Visualization of Trajectories of Electron Beams Emitted by an Ion Source with Closed Electron Drift (2008)

Visualization of Trajectories of Electron Beams Emitted by an Ion Source with Closed Electron Drift Ivan V. Bordenjuk, Oleg A. Panchenko, Sergei V. Sologub, and Ian.G. Brown, Fellow, IEEE Abstract – Trajectories of electron beams emitted by an ion source with an anode layer and Hall electron closed drift orbits were visualized using light emission from a working gas excited by electrons. Gas discharge of magnetron type, arising in the beam drift region under the influence of an electric field of a target bias potential, was visualized. Bias potential in ion-beam etching systems strongly influences the effectiveness of surface cleaning. Ion beams impinging on a target surface can charge the surface positively and cause ion beam deceleration. Ion beam charge neutralization is usually achieved by means of additional devices injecting electrons or negative ions into the positively charged beam [1] or by providing of a longitudinal (with respect to the ion beam direction) electric field, produced by additional source, that leads to the development of non-self- maintained discharge generating extra-electrons [2]. The magnetic field “sagging” out of the thruster emission slit do not effect on heavy ions that easily reach a target along ballistic trajectories. On the contrary, this field is strong for electrons, and their motion in the drift space is diffusive, and , consequently, it depends on the magnetic lines configuration. The aim of the work described here was to determine trajectories of electrons, emitted from the Hall thruster source with anode layer and moving to a target as well as localization of additional discharge of magnetron type arising in the drift space under the influence of the bias potential electric field. The basic processes in the Haal thrusters were investigated for a long time [3,4]. Crossed ExB fields provoking electron drift across the electric and magnetic field directions. Suppressed transverse mobility of electrons allows keeping strong transverse electric field. At the same time ions in the plasma are accelerated to the emission slit in the cathode, and their main part easily leave the acceleration region Trajectories of electrons are more complex, that considerably reduces electron emission current density relatively to the ion one. Subsequent electron motion is dependent on the topological Fig. 1. Magnetic field direction (small arrows) in the ion source drift region is shown in the plane perpendicular to the emission slot plane. The separatrices are shown as red lines, and the ion beams as rose,. features of the magnetic field within the of beam drift. The Hall thruster applied in experiments possesses the following characteristics: length of the racetrack emission slit - 250 mm in the straightforward part and racetrack bending radius - 40 mm, emission slit width - discharge gap – 2 mm. Magnet pole pieces were made of soft iron. The water- cooled anode with the 23x10 mm 2 rectangular cross-section was made of a stainless steel. A thin stainless steel plate (400x150 mm 2 ) was used as a target. Working gas (argon) pressure was within the range from 1x10 -3 Torr to 3x10 -4 ,Torr the base pressure in the vacuum chamber was about 5x10 -6 Torr. The power supply voltage was up to 2.5 kV, discharge current - up to 0.3 A. The magnetic field distribution within the drift region is shown in Fig. 1. Arrows indicate the magnetic field direction at that location (and not magnitude). The axial variation in the strength of the radial magnetic field has a bell-shaped distribution and correspondents to the magnetron trap configuration. In the center of the slit magnetic induction was 2 kG. Within the Manuscript received __ October 2007; revised __ _______ 200_. I.V. Bordenjuk, O.A. Panchenko, and S.V. Sologub are with the Institute of Physics, National Academy of Sciences of Ukraine, 46 Nauki Ave., Kiev, Ukraine 03028. I.G. Brown is with the Lawrence Berkeley National Laboratory, Berkeley CA 94720. Work was supported by the STCU (Science & Technology Center in Ukraine), Project #118J. Publis her Identifier S XXXX-XXXXXXX-X