Innovative Divertor Development to Solve the Plasma Heat-Flux Problem PDF Download

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Languages : en
Pages : 5

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Large, localized plasma heat exhaust continues to be one of the critical problems for the development of tokamak fusion reactors. Excessive heat flux erodes and possibly melts plasma-facing materials, thereby dramatically shortening their lifetime and increasing the impurity contamination of the core plasma. A detailed assessment by the ITER team for their divertor has revealed substantial limitations on the operational space imposed by the divertor performance. For a fusion reactor, the problem becomes worse in that the divertor must accommodate 20% of the total fusion power (less any broadly radiated loss), while not allowing excess buildup of tritium in the walls nor excessive impurity production. This is an extremely challenging set of problems that must be solved for fusion to succeed as a power source; it deserves a substantial research investment. Material heat-flux constraints: Results from present-day tokamaks show that there are two major limitations of peak plasma heat exhaust. The first is the continuous flow of power to the divertor plates and nearby surfaces that, for present technology, is limited to 10-20 MW/m2. The second is the transient peak heat-flux that can be tolerated in a short time, [tau]{sub m}, before substantial ablation and melting of the surface occurs; such common large transient events are Edge Localized Mode (ELMs) and disruptions. The material limits imposed by these events give a peak energy/[tau]{sub m}12 parameter of H"40 MJ/m2s12 [1]. Both the continuous and transient limits can be approached by input powers in the largest present-day devices, and future devices are expected to substantially exceed the limits unless a solution can be found. Since the early 90's LLNL has developed the analytic and computational foundation for analyzing divertor plasmas, and also suggested and studied a number of solid and liquid material concepts for improving divertor/wall performance, with the most recent being the Snowflake divertor concept [2] and generating Resonant Magnetic Perturbations by the SOL currents [3]. However, the specific approaches discussed here are part of a wider class of innovative divertor ideas that have come from the community in the last several years, and we certainly advocate the need to consider a range of options. Indeed, the most effective solution to the heat-flux problem may well contain features of various ideas. For example, there are the X-divertor (Kotschenreuther et al. [4]) that expands the magnetic flux surface in the vicinity of the near-X-point divertor plate, and the super X-divertor (Valanju et al. [5]) that guides the near-separatrix SOL flux tubes to a larger major radius to increase the surface area available for power deposition. These approaches have the common feature of manipulation of the edge magnetic geometry. Another approach is the use of liquid divertor surfaces that can increase the heat-flux capability by flowing the heated material to a cooling region and eventually out of the machine, and/or by being able to withstand a higher peak heat flux [6]. All of these areas are only emerging concepts that require substantially more analysis and definitive experimental tests, and given the need for a large improvement in this area, we advocate a substantial program to systematically assess the approaches. Because of space limitation here, we present some details of one of the concepts, namely the Snowflake divertor configuration. The Snowflake (SF) divertor [2] exploits a tokamak geometry in which the poloidal magnetic field varies quadratically with distance from the X-point null, [Delta]r. The name stems from the characteristic hexagonal, snowflake-like, shape of the multi-branched separatrix for this exact second-order null. In contrast, the standard X-point configuration has a poloidal field varying linearly with?r. The different variations mean that a flux expansion is much larger in the vicinity of a null of a snowflake divertor, and one can try to exploit this fact for reducing the divertor heat load. A unique feature here is also that the shear in the magnetic field near the X-point is substantially larger for the SF configuration, which may favorably affect microinstabilities and ELMs. Practical realization appears straightforward; the SF can be obtained using existing poloidal field coils in various present-day devices, and in general can be produced with coils located well outside the vacuum vessel. The SF configuration increases the flux expansion near the X-point that can be exploited by some increase in the plate wetted area, longer field-line length, and larger volume for impurity radiation. Initial UEDGE simulations comparing the SF with the standard divertor for the same conditions show a reduction in the peak heat flux for the SF of H".2-1.6 when comparing cases for the same angle of the total magnetic field to the divertor plate [7].