RF Heating and Temperature Oscillations due to a Small Gap in a PEP-II Vacuum Chamber*

Wake fields excited in a small gap of a vacuum chamber by ampere beams can have enough amplitude to heat the chamber. The electric component of these fields can be above the arcing limit. Usually flange connections in a vacuum chamber contain a vacuum gasket and an inner RF gasket. If a small gap occurs between the RF gasket and flange surface, wake fields can heat the flanges. The flanges are usually made of stainless steel, which efficiently absorbs RF power. Some flanges consist of two parts (like a vacuum valve flange) and are mechanically connected but have poor thermal contact. A temperature rise can lengthen the inner part of the flange and make firmer the thermal contact to the outer part of the flange. The heat will then flow to the outer part of the flange, which is air and water-cooled. This cooling lowers the flange temperature and the thermal contact becomes poor again. This “quasi” periodic mechanism can explain the nature of temperature oscillations observed at several locations in PEP-II, the SLAC B-factory. Contributed to Particle Accelerator Conference Portland, Oregon, USA May 12 – May 16, 2003 * Work supported by Department of Energy contract DE–AC03–76SF00515. RF HEATING AND TEMPERATURE OSCILLATIONS DUE TO A SMALL GAP IN A PEP-II VACUUM CHAMBER A. Novokhatski, J. Seeman and M. Sullivan, SLAC, Stanford, CA 94309, USA Abstract The red curve (more rapidly oscillations) presents the signal from the thermocouple attached to body of the vacuum valve. The green curve (slow function) shows the time dependence of the positron current during several top-offs. This unusual behaviour of the temperature cycling initiated the wake field study of very small gaps to understand the RF heating. Wake fields excited in a small gap of a vacuum chamber by ampere beams can have enough amplitude to heat the chamber. The electric component of these fields can be above the arcing limit. Usually flange connections in a vacuum chamber contain a vacuum gasket and an inner RF gasket. If a small gap occurs between the RF gasket and flange surface, wake fields can heat the flanges. The flanges are usually made of stainless steel, which efficiently absorbs RF power. Some flanges consist of two parts (like a vacuum valve flange) and are mechanically connected but have poor thermal contact. A temperature rise can lengthen the inner part of the flange and make firmer the thermal contact to the outer part of the flange. The heat will then flow to the outer part of the flange, which is air and water-cooled. This cooling lowers the flange temperature and the thermal contact becomes poor again. This “quasi” periodic mechanism can explain the nature of temperature oscillations observed at several locations in PEP-II, the SLAC B-factory. WAKE FIELDS AND RF HEATINGThe red curve (more rapidly oscillations) presents the signal from the thermocouple attached to body of the vacuum valve. The green curve (slow function) shows the time dependence of the positron current during several top-offs. This unusual behaviour of the temperature cycling initiated the wake field study of very small gaps to understand the RF heating. Wake fields excited in a small gap of a vacuum chamber by ampere beams can have enough amplitude to heat the chamber. The electric component of these fields can be above the arcing limit. Usually flange connections in a vacuum chamber contain a vacuum gasket and an inner RF gasket. If a small gap occurs between the RF gasket and flange surface, wake fields can heat the flanges. The flanges are usually made of stainless steel, which efficiently absorbs RF power. Some flanges consist of two parts (like a vacuum valve flange) and are mechanically connected but have poor thermal contact. A temperature rise can lengthen the inner part of the flange and make firmer the thermal contact to the outer part of the flange. The heat will then flow to the outer part of the flange, which is air and water-cooled. This cooling lowers the flange temperature and the thermal contact becomes poor again. This “quasi” periodic mechanism can explain the nature of temperature oscillations observed at several locations in PEP-II, the SLAC B-factory. WAKE FIELDS AND RF HEATING It was supposed that the gasket (RF gap ring), which is placed in the connection between the vacuum valve and the vacuum chamber, could have dimensions that are incorrect thereby producing a very small gap.