Nils Basse | RISE Research Institutes of Sweden (RISE) (original) (raw)

Papers by Nils Basse

Dielectric interruption performance of SF 6 high-voltage gas circuit breakers depends on the temp... more Dielectric interruption performance of SF 6 high-voltage gas circuit breakers depends on the temperature distribution of the gas during the post-arc phase. Understanding the details of the mixing process occurring inside the breaker during arcing is therefore fundamental for the design. Multiphysics simulations are nowadays used to predict the flow field and the energy distribution inside the interrupter during and after the arcing phase. In this paper we make use of an optical technique to observe the arc-induced SF 6 mixing process inside a dedicated test device. We extract qualitative and quantitative information and we compare this with simulations to conclude on the suitability of the models used.

We have in earlier work (Basse N P 2005 Phys. Lett. A 340 456) reported on intriguing similaritie... more We have in earlier work (Basse N P 2005 Phys. Lett. A 340 456) reported on intriguing similarities between density fluctuation power versus wavenumber on small (mm) and large (Mpc) scales.

In this paper measurements of short wavelength electron density fluctuations using collective sca... more In this paper measurements of short wavelength electron density fluctuations using collective scattering of infrared light are presented. The Wendelstein 7-AS (W7-AS) stellarator Plasma Phys. Control. Fusion 31 1579 and the diagnostic are briefly described. A series of plasma discharges with reproducible confinement transitions was created by ramping up the plasma current. Utilizing the fact that the density fluctuation wavenumber κ is anisotropic in the directions parallel and perpendicular to the local magnetic field, the diagnostic can provide a radial profile of the turbulence during both normal and degraded confinement. The found profiles display an increase of core turbulence for the reduced confinement state. The results are discussed and compared to similar tokamak measurements.

In this paper theory on collective scattering measurements of electron density fluctuations in fu... more In this paper theory on collective scattering measurements of electron density fluctuations in fusion plasmas is revisited. We present the first full derivation of the expression for the photocurrent beginning at the basic scattering concepts. Thereafter we derive detailed expressions for the auto-and crosspower spectra obtained from measurements. These are discussed and simple simulations made to elucidate the physical meaning of the findings. In this context, the known methods of obtaining spatial localization are discussed and appraised. Where actual numbers are applied, we utilize quantities from two collective scattering instruments: The ALTAIR diagnostic on the Tore Supra tokamak [A. Truc et al., "ALTAIR: An infrared laser scattering diagnostic on the Tore Supra tokamak," Rev. Sci. Instrum. 63, 3716-3724 (1992)]. and the LOTUS diagnostic on the Wendelstein 7-AS stellarator [M. Saffman et al., "CO 2 laser based two-volume collective scattering instrument for spatially localized turbulence measurements," Rev. Sci. Instrum. 72, 2579-2592 (2001)].

In this paper, measurements of small scale electron density fluctuations in low confinement (L)-m... more In this paper, measurements of small scale electron density fluctuations in low confinement (L)-mode and high confinement (H)-mode plasmas obtained by collective scattering of infrared light are presented. A single discharge in the Wendelstein 7-AS (W7-AS) stellarator (Renner H et al 1989 Plasma Phys. Control. Fusion 31 1579) was studied. The plasma developed from steady-state L-mode to a dithering mode and finally became an edge localized mode (ELM)free H-mode. These three phases are separately characterized. Autopower spectra of density fluctuations measured in the dithering and ELM-free H-modes are limited to lower frequencies than those in the dithering and steady-state L-modes. Correlations between the density fluctuations, an H α -signal and magnetic fluctuations as measured by Mirnov coils were analysed. The correlation calculations made for time windows of tens of milliseconds with time lag steps of 100 µs show that all the fluctuating quantities are highly correlated during the L-mode and dithering states, but that no correlation exists in the ELM-free H-mode. Evaluating correlations on a 20 µs timescale between magnetic and density fluctuations leads to the result that the minimum correlation timescale in steady-state and dithering L-modes is of order 100 µs, while no correlation exists for ELM-free and dithering H-modes. Finally, the time derivative of the phase of the measured complex density fluctuation signal allows us to study the structure of turbulent bursts in L-and H-mode.

Confinement transitions in the Wendelstein 7-AS stellarator [H. Renner et al., Plasma Phys. Contr... more Confinement transitions in the Wendelstein 7-AS stellarator [H. Renner et al., Plasma Phys. Controlled Fusion 31, 1579] can be induced by varying either the internal plasma current or the external magnetic field. In this paper we report on experiments where closely matched confinement states (good and bad) were constructed using the latter method. Analysis using the former scheme has been reported upon previously [S. Zoletnik et al., Plasma Phys. Controlled Fusion 44, 1581]. The electron temperature, along with the major spectral characteristics of magnetic and small-scale electron density fluctuations, changes dramatically at the transition from good to bad confinement. The fluctuation power is intermittent, and core bursts traveling in the electron diamagnetic drift (DD) direction are correlated between the bottom and top of the plasma, especially during degraded confinement. A corresponding top-bottom correlation for the edge ion DD direction turbulence feature was not found. Strong correlations are observed both between the two density fluctuation signals and between magnetic and density fluctuations in bad compared to good confinement. The correlation time of the bursts is of order 100 s, similar to the lifetime observed during edge localized modes.

The recently upgraded phase-contrast imaging ͑PCI͒ diagnostic is used to characterize the transit... more The recently upgraded phase-contrast imaging ͑PCI͒ diagnostic is used to characterize the transition from the low ͑L͒ to the enhanced D ␣ ͑EDA͒ high ͑H͒ confinement mode in Alcator C-Mod ͓I. H. Hutchinson, R. Boivin, F. Bombarda et al., Phys. Plasmas 1, 1511 ͑1994͔͒ plasmas. PCI yields information on line integrated density fluctuations along vertical chords. The number of channels has been increased from 12 to 32 and the sampling rate from 1 MHz to 10 MHz. This expansion of diagnostic capabilities is used to study broadband turbulence in L and EDA H mode and to analyze the quasicoherent ͑QC͒ mode associated with EDA H mode. Changes in broadband turbulence at the transition from L to EDA H mode can be interpreted as an effect of the Doppler rotation of the bulk plasma. Additional fluctuation measurements of D ␣ light and the poloidal magnetic field show features correlated with PCI in two different frequency ranges at the transition. The backtransition from EDA H to L mode, the so-called enhanced neutron ͑EN͒ mode, is investigated by new high frequency ͑132 and 140 GHz͒ reflectometer channels operating in the ordinary ͑O͒ mode. This additional hardware has been installed in an effort to study localized turbulence associated with internal transport barriers ͑ITBs͒. The EN mode is a suitable candidate for this study, since an ITB exists transiently as the outer density decreases much faster than the core density in this mode. The fact that the density decays from the outside inward allows us to study fluctuations progressing towards the plasma core. Our results mark the first localized observation of the QC mode at medium density: 2.2ϫ 10 20 m −3 ͑132 GHz͒. Correlating the reflectometry measurements with other fluctuating quantities provides some insight regarding the causality of the EN-mode development.

In this article measurements of small scale electron density fluctuations in dithering high confi... more In this article measurements of small scale electron density fluctuations in dithering high confinement ͑H͒-mode plasmas obtained by collective scattering of infrared light are presented. A scan of the fluctuation wavenumber was made in a series of similar discharges in the Wendelstein 7-AS ͑W7-AS͒ stellarator ͓H. Renner et al., Plasma Phys. Control. Fusion 31, 1579 ͑1989͔͒. The experimental setup and discharge properties are described. H ␣ -light observing an inner limiter was used to separate low confinement ͑L͒and H-mode phases of the plasma; the separated density fluctuations are characterized. It was found that L-͑H-͒ mode fluctuations dominate at high ͑low͒ frequencies, respectively, and that they possess well-defined and distinguishable scaling properties. Wavenumber spectra for L-and H-mode measurements are calculated and fitted by power-laws and exponential functions. The separated measurements can be fitted with the same exponents in L-and H-mode. Correlations between the density fluctuations, the H ␣ -signal and magnetic fluctuations as measured by Mirnov coils were analyzed. Correlation calculations using 50 ms time windows ͑several dithering periods͒ with time lag steps of 100 s showed that all the fluctuating quantities are highly correlated and that the maximum correlation occurs for high frequency density fluctuations. Performing separate L-and H-mode correlations on a 20 s time scale between magnetic and density fluctuations leads to the result that the minimum correlation time scale in L-mode is of order 100 s, while no correlation exists for H-mode.

We will in this Letter report on suggestive similarities between density fluctuation power versus... more We will in this Letter report on suggestive similarities between density fluctuation power versus wavenumber on small (mm) and large (Mpc) scales. The small scale measurements were made in fusion plasmas and compared to predictions from classical fluid turbulence theory. The data is consistent with the dissipative range of 2D turbulence. Alternatively, the results can be fitted to a functional form that cannot be explained by turbulence theory. The large scale measurements were part of the Sloan Digital Sky Survey galaxy redshift examination. We found that the equations describing fusion plasmas also hold for the galaxy data. The comparable dependency of density fluctuation power on wavenumber in fusion plasmas and galaxies might indicate a common origin of these fluctuations.

Turbulence plays a key role in several physical processes related to the interruption of current ... more Turbulence plays a key role in several physical processes related to the interruption of current in a gas circuit breaker (GCB). In this paper we study one aspect, namely turbulent gas mixing in the heating volume of a small-scale 3D GCB model. Mixing is observed using a shadowgraphy setup; postprocessing extracts information on the time-varying velocity field. Discharges with two different current amplitudes were studied and their repeatability investigated. A measure of mixing completeness, the largest vortex area, was investigated. The experiments reported upon in this paper were done in air at atmospheric pressure.

Understanding the dynamic processes governing gas circuit breaker physics is crucial in order to ... more Understanding the dynamic processes governing gas circuit breaker physics is crucial in order to continue to improve short-circuit current interruption performance. In this paper, we study a single arc discharge both using measurements and 3-D computational fluid dynamics simulations. The primary quantity analyzed is the pressure in the heating volume created by radiative ablation of the Polytetrafluoroethylene nozzles surrounding the arc. We use cross-correlation functions to investigate the behavior of pressure waves in the SF 6 gas, especially those induced at flow reversal where the gas flow between the arc zone and heating volume changes direction.

Intriguing parallels between density fluctuation power versus wavenumber on small (in millimeter)... more Intriguing parallels between density fluctuation power versus wavenumber on small (in millimeter) and large (in megaparsec) scales are presented. The comparative study is carried out between fusion plasma measurements and cosmological data. Based on predictions from classical fluid turbulence theory, we argue that our observations are consistent with 2-D turbulence. The similar dependencies of density fluctuations on these disparate scales might indicate that primordial turbulence has been expanded to cosmological proportions.

An overview of the diagnostics installed on the Alcator C-Mod tokamak is presented. Approximately... more An overview of the diagnostics installed on the Alcator C-Mod tokamak is presented. Approximately 25 diagnostic systems are being operated on C-Mod. The compact design of the machine and the cryostat enclosing the vacuum vessel and magnetic field coils make access challenging. Diagnostics are used to study four focus areas: transport, plasma boundary, waves, and macrostability. There is significant overlap between these topics, and they all contribute toward the burning plasma and advanced tokamak thrusts. Several advanced and novel diagnostics contribute to the investigation of C-Mod plasmas, e.g., electron cyclotron emission, phase-contrast imaging, gas puff imaging, probe measurements, and active magnetohydrodynamic antennas.

The performance of high voltage gas circuit breakers depends on the temperature distribution of h... more The performance of high voltage gas circuit breakers depends on the temperature distribution of hot gas or plasma from the arc zone mixed with cold gas that is present, for example, in the exhausts and mixing volume. Understanding the details of the mixing process is imperative to estimate the temperature distribution within the entire breaker volume. Design studies rely on computational fluid dynamics (CFD) simulations to search for the best way to achieve satisfactory mixing. One key uncertainty in the CFD simulations is the role of turbulence in this process and how to properly account for it. To gain knowledge of the mixing process between hot and cold gases, we have constructed a simplified breaker geometry that is flexible and accessible to diagnostics. Apart from standard measurements of current and arc voltage, we measure pressure in the arc zone and the mixing volume. Further, the mixing volume is specially designed to be transparent, allowing us to make shadowgraphy measurements of the turbulent mixing during and after the arcing phase. We report on experiments performed in air at atmospheric pressure.

Coriolis flowmeters operate with high accuracy when the medium metered is a single-phase incompre... more Coriolis flowmeters operate with high accuracy when the medium metered is a single-phase incompressible fluid. Multi-phase fluids lead to measurement errors because of center-of-mass motion. In this paper we review the "bubble theory" which describes errors due to phase decoupling of two-phase fluids. Examples are provided with combined phase decoupling and compressibility errors.

In this paper, we compare measured, modelled, and simulated mean velocity profiles. Smooth pipe f... more In this paper, we compare measured, modelled, and simulated mean velocity profiles. Smooth pipe flow simulations are performed for both incompressible (below Mach 0.2) and compressible (below Mach 0.1) fluids. The compressible simulations align most closely with the measurements. The simulations are subsequently used to make scaling formulae of the turbulence intensity as a function of the Reynolds number. These scaling expressions are compared to scaling derived from measurements. Finally, the found compressible scaling laws are used as an example to show how the flow noise in a flowmeter is expected to scale with the mean flow velocity. s RMS . Our objective in this paper is to derive scaling for smooth pipes. Therefore, most SEM and all CFD simulations below are made for a http://dx.

Turbulence intensity profiles are compared for smooth-and rough-wall pipe flow measurements made ... more Turbulence intensity profiles are compared for smooth-and rough-wall pipe flow measurements made in the Princeton Superpipe. The profile development in the transition from hydraulically smooth to fully rough flow displays a propagating sequence from the pipe wall towards the pipe axis. The scaling of turbulence intensity with Reynolds number shows that the smoothand rough-wall level deviates with increasing Reynolds number. We quantify the correspondence between turbulence intensity and the friction factor.