Crash in a High Temperature Tokamak written by R. V. Budny and published by . This book was released on 1994 with total page 21 pages. Available in PDF, EPUB and Kindle. Book excerpt: