Fast Extracted Beam FEB for the g-2 Experiment

Abstract

Proton bunches were extracted from the AGS synchrotron to bombard the V-target[1] for π+ production to be used in the g-2 experiment[2]. Each of the proton bunches was ~80 nsec long, contained ~6x10 protons, with a momentum p=24.02 GeV/c and was transported to the Vtarget via the U-V beam transport line[3] . This tech-note describes the acceleration process of the proton bunches, and the fast extraction mechanism of the beam bunches from the AGS synchrotron, during the the g-2 experiment which was performed in the time period, December1999 to March 2000. Acceleration stages of the proton beam The proton beam underwent the following acceleration stages to reach the final momentum of 24.02 GeV/c. 1. Proton beam in the form of H ions, was extracted from a magnetron ion source [4] at an energy of 35 keV and then was injected into a Radio Frequency Quadrupole (RFQ). 2. The RFQ [5] accelerated the H beam to an energy of 750 keV. The accelerated beam was properly with the fast chopper [6] located after the RFQ, before it was injected into the 200 MeV, 200 MHz Linear accelerator (LINAC) [7] (see item #3 below). The function of the fast chopper is to turn the beam “on and off” so that the “unchopped” section of the beam, when injected into the AGS-Booster, (see item #4 below) falls exactly into the circulating bucket of the AGS-Booster. The chopped H beam is lost but at lower energy (750 keV) and in a controlled way. 3. The LINAC accelerated the proton beam to 200 MeV, and generated a beam pulse (macro-pulse) ~500 μsec long with an average beam current of 35 mA. The macro-pulse has a fine structure which consists of bunches 1 nsec wide (micropulses) and spaced by 5 nsec (200MHz). The macro-pulse, was injected to fill the single bucket of the AGS-Booster synchrotron [8] which acts as a pre-accelerator of the AGS synchrotron. Just before the H beam is injected into the AGS-Booster, it is stripped of its two electrons into a proton, by a carbon foil which is located in the proton injection section of the AGS-Booster and intercepting a “ bumped closed orbit”. It takes few hundred turns of the bucket in the AGS-Booster to be filled by the “chopped beam” emerging from the LINAC. 1 This average beam current corresponds to a beam which is “not chopped” by the fast chopper, prior to its injection into the LINAC (see item #2 above.)