A snowflake divertor: a possible solution to the power exhaust problem for tokamaks

Ryutov, D. D.; Cohen, R. H.; Rognlien, T. D.; Umansky, M.

doi:10.1088/0741-3335/54/12/124050

Cited by 57 publications

(74 citation statements)

References 39 publications

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“…The comparison of the simulated power deposition profiles at SP 3 to the experimental ones, shows that the power flux into the private flux region predicted by the code is at least an order of magnitude too low. The measurement therefore indicates the existence of a transport channel other than diffusion with a spatially constant coefficient, possibly driven by instabilities due to the large value of β p = 2µ 0 p/B 2 θ occurring in the low B θ region around the x-point as proposed by Ryutov, who discussed these in in detail in [24,25]. Drifts may also play a role in the null-point region, however, here the flux surfaces are far away from each other and if the electric potential is constant on a flux surface, the radial electric field at the x-point is expected to be small.…”

Section: σ Scan and Transport Across The Primary Separatrixmentioning

confidence: 68%

First EMC3-Eirene simulations of the TCV snowflake divertor

Lunt

Canal

Feng

et al. 2014

Plasma Phys. Control. Fusion

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Abstract. One of the approaches to solve the heat load problem in a divertor tokamak is the so called 'snowflake' configuration, a magnetic equilibrium with two nearby x-points and two additional divertor legs. Here we report on the first EMC3-Eirene simulations of plasma-and neutral particle transport in the scrape-off layer of a series of TCV snowflake equilibria with different values of σ, the distance between the x-points normalized to the minor radius of the plasma. The constant cross-field transport coefficients were chosen such that the power-and particle deposition profiles at the primary inner strike point match the Langmuir probe measurements for the σ = 0.1 case. At the secondary strike point on the floor, however, a significantly larger power flux than that predicted by the simulation was measured by the probes, indicating an enhanced transport across the primary separatrix. As the ideal snowflake configuration (σ = 0) is approached, the density as well as the radiation maximum are predicted to move from the target plates upwards to the x-point by the simulation.

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Section: σ Scan and Transport Across The Primary Separatrixmentioning

confidence: 68%

First EMC3-Eirene simulations of the TCV snowflake divertor

Lunt

Canal

Feng

et al. 2014

Plasma Phys. Control. Fusion

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“…The cross field flux indicates the regions of high cross field transport, which is an important factor in the effectiveness of novel divertor configurations such as the snowflake [5]. Two dimensional cross correlation at y=1.5m, reference point shown as black dot.…”

Section: Transport and Cross Correlationmentioning

confidence: 99%

“…As a result, there have been studies into alternative concepts which could reduce the heat loads on divertor targets [3,4]. The effectiveness of these configurations, however, are often sensitive to cross field transport in the null region, which is currently poorly understood [5].…”

Section: Introductionmentioning

confidence: 99%

“…Figure 1 also illustrates the shape of the external field used to simulate X-point scenarios. This field was considered to be of the form shown in equation 1 [5], where θ = tan −1 z x is the azimuthal angle. The magnitude and specific geometry of this field can be arbitrarily chosen by altering A 0 , the exponent of r and corresponding coefficient of θ.…”

Section: Introductionmentioning

confidence: 99%

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X-point modelling in linear configurations using BOUT++

Shanahan

Dudson

2014

J. Phys.: Conf. Ser.

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Abstract. Magnetic X-point configurations in tokamak geometries are critical in determining edge and scrape off layer (SOL) dynamics, and hence particle and heat flux onto plasma facing components. Alternative configurations have been proposed which aim to reduce fluxes to material surfaces, but their performance depends on cross-field transport in the region of the null point which is currently poorly understood. There is therefore a need for theoretical and experimental studies of turbulence in X-point magnetic configurations. In conventional 3D turbulence simulations of tokamaks, a field-aligned coordinate system is used, which introduces numerical instabilities at the null point due to zero volume elements. As a result, X-point dynamics are often interpolated based on nearby flux surfaces, which could exclude relevant physics. Here we simulate X-point configurations in linear geometries using a non-field-aligned coordinate system, and present results of 3D drift-wave turbulence and flow simulations in Xpoint configurations using an isothermal model which evolves density, vorticity, parallel velocity and parallel current density. Simulations have been performed to explore the feasibility of experimentally studying X-point configurations in linear plasma devices which indicate that even a modest coil set carrying 300A should produce a measurable effect on the driftwave turbulence and plasma profiles near the null region. The energy dynamics of the system are also explored and indicate that an X-point causes ohmic dissipation in higher mode number turbulence.

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“…10, are an integral part of the physics of the snowflake divertor, with its large area of weak poloidal magnetic field near the second-order null (or two nearby first-order nulls), and they are of importance for standard (single X-point) divertors as well. So, attempting to relate divertor performance solely to the field structure in the common flux region of a single leg (Figs.…”

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confidence: 99%

Comment on “Magnetic geometry and physics of advanced divertors: The X-divertor and the snowflake” [Phys. Plasmas 20, 102507 (2013)]

et al. 2014

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In the recently published paper “Magnetic geometry and physics of advanced divertors: The X-divertor and the snowflake” [Phys. Plasmas 20, 102507 (2013)], the authors raise interesting and important issues concerning divertor physics and design. However, the paper contains significant errors: (a) The conceptual framework used in it for the evaluation of divertor “quality” is reduced to the assessment of the magnetic field structure in the outer Scrape-Off Layer. This framework is incorrect because processes affecting the pedestal, the private flux region and all of the divertor legs (four, in the case of a snowflake) are an inseparable part of divertor operation. (b) The concept of the divertor index focuses on only one feature of the magnetic field structure and can be quite misleading when applied to divertor design. (c) The suggestion to rename the divertor configurations experimentally realized on NSTX (National Spherical Torus Experiment) and DIII-D (Doublet III-D) from snowflakes to X-divertors is not justified: it is not based on comparison of these configurations with the prototypical X-divertor, and it ignores the fact that the NSTX and DIII-D poloidal magnetic field geometries fit very well into the snowflake “two-null” prescription.

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A snowflake divertor: a possible solution to the power exhaust problem for tokamaks

Cited by 57 publications

References 39 publications

First EMC3-Eirene simulations of the TCV snowflake divertor

First EMC3-Eirene simulations of the TCV snowflake divertor

X-point modelling in linear configurations using BOUT++

Comment on “Magnetic geometry and physics of advanced divertors: The X-divertor and the snowflake” [Phys. Plasmas 20, 102507 (2013)]

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