Wake Field Study Using Two Spaced Bunches 1

Wake eld studies using calorimetric methods are reported. Measurements show interference between wake elds generated by two bunches. The described technique can be used to determine which elements introduce bunch coupling. Introduction Due to nite resistance and irregularities of beam pipe wall the bunch of charged particles traveling through a beam pipe leaves behind a wake eld. The structure and characteristic time of this eld depend on beam pipe material as well as pipe shape. Consider several bunches circulating in a storage ring. If somewhere the wake eld exists longer than period between bunches there will be bunch to bunch interaction which in turn may cause multibunch operation problems. If the wake eld decays before the next bunch appearance, there will not be an interaction between bunches. So, the above shows the importance of having a practical method to determine the elements generating long lived wake elds. This paper reports the experimental study of wake function performed at CESR in 1994. It was based on measurements of temperature variation Work supported by the National Science Foundation On leave from BINP, Novosibirsk 1 Figure 1: The H18E scraper and CESR's beam pipe cross section. caused by wake eld energy absorption. The rst part describes the used method as well as the single bunch wake eld study. The next section reports observation of the e ect of interference of wake elds generated by two bunches. Method and tools There are several scrapers around CESR. One of them, H18E, located in west arc of ring was used in the reported experiments. It is shown at 1:1 scale in gure 1 together with the CESR beam pipe cross section. The scraper is a stainless steel tube of 0:75in diameter captured from one side. Inside there is a copper plug of 65 grams weight with air-cooling channels and a thermal sensor. The weight of scraper is mostly the mass of the plug. Air ow through plug provides cooling. Generally there are two reasons for beam pipe heating by circulating beam. The rst one is synchrotron radiation. In this case the temperature change is proportional to circulation current. The second reason is wake eld. It causes quadratic dependence of temperature on beam current. The measurements of H18E scraper temperature versus beam current, shown on gure 2 indicate pure quadratic dependence. It means that the scraper heating is due to the wake eld.