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[en] Recent experiments have noted the coexistence of multiple shearing fields in edge turbulence, and have observed that the shearing population ratios evolve as the L-H transition is approached. A novel model including zonal flows (ZFs), geodesic acoustic modes (GAMs) and turbulence as a zero-dimensional self-consistent two predator-one prey system with multiple frequency shearings is proposed. ZF with finite frequency (i.e. GAM) can have different shearing dynamics from that with zero frequency, because of the finite shearing field autocorrelation times. Decomposing the broadband ZF spectrum into the two populations enables us to assign different shearing weights to the components of the shearing field. We define states with no ZF and GAM as an L-mode-like state, that with ZF and without GAM as an ZF-only state, with GAM and without ZF as a GAM-only state and both with ZF and GAM as the coexistence state. To resolve the origins of multiple shear coexistence, mode-competition effects are introduced. These originate from higher order perturbation of wave populations. The model exhibits a sequence of transitions between various states as the net driving flux increases. For some parameters, bistability of ZF and GAM is evident, which predicts hysteretic behaviour in the turbulence intensity field during power ramp up/down studies. The presence of noise due to ambient turbulence offers a mechanism to explain the bursts and pulsations observed in the turbulence field prior to the L-H transition.
[en] After a general discussion of the experimental characteristics of the L-H transition and consideration of basic theoretical principles underlying models for it, this paper reviews the various theories of the L-H transition available in the literature, providing some background information on each theory and expressing the transition criteria in forms suitable for comparison with experiment. Some conclusions on the relevance of these models for explaining the experimental data on the transition are drawn. (author)
[en] This study presents the development of a 3D equilibrium reconstruction tool and the results of the first-ever reconstruction of an island equilibrium. The SIESTA non-nested equilibrium solver has been coupled to the V3FIT 3D equilibrium reconstruction code. Computed from a coupled VMEC and SIESTA model, synthetic signals are matched to measured signals by finding an optimal set of equilibrium parameters. By using the normalized pressure in place of normalized flux, non-equilibrium quantities needed by diagnostic signals can be efficiently mapped to the equilibrium. The effectiveness of this tool is demonstrated by reconstructing an island equilibrium of a DIII-D inner wall limited L-mode case with an n = 1 error field applied. Finally, flat spots in Thomson and ECE temperature diagnostics show the reconstructed islands have the correct size and phase.
[en] In this contribution we present results from the first N2 seeding experiments in JET performed after installation of the ITER-like Wall. Gas balance measurements for seeded L-mode discharges indicate very strong N2 retention as well as a potential increase in D2 retention. The possible influence of ammonia production on this apparent retention is discussed. Plasma parameters and impurity content were monitored throughout the seeded discharges as well as during subsequent clean-up discharges. These experiments give first insight into phenomena related to the use of nitrogen as seeding gas in JET with the ITER-like Wall, such as ammonia production and nitrogen legacy
[en] A new method for plasma boundary reconstruction, based on the toroidal multipolar expansion (TME) scheme, is applied successfully in EAST. TME applies a limited number of toroidal multipolar moments based on toroidal coordinates to treat a two-dimensional problem of axisymmetric plasma equilibrium. The plasma boundary reconstructed by TME is consistent with the results by using EFIT. The method is sufficiently reliable and fast for real time shape control. (magnetically confined plasma)
[en] The EDGE2D-EIRENE code is applied for simulation of divertor detachment during density ramp experiments in high triangularity, L-mode plasmas in both the carbon and Be/W environments in JET. Emphasis is placed on matching experimental data (upstream and in the divertor) as far as possible. The code runs without drifts and includes either C or Be as impurity, but not W, assuming that the divertor plasma is always cold enough for the W target source to be negligible and that the W targets have to some extent been coated with Be via main chamber migration. The simulations reproduce the observed particle flux detachment as density is raised in both C and Be/W, but not the experimental in/out asymmetry. The main difference between detachment in carbon and Be/W environments is a higher upstream density required in the Be/W case to obtain similar divertor conditions to those when carbon dominates
[en] Localized D2 puffing from various divertor locations has been carried out under double null (DN) and lower single null (LSN) divertor configurations to investigate the effect of gas puff locations on the divertor behavior in ohmic L-mode discharges in EAST. Localized gas puffing from the dome has a higher fueling efficiency than that from the inner and outer targets for both DN and LSN configurations. Under the DN configuration, gas puffing from the inner target exhibits a much better fueling efficiency than that from the outer target. In contrast, the gas fueling efficiency shows little difference between the inner and outer divertor gas puff locations in the LSN configuration. In LSN, localized gas puffing from the outer divertor target tends to promote detachment at the outer target. This will be employed as a means to control heat fluxes to the outer divertor target plates for high power long pulse operations.
[en] Broadband density fluctuations with peak frequency ranging from 150 to 400 kHz were measured using a multichannel microwave imaging reflectometer in core region of the low-density L-mode plasmas heated by neutral beam injection on KSTAR. These fluctuations have been studied by comparing the dominant mode scales estimated from the measurement with those predicted from linear gyrokinetic simulation. The measured poloidal wavenumbers are qualitatively comparable to those of the ‘fastest growing modes’ from simulations, whereas they are larger than those of the ‘transport-dominant modes’ by about a factor of three. The agreement on wavenumbers between the measurement and linear simulation (for the fastest growing modes) is probably due to sufficiently weak flow shear compared to the maximum linear growth rate. Meanwhile, the transport-dominant modes seem to be related to the fluctuations in lower frequencies (∼80–150 kHz) observed in some of the measurement. (paper)
[en] In this paper, we analyze heat transport in the JET tokamak using data from its high resolution ECE diagnostic and analyses based on the transfer entropy (TE). The analysis reveals that heat transport is not smooth and continuous, but is characterized by ‘trapping regions’ separated by ‘minor transport barriers’. Heat may ‘jump over’ these barriers and when the heating power is raised, this ‘jumping’ behavior becomes more prominent. To check that our results are relevant for global heat transport, we deduced an effective diffusion coefficient from the TE results. Both its value and overall radial variation are consistent with heat diffusivities reported in literature. The detailed radial structure of the effective diffusion coefficient was shown to be linked to the mentioned minor transport barriers. (paper)
[en] A campaign of pure helium discharges in the JET tokamak equipped with the MarkIIGB divertor has been performed. This paper describes some of the experimental observations of divertor detachment obtained in L-mode density ramp He discharges and presents a selection of results from the application of B2.5-Eirene code modelling to a JET He plasma. Detachment is very different from that observed in equivalent D discharges. Particle fluxes remain attached up to higher densities and, with decreasing input power, particle detachment occurs earlier in density. However, in contrast to comparable D plasmas, the low measured target Te at all but the lowest ne in helium leads to an earlier decrease in power flux. Simulations show that detachment is caused by the escape of He neutrals from the target vicinity where, together with the He+ ions they radiate along the separatrix and above the X-point area, starving regions downstream of power