RHIC Pressure Rise Case Study

Abstract

The pressure rise cases of last RHIC run are studied, some observations and analyses are presented. Given limited cases, conclusive results have to wait until next run. Our goal is to get some consensus on the pressure rise issues, which will be useful in planning next luminosity run and study. By using some illustrative examples, the following issues will be demonstrated and discussed. 1. The electron multipacting looks like the dominant mechanism in pressure rise, for both gold and proton runs. The pressure rise is more sensitive to bunch spacing and bunch intensity, rather than the total beam intensity. Also, in the limiting cases, where the beam loss effect is not dominant, the pressure rise tends to saturate for constant beam intensity, rather than to run away, indicating that the ion desorption is not a dominant factor in the RHIC pressure rise. 2. There are two types of pressure rises in the gold run. The ones caused by the beam loss have sharp rise, usually at 10 to 20 decades per minute. Since this type of rises looks like pressure run-away, it might be misidentified as the ion desorption caused pressure rise. The pressure rises caused by the electron multipacting are usually 3 to 5 decades per minute, and will reach saturation at some point. 3. For the effect of the gold ion beam loss, several pressure rise incidents have shown that the huge amount of molecules is produced at the beam loss, 15 107 . × molecules per lost gold ion. This large spattering yield has not been reported in literature, up to authors' knowledge. If this yield rate is confirmed, then small amount of gold ions lost on the chamber wall may produce enough positive ions to help electrons to survive the bunch gap, and hence support the electron multipacting. 4. For both gold and proton runs, pressure rise had only happened at the beam injection. Once the beam was accelerated, the pressure started to fall. This is explained as the result of reduced beam loss due to the reducing beam size in the acceleration, which will not produce enough positive ions to sustain the electron multipacting. 5. Intensity threshold of electron multipacting for proton run is estimated based on five 55-bunch high intensity ramps. The lowest threshold at the interaction region is about twice as high as that at either blue or yellow warm bore straight sections. Since the chambers are similar, it is speculated that the longer length of the single beam straight sections may have provided better condition for the beam halo scraping, and produced more positive ions to support the electron multipacting. Threshold of the 110-bunch gold beam at interaction is compared with the proton run. As expected, the gold beam with lower intensity, 60%, has lower threshold, about 65%, than the proton counterpart.