B. Kurzan

Doppler Reflectometry for the Investigation of Propagating Density Perturbations

Doppler reflectometry is characterized by a finite tilt angle of the probing microwave beam with respect to the normal onto the cutoff surface. According to the Bragg condition the diagnostic selects density perturbations with wave number K⊥ in the reflecting layer. From the Doppler shift of the returning microwave the propagation velocity of these perturbations v⊥ can be obtained directly. The signal intensity contains information about the perturbation amplitude. The diagnostic potential of Doppler reflectometry is demonstrated both numerically by the use of two-dimensional full-wave codes and experimentally by an antenna system with variable tilt angle installed at the W7-AS stellarator. During stationary plasma conditions the measured profile of the propagation velocity v⊥(r) is dominated by the E×B velocity of the plasma, which is obtained from passive spectroscopy. Transient states of the plasma can be followed with a temporal resolution of less than 50 µs. Thus, Doppler reflectometry allows us to investigate the interdependence of sheared flow and turbulence on that timescale.

ELM frequency control by continuous small pellet injection in ASDEX Upgrade

Injection of cryogenic deuterium pellets has been successfully applied in ASDEX Upgrade for external edge localized mode (ELM) frequency control in type-I ELMy H-mode discharge scenarios. A pellet velocity of 560 m s−1 and a size of about 6 × 1019 D-atoms was selected for technical reasons, although even lower masses were found sufficient to trigger ELMs. A moderate repetition rate close to 20 Hz was chosen to avoid over-fuelling of the core plasma. Pellet sequences of up to 4 s duration were injected into discharges close to the L–H threshold, intrinsically developing large compound ELMs at a rate of 3 Hz. With pellet injection, these large ELMs were completely replaced by smaller type-I ELMs at the much higher pellet frequency, accompanied by a slight increase of density and even of stored energy. This external ELM control could be repeatedly switched on and off by just interrupting the pellet train. ELMs were triggered in less than 200 µs after pellet arrival at the plasma edge, at which time only a fraction of the pellet has been ablated, forming a rather localized, three-dimensional plasmoid, which drives the edge unstable well before the deposited mass is spread toroidally. The pellet controlled case has also been compared with a discharge at a somewhat lower density, but with otherwise rather similar data, developing spontaneous 20 Hz type-I ELMs. Despite the different trigger mechanisms, the general ELM features turn out to be qualitatively similar, possibly because of the similarity of the two cases in terms of ELM relevant parameters. The scaling with background plasma, heating power, pellet launch parameters, etc over a larger range still remains to be investigated.

H-mode threshold and confinement in helium and deuterium in ASDEX Upgrade

In 2008, experiments have been carried out in ASDEX Upgrade to compare H-mode power threshold and confinement time in helium and deuterium. A scan in magnetic field and a wide density variation indicate that the threshold power in the two gases is very similar. The density dependence of the threshold exhibits a clear minimum. Confinement in helium is about 30% lower than in deuterium, mainly due to the reduction in the ion density caused by Z = 2 in helium.

Disruption studies in ASDEX Upgrade in view of ITER

Experiments on ASDEX Upgrade and other tokamaks have shown that the magnitude of mechanical forces and thermal loads during disruptions can be significantly reduced by raising the plasma density with massive injection of noble gases. This method should be applicable to ITER too. Nevertheless, the suppression of the runaway electron (RE) avalanche requires a much larger (two order of magnitude) density rise. This paper reports on recent experiments aimed at increasing the plasma density towards the critical value, needed for the collisional suppression of REs. An effective electron density equal to 24% of the critical density has been reached after injection of 3.3?bar?l of neon. However, the resultant large plasma density is very poloidally and toroidally asymmetric; this implies that several valves distributed around the plasma periphery become necessary at this level of massive gas injection to ensure a homogeneous density distribution.

Ultrafast broadband frequency modulation of a continuous wave reflectometry system to measure density profiles on ASDEX Upgrade

A reflectometry system has been developed for ASDEX Upgrade to measure the plasma profile from the scrape‐off layer until the bulk plasma, simultaneously at the high and low field sides. Unique features of the system are the ultrafast broadband frequency modulation of a continuous wave using solid state stable hyper abrupt tuned oscillators (down to 10 μs), high and low field side channels and fully remote control operation, via optical fiber links. Due to the special design of the transmission line, with decoupled in going and out going lines and one‐antenna configuration, the system is optimized for reception and spurious reflections are eliminated. The ultrafast operation guarantees that the effect of plasma turbulence is greatly reduced. Both features determine the high performance of the diagnostic. A dedicated data acquisition system handles the large amounts of data generated by the broadband operation. Recent developments include the operation of new channels and an automatic and accurate frequency calibration circuit. Also, advanced digital signal processing techniques were applied to obtain density profiles with high spatial and temporal (20 μs) resolutions under turbulent plasma regions, e.g., the scrape‐off layer. Experimental results are presented showing the great sensitivity of the diagnostic to plasma radial movements and its tolerance to vertical movements of the plasma. Density profiles measured in ELMy regimes illustrate the capabilities of the diagnostic to detect fast profile changes.

The spatial structure of type-I ELMs at the mid-plane in ASDEX Upgrade and a comparison with data from MAST

The radial extent and spatial structure of type-I edge localized modes (ELMs) in ASDEX Upgrade are investigated using data from a mid-plane manipulator equipped with Langmuir probes and a fast visible imaging camera and are compared to data from MAST. Plasmas with a range of toroidal magnetic fields have been studied. The radial extent of the ELM efflux is found to be largest at the smaller toroidal magnetic field. A study of a series of shots on ASDEX Upgrade with different plasma edge to wall separation suggests that the closeness of the wall does not have a stabilizing effect on the radial extent of the ELM. The data from the mid-plane manipulator and from visible imaging are consistent with non-linear ballooning mode theory, which predicts that the ELM has a filament like structure. On both devices these structures have a poloidal extent of 5–10 cm and a typical toroidal mode number of ~15 and are found to accelerate away from the plasma edge. The acceleration is ~3 times larger on MAST than on ASDEX Upgrade.

Transport into and across the scrape-off layer in the ASDEX Upgrade divertor tokamak

The elements of transport into and across the scrape-off layer in the poloidal divertor tokamak ASDEX Upgrade are analysed for different operational regimes with emphasis on enhanced confinement regimes with an edge barrier. Utilizing the existing set of edge diagnostics, especially the high-resolution multi-pulse edge Thomson scattering system, in combination with long discharge plateaus, radial sweeps and advanced averaging techniques, detailed radial mid-plane profiles of diverted plasmas are obtained. Profiles are smooth across the separatrix, indicating strong radial correlation, and there is no remarkable variation across the second separatrix either. Together with measured input, recycling, pumping and bypass fluxes, a corrected separatrix position is determined and transport characteristics are derived in the different radial zones generally identified in the profile structure. Transport in the steep gradient region inside and across the separatrix shows typical ballooning-type critical electron pressure gradient scaling and, in parallel, even a clear correlation between radial electron density and temperature decay lengths (e.g. η e = d(ln T)/d(ln n) ∼ 2 for type-I ELMy H-modes). These findings indicate the importance of stiff profiles in this region, while diffusion coefficients are secondary parameters, determined essentially by the source distribution. The outer scrape-off layer wing exhibits a more filamentary structure with preferential outward drift especially in high-performance discharges, with formal diffusion coefficients far above the Bohm value in agreement with results on the old ASDEX experiment. A basic mechanism involved there seems to be partial loss of equilibrium and fast curvature-driven outward acceleration, in principle well known from theory, investigated decades ago in pinch experiments and utilized recently in high-field-side pellet fuelling.

Survey of the H-mode power threshold and transition physics studies in ASDEX Upgrade

An overview of the H-mode threshold power in ASDEX Upgrade which addresses the impact of the tungsten versus graphite wall, the dependences upon plasma current and density, as well as the influence of the plasma ion mass is given. Results on the H–L back transition are also presented. Dedicated L–H transition studies with electron heating at low density, which enable a complete separation of the electron and ion channels, reveal that the ion heat flux is a key parameter in the L–H transition physics mechanism through the main ion pressure gradient which is itself the main contribution to the radial electric field and the induced flow shearing at the edge. The electron channel does not play any role. The 3D magnetic field perturbations used to mitigate the edge-localized modes are found to also influence the L–H transition and to increase the power threshold. This effect is caused by a flattening of the edge pressure gradient in the presence of the 3D fields such that the L–H transitions with and without perturbations occur at the same value of the radial electric field well, but at different heating powers.

L- to H-mode transitions at low density in ASDEX Upgrade

The results from ASDEX Upgrade discharges dedicated specifically to the investigation of low-density L-to-H transitions are presented. The plasmas were heated by electron cyclotron resonance heating to achieve a separation of electron and ion heat channels. Under such conditions, the ratio of electron to ion temperature at the plasma edge increases with decreasing density at the L–H transition and can be as high as 3.5. Our results strongly support the essential role of the ion channel in the L–H transition, via the diamagnetic Er provided by the ion pressure gradient.

Fast-ion redistribution and loss due to edge perturbations in the ASDEX Upgrade, DIII-D and KSTAR tokamaks

The impact of edge localized modes (ELMs) and externally applied resonant and non-resonant magnetic perturbations (MPs) on fast-ion confinement/transport have been investigated in the ASDEX Upgrade (AUG), DIII-D and KSTAR tokamaks. Two phases with respect to the ELM cycle can be clearly distinguished in ELM-induced fast-ion losses. Inter-ELM losses are characterized by a coherent modulation of the plasma density around the separatrix while intra-ELM losses appear as well-defined bursts. In high collisionality plasmas with mitigated ELMs, externally applied MPs have little effect on kinetic profiles, including fast-ions, while a strong impact on kinetic profiles is observed in low-collisionality, low q95 plasmas with resonant and non-resonant MPs. In low-collisionality H-mode plasmas, the large fast-ion filaments observed during ELMs are replaced by a loss of fast-ions with a broad-band frequency and an amplitude of up to an order of magnitude higher than the neutral beam injection prompt loss signal without MPs. A clear synergy in the overall fast-ion transport is observed between MPs and neoclassical tearing modes. Measured fast-ion losses are typically on banana orbits that explore the entire pedestal/scrape-off layer. The fast-ion response to externally applied MPs presented here may be of general interest for the community to better understand the MP field penetration and overall plasma response.