JET Scrape-off Layer Research Articles

Parametric study of dust acoustic wave destabilization in the JET scrape-off layer

Sudden inflows of W particulate are routinely observed in correlation with plasma instabilities in JET-ITER-Like-Wall experimental campaigns. Dust acoustic wave destabilization by micrometer-size tungsten dust released in such events is simulated here using realistic parameters describing the JET scrape-off layer. Such low-frequency waves may serve as a monitorable signal for dust detection in the scrape-off layer on a pulse-by-pulse basis.

Assessment of the effect of parallel temperature gradients in the JET SOL on Te measured by divertor target Langmuir probes

Higher than expected electron temperatures (Te) are often measured by divertor Langmuir probes (LP) in high recycling and detached regimes in JET and other tokamaks. As a possible mechanism to explain this discrepancy, we investigate the effect of penetration of fast, almost collisionless electrons connecting the hot upstream scrape-off layer (SOL) region to the divertor targets in JET. We simulate the electron velocity distribution function (EVDF) near the divertor targets using a simple 1D kinetic model using parallel SOL profiles from EDGE2D-EIRENE simulations. The resulting EVDF is used to construct synthetic LP current–voltage (IV) characteristics and evaluation of Te is performed in the same way as for experimental data. Results indicate that the process does not explain the anomalously high Te values estimated from the target probe measurements if the EDGE2D-EIRENE simulated parallel profiles are a good representation of reality.

Characterization of scrape-off layer transport in JET limiter plasmas

The JET scrape-off layer (SOL) has been characterized with a reciprocating probe in inner wall (IW), and outer wall (OW), limited plasmas. Experiments revealed that SOL profiles are substantially broader (by a factor of ∼5–7.5 in the power e-folding length) for IW limited than in OW limited plasmas. Results are consistent with the larger radial turbulent transport found for IW limited plasmas. Major differences are observed between IW and OW limited plasmas on the density and electron temperature e-folding lengths, parallel flow, radial turbulent transport as well as on the temporal and spatial characteristics of the fluctuations. Experimental findings on JET suggest that the differences in the SOL characteristics for both configurations are due to a combination of a poloidal asymmetry in radial transport with a reduced cross-field transport across the last closed flux surface associated with the confinement improvement observed for OW limited plasmas.The dependence of the SOL power e-folding length on the main plasma parameters was also investigated for IW limited plasmas and a modest negative dependence on both the plasma current and the line-averaged density found. Finally, it is shown that the SOL radial transport and the amplitude of the fluctuations increase with plasma current and decrease with line-averaged density for IW limited plasmas.

Non-local approach to kinetic effects on parallel transport in fluid models of the scrape-off layer

Using a non-local model, fluid simulations can capture kinetic effects in the parallel electron heat-flux better than is possible using flux limiters in the usual diffusive models. Non-local and diffusive models are compared using a test case representative of an edge-localized mode crash in the JET scrape-off layer (SOL), simulated in one dimension. The non-local model shows substantially enhanced electron temperature gradients, which cannot be achieved using a flux limiter. The performance of the implementation, in the BOUT++ framework, is also analysed to demonstrate its suitability for application in three-dimensional simulations of turbulent transport in the SOL.

Comparison of scrape-off layer transport in inner and outer wall limited JET plasmas

The JET scrape-off layer has been characterized with a reciprocating probe in inner wall, IW, and outer wall, OW, limited plasmas. Broad SOL profiles are observed for IW limited plasmas with power e-folding length substantially larger (by a factor of ∼5–7.5) than in OW limited plasmas. The properties of the fluctuations in the SOL parameters indicate larger turbulent transport for IW limited plasmas. The striking differences observed between IW and OW limited plasmas on the power e-folding length, parallel flow, turbulent transport as well as the characteristics of the fluctuations support the existence of a poloidally localized region of enhanced radial transport near the outboard midplane. The dependence of the SOL power e-folding length on the main plasma parameters was also investigated for IW limited plasmas and a modest negative dependence on both the plasma current and the line-averaged density found.

JET scrape-off-layer ionization at lower hybrid wave launching

We present in this paper modelling of the JET scrape-off layer (SOL) including direct SOL ionization by launched lower hybrid (LH) waves. For that purpose, the two-dimensional fluid code EDGE2D-NIMBUS was modified in order to account for possible enhanced ionization in the SOL, due to the LH power absorbed by the electrons in a layer extending in the radial direction from the launcher. By taking the direct LH SOL ionization into account, the observed modification of the density in front of the LH antenna during a LH power and gas injection, can be explained. The JET grill private SOL limiters, acting as sinks for the particles, are also included in the model. We compare the efficiency of gas puffing from the top and the outer mid-plane for SOL density enhancement and improvement in LH coupling. The observed reduction of the temporal variation of the LH wave reflection coefficient and of measured saturated currents in the SOL during ELMs, is explained. The density depletion, by ponderomotive forces in front of the grill, is estimated.

An analysis of VUV C IV emission from the JET divertor giving measurements of electron temperatures

A description of the radiation emitted by impurities from within a plasma is crucial if spectral line intensities are to be used in detailed studies, such as the analysis of impurity transport. The simplest and most direct check that can be made on measurements of line intensities is to analyse their ratios with other lines from the same ion. This avoids uncertainties in determining the volume of the emitting plasma and the absolute sensitivity calibration of the spectrometer and, in some cases, the need even for accurate measurements of parameters such as electron density. Consistency is required between the measured line intensity ratios and the theoretical values. The expected consistency has not been found for radiation emitted from the JET scrape-off layer (e.g. Lawson et al 2009a JINST 4 P04013), meaning that the description of the spectral line intensities of impurity emission from the plasma edge is incomplete. In order to gain further understanding of the discrepancies, an analysis has been carried out for emission from the JET divertor plasma and this is reported in this paper.Carbon was the main low Z intrinsic impurity in JET and an analysis of spectral line intensity ratios has been made for the C IV radiation emitted from the JET divertor. In this case, agreement is found between the measured and theoretical ratios to a very high accuracy, namely to within the experimental uncertainty of ∼±10%. This confirms that the description of the line intensities for the present observations is complete. For some elements and ionization stages, an analysis of line intensity ratios can lead to the determination of parameters such as the electron temperature of the emitting plasma region and estimates of the contribution of recombination to the electron energy level populations. This applies to C IV and, to show the value and possibilities of the spectral measurements, these parameters have been calculated for a database of Ohmic and additionally heated phases of a large number of pulses. The importance of dielectronic, radiative and charge-exchange recombination as well as ionization has been investigated. In addition, the development of Te throughout two example discharges is illustrated. The presented results indicate a number of areas for further investigation.

Reciprocating probe measurements of ELM filaments on JET

The ELM filamentary structure in the JET scrape-off layer (SOL) has been investigated using a multi-pin reciprocating Langmuir probe system inserted at the top, low field side of the poloidal cross-section. These measurements clearly demonstrate that both Type I and III edge localized modes (ELMs) are composed of a number of filaments. The filament propagation in the SOL is investigated and the radial velocity estimated using different methods. The large radial velocities observed during Type I ELMs (up to 6 km s−1) imply that filaments have no time to drain significant energy along the field lines, reaching the wall with a reasonable fraction of the pedestal density and temperature. In spite of the time between Type I ELMs being much longer than the ELM duration, most of the particle flux to the main chamber plasma-facing components occurs in the latter period. Furthermore, filaments in Type I ELMs are significantly larger and faster than those observed in Type III ELMs. During Type I ELMs a strong parallel flow (M ∼ 0.4) is measured towards the inner divertor, which increases during filaments. These observations are in agreement with the ballooning nature of the ELM losses.

Kinetic simulations of the parallel transport in the JET scrape-off layer

In this paper, we present kinetic simulation results of plasma parallel transport in the stationary and ELMy scrape-off (SOL) layer. We demonstrate that number of kinetic factors describing SOL are strongly nonuniform and evolve during the ELM. Power loads to the divertor obtained from the ELMy SOL simulations are in a perfect agreement with the experimental measurements at JET.

Intermittent transport in the JET far-SOL

The intermittent transport in the JET scrape-off layer has been investigated using a multi-pin reciprocating Langmuir probe system. It was found that radial transport in the far-SOL is not continuous but occurs in short bursts observed most often at the beginning of each structure. The magnitude of the parallel ion flux during the filaments quickly decreases with radius, contrary to the cross-field velocity which has a weak radial dependence. Data from the JET boundary plasma have been analyzed to characterize the properties of the intermittent events. Statistical analysis reveals that the filament duration is larger on average in L-mode when compared to that observed during ELMs, while the radial velocity and the density perturbations are smaller.

ELM transport in the JET scrape-off layer

This contribution summarizes a number of aspects of the experimental and modelling programme at JET aimed at improving the characterization and understanding of edge localized mode (ELM) transport in the scrape-off layer (SOL). Divertor target energy deposition asymmetries favouring the inner target for the ion B × ∇B drift directed towards the X-point are observed with infra-red (IR) thermography. Similar trends are seen in the ELM resolved energy radiated in the divertor volume. Particle-in-cell kinetic calculations of the parallel ELM heat transport have been made for a range of ELM energies, revealing the detailed time response of target sheath heat transmission factors and indicating that electrons deposit ∼30% of the ELM energy. The simulation results are in good agreement with experimental measurements of the integral energy deposited at the outer target up to the peak in target heat loads. A transient model of ELM filament energy evolution has been developed at JET and is able to reproduce a number of experimental observations, including the high ion energies observed in the far SOL using an electrostatic retarding field electrostatic analyser (RFA) and estimates of ELM heat fluxes deposited on main chamber limiters. During the ELM, the RFA and a second, SOL turbulence probe, clearly show the presence of coherent spikes in the hot ion flux, the plasma flux and the electron temperature. Field aligned structures have also been seen for the first time on JET in the power deposition on main wall limiters and upper dump plate surfaces using a new wide angle IR camera system. The probe signals are interpreted as the arrival of interspaced plasma filaments, with successive filaments carrying less energy. They are also consistent with the ELM out flux entering the SOL primarily on the outboard side and launching a sound wave disturbance along field lines.

A comparison of experimental measurements and code results to determine flows in the JET SOL

Two reciprocating probe systems, at the same poloidal position at the top of the JET torus but toroidally separated by 180°, have been used to measure parallel flow in the scrape-off layer (SOL) of lower single-null, diverted plasmas. One system uses the entrance slit plates of a retarding field analyser to record upstream and downstream flux densities, whilst the second employs two pins of a nine-pin turbulent transport probe. Measurements have been made for both forward and reversed toroidal field directions. The results from both probe systems are similar.In the forward field direction, that is with the ion drift direction downwards towards the divertor, a strong parallel flow is measured at the top of the machine in the direction from the outer to the inner divertor. The flow generally has a low value, Mach number M ∼ 0.2, close to the separatrix, but rises in the region of high magnetic shear close to the separatrix to a maximum of M ∼ 0.5 some 20 mm outside the separatrix. In contrast, for a reversed field, the measured flow is small (close to zero) throughout much of the SOL but rises near the separatrix to a value equal in both magnitude and direction to that observed in the forward field. There is thus some symmetry in the flow with respect to field reversal but with a symmetry axis given by a positive offset of around M ∼ 0.2. This paper presents simulations using the EDGE2D/Nimbus code, which predicts very low values of parallel flow Mach number near the probe position. The possibility of impurities released from the probe surfaces increasing the flow velocity is explored using the code.

The effect of CD4 puffing on the peripheral scrape-off layer in JET

A connection has been found between puffing of CD4 at the outer mid-plane of JET and a rise in scrape-off layer (SOL) peripheral electron density. This observation led to the proposal that CD4 puffing might be used to improve lower hybrid coupling in optimized shear discharges. Results described elsewhere have demonstrated that this is indeed the case. The focus of this paper is on the causes of this peripheral density rise using two-dimensional fluid modelling and local measurements of turbulent transport in the JET SOL.

Parallel flow in the JET scrape-off layer

Parallel plasma flows with Mach number M≈0.5 have been measured inthe JET scrape-off layer (SOL). These flows show a maximum some 10-20 mmmid-plane outside the separatrix, and decrease towards the separatrix. Theflow reverses when BT and IP are reversed, but is asymmetric aboutzero. A number of possible mechanisms for the observed flow are suggested,including ion Pfirsch-Schlüter flow, ballooning transport, momentumtransfer, reverse flow (ionization driven flow) and co-current toroidalmomentum generated in the SOL by ion ∇B⃗ and centrifugaldrifts.

Heat transport in the JET scrape-off layer

The DIVIMP/NIMBUS onion skin model (OSM) has been used to calculate 2-D plasma distributions of ne, Te and Ti in the JET edge, based on boundary conditions measured across the targets with built-in Langmuir probes. Ohmic, L mode and ELMy H mode discharges were analysed. From the calculated 2-D plasma profiles, values of χ⊥SOL(r) were extracted as a function of radial location within the SOL, showing that χ⊥SOL(r) increases with radius. At the separatrix χ⊥SOL(0) was in the range ∼0.05-1 m2·s-1, with the lowest values occurring for H modes (between ELMs). It was found that χ⊥SOL(r) ∼[Teu(r)]-2, where Teu is the electron temperature at the `upstream' end of the SOL flux tube. No clear correlation with the magnetic field, BT, was found. The results are not consistent with Bohm (χ⊥SOL ∝ Teu/B) scaling.

The effect of different ICRH heating scenarios on the jet scrape-off layer

Investigation of the scrape-off layer (SOL) at different toroidal and poloidal positions has been carried out in JET for limiter discharges with and without ion cyclotron resonance heating (ICRH). A wide range of plasma conditions ( I p ≈1−5 (rmMA), B T ≈1.8−3.5 T, 〈 n e 〉 ≈ (1−5) × 10 19 m −3 ), ICRH heating scenarios (H and He 3 minority, quadrupole, monopole, dipole antenna phasing) and different machine configurations were investigated. The total ICRH power coupled to the JET plasma was ≤ 7.2 MW, when three antennae were used, and ≤ 13 MW using eight antennae operation. Limiter discharges were produced with either eight discrete graphite limiters or two full toroidal belt limiters. Density profiles in the SOL were found to decay exponentially with an e-folding length, λ n e , which changes with total input power, wall conditioning and plasma species. The modification of the plasma edge depends on the antenna phasing. The scaling of edge temperature, edge density and impurity production with 〈 n e 〉 and P T for different scenarios is presented. Changes in SOL parameters are also compared with different torus wall and antenna conditioning.