Electron Temperature Profiles Research Articles

Electron Temperature Profiles Formed under Conditions of the Axial ECR Plasma Heating at the L-2M Stellarator

The shape is analyzed of the electron temperature profiles measured in experiments on the axial ECR heating at the L-2M stellarator at different densities and heating powers. It is ascertained that when the reduced ECR heating power exceeds some threshold value, the electron temperature profiles become broadened and flat in the core region of the plasma column. The correlation is revealed between the appearances of the flat temperature profiles and the density profiles with a dip in the axial region (the dip-shaped profiles) occurring as a result of the density pump-out effect. It is concluded that the appearance of the dip-shaped density profiles in the plasma results in a change in the mechanism of microwave radiation absorption. Hypothetically, the cyclotron absorption of the electron Bernstein waves becomes the dominant mechanism for the microwave radiation absorption that, in turn, results in the formation of the broad and flat (in the core region) electron temperature profiles.

Evidence for electron energization accompanying spontaneous formation of ion acceleration regions in expanding plasmas

We report experiments conducted in an expanding argon plasma generated in the inductive mode of a helicon source in the Hot hELIcon eXperiment–Large Experiment on Instabilities and Anisotropies facility. As the neutral gas pressure increases, the supersonic ion acceleration weakens. Increasing neutral pressure also alters the radial profile of electron temperature, density, and plasma potential upstream of the plasma expansion region. Langmuir probe measurements of the electron energy probability function (EEPF) show that heating of electrons at the plasma edge by RF fields diminishes with increasing gas pressure, yielding a plasma with a centrally peaked electron temperature, and flat potential profiles at higher neutral pressures. For neutral pressures at which ion acceleration regions develop in the expanding plasma plume, EEPFs reveal electrons with two temperature components.

Estimation and Uncertainties of Profiles and Equilibria for Fusion Modeling Codes

The results of transport modeling codes, e.g., GENE for the plasma core or SOLPS-ITER for the plasma edge, depend critically on reliable profile and equilibrium estimates. The propagation of uncertainties (UP) of input quantities to the results of modeling codes, e.g., power and particle exhaust and plasma stability, is frequently neglected because of the costs of running the codes as well as because of the missing uncertainty quantification of input quantities. The situation becomes even more cumbersome if profile gradients and their uncertainties are of major concern for transport analyses. Two different techniques are presented to estimate profiles, profile gradients, their uncertainties, and candidate profiles for UP in modeling codes. Markov Chain Monte Carlo sampling of the posterior probability density of an integrated data analysis approach is applied to estimate electron density and temperature profiles. Nonstationary Gaussian process regression is applied to estimate ion temperature and angular velocity profiles. Both methods provide in a natural way profile gradients, profile logarithmic gradients, and their uncertainties. Modeling codes benefit also from reliable equilibrium reconstructions and quantification of the uncertainty of various equilibrium parameters. For the analysis of diagnostics data, the position and uncertainty of flux surfaces as well as of the magnetic axis are important. For plasma transport and stability codes, the estimation of uncertainties of current and q-profiles is presented. For plasma edge codes the position of the separatrix contour and its uncertainty at various poloidal positions is of primary interest especially if steep profile gradients are present. Examples of uncertainties and their sources in magnetic scalar quantities, profiles, and separatrix contours are shown.

Development and characterization of thermal helium beam diagnostic with four helium lines for RFX-mod2 experiment.

Thermal Helium Beam (THB) diagnostic is widely used for measuring the electron density and temperature in the boundary region of fusion plasmas, edges, and scrape-off layers. In its standard configuration, it measures three HeI lines (667.8 nm, 706.5 nm, and 728.1 nm) and, by using a collisional-radiative model, evaluates ne and Te from the ratios of their intensities. At large neutral He density (n0 ≳ 1017 m-3), radiation re-absorption is not negligible and it has to be taken into account; it can be estimated by measuring the intensity of the fourth HeI line, λ = 501.6 nm. The original THB diagnostic of the RFX-mod experiment has been upgraded, setting up the fourth line intensity acquisition. A Czerny-Turner spectrograph separates the lines, and the old multichannel photomultiplier (PMT) detectors are replaced with the new Multi-Pixel Photon Counter (MPPC). Two 16-channel MPPC array modules allow the observation of 32 signals (4 lines × 8 spatial points). Since the MPPCs are not sensitive to the magnetic field, the whole system can be installed near the experimental device, allowing a large reduction in the optical fibers' length with a gain in the collected signal intensity. This new THB will be installed in the new experiment RFX-mod2, the upgrade of the RFX-mod device. The RFX-mod2 will operate as both reversed field pinch and tokamak, and the goal of the THB is the evaluation of the edge electron density and temperature profiles in the two magnetic configurations, in D or H plasmas. This paper describes the system, the performance of the MPPC compared with the PMTs, the alignment, and the calibration.

Temporal Profiling of Electron Temperatures Using the Hα–Hβ Line Emission and Triple Langmuir Probe Array in the Pre-Ionization Discharge of the MT-I Spherical Tokamak

Photodiodes masked with narrow-band filters have been used to obtain the temporal profiles of Hα and Hβ line emissions in the pre-ionization phase of hydrogen discharge in the MT-I Spherical Tokamak. The line ratio method relating the emission intensities of the above mentioned lines, having different excitation thresholds, provides the temporal profile of electron temperature. A triple Langmuir probe array having three individual sets of triple probes, arranged linearly, has also been used to measure the temporal profile of electron temperature at three different radial positions simultaneously. Additionally, a spectrometer HR4000+ is used to get the line-integrated emission intensity of H-Balmer lines. The objective of this experiment is to demonstrate the successful development of the optical and electric probe as a diagnostic tool for tokamak discharge.

I-mode pedestal relaxation events at ASDEX Upgrade

The I-mode confinement regime can feature small edge temperature drops that can lead to an increase in the energy deposited onto the divertor targets. In this work, we show that these events are associated with a relaxation of both electron temperature and density edge profiles, with the largest drop found at the pedestal top position. The relative energy loss is about 1 %, and is thus lower than that of type-I ELMs for the same pedestal top collisionality. Stability analysis of edge profiles reveals that the operational points are far from the ideal peeling-ballooning boundary. Also, we show that these events appear close to the H-mode transition in the typical I-mode operational space in ASDEX Upgrade, and that no further enhancement of energy confinement is found when they occur. Moreover, scrape-off layer transport during these events is found to be very similar to type-I ELMs, with regard to timescales (≈ 800 µs), filament propagation, toroidally asymmetric energy effluxes at the midplane and asymmetry between inner and outer divertor deposited energy. In particular, the latter reveals that more energy reaches the outer divertor target. Lastly, first measurements of the divertor peak energy fluence are reported, and projections to ARC—a reactor that could potentially operate in I-mode—are drawn.

Formation of flat electron density profiles at the L-2M stellarator at high ECRH powers

The shapes of the electron temperature profiles were analyzed at the L-2M stellarator based on the data obtained in experiments on the axial ECR plasma heating performed at different plasma densities and heating powers. It was ascertained that, at the reduced ECRH power exceeding some threshold value, the electron temperature profiles become broadened and flat in the axial region of the plasma column. The appearance of the flat temperature profiles correlates with the occurrence of the density profiles with a dip in the axial region forming at high reduced heating powers as a result of the density pump-out effect. It was concluded that, as a result of occurrence of the density profiles with a dip in the axial region, the mechanism for the microwave radiation absorption changes. Apparently, the cyclotron absorption of the electron Bernstein waves becomes the main mechanism for the microwave radiation absorption, which, in turn, results in the occurrence of the broad and flat (in the axial region) electron temperature profiles.

Plasma Profiles Modifications by Upgraded Power of IBW Heating System in HT-7 Tokamak

The enhancement in local and globalized electron heating has been observed after upgrading of ion Bernstien waves (IBWs) heating system in HT-7 tokamak. The overall capability of injection power has been upgraded from 350 to 600 kW (15–30 MHz) for best and active control of pressure and current density profiles, and to discover the new phenomena of plasma heating for good energy and particle confinement. A new quadruple T-type IBWs coupling antenna has been utilized which has been mounted in the toroidal direction on low field side in the device. The electron heating on both on-axis and off-axis electrons not delete has been observed. Not delete The change in direct electron heating via electron Landau damping (ELD) from IBWs has been investigated, while the bulk electron temperature showed a large rise with a heating factor, ∆(Tene)/Prf, up to 9.3 × 1019 eV m–3 kW–1 with central electron density (ne) 3.7 × 1019 m–3. The particle confinement has also been achieved with IBW injection power of 550 kW at frequency of 27 MHz and with toroidal magnetic field of 1.92 T. The modifications observed in electron temperature and other plasma profiles are discussed under various plasma conditions. The improved energy confinement has also been observed in this scenario.

Observation of helical m/n = 1/1 saturated steady mode in EAST pure electron heating scenario with q0 ≼ 1

We observed for the first time a helical m/n = 1/1 (m is the poloidal mode number and n is the toroidal mode number) saturated steady mode (SSM) in the center of the EAST electron heating dominant plasma where the core safety factor was close to but slightly below unity and the profile had an extreme peak at the plasma center ( is the electron temperature). An internal crash can be caused by the dynamics of the SSM, and its influence on the electron temperature profile is as large as that of a typical sawtooth crash (SC) in magnitude. Due to the weak magnetic shear in the core, the SSM regularly exhibits an m/n = 2/2 harmonic component. For low shear auxiliary heated plasma, the SSM cannot cause the degradation of plasma confinement ( or ), but an SC is indeed a harmful factor. Three-dimensional resistive magnetohydrodynamic simulations with a realistic EAST strong elongated magnetic configuration, a high Lundquist number () and a strong central peaked pressure profile, have been made to demonstrate the formation of the SSM in the case of and the destabilization of the SC in another case with . These nonlinear simulation results agree well with the observations of both the SSM and SC in EAST electron heating dominant discharges with an extreme central peaked . An M3D simulation found that the SC is caused by additional newly developed harmonics of the m/n = 1/1 helical instability. Nonlinear simulation also predicted a modulation of the SSM on current in the plasma core, which is favorable for maintaining in the plasma core. Furthermore, nonlinear simulation also showed that a small toroidal plasma flow can be generated by the SC.

Simulation of EAST edge plasma using SOLPS-ITER/BOUT++ coupling

The self-consistent simulation of transport and turbulence in tokamak edge plasma on the EAST tokamak is performed by coupling the fluid plasma/neutral 2D transport code SOLPS-ITER and the fluid 3D turbulence code BOUT++. The convergence of the coupling is validated. The changing rates between the two consecutive profiles of density, electron temperature and ion temperature are close to zero in the last several iteration steps, and the variations of the transport coefficients Dr, χe and χi also show the tendency to diminish as the iteration step increases, indicating that the coupling tends to be in a quasi-steady state. The edge plasma at the time t = 3400 ms in shot 36 780 with deuterium discharge on EAST is simulated by the SOLPS-ITER/BOUT++ coupling. The MHD equilibrium at the shot time with the lower single-null configuration is used for the production of the computational meshes for SOLPS-ITER and BOUT++, respectively. The computational profiles of electron density and temperature, particle and heat fluxes at the divertor targets from the SOLPS-ITER/BOUT++ coupling simulation are compared with the measured profiles, and it is found that the computational and measured profiles are similar.

Real-time applications of Electron Cyclotron Emission interferometry for disruption avoidance during the plasma current ramp-up phase at JET

Data produced by an electron cyclotron interferometer diagnostic are now available to the real-time control systems of of the joint European torus (JET) tokamak. The data consist of absolutely calibrated electron temperature profiles, covering the plasma low-field side, core and part of the high-field side, for most of the range of magnetic fields used at JET. The profiles are obtained without the need for real-time equilibrium reconstructions. A simple and robust metric for the peakedness of the profiles was defined and employed in a real-time control system. The main goal was to identify discharges at risk of disruption, after impurity accumulation in the core, and terminate them safely. This was successfully tested on the current ramp-up phase of hybrid plasmas, developed in support of high-performance scenarios for the upcoming deuterium-tritium JET campaign.

W-band system-on-chip electron cyclotron emission imaging system on DIII-D.

Monolithic, millimeter-wave "system-on-chip" (SoC) technology has been employed in heterodyne receiver integrated circuit radiometers in a newly developed Electron Cyclotron Emission Imaging (ECEI) system on the DIII-D tokamak for 2D electron temperature profile and fluctuation evolution diagnostics. A prototype module operating in the E-band (72 GHz-80 GHz) was first employed in a 2 × 10 element array that demonstrated significant improvements over the previous quasi-optical Schottky diode mixer arrays during the 2018 operational campaign of the DIII-D tokamak. For compatibility with International Thermonuclear Experimental Reactor relevant scenarios on DIII-D, the SoC ECEI system was upgraded with 20 horn-waveguide receiver modules. Each individual module contains a University of California Davis designed W-band (75 GHz-110 GHz) receiver die that integrates a broadband low noise amplifier, a double balanced down-converting mixer, and a ×4 multiplier on the local oscillator (LO) chain. A ×2 multiplier and two IF amplifiers are packaged and selected to further boost the signal strength and downconvert the signal frequency. The upgraded W-band array exhibits >30 dB additional gain and 20× improvement in noise temperature compared with the previous Schottky diode radio frequency mixer input systems; an internal 8 times multiplier chain is used to bring down the LO frequency below 12 GHz, thereby obviating the need for a large aperture for quasi-optical LO coupling and replacing it with coaxial connectors. Horn-waveguide shielding housing avoids out-of-band noise interference on each individual module. The upgraded ECEI system plays an important role for absolute electron temperature evolution and fluctuation measurements for edge and core region transport physics studies.

Parametric decay instabilities near the second-harmonic upper hybrid resonance in fusion plasmas

Parametric decay instabilities (PDIs) lead to the generation of strong frequency-shifted radiation when powerful X-mode polarized microwaves, injected for electron cyclotron resonance heating (ECRH) of fusion plasmas, cross a region of non-monotonic electron density near the second-harmonic upper hybrid resonance (UHR). For the standard second-harmonic X-mode ECRH scenarios used at the ASDEX Upgrade tokamak, the second-harmonic UHR occurs near the plasma edge, meaning that PDIs occur in connection with phenomena leading to non-monotonic edge electron density profiles, e.g. blobs, edge-localized modes (ELMs), and inter-ELM modes. We present the first study of strong signals near half the ECRH frequency in fusion-relevant plasmas and demonstrate their PDI-like features, such as an ECRH power threshold required for their occurrence, through analog modulations of the ECRH power. The signals near half the ECRH frequency are found to be far more prevalent than expected based on previous theories, and we hence present a qualitative explanation of their origin, in terms of cross-polarized microwaves generated through non-linear interactions of the waves involved in the PDIs, which captures the basic features of the experimental observations. We additionally present examples of the PDI-related microwave signals just below the ECRH frequency which indicate the location of the microwave source. Furthermore, based on a non-linear magnetohydrodynamics simulation of an ELM crash in ASDEX Upgrade, performed using the JOREK code, the duration of the PDI-like microwave spikes observed in connection with ELMs is shown to match the transit time of an ELM filament though an ECRH beam. Finally, the PDI power threshold expected theoretically, calculated using the electron density and temperature profiles from the JOREK simulation, is shown to match the experimentally observed values.

Radial control of electron temperature gradient with optimized operational configuration of double plasma device

A double plasma device (DPD) is tested for different operational configurations to identify suitable control for plasma parameters in a weakly ionized, unmagnetized plasma discharge. A separation grid is installed between the two chambers of DPD, which allows filtering of plasma from the first to the second chamber. Investigations are carried out to control the plasma parameters, especially the electron temperature by exploring the role of the grid. The grid bias is varied between −25–0 V and 0–30 V to reflect cooling and heating of plasma. The electron heating and cooling is prominent for the maximum ratio of n source/n target. The electron energy distribution function (EEDF) is obtained to describe the role of grid biasing in controlling the electron temperature in the second chamber. We demonstrated control on the radial profile of electron temperature by charging different radial cross-sections of plasma differently by using a multi-grid assembly system (MGAS). We have also identified the suitable operational regime for DPD where exercising a radial control on electron temperature is possible. Such plasmas can facilitate investigations on electron temperature control for applications in plasma processing, cold–plasma material interaction, etc where low energy electrons are desired.

Delayed ionization and excitation dynamics in a filament wake channel in a dense-gas medium

A unified theoretical description is developed for the formation of an ionized filament channel in a dense-gas medium and the evolution of electronic degrees of freedom in this channel in the laser pulse wake, as illustrated on an example of high-pressure argon. During the laser pulse, the emerging free electrons gain energy via inverse Bremsstrahlung on neutral atoms, enabling impact ionization and extensive collisional excitation of the atoms. A kinetic model of these processes produces the radial density distributions in the immediate wake of the laser pulse. After the pulse, the thermalized electron gas drives the system evolution via impact ionization (from the ground and excited states) and collisional excitation of the residual neutral atoms, while the excited atoms are engaged in Penning ionization. The interplay of these three processes determines the electron gas cooling dynamics. The local imbalance of the free-electron and ion densities induces a transient radial electric field, which depends critically on the electron temperature. The evolving radial profiles of the electron, ion, and excited-atom densities, as well as the profiles of electron temperature and induced electric field are obtained by solving the system of diffusion-reaction equations numerically. All these characteristics evolve with two characteristic timescales, and allow for measuring the electronic stage of the wake channel evolution via linear and nonlinear light-scattering experiments.

Effect of fuel isotope mass on q-profile formation in JET hybrid plasmas

The initial current ramp phase of JET hybrid plasmas is used to optimise the target q-profile for main heating to allow access to high β and avoid MHD instabilities. Mixed protium-deuterium experiments, carried out at JET since the installation of the beryllium-tungsten wall, have shown that the q-profile evolution during this Ohmic phase varies systematically with average main ion isotope mass, indicating the need for re-optimisation for future T and mixed D-T experiments. Current diffusion modelling shows that the key factor was a reduction in electron temperature profile peaking as the hydrogenic isotope mass was increased. This was correlated with an increase in plasma radiation by metallic impurities, consistent with the increased sputtering yield by higher mass isotopes during the current ramp phase. Reduced electron temperature peaking can lead to magnetic shear reversal and the appearance of a 2/1 mode, which can lock, causing the JET massive gas injection system to be triggered to avoid an unmitigated disruption. The potential for a further reduction in electron temperature peaking in T and D-T plasmas could, therefore, result in an increased risk of disruptions. To mitigate this risk, electron temperature peaking measurements have been included in the real-time control system to allow this type of disruption to be avoided by central heating, density increase or early pulse termination. These experiments indicate the need for integrated modelling of impurity behaviour, including the plasma core, scrape-off layer and plasma wall interactions, to predict q-profile evolution in the current ramp phase and anticipate the effects of isotope changes.

Measurement of Electron Temperature Profile and Fluctuation with ECE Radiometer System in Heliotron J

Measurement of Electron Temperature Profile and Fluctuation with ECE Radiometer System in Heliotron J

Observation of water-window soft x-ray emission from laser-produced Au plasma under optically thin condition

We present a systematic approach how to characterize the dynamics of a laser-produced Au plasma, generated with a commercial Joule-class laser system. By means of our diagnostic methods, we identify the dominant ionic species within the water-window spectral region, as well as the radiated energy, the x-ray emission duration and the total dimensions of the x-ray source. We present additional analysis through radiation hydrodynamic simulations to provide details about the electron temperature and density profiles and further show that the plasma exhibits nearly 20% self-absorption, while the electron temperature is limited to 400 eV. A comparison with the experimental data further allows us to verify the applied numerical code.

Initial Results From the Newly Upgraded LTX-β

LTX-β, the upgraded Lithium Tokamak Experiment, recently began its first campaign with a goal to study the transport properties of gradient-free electron temperature profile equilibria with increased toroidal field and neutral beam injection. The temperature-gradient-free equilibria in LTX were enabled by the lithium plasma-facing surface. To make the lithium surface conditions more consistent for LTX-β, a new, faster lithium evaporator system has been developed. The toroidal magnetic field has been nearly doubled to ≥0.3 T. Auxiliary heating and fueling with a high flux (35 A, 20 kV) neutral beam has begun. The beam provides core fueling without cold edge neutrals, to test whether flat temperature profiles can be sustained, and makes possible core ion temperature measurements via charge exchange recombination spectroscopy. Neutral beam fueling has been demonstrated with increases in line-integrated density. The coupling of the beam matches well with NUBEAM predictions for shine through. Improved performance has been observed following lithium evaporative coatings and an increase in field strength.

Design of GPU-based parallel computation architecture of Thomson scattering diagnostic in KSTAR

The Thomson scattering (TS) System is a diagnostic system to measure electron temperature and density profiles of tokamak plasma. The TS system requires measurement of many input signals, and the amount of raw data has significantly increased since the TS data acquisition (DAQ) system was upgraded to a fast digitizer. Research has been done on applying artificial neural network (ANN) to TS data analysis for reducing calculation time. In this paper, we propose a design of computation architecture to effectively process the increased amount of data caused by the fast digitizer and to maximize the computation performance of the ANN. In the design, the intensity values of each input signal and the ANNs can be computed in parallel by utilizing a graphical processing unit (GPU). Furthermore, we integrate the data analysis task into the TS DAQ program for real time operation. Considering stability of the integration, we separate tasks of the data acquisition and the data analysis into each thread operation, and make tasks of each digitizer board to be conducted in parallel. In the feasibility test of the design, the calculation time is shown to be appropriate for real time operation.