Soft-X-ray tomography on Alcator C

Granetz, R.; Camacho, J. F.

doi:10.1088/0029-5515/25/6/008

Cited by 58 publications

(29 citation statements)

References 9 publications

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“…The chordintegrated spectra may then be inverted 5,6 to obtain the radial emissivity profile, or compared directly to a model of the distribution function using a synthetic diagnostic. Antenna parameters such as n ʈ , forward, and reflected powers provide boundary conditions, but not information about wave propagation and damping when coupled to the plasma.…”

Section: Introductionmentioning

confidence: 99%

Hard x-ray diagnostic for lower hybrid experiments on Alcator C-Mod

Liptac

Parker

Tang

et al. 2006

Review of Scientific Instruments

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A new interferometry-based electron density fluctuation diagnostic on Alcator C-Moda) Rev. Sci. Instrum. 83, 10E301 (2012); 10.1063/1.4728090 Wide-view charge exchange recombination spectroscopy diagnostic for Alcator C-Moda) Rev. Sci. Instrum. 79, 10F529 (2008); 10.1063/1.2979865 Compact multichannel neutral particle analyzer for measurement of energetic charge-exchanged neutrals in Alcator C-Mod Rev. Sci. Instrum. 77, 083501 (2006);Alcator C-Mod's lower hybrid current drive ͑LHCD͒ system allows the exploration of advanced tokamak ͑AT͒ regimes at densities relevant to ITER and fusion reactors. The location of the LHCD is critical to AT performance and may be inferred by measuring the nonthermal bremsstrahlung emission in the hard x-ray ͑HXR͒ region. A pinhole camera using an array of 32 CdZnTe detectors is used to image energies in the 20-200 keV range. Detectors and pulse processing electronics are integrated into a compact and modular package making extensive use of printed circuit board and surface mount technology. The system also makes use of fast digitization and software signal processing techniques. An ambient environment of neutrons, gammas, and high rf power requires careful shielding. Shielding is studied using the neutron and photon transport code MCNP. The design of the diagnostic is presented along with background measurements in lieu of LHCD fast electrons. Background measurements are then compared to advanced modeling results to predict the power threshold for meaningful HXR data for a H-mode target plasma.

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Section: Introductionmentioning

confidence: 99%

Hard x-ray diagnostic for lower hybrid experiments on Alcator C-Mod

Liptac

Parker

Tang

et al. 2006

Review of Scientific Instruments

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“…Observations made on Alc_tor C-Mod using soft x-ray detector arrays and the Signals from two arrays may be use1 to form two dimensional emission profiles using tomographic reconstruction techniques [59]. Beryllium filters fifty microns thick are employed on two arrays to limit the diode's response to emissions with energies above 1 keV [59].…”

Section: Density Perturbations On the Q=l Rational Flux Surfacementioning

confidence: 99%

The design and performance of a twenty barrel hydrogen pellet injector for Alcator C-Mod

Urbahn¹

1994

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A twenty barrel hydrogen pellet injector has been designed, built and tested both in the laboratory and on the Alcator C-Mod Tokamak at MIT. The injector functions by firing pellets of frozen hydrogen or deuterium deep into the plasma discharge for the purpose of fueling the plasma, modifying the density profile and increasing the global energy confinement time.The design goals of the injector are: 1) Operational flexibility, 2) High reliability, 3) Remote operation with minimal maintenance. These requirements have lead to a single stage, pipe gun design with twenty barrels. Pellets are formed by in-situ condensation of the fuel gas, thus avoiding moving parts at cryogenic temperatures. The injector is the first to dispense with the need for cryogenic fluids and instead uses a closed cycle refrigerator to cool the thermal system components. The twenty barrels of the injector produce pellets of four different size groups and allow for a high degree of flexibility in fueling experiments. Operation of the injector is under PLC control allowing for remote operation, interlocked safety features and automated pellet manufacturing. The injector has been extensively tested and shown to produce pellets reliably with velocities up to 1400 m/sec. During the period from September to November of 1993, the injector was successfully used to fire pellets into over fifty plasma discharges. Experimental results include data on the pellet penetration into the plasma using an advanced pellet tracking diagnostic with improved time and spatial response. Data from the tracker indicates pellet penetrations were between 30 and 86 percent of the plasma minor radius. Line averaged density increases of up to 300 percent were recorded with peak densities of just under 1 × 1021/ m 3, the highest achieved on C-Mod to date. A comparison is made " between the ablation source function derived from tracker data with that predicted by four different variations of the neutral shield model. Results suggest rapid heat flow from the interior of the plasma maintains temperatures on the ablation flux surface. Localized " density perturb_ttions with a specific m=l,n=l structure and location on the q=l flux surface were observed following injection.

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“…Historically, the techniques for tomographic inversions for plasma physics applications started from a standard Abel inversion 1 method which was used for circular plasma cross sections in tokamaks. Other methods based on linear least squares techniques 2 and restricted Fourier analysis [3][4][5][6] have been developed for asymmetrically elongated plasmas. The EBITA 7 and MaxEnt 8 methods with which our results will be compared, are based on numerical iterative algorithms and able to recover structures with localized perturbations from high harmonics.…”

Section: Introductionmentioning

confidence: 99%

Bayesian soft X-ray tomography using non-stationary Gaussian Processes

Liu

Svensson

Thomsen

et al. 2013

Review of Scientific Instruments

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In this study, a Bayesian based non-stationary Gaussian Process method for the inference of soft X-ray emissivity distribution along with its associated uncertainties, has been developed. For the investigation of equilibrium condition and fast magnetohydrodynamic (MHD) behaviors in nuclear fusion plasmas, it is of importance to infer, especially in the plasma center, spatially resolved soft X-ray profiles from a limited number of noisy line integral measurements. For this ill-posed inversion problem, Bayesian probability theory can provide a posterior probability distribution over all possible solutions under given model assumptions. Specifically, the use of a non-stationary Gaussian Process to model the emission allows the model to adapt to the varying length scales of the underlying diffusion process. In contrast to other conventional methods, the prior regularization is realized in a probability form which enhances the capability of uncertainty analysis, in consequence, scientists who concern the reliability of their results will benefit from it. Under the assumption of normally distributed noise, the posterior distribution evaluated at a discrete number of points becomes a multivariate normal distribution whose mean and covariance are analytically available, making inversions and calculation of uncertainty fast. Additionally, the hyper-parameters embedded in the model assumption can be optimized through a Bayesian Occam"s Razor formalism and thereby automatically adjust the model complexity. This method is shown to produce convincing reconstructions and good agreements with independently calculated results from the Maximum Entropy (MaxEnt) and Equilibrium-Based Iterative Tomography Algorithm (EBITA) methods.

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Soft-X-ray tomography on Alcator C

Cited by 58 publications

References 9 publications

Hard x-ray diagnostic for lower hybrid experiments on Alcator C-Mod

Hard x-ray diagnostic for lower hybrid experiments on Alcator C-Mod

The design and performance of a twenty barrel hydrogen pellet injector for Alcator C-Mod

Bayesian soft X-ray tomography using non-stationary Gaussian Processes

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