New concept for measuring the edge magnetic field line angle

A new edge diagnostic for the measurement of the magnetic field line angle has been installed on the ASDEX Upgrade tokamak. The new system relies onthe motional Stark effect and is based on the simultaneous measurement ofthe polarization direction of the linearly polarized π (parallel to the electricfield) and σ (perpendicular to the electric field) lines of the Balmer spectralline Dα. In this setup, we use 3 independent observations of the same spot with polarizer angles α ≈ 0°, 45° and 90° (see figure), where 0° serves as a reference for the π lines, 90° for the σ lines, and 45° measures a combination of both σ and π lines. In order to extract the magnetic field line angle a forward model for the Stark split Dα spectra has been developed.

Fig. 1. Stark split Dα spectra of BEP diagnostics.

More details on this work can be found in:
[1] E. Viezzer et al., A new beam emission polarimetry diagnostic for measuring the magnetic field line angle at the plasma edge of ASDEX Upgrade, RSI 87, 11E528 (2016).
[2] R. Dux et al., A forward model for beam emission spectroscopy at ASDEX Upgrade, 42^nd EPS Conference, P1.121, Lisbon (2015).

Pedestal Poloidal Impurity Asymmetries

The interplay between plasma flows and transport is a key ingredient of the physics that governs the edge transport barrier (ETB) of an H-mode fusion plasma. Impurity transport is of particular importance as it is crucial for understanding and controlling the impurity content in the plasma. In the ETB, the impurity particle transporth as been observed to be determined by neoclassical theory and the question arises whether the particle transportlevel is affected if poloidal impurity asymmetries are present in the plasma edge. Using the high-resolution edge charge exchange diagnostic suite at ASDEX Upgrade we could study asymmetries on the flux surfaces at the plasma edge.

The measurements reveal that in the ETB the flow structure is asymmetric on the flux surfaces. The asymmetry in the flow pattern can be explained by an excess of impurity density at the HFS following the condition of divergence-free flows on a flux surface, which is based on the general continuity equation. Comparison of the measured flows to theoretical predictions based on the parallel momentum balance reveals the nature of theparallel impurity dynamics. The key features of the experimental data including the shape of the rotation profilesand the poloidal impurity density asymmetry are reproduced quantitatively for the first time.

Fig. 2. Poloidal asymmetries of parallel and poloidal flows (upper plot), measured impurity density at the LFS and HFS (bottom plot).

More details on this work can be found in:
[1] E. Viezzer et al., Collisionality dependence of edge rotation and in–out impurity asymmetries in ASDEX Upgrade H-mode plasmas, Nucl. Fusion 55, 123002 (2015).
[2] E. Viezzer et al., Rotation and density asymmetries in the presence of large poloidal impurity flows in the edge pedestal, PPCF 55, 124037 (2013).
[3] T. Pütterich et al., Poloidal asymmetry of parallel rotation measured in ASDEX Upgrade, Nucl. Fusion 52, 083013 (2012).
[4] R. M. Churchill et al., In–out impurity density asymmetry in the pedestal region of Alcator C-Mod, Nucl. Fusion 53, 122002 (2013).

High-resolution measurements of the edge radial electric field

The understanding of the physics relevant to the edge transport barrier (ETB) of an H-mode fusion plasma is of crucial importance as it leads to steep gradients at the plasma edge which implies a confinement gain at the boundary of the plasma. This improvement propagates into the plasma core, where a hot and dense plasma is required for fusion. The ETB is thought to be caused by a sheared plasma flow perpendicular to the magnetic field which is equivalent to a sheared radial electric field E_r. We have confirmed this mechanism as the location of the steepest ion pressure gradient ∇p_i was shown with unprecedented accuracy to match the position of the largest E_r shear. These measurements were performed using the high-resolution edge CXRS diagnostic suite available at ASDEX Upgrade. 

We have found that, in the radial force balance of impurities the poloidal rotation contribution yields the dominant term in the evaluation of E_r at the plasma edge. For the main ions, the E_r minimum coincides with the maximum pressure gradient term ∇pi/en_i supporting that the E_r well is created by the main ion species. The fact that ∇p_i/en_i matches E_r in the ETB is consistent with the main ion poloidal flow being at neoclassical levels. Quantitative comparisons between neoclassical predictions and experimental measurements of both impurity and main ion poloidal rotation show that the sign and themagnitude are in remarkably good agreement.

More details on this work can be found in:
[1] E. Viezzer et al., Evidence for the neoclassical nature of the radial electric field in the edge transport barrier of ASDEX Upgrade, NF Letter 54, 012003 (2014).
[2] E. Viezzer et al., High-accuracy characterization of the edge radial electric field at ASDEX Upgrade, NF 53, 053005 (2013).
[3] E. Viezzer et al., Parameter dependence of the radial electric field in the edge pedestal of hydrogen, deuterium and helium plasmas, PPCF 56, 075018 (2014).

The role of ion orbit losses for the edge radial electric field

Ions executing orbits that cross the separatrix have a certain probability of getting lost. These losses, which are non-ambipolar and a potential source for the radial electric field at the plasma edge, are commonly referred to as ion orbit losses (IOL). The understanding of sources and drivers of radial electric fields (Er) is very important, as the formation of a steep gradient in Er at the plasma edge is connected with a transition to high confinement.

Ion orbit loss is a very active research topic [1-4], as it has been pointed out as a potential driver for the L-H transition. While previous studies have shown that in the fully developed H-mode [5] and during the L-H transition [6] the radial electric field is well described by neoclassical theory, under certain conditions deviations from the neoclassical prediction have been observed.

The studies carried out in the PSFT group aim to shed light on the effect of ion orbit losses on the Er profile in cases with non-neoclassical poloidal rotation, such as shortly after the ELM cycle, reversed Bt/Ip, or in plasmas with externally applied magnetic perturbations which show an increased level of fast-ion losses [7].

[1] W.M. Stacey, Effect of ion orbit loss on the structure in the H-mode tokamak edge pedestal profiles of rotation velocity, radial electric field, density, and temperature, Phys. Plasmas 20, 092508 (2013).
[2] T.M. Wilks et al., Calculation of the radial electric field from a modified Ohm's law, Phys. Of Plasmas 24, 012505 (2017).
[3] J.S. deGrassie et al., Dimensionless size scaling of intrinsic rotation in DIII-D, Phys. Of Plasmas 23,  082501 (2016).
[4] R. Brzozowski et al, A geometric model of ion orbit loss under the influence of a radial electric field, Physics of Plasmas 26, 042511 (2019).
[5] E. Viezzer et al, Evidence for the neoclassical nature of the radial electric field in the edge transport barrier of ASDEX Upgrade, Nucl. Fusion 54, 012003 (2014).
[6] M. Cavedon et al, Interplay between turbulence, neoclassical and zonal flows during the transition from low to highconfinement mode at ASDEX Upgrade, Nucl. Fusion 57, 014002 (2017).
[7] M. Garcia Munoz et al., Fast-ion losses induced by ELMs and externally applied magnetic perturbations in the ASDEX Upgrade tokamak, Plasma Phys. Control. Fusion 55, 124014 (2013).

Measurements of the main ion species

In future fusion devices, the bulk plasma properties, such as temperature and density, are important since they determine plasma performance, confinement and achievable fusion power.

The edge radial electric field (E_r) shear is one of the key ingredients for achieving high plasma performance. Dedicated experiments in helium plasmas have shown that the main ion pressure gradient (∇p_i/en_i) is the main driver for E_r, as it constitutes the dominant term in the main ion radial force balance [1]. This result is consistent with poloidal main ion flows being at the neoclassical level, while impurity flows adjust as all species feel the same E_r established by the main ions.

Main ion measurements are also important for the study of energy and momentum transport in tokamak plasmas. The ion heat transport has been studied using main ion measurements on helium plasmas, and we have shown it to be at the neoclassical level in the pedestal region [2,3].

The direct measurement of deuterium main ion profiles is more challenging and, for many years, has been rarely done. Recent progress on directly measuring main ion properties has been made ([4] and references therein). The new edge main ion diagnostic at ASDEX Upgrade will allow us to directly measure the main ion properties and compare to electron and impurity properties and predictions from neoclassical theory.

[1] E Viezzer et al., High-accuracy characterization of the edge radial electric field at ASDEX Upgrade, Nucl. Fusion 53, 053005 (2013).
[2] E. Viezzer et al., Ion heat transport dynamics during edge localized mode cycles at ASDEX Upgrade, Nucl. Fusion 58,  026031 (2018).
[3] E. Viezzer et al., Dynamics of the pedestal transport during edge localized mode cycles at ASDEX Upgrade, Plasma Phys. Control. Fusion 62, 024009 (2020).
[4] S.R. Haskey et al., Main ion and impurity edge profile evolution across the L- to H-mode transition on DIII-D, Plasma Phys. Control. Fusion 60, 105001 (2018).

Plasma response

A plasma is a quasineutral ionised gas that exhibits collective effects. In the presence of externally applied magnetic perturbations, the plasma develops internal currents that, depending on the properties of the plasma equilibrium and applied perturbation, can shield or even amplify the applied external perturbation. Symmetry breaking 3D resonant magnetic perturbations are applied, nowadays to control internal MHD perturbations in most of the large magnetically confined fusion devices and are one of the most promising techniques to control Edge Localised Modes (ELMs) in ITER. The validation of plasma response models is therefore of crucial importance to be able to make predictions towards ITER. Fast ions being particularly sensitive to symmetry-breaking 3D fields, accurate fast ion measurements may help us to probe the plasma response and thus test plasma response models and numerical tools. Recent measurements in AUG have revealed the importance of the plasma collisionality and RMP poloidal spectra on the observed fast-ion losses induced by the externally applied n=2 RMP, see Fig. 4. Preliminary simulations have shown that most of the losses are induced by the applied field perturbation rather than by modifications of the NBI birth profile. However, neither the absolute ion fluxes to the wall nor the velocity-space of the escaping ions could be reproduced in simulations.

Recent AUG orbit simulations and experiments indicate that an Edge Resonant Transport Layer (ERTL) might be responsible for the observed fast-ion losses in the presence of externally applied 3D fields and that this ERTL might play an important role on the overall density pump-out and ELM suppression mechanism.

At Seville, we model plasma response employing 3D fluid/drift-kinetic simulations performed using the MEGA^[1-2] code. This model employs a non-linear dynamic approach, where changes to the plasma equilibrium accounted for through a full MHD treatment of the background plasma, while super-thermal particles are considered on fast Alvenic timescales by a drift-kinetic model.

[1] Y. Todo and T. Sato, Linear and nonlinear particle-magnetohydrodynamic simulations of the toroidal Alfven eigenmode, Physics of Plasmas 5, 1321 (1998).
[2] Y. Todo et. al., Validation of comprehensive magnetohydrodynamic hybrid simulations for Alfven eigenmode induced energetic particle transport in DIII-D plasmas, Nucl. Fusion 55, 073020 (2015).