Asymmetrical Distribution of Plasma Parameters Nearby Divertor Plates of Small-Sized Tokamaks

Annual variations of the daytime ionosphere studied with the MU radar during a full solar cycle show equinoctial asymmetry in density, temperature, and plasma velocity. The electron density (Ne) in the bottomside ionosphere is slightly greater in September equinox compared to March equinox, which arises from the asymmetry in thermospheric composition. At higher altitudes, the asymmetry in Ne reverses and becomes strong; the values of Ne in March equinox exceed those in September equinox by up to 150%. The electron temperature (Te) shows an asymmetry, which is opposite to the asymmetry in Ne; the values of Te in the topside in March equinox are less than those in September equinox by up to 300K. The asymmetry in ion temperature (Ti) is weak (about 100K) and in phase with the asymmetry in Ne. The northward perpendicular plasma velocity (V⟂) is slightly greater in March equinox compared to September equinox. The meridional component of the thermospheric neutral wind velocity (Uθ), derived from the field‐parallel plasma velocity V∥, is up to 50% lower in March equinox compared to September equinox. The asymmetry in neutral winds accounts for the asymmetry in ionospheric density and temperature in the topside, with minor contribution from the asymmetry in plasma drifts.

Reduction of the heat load to plasma-facing components is a crucial problem for future fusion reactors like Divertor Test Tokamak (DTT). Mitigation of the power load via increased plasma radiation with the use of puffed ions of impurities would be one way to mitigate power in the scrape-off layer. This paper presents a numerical investigation of the impact of seeded impurities on the radiation pattern and the power load to the divertor plates of the high-field DTT reactor in the single null (SN) configuration. The simulations have been done with the use of the TECXY code, which solves multi-species plasma transport equations for multiple impurity species simultaneously and all associated ionization stages in a two-dimensional poloidal geometry. TECXY represents the model of plasma transport in the scrape-off layer region by a classical set of transport equations of multi-species plasma derived by Braginskii. The paper aims to compare the mitigation capabilities of neon and argon impurities seeded in the plasma of the DTT device and to obtain a significant energy flux reduction to the target plate at the smallest possible impurity concentration. Performed investigations showed the effects of neon and argon impurities seeding separately for constant electron density at the separatrix. It has been found that the decrease in electron temperature on the divertor plates up to 3 eV at the outer and the inner divertor plates and the peak power load below 15 MW m−2 at the outer divertor plate can be achieved with argon seeding and much lower impurity concentration than that in the case of neon impurity seeding. Studies have shown the complexity of the effect of neon and argon impurities on the boundary plasma. It was found that the reduction of temperature and the power on both divertor plates was the most effective for the high upstream plasma densities. The results show also that diffusive perpendicular transport strongly affects impurity radiation and thus plasma condition at divertor plates.

Impurity profiles have been measured with the edge high field side (HFS) and low field side (LFS) charge exchange recombination spectroscopy suite at ASDEX Upgrade enabling the study of the poloidal structure of the edge parallel flow in H-mode, L-mode and I-mode. In H-mode, asymmetries in the impurity density, toroidal and poloidal rotations are found. In I-mode, only toroidal rotation asymmetries have been measured while in L-mode no asymmetries have been observed. The measured parallel flow can be divided in two components, the Pfirsch–Schlüter (PS) flow and the symmetric flow. Two different methods have been followed to determine both contributions to the parallel flow. The first method is based on the calculation of the PS flow at the HFS and LFS from the radial electric field. The second method directly provides the symmetric flow from the flux surface average (FSA) of the parallel flow. In H-mode, the methods provide different results, while in L-mode and I-mode they agree. The differences observed in H-mode between the two methods could be explained by the existence of asymmetries in the impurity density, by the non-negligible particle sources and radial losses, or by the approximations made in the calculation of the FSA of the parallel flow from measurements in two poloidal positions (midplane HFS and LFS) only.

To understand the nightside plasma sheet structure under different interplanetary magnetic field (IMF) Bz conditions, we have investigated statistically the equatorial distributions of ions and magnetic fields from Geotail when the IMF has been continuously northward or southward for shorter or longer than 1 hour. A dawn‐dusk density (temperature) asymmetry with higher density (temperature) on the dawn (dusk) side is seen in the near‐Earth plasma sheet during northward IMF, resulting in roughly dawn‐dusk symmetric pressure. As southward IMF proceeds, the density asymmetry weakens while the temperature asymmetry maintains, resulting in higher pressure on the dusk side. The plasma sheet is relatively colder and denser near the flanks than around midnight. The flux distributions show that the density asymmetry is due to ions <∼3 keV, and the temperature asymmetry is due to ions above thermal energy. The perpendicular flow shows that ions divert around the Earth mainly through the dusk side in the inner plasma sheet because of westward diamagnetic drift. The magnetic fields indicate that field lines are more stretched during southward IMF. Ions' electric and magnetic drift paths evaluated from the observations show that for thermal energy ions, magnetic drift is as important as electric drift. Comparison of the distributions of the observed phase space density with the evaluated drift paths at different energies indicates that the electric and magnetic drift transport is responsible for the observed dawn‐dusk asymmetries in the plasma sheet structure.

Measurement of edge parameters in TEXTOR-94 at the low and high field side with atomic beams

The infrared diagnosis system is an efficient tool to provide the immediate temperature distribution on the divertor plate, which can be converted to heat flux by solving the heat conduction equation with the finite difference method. This paper concentrates on the heat flux on the divertor with different discharge conditions on EAST. Experimental results show that the heat flux on the divertor plate has a strong in–out asymmetry under different configurations. The in–out asymmetry in double null configuration is stronger than that in upper single null configuration. Meanwhile, Argon gas puffing has been introduced in EAST to control the divertor heat flux, which shows that argon injection from either the upper outer divertor or the upper dome is very effective at reducing the divertor heat flux. Whether the argon gas is introduced from the upper outer divertor or the upper dome, heat load on the inner divertor plate tends to be more easily reduced than that on the outer divertor plate, the reason for which is discussed in this paper.

Simulation of an H-mode ASDEX Upgrade shot with boron impurity was done with the B2SOLPS5.2 transport code. Simulation results were compared with the unique experimental data available for the chosen shot: radial density, electron and ion temperature profiles in the equatorial midplanes, radial electric field profile, radial profiles of the parallel velocity of impurities at the low-field side (LFS) and high-field side (HFS), radial density profiles of impurity ions at LHS and HFS. Simulation results reproduce all available experimental data simultaneously. In particular strong poloidal HFS–LFS asymmetry of B5+ ions was predicted in accordance with the experiment. The simulated HFS B5+ density inside the edge transport barrier is twice larger than that at LFS. This is consistent with the experimental observations where even larger impurity density asymmetry was observed. A similar effect was predicted in the simulation done for the MAST H-mode. Here the HFS density of He2+ is predicted to be 4 times larger than that at LHS. Such a large predicted asymmetry is connected with a larger ratio of HFS and LFS magnetic fields which is typical for spherical tokamaks. The HFS/LFS asymmetry was not measured in the experiment, however modelling qualitatively reproduces the observed change of sign of He+parallel velocity to the counter-current direction at LFS. The understanding of the asymmetry is based on neoclassical effects in plasma with strong gradients. It is demonstrated that simulation results obtained with account of sources of ionization, realistic geometry and turbulent transport are consistent with the simplified analytical approach. Difference from the standard neoclassical theory is emphasized.

Investigations on density and temperature asymmetries due to drift motions in the boundary layer of TEXTOR-94

The parallel and radial transport properties of the plasma edge of TEXTOR are studied using radial electron density and temperature profiles as well as ion temperatures and poloidal velocities in the scrape-off layer (SOL). These quantities are measured by thermal helium beams at the low (LFS) and high field side (HFS) by emission and beam driven charge exchange recombination spectroscopy. We investigate the influence of the safety factor and of a magnetic field reversal on these edge parameters. Especially the field reversal leads to clear effects: a decrease of the density at the LFS, a significant change in the poloidal density distribution, which is identified by comparing LFS and HFS densities, and an increase in the density e-folding length at both poloidal positions. The poloidal ion rotation changes sign in reversed field configuration and thus gives a hint on the role of poloidal drifts in the SOL. For further analysis, we present a simple fluid model including poloidal drift velocities and local ionization sources. With this model we can show that the poloidal E×B drift clearly influences the poloidal density distribution. However, although the model results show the same tendencies as the experimental findings, the impact of the field reversal on the density asymmetry is not that pronounced as in the experiment. The dependence of the density e-folding length on the poloidal and radial drifts as well as on the source distribution is discussed within the frame of analytical estimates.

Study of power load on TdeV divertor plates with an infrared camera

Asymmetric cold pulse perturbation has been observed by means of electron–cyclotron emission (ECE) and soft-x-ray array during pulse-modulated molecular beam injection (MBI) experiments on HL-2A. The perturbation depth is about 30 cm from the low field side and only about 10 cm from the high field side. The cold pulses cannot propagate to the plasma centre from either the low or high field side. The electron temperature in the plasma centre does not change during MBI, but electron density pulse perturbations can be observed in the plasma centre from the ECE 3rd harmonic measurements, which corresponds to the results of the far-infra-red laser interferometer. The experimental results indirectly provide evidence of the shielding mechanisms of the MBI physics.

Modeling of asymmetric redeposition distribution between inner and outer regions of the W-shaped divertor in JT-60U

A synthetic electron cyclotron emission (ECE) diagnostic is used to interpret ECE signals from preset plasma equilibrium profiles, including magnetic field, electron density, and electron temperature. According to the simulation results, the electron temperature (Te) profile covering the harmonic overlap region can be obtained by receiving ECE signals at the high field side (HFS) of the HL-2M plasma. The third harmonic ECE at the low field side (LFS) cannot pass through the second harmonic resonance layer at the HFS unless the optical thickness (τ) of the second harmonic becomes gray (τ ≤ 2). In addition, the impact of the relativistic frequency down-shift has been evaluated and corrected. The measurable range of the HFS ECE has been calculated by scanning different parameters (electron density, temperature, and magnetic field). Higher plasma parameters allow a wider radial range of electron temperature measurements. The minimum inner measurable position can reach R = 120cm (r/a = -0.89) when the product of core temperature (Te0) and density (ne0) is greater than 35 × 1019keVm-3, which is extended by more than 30cm inward compared with that of the LFS measurement. The HFS ECE will greatly improve the diagnostic ability of ECE systems on the HL-2M tokamak.

Novel flow rotation measurements based on charge exchange recombination spectroscopy at the inboard midplane of the ASDEX Upgrade tokamak reveal the existence of an asymmetric flow structure at the H-mode edge, which is shown to arise due to a poloidal impurity density asymmetry. A quantitative evaluation of the impurity density at the inboard side demonstrates that the impurities redistribute along the flux surface, resulting in a poloidal dependency of the impurity density. The poloidal and toroidal impurity flows measured at the high-field side (HFS) and low-field side (LFS) are compared to theoretical predictions based on the parallel momentum balance, which includes friction, inertia, pressure and electric force. Both a fluid and a kinetic approach are used, showing good agreement with each other. The measured impurity flow structure is described by the model quantitatively when a finite poloidal main ion flow of ∼2 km s−1 arises, which is in keeping with the standard neoclassical prediction. The interplay of all terms, in particular the inclusion of the impurity inertia term, is important in reproducing the observed flow structure and results in an impurity accumulation at the HFS. The existence of a poloidal impurity density asymmetry in the edge transport barrier slightly reduces the drift parameter v/D, however, the experimental value is consistent with standard neoclassical theory. This demonstrates that despite the asymmetry in the impurity density, the impurity particle transport is at the neoclassical level.

The nature of the multi-modal n = 2 plasma response and its impact on global confinement is studied as a function of the axisymmetric equilibrium pressure, edge safety factor, collisionality, and L-versus H-mode conditions. Varying the relative phase () between upper and lower in-vessel coils demonstrates that different n = 2 poloidal spectra preferentially excite different plasma responses. These different plasma response modes are preferentially detected on the tokamak high-field side (HFS) or low-field side (LFS) midplanes, have different radial extents, couple differently to the resonant surfaces, and have variable impacts on edge stability and global confinement. In all equilibrium conditions studied, the observed confinement degradation shares the same dependence as the coupling to the resonant surfaces given by both ideal (IPEC) and resistive (MARS-F) MHD computation. Varying the edge safety factor shifts the equilibrium field-line pitch and thus the dependence of both the global confinement and the n = 2 magnetic response. As edge safety factor is varied, modeling finds that the HFS response (but not the LFS response), the resonant surface coupling, and the edge displacements near the X-point all share the same dependence. The LFS response magnitude is strongly sensitive to the core pressure and is insensitive to the collisionality and edge safety factor. This indicates that the LFS measurements are primarily sensitive to a pressure-driven kink-ballooning mode that couples to the core plasma. MHD modeling accurately reproduces these (and indeed all) LFS experimental trends and supports this interpretation. In contrast to the LFS, the HFS magnetic response and correlated global confinement impact is unchanged with plasma pressure, but is strongly reduced in high collisionality conditions in both H- and L-mode. This experimentally suggests the bootstrap current drives the HFS response through the kink-peeling mode drive, though surprisingly weak or no dependence on the bootstrap current is seen in modeling. Instead, modeling is revealed to be very sensitive to the details of the edge current profile and equilibrium truncation. Holding truncation fixed, most HFS experimental trends are not captured, thus demonstrating a stark contrast between the robustness of the HFS experimental results and the sensitivity of its computation.