Dα Emission Research Articles

Deuterium Emission in Laser Plasma Induced by Transversely Excited Atmospheric Pressure CO2 Laser in Low-Pressure of Helium Surrounding Gas

A transversely excited atmospheric pressure (TEA) CO2 laser was focused on samples containing hydrogen of high concentration in ambient gas of low pressure. The optimum conditions for reducing the spectral width of Hα emission have been investigated. As a result, it was found that a film sample is superior to a bulk sample, and He gas should be used as the ambient gas at approximately 1 Torr, and for spectral measurement, only the front area of the plasma should be selected. We successfully demonstrated the detection of Dα emission, of which peak shifts only 0.179 nm from the Hα emission, by employing a D2O liquid film sandwiched between a Cu plate and a thin plastic wrapping film.

Interpretation of the Dα emission from the high field side of Alcator C-Mod

Measurements from an inner wall mounted scanning Langmuir probe and modelling of Dα emission show a sharp fall-off of plasma density in the inboard private flux region. Dα emission is modelled using a one-dimensional space, two-dimensional velocity kinetic neutral code in order to track the changes in the density profile while the magnetic topology is changed from single to double null. A sharp density decay is observed to track with the location of the secondary separatrix. The data presented are consistent with a lower level of cross-field transport on field lines in the private flux region restricted to the high field side when compared to cross-field transport on field lines in the low field side scrape-off layer.

Synchronization of L-mode to H-mode transitions on the sawtooth cycle in Ohmic TCV plasmas

L-mode to H-mode transitions (LH transitions) are obtained in a wide range of plasma parameters in TCV Ohmic discharges. LH transitions either occur directly, indicated by a single abrupt drop in Dα emission, or present an intermediate ‘dithering’ phase before the H-mode state is attained, in which Dα emission shows rapid excursions between L-mode and H-mode levels. The type of transition, with or without a dithering phase, depends on the plasma current and plasma triangularity. It is less sensitive to plasma density and divertor geometry. The relative direction of the ion ∇ B drift also plays a role in the characteristics of the LH transition.The time delay between an LH transition and the preceding sawtooth crash has been measured in every TCV H-mode discharge. Most LH transitions are found to occur in the first 30% of the sawtooth cycle, i.e. in the first 2 ms after a sawtooth crash. Moreover, LH transitions often occur soon after a large crash of a ‘double sawtooth’. The distribution function of these time delays clearly indicates a synchronization of these two processes. However, the heat pulse released during a sawtooth crash has a transit time from the q = 1 surface to the plasma edge, which is much shorter than the time delay between the sawtooth crash and the LH transition.

A Comparison between Divertor Heat Loads in ELMy and HRS H-Modes on JFT-2M

A comparison between divertor heat loads in ELMy and High Recycling Steady (HRS) H-modes was carried out by fast measurements of the two dimensional Dα emission, and the divertor electron temperature and ion saturation current on JFT-2M. The heat load for the HRS H-mode was maintained at the same level as the time-averaged heat load for the ELMy H-mode, indicating that the HRS H-mode has the attractive advantage of avoiding the severe ELM heat load. An enhancement of particle transport dominated the heat load for the HRS H-mode, whereas the large ELM heat load was induced by abrupt increases of both heat and particle transport.

Use of one dimensional Dα camera to measure edge electron density gradients

One of the clearest signatures of the transitions from L- to H-mode plasmas is the steepening of the edge density gradient. A concomitant narrowing of the visible radiation, usually dominated by Balmer Dα emission, is generally observed. Typically, the Dα emissivity width is 4–6 cm for the L-mode, similar for ELMs, and 1–3 cm for H-mode MAST plasmas, comparable with typical edge ne scale lengths. A means of extracting electron densities from this narrowing of the radial Dα emissivity profile was pioneered on the START device and developed further on MAST. It takes advantage of the near insensitivity to electron temperature of both the ionization and emission rates of hydrogen at temperatures well in excess of the ionization potential (13.6 eV). The Dα emissivity profile is obtained from spatial inversion of line-of-sight integrated intensities recorded by a linear camera (Δt=125 μs, 256 pixels). The 300 point Thomson scattering diagnostic has been used as a yardstick in comparing experimental data from a wide range of edge density gradients and a range of edge temperatures in checking the models validity and assumptions.

Direct removal of edge-localized pollutant emission in a near-infrared bremsstrahlung measurement

Visible and near-infrared electron–ion bremsstrahlung measurements in fusion research devices, used to determine the effective ionic charge (Zeff), are often plagued by pollutant emission from the cool-edge region. The primary sources of visible and near-infrared non-bremsstrahlung continuum emission in the Madison Symmetric Torus arise from electron–neutral interactions, and the pollutant emission is hence directly proportional to the bulk neutral density. Simultaneously monitoring the total emission at 1040 nm and the neutral contaminant (via Dα emission) has enabled an extraction of the electron–ion bremsstrahlung and a measurement of Zeff at comparatively low electron density (1013 cm−3).

Diagnosis of edge localized mode evolution in DIII-D using fast-gated CID and infrared cameras

The tangentially viewing visible and vertically viewing infrared camera systems on DIII-D were upgraded to permit emission measurements during edge localized modes (ELMs) with integration times as short as 1 and 100 μs, respectively. The visible system was used to obtain two-dimensional poloidal profiles of CIII (465 nm) and Dα (656.3 nm) emission with 20 μs integration during various stages of ELM events in the lower DIII-D divertor. The infrared system was used to measure the heat flux to the divertor targets at 10 kHz with 100 μs exposure. Upgrades to the data processing and storage systems permitted efficient comparison of the temporal evolution of these measurements.

Peculiarities of electrode biasing experiments on the T-10 tokamak in regimes with Ohmic and ECR heating

Two types of electrode biasing H-mode were observed on the T-10 tokamak in regimes with electron–cyclotron resonant heating (ECRH). These types differ mainly by the dynamics of the electron temperature. Both types are characterized by a strong (130 V cm−1) radial electric field formation, a decrease of Dα emission intensity, a rise of line-average plasma density and an increase in energy confinement time. However, distinguishing features of the type II H-mode are a time delay of 70–80 ms in the electron temperature growth, a slower increase of the plasma density, and a weak drop of Dα intensity. The differences between the two types of biased H-mode are likely to be connected with the conditioning of the vacuum chamber.

Improved performance of ELMy H-modes at high density by plasmashaping in JET

We present the results of experiments in JET to study the effect of plasmashape on high density ELMy H-modes, with geometry of the magnetic boundarysimilar to that envisaged for the standard Q = 10 operation in ITER. Theexperiments described are single lower null plasmas, with standard qprofile, neutral beam heating and gas fuelling, with average plasmatriangularity δ calculated at the separatrix ~0.45-0.5and elongation κ~1.75. In agreement with the previous resultsobtained in JET and other divertor Tokamaks, the thermal energyconfinement time and the maximum density achievable in steady state for agiven confinement enhancement factor increase with δ. The newexperiments have confirmed and extended the earlier results, achieving amaximum line average density ne~1.1nGR for H98~0.96. Inthis plasma configuration, at 2.5 MA/2.7 T (q95~2.8), a lineaverage density ~95% nGR with H98 = 1 and βN~2are obtained, with plasma thermal stored energy content Wth beingapproximately constant with increasing density, as long as the dischargemaintains Type I ELMs, up to nped~nGR (and ne~1.1nGR).A change in the Type I ELMs behaviour is observed for pedestal densities nped≳70% nGR, with their frequency decreasing with density(at constant Psep), enhanced divertor Dα emission and increasedinter-ELM losses. We show that this change in the ELM character at highpedestal density is due to a change in transport and/or stability in thepedestal region, with the ELMs changing from Type I to mixed Type I and TypeII. The similarity of these observations with those in the Type II ELMregime in ASDEX Upgrade and with other small ELM regimes in DIII-D, JT-60U and Alcator C-MOD is discussed.Finally, we present the first results of experiments by studying in more detailthe effects of the plasma boundary geometry, in particular by investigatingseparately the effect of the upper and lower triangularity, at high average δ. We show that the changes to the lower δ (or of the radialposition of the x-point) affect the pedestal parameters, the size of ELMenergy losses as well as the global energy confinement of the plasma.

Fast ELM dynamics in JT-60U

Simultaneous fast measurements of electron density and temperature, Dα emission and Mirnov probe fluctuations in JT-60U during type I ELMs have been performed. Density fluctuations, measured with two vertical chords of the far infrared interferometer (FIR) at low and high field sides, have been found instrumental in determining spatial structure of type I ELMs (fluctuations localized mainly at the low field side) and duration of its MHD phase. Two different mechanisms for the initial phase of the interaction between the ELM-induced power and particle flows into the scrape-off layer (SOL) and the divertor target have been considered. They may explain the observed difference in the behaviour of Dα emission spikes at the inner and outer targets, which is related to the value of electron collisionality at the pedestal before the ELM. In ELMs with low pedestal collisionality, sharp fronts of Dα emission spikes at the inner and outer strike points and their simultaneous occurrence, coincident with the start of large-scale oscillations observed on the FIR channels, is observed. It is concluded that in such ELMs, plasma-wall interaction is initiated by a power flow conducted to the target through electron channel. In ELMs with high pedestal collisionality (but still of type I), slower evolution of fast signals was observed, with more gradual Dα spikes and clear time delays between outer and inner Dα channels, consistent with the mechanism of convective parallel energy fluxes with the ion sound speed proposed at JET.

Edge turbulence imaging in the Alcator C-Mod tokamak

The two-dimensional (2D) radial vs poloidal structure of edge turbulence in the Alcator C-Mod tokamak [I. H. Hutchinson, R. Boivin, P. T. Bonoli et al., Nucl. Fusion 41, 1391 (2001)] was measured using fast cameras and compared with three-dimensional numerical simulations of edge plasma turbulence. The main diagnostic is gas puff imaging, in which the visible Dα emission from a localized D2 gas puff is viewed along a local magnetic field line. The observed Dα fluctuations have a typical radial and poloidal scale of ≈1 cm, and often have strong local maxima (“blobs”) in the scrape-off layer. The motion of this 2D structure motion has also been measured using an ultrafast framing camera with 12 frames taken at 250 000 frames/s. Numerical simulations produce turbulent structures with roughly similar spatial and temporal scales and transport levels as that observed in the experiment; however, some differences are also noted, perhaps requiring diagnostic improvement and/or additional physics in the numerical model.

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.

Study of particle pumping characteristics for different pumping geometries in JT-60U and DIII-D divertors

Particle pumping characteristics were compared between pumping from the inner side private flux region (IPP) and pumping from both sides of the private flux region (BPP) in the JT-60U W shaped divertor, and between JT-60U IPP and pumping in the DIII-D lower baffled divertor. The pumping flux for BPP is smaller than that for IPP by about a factor of 2 with weak in-out asymmetry of recycling neutral flux and by a factor of 3.5-6.5 with strong in-out asymmetry. The reduction of the pumping flux for BPP is consistent with Monte Carlo simulations, where backflow at the outer pumping slot is observed due to in-out recycling asymmetry. The pumping flux in DIII-D at Ip = 0.8 MA and BT = 1.6 T is comparable to or smaller than that for JT-60U IPP at Ip = 1.0 MA, BT = 3.8 T and Ip = 1.5 MA, BT = 3.5 T in the same density regime. In the DIII-D divertor with pumping from the private flux region, the pumping flux decreases with increasing in-out asymmetry. The pumping flux normalized by the integrated Dα emission over the whole plasma exhibits a similar dependence on the distance between the pumping slot and the strike point in JT-60U IPP and the DIII-D lower divertor with pumping through the outer divertor plasma region.

Dynamics of edge localized modes in the TCV tokamak

The Dα emission time-series from the TCV tokamak wasinvestigated for a large set of ELMy discharges, to determine whetherfluctuations in the time delay between ELMs are the result of noiseonly, or whether a deterministic process in a chaotic state exists, assuggested by results on JT-60U (Bak \ extit{et al} 1999 \ extit{Phys. Rev. Lett.} \ extbf{83} 1339). This study was carried out by searchingthe time-series of each discharge for transient sequences that aregeneric to chaotic systems, called unstable periodic orbits (UPOs). Bytaking a statistical approach, a subset of discharges with q95greater than 2.6 were shown to contain many more UPOs than could beexpected if the dynamical system was noise dominated, and the propertiesof these UPOs were found to exhibit systematic variations with plasmaparameters. Specifically, the period of the unstable fixed point ΔT* was found to scale independently with the plasma current,density and the inner plasma-wall distance. In a number of cases, twoseparate fixed points were found to co-exist during the same discharge.

Formation of electron transport barriers under ECR control of the q(r) profile in the T-10 tokamak

Abstract-the formation of transport barriers under electron cyclotron resonance heating and current drive in the t-10 tokamak is studied. in regimes with off-axis co-eccd and qL<4 at the limiter, a spontaneous transition to improved confinement accompanied by the formation of two electron transport barriers is observed. the improvement resembles an L-H transition. It manifests itself as density growth, a decrease in the Dα emission intensity, and an increase in the central electron and ion temperatures. Two deep wells on the potential profile (the first one at r/aL≈0.6, where aL is the limiter radius, and the second one near the edge) arise during the transition. the internal barrier is formed when dq/dr∼0 with q≈1 in the barrier region.

First results of biasing experiments on the T-10 tokamak.

The first results of the electrode biasing experiments performed on the T-10 tokamak are presented. Positive voltage applied to an electrode inserted inside the tokamak limiter results in the H-mode transition characterised by a decrease of Dα emission intensity, a rise of line-average plasma density and an increase in energy lifetime. Growth of core electron and ion temperatures during the electrode biasing implies the formation of the thermal barrier in addition to the particle transport barrier.

Spatial variation of Dα line spectral profile and emission process inthe divertor region of JT-60U

Spatial variation of the Dα line spectral profiles emittedfrom the divertor region has been measured with a high-resolutionvisible-light spectrometer. The observed Dα line spectral profiles arecompared with the calculation of a neutral particle transport code. Goodagreement is obtained under the attached-plasma conditions. The components ofthe observed spectral profiles are identified at each measured position. Thedissociative excitation (direct production of D*(n = 3) by thedissociation of D2 and D2+) is important near the divertor target. Atpositions away from the divertor target, the emission due to the electroncollisional excitation of the dissociated atoms contributes significantly toboth the Dα line intensity and its spectral profile. The contributionof atoms produced by charge exchange and particle reflection are alsoimportant in the upstream of the divertor plasma. These fractions increasewith the increase in the electron density, broadening the Dα linespectral profile. The Dα line spectral profiles are measured underseveral plasma conditions. The Dα line spectral profile near thetarget becomes broader during the ELMy phase. Under the detached-plasmacondition, the Dα line spectral profile is symmetric to the linecentre and is narrow, dominated by the low-temperature components. A similarfeature is observed at the MARFE region.

Self-consistent plasma-neutral modeling in tokamak plasmas with a large-area toroidal belt limiter

Plasma-neutral phenomena in the edge plasma and scrape-off layer of the Torus Experiment for Technology Oriented Research [G.H. Wolf and the TEXTOR Team, J. Nucl. Mater. 122&amp;123, 1124 (1984)] with the toroidal belt Advanced Limiter Test (ALT-II) [D.M. Goebel et al., J. Nucl. Mater. 162–164, 115 (1989)] are simulated using the code package B2-EIRENE [D. Reiter et al., Plasma Phys. Controlled Fusion 33, 1579 (1991)]. Spatially-constant, anomalous radial transport coefficients (D,V,χ) are used for fitting measured electron temperature and density profiles. Primary neutral fluxes are determined by plasma fluxes to material surfaces, and Dα emissions are predicted from them. Comparison of the predicted Dα emission with measurements indicates a critical need, in predictive modeling, for a self-consistent model of fluxes to material surfaces that are parallel to the magnetic field. Appropriate factors are calculated for deducing D+ source rates from Dα emissions measured in various locations, taking into account molecular processes and spatially varying plasma parameters; values range from 17 to 28 ions/photon. Ion fluxes lost to pumps or the wall must be explicitly re-introduced as neutral fluxes at the outer boundary.

Instrumentation for the joint European torus motional Stark effect diagnostic

A motional Stark effect magnetic field pitch angle diagnostic has been implemented on the joint European torus (JET) tokamak. The instrumentation designed following the study by Hawkes et al. (these proceedings) is described. Dα emission from the Octant 4 neutral beams is collected by optics which transport the plasma image outside the vacuum vessel and through a pair of photoelastic modulators (PEMs) and a linear polarizer. The light is fiber-optically coupled to interference filter spectrometers, which incorporate a remotely controlled filter tilting mechanism. This allows the center wavelength of the filter bandpass to be tuned over a range sufficient for observation of the σ and π lines of the Stark spectrum emitted by the full- and half-energy components of the beam, providing flexibility to make measurements with a variety of beam configurations. The detectors are low-noise avalanche photodiode modules. Fast digital signal processing techniques are used to extract the Fourier components of the signal at the PEM first and second harmonic frequencies. Analysis of these signals will yield the magnetic field pitch angle, which will be used as a constraint on EFIT equilibrium reconstruction modeling to obtain the q(r) profile. The system has 25 spatial channels covering the outer-half of a JET plasma with spatial resolution of 0.03–0.07 m per channel with ∼0.05 m channel-to-channel separation. Time resolution is expected to be 1–10 ms.

Simultaneous measurement of q and Er profiles using the motional Stark effect in high-performance DIII-D plasmas (invited)

The motional Stark effect (MSE) diagnostic was developed to provide a measurement of the magnetic pitch angle or q profile in tokamaks. The technique relies upon polarization measurements of Stark broadened Dα emission to determine the pitch angle of the Lorentz vb×B electric field, where vb is the injected neutral beam particle velocity and B is the total magnetic field. However, in many advanced confinement regimes, large values of the plasma radial electric field, Er, are observed and can affect the interpretation of MSE measurements. Viewing fixed locations in the plasma from two different viewing angles allows one to separate the Er field from the vb×B field, thus providing simultaneous measurement of the Er and q profiles. To achieve this measurement, the DIII–D MSE diagnostic was recently upgraded from 16 to 35 channels with three independent viewing angles. The new instrument provides an Er resolution of 5–10 kV/m with a time response of 1 ms. Measurement results from the VH mode, reverse shear, and H mode plasmas are presented.