Concept of the Beam Exit and Entrance Windows for the TESLA Water based Beam Dumps.pdf

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Concept of the Beam Exit and Entrance Windows for the TESLA Water based Beam Dumps.pdf

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Concept of the Beam Exit and Entrance Windows for the TESLA Water based Beam Dumps

DESY, February 2001, TESLA Report 2001-07 Concept of the Beam Exit and Entrance Windows for the TESLA Water based Beam Dumps and its related Beam Lines M. Maslov, M. Schmitz, V. Sytchev 1Concept of the Beam Exit and Entrance Windows for the TESLA Water based Beam Dumps and its related Beam Lines M. Maslov, M. Schmitz, V. Sytchev Introduction and Requirements In the context of the beam abort systems for the TESLA project, beam windows are required for two purposes. The beam leaves the vacuum system through an exit window at the end of the abort line before it enters the water absorber through a separate entrance window. These windows are heated by about 1 ms long beam pulses (bunch trains), which have a spot size in the order of ≈1 mm2 and carry a total number of Nt particles ranging between 13104 ? at the Tesla Test Facility (TTF) and 13102.7 ? for TESLA FEL operation parameters. They repeat at a rate of ν (5 to 10 Hz), which results in an overall average beam current ν??= tave NeI in the range of 45μA (TESLA main linac) to 64μA (TTF). Typically the window thickness is chosen to be small compared to its radiation length X0, thus shower development can be neglected. In that case energy deposition in the window is dominated by ionisation losses and therefore independent of the incident particle energy. As shown later, thermal diffusion within the bunch train passage time of 1 ms is negligible. Each bunch train causes an instantaneous temperature jump ?Tinst in the window, which, for a given material, only depends on the incident particle density dN/dA, i.e. spot size and Nt. After the passage of the bunch train this temperature decays to a certain amount until the next one arrives. Therefore in steady state an equilibrium temperature rise ?Teq is reached, around which level the temperature varies with time in a sawtooth-like manner. The equilibrium temperature rise is determined by the average beam current Iave , the beam size and the heat transport towards the hea