THE UNIVERSITY OF MARYLAND ELECTRON RING (UMER) ENTERS A NEW REGIME OF HIGH-TUNE-SHIFT RINGS * R. A. Kishek @ , G. Bai, B. Beaudoin, S. Bernal, D. Feldman, R. Feldman, R. Fiorito, T.F. Godlove, I. Haber, T. Langford, P.G. O'Shea, B. Quinn, C. Papadopoulos, M. Reiser, D. Stratakis, D. Sutter, K. Tian, J.C.T. Thangaraj, M. Walter, and C. Wu, Institute for Research in Electronics & Applied Physics, University of Maryland, College Park, MD 20742, USA Abstract Beams with a high phase space density are useful for many modern applications such as free electron lasers, pulsed neutron sources, high-energy-density physics, and high-luminosity colliders. Production of such beams requires understanding the complex space charge dynamics at the low-energy end of the accelerator. The University of Maryland Electron Ring (UMER) has been designed and built with the purpose of investigating space charge effects using scaled low-energy electron experiments. We have recently circulated the highest- space-charge beam in a ring to date, achieving a breakthrough both in the number of turns and in the amount of current propagated. We have propagated a beam with an integer tune shift for over 100 turns, and other, even higher-current beams, for 5-50 turns albeit with some beam loss. One beam had a tune shift at injection of 5.0, which is several factors higher than anything propagated in the past. We report here as well on other interesting aspects of the UMER work. INTRODUCTION Modern accelerator applications require high-quality beams that have a high phase space density. Traditional applications such as high-energy colliders [1-2] can benefit from increased luminosity that allows detection of rare particles. Another class of accelerators at medium energies requires beams with a high brightness to accurately image matter at the molecular and atomic scales. Examples are accelerator-driven neutron sources [3] and high-power free-electron-lasers and light sources [4-7]. Dense beams of heavy ions [8] can also be used to produce exotic states of matter for high-energy density studies, and eventually can be used to drive inertial fusion reactions for energy production. Production of the high-quality beams needed for such applications represents a major scientific challenge due to the complex dynamics of charged particle collections with high phase-space densities. Particularly at the low-energy end of these accelerators, space charge forces lead to collective behavior that is difficult to analyze self- consistently and is often destructive to the beam. In ______________________________ * Work supported by US Dept. of Energy grant numbers DE-FG02-94ER40855 and DE-FG02-92ER54178, and by US DOD Office of Naval Research and JTO. @ ramiak@umd.edu practical terms, space charge interactions often result in emittance growth and halo formation, i.e., the dilution of the beam phase space and reduction of quality. Beam losses in high-power accelerators due to halos also increase the costs of the accelerator due to the radiation and health issues involved. The key to increasing beam brightness is to understand space charge dynamics sufficiently to be able to accurately predict beam evolution. In this paper we report on the University of Maryland Electron Ring (UMER) [9], a model accelerator with low-energy electrons designed to enhance space charge forces and study their interactions over relatively long time scales. The parameters of the UMER beam are adjustable over a wide range of intensities, and are scalable to other machines. The UMER effort is further supported by a theoretical and computational modeling effort that is coordinated closely with experiments in order to reveal useful physical insights. UMER is currently in the multi-turn commissioning phase. Due to the intense space charge in even the lowest-current UMER beams, our commissioning goals have been limited to achieving 100 turns at low-current and 10 turns at the highest beam currents. We are well on our way towards achieving these goals, thus demonstrating the possibility of operating a ring with extreme tune shifts, and in the process learning much about the dynamics of beams with space charge. Of interest are processes leading to emittance growth, halo formation, increase in energy spread, and instability. This paper serves to summarize the latest developments in the UMER project, including the results of the multi- turn commissioning effort. SPACE CHARGE INTENSITY In order to put the UMER effort in context, we will quantify what we mean by space charge intensity. Beam brightness is a commonly used measure of phase-space density and is a good measure of the inherent quality of the beam at the source or target. From the point of view of beam transport, however, what matters is the relative strength of the space charge force to other forces in the system, such as the applied external force or the thermal pressure due to emittance. By normalizing the rms envelope equation, several related dimensionless parameters can be derived that describe this relationship. An example is the tune depression, defined as the ratio of TUZBAB03 Proceedings of PAC07, Albuquerque, New Mexico, USA 05 Beam Dynamics and Electromagnetic Fields 820 D03 High Intensity - Incoherent Instabilities, Space Charge, Halos, Cooling 1-4244-0917-9/07/$25.00 c 2007 IEEE