Stability analyses of the beam tube cooling system in the KATRIN source cryostat

The Karlsruhe Tritium Neutrino Experiment (KATRIN) will measure the mass of the electron antineutrino with a sensitivity of 0.2eV/c2, based on the precise measurement of the T2βT2β spectrum in a region close to the endpoint. This requires a T2T2 source, which can provide 1011β1011β decay electrons per second. The KATRIN source cryostat consists in its centre of a 10 m long beam tube of 90 mm inner diameter, operated at 30 K. Molecular T2T2 is injected in the beam tube through a central injection chamber and pumped at either tube end. The T2T2 density profile must have a stability of 10-310-3 in order to limit the systematic errors, yielding stringent requirements on the beam tube temperature homogeneity and stability of ±30mK. This shall be achieved with a design, where the thermal radiation from the vacuum pumps is almost entirely absorbed by LN2LN2 and He heat exchangers on the pump ports. The beam tube itself is cooled with two-phase tubes that are part of a Ne thermosiphon. After describing the thermal environment of the beam tube, the design parameters and the operational limits of the thermosiphon will be discussed. This is followed by a detailed analysis of its dynamic behaviour, based on experimental data taken in the primary He cooling system. A “tailor-made” Ne condenser design is presented, enabling the suppression of the primary He temperature variations by two orders of magnitude, from c. ±0.3K to below ±3mK.