Double-null Discharges Research Articles

SOLPS-ITER modeling of divertor detachment in MAST-U’s super-X double null configuration and comparison to experiment

The MAST-U tokamak has recently undergone upgrades to investigate any detachment advantages of the “Super-X Double-Null” magnetic configuration, a concept featuring up-down symmetry with two magnetic nulls and elongated divertor legs with strong wall baffling. This study presents SOLPS-ITER simulations of MAST-U Super-X Double Null discharges, in H-mode conditions, compared to initial 2D imaging measurements of divertor detachment characteristics. In the simulations, the Super-X divertor targets achieve detachment at a substantially lower upstream separatrix density compared to a shorter leg, conventional configuration (CD). As the density increases further in the Super-X configuration, the cold detached front moves upstream towards the X-point. Its speed in the poloidal plane, calculated as the change in front location divided by the corresponding increase in upstream plasma density, decreases by a factor of ∼ 3–4 as the front transitions from the baffled divertor chamber to the main plasma chamber, reaching a speed comparable to that observed in the CD configuration. In a chosen experimental Super-X density ramp discharge, the ionization front, identified using D2 Fulcher emission as a proxy, is already displaced from the target at the start of the divertor fueling ramp, whereas modeling of the same initial phase shows the ionization front still close to the target. As divertor fueling increases, both experiment and modeling show the ionization front moving further upstream towards the divertor throat. However, the associated variation in upstream density is ∼ 2 times larger in the modeling compared to the experiment, resulting in a broader detachment window. Comprehensive modeling efforts are underway to address these discrepancies, with several inaccurate modeling assumptions identified as potential causes of the deviation.

High-field side scrape-off layer investigation: Plasma profiles and impurity screening behavior in near-double-null configurations

New experiments on Alcator C-Mod reveal that the favorable impurity screening characteristics of the high-field side (HFS) scrape-off layer (SOL), previously reported for single null geometries, is retained in double null configurations, despite the formation of an extremely thin SOL. In balanced double-null, nitrogen injected locally into the HFS SOL is better screened by a factor of 2.5 compared to the same injection into the low field side (LFS) SOL. This result is insensitive to plasma current and Greenwald fraction. Nitrogen injected into the HFS SOL is not as well screened (only a factor of 1.5 improvement over LFS) in unbalanced double-null discharges, when the primary divertor is in the direction of B×∇B. In this configuration, impurity ‘plume’ emission patterns indicate that an opposing E × B drift competes with the parallel impurity flow to the divertor. In balanced double-null plasmas, the dispersal pattern exhibits a dominant E × B motion. Unbalanced discharges with the primary divertor opposite the direction of B×∇B exhibit excellent HFS screening characteristics – a factor of 5 enhancement compared to LFS. These data support the idea that future tokamaks should locate all RF actuators and close-fitting wall structures on the HFS and employ near-double-null magnetic topologies, both to precisely control plasma conditions at the antenna/plasma interface and to maximally mitigate the impact of local impurity sources arising from plasma-material interactions.

Resistive MHD simulation of edge-localized-modes for double-null discharges in the MAST device

Recent development of the nonlinear magneto hydrodynamic (MHD) code JOREK has enabled the alignment of its two-dimensional finite-element grid along poloidal flux surfaces for double-null Grad–Shafranov equilibria. In previous works with the JOREK code, only single X-point plasmas were studied. The fast-camera diagnostic on MAST, which gives a global view of the pedestal filamentation during an ELM crash, clearly shows filaments travelling far into the scrape-off layer, as far as the first wall. Simulation of such a filament dynamics in MAST double-null plasmas is presented here and compared with experimental observations. In addition to direct comparison with the fast-camera images, general aspects of filaments are studied, such as their radial speed and composition. A qualitative validation of simulations is carried out against other diagnostics, such as the Thomson-scattering profiles or the infra-red camera images. Simulations are found to reproduce experimental edge localized modes in a reasonable manner, with similar energy losses and divertor heat-flux profiles. However, the MHD model used for those simulations is a reduced MHD model, which is likely approaching the limit of its applicability for the MAST device. Also, the absence of diamagnetic drift terms in the present MHD model results in nonlinear simulations being dominated by the highest mode number, and thus coupling with lower mode numbers is not observed.

Comparison of small edge-localized modes on MAST and ASDEX Upgrade

Results from a set of ITPA-coordinated experiments on ASDEX Upgrade and MAST to compare the characteristics of small edge-localized modes (ELMs) are presented. In MAST a small ELM regime is established in connected double null discharges in a limited region of normalized pedestal pressure and collisionality. Type II ELMs on ASDEX Upgrade have high frequency and low energy loss and occur at high triangularity and close to double null. On both devices the transition from type I to small ELMs is connected with a similar threshold value of the pedestal collisionality. For the first time the temporal and spatial evolution of the filament structures observed during these small ELMs have been studied. The radial and toroidal velocities of the filaments in these small ELMs are slower compared with type I ELMs on both devices. The observations are compatible with the filaments in small ELMs originating closer to the last closed flux surface than is the case in type I ELMs. The toroidal mode number of the small ELMs, derived from the temporal evolution of the filaments, is typically a factor of two larger than for type I ELMs. The small ELMs on MAST have sufficient similarities to type II ELMs on ASDEX Upgrade that they should be classified as the same.

Spontaneous Toroidal Rotation in Alcator C-Mod Plasmas with No Momentum Input

Spontaneous toroidal rotation of impurity ions has been observed in the core of Alcator C-Mod plasmas with no external momentum input. The magnitude of the rotation ranges from -60 km/s (countercurrent) in limiter L-mode discharges to +140 km/s (cocurrent) in ion cyclotron range of frequencies-heated H-mode plasmas. The core rotation in L-mode plasmas is generally countercurrent and is found to depend strongly on the magnetic topology; in near double null discharges, the core rotation changes by 25 km/s with a variation of a few millimeters in the distance between the primary and secondary separatrices. In H-mode plasmas, the rotation increments in the cocurrent direction with the toroidal rotation velocity increase proportional to the corresponding stored energy increase, normalized to the plasma current. These discharges exhibit a positive Er in the core. Immediately following the transition from L-mode into enhanced Dα (EDA) H-mode, the cocurrent rotation appears near the plasma edge and propagates to the center on a time scale similar to the energy confinement time but much less than the neoclassical momentum diffusion time, indicating both the role of the plasma boundary in the dynamics of the H-mode transition and the anomalous nature of momentum transport. Rotation velocity profiles are flat in EDA H-mode plasmas and centrally peaked for edge-localized mode-free H-modes, demonstrating the effects of an inward momentum pinch. In EDA H-mode discharges that develop internal transport barriers, the core toroidal rotation inside the barrier foot is observed to drop on a time scale similar to the core pressure profile peaking (hundreds of milliseconds), indicating a negative Er well in the core region.

H-mode pedestal, ELM and power threshold studies in NSTX

H-mode operation plays a crucial role in National Spherical Torus Experiment (NSTX) research, allowing higher beta limits due to reduced plasma pressure peaking, and long pulse operation due to high bootstrap current fraction. Here, new results are presented in the areas of edge localized modes (ELMs), H-mode pedestal physics and power threshold studies. ELMs of several types as reported by higher aspect ratio tokamaks have been observed: (1) large, Type I ELMs, (2) intermediate-sized Type III ELMs and (3) tiny ELMs. Many high performance discharges in NSTX have the tiny ELMs (recently termed Type V), which have some differences as compared with small-magnitude ELM types in the published literature. A divertor multifaceted axisymmetric radiation from the edge (MARFE) on the inboard leg provides an effective light source to examine the effect of the ELMs on the divertor plasma; it is clear that only the large ELMs burn through the MARFE. The time difference between observation of the ELM flux at the outer and inner targets is substantially longer for the smallest ELMs as compared with the large ELMs. In addition, the visible light patterns show ‘finger-like’ striations during the tiny ELMs. H-mode pedestal studies have commenced, with the observation that the pedestal contains between 25% and 33% of the total stored energy, and the NSTX pedestal energy agrees reasonably well with a recent international multi-machine scaling. A power threshold identity experiment between NSTX and the Mega-Amp Spherical Tokamak shows comparable loss power at the L–H transition in balanced double-null discharges. Both machines require more power for the L–H transition as the balance is shifted toward lower-single null. High-field side gas fuelling enables more reliable H-mode access in NSTX, but does not always lead to a lower power threshold, e.g. with a reduction of the duration of early heating.

Plasma profiles and flows in the high-field side scrape-off layer in Alcator C-Mod

A novel, magnetically-driven swing probe was recently installed near the midplane on the high-field side SOL in Alcator C-Mod. The probe collects plasma from co- and counter-current directions during its respective 0–90° and 90–180° of motion, thus providing profiles of density, electron temperature and plasma flow parallel to magnetic field lines (Mach number, M∥) up to the separatrix. Results are reported from discharges with different magnetic topologies: lower single-null, upper single-null, and double-null. In single-null, a strong parallel flow (∣M∥∣∼1) is detected, which is always directed from the low- to high-field SOL. In double-null discharges, e-folding lengths in the high-field SOL are a factor of ∼4 shorter than the low-field SOL. Thus, plasma appears to ‘fill-in’ the high-field SOL in single-null plasmas, not by cross-field transport but by parallel flow from the low-field SOL – a picture consistent with a very strong ballooning-like component to the cross-field transport.

Accounting of the power balance for neutral-beam heated H-mode plasmas in NSTX

Power balance accountability was investigated for neutral beam heated plasmas in NSTX. Measurements of heat to the divertor strike plates and radiation in the divertor and the core were made as a function of heating power in lower single-null (LSN) and double-null (DN) configurations. Up to 70% of the net input power is accounted in the LSN discharges with ≈15–20% of power lost as fast ions, ≈25–35% incident on the divertor plates, ≈5% radiated in the core, and ≈5–10% radiated in the divertor. In DN discharges, the fast ion loss is comparable to LSN discharges, the core radiated power fraction is ≈10%, and the power deposited in the lower divertor as radiation and heat flux is ≈20–25% of the total. Assuming up/down symmetry, the power accountability in DN is ≈85–90%.

A comparison of mid-plane scrape-off-layer measurements with model predictions in MAST and the calculation of cross-field transport coefficients

The outboard mid-plane values of electron density (ne) and temperature (Te) in the scrape-off-layer (SOL) of MAST are derived from target Langmuir probe measurements using OSM2-EIRENE and also using a simple two-point model. The values are compared with mid-plane measurements of ne and Te from a reciprocating probe and a Thomson scattering system for Ohmic, L- and H-modes single-null divertor (SND) discharges. The cross-field heat (χ⊥) diffusion coefficient at the low field side of SND discharges is derived both using the OSM2-EIRENE model and a two-point model coupled with a simple theory. The results are found to be similar within the errors. Typically χ⊥ values are found to lie in the range 0.5–2.0 m2 s−1 for L-mode and 0.1–0.5 m2 s−1 for H-mode. A comparison is made of the transport coefficients derived at the low and high field sides for a set of connected double-null discharges. These results are used to make preliminary observations of the effect of magnetic fields on SOL transport in MAST.

H-mode pedestal characteristics on MAST

The H-mode pedestal characteristics on the mega ampère spherical tokamak (MAST) are measured in a variety of disconnected double null discharges and the effect of edge localized modes (ELMs) on the pedestal is presented. The edge density pedestal width in spatial co-ordinates is similar on both the inboard and outboard sides. Neutral penetration may be able to explain the density pedestal width but it alone cannot explain the characteristics of the temperature pedestal. The data from MAST can be used to improve temperature pedestal width scalings by extending the ranges in pedestal collisionality, magnetic field, elongation and aspect ratio studied by other machines. Convective transport is found to dominate energy losses during ELMs and the fractional loss of pedestal energy during an ELM on MAST correlates better with SOL ion transit time than with pedestal collisionality.

Effect of gas fuelling location on H-mode access in NSTX

The dependence of H-mode access on the poloidal location of the gas injection source has been investigated in NSTX. We find that gas fuelling from the centre stack midplane area produces the most reproducible H-mode access, with generally the lowest L–H threshold power in the lower single-null configuration. The edge toroidal rotation velocity of C2+ is largest (in the direction of the plasma current) just before the L–H transition with centre stack midplane fuelling and then reverses direction after the L–H transition. Simulation of these results with a guiding centre Monte Carlo neoclassical transport code (XGC) is qualitatively consistent with the trends in the measured velocities. Double-null discharges exhibit H-mode access with gas fuelling from either the centre stack midplane or centre stack top locations, indicating a reduced sensitivity of H-mode access to fuelling location in that shape.

A review of plasma boundary phenomena in the mega ampere spherical tokamak

Enhanced diagnostics and an expanded operational space in Mega Ampère Spherical Tokamak (MAST) has lead to a wealth of data on the plasma boundary becoming available over the last year of operations and has significantly advanced the understanding of this region in the spherical tokamak and, more generally, is contributing to an improved understanding of the underlying plasma phenomena for this important region of the tokamak. Amongst the observations reported in this paper are that MAST is able to achieve simultaneous high confinement and high density with Type III ELMs for connected double-null discharges in which most (>93%) of the power efflux from the core observed at the targets (including during ELMs) arrives at the large wetted area outboard targets and is evenly distributed between the upper and lower divertor regions. Significant particle fluxes are reported well beyond the outboard separatrix during ELMs (up to 20 cm distant from the plasma) and appear to be associated with the radial expansion of a localised structure at around 1 km s −1. The result of preliminary experiments to broaden the SOL by asymmetric divertor biasing are also presented which show an encouraging, qualitative agreement with theory and demonstrate clear evidence for both SOL broadening and target power reduction.

Particle transport in diverted TdeV

The particle confinement time of ohmic double-null discharges in Tokamak de Varennes (TdeV) is determined by two different techniques, the conventional method and a new technique based on the temporal decay of the total core population following the injection of a gas puff. Both methods show a confinement time increasing with density up to a maximum of 13 ms at and decreasing at larger densities, with very little dependence on plasma current. Particle transport is analysed using fast gas puffing and Abel inversion of the seven-chord submillimetre (SMM) interferometer together with the source profiles determined by measurements. The incremental transport coefficients are obtained by testing the standard form of the particle flux function against the data during the transitory period towards equilibrium. Both perturbed diffusion and convection coefficients are found to vary approximately as the inverse of the density and almost proportionally to the plasma current. The equilibrium transport coefficients are then deduced from the experimental equilibrium density profiles and the measured incremental coefficients using a transport model developed from the data. The model is finally used to predict confinement times to be compared with experiment. The effect of divertor plate biasing on transport is also discussed.

Ohmic H-modes in the TCV tokamak

The TCV tokamak has obtained ohmic H-modes in virtually all diverted plasmas with the ion drift directed towards an X-point and in several elongated limiter plasmas. Troyon factors up to 2 and line average densities up to , corresponding to Ithe Greenwald limit, have been obtained in diverted ELM-free H-modes. Quasi-stationary H-modes lasting for the entire current flat top (1.5 s) have been obtained in the presence of regular ELMs. The occurrence and magnitude of ELMs have been found to depend on configurational parameters such as the position of the `active' X-point in unbalanced double-null discharges and the plasma - wall separation in single-null discharges. These dependencies have permitted active control of ELM behaviour in TCV. A continuous spectrum of ELM amplitudes and frequencies has been observed, ranging from clearly identifiable type III ELMs to large, low-frequency ELMs which expel up to 12% of the stored energy and up to 7% of the particle content and are reminiscent of type I ELMs. A previously unknown, benign kind of ELM, with a maximum amplitude in the divertor region, has also been observed.

Study of the conditions for spontaneous H(high)-mode transitions in DIII-D

A series of scaling studies attempting to correlate the H(high)-mode power threshold (PTH) with global parameters have been conducted. Data from these discharges is also being used to look for dependence of PTH on local edge parameters and to test theories of the transition. Boronization and better operational techniques have resulted in lower-power thresholds and weaker density scaling. Neon impurity injection experiments show that radiation also plays a role in determining PTH. A low-density threshold for the L(low)–H(high) transition has been linked with the locked mode low-density limit, and can be reduced with the use of an error field correcting coil. Highly developed edge diagnostics, with spatial resolution as low as 5 mm, are used to evaluate how the power threshold depends on local edge conditions. Preliminary analysis of local edge conditions for parameter scans of ne, BT, and Ip in single-null discharges, and the X-point imbalance in double-null discharges show that, just before the transition to the H mode, the edge temperatures near the separatrix are approximately constant at 100<Ti<220 eV and 35<Te<130 eV, even though the threshold power varied from 1.5 to 14 MW. During a density scan, the edge ion collisionality, ν*i, varied from 2 to 17, demonstrating that a transition condition as simple as ν*i=const is inconsistent with the data. The local edge parameters of ne, Te, and Ti do not always follow the same global scaling as PTH. Therefore, theories of the L–H transition need not be constrained by these scalings.

Ohmic H-mode and confinement in TCV

The unique flexibility of TCV for the creation of a wide variety of plasma shapes has been exploited to address some aspects of tokamak physics for which the shape may play an important role. The electron energy confinement time in limited ohmic L-mode plasmas whose elongation and triangularity have been varied ( kappa =1.3-1.9, delta =0.1-0.7) has been observed to improve with elongation as kappa 0.5 but to degrade with triangularity as (1-0.8 delta ), for fixed safety factor. Ohmic H-modes have been obtained in several diverted and limited configurations, with some of the diverted discharges featuring large ELMs whose effects on the global confinement have been quantified. These effects depend on the configuration: in double null (DN) equilibria, a single ELM expels on average 2%, 6% and 2.5% of the particle, impurity and thermal energy content respectively, whilst in single null (SN) configurations, the corresponding numbers are 3.5%, 7% and 9%, indicative of larger ELM effects. The presence or absence of large ELMs in DN discharges has been actively controlled in a single discharge by alternately forcing one or other of the two X-points to lie on the separatrix, permitting stationary density and impurity content (Zeff=1.6) in long H-modes (1.5 s).

Interior microturbulence characteristics during H- and VH-mode in the DIII-D tokamak

Measurements of interior microturbulence in the DIII-D tokamak have indicated that during double null discharges with low radiated power and strong neutral beam injection power, interior microturbulence is observed to reduce prior to the onset of the so-called VH-mode, when the peak energy confinement time is greater than two times that predicted by the JET/DIII-D H-mode scaling law. The core broadband turbulence level begins to modulate at frequencies of hundreds of Hertz, and over 100-200 ms following the L to H transition, the modulation period increases while the average fluctuation level reduces. The bursts are associated with momentum transfer events because the toroidal rotation increases significantly after their suppression. The associated Er*B convective drift increases significantly as well. During this period, confinement increases continuously and is often observed to peak several hundred ms after the modulation ends. The question arises as to whether shear flow stabilization might be reducing core turbulence in VH-mode and reducing the fluctuation driven energy transport. In recent experiments, it has been shown that independently reducing Er in the core limits the fluctuation reduction and prevents access to the VH-mode regime.

Experimental survey of the L-H transition conditions in the DIII-D tokamak

We present the global analysis of a recent survey of the H-mode power threshold in DIII-D using D degrees to D+ NBI after boronization of the vacuum vessel. Single parameter scans of BT, Ip, density, and plasma shape have been carried out on the DIII-D tokamak for neutral beam heated single-null and double-null diverted plasmas. In single-null discharges, the power threshold is found to increase approximately linearly with BT and ne but remains independent of Ip. In double-null discharges, the power threshold is found to be approximately independent of both BT and ne. Various shape parameters such as plasma-wall gaps had only a weak effect on the power threshold. Imbalancing the double null configuration resulted in a large increase in the threshold power.

Impurity fluxes in ASDEX studied with a solid graphite collector probe

A high-purity graphite probe for collecting impurities in ASDEX has been used for the first time. The probe head consists of a rotatable graphite cylinder housed in a fixed molybdenum shield containing an exposure slit. Measurements were made on the scrape-off layer plasma streaming to the upper outer divertor plate and, after extraction of the probe, collected impurities were identified and quantified using surface physics techniques. Single null ohmic discharges produce a threefold increase in erosion of the Cu plate of the active upper divertor compared to similiar double null discharges. The radial distributions of scrape-off layer impurities change in both shape and intensity with vertical plasma position. Boronization of the main vessel reduces the amount of Fe collected by more than a factor of two but the oxygen deposit increases. Radial distributions of Cu and Fe deposits as a function of time for a neutral beam heated discharge are also discussed.

Microwave measurements of moving striations in d.c. glow discharge : K.F. Sodomsky, J. Appl. Phys., 34 (7), July 1963, 1860

The divertor of the DIII-D tokamak is being modified to operate as a slot type, disipative divertor. This modification, called the Radiative Divertor Program (RDP) is being carried out in two phases. The design and analysis is complete and hardware is being fabricated for the first phase. This first phase consists of an upper divertor baffle and cryopump to provide some density control for high triangularity, single or double null discharges. Installation of the first phase is scheduled to start in October, 1996. The second phase provides pumping at all four divertor strike points of double null high triangularity discharges and baffling of the neutral particles from transport back to the core plasma. Studies of the effects of varying the slot length and width of the divertor can be easily accomplished with the design of RDP hardware.Static and dynamic analyses of the baffle structures, new cryopumps, and feedlines were performed during the preliminary and final design phases. Disruption loads and differential thermal displacements must be accommodated in the design of these components. With the full RDP hardware installed, the plasma current in DIII-D will be a maximum of 3.0 MA. Plasma disruptions induce toroidal currents in the cryopump, producing complex dynamic loads. Simultaneously, the vacuum vessel vibrations impose a sinusoidal base excitation to the supports for the cryopump. Static and dynamic analyses of the cryopump demonstrate that the stresses due to disruption and thermal loadings satisfy the stress and deflection criteria.