SCL 1 2 3 4 5 32 17 12 13 14 15 250 m 227 m 25 m 160 m Camera Mirror Power meter LR LR Laser room Laser wire station Cryomodule number 32 SNS Baseline SCL 1 2 3 4 5 32 17 12 13 14 15 250 m 227 m 25 m 160 m Camera Mirror Power meter LR LR Laser room Laser wire station Cryomodule number 32 LR Laser room Laser wire station Cryomodule number 32 SNS Baseline LASER WIRE BEAM PROFILE MONITOR AT SNS* Yun Liu # , Cary Long, Warren Grice, Willem Blokland, Saeed Assadi Spallation Neutron Source, ORNL, Oak Ridge, TN 37831, U.S.A. Abstract We report the experimental implementation, measurement results and analysis of the hydrogen ion (H - ) beam profile diagnostics with a laser wire in the spallation neutron source (SNS) superconducting linear accelerator at the Oak Ridge National Laboratory. The advantages of the laser beam profile monitor at SNS include non-invasive measurement, longitudinal beam scan over minipulses, and the capability of measuring H - beam profiles over multiple energy levels (200 MeV - 1 GeV) using a single laser source. INTRODUCTION The superconducting linear accelerator (SCL) at the SNS accelerates the negative hydrogen ion (H - ) beam pulse from 187 MeV to 1 GeV with a peak beam current of 38 mA [1]. For the diagnostics of high-brightness particle beams as necessary for the SNS, nondestructive methods have to be developed [2]. Since the outer electron of H - is bound only by 0.75 eV, photons with an energy above this threshold level can be used to partially neutralize H - beams. This property can be exploited to determine the transverse phase space distribution with high time and space resolutions. The interaction of laser light and H - ions detaches a small number of electrons which are separated magnetically from the rest of the beam and are measured by an electron detector. We call such a laser-based measurement system a laser wire in comparison with the physical carbon wire. The main merits of the laser wire over a solid wire scanner are that it does not use a physical wire that could contaminate the superconducting surface and it can operate at full operational H - beam power. LASER WIRE SYSTEM SETUP Figure 1 shows an outline of the entire laser wire system. The SNS SCL consists of 23 cryomodules which boosts the H - energy from 187 MeV to 1 GeV. There are 9 laser wire (LW) stations installed along the SCL beam line. The first four LW stations are installed after the first four cryomodules (each cryomodule drives three medium- beta cavities), the next four LW stations are located after cryomodules 12-15 (each drives four high-beta cavities), and the last LW station is placed at the end of the SCL. The LW stations are arranged so that the H - beam at different energy levels (200 MeV – 1 GeV) can be measured. In our system, a Q-switched Nd:YAG laser operating at 1.06 um is used as the light source. The laser has a repetition rate of 30 Hz and a pulse width of 7 ns. The laser pulse energy is controlled by a set of polarization optics elements and typical pulse energy level is 50 - 200 mJ. Using a Q-switched laser source will provide enough pulse energy within a short time period so that a sufficient number of electrons can be detached, which significantly increases the detection efficiency and accuracy. The laser is located outside the SCL tunnel and the laser beam is directed by mirrors through the laser transport line (LTL) pipe to one of the 9 LW stations. Along the LTL path, three cameras are installed to monitor the position of the laser beam. The first camera is installed at the entrance of the linac tunnel and the cameras 2 and 3 are located at about 125 m and 210 m away from the tunnel entrance, respectively. The mirror at the entrance box is controlled in both axes with a feedback signal from cameras 2 or 3 to stabilize the beam position. At the end of the LTL pipe, a power meter is Figure1: Outline of the laser wire system installed in the SNS superconducting linac. Distances in the figure are from the laser room. ____________________________________________ *This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-96OR22464 with the U.S. Department of Energy. # liuy2@ornl.gov Proceedings of EPAC08, Genoa, Italy TUPC061 06 Instrumentation, Controls, Feedback & Operational Aspects T03 Beam Diagnostics and Instrumentation 1197