Direction Of Toroidal Magnetic Field Research Articles

Reconstruction and interpretation of ionization asymmetry in magnetic confinement via synthetic diagnostics

Strong poloidal refueling asymmetry in the DIII-D tokamak is inferred from line radiation measurements. Synthetic diagnostics in neutral transport modeling coupled to gyrokinetic simulations illuminate implications for the plasma flow profile in the scrape-off layer of single-null beam-driven discharges. Recycling occurs primarily either on the inner or outer divertor legs, depending on the toroidal magnetic field direction. By reversing the toroidal magnetic field, the observed line radiation asymmetry is nearly eliminated or reversed. It is determined that, while relatively simple physics can describe the observed ionization asymmetry, predicting the overall brightness of the hydrogenic Lyman-α signal requires detailed simulation of the plasma and resulting turbulence. To this end, kinetic plasma simulations fully coupled to comprehensive neutral transport calculations—a novel capability—provide first-principles reproduction of Lyman-α observations on DIII-D.

SOLPS-ITER drift modeling of neon impurity seeded plasmas in EAST with favorable and unfavorable toroidal magnetic field direction

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.

Modeling study of divertor particle flow pattern and in–out plasma density asymmetry due to drifts with SOLPS and BOUT++

A study of the effects of drifts on the particle flow pattern and in–out divertor plasma density asymmetry for L-mode and H-mode plasmas is carried out for EAST discharges by the edge plasma transport codes SOLPS and BOUT++ . The simulation of L-mode plasmas is done by SOLPS while the simulation of H-mode plasmas is done by BOUT++ . The toroidal magnetic field direction for the simulated discharge is artificially reversed in the codes to study the effects of different drift directions on the divertor particle flow pattern and the in–out asymmetry of divertor plasma density. The divertor particle flows induced by diamagnetic and E × B drifts are found to have similar directions in the divertor region for the same discharge. The directions of the flows induced by drifts would be reversed with the reverse of toroidal magnetic field direction. The diamagnetic drift seems to have no effect on the in–out asymmetry of divertor plasma density due to its divergence-free nature. However, the E × B drift could result in a pronounced asymmetry of plasma density between the inner and outer divertor targets. The density in–out asymmetry caused by E × B drift is reversed with the reverse of E × B drift flow direction. Detailed analysis shows that the radial component of the E × B drift flow is the main cause of density asymmetry. The results from the simulation of H-mode plasmas with BOUT++ are similar to those of the L-mode plasmas with SOLPS except that the drift effects seem to be slightly larger in the H-mode plasmas compared to the L-mode plasmas.

Validation of edge turbulence codes against the TCV-X21 diverted L-mode reference case

Self-consistent full-size turbulent-transport simulations of the divertor and scrape-off-layer (SOL) of existing tokamaks have recently become feasible. This enables the direct comparison of turbulence simulations against experimental measurements. In this work, we perform a series of diverted ohmic L-mode discharges on the tokamak à configuration variable (TCV) tokamak, building a first-of-a-kind dataset for the validation of edge turbulence models. This dataset, referred to as TCV-X21, contains measurements from five diagnostic systems from the outboard midplane (OMP) to the divertor targets—giving a total of 45 one- and two-dimensional comparison observables in two toroidal magnetic field directions. The experimental dataset is used to validate three flux-driven 3D fluid-turbulence models—GBS, GRILLIX and TOKAM3X. With each model, we perform simulations of the TCV-X21 scenario, individually tuning the particle and power source rates to achieve a reasonable match of the upstream separatrix value of density and electron temperature. We find that the simulations match the experimental profiles for most observables at the OMP—both in terms of profile shape and absolute magnitude—while a comparatively poorer agreement is found towards the divertor targets. The match between simulation and experiment is seen to be sensitive to the value of the resistivity, the heat conductivities, the power injection rate and the choice of sheath boundary conditions. Additionally, despite targeting a sheath-limited regime, the discrepancy between simulations and experiment also suggests that the neutral dynamics should be included. The results of this validation show that turbulence models are able to perform simulations of existing devices and achieve reasonable agreement with experimental measurements. Where disagreement is found, the validation helps to identify how the models can be improved. By publicly releasing the experimental dataset and validation analysis, this work should help to guide and accelerate the development of predictive turbulence simulations of the edge and SOL.

Determination of the characteristic magnetic pre-sheath length at divertor surfaces using micro-engineered targets on DiMES at DIII-D

The magnetic pre-sheath (MPS) length, L MPS, is a critical parameter to define the sheath potential, which controls the ion trajectory of low-Z species (D, T, He, and C), as well as the prompt re-deposition of high-Z species. To determine L MPS, we fabricated micro-trenches (30 × 30 × 4 μm) via focused ion beam milling on a silicon surface and exposed them to L-mode deuterium plasmas in DIII-D via the divertor material evaluation system (DiMES) removable sample exposure probe. The areal distribution of impurity depositions, mainly consisting of carbon, was measured by energy-dispersive x-ray spectroscopy (EDS) to reveal the deuterium ion shadowing effect on the trench floors. The carbon deposition profiles showed that the erosion was maximized for the azimuthal direction of φ = −40° (referenced to the toroidal magnetic field direction) as well as the polar angle of θ = 80°. A Monte Carlo equation-of-motion (EOM) model, based on a collisionless MPS, was used to calculate the azimuthal and polar deuterium ion angle distributions (IADs) at the surface for a range of L MPS = k × ρ i, where ρ i is the ion gyro radius and k = 0.5–4. Then, gross erosion profiles were calculated by a Monte Carlo micro-patterning and roughness (MPR) code for ion sputtering using as input the calculated azimuthal and polar IADs for each value of k. Good agreement with the experimental C deposition profiles was obtained for the case k = 2.5–3.5. This result is consistent with a previous kinetic modeling prediction of k ∼ 3, as well as previous analytical investigations that predicted the L MPS to be several ion gyro radii. A validation of theoretical sheath models supports its applicability to ITER and pilot plant divertors to successfully predict plasma–materials interactions.

Assessment of particle and heat loads to the upper open divertor in ASDEX Upgrade in favourable and unfavourable toroidal magnetic field directions

Abstract Pairs of ASDEX Upgrade L-mode discharges with the toroidal magnetic field, BT, in the forward and reverse directions have been used to study the impact of neoclassical drifts on the divertor plasma conditions and detachment. The evolution of the peak heat flux and the total power loads onto both the outer and the inner targets depends significantly on the toroidal field direction: increasing the core plasma density affects mainly the heat loads in the BT 0 (favourable). Ion saturation current measurements show similar trends to those of the IR heat flux data. These discrepancies are not only caused by drifts but also by different levels of radiated power in the core, thus the power across the separatrix, Psep. Tomographic reconstructions show that Psep is not constant within the entire dataset. Finally, at I p = 0.8 MA , a significant reduction of the peak heat flux is observed at both targets for both field directions. On the other hand, at I p = 0.6 MA , a reduction of the peak heat flux is only observed for BT I p = 0.8 MA .

Effects of drifts on divertor plasma transport in LHD

An asymmetric plasma particle load on divertor tiles located at positions of symmetric magnetic field lines structure has been investigated in the Large Helical Device (LHD). The asymmetry reverses with reversing the toroidal magnetic field direction. This suggests that drifts affect the asymmetry. It is found that the degree of the asymmetry depends on the electron temperature in the edge plasma and on the toroidal magnetic field strength. The degree of the asymmetry becomes large and then saturates with an increase of electron temperature. In the lower magnetic field discharge, the degree of the asymmetry is larger. The mass dependence of the asymmetry is not clear among hydrogen, deuterium and helium. Results presented suggests that E × B drift plays a role in formation of the asymmetry.

Enhanced drift effects in the TCV snowflake divertor

Measurements of various plasma parameters at the divertor targets of snowflake (SF) and conventional single-null configurations indicate an enhanced effect of the drift in the scrape-off layer of plasmas in the SF configuration. Plasma boundary transport simulations using the EMC3-Eirene code show that the poloidal gradients of the kinetic profiles in the vicinity of the null-point of a SF divertor are substantially larger than those of a conventional single-null configuration. These gradients are expected to drive larger flows in the SF divertor and are thought to be responsible for the formation of the double-peaked particle and heat flux target profiles observed experimentally. Experiments in forward and reversed toroidal magnetic field directions further support this conclusion. The formation of such a double-peaked profiles is enhanced at higher plasma densities and may have beneficial effects on the divertor heat loads since they lead to broader target profiles and lower peak heat fluxes.

Change of the scrape-off layer power width with the toroidal B-field direction in ASDEX upgrade

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.

Characterization of the poloidal asymmetries in the ISTTOK edge plasma

Edge plasma in/out and up/down asymmetries have been directly investigated on ISTTOK with a probe system that simultaneously samples the plasma at four poloidal angles. An experimental investigation of the asymmetries is presented as a function of the direction of plasma current and toroidal magnetic field. The edge plasma parameters show significant in/out asymmetries in both equilibrium and fluctuating parameters. Asymmetries favoring the outboard side are observed in density, turbulent particle flux and diffusion coefficient independently of the toroidal magnetic field and plasma current directions, suggesting a ballooning-like transport. This interpretation is supported by the characteristics of the fluctuations as they are found to be more intermittent on the outboard side. In addition, smaller up/down asymmetries reversing with the reversal of the toroidal magnetic field direction and favoring the ion ∇B drift direction are found. Significant poloidal asymmetries were also measured in the parallel flow that can be attributed mainly to Pfirsch–Schluter flows.

Simulation of small size divertor tokamak plasma edge under effect of toroidal magnetic field reversal

Asymmetries between the divertor legs of small size divertor (SSD) tokamak plasma edge are noticed to reverse when the direction of toroidal magnetic field is reversed. In the present paper the small size divertor tokamak plasma edge under effect of toroidal magnetic field reversal is simulated by B2SOLPS0.5.2D fluid transport code. The simulation demonstrate the following results: 1) Parallel (toroidal) flow flux and Mach number up to 0.6 at higher plasma density reverse with reverse toroidal magnetic direction in the edge plasma of small size divertor tokamak. 2) The radial electric field is toroidal magnetic direction independence in edge plasma of small size divertor tokamak. 3) For normal and reverse toroidal magnetic field, the strong ITB is located between the positions of the maximum and minimum values of the radial electric field shear. 4) Simulation result shows that, the structure of radial electric field at high field side (HFS) and low field side (LFS) is different. This difference result from the change in the parallel flux flows in the scrape off layer (SOL) to plasma core through separatrix. 5) At a region of strong radial electric field shear, a large reduction of poloidal rotation was observed. 6) The poloidal rotation is toroidal magnetic field direction dependence.

Poloidal asymmetries in the ISTTOK edge plasma

The ISTTOK edge plasma parameters show significant poloidal asymmetries in both equilibrium and fluctuating parameters. Asymmetries favoring the outboard side are observed in density, turbulent particle flux and diffusion coefficient independently of the toroidal magnetic field direction, suggesting a ballooning-like transport. This interpretation is supported by the characteristics of the fluctuations as they are found to be more intermittent on the outboard side. In addition, up/down asymmetries reversing with the reversal of the toroidal magnetic field direction are found. Significant poloidal asymmetries were also measured in the parallel flow that can be attributed mainly to Pfirsch–Schluter flows.

ICRF antenna-plasma interactions and its influence on W sputtering in ASDEX upgrade

Analysis of the W concentration during ICRF over AUG experimental campaigns confirms the critical role of W antenna limiters for the W content in plasma, though other structures connected to antennas along magnetic field lines cannot be neglected as W sources.Abrupt changes of spectroscopically measured W sputtering patterns are observed which correlate with step-wise changes of connection lengths at antenna limiters. Analysis of discharges with the reversed direction of toroidal magnetic field shows less W release compared to identical discharges with the normal direction. The lower W release is accompanied by lower intensity of fluctuations of reflected ICRF power in the 1–60kHz range. The observations suggest that local magnetic geometry and density convection at the antennas are at least as important for the W sputtering as the distribution of RF near-fields at the antenna.Measurements of DC currents flowing through the antenna limiters show that the limiters at the active antenna collect predominantly negative DC currents whereas those distant from the active antenna collect predominantly positive DC currents. The latter decrease and become more negative when the intensity of the RF pickup measured at the limiters increases. The mutual compensation between the positive and negative currents can lead to lower values of the DC current than those expected from simplified theoretical models of the RF/DC circuit.

Simulation of Electric Fields in Small Size Divertor Tokamak Plasma Edge

The fluid simulation of Small Size Divertor Tokamak (SSDT) plasma edge by the B2-SOLPS5.0 2D [1] transport code gives the following results: First, in the vicinity of separatrix the radial electric field result is not close to the neoclassical electric field. Second, the shear of radial electric field is independent on plasma parameters. Third, switching on poloidal drifts (E×B and diamagnetic drifts) leads to asymmetric parallel and poloidal fluxes from outer to inner plates and upper part of SOL for normal direction of toroidal magnetic field. Fourth, for the normal direction of toroidal magnetic, the radial electric field of SSDT is affected by the variation in temperature heating of plasma. Fifth, the parallel flux is directed from inner to outer plate in case of discharge without neutral beam injection (NBI).

EDGE2D code simulations of SOL flows and in–out divertor asymmetries in JET

EDGE2D simulations show that high SOL flows can be generated when an additional radial convective transport is applied in the SOL and pedestal region along with a ‘ballooning-like’ poloidal variation in transport. This model produces an inward particle flux at the inboard side of the plasma along with an enhanced outward flux at the outer mid-plane. Parallel Mach numbers can be produced at the top of the machine which are comparable in magnitude to those observed by the JET Mach probes. For the normal toroidal magnetic field direction, the parallel flow Mach number is about M=0.32 in this case. Applying these two additional mechanisms also allows experimental outer to inner target power and out–in divertor line emission intensity asymmetries to be qualitatively reproduced.

Simulation of power and particle flows in the NSTX edge plasma

We simulate edge plasmas in NSTX double-null divertor configurations with the UEDGE two-dimensional fluid code. The carbon impurity density in the core plasma and total radiated power increase with the core heating power and the magnitude of the physical and chemical sputtering in the divertor. Up/down plasma asymmetries are generated by classical cross-field particle drifts. With the standard toroidal magnetic field direction (ion ∇ B drift toward the lower x -point), the highest density occurs at the lower inboard divertor plate and the highest heat flux at the upper outboard divertor plate. Simulations of three-dimensional edge plasma turbulence with the BOUT code show that anomalous radial transport at the outboard midplane should be similar in magnitude for NSTX and DIII-D.

Poloidal SOL asymmetries and toroidal flow in DITE

In this paper an experimental investigation of asymmetries in the DITE plasma boundary will be presented for a limiter configuration consisting of a pumped limiter centred on the outside midplane and an inner bumper limiter displaced ~ 70 ° toroidally. A number of experiments have been performed in which the poloidal variation of the edge T e, n e and j sat has been monitored as a function, principally, of the direction of plasma current and toroidal magnetic field. Poloidal asymmetries in particle flux are observed with the measured flux increasing towards the outside midplane. The poloidal variation of T e is less marked. Considerable e-i side asymmetries are evident in the particle flux which, when interpreted as indicative of net plasma flow, give poloidally varying Mach numbers and flow directions dependent on the toroidal magnetic field direction and line-averaged central density. The persistence of the observed asymmetries when the magnetic field and plasma current directions are reversed show the effect to be related to plasma properties and not simply a consequence of varying connection lengths.

Electron cyclotron wave propagation and absorption in the Compact Ignition Tokamak

The possibility of heating the Compact Ignition Tokamak (CIT) with electron cyclotron power at 280 GHz (fundamental cyclotron resonance) is examined by simple analytic estimates and by detailed computational means. Efficient single-pass absorption of the ordinary polarization is accomplished by scanning the angle of injection of the microwave power from 60° to 90° relative to the toroidal magnetic field direction as the field intensity is ramped from 7 or 8 T to 10 T and the volume average beta increases from 0.4% to 2.8%. Efficient absorption occurs for the ranges of central density of (2-8) × 1020 m−3 and central temperatures of 5-20 keV. Thus, electron cyclotron resonance heating, using either gyrotrons or free-electron lasers, offers a reasonable heating scenario from the Ohmic phase to the ignition phase, even with a single-frequency radiofrequency source.