T. Munsat

Simultaneous microwave imaging system for density and temperature fluctuation measurements on TEXTOR (invited)

Diagnostic systems for fluctuation measurements in plasmas are, of necessity, evolving from simple one-dimensional (1D) systems to multidimensional systems due to the complexity of the magnetohydrodynamics (MHD) and turbulence physics of plasmas as illustrated by advanced numerical simulations. Using the recent significant advancements in millimeter wave imaging technology, microwave imaging reflectometry (MIR) and electron cyclotron emission imaging (ECEI), simultaneously measuring density and temperature fluctuations, have been developed for Toroidal EXperiment for Technology Oriented Research (TEXTOR). The MIR system was installed on Textor and the experiment was performed in September, 2003. Subsequent MIR campaigns have yielded poloidally resolved spectra and assessments of poloidal velocity. The 2D ECE imaging system (with a total of 128 channels), installed on TEXTOR in December, 2003, successfully captured the first true 2D images of Te fluctuations of m=1 oscillations (“sawteeth”) near the q∼1 su...

Commissioning of electron cyclotron emission imaging instrument on the DIII-D tokamak and first data.

A new electron cyclotron emission imaging diagnostic has been commissioned on the DIII-D tokamak. Dual detector arrays provide simultaneous two-dimensional images of T(e) fluctuations over radially distinct and reconfigurable regions, each with both vertical and radial zoom capability. A total of 320 (20 vertical×16 radial) channels are available. First data from this diagnostic demonstrate the acquisition of coherent electron temperature fluctuations as low as 0.1% with excellent clarity and spatial resolution. Details of the diagnostic features and capabilities are presented.

2D electron cyclotron emission imaging at ASDEX Upgrade (invited)

The newly installed electron cyclotron emission imaging diagnostic on ASDEX Upgrade provides measurements of the 2D electron temperature dynamics with high spatial and temporal resolution. An overview of the technical and experimental properties of the system is presented. These properties are illustrated by the measurements of the edge localized mode and the reversed shear Alfvén eigenmode, showing both the advantage of having a two-dimensional (2D) measurement, as well as some of the limitations of electron cyclotron emission measurements. Furthermore, the application of singular value decomposition as a powerful tool for analyzing and filtering 2D data is presented.

Structure and motion of edge turbulence in the National Spherical Torus Experiment and Alcator C-Mod

In this paper we compare the structure and motion of edge turbulence observed in L-mode vs. H-mode plasmas in the National Spherical Torus Experiment (NSTX) [M. Ono, M. G. Bell, R. E. Bell et al., Plasma Phys. Controlled Fusion 45, A335 (2003)]. The radial and poloidal correlation lengths are not significantly different between the L-mode and the H-mode in the cases examined. The poloidal velocity fluctuations are lower and the radial profiles of the poloidal turbulence velocity are somewhat flatter in the H-mode compared with the L-mode plasmas. These results are compared with similar measurements Alcator C-Mod [E. Marmar, B. Bai, R. L. Boivin et al., Nucl. Fusion 43, 1610 (2003)], and with theoretical models.

Quiet periods in edge turbulence preceding the L-H transition in the National Spherical Torus Experiment

This paper describes the first observations in the National Spherical Torus Experiment (NSTX) [S. M. Kaye et al., Phys. Plasmas 8, 1977 (2001)] of “quiet periods” in the edge turbulence preceding the low-to-high (L-H) mode transition, as diagnosed by the gas puff imaging (GPI) diagnostic near the outer midplane separatrix. During these quiet periods the GPI Dα light emission pattern was transiently similar to that seen during H-mode, i.e., with a relatively small fraction of the GPI light emission located outside the separatrix. These quiet periods had a frequency of ∼3 kHz for at least 30 ms before the L-H transition, and were correlated with changes in the direction of the local poloidal velocity. The GPI turbulence images were also analyzed to obtain an estimate for the dimensionless poloidal shearing S=(dVp/dr)(Lr/Lp)τ. The values of S were strongly modulated by the quiet periods but did not significantly vary during the ∼30 ms preceding the L-H transition. Since neither the quiet periods nor the shea...

Microwave imaging reflectometer for TEXTOR (invited)

Understanding the behavior of fluctuations in magnetically confined plasmas is essential to the advancement of turbulence-based transport physics. Though microwave reflectometry has proven to be an extremely useful and sensitive tool for measuring small density fluctuations in some circumstances, this technique has been shown to have limited viability for large amplitude, high kθ fluctuations and/or core measurements. To this end, a new instrument based on two-dimensional imaging reflectometry has been developed to measure density fluctuations over an extended plasma region in the TEXTOR tokamak. This technique is made possible by collecting an extended spectrum of reflected waves with large-aperture imaging optics. Details of the imaging reflectometry concept, as well as technical details of the TEXTOR instrument, are presented. Data from proof-of-principle experiments on TEXTOR using a prototype system is presented, as well as results from a systematic off-line study of the advantages and limitations of...

Fluctuation measurements in tokamaks with microwave imaging reflectometry

To study the mechanism of anomalous transport in tokamaks requires the use of sophisticated diagnostic tools for the measurement of short-scale turbulent fluctuations. In this article, we describe an attempt at developing a technique capable of providing a comprehensive description of plasma fluctuations with k⊥ρi<1, such as those driven by the ion temperature gradient mode in tokamaks. The proposed method is based on microwave reflectometry, and stems from a series of numerical calculations showing that the spatial structure of fluctuations near the cutoff could be obtained from the phase of reflected waves when these are collected with a wide aperture optical system forming an image of the cutoff onto an array of phase sensitive detectors. Preliminary measurements with a prototype apparatus on the Torus Experiment for Technology Oriented Research 94 (TEXTOR-94) [U. Samm, Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (IEEE, Piscataway, NJ, 1995), p. 470] confirm the validity of these...

3 MV hypervelocity dust accelerator at the Colorado Center for Lunar Dust and Atmospheric Studies

A hypervelocity dust accelerator for studying micrometeorite impacts has been constructed at the Colorado Center for Lunar Dust and Atmospheric Studies (CCLDAS) at the University of Colorado. Based on the Max-Planck-Institüt für Kernphysik (MPI-K) accelerator, this accelerator is capable of emitting single particles of a specific mass and velocity selected by the user. The accelerator consists of a 3 MV Pelletron generator with a dust source, four image charge pickup detectors, and two interchangeable target chambers: a large high-vacuum test bed and an ultra-high vacuum impact study chamber. The large test bed is a 1.2 m diameter, 1.5 m long cylindrical vacuum chamber capable of pressures as low as 10(-7) torr while the ultra-high vacuum chamber is a 0.75 m diameter, 1.1 m long chamber capable of pressures as low as 10(-10) torr. Using iron dust of up to 2 microns in diameter, final velocities have been measured up to 52 km/s. The spread of the dust particles and the effect of electrostatic focusing have been measured using a long exposure CCD and a quartz target. Furthermore, a new technique of particle selection is being developed using real time digital filtering techniques. Signals are digitized and then cross-correlated with a shaped filter, resulting in a suppressed noise floor. Improvements over the MPI-K design, which include a higher operating voltage and digital filtering for detection, increase the available parameter space of dust emitted by the accelerator. The CCLDAS dust facility is a user facility open to the scientific community to assist with instrument calibrations and experiments.

Comparison of scrape-off layer turbulence simulations with experiments using a synthetic gas puff imaging diagnostic

A synthetic gas puff imaging (GPI) diagnostic has been added to the scrape-off layer turbulence (SOLT) simulation code, enabling comparisons with GPI data from the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)]. The edge and scrape-off layer are modeled in the radial and poloidal (bidirectional) dimensions of the outboard midplane region of NSTX. A low-confinement mode discharge is simulated by choosing reference parameters, including radial density and temperature profiles, to be consistent with those of the shot (no. 112825). NSTX and simulation GPI data are submitted to identical analyses. It is demonstrated that the level of turbulent fluctuations in the simulation may be adjusted to give synthetic GPI radial intensity profiles similar to those of the experiment; for a “best-case” simulation, SOLT and NSTX probability distribution functions of blob radial locations, widths, and GPI image velocities are compared. For the simulation, synthetic GPI image velocity ...

Derivation of time-dependent two-dimensional velocity field maps for plasma turbulence studies

Time-resolved two-dimensional (2D) velocity maps have been derived for fluctuation measurements in the edge plasma of the National Spherical Torus Experiment (NSTX). The maps have been derived from time sequences of 2D images recorded with the gas puff imaging diagnostic. A hybrid technique combining optical flow and local pattern matching has been implemented to overcome the individual limitations of each when used with data of limited temporal and/or spatial resolution. Local flow velocities of up to ∼8km∕s and average poloidal flow velocities of up to ∼5km∕s are found. Results are compared to previous velocity extraction techniques and NSTX results.