Microwave Ion source for low charge state Ion production

Microwave Ion Source for Low Charge State Ion Production * J. Reijonen † , M. Eardley, R. Gough, K. Leung, R. Thomae Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 5-119, Berkeley, CA 94720, USA Abstract. The Plasma and Ion Source Technology Group at LBNL have developed a microwave ion source. The source consists of a stainless-steel plasma chamber, a permanent-magnet dipole structure and a coaxial microwave feed. Measurements were carried out to characterize the plasma and the ion beam produced in the ion source. These measurements included current density, charge state distribution, gas efficiency and accelerated beam emittance measurements. Using a computer controlled data acquisition system a new method of determining the saturation ion current was developed. Current density of 3-6 mA/cm 2 was measured with the source operating at the over dense mode. The highest measured charge-states were Ar 5+ , O 3+ and Xe 7+ . Gas efficiency was measured using a calibrated argon leak. Depending on the source pressure and discharge power, more than 20% total gas efficiency was achieved. The emittance of the ion beam was measured by using a pepper-pot device. Certain spread was noticed in the beam emittance in the perpendicular direction to the source dipole field. For the parallel direction to the magnetic field, the normalized rr’ emittance of 0.03 π-mm-mrad at 13 kV of acceleration voltage and beam exit aperture of 3-mm-in-diameter was measured. This compares relatively well with the simulated value of 4rms, normalized emittance value of 0.024 π-mm- mrad. INTRODUCTION Single to low charge state ion beams with good beam quality and ion sources with good gas efficiency are needed in various applications, ranging from radio- active ion beam production to high energy ion implanters. A simple permanent-magnet microwave ion source has been developed to fulfill the various requirements. 1. SOURCE DESIGN The plasma chamber of the microwave ion source was made from stainless-steel. Three vacuum feed- through ports were designed: one to the opposite side of the extraction and two facing each other in the middle of the magnet-rings. The two facing ports were designed to be different in length. This enabled one to test different lengths of the coaxial antenna inside the source. The other two ports were used for gas feed and for a window port. The magnets were attached around the source using an iron yoke. They were floating around the source chamber so that the cooling air was flowing between the source body and the magnets. This arrangement maintained the temperature of the magnets reasonably low. The magnet rings were constructed from a SmCo magnet blocks measuring 1.3 cm, 2.5 cm and 3.8 cm in width, height and depth respectively. To have a ring configuration, using bar magnets, the bar ends close to the feed-trough port were touching each other, but the other ends of the magnet bars were separated. To achieve a fairly uniform field, the magnet poles of the opposite sides were rotated slightly so that the magnets in the poles did not face each other. In figure 1 a schematic drawing of the micro-wave ion source is shown. Initially the magnetic field (B dip (d)) using a yoke, as a function of the distance between the poles was approximately given by This Work was supported by Department of Energy under Contract No. DE-AC03-76SF00098 email: jreijonen@lbl.gov