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Recent progress towards an increased understanding of the physical processes in the divertor and scrape-off layer (SOL) plasmas in DIII-D has been made possible by a combination of new diagnostics, improved computational models and changes in divertor geometry. The work focused primarily on ELMing H mode discharges. The physics of partially detached divertor plasmas, with divertor heat flux reduction by divertor radiation enhancement using D2 puffing, was studied in two dimensions, and a model of the heat and particle transport was developed that includes conduction, convection, ionization, recombination and flows. Plasma and impurity particle flows were measured with Mach probes and spectroscopy and compared with the UEDGE model. The model now includes self-consistent calculations of carbon impurities. Impurity radiation was increased in the divertor and SOL with `puff and pump' techniques using SOL D2 puffing, divertor cryopumping and argon puffing. The important physical processes in plasma-wall interactions were examined with a DiMES (divertor material evaluation system) probe, plasma characterization near the divertor plate and the REDEP code. Experiments comparing single null plasma operation in baffled and open divertors demonstrated a change in the edge plasma profiles. These results are consistent with a reduction in the core ionization source calculated with UEDGE. Divertor particle control in ELMing H mode with pumping and baffling resulted in a reduction in H mode core densities to ne/nGr ≈ 0.25 (with nGr the Greenwald density). Divertor particle exhaust and heat flux were studied as the plasma shape was varied from a lower single null to a balanced double null, and finally to an upper single null.

We previously reported electrostatic energy analyzer (GEA) measurements of the parallel energy distribution of ions incident on a floating target in PISCES-A [J. Cuthbertson, R. Lehmer, M. Jakubowski, S.C. Luckhardt, Bull. Am. Phys. Soc. 41 (1996) 1575]. The principal finding was that ion energy spectrum was significantly broader (6–8 eV) than the ion temperature expected from classical collision processes (Ti ≅ 2–3 eV). In this paper, we present further gridded energy analyzer measurements, and Doppler broadening measurements of ion spectral lines. Moreover, we have varied plasma conditions such that the turbulent plasma potential fluctuation amplitudes covered the range 3 eV

A highly radiating zone (MARFE) just above the divertor X-point has been used to access the marginal transition regime to study the existence of a critical point for the L to H transition. Phase transition models predict that at the critical point the transition duration increases and the plasma parameters vary continuously between the L- and H-mode. In these experiments, the L to H transition duration increased 50-100 times over fast transitions. However, the evolution of shear, the edge density gradient, the H-mode pedestal, and fluctuations is essentially unchanged from that in fast transitions. The only difference is in the speed with which and the degree to which the fluctuation amplitudes are transiently reduced. This difference is understandable in terms of the time scales for fluctuation amplitude reduction and edge pressure gradient increase (several ms), provided the edge fluctuations are pressure-gradient driven.

In a magnetically diverted tokamak, the scrape-off layer (SOL) and divertor plasma separates the first wall from the core plasma, intercepting impurities generated at the wall before they reach the core plasma. The divertor plasma can also serve to spread the heat and particle flux over a large area of divertor structure wall using impurity radiation and neutral charge exchange, thus reducing peak heat and particle fluxes at the divertor strike plate. Such a reduction will be required in the next generation of tokamaks, for without it the divertor engineering requirements are very demanding. To successfully demonstrate a radiative divertor, a highly radiative condition with significant volume recombination must be achieved in the divertor, while maintaining a low impurity content in the core plasma. Divertor plasma properties are determined by a complex interaction of classical parallel transport, anomalous perpendicular transport, impurity transport and radiation, and plasma - wall interaction. In this paper we will describe a set of experiments on DIII-D designed to provide detailed two-dimensional documentation of the divertor and SOL plasma. Measurements have been made in operating modes where the plasma is attached to the divertor strike plate and in highly radiating cases where the plasma is detached from the divertor strike plate. We will also discuss the results of experiments designed to influence the distribution of impurities in the plasma using enhanced SOL plasma flow. Extensive modelling efforts will be described which are successfully reproducing attached plasma conditions and are helping to elucidate the important plasma and atomic physics involved in the detachment process.

A series of experiments was conducted on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] to investigate the physical processes which limit density in high confinement mode (H-mode) discharges. The typical H-mode to low confinement mode (L-mode) transition limit at high density near the empirical Greenwald density limit [M. Greenwald et al., Nucl. Fusion 28, 2199 (1988)] was avoided by divertor pumping, which reduced divertor neutral pressure and prevented formation of a high density, intense radiation zone (MARFE) near the X-point. It was determined that the density decay time after pellet injection was independent of density relative to the Greenwald limit and increased nonlinearly with the plasma current. Magnetohydrodynamic (MHD) activity in pellet-fueled plasmas was observed at all power levels, and often caused unacceptable confinement degradation, except when the neutral beam injected (NBI) power was ⩽3 MW. Formation of MARFEs on closed field lines was avoided with lo...

Recent measurements of the two-dimensional (2-D) spatial profiles of divertor plasma density, temperature, and emissivity in the DIII-D tokamak [J. Luxon et al., in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1987), p. 159] under highly radiating conditions are presented. Data are obtained using a divertor Thomson scattering system and other diagnostics optimized for measuring the high electron densities and low temperatures in these detached divertor plasmas (ne⩽1021 m−3, 0.5 eV⩽Te). D2 gas injection in the divertor increases the plasma radiation and lowers Te to less than 2 eV in most of the divertor volume. Modeling shows that this temperature is low enough to allow ion–neutral collisions, charge exchange, and volume recombination to play significant roles in reducing the plasma pressure along the magnetic separatrix by a factor of 3–5, consistent with the measurements. Absolutely calibrated vacuum ultravi...

Reciprocating Langmuir probes have been used to investigate turbulent transport in DIII-D edge and divertor plasmas. The turbulent particle flux on the outboard midplane exceeds the particle flux from the 2-D local transport code UEDGE by a large enough factor to allow for substantial poloidal asymmetry in the turbulent transport. Changes in the turbulent effective diffusion coefficient in the edge agree qualitatively with particle confinement changes. Potential fluctuations eφf/kTe in the divertor in attached ELM-free conditions are comparable to eφf/kTe on the outboard midplane. ELMs transiently enhance eφf/kTe to 2 in attached plasmas and to 2–20 in detached plasmas. Such strong turbulence (eφf/kTe > 1) in the divertor may affect the mean energy of ions striking the divertor target plates by altering the sheath potential and therefore affect target plate erosion rates.

In this paper the authors present measurements of the global power and particle balance in the high-performance phase of negative central magnetic shear (NCS) discharges and compare with reference VH-mode discharges. The principal differences observed are that NCS has a much lower fraction of the total input power flowing into the boundary, less core radiation, and larger rate of stored energy increase as a fraction of total power. Scrape-off layer (SOL) temperature and divertor heat flux profiles, and radiation profiles at the midplane, are similar to VH-mode. Due to the good core particle confinement and efficient fueling by neutral beam injection (NBI), with little gas puffing, the gas load on the walls and the recycling are very low during the NCS discharges. This results in a rate of density rise relative to beam fueling at the L to H transition time which is 1/3 of the value for VH transitions, which is in turn 1/2 that for L-to-ELMing-H-mode transitions.

This paper describes divertor density and temperature measurements using both a new reciprocating Langmuir probe (XPT-RCP) which plunges vertically above the divertor floor up to the X-point height and swept, single, Langmuir probes fixed horizontally across the divertor floor. These types of measurements are important for testing models of the SOL and divertor which then are used to determine engineering design criteria for plasma facing components in reactor size tokamaks. The 6 mm diameter fixed single probes (19 domed and 2 flat at radially equivalent locations) are incorporated into the lower divertor floor at 19 radial locations and swept at 250 Hz. These probes are critical for determining plasma detachment from the floor during operation with high density, highly radiating divertors. By sweeping the divertor strike point across the fixed probes, different regions of the target plate incident flux profile can be sampled and a high resolution spatial profile can be obtained from each probe tip as the strike point moves past. The X-point reciprocating probe (XPT-RCP) provides ne and Te profiles with high spatial (2 mm) and temporal (0.5 ms) resolution from the target plate to the X-point along a single vertical chord at the same radial location as a fixed probe tip at a different azimuthal location. The probe ne and Te are compared to the divertor Thomson scattering (DTS) ne and Te (eight vertical points at 20 Hz, RThomson = RX-point-rcp). Recent observations have also shown divertor densities from 3 × 1019 to 4 × 1020 m−3 near the target plate with the highest densities observed with D2 gas puffing. Electron temperature is typically of the order of 15–25 eV at the target rising to about 70 eV near the X-point. Lower temperature, higher density plasmas are observed along the inner leg. Generally good agreement among the XPT-RCP, the fixed floor probes, and the DTS is observed. Differences between these diagnostic measurements will also be discussed with respect to different operating regimes and conditions.

A comprehensive set of experiments has been carried out on the DIII-D tokamak to measure the 2-D (R,Z) structure of the divertor plasma in a systematic way using new diagnostics. Measurements cover the divertor radially from inside the X-point to the outer target plate and vertically from the target plate to above the X-point. Identical, repeatable shots were made, each having radial sweeps of the X-point and divertor strike points, to allow complete plasma and radiation profile measurements. Data have been obtained in ohmic, L-mode, ELMing H-mode, and reversed B{sub T} operation ({gradient}B drift away from the X-point). In addition, complete measurements were made of radiative divertor plasmas with a Partially Detached Divertor (PDD) induced by D{sub 2} injection and with a Radiating Mantle induced by Impurity injection (RMI) using neon and nitrogen. The data set includes first observations of the radial and poloidal profiles of the X-point, inner and outer leg plasmas in PDD and RMI radiative divertor operation. Preliminary data analysis shows that intrinsic impurities play a critical role in determining the SOL and divertor conditions.

DIII-D currently operates with a single- or double-null open divertor and graphite walls. Active particle control with a divertor cryopump has demonstrated density control, efficient helium exhaust, and reduction of the inventory of particles in the wall. Gas puffing of D{sub 2} and impurities has demonstrated reduction of the peak divertor beat flux by factors of 3--5 by radiation. A combination of active cryopumping and feedback-controlled D{sub 2} gas puffing has produced similar divertor heat flux reduction with density control. Experiments with neon puffing have shown that the radiation is equally-divided between a localized zone near the X-point and a mantle around the plasma core. The density in these experiments has also been controlled with cryopumping. These experimental results combined with modeling were used to develop the new Radiative Divertor for DIII-D. This is a double-null slot divertor with four cryopumps to provide particle control and neutral shielding for high-triangularity advanced tokamak discharges. UEDGE and DEGAS simulations, benchmarked to experimental data, have been used to optimize the design.

Results are presented from an experiment in which a large amount of deuterium gas was continuously puffed at the scrape-off layer and exhausted from the DIII-D pump divertor ('puff and pump' technique). The experiments were conducted with a single null divertor configuration during edge localized mode (ELM)ing H mode. Steady state density conditions were attained during the strong deuterium puff. The deuterium puff reduced the central plasma density of argon, puffed continuously as a trace impurity, by as much as a factor of 20, and the central argon radiated power density by as much as a factor of 10

Net erosion/deposition rates of carbon and tungsten were measured at the outer strike point of the divertor plasma on the floor of the DIII-D tokamak during deuterium H-mode operation at an average power deposition of about 45 W/cm 2 . For carbon, net erosion rates of up to 4 nm/s were found. For a tungsten film, no appreciable erosion was detected. However, measurements of deposited tungsten on adjacent carbon surfaces indicated a net W erosion rate of 0.06 nm/s.

We present a study of the impact of H-mode operation at low density on divertor plasma parameters on the DIII-D tokamak. The line-average density in H-mode was scanned by variation of the particle exhaust rate, using the recently installed divertor cryo-condensation pump. The maximum decrease (50%) in line-average electron density was accompanied by a factor of 2 increase in the edge electron temperature, and 10% and 20% reductions in the measured core and divertor radiated power, respectively. The measured total power to the inboard divertor target increased by a factor of 3, with the major contribution coming from a factor of 5 increase in the peak heat flux very close to the inner strike point. The measured increase in power at the inboard divertor target was approximately equal to the measured decrease in core and divertor radiation.

We account for ≥85% of the injected power in DIII-D during single-null diverted ELMing H-mode discharges. Core plasma radiation accounts for ≤18% with the rest flowing through the scrape-off layer to the inboard and outboard divertors. The total power roughly splits in an in: out ratio of 1:1.3. Of the power entering the outboard divertor 75% leaves as direct heat flux to the divertor target plates with the rest in the form of radiation. In the inboard divertor, however, 75% of the power leaves as radiation with the rest as divertor target plate heat flux and charge exchange losses. The in: out ratio remains constant over a range of injected power and plasma current and even as divertor conditions change due to gas puffing.

In this paper, we examine a denser and broader scrape-off layer (SOL) plasma, one class of which was first seen in VH mode, in the DIII-D tokamak. An enhanced SOL appears in many types of double-null (DN) discharges and is not a property of VH mode only. The L and H mode DN enhanced SOL density and temperature profiles exhibit a 5–6 cm broad profile outside the separatrix. The VH mode enhanced SOL is associated with a higher separatrix density and temperature and is only 1.5–2 cm wide. Both types of SOL enhancement are only observed in high triangularity discharges. The origin of the enhanced SOL is, however, not yet understood.

In this paper, scrape-off layer measurements in DIII-D are presented as a function of the main discharge plasma parameters. A systematic study is under way to understand and predict the behavior of the edge and divertor plasma in DIII-D and this scaling behavior will be crucial for the design of ITER. To facilitate the studies, a fast reciprocating Langmuir probe incorporating five graphite tips was installed at the midplane of DIII-D which has the capability of performing multiple plunges 1 cm inside the separatrix during 5 MW of NBI. Density and temperature profiles in the midplane (reciprocating probe), near the top (Thomson scattering) and at the lower divertor plate (fixed Langmuir probe array) are compared by mapping the measurements into magnetic flux coordinates. Local pressure measurements are compared on different parts of a flux surface. The three different local measurements also indicate the spatial evolution of plasma conditions as plasma approaches the divertor plate. Ohmic and L-mode discharges exhibit similar (exponential) density and temperature decay in the scrape-off layer. H-mode discharges, however, display a faster spatial decay reflecting at least a factor of 3 decrease in the perpendicular diffusion coefficient. Consistency of the magnitude and scaling behavior of the edge profile parameters with models of the scrape-off layer is examined.

An array of fixed Langmuir probes spanning the inner and outer divertor strike points has been used to measure the electron temperature, plasma density, and particle/heat flux profiles of the divertor plasma in DIII-D. We report here on observations made during the steady state phase of high-power neutral-beam heated H-mode plasmas. These data are important for predicting the expected impurity sources and target plate material lifetimes in future high power tokamaks such as ITER. H-mode plasmas formed with a high divertor neutral pressure in the absence of external gas feed in DIII-D were found to have beneficial consequences in reducing the peak values of divertor electron temperature without loss of core energy confinement. We have observed a steady state, high density ( n pl,d ≈10 14 cm −3 ), moderate temperature ( 2 . A better understanding of the importance of collisional effects may be needed to interpret these results.

In this paper we report results of scaling studies aimed at determining how the divertor conditions vary with plasma current, toroidal field, and neutral beam heating power in H-mode discharges with ELMs in the DIII-D tokamak. We find that ELMs produce relatively more direct particle losses (50% or more of the total) than energy losses (≤20%). The time-average peak divertor heat flux in these plasmas is found to scale as d α ( P NBI I p )( B p,mp / B p,div ). The linear power dependence suggests that the plasma sheath at the targets is primarily responsible for limiting the parallel energy flow, while the I p variation may mean that the radial energy transport in the SOL decreases with increasing plasma current, just as it does in the core.

We describe a new system for the production of high flux neutral beams of low energy (5-10 eV) atoms, for use in beam-surface interaction and surface modification studies. This source can recreate the flux of superthermal atomic oxygen and other neutral species to which spacecraft surfaces are exposed in low Earth orbit. The system is based upon a magnetically confined (3-4 kG) co-axial plasma source driven by 1 kW RF at 2.45 GHz. The beam is. produced by the acceleration of plasma ions onto a negatively biased plate of high-Z metal (Mo or Pt); the ions are neutralized and reflected by the surface, retaining a large fraction of their incident kinetic energy, forming a beam of superthermal atoms. The source can provide neutral flux > 5 × 10 16 /cm2 s and fluence > 1020 cm2 in a five hour exposure. Such a high fluence source can provide accelerated simulation testing of materials and coatings for space applications. To date, samples of carbon film, carbon-based paint (Z-302), Kapton, mylar, teflon,...

A Two-Reservoir Model for recycling of hydrogenic atoms at graphite surfaces in JET is presented. Analysis of density transients induced by Neutral Beam Injection (NBI) and other modes of operation are used to determine the parameters of the model. In addition to describing many observed features of transient density and recycling behaviour, the model also reproduces an asymptotic external fuelling efficiency, ϵ s ≈ 1 s, for H/D flux incident on graphite, is determined. Stronger, saturable pumping observed after helium-discharge conditioning is also described. The importance of these pumping mechanisms for control of density and its profile, and for optimization of Q dt in JET, is discussed.

The effects of a series of organic additions and of two inorganic additions on the structure of Watts nickel as revealed by electron microscopy are discussed in terms of their previously reported influence on surface reflectivity. A comparison with bright nickel—cobalt deposits has been made. The morphology of growth is discussed and tentative suggestions advanced in an attempt to explain and account for the different structures that have been observed. It is concluded that the most effective brighteners are those which not only bring about considerable reduction in grain size but in addition, and possibly even more important, can exercise a smoothing influence by favouring the development of a uniform polycrystalline surface as distinct from one showing pyramidal, truncated pyramidal, or block types of growth.

Earlier work on the structure of deposits from the Watts nickel solution has been extended to an examination of the structure of deposits from the nickel sulphamate bath, which can be operated at very high cathode current density and overpotential. Even at overpotentials > 1 V, very short deposition times produced structures very similar to those found in Watts nickel deposits formed at much lower overpotentials.

For a conducting body experiencing superimposed changing and steady magnetic field, for example a limiter in a tokamak during plasma quench, the induced eddy currents and the deflections resulting from those eddy currents are coupled. Experimental study of these coupled deflections and currents can be performed with the FELIX (Fusion Electromagnetic Induction Experiment) facility nearing completion at ANL. Predictions of the coupling are described, as computed with the code EDDYNET, which has been modified for this purpose. Effects of the coupling will be readily observable experimentally. In the FELIX plate experiment, the coupling between deflection and eddy currents was readily calculated because the rigid-body rotation of the plate is equivalent to a contrarotation of the applied magnetic fields. For a geometry such as a plasma limiter, in which the eddy currents would cause a deformation of the conducting body, an analysis of the coupling between eddy currents and deformation would require a structural-analysis code and an eddy current code to be simultaneously computing from the same mesh.

SUMMARY Progress in the development of alloy plating is reviewed with some reference to theoretical factors and to new research approaches. The advantages of polarography in studying alloy deposition are discussed with particular reference to the co-deposition of tin and nickel. Alternating current polarography is proving helpful in studying the mechanism whereby organic additions to nickel plating electrolytes can induce brightness. Some preliminary results obtained with this technique for specific additions have enabled a tentative explanation of the mode of reduction of these additions during actual electrodeposition to be advanced. The possible role of sulphur in organic brighteners is briefly discussed with reference to some comparative results for sulphur-containing and for sulphur-free additions. The principles of electrochemical machining, involving the removal of metal by high-speed anodic dissolution, are briefly discussed and the advantages of the process as compared with conventional methods of machining outlined.

This paper presents a comparison of the total radiated power profile and impurity line emission distributions in the SOL and divertor of DIII-D. This is done for ELMing H-mode plasmas with heavy deuterium injection (partially detached divertor operation, PDD) and those without deuterium puffing. Results are described from a series of dedicated experiments performed on DIII-D to systematically measure the 2D ( R, Z ) structure of the divertor plasma. The discharges were designed to optimize measurements with new divertor diagnostics including a divertor Thomson scattering system. Discharge sequences were designed to produce optimized data sets against which SOL and divertor theories and simulation codes could be benchmarked. During PDD operation the regions of significant radiated power shift from the inner divertor leg and SOL to the outer leg and X-point regions. D α emission shifts from the inner strikepoint to the outer strikepoint. Carbon emissions (visible CII and CIII) shift from the inner SOL near the X-point to a distributed region from the X-point to partially down the outer leg during moderate D 2 puffing. In heavy puffing discharges the carbon emission coalesces on the outer separatrix near the X-point and for very heavy puffing it appears inside the last closed flux surface above the X-point. Calibrated spectroscopic measurements indicate that hydrogenic and carbon radiation can account for all of the radiated power. L α and CIV radiation are comparable and when combined account for as much as 90% of the total radiated power along chords viewing the significant radiating regions of the outer leg.

A major portion of the DIII-D program includes studies of the L-H transition, of the VH-mode, of particle transport and control and of the power-handling capability of a divertor. Significant progress has been made in all of these areas and the purpose of this paper is to summarize the major results obtained during the last two years. An increased understanding of the origin of improved confinement in H-mode and in VH-mode discharges has been obtained, good impurity control has been achieved in several operating scenarios, studies of helium transport provide encouraging results from the point of view of reactor design, an actively pumped divertor chamber has controlled the density in H-mode discharges and a radiative divertor is a promising technique for controlling the heat flux from the main plasma.

The uses of electrolysis, for the surface cleaning and descaling of metals, for improving the resistance of aluminium and its alloys to oxidation and abrasion, and for the polishing of metals, are discussed.Compared with direct-immersion processes, electrolytic cleaning and descaling is often speedier, more efficient and easier to control. In cathodic cleaning, adventitious matter is loosened by the nascent hydrogen evolved on the surface of the work, while the gas itself is a powerful reducing agent. Anodic pickling in acid electrolytes is precise in action and avoids the dangers of hydrogen embrittlement and of over-pickling. Alternate anodic and cathodic procedures offer certain advantages that are leading to their wider adoption.Theory and practice in the anodic oxidation of aluminium are discussed; reference is made to the use of this type of process to facilitate photographic reproduction on the surface of the metal, and to modifications of anodizing technique designed to produce especially hard, wear-resisting films.The principles of electrolytic polishing are explained. Industrial practice in the electrolytic polishing of steel, nickel, copper and aluminium is outlined. A comparison is drawn between electrolytic and mechanical polishing, and some reference is made to the economics of the electrolytic process.