Secondary Electron Yield on Cryogenic Surfaces as a Function of Physisorbed Gases

In LHC the electron cloud induced by photoelectrons, gas ionization and secondary electrons emitted from the beam pipe walls could be a limitation of the performance. The electron cloud induce heat load on the cryogenic system, cause pressure rise, emittance growth and beam instabilities, which in the end will limit the beam’s lifetime. Beam- induced multipacting, which can arise through oscillatory motion of photoelectrons and low-energy secondary electrons bouncing back and forth between opposite walls of the vacuum chamber during successive passage of proton bunches, represent therefore a potential problem for the machine. The secondary electron yield (SEY) is one of the key parameters for the electron cloud build up and multipacting phenomenon. An electron cloud occurs if the metal surface secondary electron yield is high enough for electron multiplication. This parameter has been extensively studied on room temperature samples but uncertainties remain for samples at cryogenic temperature. Indeed, at low surface temperature SEY is strongly dependent on the nature of the physisorbed gases and on the surface coverage. In this work the secondary electron yield (SEY) at cryogenic temperatures has been measured and the results are presented. Of particular interest is the variation of the SEY with the gas coverage as most gases especially CO, CO2, and CH4 in case of LHC condense on the cryogenic parts of the machine. In addition to these gases measurements have been performed with N2, because of its importance as calibration gas for most of the pressure gauges and pumps. Also measurements with Kr have been done, to compare the results with existing works. In order to acquire a better understanding about the behaviour of SEY as a function of different gas coverage, measurements of work function have been made. The measurements of the SEY at cryogenic surfaces require a UHV system with a sample holder which can be cooled down to cryogenic temperatures, a source of primary electrons and detection of sample current and secondary electron current. In addition a gas injection system is necessary to produce variable coverage with different gases. In the present system the sample (Cu, Al, electro-polished Cu, Nb and C) was cooled down to 4.7K, irradiated with an electron gun and currents were measured on the sample and on a collector electrode. Work function was measured with a Kelvin Probe.