M. Cavedon

Experimental studies of high-confinement mode plasma response to non-axisymmetric magnetic perturbations in ASDEX Upgrade

The interaction of externally applied small non-axisymmetric magnetic perturba16 tions (MP) with tokamak high-confinement mode (H-mode) plasmas is reviewed and illus17 trated by recent experiments in ASDEX Upgrade. The plasma response to the vacuum MP 18 field is amplified by stable ideal kink modes with low toroidal mode number n driven by the 19 H-mode edge pressure gradient (and associated bootstrap current) which is experimentally 20 evidenced by an observable shift of the poloidal mode number m away from field alignment 21 (m = qn, with q being the safety factor) at the response maximum. A torque scan experiment 22 demonstrates the importance of the perpendicular electron flow for shielding of the resonant 23 magnetic perturbation, as expected from a two-fluid MHD picture. Two significant effects of 24 MP occur in H-mode plasmas at low pedestal collisionality, νped ≤ 0.4: (a) a reduction of the 25 global plasma density by up to 50% and (b) a reduction of the energy loss associated with edge 26 localised modes (ELMs) by a factor of up to 10. A comprehensive database of ELM mitigation 27 pulses at low ν∗ in ASDEX Upgrade shows that the degree of ELM mitigation correlates with 28 the reduction of pedestal pressure which in turn is limited and defined by the onset of ELMs, 29 i. e. a modification of the ELM stability limit by the magnetic perturbation. 30 PACS numbers: 52.55.Fa, 52.55.Tn, 52.65.Kj 31 2

The role of the density profile in the ASDEX-Upgrade pedestal structure

Experimental evidence for the impact of a region of high density localised in the high-field side scrape-off layer (the HFSHD) on plasma confinement is shown in various dedicated experiments on ASD ...

Plasma response measurements of external magnetic perturbations using electron cyclotron emission and comparisons to 3D ideal MHD equilibrium

The plasma response from an external n = 2 magnetic perturbation field in ASDEX Upgrade has been measured using mainly electron cyclotron emission (ECE) diagnostics and a rigid rotating field. To interpret ECE and ECE-imaging (ECE-I) measurements accurately, forward modeling of the radiation transport has been combined with ray tracing. The measured data is compared to synthetic ECE data generated from a 3D ideal magnetohydrodynamics (MHD) equilibrium calculated by VMEC. The measured amplitudes of the helical displacement around the outboard midplane are in reasonable agreement with the one from the synthetic VMEC diagnostics. Both exceed the predictions from the vacuum field calculations and indicate the presence of a kink response at the edge, which amplifies the perturbation. VMEC and MARS-F have been used to calculate the properties of this kink mode. The poloidal mode structure of the magnetic perturbation of this kink mode at the edge peaks at poloidal mode numbers larger than the resonant components vertical bar m vertical bar g vertical bar nq vertical bar, whereas the poloidal mode structure of its displacement is almost resonant vertical bar m vertical bar approximate to vertical bar nq vertical bar. This is expected from ideal MHD in the proximity of rational surfaces. The displacement measured by ECE-I confirms this resonant response.

High frequency magnetic fluctuations correlated with the inter-ELM pedestal evolution in ASDEX Upgrade

In order to understand the mechanisms that determine the structure of the high confinement mode (H-mode) pedestal, the evolution of the plasma edge electron density and temperature profiles between edge localised modes (ELMs) is investigated. The onset of radial magnetic fluctuations with frequencies above 200 kHz is found to correlate with the stagnation of the electron temperature pedestal gradient. During the presence of these magnetic fluctuations the gradients of the edge electron density and temperature are clamped and stable against the ELM onset. The detected magnetic fluctuation frequency is analysed for a variety of plasma discharges with different electron pressure pedestals. It is shown that the magnetic fluctuation frequency scales with the neoclassically estimated velocity at the plasma edge. This points to a location of the underlying instability in the gradient region. Furthermore, the magnetic signature of these fluctuations indicates a global mode structure with toroidal mode numbers of approximately 10. The fluctuations are also observed on the high field side with significant amplitude, indicating a mode structure that is symmetric on the low field side and high field side. The associated fluctuations in the current on the high field side might be attributed to either a strong peeling part or the presence of non-adiabatic electron response.

Parameter dependence of the radial electric field in the edge pedestal of hydrogen, deuterium and helium plasmas

The characteristics of the edge radial electric field (Er) are studied in deuterium, hydrogen and helium plasmas at ASDEX Upgrade. The minimum of the Er well is analyzed as a function of pedestal parameters and the best correlation is found between the depth of the Er well and the ion pressure at the pedestal top. This result is consistent with Er being balanced by the main ion pressure gradient term. Studying the radial position of the Er minimum reveals that the Er well moves closer to the last closed flux surface the deeper the Er well is. This suggests that for deeper Er wells the distance between the steepest gradients in the ion temperature and ion density profile is reduced. The width of the Er well shows no significant variations despite changing the ion temperature, magnetic field and plasma particle species. At AUG, the Er well is on average 1.2 cm wide. A multi-machine comparison supports a machine size scaling of Er and indicates that the Er shear layer covers the outer 2% of the plasma minor radius independent of the size of the machine. Based on this scaling, the width of the Er well in ITER is estimated to ~4 cm.

The role of carbon and nitrogen on the H-mode confinement in ASDEX Upgrade with a metal wall

Carbon (CD4) and nitrogen (N2) have been seeded in ASDEX Upgrade (AUG) with a tungsten wall and have both led to a 20–30% confinement improvement. The reference plasma is a standard target plasma with I p /B T = 1 MA/2.5 T, total input power P tot ~ 12 MW and normalized pressure of β N ~ 1.8. Carbon and nitrogen are almost perfectly exchangeable for the core, pedestal and divertor plasma in this experiment where impurity concentrations of C and N of 2% are achieved and Z eff only mildly increases from ~1.3 to ~1.7. As the radiation potentials of C and N are similar and peak well below 100 eV, both impurities act as divertor radiators and radiate well outside the pedestal region. The outer divertor is purposely kept in an attached state when C and N are seeded to avoid confinement degradation by detachment. As reported in earlier publications for nitrogen, carbon is also seen to reduce the high field side high density (the so-called HFSHD) in the scrape off layer above the inner divertor strike point by about 50%. This is accompanied by a confinement improvement for both low (δ ~ 0.25) and high (δ ~ 0.4) triangularity configurations for both seeding gases, due to an increase of pedestal temperature and stiff core temperature profiles. The electron density profiles show no apparent change due to the seeding. As an orthogonal effect, increasing the triangularity leads to an additionally increased pedestal density, independent of the impurity seeding. This experiment further closes the gap in understanding the confinement differences observed in carbon and metal wall devices; the absence of carbon can be substituted by nitrogen which leads to a similar confinement benefit. So far, no definite physics explanation for the confinement enhancement has been obtained, but the experimental observations in this paper provide input for further model development.

ELM behavior in ASDEX Upgrade with and without nitrogen seeding

The Type I ELM behavior in ASDEX Upgrade with full W plasma facing components is studied in terms of time scales and energy losses for a large set of shots characterized by similar operational para ...

Pellet refuelling of particle loss due to ELM mitigation with RMPs in the ASDEX Upgrade tokamak at low collisionality

The complete refuelling of the plasma density loss (pump-out) caused by mitigation of edge localised modes (ELMs) is demonstrated on the ASDEX Upgrade tokamak. The plasma is refuelled by injection of frozen deuterium pellets and ELMs are mitigated by external resonant magnetic perturbations (RMPs). In this experiment relevant dimensionless parameters, such as relative pellet size, relative RMP amplitude and pedestal collisionality are kept at the ITER like values. Refuelling of density pump out of the size of requires a factor of two increase of nominal fuelling rate. Energy confinement and pedestal temperatures are not restored to pre-RMP values by pellet refuelling.

Collisionality dependence of edge rotation and ion-out impurity asymmetries in ASDEX Upgrade H-mode plasmas

The poloidal and toroidal impurity flows in the edge transport barrier of H-mode plasmas have been studied over a wide range of pedestal top ion collisionalities. A comparison of the edge poloidal rotation measurements to neoclassical predictions shows good agreement in all cases. The measured edge impurity toroidal rotation is observed to change sign from co-current to counter-current with decreasing collisionality. The switch occurs at the same collisionality at which neoclassical theory predicts the main ion poloidal rotation to change from the electron to the ion diamagnetic direction. The behaviour of these two species, when used to calculate the main ion toroidal rotation via the radial force balance equation, leads to fairly constant co-current main ion toroidal rotation. Hence, at low collisionality, due to a reduced frictional coupling, the main ion-impurity differential rotation can be quite large. The behaviour of impurity ion flows on a flux surface has also been investigated in detail and it was found that the measurements are consistent with the continuity equation only if the poloidally asymmetric impurity density distribution is taken into account. The asymmetry is found to be the result of the interplay of all forces in the parallel momentum balance, with the friction force providing the dominant drive. Close to the separatrix the poloidal centrifugal force, which is usually neglected, also gives an additional contribution to the impurity density asymmetry. Within the experimental uncertainties the ion temperature and the electrostatic potential are simultaneous flux functions, despite the presence of a poloidally asymmetric impurity density profile.

Fast piezoelectric valve offering controlled gas injection in magnetically confined fusion plasmas for diagnostic and fuelling purposes

In magnetically confined fusion plasmas controlled gas injection is crucial for plasma fuelling as well as for various diagnostic applications such as active spectroscopy. We present a new, versatile system for the injection of collimated thermal gas beams into a vacuum chamber. This system consists of a gas pressure chamber, sealed by a custom made piezo valve towards a small capillary for gas injection. The setup can directly be placed inside of the vacuum chamber of fusion devices as it is small and immune against high magnetic fields. This enables gas injection close to the plasma periphery with high duty cycles and fast switch on/off times ≲ 0.5 ms. In this work, we present the design details of this new injection system and a systematic characterization of the beam properties as well as the gas flowrates which can be accomplished. The thin and relatively short capillary yields a small divergence of the injected beam with a half opening angle of 20°. The gas box is designed for pre-fill pressures of 10 mbar up to 100 bars and makes a flowrate accessible from 1018 part/s up to 1023 part/s. It hence is a versatile system for both diagnostic as well as fuelling applications. The implementation of this system in ASDEX Upgrade will be described and its application for line ratio spectroscopy on helium will be demonstrated on a selected example.