Carbon Foam for Beam Stop Application
The Next Generation Light Source (NGLS) is a transformative new user facility currently in the planning stages at DoE. This facility would use a LINAC to accelerate electrons up to 2.5GeV in order to produce the free-electron lasers (FELs) used for the experiments. After the FELs, an approximately 750kW electron beam has to be stopped. This amount of power has been conventionally stopped using water baths (SLAC, CEBAF). Dissipating this amount of energy in water introduces a mobile radionuclide management issue that could be avoided with a solid core beam stop. Largely due to radionuclide issues, DESY (Europes XFEL host) is planning on using conventional nuclear grade graphite solid core to stop ~600kW of power. The graphite stops the electrons, and the associated heat is conducted to a copper shell that is eventually cooled with water. However, the density of the graphite at ~1.8 g/cc prevents the use of a single beam stop; it would get too hot and requires active switching between two separate dumps. Active switching at NGLSs high repetition rates would be a very challenging task. Conductive POCO graphite foam, pioneered by ORNL in the late1990s, was initially thought to offer a promising solution to this beam stop problem. Graphite foam, available at density from 0.1 to 0.9 g/cc, certainly increases the radiation length as desired. As a result the energy dissipation per unit length of the dump (the average heat flux) is reduced. Preliminary analysis indicates that graphitic foams (in an XFEL like copper jacket) would allow the use of a single foam core dump per high power beam line. However, the commercially available graphite foams, like POCO, are anisotropic, with thermal conductivities much higher in one direction. These foams do not have the ideal properties for the NGLS application, due to a combination of available densities, and the level of anisotropy. This would result in a worst case, material failure, and less efficient heat transfer. Allcomps isotropic low-density, high conductivity foam has unique potential for solving this problem. Being highly thermally conductive and tailorable in density, it is possible to reach the appropriate beam stop design parameters. As a demonstration in Phase I of its uniqueness, Allcomp proposes to produce a higher density version of its foam and to test foam specimens in the temperature regime of the beam stop operation. Foam blocks of relevant size, in diameter and length, will be produced and foam properties will be established and compared to predictions. In Phase II irradiation tests will be performed and any effects of radiation displacement damage will be factored into the beam stop design analysis. Commercial Applications: In addition to this beam stop application Allcomps high conductivity foam opens new opportunities in commercial applications such as a heat spreader in computers, avionics racks, and spaced-based electronics. Next generation inner tracking detectors are evaluating POCO foam for cooling stave like structures. Our isotropic highly machinable graphitic foam is an ideal replacement. Accelerator-based experiments will also be a commercial outlet for this material.
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