INITIAL DENSITY PROFILE MEASUREMENTS USING A LASER-INDUCED FLUORESCENCE DIAGNOSTIC IN THE PAUL TRAP SIMULATOR EXPERIMENT ∗ M. Chung † , E. P. Gilson, R. C. Davidson, P. Efthimion, R. Majeski Plasma Physics Laboratory, Princeton University, Princeton, New Jersey, 08543 USA Abstract Installation of a laser-induced fluorescence (LIF) diag- nostic system has been completed and initial measurements of the beam density profile have been performed on the Paul Trap Simulator Experiment (PTSX). The PTSX de- vice is a linear Paul trap that simulates the collective pro- cesses and nonlinear transverse dynamics of an intense charged particle beam propagating through a periodic fo- cusing quadrupole magnetic configuration. Although there are several visible transition lines for the laser excitation of barium ions, the transition from the metastable state has been considered first, mainly because a stable, operating, broadband, and high-power laser system is available for experiments in this region of the red spectrum. The LIF system is composed of a dye laser, fiber optic cables, a line generator, which uses a Powell lens, collection optics, and a CCD camera system. Single-pass mode operation of the PTSX device is employed for the initial tests of the LIF sys- tem to make optimum use of the metastable ions. By min- imizing the background light level, it is expected that ade- quate signal-to-noise ratio can be obtained to re-construct the radial density profile of the beam ions. INTRODUCTION Understanding the physics of high-intensity beams is important because of the wide variety of accelerator ap- plications, including high energy physics, heavy ion fu- sion, ion-beam-driven high energy density physics, nu- clear waste transmutation, and spallation neutron sources to mention a few examples [1]. To address the many impor- tant issues in high-intensity beams experimentally, the Paul Trap Simulator Experiment (PTSX) device was built at the Princeton Plasma Physics Laboratory (PPPL) in 2002, and demonstrated quiescent beam propagation over equiv- alent distances of tens of kilometers [2]. The PTSX de- vice is a compact laboratory facility that investigates in- tense beam dynamics by taking advantage of the similar- ity between the dynamics of an intense beam propagating through a periodic focusing quadrupole magnetic field, and a one-component nonneutral plasma trapped in an oscillat- ing quadrupole electric field [3]. Currently, three diagnos- tic systems are installed on PTSX, which include: a Fara- day cup, capacitive pick-ups, and a laser-induced fluores- cence (LIF) diagnostic. The LIF diagnostic is expected to be very useful for the in-situ measurement of the transverse ∗ Research supported by the U.S. Department of Energy † mchung@princeton.edu beam density profile, velocity distribution measurements, and halo particle detection. Because the atomic spectrum of barium ions is amenable to the present LIF experimen- tal setup, barium ions have been chosen as the preferred ion species. In this paper, the installation of the barium ion source and the LIF system are summarized, together with the initial test results. BARIUM ION SOURCE Barium ions are produced at a hot metal surface by con- tact ionization. Traditionally, rhenium and tungsten have been used for the hot metal plate to produce both ions by contact ionization, and electrons by thermionic emission. Because electrons are not required for the PTSX device, platinum is a more favorable choice for the hot metal plate because of its higher work function than rhenium and tung- sten. Platinum’s work function is 5.65 eV, and its melting point is 1768 ◦ C. The design of the barium ion source is based on the com- pact metal-ion source developed for heavy ion beam probes used for plasma diagnostics [4]. The ion source is com- posed of a beam material oven and a metal ionizer. The oven is a tantalum tube with 0.5” in diameter and 4” in length. The radial tube size is adequate to make the beam RMS-matched to the externally applied focusing field for the nominal operating conditions of PTSX. The length of the tantalum tube is chosen in such a way that heat con- duction and radiation processes sustain the proper temper- ature distribution along the tube. Normally, the tantalum tube is maintained higher than 400 ◦ C to decompose any barium oxide layer. The ionizer consists of a stack of plat- inum meshes which are woven from 0.1 mm platinum wires and have 62.7% open area. The platinum meshes are in- serted into the open end of the oven tube, and the vapor of the beam material is ionized on the hot platinum wire sur- faces as it passes through the tube. Two tungsten heaters are wrapped around the tube and the ionizer, respectively. Each heater has its own heat shield to increase the thermal efficiency and is connected to a high-current power supply through thick copper rods [Fig. 1(left)]. The temperature of the oven and the ionizer are controlled by adjusting the cur- rents of the power supplies and monitored by two indepen- dent K-type thermocouples attached to these components. To minimize oxidization, barium is loaded into the oven in- side an argon-filled tent. About 6 g of barium allowed 2-3 months of operations in the initial experiments. The ionizer is surrounded by a Pierce electrode, fol- THPAS080 Proceedings of PAC07, Albuquerque, New Mexico, USA 05 Beam Dynamics and Electromagnetic Fields 3666 D03 High Intensity - Incoherent Instabilities, Space Charge, Halos, Cooling 1-4244-0917-9/07/$25.00 c 2007 IEEE