1,720,967 research outputs found
Development of fast RF spectrometer system for MHD detection
No full textDuring the 2010 KSTAR plasma experiment, unidentified spiky RF pulses (< 3 GHz, similar to 10 mu s) were detected by an electron cyclotron emission imaging (ECEI) system at the occurrence of large MHD instabilities such as edge localized modes (ELMs). A fast RF spectrometer system has been constructed for direct detection of the RF radiation in the range of 0.5 similar to 3.0 GHz to study the spectral evolution of the RF spikes in more detail.X1153sciescopu
Large-Aperture Broadband Polarization Rotator for the KSTAR ECE Imaging System
No full textThe electron cyclotron emission (ECE) imaging system installed in the KSTAR tokamak has been designed for 2D electron temperature measurement based on the X-mode 2nd harmonic ECE radiation at a magnetic field similar to 2 T. For operation at higher magnetic fields (similar to 3-3.5 T) in the future, the frequency of the X-mode 2nd harmonic ECE (> 170 GHz) exceeds the detectable range of the system. Instead of extending the detector frequency range, large-aperture half-wave plates for polarization rotation have been developed to utilize the O-mode fundamental ECE, which is within the detectable range (similar to 65-135 GHz) of the present system.X116sciescopu
Sawtooth Precursor Oscillations on DIII-D
No full textThe sawtooth oscillation, observed in tokamak plasmas with a central safety factor of less than unity, is a periodic disruptive instability characterized by a slow ramping of central plasma density and temperature, followed by a fast relaxation resulting in flattening of both profiles. Elongated neutral-beam-heated discharges on the DIII-D tokamak exhibit multiple precursor oscillations with mode number m/n = 1/1. The dominant m/n = 1/1 mode oscillates at the plasma rotation frequency. A downshifted mode also appears early in the sawtooth ramp. A normalization of electron cyclotron emission imaging data that removes the contribution of slow electron temperature profile evolution reveals that both modes are consistent with an underlying quasi-interchange plasma displacement.X1132sciescopu
Design of the reflective optics for Tore Supra ECEI system
No full textA 2D electron cyclotron emission (ECE) imaging system for Tore Supra is under design for studying the MHD physics of the magnetically confined plasma such as sawteeth, tearing modes, and turbulent fluctuations. Complex beam path due to the tight access in Tore Supra led to the design of reflective optics made of 6 or more large cylindrical/flat mirrors. The total path length of the ECE beam is about 11 m, including almost 4 m inside the vacuum vessel. The imaging property of the optics has been estimated using the Gaussian beam simulation and ray transfer analysis. The possible setups for the optical alignment of the diagnostic and the operation scenarios with single-or dual-array measurement system are discussed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4732852]X1132sciescopu
Evaluation of the imaging properties of Microwave Imaging Reflectometry
No full textMicrowave Imaging Reflectometry (MIR) has been developed for unambiguous measurement of electron density fluctuations in fusion plasmas. The loss of phase information limiting the use of conventional reflectometry can be minimized by a large aperture imaging optics and an array of detectors in the MIR embodiment. The evaluation of the optical system is critical for precise reconstruction of the fluctuations. The optical systems of the prototype TEXTOR MIR [2] and newly-designed KSTARMIR [5] systems have been tested with a corrugated target simulating density fluctuations at the cut-off surface. The reconstructed phase from the MIR system has been compared to the directly measured phase of corrugations taking into account the rotational speed of the target. The effects of optical aberrations and interference between lenses on the phase reconstruction have been investigated by the 2D amplitude measurement of the reflected waves and the diffraction-based optical simulations. (CODE V) A preliminary design of the KSTAR MIR optics has been suggested which can minimize the aberration and interference effects.X112sciescopu
Imaging Techniques for Microwave Diagnostics
No full textImaging diagnostics, such as Electron Cyclotron Emission Imaging (ECEI) and Microwave Imaging Reflectometry (MIR), exhibit unique characteristics that make them particularly well suited to the validation of theoretical models for plasma instabilities and turbulent fluctuations. A 2-D picture of plasma phenomena is provided unambiguously, from localized, time-resolved measurements. After more than a decade of development and successful demonstrations on RTP [1,2] and TEXTOR [3, 4, 5, 6], ECEI has come into maturity as an electron temperature diagnostic technique, and systems at ASDEX-UG [7] and DIII-D [8] are making regular contributions to plasma physics. The next generation ECEI diagnostic is currently being installed on KSTAR [9, 10]. MIR is a radar reflectometric density fluctuation diagnostic, and hence the perfect complement to ECEI when realized to simultaneously image the same plasma volume. Experiments with MIR at TEXTOR have guided a recent surge in analysis and laboratory experiments aimed at resolving remaining issues [11, 12]. Both techniques are discussed in this tutorial with brief examples of data which illustrate the capabilities of these techniques and motivate future development for application on ITER and burning plasma experiments to come. (c) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimX1199sciescopu
Microwave imaging reflectometry for KSTAR
No full textA microwave imaging reflectometry (MIR) system with two probing frequencies is being developed for 2D measurement of electron density fluctuations for KSTAR plasmas. The two-frequency probe beam enables simultaneous measurement of density fluctuations at two cutoff layers. Laboratory test of two approaches (reflective and refractive optics) has suggested an extreme care is needed in designing the optics considering problems such as aberration issues and standing waves. Currently, an optics based on refractive element is being designed with the minimum number of lens. The detector system consists of array detectors and electronics for the fluctuation phase reconstruction. The two-frequency MIR system will be installed for the 2012 KSTAR campaign and will be expanded to five frequencies by 2014.X113sciescopu
Characteristics of the first H-mode discharges in KSTAR
No full textTypical ELMy H-mode discharges have been obtained in the KSTAR tokamak with the combined auxiliary heating of neutral beam injection (NBI) and electron cyclotron resonant heating (ECRH). The minimum external heating power required for the L-H transition is about 0.9MW for a line-averaged density of similar to 2.0 x 10(19) m(-3). There is a clear indication of the increase in the L-H threshold power with decreasing density for densities lower than similar to 2 x 10(19) m(-3). The L-H transitions typically occurred shortly after the beginning of plasma current flattop (I-p = 0.6 MA) period and after the fast shaping to a highly elongated double-null divertor configuration. The maximum heating power available was marginal for the L-H transition, which is also implied by the relatively slow transition time (>10 ms) and the synchronization of the transition with large sawtooth crashes. The initial analysis of thermal energy confinement time (tau(E)) indicates that tau(E) is higher than the prediction of multi-machine scaling laws by 10-20%. A clear increase in electron and ion temperature in the pedestal is observed in the H-mode phase but the core temperature does not change significantly. On the other hand, the toroidal rotation velocity increased over the whole radial range in the H-mode phase. The measured ELM frequency was around 10-30 Hz for the large ELM bursts and 50-100 Hz for the smaller ones. In addition, very small and high frequency (200-300 Hz) ELMs appeared between large ELM spikes when the ECRH is added to the NBI-heated H-mode plasmas. The drop of total stored energy during a large ELM is up to 5% in most cases.X112121sciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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