Electron cloud observations and cures in the Relativistic Heavy Ion Collider

Abstract

Since 2001, the Relativistic Heavy Ion Collider has experienced electron cloud effects, some of which have limited the beam intensity. These include dynamic pressure rises (including pressure instabilities), tune shifts, a reduction of the instability threshold for bunches crossing the transition energy, and possibly incoherent emittance growth. We summarize the main observations in operation and dedicated experiments as well as countermeasures including baking, nonevaporable getter coated warm beam pipes, solenoids, bunch patterns, antigrazing rings, prepumped cold beam pipes, scrubbing, and operation with long bunches.

Reduction of secondary electron yield for E-cloud mitigation by laser ablation surface engineering

“…The results of measurements reported in the literature show that energy of electrons impinging on the beam chamber is not greater than 600 eV in the LHC [26] and SPS [27], and not greater than 500 eV in RHIC [18]. The material parameters influencing the SEY are the penetration depth of the primary electron, the stopping power, the escape depth of the secondary electrons, the atomic number Z and, to a lesser extent, the work function [28].…”

“…making grooves) [3,16]; (c) Coating with low SEY materials (such as TiN [17], NEG [18] and amorphous carbon (a-C) [19]); (d) Coating with low SEY microstructure (ex. : copper black, gold black; columnar NEG) [20,21,22]; (f) Various combinations of above.…”

Ion Colliders

“…11 and 12, and Ref. [45]). The most limiting effect was seen at store, when electron clouds were formed in some of the experiments after the bunches are transferred from the accelerating to the storage RF system.…”

“…11 the measured electron flux into the wall and pressure is shown as a function of the total filled proton intensity. An analysis of the dynamic pressure rises in RHIC concluded that all operationally relevant observation are caused by electron clouds [45].…”

“…12). The pressure grew exponentially until the beams were aborted by the machine permit system [45]. After the vacuum upgrades in the warm sections (NEG coated beam pipes and baking), these pressure instabilities were suppressed.…”