Erosion, Transport, and Tritium Codeposition Issues of a Beryllium Wall Tokamak

Fusion Engineering and Design, 72, 363-375 (2005).

Brooks, J. N., Allain, J. P., Alman, D. A., Ruzic, D. N.

We analyzed beryllium first wall sputtering erosion, sputtered material transport, and T/Be codeposition for a typical next-generation tokamak design—the fusion ignition research experiment (FIRE). The results should be broadly applicable to any future tokamak with a beryllium first wall. Starting with a fluid code scrapeoff layer attached plasma solution, plasma D⁰ neutral fluxes to the wall and divertor are obtained from the DEGAS2 neutral transport code. The D⁺ ion flux to the wall is computed using both a diffusive term and a simple convective transport model. Sputtering coefficients for the beryllium wall are given by the VFTRIM-3D binary-collision code. Transport of beryllium to the divertor, plasma, and back to the wall is calculated with the WBC+ code, which tracks sputtered atom ionization and subsequent ion transport along the SOL magnetic field lines. Then, using results from a study of Be/W mixing/sputtering on the divertor, and using REDEP/WBC impurity transport code results, we estimate the divertor surface response. Finally, we compute tritium codeposition rates in Be growth regions on the wall and divertor for D–T plasma shots using surface temperature dependent D–T/Be rates and with different assumed oxygen contents. Key results are: (1) peak wall net erosion rates vary from about 0.3 nm s⁻¹ for diffusion-only transport to 3 nm s⁻¹ for diffusion plus convection, (2) T/Be codeposition rates vary from about 0.1 to 10.0 mg T s⁻¹ depending on the model, and (3) core plasma contamination from wall-sputtered beryllium is low in all cases (< 0.02%). Thus, based on the erosion and codeposition results, the performance of a beryllium first wall is very dependent on the plasma response, and varies from acceptable to unacceptable.