Laser Wire Based Parallel Profile Scan of H Beam at the Superconducting Linac of Spallation Neutron Source*

We report on the world’s first experiment of a parallel profile scan of the hydrogen ion (H) beam using a laser wire system. The system was developed at the superconducting linac (SCL) of the Spallation Neutron Source (SNS) accelerator complex. The laser wire profile scanner is based on a photo-detachment process and therefore can be conducted on an operational H beam in a nonintrusive manner. The parallel profile scanning system makes it possible to simultaneously measure profiles of the 1-MW neutron production H beam at 9 different locations (corresponding to energy levels of 200 MeV – 1 GeV) of beamline using a single light source. INTRODUCTION In recent years, laser based nonintrusive H beam profile/emittance measurement systems have been developed at the SNS which is an accelerator based, world’s most intense short-pulse neutron scattering facility [1,2]. Although laser wire profile monitors were installed at multiple locations along the SCL beamline, previously profile measurements have only been performed serially since a single light source is used. On the other hand, physics study such as the SCL modeling at SNS requires the measurement of H beam profiles at different locations along the acceleration path and usually such measurement needs to be conducted many times on different accelerator settings. A simultaneous profile measurement would be especially helpful to improve the efficiency and accuracy of the physics studies. We show that through a series of improvements on the optical transfer line, timing control and scanning software, it is possible to simultaneously measure H beam profiles at 9 different locations along the SCL. The measurement was conducted completely in a nonintrusive manner on the neutron production H beam. The entire measurement process takes less than 5 minutes to complete. Together with the hardware modification, we have also upgraded our user interface to provide a push-button style, informative platform to make the laser wire system a truly convenient and useful tool for the accelerator operators and physicists. SYSTEM CONFIGURATION Laser Wire System Layout The 230-meter long SCL is the longest section of the SNS accelerator complex and it accelerates the H from ~ 200 MeV to 1 GeV. The SNS laser wire system consists of 9 profile monitors that cover from the beginning to the end of the SCL. Figure 1 shows a layout of the system. Figure 1: Outline of the SNS laser wire system. PZT Mirror: piezo-electric transducer driven mirror for beam position stabilization. The numbers in the figure indicate the SCL cryomodule and distances are from the laser source. Inset box A: diagram of an individual profile measurement station. BS: beam splitter, M: mirror, FM: flipper mirror, BD: beam dump, VM: vacuum window. A schematic illustration of the laser wire measurement station is shown in the inset box A of Fig. 1. At each measurement station, a stepper motor driven pick-up mirror intercepts the free-space laser transport line (LTL) and redirects the laser light to the measurement box. Inside the measurement box, a motorized flip mirror switches the laser light between the horizontal and vertical scanning axes. The laser beam enters a vacuum chamber through a vacuum window (laser port) and is designed to interact with the ion beam close to the focal point of the laser beam. When the H beam interacts with the laser light, a small portion of the ions illuminated by the laser pulse are neutralized and the liberated electrons are collected by an electron detector. The measurement of the resulting electron density leads to the determination of the negative ion density. By scanning the incident laser beam in horizontal or vertical directions, profiles of the ion beam along the correspondent axis can be obtained.