Edge Electron Temperature Research Articles

A tungsten-wall sputtering model for the plasma start-up simulation in tokamaks

Abstract Tungsten (W) is the most probable material for the plasma-facing components of fusion reactors due to its excellent thermal and physical properties. A W-wall sputtering model has been established to simulate the start-up of a tokamak plasma using the 0D simulation code DYON. This model incorporates the revised Bohdansky formula to calculate the sputtering yield and a modified formula for calculating the energy impacting the walls. This formula integrates the temporal behavior of electron and ion temperatures at the plasma edge, which has been partially verified by the Thomson scattering diagnostic data. With the new model in place, predictive simulations were conducted for KSTAR’s Ohmic plasma under two W-wall scenarios: one with the entire wall surface covered by W and the other with 95% coverage of W and 5% coverage of carbon (C). The results indicate that the full-W wall may perform better from the perspective of start-up performance. The disparity can primarily be attributed to impurities generated through sputtering and recycling on the C wall. The validity of this model will be finally confirmed when the Thomson diagnostic system is able to precisely measure the edge electron temperature during plasma start-up.

Status of edge electron density and temperature measurements with Helium Beam Emission Spectroscopy (He-BES) on EAST

Helium Beam Emission Spectroscopy (He-BES) diagnostic has been developed on EAST, which is able to measure the edge electron density and temperature profiles simultaneously using a helium line intensity ratio method. The diagnostic includes the beam injector and the detection system. There are 20 observation channels within an observation range of 80 mm in the detection system at the low filed side, which can cover the whole scrape-off layer (SOL) and part of the pedestal region of EAST. The beam injector system has been upgraded to Supersonic Molecular Beam Injector (SMBI) system to realize deeper helium injection since the 2021 campaign. Four spectral lines at wavelengths of 728.1 nm, 706.5 nm, 667.8 nm and 656.3 nm are detected by the He-BES. The first three spectral lines, including 728.1 nm, 706.5 nm, 667.8 nm, are measured for calculating edge n e and T e profiles based on the collisional-radiative model (CRM) model, and the last spectral line (656.3 nm) is used for the measurement of D α emission. The edge electrostatic fluctuations can be obtained from the power spectrum of D α emission. The electron density and temperature profiles calculated from the 667.8/728.1 and 728.1/706.5 nm line ratios are in good agreement with those from other diagnostics in the edge region of plasma. The self-consistency of He-BES diagnostic is also verified, such as the density pump out caused by LHW and the lower edge temperature caused by the lower heating power.

Investigation on the edge cooling threshold of the density limit in the J-TEXT tokamak with limiter and divertor configurations

Cooling of the plasma edge is widely considered to be a key element in the density limit (DL) of tokamaks. This paper investigates the critical edge cooling threshold of the DL by exploring various plasma configurations in the joint-Texas experimental tokamak. Notably, significant differences in the edge electron temperature in the vicinity of the last closed flux surface were observed between the limiter and divertor configurations. However, the electron temperature drops to a similar level in the vicinity of q= 3 surface close to DL, independent of the magnetic field configuration. In addition, to evaluate the reliability of the critical edge cooling threshold, experiments were conducted by increasing the carbon impurity content to enhance the edge cooling rate. These experiments involved two approaches to increase the carbon impurity content: methane injection and penetration of a graphite solid source. Results from these experiments indicate that the temperature threshold of the q = 3 surfaces remains consistent even with a stronger edge cooling rate. The consistency observed in the electron temperature threshold near the q = 3 surface, regardless of magnetic configuration or edge cooling rate, could help refine existing theoretical and simulation works and improve the prediction accuracy of the DL disruption.

Estimates of global recycling coefficients for LTX-β discharges

We report the first observation of global recycling coefficient R near 0.5 in the Lithium Tokamak eXperiment-β (LTX-β), significantly below the minimum R previously reported in other devices. In a series of experiments with varied Li wall conditioning, estimates of the recycling coefficient have been made using a Lyman-α array and DEGAS2 modeling. A progressive reduction in Lyman-α emission with increased lithium and an increase in edge electron temperature are observed. It is also observed that with increasing Li coating thickness, the effective particle confinement time τp* is reduced and approaches TRANSP calculated energy confinement time (τE), with τp* near τE,TRANSP for the lowest recycling coefficients. Edge temperatures approaching core plasma temperatures, first reported in LTX, can now be directly connected to estimates of the recycling coefficient and qualitatively agree with previous UEDGE simulations. The particle flux to the limiting surfaces appears to be significantly reduced in comparison with fluid scrape-off layer (SOL) models, indicating that a large fraction of the SOL ions are mirror trapped. SOL collisionality drops more than an order of magnitude below the banana regime boundary, indicating the importance of kinetic effects. Full-f 1x2v gyrokinetic simulations of SOL field lines with the GKEYLL code indicate that the fraction of ions trapped along field lines increases as collisionality drops, as a result of increased lithium evaporation.

Development of dual-path multi-pass Thomson scattering system in GAMMA 10/PDX

A dual-path Thomson scattering (DPTS) system was developed to measure electron temperature and density in a GAMMA 10/PDX central cell (CC) and an end cell (EC). The DPTS comprises CC Thomson scattering system (CC-TS) and an EC Thomson scattering system (EC-TS). In the CC-TS, we developed a multipass system to improve the signal intensities and time resolution. In the EC-TS, a double-pass system was developed as a preliminary step in developing a multipass system. In a previous EC-TS, the optical components for the double-pass configuration were placed in a vacuum vessel. To reduce the influence of stray light and facilitate alignment, a hole for the double-pass laser path was made on the partition wall in the vacuum chamber such that the probe laser could be extracted into the atmosphere and the double-path configuration could be easily adjusted. We applied DPTS to detached plasma experiments and successfully measured the core and edge electron temperatures and densities with the detached plasma experiments with the additional application of supersonic molecular beam injection and central electron cyclotron heating in the core plasma to simultaneously introduce intermittent particle flux.

Extending the low-recycling, flat temperature profile regime in the lithium tokamak experiment-β (LTX-β) with ohmic and neutral beam heating

Recent experiments in the lithium tokamak experiment-β (LTX-β) have extended the duration, performance, operating conditions, and diagnosis of the flat-temperature profile, low-recycling regime first observed in LTX. As expected, Li retains hydrogen and suppresses edge neutral cooling, allowing increased edge electron temperature, roughly equal to the core T e. Flat temperature profiles had been obtained transiently in LTX, as the plasma density decayed following the cessation of edge gas puffing. Careful control over the fueling in LTX-β has now been shown to sustain the flat T e profile and hot edge unique to the low-recycling regime for multiple confinement times in high performance discharges with decaying or steady density. With low density, the flat T e profile is also seen to extend into the scrape-off layer. Neutral beam heating is observed in target discharges with relatively flat electron temperature profiles (T edge ∼ T core/2), though beam heating is stronger in discharges with higher fueling, higher density, and depressed edge T e. Beam heating produces additional peaking of the T e profile, without degradation of the energy confinement time. Neutral beam heating of target discharges with relatively flat electron temperature profiles similarly results in broad beam heated temperature profiles. Energy confinement in LTX-β generally compares favorably to ohmic and H-mode scalings, frequently exceeding them by factors of 2–4. New and improved diagnostics in LTX-β enable better characterization of this unique regime, including measurements of ion temperature and high field side Thomson scattering profiles. As an initial step toward characterizing turbulence with no T e gradient and roughly equal density and pressure gradient, core fluctuation spectra have been measured in peaked T e discharges using far-forward scattering and fluctuation reflectometry.

A new indirect measurement method of the electron temperature for the Proto-sphera's pinch plasma

This article presents a new method for estimating the electron temperature of the Proto-sphera's screw pinch. The temperature radial profile is obtained by a self-consistent modeling of a 1D MHD equilibrium along with a 0D power balance of the plasma column, given measurements and estimates of the axial pinch plasma current, of the plasma rotational frequency and, at the equatorial plane, of the electron density radial profile, of the edge poloidal magnetic field, of the edge electron temperature and of the neutrals pressure in the vacuum vessel. The plasma is considered in equilibrium with its neutral phase and in constant rotation. A MATLAB code has been developed with the aim of estimating the MHD radial equilibrium profiles, the thermodynamic plasma state and the neutrals profile. The numerical estimates are compared with available experimental data showing a good agreement.

Improved neutral and plasma density control with increasing lithium wall coatings in the Lithium Tokamak Experiment-[formula omitted] (LTX-[formula omitted

A comparison of three sets of Lithium Tokamak Experiment-β (LTX-β) discharges is presented, each with progressively more lithium evaporatively deposited on the stainless steel plasma facing components (PFCs). Multiple observations independently indicate a reduction in recycling with increasing lithium deposition - plasma current, discharge duration, density pumpout, and edge electron temperatures increase while plasma density and neutral influx decrease. These measurements make use of several new operational and diagnostics upgrades that have been installed on LTX-β to enable recycling analysis and quantification. Installation of an upgraded Supersonic Gas Injector (SGI) provides access to rapid density pumpout, enabling estimation of effective particle confinement times. Edge density and temperature are measured using a new, movable, low field side, off-midplane, swept single langmuir probe in addition to the LTX-β core Thomson scattering system. The neutral particle influx from the high field side limiter is measured using a hydrogen Lyman-α array. Prospects for future modeling and analysis integrating all of these measurements is discussed.

Comparison of dynamical features between the fast H-L and the H-I-L transition for EAST RF-heated plasmas

In this paper, a comparison of dynamical features between the fast H-L and the H-I-L transition, which can be identified by the intermediate phase, or ‘I-phase’, has been made for radio-frequency (RF) heated deuterium plasmas in EAST. The fast H-L transition is characterized by a rapid release of stored energy during the transition transient, while the H-I-L transition exhibits a ‘soft’ H-mode termination. One important distinction between the transitions has been observed by dedicated probe measurements slightly inside the separatrix, with respect to the radial gradient of the floating potential, which corresponds to the E × B flow and/or the electron temperature gradient. The potential gradient inside the separatrix oscillates and persists during the stationary I-phase, and shows a larger amplitude than that before the fast H-L transition. The reduction of the gradient leads to the final transition to the L-mode for both the fast H-L and the H-I-L transition. These findings indicate that the mean E × B flow shear and/or edge electron temperature gradient play a critical role underlying the H-L transition physics. In addition, the back transition in EAST is found to be sensitive to magnetic configuration, where the vertical configuration, i.e., inner strike-point located at vertical target, favours access to the H-I-L transition, while the horizontal shape facilitates achievement of the fast H-L transition. The divertor recycling level normalized to electron density is higher before the fast H-L transition, as compared to that before the I-phase, which strongly suggest that the density of the recycled neutrals is an important ingredient in determining the back transition behaviour.

Design of an edge TV Thomson scattering diagnostic system on EAST

The study of the edge plasma has significant effect for the performance improvement of magnetic confinement. However, the Thomson scattering system, which has been used to provide electron temperature and electron density on EAST, do not meet the requirements for the edge plasma measurements in measurement region, spatial and spectral resolution. Herein, a TV Thomson scattering diagnostic system has been constructed for the measurements of edge electron temperature and electron density profiles on EAST. A new laser system, optical collection system and data acquisition system are designed to enable the TV Thomson system operate at the measurement region of 0.65–1.1 ρ (“ρ” is the unitary radius in flux coordinate) with spectral resolution of 5 nm and spatial resolution as low as 2 mm. And the theoretical analysis proves that the system has the measurability at electron temperature of 20–1000 eV and density beyond 0.2 × 1019 m-3, which fulfills research of the pedestal microstructure on EAST. The establishment of the edge TV Thomson system can create the conditions for the edge physical study on EAST.

Development and preliminary test of a space-resolved vacuum-ultraviolet spectroscopy in EAST.

The impurity radiation from the divertor region of the EAST tokamak is dominantly in the wavelength range of vacuum ultraviolet (VUV) due to the elevated edge electron temperature. A space-resolved VUV spectroscopy is developed to measure impurity radiation in the divertor region. An eagle-type VUV spectrometer with a focal length of 1m is adopted in this system, equipped with a spherical grating and a charged-coupled device (CCD) detector. The performance of the VUV spectrometer is preliminarily tested on a synchrotron radiation facility. The wavelength calibration is conducted near 65nm. It is found that the wavelength range observed by the CCD detector is about 11.07nm around the central wavelength of about 65nm. With a linear dispersion of 0.0053 nm/pixel, it is possible to measure the ion temperature lower than 20eV at the edge region by analyzing the Doppler broadening of a carbon line. These test results show that the performance of the VUV spectrometer is capable of measuring divertor radiation and analyzing the ion temperature of edge impurity ions.

Lithium splashing from flowing liquid lithium limiter and its effect on high confinement plasma performance in EAST tokamak

Abstract To study the potential applications of liquid lithium (Li) as plasma facing material, the third generation of flowing liquid Li (FLiLi) limiter has been designed and successfully tested in EAST tokamak H-mode discharges in the 2018 run campaign. The stability of liquid Li on FLiLi surface, and Li splashing and its effect on the plasma performance are investigated in detail. Li splashing was observed in L-mode and H-mode discharges due to Rayleigh-Taylor (R-T) instability, resulting from electromagnetic forces J → × B → . Li splashing in low-parameter plasma decreased both core and edge electron temperatures, resulting in the degradation of plasma energy confinement and even H-L back transition, however, it almost did not influence plasma confinement performance in high-parameter plasma. These findings indicate that high-parameter plasma has higher tolerance to Li impurities than low-parameter plasma. By contrast, tungsten splashing from the melted layer on the upper divertor was also induced by the R-T instability but generally resulted in plasma disruption, which suggests lower tolerance to tungsten impurities in EAST tokamak.

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.

Study of power width scaling in scrape-off layer with 2D electrostatic turbulence code based on EAST L-mode discharges

The study of the power width in the scrape-off layer (SOL) is very important for the design and operation of ITER. In this paper, a 2D electrostatic turbulence code is employed to study the power width scaling in L-mode plasmas. It is found that the electron and ion turbulent transports dominate the radial heat fluxes, and the electron heat conduction and the ion heat advection dominate the parallel heat flux in the near and far SOL in L-mode plasmas. The simulated SOL power width agrees well with the Eich scaling [T. Eich et al., Nucl. Fusion 53, 093031 (2013)] and the predictions by the heuristic drift-based model [R. J. Goldston, Nucl. Fusion 52, 013009 (2012)] for selected EAST L-mode discharges. A numerical scaling has been performed based on one of these discharges. The scaling dependence on the safety factor is consistent with the Eich scaling and the scaling exponent of the edge electron temperature is close to that in the ASDEX-Upgrade L-mode scaling [B. Sieglin et al., Plasma Phys. Controlled Fusion 58, 055015 (2016)]. The investigation of the obtained numerical scaling for L-mode plasmas reveals that the SOL power width is influenced by the safety factor, the edge electron density, and the edge electron temperature through the parallel heat transports, the radial turbulent heat transports, and both the parallel and radial heat transports, respectively. The formulation of the turbulence model suggests that the scaling dependence on the poloidal magnetic field (or the plasma current) for the experimental scalings is essentially the scaling dependence on the ballooning length, q95R. Based on this idea, a further numerical scaling gives λq∝q951.30Bt−0.33R1.32∝Bt−0.29(q95R)1.33, which has a strong scaling dependence on the major radius that is different from the Eich scaling.

Estimation of carbon impurity from the guard limiter of the lower hybrid wave antenna on EAST tokamak

Hot spots on the EAST tokamak graphite guard limiters of the lower hybrid wave (LHW) antenna may cause a sudden increase of impurity influx and even ending with disruption. A sheath model is developed by taking into account the energetic electron component from the plasma-LHW interaction via electron Landau damping, and then a self-consistent method is used to study the interaction of the plasma across the sheath at the material surface and material thermal response. It is found that the fast electron fluxes driven by both the 2.45 and 4.6 GHz LHWs modify strongly the sheath potential, which then influence on the bombarding ion energy at the material surface and the surface temperature. By calculating the carbon physical sputtering and chemical erosion from guard limiter of the LHW antenna, the carbon production from the guard limiter in two LHW systems is found to have different behavior for the same edge plasma density and electron temperature. Our results show that carbon impurity is mainly from physical sputtering, except for the 4.6 GHz LHW in a narrow range of the high electron temperature where the chemical erosion has a very sharp increase. Due to modification of the sheath potential induced by the fast electrons, the power density is higher with the 4.6 GHz than the 2.45 GHz LHW system. As a consequence the surface temperature is higher at 4.6 GHz and susceptible to reach several hundred of degrees which is prone to chemical erosion.

Upgrade of the multi-energy soft x-ray diagnostic system for studies of ELM dynamics in the EAST tokamak

The multi-energy soft x-ray (ME-SXR) diagnostic system has been calibrated to study the evolution of the edge electron temperature profile during the edge localized mode (ELM) events. The electronic bandwidth of the Mazet MTi04 multi-channel variable gain trans-impedance amplifier measured in the laboratory is consistent with the nominal value. As the gain of the preamplifier is increased from 105 to 2 × 107, the bandwidth is decreased from 100 kHz to 10 kHz. The relative gain errors for different channels (5 arrays ×20 channels) were also calibrated in situ after the system was installed on the EAST tokamak. The results showed that the relative gain error (RGE) in each array was less than 10% compared to the reference channel of the array, and was insensitive to the preamplifier gains. A feedback loop was designed to automatically adjust the programmable gains of the preamplifiers based on the signal amplitudes measured in the previous discharge. The reconstructed edge electron temperature (Te) profile was also uploaded automatically to the MDSplus server between discharges. Utilizing the fast temporal resolution of Te (∼5 kHz), the dynamics of edge electron temperature evolution during Type I and high frequency ELM (HF-ELM) events were compared. The results showed that the edge Te profile collapsed during Type I ELMs, while for the HF-ELM regime, the edge Te profile pedestal structure sustains stiffness.

Investigation of electromagnetic geodesic acoustic mode in EAST RF-heating plasma

The magnetic fluctuations of geodesic acoustic modes (GAMs) have been investigated using a Doppler backscattering system and Mirnov probes during ion cyclotron resonance heating and lower hybrid wave heating in the edge plasma in the experimental advanced superconducting tokamak, and the magnetic component of the GAM was observed. The frequency of the GAM has a clear relationship with the edge electron temperature, and the mode numbers of the poloidal component of the magnetic structure of the GAM are approximately m=2 (sin (2θ)) and n = 0. A more detailed investigation shows that the maximum values of the amplitude of the poloidal magnetic field fluctuations and poloidal electric field fluctuations are approximately 10−6 T and 200 V/m, respectively.

Physics of increased edge electron temperature and density turbulence during ELM-free QH-mode operation on DIII-D

For the first time, we report increased edge electron temperature and density turbulence levels (T̃e and ñe) in Edge Localized Mode free Quiescent H-mode (ELM-free QH-mode) plasmas as compared to the ELMing time period. ELMs can severely damage plasma facing components in fusion plasma devices due to their large transient energy transport, making ELM-free operation a highly sought after goal. The QH-mode is a candidate for this goal as it is ELM-free for times limited only by hardware constraints. It is found that the driving gradients decrease during the QH-mode compared to the ELMing phase, however, a significant decrease in the ExB shearing rate is also observed that taken together is consistent with the increased turbulence. These results are significant as the prediction and control of ELM-free H-mode regimes are crucial for the operation of future fusion devices such as ITER. The changes in the linear growth rates calculated by CGYRO [Candy et al., J. Comput. Phys. 324, 73 (2016)] and the measured ExB shearing rate between ELMing and QH-mode phases are qualitatively consistent with these turbulence changes. Comparison with ELMing and 3D fields ELM suppressed H-mode finds a similar increase in T̃e and ñe, however, with distinctly different origins, the increased driving gradients rather than the changes in the ExB shearing rate in 3D fields ELM suppressed the H-mode. However, linear gyrokinetic calculation results are generally consistent with the increased turbulence in both ELM-controlled discharges.

Analysis of electron temperature, impurity transport and MHD activity with multi-energy soft x-ray diagnostic in EAST tokamak

A new edge tangential multi-energy soft x-ray (ME-SXR) diagnostic with high temporal (≤ 0.1 ms) and spatial (∼1 cm) resolution has been developed for a variety of physics topics studies in the EAST tokamak plasma. The fast edge electron temperature profile (approximately from to the scrape-off layer) is investigated using ME-SXR diagnostic system. The data process was performed by the ideal ‘multi-foil’ technique, with no priori assumptions of plasma profiles. Reconstructed ME-SXR emissivity profiles for a variety of EAST experimental scenarios are presented here for the first time. The applications of the ME-SXR for study of the effects of resonant magnetic perturbation on edge localized modes and the first time neon radiating divertor experiment in EAST are also presented in this work. It has been found that neon impurity can suppress the 2/1 tearing mode and trigger a 3/1 MHD mode.

Observation of Flat Electron Temperature Profiles in the Lithium Tokamak Experiment.

It has been predicted for over a decade that low-recycling plasma-facing components in fusion devices would allow high edge temperatures and flat or nearly flat temperature profiles. In recent experiments with lithium wall coatings in the Lithium Tokamak Experiment (LTX), a hot edge (>200 eV) and flat electron temperature profiles have been measured following the termination of external fueling. Reduced recycling was demonstrated by retention of ∼60% of the injected hydrogen in the walls following the discharge. Electron energy confinement followed typical Ohmic confinement scaling during fueling, but did not decrease with density after fueling terminated, ultimately exceeding the scaling by ∼200%. Achievement of the low-recycling, hot edge regime has been an important goal of LTX and lithium plasma-facing component research in general, as it has potentially significant implications for the operation, design, and cost of fusion devices.