Edge Electron Temperature Research Articles

Edge Plasma Transport in the Helical Divertor Configuration in LHD

AbstractTo understand the edge plasma transport in the complex magnetic field lines structure in the LHD heliotoron, the heat and particle fluxes profiles on divertor plates were investigated experimentally and theoretically by using EMC3‐EIRENE code. The heat flux profiles on a divertor plate in different magnetic configuration were measured and the main channel of the heat transport was revealed to be the field lines with long connection lengths which connect the divertor plate and the stochastic field lines layer. A comparison of the heat and the ion fluxes profiles on the divertor plates between the experimental data and the results of the calculation using EMC3‐EIRENE code was conducted. It was revealed that the change of the cross‐field transport in the edge plasma region causes not only the change of the heat and particle flux profiles on divertor plates, but also the changes of the intrinsically non‐uniform global heat and particle deposition profiles on the helical diveror. The cross‐field transport seems to be larger in the higher edge electron temperature discharge in the experimental conditions in this paper (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Rotation dependence of a phase delay between plasma edge electron density and temperature fields due to a fast rotating, resonant magnetic perturbation field

Measurements of the plasma edge electron density ne and temperature Te fields during application of a fast rotating, resonant magnetic perturbation (RMP) field show a characteristic modulation of both, ne and Te coherent to the rotation frequency of the RMP field. A phase delay Φ between the ne(t) and Te(t) waveforms is observed and it is demonstrated that this phase delay Φ is a function of the radius with Φ(r) depending on the relative rotation of the RMP field and the toroidal plasma rotation. This provides for the first time direct experimental evidence for a rotation dependent damping of the external RMP field in the edge layer of a resistive high-temperature plasma which breaks down at low rotation and high resonant field amplitudes.

Investigation on the influence of plasma properties and SOL transport on the particle flux profiles on divertor plates in the Large Helical Device

Particle flux profiles on the divertor plates and the electron temperature profiles in the scrape-off layer (SOL) in the Large Helical Device (LHD) heliotron were investigated with EMC3-EIRENE code. These profiles are modified during a discharge due to the changes of the edge plasma density and temperature those can cause the change of transport coefficient. Comparison of the edge electron temperature profiles between the measurements and the simulations revealed that the cross field transport coefficients in the LHD scrape-off layer depend on plasma parameters, especially electron temperature. For the particle flux profile on the divertor plates, the absolute value of the simulation results with the transport coefficient consistent with the edge temperature profile analysis were well agree with the experimental data, though the profile shapes of experimental data were not necessarily reproduced well by the simulation.

Analysis of difference in Hα spectral line profiles between attachment and detachment plasmas in LHD

The sustainable detachment has been achieved in LHD. The line-averaged electron density of the detachment plasmas is typically over 1.0×1020m−3. The difference in the broad component of the Hα spectral line profiles between the attachment and detachment plasmas has been observed. In attachment, the divertor flux much affects the neutral hydrogen generation and the large inward flow up to 1.5×104m/s seemed to be due to the reflection is observed. In detachment, the much smaller flow velocity along the sight lines and the lower Doppler temperature are observed. These observations represent the drastic reduction of the divertor flux and the decrease of the edge electron temperature involved with detachment.

Plasma flow and carbon production and circulation with the ergodic divertor of Tore Supra

This paper presents a detailed study of carbon production and transport from the ergodic divertor (ED) target plates to the plasma core in the Tore Supra tokamak. Adapted experimental and numerical modelling techniques have been used to describe each of the main phenomena in play. Edge electron density and temperature are measured with Langmuir probes. The C II, C III and Hα emission is measured with optical fibres and cameras. The background plasma flow is calculated consistently with the observed recycling pattern by the neutral transport code EDCOLL for the two magnetic connection schemes of interest (short or long connection lengths). 3D Monte-Carlo modelling of carbon near the neutralizer plate (BBQ code) shows that the transport of carbon ions is governed by the friction force in addition to the electric field. Finally, a simplified 3D test particle model is used to estimate the core penetration fraction of carbon. A high value is found for the carbon screening efficiency (fraction of particles that does not penetrate in the plasma core), in the range 95–97% depending on the edge plasma conditions. This value, combined with the calculated carbon influxes, yields the first quantitative estimate of the carbon core contamination during ED operation. The paper shows that the screening of carbon and core contamination are mainly dependent on the carbon source (partially controlled with the ED) and the plasma flow distribution in the laminar region (magnetic topology and particle drifts).

Impurity identifications, concentrations and particle fluxes from spectral measurements of the EXTRAP T2R plasma

An absolute intensity calibrated 0.5 m spectrometer with optical multi-channel analyser detector was used to observe the visible-UV radiation from the plasma in the EXTRAP T2R reversed field pinch experiment. Spectral lines were identified indicating the presence of oxygen, chromium, iron and molybdenum impurities in the hydrogen plasma. Certain regions of interest were examined in more detail and at different times in the plasma discharge. Impurity concentration calculations were made using the absolute intensities of lines of OIV and OV measured at 1–2 ms into the discharge generating estimates of the order of 0.2% of ne in the central region rising to 0.7% of ne at greater radii for OIV and 0.3% rising to 0.6% for OV. Edge electron temperatures of 0.5–5 eV at electron densities of 5–10×1011 cm−3 were calculated from the measured relative intensities of hydrogen Balmer lines. The absolute intensities of hydrogen lines and of multiplets of neutral chromium and molybdenum were used to determine particle fluxes (at 4–5 ms into the plasma) of the order 1×1016, 7×1013 and 3×1013 particles cm−2 s−1, respectively.

Formation of edge transport barrier in the ergodic field layer of helical divertor configuration on the Large Helical Device

On the Large Helical Device (LHD), low to high confinement (L–H) transition and edge transport barrier (ETB) formation were observed in the low beta regime (⟨βdia⟩ < 1%, ⟨βdia⟩: volume-averaged beta derived from diamagnetic measurement) as well as in relatively high beta regime (>1.5%). In most of ETB plasmas electron density preferentially increases in the edge region without a substantial rise of the edge electron temperature. The ETB zone develops inside the ergodic field layer calculated in the vacuum field. The ETB formation strongly destabilizes edge coherent modes such as m/n = 2/3 or 1/2 (m, n: poloidal and toroidal mode numbers), because the plasma edge region is in the magnetic hill. The ETB is partially destroyed by the combination of these edge MHD modes and ELM-like activities. For a particular experimental condition, the forced generation of a sizable m/n = 1/1 magnetic island near the edge by application of external field perturbations facilitates the L–H transition at a lower electron density and suppresses edge MHD modes and ELM-like activities to lower levels.

Analyses of edge plasma characteristics in HL-2A

The edge plasma characteristics are studied by both a movable array of Mach/Reynolds stress/Langmuir 10-probes in the boundary region and the fixed flush probe arrays on the 4 divertor neutralization plates at the same toroidal cross-section in the HL-2A tokamak. The dependence of the Reynolds stress on poloidal flow in the edge plasma is analysed. The result indicates that the sheared poloidal flow in tokamak plasma can be induced by the radial gradient of Reynolds stress. In the divertor experiments of HL-2A, the profiles of the electron temperature, density and floating potential on divertor plates are measured by the flush probe arrays. The edge electron temperature in divertor configuration is higher than that in limiter configuration. The temperature asymmetry between outer and inner target plates is observed. The result of magnetic surface reconstructed from 18 Mirnov coils signals is presented. Both the particle recycling and the impurity flux in the bulk plasma during divertor discharges are discussed. Neutral gas pressure in divertor chamber, measured by fast ionization gauge during divertor discharge, is given.

Characterization of the H-mode edge barrier at ASDEX Upgrade

The scaling of the edge transport barrier (ETB) that sustains H-mode plasmas is crucial for the performance of next step tokamaks. At ASDEX Upgrade, the suite of edge diagnostics has been significantly improved to address this issue. High spatial resolution profiles of most of the key edge plasma parameters necessary to determine the magneto hydrodynamic (MHD) stability are now available. New high temporal resolution measurements give clear indications of the nonlinear evolution of the ELM crash. The correlation lengths of edge turbulence have been shown to be correlated with the edge radial electric field shear using a new correlation Doppler reflectometer system. The measured pressure gradient in the ETB is found to be consistent with ideal MHD stability limits, both for Type I and II ELMs. In addition, the edge electron temperature and density gradient lengths are found to be strongly correlated, leaving only the ETB width as a free parameter. In ASDEX Upgrade, the ETB width does not vary significantly over the entire H-mode edge database. Modelling of the transport of comparison discharges in hydrogen and deuterium shows that the expected mass effect on neutral penetration is largely compensated by more efficient heating of deuterium neutrals but requires a transport barrier in both the energy and particle channels in order to reproduce the measured edge temperature and density profiles.

Advances in understanding quiescent H-mode plasmas in DIII-D

Recent QH-mode research on DIII-D [J. L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research 1996 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] has used the peeling-ballooning modes model of edge magnetohydrodynamic stability as a working hypothesis to organize the data; several predictions of this theory are consistent with the experimental results. Current ramping results indicate that QH modes operate near the edge current limit set by peeling modes. This operating point explains why QH mode is easier to get at lower plasma currents. Power scans have shown a saturation of edge pressure with increasing power input. This allows QH-mode plasmas to remain stable to edge localized modes (ELMs) to the highest powers used in DIII-D. At present, the mechanism for this saturation is unknown; if the edge harmonic oscillation (EHO) is playing a role here, the physics is not a simple amplitude dependence. The increase in edge stability with plasma triangularity predicted by the peeling-ballooning theory is consistent with the substantial improvement in pedestal pressure achieved by changing the plasma shape from a single null divertor to a high triangularity double null. Detailed ELITE calculations for the high triangularity plasmas have demonstrated that the plasma operating point is marginally stable to peeling-ballooning modes. Comparison of ELMing, coinjected and quiescent, counterinjected discharges with the same shape, current, toroidal field, electron density, and electron temperature indicates that the edge radial electric field or the edge toroidal rotation are also playing a role in edge stability. The EHO produces electron, main ion, and impurity particle transport at the plasma edge which is more rapid than that produced by ELMs under similar conditions. The EHO also decreases the edge rotation while producing little change in the edge electron and ion temperatures. Other edge electromagnetic modes also produce particle transport; this includes the incoherent, broadband activity seen at high triangularity. Pedestal values of ν* and βT bracketing, those required for International Experimental Thermonuclear Reactor [Nucl. Fusion 39, 2137 (1999)] have been achieved in DIII-D, demonstrating the QH-mode edge densities are sufficient for future devices.

High frequency MHD activity related to type I ELMs in ASDEX Upgrade

High frequency coherent magnetic fluctuations related to the type I edge localized mode (ELM) cycle have been found in ASDEX Upgrade. Typical frequencies are in the range 300–500 kHz, and the poloidal mode number, m, measured at the outboard midplane, is estimated to be 10–15. These MHD modes are present in many low density (low collisionality) discharges, while they have not been found in high density ones yet. New findings on already known low frequency type I ELM precursors confirm that in ASDEX Upgrade type I ELM mode precursors are normally phase locked to the wall in discharges with NBI heating parallel to and in the direction of the plasma current (co-injection) and can be seen with a non-zero rotation frequency only in shots with anti-parallel NBI (counter-injection). Additional information is given by ECE data. The existence of low and high frequency MHD events suggests an interpretation in the framework of the peeling–ballooning model. The high frequency magnetic activity seems to be related strictly to the increase in (and eventual saturation of) edge electron temperature, while low frequency modes are related directly to the ELM crash itself.

Edge ion parameters at the L–H transition on JET

L–H transition experiments have recently been performed on JET to characterize the local edge ion parameters. Evidence of a threshold in the edge ion temperature to access the H-mode is presented and comparisons are made with edge electron temperature. These results provide the first scaling of pedestal ion temperature on JET. The threshold edge ion temperature scales approximately linearly with toroidal magnetic field and shows no dependence on electron density in the range explored. Reference shots have been repeated with the ion ∇ B drift away from the X-point and counter neutral beam injection. The L–H transition is also considered in terms of dimensionless parameter space. The effective edge ion collisionality does not appear to play a significant role in the L–H transition, providing no clear distinction between the L- and H-modes. In contrast, the normalized poloidal ion gyroradius provides a sharp H-mode boundary at any given field and current, consistent with a threshold ion temperature for H-mode access being dependent on toroidal field but not on density. However, comparison of ρ* and with forward and reversed fields across a range of fields and currents demonstrates that these dimensionless parameters simply describe an operation trajectory for the JET L–H transition.

Ergodic edge region of large helical device

Ergodicity in various vacuum magnetic configurations in large helical device was quantitatively estimated, using the Kolmogorov length as a measure. It is found that the edge electron temperature profile changes its gradient at the position where the ergodicity begins to increase several cm outside the last closed flux surface (LCFS). In the profiles no distinguished change of plasma parameters was seen at the LCFS, which suggests the existence of a region just outside the LCFS where the confinement performance is relatively as good as that in the closed region. The radial electron heat conductivity was also estimated using a simple energy balance equation and it was confirmed that the conductivity is strongly affected by the ergodicity. Using the relationship between ergodicity and electron heat conductivity, a new practical definition of the LCFS based on the experiments is proposed.

D Experimental Study in Tore-Supra Ergodic Divertor Plasmas: Molecular Deuterium Density Measurements

The deuterium Dα line emitted in front of an equatorial neutraliser plate in Ergodic Divertor Tore Supra plasmas is a common diagnostic of recycling particles in present Tokamaks. First, the possible perturbation of the Dα spectral lineshape by the nearby HeII line Brβ has been studied. Although its effect on the Dα line profile shape is evaluated to be non negligible in plasmas with large Helium contents, it is concluded that it should be of no influence in usual fusion plasmas. Second, a detailed study of the Dα spectral lineshape has been performed to obtain temperatures and densities of the different neutral deuterium species. A synthetic profile method taking into account Zeeman and Doppler effects has been used. This shows that two deuterium populations with specific behaviours have to be considered: the first one (‘cold atoms’) is composed of atomic deuterium coming from D2 desorbed from the neutraliser plate, while the second one (‘warm atoms’) is composed of charge exchange (CX) and reflected deuterium. The Doppler temperature of cold atoms increases with the edge electron density, nedgee (1.4 eV < TcD < 2.5 eV). The possible heating of cold atoms by D+ ions is evaluated to explain this effect. Molecular and atomic deuterium densities behaviour versus edge electron temperature Te are analysed with the use of the warm and cold Dα brightnesses coupled to a collisional radiative deuterium model. The results are compared to the numerical code EDCOLL. An important molecular density (> 1018 m-3) is found for the whole range of edge electron temperature (10 eV < Te < 40 eV) while the ratio of the atomic to the molecular densities is around 1.8. The density of atoms coming from molecular dissociation is ∼ 0.8 times the density of atoms created by charge exchange or re.ection of deuterons on the neutraliser.

Analysis of asymmetric Dα spectra emitted in front of a neutralizer plate of the Tore-Supra ergodic divertor

A spectroscopic analysis of the deuterium Dα line (λ = 6561.0 Å) emitted in the edge of Tore-Supra just in front of the equatorial neutralizer plate of the ergodic divertor is presented. This analysis, which relies on a line shape modelling, is focused on asymmetric Dα spectra which are obtained in some diverted discharges. It will be shown that three populations of neutrals are necessary to interpret such spectra. A cold atoms population results from the dissociation of molecules desorbed from the surface, and a warm one from charge-exchange reactions with the plasma ions. The last population of neutrals is produced by normal reflection of plasma ions on the neutralizer V-shaped notches \\mbox{(V-points)} and has an anisotropic velocity distribution function. The anisotropy of this velocity distribution function is due to its initial form and to its partial relaxation by neutral-ion elastic collisions. The analysis based on a line shape fitting program including a relaxation model allows one to estimate the relative contributions of the neutral populations of recycled deuterium to the Dα emission and their Doppler temperatures. Finally, the dependence of the ion and neutral temperatures deduced from the fits and the dominant molecular dissociation channels are discussed as a function of the edge electron density and temperature.

Atomic and molecular deuteriumedge density evaluation from spectral analysis of the Dα lineshape

A method for molecular and atomic deuterium density evaluation, based on the analysis of the Dα spectral line shape coupled to a collisional radiative model, is presented. It is illustrated by its application to the plasma edge in the ergodic divertor configuration of the tokamak Tore Supra. An important molecular density (nD2>1018 m-3, 1 cm radially away from the neutralizer plate), of the same order as the total atomic density nD (nD/nD2~1.4), is found for the entire range of edge electron temperatures (10 eV<Te<40 eV). The density ratio of D atoms created by charge exchange or reflection of deuterons on the neutralizer plate and D atoms coming from D2 dissociation is approximately constant for electron temperatures larger than 20 eV (~0.9). A reasonable agreement is found with the results of the multi-one-dimensional code EDCOLL and the three-dimensional Monte Carlo code BBQ. The influence of the electron temperature and density distributions along the spectroscopic line of sight on the measurements is discussed in the frame of simulations with the numerical code EDCOLL.

Feedback control on edge plasma parameters with ergodic divertor in Tore Supra

In order to control highly radiative plasmas with the ergodic divertor, feedback procedures based on real time Langmuir probes measurements have been implemented in Tore Supra. The prediction of the detachment density threshold, which leads to disruption, is a major experimental difficulty. On the other hand, it has been observed that the edge electron temperature limit, measured by probes, is quite reproducible in many different operating scenarios. Therefore, feedback procedures on the edge electron temperature have been developed to control the gas injection. When the plasma reaches detachment, the fuelling efficiency abruptly increases and the feedback loop can become unstable. A detachment criterion, defined as the ratio of the saturation current of the probes scaled from the high recycling regime phase to its current value, has been used to manage detached plasmas. These techniques have been successfully applied in Tore Supra discharges with ergodic divertor for the study of edge density regimes and plasma detachment in ohmic, ICRH heating, and impurity seeded experiments.

Spectroscopic study of neon emission and retention in the Tore Supra ergodic divertor

In order to assess the capability of the Tore Supra ergodic divertor (ED) to retain impurities in the low confinement edge region, spectroscopic observations of a divertor neutraliser plate are reported. The neutral neon density is deduced from these measurements; it increases strongly (up to 1.5×10 17 m −3 per injected Pa l) when the plasma approaches detachment. The central neon density is approximately independent of the plasma edge conditions. A 2D model confirms the relatively weak measured dependence of the neutral neon penetration on edge electron density and temperature. Comparison of BBQ (3D scrape-off layer Monte-Carlo code) results with 1D impurity radial transport modelling suggests a possible mechanism for the observed weak dependence of core content on edge impurity influx: enhanced exchange between the ergodized layer of the core and the neutraliser region.

Chemical erosion yields and photon efficiency measurements in the JET gas box divertor

Hydrocarbon injection into the JET gas box divertor in ohmic and L-mode discharges has been used to investigate the intrinsic hydrocarbon erosion yields and to determine the variation of the photon efficiency with the edge ion flux. Constant methane erosion yields were measured (5% for CD 4, 3% for CH 4) with no sign of any flux dependence. The erosion yield of ethane/ethene was lower but increased with plasma density (increasing edge ion flux, decreasing edge electron temperature) such that it dominated the molecular carbon source at high density. The measured photon efficiencies (the number of molecular dissociations per emitted photon) were not constant, but were found to increase with increasing density (increasing edge ion flux, decreasing edge electron temperature).

Diagnosis for the Interaction of Supersonic Molecular Beam with Plasma

Supersonic Molecular Beam Injection (SMBI) is a new fuelling method for Tokamaks and has recently been improved to enhance the flux of the beam and to make a survey of the cluster effect within the beam. There are a series of new phenomena, which implicate the interaction of the beam (including clusters) with the toroidal plasma of HL-1M Tokamak. The Hα signals from the edge show a regular variation around the torus. Around the injection port, the edge Hα signals are positive rectangular wave, which is consistent with that of the injection beam pulses. The edge electron temperature, measured with movable Langmuir probes, decreases by an order of magnitude and the density increases by an order of magnitude. Hα emission at the beam injection port, measured with CCD camera at an angle of 13.4 degrees to the SMBI line, shows many separate peaks within the contour plot. These peaks may show the strong emission produced by the interaction of the hydrogen clusters with the plasma. Hydrogen clusters may be produced in the beam according to the empirical scaling (Hagena) law of clustering onset, Γ* = kd0.85P0/T02.29. Here d is the nozzle diameter in μm, P0 the stagnation pressure in mbar, T0 the source temperature in K, and k is a constant related to the gas species. If Γ* > 100, clusters will be formed. In present experiment Γ* is about 127.