Electron Temperature Fluctuations Research Articles

An eight-channel correlation electron cyclotron emission diagnostic for turbulent electron temperature fluctuation measurement in HL-2A tokamak.

A new correlation electron cyclotron emission (CECE) diagnostic has recently been installed on the HL-2A tokamak in order to study electron temperature fluctuations. Eight radial locations are measured simultaneously through eight pairs of correlated channels. Multiplexers are employed in the intermediate frequency section instead of the conventional separated filter banks to meet strict cross-isolation specifications and lower insertion loss. Relative electron temperature fluctuations are observed by CECE for the first time on the HL-2A by using the spectral decorrelation method. The achieved minimum detectable fluctuation level is up to (T̃e/Te)min∼0.5%. When studying electron temperature fluctuations in the core region with gas puffing, the cross-power spectra show that the amplitude of the electron temperature fluctuation increases in a high temperature and low density plasma. Further analysis demonstrates that the electron temperature gradient ∇Te drives the electron temperature fluctuations together with electron heat transport.

Localized modulation of turbulence by m/n = 1/1 magnetic islands in the HL-2A tokamak

Modulation of turbulent electron temperature fluctuations () and density fluctuations () by an m/n = 1/1 tearing mode island was observed in the core plasma region of the HL-2A tokamak. High spatiotemporal resolution two-dimensional images of show the first evidence that the turbulence modulation occurs only when the island width exceeds a certain threshold value ( cm) and the modulation is localized merely in the inner area of the island due to significant alteration of local profiles and turbulence drives. Evidence also reveals that for large islands turbulence spreading takes place across the island region. The results are generally consistent with theories and simulations.

Electron temperature gradient turbulence induced energy flux in the large volume plasma device

The Large Volume Plasma Device (LVPD) has successfully demonstrated excitation of the Electron Temperature Gradient (ETG) driven turbulence in the finite plasma beta (β∼0.06−0.4) condition, where the threshold condition for ETG turbulence is, ηETG=Ln/LT>2/3 satisfied, where, Ln=1ndndx−1 is the density scale length and LTe=1TedTedx−1 is the temperature scale lengths [Mattoo et al., Phys. Rev. Lett. 108, 255007 (2012)]. The observed mode follows wave vector scaling and frequency ordering as k⊥ρe≤1 ≪ k⊥ρi, Ωi<ω ≪ Ωe, where k⊥ is the perpendicular wave vector, ρe, ρi are Larmor radii of the electron and ion, respectively, and Ωi, Ωe, ω are the ion, electron gyro frequencies and the mode frequency, respectively. Simultaneous measurement of fluctuations in electron temperature, δTe ∼ (10−30) %, plasma density, δne ∼ (5−12) %, and potential δVf ∼ (1−10) % are obtained. A strong negative correlation with correlation coefficients Cδn−δφ ∼−0.8 and CδT−δφ ∼−0.9 is observed between the density and potential and temperature and potential fluctuations, respectively. These correlated density, temperature, and potential fluctuations lead to the generation of turbulent heat flux. The measured heat flux is compared with the theoretically estimated heat flux from ETG model equations. The experimental result shows that the net heat flux is directed radially outward.

Verification and validation of integrated simulation of energetic particles in fusion plasmas

This paper reports verification and validation of linear simulations of Alfvén eigenmodes in the current ramp phase of DIII-D L-mode discharge #159243 using gyrokinetic, gyrokinetic-MHD hybrid, and eigenvalue codes. Using a classical fast ion profile, all simulation codes find that reversed shear Alfvén eigenmodes (RSAE) are the dominant instability. The real frequencies from all codes have a coefficient of variation of less than 5% for the most unstable modes with toroidal mode number n = 4 and 5. The simulated RSAE frequencies agree with experimental measurements if the minimum safety factor is adjusted, within experimental errors. The simulated growth rates exhibit greater variation, and simulations find that pressure gradients of thermal plasmas make a significant contribution to the growth rates. Mode structures of the dominant modes agree well among all codes. Moreover, using a calculated fast ion profile that takes into account the diffusion by multiple unstable modes, a toroidal Alfvén eigenmode (TAE) with n = 6 is found to be unstable in the outer edge, consistent with the experimental observations. Variations of the real frequencies and growth rates of the TAE are slightly larger than those of the RSAE. Finally, electron temperature fluctuations and radial phase shifts from simulations show no significant differences with the experimental data for the strong n = 4 RSAE, but significant differences for the weak n = 6 TAE. The verification and validation for the linear Alfvén eigenmodes is the first step to develop an integrated simulation of energetic particles confinement in burning plasmas incorporating multiple physical processes.

H-mode grade confinement in L-mode edge plasmas at negative triangularity on DIII-D

Plasmas with a negative triangularity shape have been created on the DIII-D tokamak that, despite maintaining standard L-mode edge radial profiles, reach volume averaged pressure levels typical of H-mode scenarios. Within the auxiliary power available for these experiments, plasmas exhibit near-zero power degradation while sustaining βN = 2.7 and H98,y2 = 1.2 for several energy confinement times. Detailed comparison with matched discharges at positive triangularity indicates that Trapped Electron Modes are weakened at negative triangularity, consistent with increased confinement and reduced intensity of fluctuations in electron density, electron temperature, and ion density. These results indicate that a negative triangularity plasma operating without an edge pedestal might provide an attractive scenario for operations in future reactors.

Asymmetric and endogenous magnetic reconnection processes in quasi-collisionless plasmas

We define an endogenous magnetic reconnection process as the one having a driving factor that lies within the region where a drastic change in magnetic field topology occurs. A process of this kind is shown to take place when an electron temperature gradient is present in a plasma column and when, referring to quasi-collisionless regimes, the evolving electron temperature fluctuations are anisotropic. A two-fluid theory, for weakly collisional plasma regimes and confinement configurations which have sheared magnetic fields, is formulated, and two classes of reconnecting modes are identified. The localized class of modes is characterized by a transverse reconnecting magnetic field B̃x of odd parity, as a function of the radial variable (across the reconnection layer). The width of this remains significant even when large macroscopic distances, such as those of interest to space and astrophysics, are considered. The relevant reconnection can be referred to as antisymmetric. In fact, the (additional and realistic) presence of a finite current density gradient leads to finding modes with mixed B̃x parity which are no longer localized within the reconnection layer. Then, the resulting magnetic islands are not symmetric. In view of the fact that there are plasmas in the Universe with considerable electron thermal energy contents, the features of the novel reconnecting modes can be relied upon in order to produce generation of magnetic fields or conversion of magnetic energy into particle energy (through a sequence of mode-particle resonances).

Comparison of probe and narrow-band imaging measurements in a magnetized cylindrical plasma

We analyze similarities and differences between electrostatic probe measurements and high-speed visible light imaging measurements in a low-temperature argon helicon plasma. Electrostatic probes are the usual diagnostics for making fluctuation measurements in low-temperature plasmas, although fast-imaging of visible emission is becoming increasingly common. The two diagnostics are highly complementary, and typical analysis takes fluctuations in both ion saturation current and visible light intensity to be proxies for plasma density fluctuations. The observations presented here indicate that care is needed in applying this approximation. The plasma is produced with a helicon antenna, and we measure fluctuations in ion saturation current, floating potential, and visible emission near wavelengths of 488 nm (argon ions) and 750 nm (argon neutrals). Simultaneous measurements show relatively low correlation values between ion saturation current and ion line emission and an anti-correlation between floating potential and ion line emission. Variations in cross-coherence between ion emission and probe quantities indicate the frequency dependence in the relationship between the measurements. For various frequency bands where probe/imaging coherence is relatively high, fluctuation intensity profiles differ significantly and coherent fluctuations are observed in ion light which are not apparent in the probe measurements. A sensitivity analysis indicates that electron temperature fluctuations, although nearly always neglected when analyzing data from low-temperature plasma experiments, may play a role in the discrepancy between probe and narrow-band imaging measurements.

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.

Development of ECE/ECEI diagnostics and MHD-related studies on HL-2A tokamak

A novel 60-channel electron cyclotron emission (ECE) radiometer has been designed and tested for the measurement of electron temperature profiles on the HL-2A tokamak. This system is based on the intermediate frequency division technique, and has the features of wide working frequency range (60−90 GHz) and high temporal-spatial resolution (3 µs, 1 cm), which covers almost the entire plasma region. Also, an electron cyclotron emission imaging (ECEI) system has been developed for studying two dimensional electron temperature fluctuations. It is comprised of several front-end quasi-optical lenses, a 24 channel heterodyne imaging array with a tunable RF frequency range spanning 60−135 GHz, and a set of back-end ECEI electronics that together generate two 24×8 array images of the 2nd harmonic X-mode electron cyclotron emission from the HL-2A plasma. The measurement region can be flexibly shifted due to two independent local oscillator sources, and the field of view can be adjusted easily by changing the position of the zoom lenses as well. The temporal resolution is about 2.5 µs and the achievable spatial resolution is 1 cm. The ECE/ECEI diagnostics have been demonstrated to be powerful tools to study MHD-related physics including the multi-scale interaction between macro-scale MHD and micro-scale turbulence on the HL-2A tokamak.

Overview of the ECE measurements on EAST

Radiometer systems and a Michelson interferometer, have been operated routinely to detect the elec-tron cyclotron emission (ECE) from EAST plasmas for diagnosing the local electron temperature. A common quasi-optical antenna placed inside the vacuum vessel is employed to collect and focus the plasma emission, and the line of sight is along a radial chord. All of the systems are located in a diagnostic room where the plasma emission is transmitted by overmoded corrugated waveguide. In-situ absolute intensity calibration has been carried out for both the radiometer systems and the Michelson interferometer independently, to ensure that the ECE diagnostic provides an independent electron temperature measurement. In order to diagnose the small-amplitude electron temperature fluctuation, a correlation ECE (CECE) diagnostic has been designed and commissioned recently. The CECE diagnostic employs an independent antenna system which has improved poloidal resolution. A synthetic diagnostic is realized by using the simulation code SPECE to interpret the ECE data in plasmas with non-Maxwellian distribution, and preliminary results imply that the ECE data could be still useful as a localized measurement in plasmas with non-thermal electrons, such as the LHW-heated plasmas on EAST.

Statistical properties of the plasma fluctuations and turbulent cross-field fluxes in the outboard mid-plane scrape-off layer of Alcator C-Mod

The Alcator C-Mod mirror Langmuir probe diagnostic is used to characterize plasma fluctuations in the outboard mid-plane scrape-off layer (SOL). Ohmically heated L-mode plasmas with Greenwald fractions from 0.1 to 0.45 are analyzed. In plasmas with a low line-averaged core density, the radial profiles of the average electron density and temperature feature a two-scale structure with a strong gradient in the near-SOL and a weak gradient in the far-SOL. The relative fluctuation levels of the electron density are below 25% throughout the entire SOL and the fluctuation driven heat flux is dominated by plasma density fluctuations. In the plasma with the highest line-averaged core density, the radial profiles of the electron density and temperature feature only a single weak gradient length scale across the SOL. The plasma density at the limiter radius is approximately 10 times larger than in low-density discharges. The average electron temperature at the separatrix falls to approximately 30 eV while the magnitude of the relative temperature fluctuations increases with the line-averaged core plasma density. With increasing plasma density, the fluctuation-driven heat fluxes increase by more than one order of magnitude while retaining a relative fluctuation level of approximately 3. Even though the SOL is on average cooler than in low density discharges, large electron temperature fluctuations govern the turbulence driven heat flux towards the wall while contributions from density fluctuations and triple correlations contribute approximately 35% and 15%, respectively.

In situ relative self-dependent calibration of electron cyclotron emission imaging via shape matching.

Electron Cyclotron Emission Imaging (ECEI) is a diagnostic system which measures 2-D electron temperature profiles with high spatial-temporal resolution. Usually only the normalized electron temperature fluctuations are utilized to investigate the magnetohydrodynamics modes due to the difficulties of ECEI calibration. In this paper, we developed a self-dependent calibration method for 24 × 16 channel high-resolution ECEI on the Experimental Advanced Superconducting Tokamak. The technique of shape matching is applied to solve for the matrix of the calibration coefficients. The calibrated area is further expanded to an occupation ratio of 88% observation area by utilizing the features of sawtooth crash. The result is self-consistent and consistent with calibrated 1D ECE measurement.

Variable location channels to improve efficiency and precision for direct ∇T e measurements and high spatial resolution T e -profile measurements using electron cyclotron emission.

Electron cyclotron emission (ECE) diagnostics that use variable location channels based on yttrium iron garnet (YIG) bandpass filters improve the precision and the efficiency of measurements of electron temperature (T e ) profiles and fluctuations (δT e ). These variable frequency filters were substituted for fixed frequency filters in the intermediate frequency section to achieve the required higher resolution over a target radial range, just before the experiment. Here, we present the proof-of-principle for high temporal resolution measurement of the electron temperature gradient, via real-time slewing of a YIG filter for relocation of an ECE channel during a long pulse. The key component is the application of YIG tunable filters with their narrow bandwidth and capability for a high slew rate of their center frequency. This application permits fast relocation of the ECE channels for direct measurement of the gradient and close spacing of channels to investigate the magnetic island's dynamic behavior.

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.

A reduced Landau-gyrofluid model for magnetic reconnection driven by electron inertia

An electromagnetic reduced gyrofluid model for collisionless plasmas, accounting for electron inertia, finite ion Larmor radius effects and Landau-fluid closures for the electron fluid is derived by means of an asymptotic expansion from a parent gyrofluid model. In the absence of terms accounting for Landau damping, the model is shown to possess a non-canonical Hamiltonian structure. The corresponding Casimir invariants are derived and use is made thereof, in order to obtain a set of normal field variables, in terms of which the Poisson bracket and the model equations take a remarkably simple form. The inclusion of perpendicular temperature fluctuations generalizes previous Hamiltonian reduced fluid models and, in particular, the presence of ion perpendicular gyrofluid temperature fluctuations reflects into the presence of two new Lagrangian invariants governing the ion dynamics. The model is applied, in the cold-ion limit, to investigate numerically a magnetic reconnection problem. The Landau damping terms are shown to reduce, by decreasing the electron temperature fluctuations, the linear reconnection rate and to delay the nonlinear island growth. The saturated island width, on the other hand, is independent of Landau damping. The fraction of magnetic energy converted into perpendicular kinetic energy also appears to be unaffected by the Landau damping terms, which, on the other hand, dissipate parallel kinetic energy as well as free energy due to density and electron temperature fluctuations.

Evolution of space open electric arc burning in the external axial magnetic field

The calculation was made for open DC electric arc burning in an external uniform axial magnetic field. It was performed within the framework of a nonstationary three-dimensional mathematical model in approximation of partial local thermodynamic equilibrium of plasma. A “schematic” analog of electron temperature fluctuations was proposed for numerical realization of the open electric arc column of a helical shape. According to calculations, it was established that the column of the open electric arc takes a helical space shape. Plasma rotates around a longitudinal axis of the arc, at that the directions of plasma rotation near the cathode and the anode are opposite. In the arc cross-sections, the velocity of plasma rotation is unequal and the deviation value of the same part of the arc from the central axis varies in time. A helical shape of the open arc is not stable and varies in time. Apparently, the open arc cannot remain stable and invariable in the time helical shape in the external axial magnetic field.

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.

Intermittent electron density and temperature fluctuations and associated fluxes in the Alcator C-Mod scrape-off layer

The Alcator C-Mod mirror Langmuir probe system has been used to sample data time series of fluctuating plasma parameters in the outboard mid-plane far scrape-off layer. We present a statistical analysis of one second long time series of electron density, temperature, radial electric drift velocity and the corresponding particle and electron heat fluxes. These are sampled during stationary plasma conditions in an ohmically heated, lower single null diverted discharge. The electron density and temperature are strongly correlated and feature fluctuation statistics similar to the ion saturation current. Both electron density and temperature time series are dominated by intermittent, large-amplitude burst with an exponential distribution of both burst amplitudes and waiting times between them. The characteristic time scale of the large-amplitude bursts is approximately 15 . Large-amplitude velocity fluctuations feature a slightly faster characteristic time scale and appear at a faster rate than electron density and temperature fluctuations. Describing these time series as a superposition of uncorrelated exponential pulses, we find that probability distribution functions, power spectral densities as well as auto-correlation functions of the data time series agree well with predictions from the stochastic model. The electron particle and heat fluxes present large-amplitude fluctuations. For this low-density plasma, the radial electron heat flux is dominated by convection, that is, correlations of fluctuations in the electron density and radial velocity. Hot and dense blobs contribute only a minute fraction of the total fluctuation driven heat flux.

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.

Overview of the electron cyclotron emission measurements on EAST

Radiometer systems and a Michelson interferometer, have been operated routinely to detect the electron cyclotron emission (ECE) from EAST plasmas for diagnosing the local electron temperature. A common quasi-optical antenna placed inside the vacuum vessel is employed to collect and focus the plasma emission, and the line of sight is along a radial chord. All of the systems are located in a diagnostic room where the plasma emission is transmitted by overmoded corrugated waveguide. In situ absolute intensity calibration has been carried out for both the radiometer systems and the Michelson interferometer independently, to ensure that the ECE diagnostic provides an independent electron temperature measurement. In order to diagnose the small-amplitude electron temperature fluctuation, a correlation ECE (CECE) diagnostic has been designed and commissioned recently. The CECE diagnostic employs an independent antenna system which has improved poloidal resolution.