Giant ELM Research Articles

A new pathogenic polypore on urban trees: The first record of Rigidoporus ulmarius (Rigidoporaceae, Hymenochaetales) in Hungary

AbstractThe Giant Elm Bracket (Rigidoporus ulmarius) is a widely-distributed necrotrophic polypore species that causes white heart rot in deciduous trees. Despite its recognition as one of the largest species known for forming basidiomata, this perennial polypore had not been documented in Hungary. However, in recent years, two specimens macroscopically resembling this species were collected on old horse chestnut (Aesculus hippocastanum) trees from two different places in Hungary by amateur mycologists. In this study, subsequent morphological and molecular-genetic analyses of these fungal samples confirmed their identity as R. ulmarius. This study represents the first documented occurrence of this plant pathogenic polypore species in Hungary.

Grassy ELM regime at low pedestal collisionality in high-power tokamak plasma

Spontaneous mitigation of giant ELMs and appearance of grassy ELMs have been observed repeatedly at low pedestal collisionality () in the high-power (P inj > 13 MW) hybrid scenario in DIII-D. Higher β p and higher q 95 appear to be beneficial to achieving the grassy ELM regime. The grassy ELM H-mode plasma shows high energy confinement performance (H 98y2 up to over 1.5) under the conditions of high neutral beam torque and high core rotation. The pedestal width appears to exceed the EPED1.0 model prediction by more than 50%. Pedestal stability analysis performed with the ELITE code indicates that the stability against low-n kink/peeling modes is improved with increased auxiliary heating power and the operational point in the grassy ELM regime is located near the ballooning boundary. The pedestal stability characteristics during the grassy ELM crashes have been investigated in comparison with the giant ELM crashes based on plasma profiles experimentally measured with high resolution and accuracy. It has been found that the underlying mechanism for the observed small-amplitude ELM crashes is mainly the expansion of the ballooning stability boundary induced by an initial radially localized collapse in the pedestal, which helps to stop the growth of instabilities and further collapse of the pedestal. The effect of electron density pedestal on mitigating edge localized instabilities has been analyzed by numerical simulation, suggesting that the electron density pedestal characterized by high n e,sep/n e,ped and low density gradient helps to stabilize peeling-ballooning modes because of a low pressure pedestal gradient and to lower the ballooning boundary mainly because of a low ion diamagnetic frequency in the pedestal region, thus triggering ballooning instabilities and producing the intrinsic grassy ELMs. Numerical simulation of the Chinese fusion engineering test reactor (CFETR) with the SOLPS code indicates that the separatrix density might be insensitive to the electron diffusivity in the pedestal region and increase with the power flowing from the core region to the edge region. Pedestal stability analysis suggests that the flat density pedestal with high separatrix density obtained in the high-power plasma in CFETR would make the operational point close to the ballooning boundary, which is considered to help destabilize ballooning instabilities and facilitate the access to the grassy ELM regime.

Surface modification and droplet formation of tungsten under hot plasma irradiation at the GOL-3

The paper presents experimental investigations of tungsten surface modification after plasma loads at the multimirror trap GOL-3. Energy loads on tungsten surface varied from 2 up to 4 MJ/m 2 per shot with sets from 1 to 9 repetitive irradiations that corresponds to loads higher than tungsten melting threshold and is close to ITER giant ELM loads. Targets surface modification and transverse microsections after irradiation was studied by optical microscopy , SEM and hardness tester. Formation on tungsten surface of three different crack networks with typical cell sizes of 1000, 10 and 0.3 μm and bubbles are identified. The network of large cracks extend perpendicularly to the irradiated sample surface to a depth of 50–350 μm. Erosion depth depends on energy loads – rises from 20 to 200 μm at 2 and 4 MJ/m 2 correspondingly. Cracking, development of tungsten surface morphology and droplets splashing are discussed.

Toroidal and poloidal momentum transport studies in JET

This paper reports on the recent studies of toroidal and poloidal momentum transport in JET. The ratio of the global energy confinement time to the momentum confinement is found to be close to τE/τϕ = 1 except for the low density or low collisionality discharges where the ratio is τE/τϕ = 2–3. On the other hand, local transport analysis of around 40 discharges shows that the ratio of the local effective momentum diffusivity to the ion heat diffusivity is χϕ/χi ≈ 0.1–0.4 (averaged over the radial region r/a = 0.4–0.7) rather than unity, as expected from the global confinement times and used often in ITER predictions. The apparent discrepancy in the global and local momentum versus ion heat transport can be at least partly explained by the fact that momentum confinement within edge pedestal is worse than that of the ion heat and thus, momentum pedestal is weaker than that of ion temperature. In addition, while the ion temperature profile shows clearly strong profile stiffness, the toroidal velocity profile does not exhibit stiffness, as exemplified here during a giant ELM crash. Predictive transport simulations with the self-consistent modelling of toroidal velocity using the Weiland model and GLF23 also confirm that the ratio χϕ/χi ≈ 0.4 reproduces the core toroidal velocity profiles well and similar accuracy with the ion temperature profiles. Concerning poloidal velocities on JET, the experimental measurements show that the carbon poloidal velocity can be an order of magnitude above the neo-classical estimate within the ITB. This significantly affects the calculated radial electric field and therefore, the E × B flow shear used for example in transport simulations. Both the Weiland model and GLF23 reproduce the onset, location and strength of the ITB well when the experimental poloidal velocity is used while they do not predict the formation of the ITB using the neo-classical poloidal velocity in time-dependent transport simulation. The most plausible explanation for the generation of the anomalous poloidal velocity is the turbulence driven flow through the Reynolds stress. Both CUTIE and TRB turbulence codes show the existence of an anomalous poloidal velocity, being significantly larger than the neo-classical values. And similarly to experiments, the poloidal velocity profiles peak in the vicinity of the ITB and seem to be dominantly caused by flow due to the Reynolds stress. However, it is important to note that both the codes treat the equilibrium in a simplified way and this affects the geodesic curvature effects and geodesic acoustic modes (GAMs). Therefore, the results should be considered as indicative, and most probably provide an upper bound of the mean poloidal velocity as results from other codes including GAM dynamics show that they often serve as a damping mechanism to flows.

Propagation velocity of changes to confinement

Transient and non-transient changes to confinement properties of a tokamak equilibrium arise from several different plasma phenomena: a heat pulse triggered by a sawtooth; a cold pulse triggered by impurity injection or a giant ELM; the L ↔ H transitions triggered at the edge can produce a fast response in temperature towards the centre; MHD instabilities like sawtooth collapse, ELM or ‘outer’ mode cause negative-positive changes to the local energy density. Such changes to the confinement properties propagate through the plasma on timescales which are much smaller than the confinement time itself. A model to describe this based on turbulent guiding centre drift motion is developed. The theory for the non-linear model focuses on the propagation of perturbations to the plasma temperature and density. The model predicts a fluctuation-turbulence spectrum which agrees with that observed in experimental fluctuation measurements, apart from the predicted long wavelength part, which is not measured experimentally. A turbulence theory for the guiding centre velocity correlation function is developed in five steps, three of which have often been used in work on turbulence. Step 1 is the random phase approximation. Step 2 is the Markovian approximation. Step 3 is the discrete to continuum approximation. Step 4 is the solution of a prototype non-linear equation for one single harmonic. Step 5 is the solution of a non-linear equation for the entire spectrum of fluctuations using statistical techniques. The solution can be studied in two regimes. The long term or steady state regime has commonly been used in many turbulence theories. On the other hand the short term or ballistic regime predicts that perturbations propagate by advective or wave-like transport relevant to transient changes. The propagation speed is the characteristic guiding centre fluid drift velocity scaled by the ratio between the energy in the fluctuations and the thermal energy. Its magnitude agrees with experimental observations. The validity of the approximations in each of the five steps is discussed, together with how appropriate these are to a tokamak equilibrium with shear.

Collapse of density pedestal by giant ELM on JT-60U

In JT-60U ELMy H-mode discharges, the detailed behaviour of giant (type I) ELMs wasmeasured using a heterodyne reflectometer system in order to understandthe collapse mechanism of the pedestal structure and consequences on the core/edgeplasma. The phase signal of the reflectometer exhibits the movement of the cutofflayer (density layer) due to the collapse of the pedestal in a density profile byan ELM. An ELM event can be classified into a precursor phase, collapse phase,recovery phase and a relaxation phase. A certain density layer measured near theshoulder of the pedestal moves about 7 cm inside the plasma in the collapse phase. Theprecursor oscillation in a pedestal density and the relationship between the collapseof the pedestal structure and a Dα burst are also studied.

Lateral deflection of the SOL plasma during a giant ELM

In recent H-mode experiments at JET with giant ELMs a lateral deflection of hot tokamak plasma striking the divertor plate has been observed. This deflection can effect the divertor erosion caused by the hot plasma irradiation. Based on the MHD model for the vapor shield plasma and the hot plasma, the Seebeck effect is analyzed for explanation of the deflection. At t=−∞ both plasmas are at rest and separated by a boundary parallel to the target. The interaction between plasmas develops gradually (`adiabatically') as exp( t/ t 0) with t 0∼10 2 μs the ELM duration time. At inclined impact of the magnetized hot plasma a toroidal current develops in the interaction zone of the plasmas. The J × B force accelerates the interacting plasmas in the lateral direction. The cold plasma motion essentially compensates the current. The magnitude of the hot plasma deflection is comparable to the observed one.

The Old Swimmin' Hole

The Old Swimmin' Hole G.C. Compton The bees are in the locusts. The mud dobber builds on the pump house wall. Every year about this time I get an overwhelming urge to strip off naked, sling my britches across a willow limb and dive into the old Pine Hole. They were under bridges, beneath cliffs, in coves and bends in the creek. Some were formidably deep and mysterious, measured by the number of breast strokes it took to cross or how high the water came "over a tall man's head." Others were shallow streams dammed with rocks, tree limbs and old automobile tires. With names like the Splash Dam, Dock Bill, Number Seven and the Swirl Hole, they were the public swimming holes of the 50s and 60s and part of growing up in Pike County, Kentucky. How well I remember the Pine Hole at Jonancy! How it got its name has remained a mystery, for not a single pine grew near this horseshoe bend on Shelby Creek. Nor did grass or flowers adorn its sandy banks. The most pernicious weed was quickly trampled by a hundred bare feet hurrying to the big diving rock in the shade of a giant elm. In the spring and summer they came, boys of all ages—and sometimes girls—from Elwood, Dock Bill, Longfork and Virgie. We all came: Cobb Holbrook, who proudly carried six buckshots in his rear, victim of an aborted watermelon theft and one-eyed Benton Addington's Winchester; the enviable Elmo Music, a waterhead who didn't have to go to school and owned a Schwinn bike with chrome fenders; Roman Calhoun, who favored Howdy Doody and once drank a whole case of Carnation cream; and Puny Blankenship, who never bathed but had a coveted six and three-quarters inch tongue and webbed feet. There was also Wendell Johnson, born with a caul over his face, who could predict earthquakes and the best days to play hooky and go fishing; and Fat Osborne, a pugnacious lad whose dream of becoming a great soldier would end in a rice paddy in Vietnam. We came, tripping barefoot down the railroad track, shirtless with sunburned backs, skipping down the highway, crossing cornfield and bottom. We dived, swam, frolicked and showed off. We argued, fought and forgave. We regaled one another with stories of feats and infidelities, less often real than imagined. But there was one thing we 34 shared. We had all been warned by our mothers never ever to go near the Pine Hole. Therefore, we frequented its forbidden waters with great regularity and relish. My own mother continuously wagged her finger against the common society of the Pine Hole. She preached about my chances of drowning, cutting my feet on broken glass or splitting my skull on unseen objects in the water. She warned me about the toilets whose foul effluence fed the creek and said I was bound to catch typhoid fever, diphtheria, lockjaw or all three. As far as I know, Mother had never in her life witnessed a case of lockjaw, yet she was certain I would get the dread disease if I got cut in the Pine Hole. Once after slicing my knee on a rusty pipe, I ate a whole chocolate cake against the day my jaws would be sealed forever. Well aware of my tolerance for temptation, Mother made me promise that if I ever did go around the Pine Hole I would never try diving off "that old rock." Of course, I obeyed. Trespassing, accidentally, on this forbidden ground I stripped off naked, climbed the big elm tree instead, twenty feet up, and bailed off an overhanging limb. Lonzo Johnson, in obedience to his mother, shed his clothes and did the same. Lonzo had a solitary rotten tooth before he dived off the limb—but not afterwards. Following a poorly executed swan dive, he left the cavernous canine sticking in Edgil Roberts's head. It was an incident that left Edgil's mother in something of a quandary that day. She had to decide, under duress ofthe moment, whether to take herboy to Pikeville to see a doctor or enlist the...

Discharge optimization and the control of edge stability

Discharge optimization for improving MHD stability of both core and edge was essential for the achievement of record fusion power discharges, in the ELM-free hot ion H mode regime, in the recent JET DT operation. The techniques used to increase edge stability are described. In particular the successful technique of current rampdown used to suppress the outer mode is reported. The increased stability of the outer mode by decreasing the edge current density confirms its identification as an n = 1 external kink. Decreasing the plasma current, however, decreases the ELM-free period, which is consistent with stability calculations that show an earlier onset of the ballooning limit. In order to increase external kink stability without causing a deterioration in the ELM-free period, a compromise was achieved by using plasma current rampdown, while working at the highest plasma current values possible. Results from a plasma current scan show that at the time of occurrence of the first giant ELM, the plasma stored energy, as well as the pressure measured at the top of the edge pedestal increase linearly with plasma current, for a given plasma configuration and power. This is consistent with models of the edge transport barrier, where the transport barrier width is proportional to the ion (or fast ion) poloidal Larmor radius. The MHD observations in DT and deuterium only discharges were found to be similar. Thus the experience gained on the control of MHD modes in deuterium plasmas could be fully exploited in the DT campaign.

β scaling of confinement

Empirical scaling laws (Goldston, Kaye-Goldston, ITER89P and ITER93H) for the confinement time τE in tokamaks depend on dimensionless global plasma parameters in a way which is difficult to associate with some general plasma physics model. It is shown that such dependences can arise from artefacts in the database used to derive the scaling expression. The artefacts involve inner relationships, for example, collinearities between variables. For the ITER L mode and H mode databases, it is shown that subsets of the scaling expressions are approximately constant. An alternative description of L mode, ELM-free, small ELM and giant ELM H mode data is given. The alternative description does not include the dependences upon plasma β, elongation and safety factor which are present in the above scaling laws; extrapolations to ITER parameters by this alternative data description are virtually the same as those by ITER89P and ITER93H. The alternative description is based on a simple electrostatic physics model (plateau scaling) in which confinement is degraded by non-collisional processes. It can describe global confinement data for all four confinement regimes; the common scaling features a multiplier C which is assigned a different value for each regime. The complex physics associated with MHD instabilities (ELMs, sawteeth, outer modes) is hidden in the multiplier C, and attempts are made to uncover such physics. In these attempts global confinement is made to scale with physics models rather than physics variables.

Studies of giant ELM interaction with the divertor target in JET

Studies of giant ELM interaction with the divertor target in JET

Studies of giant ELM interaction with the divertor target in JET

In this paper we present an overview of recent studies of giant ELM interaction with the divertor target in JET. With ID modelling, we reproduce the experimentally found time behaviour of the divertor plasma parameters and show the non-linear dependence of the particle flux and D α intensity peak on the ELM power loss. Measurements of the impurity influx from the target plates show non-sputtering contributions. During ELMs we find a strong current of the order of magnitude of the ion saturation current flowing from the plasma to the target plate near both strike points. The magnitude of the total current is high enough to account for the observed movement of the strike points. The main plasma moves during the ELM vertically and horizontally in a characteristic way. The movement is best described by an n = 0, m = 1 mode.

Model of a giant ELM

A model of the giant ELMs (type-I ELM) is developed. Theory of the self-sustained turbulence of the current-diffusive ballooning mode (CDBM) is developed, and multifold states for the L-mode, H-mode and the third state with magnetic braiding, M-mode, are obtained. The transition to M-mode occurs if the pressure gradient reaches a critical value. The nonlinear excitation of the magnetic perturbation takes place, the growth time of which is of the order of the poloidal-Alfvén transit time, followed by catastrophic enhancement of the transport coefficient. Avalanche of the transport catastrophe is also analysed, showing a very rapid radial propagation velocity. The M-state terminates if the pressure gradient becomes small, leading to the back-transition to the H-(L-)mode. Under the constant power supply, these processes can repeat themselves, causing periodic bursts. The period becomes shorter as the average power flux increases.

Study of giant edge-localized modes in DIII-D and comparison with ballooning theory.

In DIII-D H-mode plasmas, large amplitude modulation of steep edge electron pressure gradients are observed during the occurrences of giant edge-localized modes (ELM's). For a considerable range of plasma currents (1 to 2 MA), the experimental pressure gradient before a giant ELM is close to the theoretically predicted marginal stability limit for the first regime of the ideal ballooning mode. These results, together with observations showing that ELM's originate in the region of bad curvature, strongly suggest that ELM's are triggered by ballooning mode instabilities.