R.D. Monk

Interpretation of ion flux and electron temperature profiles at the JET divertor target during high recycling and detached discharges

Abstract Detailed experiments have been carried out with the JET Mark 1 pumped divertor to characterise high recycling and detached plasma regimes. This paper presents new measurements of high resolution divertor ion flux profiles that identify the growth of additional peaks during high recycling discharges. These ion flux profiles are used in conjunction with Dα and neutral flux measurements to examine the physics of divertor detachment and compare against simple analytic models. Finally, problems are highlighted with conventional methods of single and triple probe interpretation under high recycling conditions. By assuming that the single probe behaves as an asymmetric double probe the whole characteristic may be fitted and significantly lower electron temperatures may be derived when the electron to ion saturation current ratio is reduced. The results from the asymmetric double probe fit are shown to be consistent with independent diagnostic measurements.

Parallel electron temperature and density gradients measured in the JET MkI divertor using thermal helium beams

Abstract This paper describes the first application of a thermal helium beam diagnostic to a divertor. The helium beam is used to determine spectroscopically the electron temperature and density from the inner and outer strike points up to the X-point, using helium line ratios which are primarily sensitive to electron density and temperature, as reported by Schweer et al. [J. Nucl. Mater. 196–198 (1992) 174]. Measurement of the neutral helium line intensities in the outer divertor target were performed under attached, high recycling and detached plasma conditions in Ohmic and L-mode discharges. An interpretative model has been developed using the DIVIMP code at JET which incorporates the helium injection point, the nozzle divergence and the viewing arrangement of the periscope for a particular equilibrium.

Volume recombination and detachment in JET divertor plasmas

The spatial distribution of recombination has been studied in the detaching phases of L and H-mode discharges. Recombination is detected by the sudden increase in intensity of the high quantum number states of the Balmer series of hydrogen. In L-mode, detachment occurs completely and the radiation moves to a region near the X-point. In H-mode, detachment only occurs between ELMs. There is a marked decrease in the pedestal T c inside the separatrix and loss of confinement before detachment takes place.

Determination of JET scrape-off layer transport coefficients using an interpretative “onion-skin” plasma model

Abstract Currently there is much interest in the application of sophisticated 2D codes to model the scrape-off layer (SOL) plasma in divertor tokamaks. These models generate the plasma solution at the divertor target starting from boundary conditions defined on a closed flux surface near the separatrix. An alternative approach, used within the DIVIMP code [1], is to anchor the boundary conditions at the divertor target using experimental Langmuir probe measurements and then solve the 1D fluid equations along each flux tube in the SOL generating an “onion-skin” model. Interaction of the plasma with recycled neutrals is included by coupling to a 2D neutral transport code. Such a model has been applied to analyse the SOL plasma transport in JET divertor discharges from the previous experimental campaign. Following the approach of Shimizu et al. [2], the radial variation of the anomalous cross-field transport coefficients ( D ⊥ , χ ⊥ ) are evaluated over a range of discharges and compared with results from a 2D predictive code.

Modelling of deuterium emission in high density divertor plasmas in JET

Abstract Simulations of neutral deuterium emission from high density Ohmic and L-mode JET divertor discharges with the EDGE2D/NIMBUS codes are presented. At high density the codes fail to reproduce quantitatively the steady increase of divertor D 0 emission with rising plasma density observed in the experiment. However, good agreement is obtained with the measured D γ /D α ratio in inner and outer divertor despite the total plasma volume recombination being small in the simulations. The asymmetry between inner and outer divertor detachment is found to depend on the strength of the charge exchange momentum losses. Recombination alone is not sufficient to obtain strong detachment in the inner divertor. Measurements of Ly β /D α emission profiles across the JET divertor show signs of Lyman radiation reabsorption in the inner divertor at high density. The first attempt at including divertor plasma opacity corrections selfconsistently in EDGE2D/NIMBUS simulations of high density JET L-mode discharges is presented. The effect of opacity on the simulated deuterium emission and on the 2D plasma solution is discussed.

Deduction of SOL Transport Coefficients Using 2D Modelling for Hot-Ion ELM-free H-Modes in JET

Abstract Profiles of Te and Jsat measured by Langmuir probes at the target plates of the JET MkI divertor and the target plate power loading are modelled via the EDGE2D/NIMBUS codes. The low-density ( n e ∼ 1 × 10 19 m −3 ) OH plasma preceding high-power NBI, as well as the ELM-free high performance H∗ and high performance rollover HRO phases are considered. Experimentally, the power splitting between ions (Pi ≥ 7 MW) and electrons (Pe n e can increase up to a factor of three as the H∗-phase progresses. In modelling χi is taken as 1 m2/s. To match the divertor Te-profiles, χe must be varied from the strike point outwards as ∼ 0.5 → 3 m2/s in all phases. Perpendicular particle transport is assumed to be purely diffusive or with a pinch. Duplication of Jsat(R) requires: D ⊥ ∼ 0.03 → 0.015 m 2 /s (OH → H ∗ ) , or ν pinch D ⊥ ∼ 15 → 45 → 25 m−1 (OH → H∗ → HRO) with D⊥ = 0.1 m2/s. Sensitivity studies involving different recycling scenarios, deep and shallow computational grids, wall material and the thermal transport barrier are carried out to judge the possible influences on the deduced transport coefficients.

Influence of active pumping on density and confinement behaviour of JET plasmas

This paper describes the confinement characteristics of high density Elmy H-modes obtained in the JET Mark I experimental campaign. Plasma pulses carried out with and without the in-vessel cryopump operational (≈ 240 m3 s−1 pumping speed for deuterium) are compared. In all cases, it is found that the H-mode density limit is not disruptive, but manifests itself as a loss of confinement, accompanied by an increase of neutral pressure at the plasma edge. The interpretation of the H-mode density limit in terms of the Greenwald scaling is discussed. It is also shown that the loss of confinement correlates to the onset of detachment in the divertor, and that operation at high density in H-mode is compatible with partial detachment but not with full detachment.

Plasma Ion Mass-Spectrometry in the Textor Boundary

In this paper we present the first results obtained with a plasma ion mass spectrometer in the boundary of the TEXTOR tokamak. Measurements were made several days after TEXTOR was boronised. Comparisons are made with results obtained in DITE under similar discharge conditions but in an unboronised machine. The measured charge state distributions are compared with simulations by the Monte Carlo impurity transport code LIM for carbon, boron, oxygen and injected neon impurities.

Assessment of the DIVIMP ‘onion-skin’ model in the JET Mark I divertor

Abstract The DIVIMP-NIMBUS [1] ‘Onion-Skin’ model has been used to predict upstream plasma parameters using as input measurements by Langmuir probes in the divertor target tiles. Measurements of density and temperature (both Ti and Te) across the SOL close to the stagnation point at the top of the torus have been made using a reciprocating probe carrying both Langmuir and retarding field analyzer (RFA) heads. In addition, a lithium beam diagnostic has been used to measure radial distributions of density upstream. Ohmic, L-mode and H-mode discharges have been studied, and good agreement with code predictions in the three cases has been found. The heat transport coefficient, χ⊥(r), has been evaluated across the SOL, and is found to increase with increasing distance from the separatix.

Neutral particle retention in the JET MK I divertor

Abstract Retention of neutral deuterium and nitrogen in the JET MK I divertor has been investigated. Results show that ohmic plasma detachment reduces deuterium retention, that the magnetic divertor configuration has some influence on the achievable deuterium retention, and that nitrogen in nitrogen-seeded steady state detached H-mode discharges accumulates in the divertor.