Measurement Of Electron Temperature Fluctuations Research Articles

Electron temperature fluctuation measurements with Correlation Electron Cyclotron Emission in L-mode and I-mode plasmas at ASDEX Upgrade

The Correlation Electron Cyclotron Emission (CECE) diagnostic at ASDEX Upgrade (AUG) is used to investigate the features of outer core and pedestal (ρpol = 0.85-1.0) turbulence across confinement regime transitions. The I-mode confinement regime is a promising operational scenario for future fusion reactors because it features high energy confinement without high particle confinement, but the nature of the edge and pedestal turbulence in I-mode plasmas is still under investigation. The edge Weakly Coherent Mode (WCM) appears in the I-mode pedestal and may play a role in transport. In this work we explore electron temperature (Te) fluctuations in the plasma outer core and pedestal using a 24-channel high radial resolution CECE radiometer. CECE measurements provide turbulence information including the Te fluctuation amplitude, turbulent spectra, and radial localization of turbulent features. With CECE measurements we show that the WCM is localized in the pedestal region in both L-mode and I-mode and is measured in optically thick plasmas with a Te fluctuation amplitude of 2.3%. Broadband drift wave turbulence is measured in the outer core with a Te fluctuation amplitude of <1%. A second CECE system recently installed at AUG allowed for non-standard fluctuation measurements during L-mode and I-mode experiments. The second CECE system was toroidally separated from the primary system, allowing measurements of the long-range toroidal correlation of the WCM indicating its low toroidal mode number. A reflectometer sharing a line of sight with the second CECE system enabled density-temperature cross-phase (αne Te ) measurements. The WCM αne Te changes between L-mode and I-mode as the Te gradient steepens.

Database study of turbulent electron temperature fluctuation measurements at ASDEX Upgrade

In this work, an automated method for the analysis of data from the correlation electron cyclotron emission (CECE) diagnostic is applied to discharges in the ASDEX Upgrade (AUG) tokamak. This recently developed, automated method provides an efficient means of accurately analysing large quantities of experimental turbulence data, enabling the development of the largest database of CECE measurements of tokamak plasmas to-date. The turbulence database provides the opportunity to search for large-scale trends in experimental data to improve our understanding of transport-relevant plasma turbulence. The results of physics-based investigations utilizing this turbulence database will be reported on separately from this work.

Development of a correlation ECE radiometer for electron temperature fluctuation measurements in Heliotron J

A radial correlation ECE radiometer diagnostic has been developed for electron temperature fluctuation measurements in the helical-axis heliotron device, Heliotron J. The radiometer consists of two heterodyne detection systems. One system scans the frequency of a local oscillator from 52 to 64 GHz with a single intermediate frequency filter, and the second system has a fixed frequency, 56 GHz local oscillator with four intermediate frequency filters. This frequency range covers measurement positions spanning from the plasma core to the half radius. Laboratory tests indicate that each system has narrow intermediate frequency bandwidth and high-sensitivity over a large dynamic range. During plasma experiments with NBI heating, radiation temperature fluctuation measured by the CECE radiometer decrease with increasing ECCD commensurate with previous measurements of energetic particle driven modes on Heliotron J.

Measurement of electron temperature fluctuations on J-TEXT via correlation ECE.

The capabilities of the joint-Texas experimental tokamak correlation electron cyclotron emission (CECE) diagnostic have recently been extended with an upgrade. Four new yttrium iron garnet (YIG) filters from 4 GHz to 18 GHz with a bandwidth of 90 ∼ 230 MHz are added to the previous 4 channels. Optical optimization of the transmission line has improved the poloidal resolution, which allows k θ < 3.08 cm-1. The improvement of video amplifiers allows the frequency and amplitude gain to be adjusted discretely from 200 kHz to 1 MHz and from 200 to 1000, respectively, for different situations. A controller is designed to remotely adjust the center frequency of the YIG filters. Based on the CECE, the distribution and the effect of magnetohydrodynamic instabilities on electron temperature fluctuations have been observed. The experiment results show good performance of the upgraded CECE diagnostic.

Correlation electron cyclotron emission diagnostic and improved calculation of turbulent temperature fluctuation levels on ASDEX Upgrade.

A newly upgraded correlation electron cyclotron emission (CECE) diagnostic has been installed on the ASDEX Upgrade tokamak and has begun to perform experimental measurements of electron temperature fluctuations. CECE diagnostics measure small amplitude electron temperature fluctuations by correlating closely spaced heterodyne radiometer channels. This upgrade expanded the system from six channels to thirty, allowing simultaneous measurement of fluctuation level radial profiles without repeat discharges, as well as opening up the possibility of measuring radial turbulent correlation lengths. Newly refined statistical techniques have been developed in order to accurately analyze the fluctuation data collected from the CECE system. This paper presents the hardware upgrades for this system and the analysis techniques used to interpret the raw data, as well as measurements of fluctuation spectra and fluctuation level radial profiles.

Validation of gyrokinetic simulations with measurements of electron temperature fluctuations and density-temperature phase angles on ASDEX Upgrade

Measurements of turbulent electron temperature fluctuation amplitudes, δTe⊥/Te, frequency spectra, and radial correlation lengths, Lr(Te⊥), have been performed at ASDEX Upgrade using a newly upgraded Correlation ECE diagnostic in the range of scales k⊥&amp;lt;1.4 cm−1, kr&amp;lt;3.5 cm−1 (k⊥ρs&amp;lt;0.28 and krρs&amp;lt;0.7). The phase angle between turbulent temperature and density fluctuations, αnT, has also been measured by using an ECE radiometer coupled to a reflectometer along the same line of sight. These quantities are used simultaneously to constrain a set of ion-scale non-linear gyrokinetic turbulence simulations of the outer core (ρtor = 0.75) of a low density, electron heated L-mode plasma, performed using the gyrokinetic simulation code, GENE. The ion and electron temperature gradients were scanned within uncertainties. It is found that gyrokinetic simulations are able to match simultaneously the electron and ion heat flux at this radius within the experimental uncertainties. The simulations were performed based on a reference discharge for which δTe⊥/Te measurements were available, and Lr(Te⊥) and αnT were then predicted using synthetic diagnostics prior to measurements in a repeat discharge. While temperature fluctuation amplitudes are overestimated by &amp;gt;50% for all simulations within the sensitivity scans performed, good quantitative agreement is found for Lr(Te⊥) and αnT. A validation metric is used to quantify the level of agreement of individual simulations with experimental measurements, and the best agreement is found close to the experimental gradient values.

Measurement of electron temperature fluctuations using a tunable correlation electron cyclotron emission system on Alcator C-Mod.

A tunable correlation electron cyclotron (CECE) system was recently installed on the Alcator C-Mod tokamak to provide local, quantitative measurement of electron temperature fluctuations in the tokamak core. This system represents a significant upgrade from the original CECE system, expanding the measurement capabilities from 4 to 8 total channels, including 2 remotely tunable YIG filters (6-18 GHz; 200 MHz bandwidth). Additional upgrades were made to the optical system to provide enhanced poloidal resolution and allow for measurement of turbulent fluctuations below kθρs < 0.3. These expanded capabilities allow for single shot measurement of partial temperature fluctuation profiles in the region ρ = 0.7 - 0.9 (square root of normalized toroidal flux) in a wide variety of plasma conditions. These measurements are currently being used to provide stringent tests of the gyrokinetic model in ongoing model validation efforts. Details of the hardware upgrades, turbulent fluctuation measurements, and ongoing comparisons with simulations are presented.

Development of electron cyclotron emission imaging system on the HL-2A tokamak

A 2D electron cyclotron emission imaging (ECEI) system has been developed for measurement of electron temperature fluctuations in the HL-2A tokamak. It is comprised of a front-end 24 channel heterodyne imaging array with a tunable RF range spanning 75-110 GHz, and a set of back-end ECEI electronics that together generate 24 × 8 = 192 channel images of the 2nd harmonic X-mode electron cyclotron emission from the HL-2A plasma. The simulated performance of the local oscillator (LO) optics and radio frequency (RF) optics is presented, together with the laboratory characterization results. The Gaussian beams from the LO optics are observed to properly cover the entire detector array. The ECE signals from the plasma are mixed with the LO signal in the array box, then delivered to the electronics system by low-loss microwave cables, and finally to the digitizers. The ECEI system can achieve temporal resolutions of ~μs, and spatial resolutions of 1 cm (radially) and 2 cm (poloidally).

Boundary perturbations coupled to core 3/2 tearing modes on the DIII-D tokamak

High confinement (H-mode) discharges on the DIII-D tokamak are routinely subject to the formation of long-lived, non-disruptive magnetic islands that degrade confinement and limit fusion performance. Simultaneous, 2D measurement of electron temperature fluctuations in the core and edge regions allows for reconstruction of the radially resolved poloidal mode number spectrum and phase of the global plasma response associated with these modes. Coherent, n = 2 excursions of the plasma boundary are found to be the result of coupling to an n = 2, kink-like mode which arises locked in phase to the 3/2 island chain. This coupling dictates the relative phase of the displacement at the boundary with respect to the tearing mode. This unambiguous phase relationship, for which no counter-examples are observed, is presented as a test for modeling of the perturbed fields to be expected outside the confined plasma.

Electron temperature fluctuations associated with the weakly coherent mode in the edge of I-mode plasmas

New measurements of electron temperature fluctuations associated with the weakly coherent mode (WCM) during improved mode, or I-mode plasmas (Whyte et al 2010 Nucl. Fusion. 50 105005) at Alcator C-Mod (Marmar et al 2007 Fusion. Sci. Technol. 51 3261) are presented in this paper. The measurements are made with a 32-channel, high-resolution profile electron cyclotron emission radiometer. The WCM electron temperature fluctuations are localized to a 1 cm region inside the last closed flux surface. The WCM electron temperature fluctuation level is measured in several different I-mode discharges and is in the range , which is an order of magnitude smaller than the WCM density fluctuation level. The WCM edge fluctuations observed in I-mode are believed to play a role in increasing particle transport but not energy transport in the edge of I-mode plasmas. The large difference between normalized density and electron temperature fluctuation amplitudes provides new evidence that the WCM fluctuations can separately affect energy and particle transport.

Calibration of a Thomson scattering diagnostic for fluctuation measurements

Detailed calibrations of the Madison Symmetric Torus polychromator Thomson scattering system have been made suitable for electron temperature fluctuation measurements. All calibrations have taken place focusing on accuracy, ease of use and repeatability, and in situ measurements wherever possible. Novel calibration processes have been made possible with an insertable integrating sphere (ISIS), using an avalanche photodiode (APD) as a reference detector and optical parametric oscillator (OPO). Discussed are a novel in situ spatial calibration with the use of the ISIS, the use of an APD as a reference detector to streamline the APD calibration process, a standard dc spectral calibration, and in situ pulsed spectral calibration made possible with a combination of an OPO as a light source, the ISIS, and an APD used as a reference detector. In addition a relative quantum efficiency curve for the APDs is obtained to aid in uncertainty analysis.

A Review of ECE Correlation Radiometry Techniques for Detection of Core Electron Temperature Fluctuations

Over the past 15 years, correlation radiometry techniques have been developed to detect the small-amplitude, high-frequency electron temperature fluctuations to which standard electron cyclotron emission measurements are insensitive. In this paper we review the principles of these techniques and summarize recent core electron temperature fluctuation measurements and their impact on understanding turbulent transport.

First Results of Correlation Electron Cyclotron Emission on Tore Supra

Measurements of electron temperature fluctuations by means of correlation electron cyclotron emission (ECE) diagnostics aid in understanding the nature of the turbulent transport in fusion plasmas. On Tore Supra tokamak, a 32-channel heterodyne ECE radiometer has been upgraded to include two channels for temperature fluctuation measurements. The central frequency of the yttrium iron garnet filter on each channel is remotely monitored by a driver, allowing one to shift the observation volume in the plasma radially. To resolve the fluctuation amplitude of 0.1%, integration times of ~3 s at 1 MHz sampling rate are required for the statistical error level of 0.05%. Together with the 32-channel profile radiometer, correlation ECE is a powerful and unique tool for simultaneous observation of the magnetohydrodynamic phenomena and studies of the plasma transport properties in various heating regimes during long-shot multimegawatt operation. First measurements of temperature fluctuations, of various origins, on Tore Supra (including the observation of the toroidicity-induced Alfvén eigenmodes), as well as a description of the analysis methods used in the data evaluation, are reported in this paper.

Two-dimensional correlation measurements of electron cyclotron emission fluctuations on the stellarator Wendelstein 7-AS

The miniaturization of receiver arrays permits the arrangement of numerous poloidally staggered radial sightlines of an electron cyclotron emission (ECE) diagnostic for the measurement of the electron temperature and its fluctuations, making two-dimensional (2D) imaging of the electron temperature and its fluctuations caused by plasma turbulence possible. For the stellarator W7-AS, the development of a fully monolithic microwave integrated circuit 150 GHz subharmonic mixer array is under development. As a first step, a 2D ECE system for the measurement of electron temperature fluctuations using four individual horn-reflector arrangements in conjunction with multichannel heterodyne radiometers was installed and set into operation. With Gaussian beam optics and four poloidally staggered sightlines, electron temperature fluctuations could be characterized in radial and poloidal directions simultaneously. First observations in purely electron cyclotron resonance heated stellarator plasmas reveal a broadband drift-wave feature. Earlier experiments showing a decrease of the electron temperature fluctuation level with increasing heating power were confirmed. Additionally, it was revealed, using the 2D ECE correlation radiometer, that an increased velocity shear might account for the decrease of the coherence length and thus for the reduction of the electron temperature fluctuation level.

Diagnostic of electron temperature fluctuations in a turbulent plasma

Measurements of fluctuations have been performed in the steady-state magnetoplasma BLAAMANN characterized by a turbulent regime. In such a plasma electron temperature fluctuations can be of the same order of the floating potential ones, when the plasma is produced at high neutral pressure P&amp;gt;5×10−4 mbar. Measurements of electron temperature fluctuations are performed through triple probe diagnostic.

Electron temperature fluctuation measurements and techniques in the DIII-D tokamak

Several techniques for determining the level and characteristics of electron temperature fluctuations via measurements and analysis of electron cyclotron emission (ECE) have recently been proposed and demonstrated [S. Sattler and H. J. Hartfuss, Plasma Phys. Cont. Fus. 35, 1285 (1993) and G. Cima et al., Phys. Plasmas (1994)]. A technique relying on the correlation of ECE in orthogonal frequency bins but lying within the line broadening of the local emission spectrum is being pursued at DIII-D. Special concerns include the effect of large rotation in the discharges due to neutral beam injection and extremely low fluctuation levels in enhanced confinement regimes such as negative central shear discharges. An antenna design optimized for fluctuation studies is also being developed.

Edge Plasma Turbulence Diagnosis by Langmuir Probes

A view of the experimental techniques used in the characterization of fluctuations in the plasma boundary region of magnetic confinement systems by Langmuir probes is given. In particular, recent experimental developments for the measurement of electron temperature fluctuations are discussed.

Edge turbulence and anomalous transport in fusion plasmas

A view of the latest experimental results and progress in the characterization and understanding of edge turbulence is given. The development of new fluctuation analysis techniques has provided a new bridge between experimental measurements and theoretical turbulence models. At the same time, the characterization of fluctuation and turbulent transport has stimulated new experimental developments in the measurement of electron temperature fluctuations; it has been shown that substantial levels of electron temperature fluctuations are present in the boundary region of magnetically confined plasmas. Several driving (curvature, atomic physics, .) and damping (electric field) mechanisms of the edge turbulence have been investigated. The development of methods to modify and control plasma turbulence and confinement appears as a very promising area of research. Recent results emphasize the importance of comparative studies between the structure of the turbulence in toroidal confinement systems to test critically edge turbulence models.

Intensity interferometry for measurement of electron temperature fluctuations in fusion plasmas

Transport-relevant fluctuations of the electron temperature can in principle be measured by radiometry of the electron cyclotron emission (ECE), but they might be buried completely in natural fluctuations of the ECE due to the thermal nature of this radiation. The spatial coherence properties of thermal radiation can be utilized in correlation experiments to overcome the principal difficulties and to make temperature fluctuations accessible to measurement. On the basis of these considerations an intensity interferometer was built and installed on the Wendelstein 7-AS stellarator. The interferometer consists of two identical but independent multichannel heterodyne radiometers viewing the same emitting plasma volume along crossed lines of sight. The angle between the sightlines is chosen sufficiently large to decorrelate the natural fluctuations of the radiation field. A quasithermal microwave radiation source is used to test the intensity interferometer and to simulate the correlation experiment. Measurements of the spatial coherence and the statistical properties are in agreement with theoretical predictions.

Electron temperature fluctuation measurements with high temporal resolution in the TEXTOR boundary plasma

Using an oscillating Langmuir probe technique power spectra of the electron temperature have been obtained in the range from 0 to 100 kHz. It is found that the major part of the power is concentrated below 20 kHz. Relative levels of electron temperature fluctuations between 10% and 30% have been observed in ohmically heated plasmas.