Scrape-off Layer Flow Research Articles

Transport-driven scrape-off layer flows and the x-point dependence of the L-H power threshold in Alcator C-Mod

Factor of ∼2 higher power thresholds for low- to high-confinement mode transitions (L-H) with unfavorable x-point topologies in Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] are linked to flow boundary conditions imposed by the scrape-off layer (SOL). Ballooning-like transport drives flow along magnetic field lines from low- to high-field regions with toroidal direction dependent on upper/lower x-point balance; the toroidal rotation of the confined plasma responds, exhibiting a strong counter-current rotation when B×∇B points away from the x point. Increased auxiliary heating power (rf, no momentum input) leads to an L-H transition at approximately twice the edge electron pressure gradient when B×∇B points away. As gradients rise prior to the transition, toroidal rotation ramps toward the co-current direction; the H mode is seen when the counter-current rotation imposed by the SOL flow becomes compensated. Remarkably, L-H thresholds in lower-limited discharges are identical to lower x-point discharges; SOL flows are also found similar, suggesting a connection.

Edge and divertor physics with reversed toroidal field in JET

Asymmetries are a ubiquitous feature of the scrape-off layer (SOL) and divertor plasmas in any tokamak and are thought to be driven primarily by a variety of drift flows, the directions of which reverse with reversal of the main toroidal field. The understanding of precisely how these field dependent drifts combine to yield any given experimental observation is still very much incomplete. A recent campaign of reversed field operation at JET designed to match a variety of discharges to their more frequently executed forward field counterparts has been executed in an attempt to contribute to this understanding. This paper summarises the most important findings from these experiments and includes some new EDGE2D simulation results describing the SOL flow.

13C transport studies in L-mode divertor plasmas on DIII-D

13CH4 was injected with a toroidally-symmetric gas system into 22 identical lower-single-null L-mode discharges on DIII-D. The injection level was adjusted so that it did not significantly perturb the core or divertor plasmas, with a duration of ∼3s on each shot, for a total of ∼300TL of injected particles. The plasma shape remained very constant; the divertor strike points were controlled to ∼1cm at the divertor plate. At the beginning of the subsequent machine vent, 29 carbon tiles were removed for nuclear reaction analysis of 13C content to determine regions of carbon deposition. It was found that only the tiles inboard of the inner strike point had appreciable 13C above background. Visible spectroscopy measurements of the carbon injection and comparisons with modeling are consistent with carbon transport by means of scrape-off layer flow.

Particle control and SOL plasma flow in the W-shaped divertor of JT-60U tokamak

Effect of divertor pumping on the scrape-off layer (SOL) plasma and impurities has been studied in many divertor tokamaks. Recent experimental studies of particle exhaust and SOL flow in JT-60U are reviewed. Pumping flux at the inner and outer pumping slots of the W-shaped divertor was investigated under the attached and detached divertor conditions. The SOL flow pattern was determined, for the first time, at three locations, i.e. at the low-field-side SOL (the outer midplane and divertor X-point) and at the high-field-side SOL (above the inner baffle), using three reciprocating Mach probes. The measured profiles were compared with modelling results from two-dimensional plasma simulations, UEDGE-code, with the ion drift effects included.

Impurity enrichment and radiative enhancement using induced SOL flow in DIII-D

Experiments on DIII-D have demonstrated the efficacy of using induced scrape-off-layer (SOL) flow to preferentially enrich impurities in the divertor plasma. This SOL flow is produced through simultaneous deuterium gas injection at the midplane and divertor exhaust. Using this SOL flow, an improvement in enrichment (defined as the ratio of impurity fraction in the divertor to that in the plasma core) has been observed for all impurities in trace-level experiments (i.e., impurity level is non-perturbative), with the degree of improvement increasing with impurity atomic number. In the case of argon, exhaust gas enrichment using modest SOL flow is as high as 17. Using this induced SOL flow technique and argon injection, radiative plasmas have been produced that combine high radiation losses ( P rad/ P input > 70%), low core fuel dilution ( Z eff < 1.9), and good core confinement ( τ E > 1.0 τ E,ITER93H).

Noble gas exhaust with a strongly baffled divertor in ASDEX-Upgrade

Since spring of 1997 ASDEX Upgrade has operated with the new divertor II (LYRA), which is characterised by vertical target plates, tight baffling towards the main chamber, and a dome baffle in the private flux region. The main goal of this divertor modification was to increase the divertor density (of the plasma and of the neutrals) for a given plasma density, whereas the neutral density in the main chamber should be similar or even lower. Pumping occurs now from the private flux region which is linked to the pump chamber by finite conductance below the outer target. A cryo pump (about 100 m 3/s) has been installed in the outer divertor chamber in addition to the turbo pumps. This divertor scheme is very similar to the one planned for ITER. First experiments with the new divertor indicate that the compression of helium, C He (the ratio of neutral He density in the divertor to the He density at the plasma edge) is much higher in the new divertor, while the neon exhaust efficiency has decreased. The cryo pump can strongly enhance the scrape-off layer flow of deuterium (by a factor of about 3), resulting in a small improvement of the He exhaust rate.

Impurity enrichment studies with induced scrape-off layer flow on DIII-D

A series of controlled experiments has been carried out in DIII-D to induce a bulk ion flow in the SOLand evaluate its effect on the localization of impurities in the divertor. This induced SOL flow was created bysimultaneous deuterium puffing and divertor exhaust using a divertor cryopump, and the impurity enrichment was measured directly. The experiments were designed to compare enrichment in discharges with and without induced flow having otherwisesimiliar divertor parameters. Significant increases in impurity compression and enrichment are observed when flow is induced, and the degree of impurity enrichment in the divertor is found to be dependent on the impurity of interest. Detailed particle measurements made possible by the direct measurement of impurity densities in several reservoirs indicate reasonable particle balance for helium throughout the duration of the discharge. Conversely, while the total input of neon is balanced by the total exhaust by the end of a discharge, particle balance is not observed during the course of the discharge. A significant wall inventory with a short release time (∼10 ms) is surmised.

Fueling of ITER-Scale Fusion Plasmas

Fueling system functions for the International Thermonuclear Engineering Reactor (ITER) and similar scale devices are to provide hydrogenic fuel to maintain the plasma density profile for a specified fusion power, to replace the deuterium-tritium (D-T) ions consumed in the fusion reaction, to establish a density gradient for plasma particle (especially helium ash) flow to the edge, and also to supply hydrogenic edge fueling for increased scrape-off layer flow for optimum divertor operation. An additional function is to inject impurity gases at lower flow rates for divertor plasma radiative cooling, for wall conditioning, and for plasma discharge termination on demand. The burn fraction of ITER is about 1%, which is more than an order of magnitude lower than values typically assumed in fusion reactor studies. This low burn fraction results in large vacuum pumping and fuel processing systems to handle the larger D-T throughput. Gas and pellet fueling efficiency data from past tokamak experiments are reviewed; pellet fueling efficiency is significantly larger than that of gas injection. An overview of the current research and development status of gas and pellet fueling technology is presented.