Dependence of divertor asymmetries on the toroidal magnetic field

The BOUT++ transport code is run to study the effects of plasma drifts on the divertor out-in asymmetries (DOIAs) of particle and heat fluxes and their decay widths for EAST lower single null H-mode discharges. The diamagnetic drift seems to have no effects on the DOIAs of total particle and heat fluxes due to its divergence-free nature. However, it could significantly increase the DOIAs of peak particle and heat fluxes and the flux decay widths. The E × B drift is found to induce a large plasma flow to the divertor region, enhancing the DOIAs of both total and peak particle and heat fluxes and the flux decay widths. Both the radial and poloidal components of the E × B drift are necessary in increasing the DOIAs, however, the radial E × B drift seems to play a more important role. The effects on the DOIAs caused by both diamagnetic and E × B drifts are reversed with the reverse of toroidal magnetic field. The heat flux decay width λq and spreading width Sq are important physical and engineering parameters for the divertors and could be obtained by fitting the heat flux profiles at divertor targets. The λq at the outer target from the simulation case with all drifts could well match with the multi-machine scaling proposed by Eich and the DOIA of λq is in reasonable agreement with the scaling proposed by Goldston.

The divertor asymmetry in particle and power fluxes has been investigated on the EAST superconducting tokamak [S. Wu and EAST Team, Fusion Eng. Des. 82, 463 (2007)] for both single null (SN) and double null (DN) divertor configurations. D2 and Ar puffing at various divertor locations has also been explored as an active means to reduce peak target heat load and control divertor asymmetry. For SN, peak heat load on the outer divertor target is 2–3 times that on the inner divertor target under typical ohmic plasma conditions. DN operation leads to a stronger in-out asymmetry favoring the outer divertor. D2 and Ar puffing promotes partial detachment near the strike points, greatly reducing peak target heat load (over 50%), while the far-SOL divertor plasma remains attached. What is remarkable is that the particle flux is even increased away from the strike points when the B×∇B drift is directed toward the divertor target, thus facilitating particle removal.

Where is the solar dynamo located and what is its modus operandi? These are still open questions in solar physics. Helio- and asteroseismology can help answer them by enabling us to study solar and stellar internal structures through global oscillations. The properties of solar and stellar acoustic modes are changing with the level of magnetic activity. However, until now, the inference on subsurface magnetic fields with seismic measures has been very limited. The aim of this paper is to develop a formalism to calculate the effect of large-scale toroidal magnetic fields on solar and stellar global oscillation eigenfunctions and eigenfrequencies. If the Lorentz force is added to the equilibrium equation of motion, stellar eigenmodes can couple. In quasi-degenerate perturbation theory, this coupling, also known as the direct effect, can be quantified by the general matrix element. We present the analytical expression of the matrix element for a superposition of subsurface zonal toroidal magnetic field configurations. The matrix element is important for forward calculations of perturbed solar and stellar eigenfunctions and frequency perturbations. The results presented here will help to ascertain solar and stellar large-scale subsurface magnetic fields, and their geometric configuration, strength, and change over the course of activity cycles.

An experimental investigation on some effects of the toroidal magnetic field on an annular plasma produced by a modified hard-core θ-pinch is described. The length of the annular plasma could by varied by a pair of end coils. A high speed camera, a floating double probe and a magnetic probe were used for the investigation. The experimental results show that the toroidal magnetic field has some effects to suppress the macroscopic instabilities, such as the positional instability, and the axial contraction of the annular plasma, where the former was observed in the case with the shorter annular plasma and the latter in the case with the longer one when the toroidal field was not applied.

Density dependence of SOL power width in ASDEX upgrade L-Mode

Compatibility of the radiating divertor with high performance plasmas in DIII-D

Simultaneous control of the transient heat load induced by the large-amplitude edge-localized modes (ELMs) and the time-averaged heat and particle fluxes to the divertor targets is a critical issue for the steady-state operation of a future tokamak fusion reactor. The combination of divertor detachment and grassy-ELM regime provides a candidate solution to this issue. The strong particle exhaust capability of the grassy-ELM regime greatly facilitates the achievement of steady-state operation of a detached plasma with divertor impurity seeding. Stable complete detachment at inner target and partial detachment at outer target in the grassy-ELM regime have been achieved with seeding of 5% neon (Ne) and 95% D2 mixture since 2018 in the EAST superconducting tokamak with ITER-like tungsten monoblock upper divertor. The peak ion fluxes (Γion) on the upper outer and inner divertor targets were reduced by 55% and 92%, respectively. However, it was accompanied by confinement degradation with 18% reduction in the plasma stored energy WMHD. In contrast, partial detachment on the upper outer divertor target with confinement improvement has been achieved with the power across the separatrix (Psep) around the H-mode threshold power. In addition, at relatively lower q95 (∼5.7) with unfavorable Bt direction, Ne seeding leads to a transition from mixed (large/small) ELM regime into grassy-ELM regime with significantly increased ELM frequency, which extends the accessible parameter range of the grassy-ELM regime towards lower q95.

Divertor asymmetry is a major challenge in achieving high-power, long-pulse discharges in future fusion reactors. Impurity seeding is the most common method for achieving divertor detachment in fusion devices. In this study, the SOLPS-ITER code is used to investigate the impact mechanisms of nitrogen (N) and neon (Ne) impurity seeding on the asymmetry between the inner and outer divertor in HL-2A under the attached and detached divertor conditions. Results indicate that N and Ne impurity seeding can increase asymmetry of energy and particle flux between the inner and outer targets. The trend in the energy flux ratio between the inner and outer targets is consistent with that of the particle flux ratio. Research indicates that under the attached divertor condition, the increase in energy flux asymmetry due to impurity seeding is primarily influenced by the electron temperatures between the inner and outer targets. However, when under the detached divertor condition, the increased asymmetry in energy and particle flux is primarily attributed to impurity seeding, which narrows the ion source generation in the inner divertor while broadening the ion sink region compared to the outer divertor.

Radial electric fields up to ∼4 kV m−1 are observed in the boundary between the private flux region (PFR) and the scrape-off layer (SOL) driving E × B drifts between the inner and outer targets at speeds up to 2.8 km s−1 in the Tokamak à configuration variable divertor. The resulting E × B fluxes, located in a narrow region ( ΔρΨ<0.012 in normalized radius or Δ R − R sep <4 mm mapped to the outer midplane) are equivalent to around 20% of the total heat and particle flux to the divertor targets (inner + outer). At the peak E r, the E × B poloidal transport is equivalent to parallel flows with M ∥ ∼ 3. In the snowflake divertor with a second X-point in the outer SOL, the drifts in the PFR-SOL boundary were equivalent to around 30% of the total heat and particle flux to the divertor targets and cover a region ∼50% wider than in the single null ( ΔρΨ ∼ 0.018, Δ R − R sep ∼ 6 mm). The location of the PFR-SOL boundary drift shifts radially in the E ∥ × B direction when reversing the toroidal field direction. Peaks in density and electron pressure have been identified near the primary X-point along with large gradients in density, temperature, and potential, the latter resulting in a local electric field ∼2.7 kV m−1 which drives a drift (1.9 km s−1) upwards towards the closed flux surfaces. Floating potential (V f) magnitudes up to 75 V (∼2 kTe) were measured, indicating that V f and parallel currents should not be neglected when estimating plasma potential.

We have succeeded in obtaining magnetized equilibrium states with differential rotation and differential toroidal magnetic fields. If an internal toroidal field of a proto-neutron star is wound up from the initial poloidal magnetic field by differential rotation, the distribution of the toroidal magnetic field is determined by the profile of this differential rotation. However, the distributions of the toroidal fields in all previous magnetized equilibrium studies do not represent the magnetic winding by the differential rotation of the star. In this paper, we investigate a formulation of a differential toroidal magnetic field that represents the magnetic field wound up by differential rotation. We have developed two functional forms of differential toroidal fields which correspond to a v-constant and a j-constant field in analogy to differential rotations. As the degree of the differential becomes very high, the toroidal magnetic field becomes highly localized and concentrated near the rotational axis. Such a differential toroidal magnetic field would suppress the low-T/|W| instability more efficiently even if the total magnetic field energy is much smaller than that of a non-differential toroidal magnetic field.

The change of the scrape-off layer power width in dependence of the toroidal magnetic field direction is investigated in L-mode discharges in the upper single null (USN) configuration in ASDEX Upgrade. The heat flux onto the outer and inner divertor plates is measured using a fast 2D infrared camera. The heat flux distribution is described by an exponential power fall-off length and a diffusive broadening in the divertor region . In this paper the parameters, and , for the inner and outer divertor target are compared for both toroidal magnetic field directions. For the divertor broadening no dependence on the toroidal magnetic field direction is observed. The comparison between the near scrape-off layer electron temperature fall-off length and the power fall-off length are in agreement with the 2-point model. It is concluded that electron conduction is the main contribution for the scrape-off layer parallel transport in these discharges. The ratio between inner, , and outer, , power fall-off length is dependent on the toroidal magnetic field direction. The numerical values are for favourable ion drift direction and / for non-favourable drift direction. The different ratios are explained by vertical drifts, which are dependent on the toroidal magnetic field direction.

The HL-2A tokamak has a very closed divertor geometry, and a new infrared camera has been installed for high resolution studies of edge-localized mode (ELM) heat load onto the outer divertor targets. The characteristics of power deposition patterns on the lower outer divertor target plates during ELMs are systematically analysed with infrared thermography. The ELM energy loss is in the range of 3%–8% of the total plasma stored energy. The peak heat flux on the outer divertor targets during ELMs currently achieved in HL-2A is about 1.5–3.2 MW m−2, the wetted area is about 0.5–0.7 m2, and the corresponding integrated power decay length at the midplane is about 25–40 mm. The rise time of the ELM power deposition is in the range of about 100 μs to 400 μs, and the decay time is typically 1.5 to 4 times longer than the corresponding rise time. Convective transport along open field lines during the ELM rise phase from the midplane towards the divertor targets is implied due to the correlation of parallel transport time in the scrape-off layer (SOL) and ELM power rise time. The peak ELM energy fluence is compared with those predicted by models and with experimental data from JET, ASDEX Upgrade, MAST, and COMPASS. The results, as a whole, show a good agreement.

The effect of divertor magnetic balance on H-mode performance in DIII-D

The Surface Eroding Thermocouple (SETC) is a robust diagnostic utilized in DIII-D to provide fast, edge-localized modes (ELMs) resolved heat flux measurements, in particular in geometric regions that are too shadowed for traditional infrared thermography. In order to further investigate the power dissipation in the divertor region, a combination of flush-mounted and recessed SETCs was developed to assess the effect on surface heating from non-charged particles at the divertor target. Utilizing the Divertor Materials Evaluation System sample exposure platform, the first demonstration of the feasibility of using this new method to distinguish between the heat flux from charged particles and that from neutrals and radiative heating was achieved. This paper details the process of using the combination of flush SETCs and recessed SETCs to measure the multiple heat flux components at the divertor target and further discusses how to determine two important ratios, α (ratio of heat flux from charged particles deposit on recessed SETC to that deposit on flush SETC) and β (ratio of heat flux from non-charged particles deposit on recessed SETC to that deposit on flush SETC), in the estimation of the heat flux from non-charged particle sources. Using a time dependent ratio α, it was found that ∼50% of the total incident heat flux is attributable to the non-charged particles in the fully detached open divertor in DIII-D. Finally, the new application of similar SETC diagnostics in the Small Angle Slot divertor with a V-like configuration and partial tungsten coated surface (SAS-VW) is also introduced.

To better understand divertor detachment and asymmetry in the Experimental Advanced Superconducting Tokamak (EAST), drift modeling via the comprehensive edge plasma code SOLPS-ITER of neon impurity seeded plasmas in favorable/unfavorable toroidal magnetic field (B T) has been performed. Firstly, electrostatic potential/field (/E) distribution has been analyzed, to make sure that and E are correctly described and to better understand drift-driven processes. After that, drift effects on divertor detachment and asymmetry have been focused on. In accordance with the corresponding experimental observations, simulation results demonstrate that in favorable B T the onset of detachment is highly asymmetric between the inner and outer divertors; and reversing B T can significantly decrease the magnitude of in-out asymmetry in the onset of detachment, physics reasons for which have been explored. It is found that, apart from the well-known E × B drift particle flow from one divertor to the other through the private flux region, scrape-off layer (SOL) heat flow, which is much more asymmetrically distributed between the high field side and low field side for favorable B T than that for unfavorable B T, is also a critical parameter affecting divertor detachment and asymmetry. During detachment, upstream pressure (P u) reduction occurs and tends to be more dramatical in the colder side than that in the hotter side. The convective SOL heat flow, emerging due to in-out asymmetry in P u reduction, is found to be critical for understanding divertor detachment and asymmetry observed in EAST. To better understand the calculated drastic power radiation in the core and upstream SOL, drift effects on divertor leakage/retention of neon in EAST with both B T directions have been addressed for the first time, by analyzing profile of poloidal neon velocity and that of neon ionization source from atoms. This work can be a reference for future numeric simulations performed more closely related to experimental regimes.