Showing papers by "A. J. H. Donné published in 2012"

Advanced Thomson scattering system for high-flux linear plasma generator

Abstract: An advanced Thomson scattering system has been built for a linear plasma generator for plasma surface interaction studies. The Thomson scattering system is based on a Nd:YAG laser operating at the second harmonic and a detection branch featuring a high etendue (f/3) transmission grating spectrometer equipped with an intensified charged coupled device camera. The system is able to measure electron density (n(e)) and temperature (T(e)) profiles close to the output of the plasma source and, at a distance of 1.25 m, just in front of a target. The detection system enables to measure 50 spatial channels of about 2 mm each, along a laser chord of 95 mm. By summing a total of 30 laser pulses (0.6 J, 10 Hz), an observational error of 3% in n(e) and 6% in T(e) (at n(e) = 9.4 × 10(18) m(-3)) can be obtained. Single pulse Thomson scattering measurements can be performed with the same accuracy for n(e) > 2.8 × 10(20) m(-3). The minimum measurable density and temperature are n(e) < 1 × 10(17) m(-3) and T(e) < 0.07 eV, respectively. In addition, using the Rayleigh peak, superimposed on the Thomson scattered spectrum, the neutral density (n(0)) of the plasma can be measured with an accuracy of 25% (at n(0) = 1 × 10(20) m(-3)). In this report, the performance of the Thomson scattering system will be shown along with unprecedented accurate Thomson-Rayleigh scattering measurements on a low-temperature argon plasma expansion into a low-pressure background.

Two-dimensional imaging of edge-localized modes in KSTAR plasmas unperturbed and perturbed by n=1 external magnetic fields

Abstract: The temporal evolution of edge-localized modes (ELMs) has been studied using a 2-D electron cyclotron emission imaging system in the KSTAR tokamak. The ELMs are observed to evolve in three distinctive stages: the initial linear growth of multiple filamentary structures having a net poloidal rotation, the interim state of regularly spaced saturated filaments, and the final crash through a short transient phase characterized by abrupt changes in the relative amplitudes and distance among filaments. The crash phase, typically consisted of multiple bursts of a single filament, involves a complex dynamics, poloidal elongation of the bursting filament, development of a fingerlike bulge, and fast localized burst through the finger. Substantial alterations of the ELM dynamics, such as mode number, poloidal rotation, and crash time scale, have been observed under external magnetic perturbations with the toroidal mode number n = 1.

Characterization of broadband MHD fluctuations during type-II edge localized modes as measured in 2D with ECE-imaging at ASDEX Upgrade

Abstract: The characterization of a broadband fluctuation that is typical for the type-II ELM regime at ASDEX Upgrade has been improved using the 2D capabilities of ECE-imaging. During the transition from the type-I to type-II ELMy phase, it has been found that electron temperature fluctuations form a broadband peak in the 19?65?kHz range. In the type-II phase, this broadband fluctuation reaches a maximum relative amplitude of almost 20% just inside the top of the pedestal. Simultaneously, the electron temperature profile is completely flattened at this location. The 2D distribution of the amplitude of this broadband fluctuation is such that, when averaged over time, a minimum occurs around the mid-plane. From the measurements of the nearby magnetic pickup coils, a similar broadband fluctuation seems visible in the same frequency range. However, this is peaked at a slightly lower frequency and does not show a similar minimum. From the analysis of the fluctuations on small timescales, the poloidal and toroidal mode numbers are estimated to be m???100 and n???21. Furthermore, activity reminiscent of beat waves has been observed, which might partially account for the fluctuation's broadband nature and the seeming velocity variation of single fluctuation passages. Overall, similarities between the characteristics of this broadband fluctuation and various precursors to type-I ELMs suggest that this fluctuation can play an important role in regulating the ELM cycle.

Commissioning of inline ECE system within waveguide based ECRH transmission systems on ASDEX upgrade

Abstract: A CW capable inline electron cyclotron emission (ECE) separation system for feedback control, featuring oversized corrugated waveguides, is commissioned on ASDEX upgrade (AUG). The system is based on a combination of a polarization independent, non-resonant, Mach-Zehnder diplexer equipped with dielectric plate beam splitters [2, 3] employed as corrugated oversized waveguide filter, and a resonant Fast Directional Switch, FADIS [4, 5, 6, 7] as ECE/ECCD separation system. This paper presents an overview of the system, the low power characterisation tests and first high power commissioning on AUG. © Owned by the authors, published by EDP Sciences, 2012.

Feasibility study for a new high resolution Thomson scattering system for the ASDEX Upgrade pedestal

Abstract: A new Thomson scattering diagnostic is proposed for the study of fast plasma dynamics in the pedestal of ASDEX Upgrade. The diagnostic will measure electron temperature and density profiles over a ~ 3 cm wide area in the edge transport barrier region, with ~ 1?2 mm spatial resolution and ~ 10 kHz sampling rate. A challenging goal of the project is the study of the bootstrap current in the plasma pedestal by measuring the distortion and shift of the electron distribution along the toroidal direction. Expected spatial and time resolutions of the current density measurements are ~ 3 mm and ~ 1 ms correspondingly. The new diagnostic will be used to study the fast dynamic behaviour of the pedestal bootstrap current, where models indicate that it plays a key role in regulating edge stability, e.g. during ELMs. The diagnostic design is based on the intra-cavity multi-pass system currently in operation in TEXTOR, which uses a probing ruby laser, a grating spectrometer and two fast CMOS cameras for scattered light detection, and has achieved measuring accuracies of the order of ~ 1% for ne and ~ 2% for Te. Parts of that system will be reused in ASDEX Upgrade (some with significant modifications), but the laser multi-pass and light collection systems are entirely redesigned. Restrictions in space and line-of-sight availability have led to the adoption of a design which uses in-vessel multi-pass mirrors and light collection optics, requiring a number of innovative technical solutions to permit remote laser alignment and light collection. We give an overview of the project, discuss the underlying physics basis and present a number of technical solutions employed.

Comparative Study of Sawtooth Physics and Alfvén Waves via 2-D ECE Imaging on KSTAR, DIII-D, AUG and TEXTOR

Abstract: Full text: A new and improved 2D microwave imaging system on TEXTOR tokamak plasma demonstrated the unprecedented advantage of high temporal and spatial resolution of 2D images over the conventional 1D data in studies of the physics of the sawtooth crash; The major new findings are; a) The high field side sawtooth crash was re-confirmed. b) The role of the core current density in the theoretical explosive growth rate of the ideal kink or resistive instability may have to be considered to explain the observed multiple reconnection processes and stagnated growth rate of the island during the post-cursor phase of the sawtooth oscillation. (c) The rigid body rotation assumption is well valid and this validation was achieved through variation of the toroidal rotation speed controlled by the toroidal momentum input by the heating beam. Following the successful findings of new physics from the TEXTOR experiments, a state-of-the-art optics design for KSTAR and DIII-D was launched for detailed study of physics. The DIII-D system has already been deployed to study the physics of sawteeth as well as of other MHD modes. The sawtooth results will be compared with the previous studies from TEXTOR where the plasma shape is circular. The first 2D structure of Alfven eigenmodes from the reverse shear regime in DIII-D discharge is a significant preliminary outcome; the results from the 2010 KSTAR campaign will be compared with those from DIII-D as well as TEXTOR. Abundant spatial ({approx} 200 channels), temporal, and frequency information will be utilized to study the fine structure of these modes. The KSTAR system featuring an optical system with a high zoom capability of up to a factor of 3 will be deployed for the 2010 campaign. This system is equipped with twin imaging arrays for simultaneous observation of two different plasma regions and will be used for comparative study of sawtooth physics, Alfven waves, NTMs and ELMs associated with the L/H transition. One of the prime physics goals of the KSTAR system is to explore the fundamentals of the disruption physics in a superconducting tokamak device. The global as well as the detailed local images of the thermal quenching phase of the disruption will assist the construction of a full picture of disruption physics and may provide a preventive measure of this event which is one of the most important issues for ITER as well as DEMO. (author)

Visualization of instabilities in hot magnetized plasmas by ECE Imaging

Abstract: Summary form only given. Electron cyclotron emission radiometry is a standard technique on most magnetically confined plasmas to measure the electron temperature profile, usually along the plasma mid plane. By combining heterodyne radiometry with optical imaging techniques it is possible to measure the electron temperature and fluctuations in this quantity in a two-dimensional section of the poloidal plasma plane.