Electron Temperature Fluctuations Research Articles

Natural width of the superconducting transition in epitaxial TiN films

Abstract We investigate the effect of various fluctuation mechanisms on the dc resistance in superconducting devices based on epitaxial titanium nitride (TiN) films. The samples we studied show a relatively steep resistive transition (RT), with a transition width δT/Tc ~ 0.002-0.025, depending on the film thickness (20 nm, 9 nm, and 5 nm) and device dimensions. This value is significantly broader than expected due to conventional superconducting fluctuations ( δT/Tc < 10-3). The shape and width of the RT can be perfectly described by the well-known effective medium theory, which allows us to understand the origin of the inhomogeneity in the superconducting properties of TiN films. We propose that this inhomogeneity can have both dynamic and static origins. The dynamic mechanism is associated with spontaneous fluctuations in electron temperature (T-fluctuations), while the static mechanism is due to a random spatial distribution of surface magnetic disorder (MD). Our analysis has revealed clear correlations between the transition width and material parameters as well as device size for both proposed mechanisms. While T-fluctuations may contribute significantly to the observed transition width, our findings suggest that the dominant contribution comes from the MD mechanism. Our results provide new insights into the microscopic origin of broadening of the superconducting transition and inhomogeneity in thin superconducting films.

Experimental observation of micro-tearing modes in the tokamak pedestal

A micro-tearing mode has been observed in electron temperature fluctuations and magnetic fluctuations with the frequency range of 80–120 kHz in the pedestal of the Experimental Advanced Superconducting Tokamak (EAST) using Electron Cyclotron Emission Imaging (ECEI) and Mirnov magnetic coil measurement. It appears in the type-I ELMs period after the L-H transition. The measured poloidal phase velocity of the electron temperature fluctuations is ∼40 km/s (f = 80–120 kHz) in the electron diamagnetic direction in the laboratory frame with the poloidal wave number around kΘ ∼0.15 /cm. The two-dimensional structure of MTMs is directly observed using two-dimensional Electron cyclotron emission imaging. The experiment confirmed that the electron temperature (ECEI) and radial electric field (Backward Doppler reflectometer) during MTM meet the pressure balance equation. Until now, the theory and simulation results showed that transport is in heat flux, small particle flux, while the experimental results of this paper indicate that the particle flux is small.

Long radial coherence of electron temperature fluctuations in non-local transport in HL-2A plasmas

The dynamics of long-wavelength (kθ < 1.4 cm−1), broadband (20 kHz–200 kHz) electron temperature fluctuations ( T∼e/Te ) of plasmas in gas-puff experiments are observed for the first time in HL-2A tokamak. In a relatively low density (n e(0) ≃ 0.91 × 1019m−3–1.20 × 1019m−3) scenario, after gas-puffing the core temperature increases and the edge temperature drops. On the contrary, temperature fluctuation drops at the core and increases at the edge. Analyses show the non-local emergence is accompanied with a long radial coherent length of turbulent fluctuations. While in a higher density (n e(0) ≃ 1.83 × 1019 m−3–2.02 × 1019 m−3) scenario, the phenomena are not observed. Furthermore, compelling evidence indicates that E × B shear serves as a substantial contributor to this extensive radial interaction. This finding offers a direct explanatory link to the intriguing core-heating phenomenon witnessed within the realm of non-local transport.

Study on the Physical Characteristics of Plasma and Its Relationship with Pore Formation during Laser-Metal Active Gas Arc Hybrid Welding of 42CrMo Steel

The automobile industry puts forward higher requirements for the design and manufacture of steel pistons. However, the welding of 42CrMo steel pistons still has unsolved technical problems, especially welding defects that cannot be directly detected, such as pores, which are easily generated inside the weld. A plasma experiment of laser-metal active gas arc (MAG) hybrid welding 42CrMo steel was conducted in this paper, and plasma signals inside and outside the keyhole were detected during the laser welding, leading laser laser-MAG hybrid welding, and leading arc laser-MAG hybrid welding of 42CrMo steel. The characteristic parameters such as electron temperature and electron density were calculated and analyzed to investigate the relationship between plasma behavior and the formation of weld porosity in the welding process of 42CrMo steel. Based on the fluctuations in plasma electron temperature and electron density, the prediction of pore formation in the weld of 42CrMo steel was made, aiming to provide guidance for achieving a stable and reliable laser-MAG hybrid welding process for 42CrMo steel.

The observation of avalanche-like transport during confinement power degradation in Heliotron J

In this study, we observe avalanche-like electron thermal transport in Heliotron J under plasma conditions that exhibit power degradation to the central electron cyclotron heating (ECH). The newly installed GHz sampling electron cyclotron emission diagnostic allows for the observation of the long-distance radial propagation of electron temperature fluctuations. We find that the electron temperature fluctuations are associated with avalanches, because the Te fluctuations (i) propagate from the core to the edge with a speed comparable to the diamagnetic drift velocity, (ii) exhibit a 1/f power-law scaling in the frequency spectrum and a Hurst exponent close to 1, and (iii) dominate in the ECH deposition location and spread to the edge as the heating power increases. Furthermore, the electron heat avalanches can spread to the scrape off layer region when they are enhanced, i.e., the Te fluctuations correlate with the Dα emission, which also has a frequency spectrum that scales with a 1/f power law.

Effects of ion trapping and fluctuations of electron temperature and plasma flow on cross-beam energy transfer

The influences of ion trapping and fluctuations of electron temperature and plasma flow on cross-beam energy transfer (CBET) are examined using two- and three-dimensional particle-in-cell simulations in parameter regimes relevant to recent CBET experiments at the OMEGA laser facility. In mid-Z plasma irradiated by an intense pump beam and weaker probe beam, ion trapping, collisional de-trapping, and plasma flow induced by thermal effects are shown to affect the CBET gain. Ion trapping can enhance or detune the CBET resonance [Nguyen et al., Phys. Plasmas 28, 082705 (2021)]. Collisional de-trapping can affect the CBET gain at low seed beam intensity near the onset threshold for ion trapping. Thermal-effects-induced flow can also detune the CBET resonance at a level comparable to that from trapping at low seed beam intensity. As a consequence, the CBET gain is sensitive to collisions and dimensionality at low seed beam intensity where ion trapping is weak but is insensitive to collisions and dimensionality at high seed beam intensity where ion trapping is strong.

Development of correlation ECE system for electron temperature fluctuation measurement in LHD

Correlation-ECE (C-ECE) is a standard method for investigating turbulence driven transport. This method allows electron temperature fluctuations that contain information on turbulent transport and independent thermal noise. The turbulence feature is extracted from a correlation analysis from two close locations. The A C-ECE system is utilized on the large helical device (LHD) to measure emission within the frequency range of 74–79.6 GHz. This system employs the spectral decorrelation method and serves as a collective Thomson scattering diagnostic receiver in the LHD. The C-ECE receiver system is comprised of a filter bank system with 32 band-pass filters and a fast digitizer system operating at a sampling rate of 12.5 GHz in the intermediate frequency (IF) stage. This study presents initial experimental results on temperature fluctuation spectra in the LHD, obtained through the C-ECE system using a coherency-based analysis method. An MHD mode at 5 kHz is excited from the onset of neutral beam injection in a magnetic probe, and coherence spectra are obtained from two C-ECE receiver systems. The temperature fluctuation results are derived from the coherence spectrum after bias removal and indicate a level of approximately 3% in the frequency range of 0 to 400 kHz. Further investigations will be conducted to explore drift wave turbulence activities and reconstruct the radial profile of temperature fluctuation in the LHD using the C-ECE receiver systems.

Spontaneous formation of a transport barrier in helium plasma in a tokamak with circular configuration

We report on the first experimental observation of a spontaneously formed transport barrier in the tokamak with a circular configuration in helium plasmas. There was no external polarization of the plasma by electric field or other technique to form the barrier as it is typically used in tokamaks with circular plasma. In general, the transport barriers play an important role in plasma confinement especially in tokamaks with divertor configuration. In our experiments, we clearly observe distinct characteristics of a transport barrier, including a steep gradient of the electron temperature and an enhanced radial electric field along with the change in the plasma potential, floating potential, and electron temperature fluctuation. The electron temperature and the plasma potential are obtained by a combination of the ball-pen and Langmuir probe measurements with high temporal resolution on a shot-to-shot basis. This first experimental observation of the spontaneously formed transport barrier might bring new possibilities to obtain a fusion-relevant study of the edge plasma parameters and transport in helium plasmas even on small tokamaks.

Isotope effects on energy transport in the core of ASDEX-Upgrade tokamak plasmas: Turbulence measurements and model validation

Design and operation of future tokamak fusion reactors using a deuterium–tritium 50:50 mix requires a solid understanding of how energy confinement properties change with ion mass. This study looks at how turbulence and energy transport change in L-mode plasmas in the ASDEX Upgrade tokamak when changing ion species between hydrogen and deuterium. For this purpose, both experimental turbulence measurements and modeling are employed. Local measurements of ion-scale (with wavevector of fluctuations perpendicular to the B-field k⊥&amp;lt; 2 cm−1, k⊥ρs&amp;lt; 0.2, where ρs is the ion sound Larmor radius using the deuterium ion mass) electron temperature fluctuations have been performed in the outer core (normalized toroidal flux ρTor=0.65−0.8) using a multi-channel correlation electron cyclotron emission diagnostic. Lower root mean square perpendicular fluctuation amplitudes and radial correlation lengths have been measured in hydrogen vs deuterium. Measurements of the cross-phase angle between a normal-incidence reflectometer and an ECE signal were made to infer the cross-phase angle between density and temperature fluctuations. The magnitude of the cross-phase angle was found larger (more out-of-phase) in hydrogen than in deuterium. TRANSP power balance simulations show a larger ion heat flux in hydrogen where the electron-ion heat exchange term is found to play an important role. These experimental observations were used as the basis of a validation study of both quasilinear gyrofluid trapped gyro-Landau fluid-SAT2 and nonlinear gyrokinetic GENE codes. Linear solvers indicate that, at long wavelengths (k⊥ρs&amp;lt;1), energy transport in the deuterium discharge is dominated by a mixed ion-temperature-gradient (ITG) and trapped-electron mode turbulence while in hydrogen transport is exclusively and more strongly driven by ITG turbulence. The Ricci validation metric has been used to quantify the agreement between experiments and simulations taking into account both experimental and simulation uncertainties as well as four different observables across different levels of the primacy hierarchy.

Magnetospheric Multiscale measurements of turbulent electric fields in earth's magnetosheath: How do plasma conditions influence the balance of terms in generalized Ohm's law?

Turbulence is ubiquitous within space plasmas, where it is associated with numerous nonlinear interactions. Magnetospheric Multiscale (MMS) provides the unique opportunity to decompose the electric field (E) dynamics into contributions from different linear and nonlinear processes via direct measurements of the terms in generalized Ohm's law. Using high-resolution multipoint measurements, we compute the magnetohydrodynamic (EMHD), Hall (EHall), electron pressure (EPe), and electron inertia (Einertia) terms for 60 turbulent magnetosheath intervals, to uncover the varying contributions to the dynamics as a function of scale for different plasma conditions. We identify key spectral characteristics of the Ohm's law terms: the Hall scale, kHall, where EHall becomes dominant over EMHD; the relative amplitude of EPe to EHall, which is constant in the sub-ion range; and the relative scaling of the nonlinear and linear components of EMHD and of EHall, which are independent of scale. We find expressions for the characteristics as a function of plasma conditions. The underlying relationship between turbulent fluctuation amplitudes and ambient plasma conditions is discussed. Depending on the interval, we observe that EMHD and EHall can be dominated by either nonlinear or linear dynamics. We find that EPe is dominated by its linear contributions, with a tendency for electron temperature fluctuations to dominate at small scales. The findings are not consistent with existing linear kinetic Alfvén wave theory for isothermal fluctuations. Our work shows how contributions to turbulent dynamics change in different plasma conditions, which may provide insight into other turbulent plasma environments.

Energetic ion and plasma oscillation measurements during plume mode operation of a hollow cathode

Hollow cathodes are important devices used for spacecraft electric propulsion. The hollow cathode has two operational modes. One mode is a stable mode called the spot mode, and the other is an unstable mode called the plume mode. Operation in plume mode should be avoided since the instability causes high-energy ions that sputter-erode the cathode parts. In this study, the relationship between discharge oscillations and ion energy distribution in plume mode was investigated using a triple Langmuir probe and retarding potential analyzer for a 40-A class xenon hollow cathode with a lanthanum hexaboride emitter. The triple probe can measure unsteady electron temperature and plasma density oscillations. The electron temperature was not so high, 1 to 2 eV. Some instabilities were observed in the plume mode. The ionization instability with a low frequency oscillation of 30 kHz was the dominant mode. A broad spectrum around 330 kHz due to ion acoustic turbulence was observed. In addition, in the downstream plume region, oscillations around 120 kHz were observed owing to temporal change in anomalous resistivity. The 95% ion population voltage found to be 20 and 30 eV in spot and plume modes, respectively. The magnitude of the low frequency ionization oscillation was found to be inversely proportional to ion energy in plume mode. This indicates that the resonant energy transfer from the oscillation to the ion energy through Landau damping probably plays an important role in high energy ion generation in plume mode. A clear correlation between discharge current and electron temperature waveforms was found. The larger the electron temperature fluctuation, the stronger the correlation between discharge current and electron temperature, and the larger the phase difference deviation from 180°.

Magnetic fields from compensated isocurvature perturbations

Compensated isocurvature perturbations (CIPs) are perturbations to the primordial baryon density that are accompanied by dark-matter-density perturbations so that the total matter density is unperturbed. Such CIPs, which may arise in some multi-field inflationary models, can be long-lived and only weakly constrained by current cosmological measurements. Here we show that the CIP-induced modulation of the electron number density interacts with the electron-temperature fluctuation associated with primordial adiabatic perturbations to produce, via the Biermann-battery mechanism, a magnetic field in the post-recombinaton Universe. Assuming the CIP amplitude saturates the current BBN bounds, this magnetic field can be stronger than $10^{-15}\,\mathrm{nG}$ at $z\simeq20$ and stronger by an order of magnitude than that (produced at second order in the adiabatic-perturbation amplitude) in the standard cosmological model, and thus can serve as a possible seed for galactic dynamos.

Coherent mode induced by supersonic molecular beam injection in EAST Ohmic plasmas

A coherent mode (CM) induced by the supersonic molecular beam injection (SMBI) was observed in Ohmic plasmas of EAST tokamak. The CM existed in the spectra of the electron temperature fluctuation and density fluctuation, but not in the spectrum of Mirnov fluctuation. The radial location of the CM was about 10 cm inside the separatrix (normalized minor radius ). In most cases, the CM has multiple discrete frequencies between with an interval of ∼1 kHz. Occasionally, there is only one dominant frequency. The poloidal scale of the CM (poloidal wavenumber , poloidal mode number ) is between that of typical ion turbulence and macroscopic magnetohydrodynamic activities. Statistical analysis results suggest that the onset of the CM requires a high temperature gradient and a relatively low density gradient. Furthermore, the transport analysis shows that the SMBI-caused perturbation propagates inward faster in plasmas with stronger CM, which implies that the CM might be correlated to a high local transport level. However, the impact of the CM on macroscopic plasma parameters including stored energy, energy confinement time, and SMBI-caused mean density fluctuation is too weak to be found in comparison with the random fluctuation and the measurement errors, which suggests that the CM does not have a clear influence on the global confinement.

A Proposal to Evaluate Electron Temperature and Electron Density Fluctuations Using Dual Wavelength Emission Intensity Tomography in a Linear Plasma

A procedure to calculate normalized electron temperature and electron density fluctuations using dual wavelength emission intensity tomography in a linear magnetized plasma is proposed. Expression of emission intensity is modeled as \(\epsilon_{k} \propto T_{\text{e}}^{\alpha_{k}}n_{\text{e}}^{2}\) where \(\epsilon_{k}\) is emission intensity, Te is electron temperature, ne is electron density, k = 1 for ArI, and k = 2 for ArII. Using the model, normalized Te and ne fluctuations can be expressed as linear function of both the normalized emission intensity fluctuations. To calculate αk, we perform least squares method between real \(\epsilon_{k}\) measured with a tomography system and artificial \(\epsilon_{k}\) reconstructed using Te and ne measured with a Langmuir probe located downstream of the tomography system. The results show that α1 = 2.2 and α2 = 2.6, and normalized Te and ne fluctuations are preliminarily evaluated from emission intensity tomography data.

Physics Studies for Assessment of Requirement of Displacement Compensation System for ITER ECE Diagnostic

The Electron Cyclotron Emission (ECE) diagnostic has the key function of measuring the core electron temperature profile and electron temperature fluctuation, from the intensity of electron cyclotron radiation emitted from the plasma along the major radius. The ECE diagnostic consists of three main systems: (1) front-end optics, which collects the radiation from the plasma, (2) transmission lines including polarizer splitter unit, which transports the ordinary and extraordinary ECE emission modes separately from the front-end and distributes it to the instrumentation, and (3) detection and analysis instrumentation which is housed at a distance from the tokamak, in the diagnostics building [1]. With its high electron temperatures and harsh environment, ITER presents various challenges for the diagnostic system. One of the most insidious is the misalignment between the in-vessel front-end optics and the ex-vessel transmission line which is caused by vibration of the vacuum vessel during operational and baking phases. Since the electron temperature is inferred from the intensity of the ECE, transient misalignment may lead to poor accuracy in this critical measurement. These displacements are expected to be ~ 15 mm in vertical (z) and horizontal (x) directions, and ~ 5 mm in the toroidal (y) direction. It is important to minimize the effect of these displacements, so that the system maintains alignment during operation, and reliable temperature information is attained. Our objective is to first study the coupling losses due to imperfect coupling of Gaussian beams owing to port plug displacements. Measurements are done to determine the power loss due to coupling of offset beams experimentally. The measured value for coupling loss is ~2 dB at 120 GHz, which is quite high, and it is therefore concluded that a mechanism is needed to compensate for the displacements.

Design of Stray Radiation Sensor for ITER ECE Diagnostic

The Electron Cyclotron Emission (ECE) diagnostic has a primary role in the measurement of electron temperature profile and electron temperature fluctuations in ITER. This diagnostic shall be exposed to significant power due to unabsorbed Electron Cyclotron Heating (ECH) power in the plasma. The expected stray power loads could be a few tens of watts, and therefore, the protection of millimetre wave components is one of the design challenges of ITER ECE diagnostic. This protection system includes sensors, a band stop notch filter, and a shutter to stop the RF stray radiation from being incident on the sensitive components. The sensors will be positioned along the ECE transmission line, and shall be used for real-time power monitoring of the stray radiation. Here, we describe a novel design of a sensor for monitoring the stray radiation power. This sensor is a Schottky Diode rectenna, known for high-power and high-speed millimetre wave detection capability. It consists of a 2x2 microstrip patch antenna array, a matching circuit, a diode, and a low pass filter. The antenna array is designed analytically and optimized in CST Microwave Studio, for wide reception angle, high gain, and low side lobe levels. Furthermore, the rectifying circuit is optimized using Agilent Advanced Design System (ADS) software to get better rectification and impedance matching of the signal, thereby improving its detection sensitivity. The ADS simulation results show that the detection sensitivity is about 1000V/W for input power of -30 dBm at 170 GHz, thereby achieving the required performance of the sensor.

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 &lt;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.

New Q and V-band ECE radiometer for low magnetic field operation on LHD

To meet the demand for information on electron temperature fluctuations in low magnetic field experiments in the Large Helical Device (LHD), a new ECE radiometer covering the Q and V bands has been installed. Combination mirrors are installed in the vacuum vessel to focus the beam and efficiently propagate the radiated electron cyclotron waves. Notch filters are used to eliminate stray light from the gyrotron, and a 32-channel heterodyne radiometer is constructed using a filter bank system. As a result, oscillations of electron temperature and both electromagnetic and electrostatic fluctuations were successfully observed.

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.

Electron temperature fluctuation levels of the quasi-coherent mode across the plasma radius

EDA H-mode is an ELM-free regime in which the edge quasi-coherent mode (QCM) replaces the ELMs. The estimated location of the quasi-coherent mode is in a partly optically thin region of steep gradients localized between ρpol = 0.96 -1. Relative fluctuations of radiation temperature between 15 and 80 kHz are about 7% with significant density contribution. In the electron cyclotron emission (ECE) channels with resonances in the plasma core, a mode with the same frequency as the quasi-coherent mode is measured. The peak amplitude of both core and edge modes matches the strongest electron temperature gradient in the core and the edge, respectively. The ECE core and edge signals are out of phase. The radiation transport forward model (ECRad) shows that the refraction explains the phase relation between the edge and the core ECE channels. The phase correlates with the sign of the core Te. The amplitude of the fluctuations in the core decreases with decreasing gradients, which is the trend seen in the experiment. The amplitude ratio of the core and edge fluctuation is a factor of five in the experiment; this ratio remains a factor of a hundred in the modeling.