PSR electron cloud detector and suppressor mechanical design and fabrication

Abstract

In order to better understand the two stream e-p instability issue in the LANSCE Proton Storage Ring (PSR), a new diagnostic instrument has been developed to measure the electron cloud formation and trapping in a quadrupole magnet at the LANSCE PSR. The device called the Electron Cloud Detector (ECD) was fabricated and has successfully been installed in the PSR. Along with the Electron Cloud Detector, an additional device was developed to manipulate electrons ejected from the quadrupole and allow additional information to be obtained from ECD measurements. This paper will discuss the mechanical design and fabrication issues encountered during the course of developing both devices. ELECTRON CLOUD DETECTORA diagnostic has been developed to study the electron cloud generation and trapping in a PSR quadrupole [1]. The concept is to measure the electron flux that strikes the beam pipe wall during beam induced multipactoring. The device also features an electrode that can be pulsed after the beam pulse has left the magnet to sweep out any electrons that are trapped in the quadrupole. This is a second generation of an electron sweeping device [2] that had been used in drift spaces in the PSR. This new device will reside within a quadrupole in sector four of the PSR. Figure 1 shows the ECD installed in the PSR quadrupole. The existing magnet in that location was removed along with its beam position monitor and a spare quadrupole containing the new ECD was installed. Figure 1: Electron Cloud Detector Beam-line Device. ECD FEATURESThe ECD is made up of three main parts, a Retarding Field Analyzer (RFA) [3] chamber, a sweeping electrode, and the vacuum housing. The ECD is oriented within the quadrupole so that the sweeping electrode and the edge of the RFA collector are normal to the quadrupole magnetic field lines. Angular alignment of the ECD in the quadrupole ±1 degree was specified. The RFA assembly consists of an electron filter, an electron repeller, and an electron collector. The operating concept is that electrons within the quadrupole will spiral around the quadrupole magnetic field lines and eventually move into the RFA through a series of holes in the beam pipe wall. The repeller grid will have a negative bias applied to it and only electrons of sufficient velocity will be able to pass through and onto the collector where the electron signal is detected.The ECD is designed to detect two types of electron signals; prompt electrons and swept electrons. The prompt electrons are those produced by the beam induced multipactoring while the sweeping electrode is grounded. The swept electrons are those that are stay trapped in the quadrupole after the beam has passed through and are then driven into the RFA collector by applying a fast rising voltage pulse of -485 volts to the sweeping electrode.