Intense beam transport experiments in a multi-bend system at the University of Maryland Electron Ring

Abstract

We report on the results of beam transport experiments at the University of Maryland Electron Ring (UMER) over a distance of approximately $\text{4}\text{m}$. The experiments involve a $\text{10}\text{keV}$, 25–$\text{100}\text{mA}$, $\text{100}\text{ns}$ electron beam in a lattice consisting of one short solenoid, 24 printed-circuit magnetic quadrupole lenses, and a number of bending and steering dipole magnets. The diagnostics include capacitive beam-position monitors, phosphor screens, slit-wire emittance meters and others. We discuss the conditions required for matching the space-charge-dominated beam into the UMER lattice, as well as emittance measurements and initial studies of longitudinal dynamics in the ring.

Introduction

The University of Maryland Electron Ring (UMER) is designed for scaled experiments employing low energy (up to $\text{10}\text{keV}$), high current (up to $\text{100}\text{mA}$) electron beams [1], [2]. With a strong-focusing (FODO) lattice consisting of 36 periods over a $\text{11.52}\text{m}$ circumference (see Table 1), UMER can operate in a regime of unprecedented high beam intensity. Beam transport in this regime is dominated by space charge and is characterized by very low tune depression, i.e., the ratio of phase advances per lattice period of the betatron oscillations with and without space charge [3]. As an example, the nominal $\text{100}\text{mA}$ at $\text{10}\text{keV}$ in UMER models the beam physics of a kA beam of heavy ions at GeV energies in a future heavy-ion fusion driver. Construction of a machine with a layout similar to UMERs was undertaken a few years ago at Lawrence Livermore National Laboratory. In the Livermore ring, a $\text{80}\text{keV}$ (initial energy) potassium-ion beam was successfully transported and accelerated over one-quarter turn [4]. While an electron ring can address beam physics issues in a low-cost way, the LLNL ring did also address issues peculiar to ions. At the time of writing, UMER is also approaching completion of a one-quarter turn. Figs. 1a and b show the current experiment layout.

In the next section, we briefly describe the UMER electron gun. The main results from matching experiments in a straight section about $\text{1}\text{m}$ long are summarized in Section 3. In Section 4, the first results of bend experiments over eight FODO periods, or $\text{2.5}\text{m}$, approximately, are presented. The last section is devoted to conclusions.