J. Neumann

Simulation of prebunching in free-electron lasers

Prebunched beams are used in many coherent radiation sources. If the beam is bunched on scale lengths comparable or shorter than the desired wavelength, then the resonant wavelength is excited without a drive signal and grows faster than exponential. A free-electron laser with a prebunched beam combines the best characteristics of amplifiers and oscillators. In comparison with oscillators, no drive signal is needed and the wiggler is short. In comparison with amplifiers, no resonator is needed. Simulation of enhanced bunching has been demonstrated using the MEDUSA simulation code (IEEE J. Quantum Electron. 36 (2000) 275; Nucl. Instr. and Meth. A 475 (2001) 381; Nucl. Instr. and Meth. A 475 (2001) 118) for optical klystrons (Nucl. Instr. and Meth. A 475 (2001) 381) and high-gain harmonic generation (Nucl. Instr. and Meth. A 475 (2001) 118). In the latter case, modulation of the beam in one wiggler was enhanced in a magnetic chicane prior to injection into a second wiggler. We describe modifications to MEDUSA to simulate prebunching, and the application of the code to study requirements and limitations on the process.

Production of photoemission-modulated beams in a thermionic electron gun

The generation and evolution of perturbations and modulations in intense charged particle beams are of key importance for many accelerator applications. Prior work focused on perturbations and modulations produced in gridded electron guns with thermionic cathodes. By using a drive laser, photoemission can produce perturbations within a longer beam generated by thermionic emission. These perturbations affect beam density only, while previous experiments with gridded guns produced perturbations in both beam density and velocity. We have extended these capabilities by developing a flexible system to produce multiple perturbations whose timing and amplitude can be easily adjusted for beam research applications. In this article we describe this apparatus and give preliminary results.

Electron beam modulation using a laser-driven photocathode

Coherent synchrotron radiation may lead to a microwave instability on an electron bunch at wavelengths much smaller than the bunch length. It is possible that ripples (prebunching) on the electron bunch distribution may seed this instability. We report on research exploring this effect using a longitudinally modulated drive laser to generate a modulated electron beam. Our first step is to develop simulations that will help us study the beam generation process using PARMELA. Preliminary experiments on laser beam and electron beam modulation, conducted at the Source Development Laboratory at the National Synchrotron Light Source, show modulation at frequencies in the terahertz regime is attainable. Longitudinal prebunching may enhance the performance of FEL or other radiative devices in the terahertz regime. Alternatively, longitudinal control over the electron beam might be an effective method of suppressing coherent synchrotron radiation instabilities that cause beam break-up.

Simulations and Experiments of Electron Beams Pre-Modulated at the Photocathode

The University of Maryland and the Source Development Laboratory at Brookhaven National Laboratory have been collaborating on a project that explores the use of electron beam pre-modulation at the cathode to control the longitudinal structure of the electron beam. This technique could be applied to creating deliberate modulations which can lead to the generation of terahertz radiation, or creating a smooth profile in order to suppress radiation. This paper focuses on simulations that explore some of the pre-modulated cases achieved experimentally.

Combined thermionic and photoelectric emission from dispenser cathodes

Photoelectric emission from dispenser cathodes has been studied earlier. Photoelectric emission in conjunction with themionic emission can provide a convenient and flexible means of modulating emission from thermionic cathodes with programmable formats and high bandwidth. This may be useful for experimental studies the beam dynamics of space charge dominated beams, such as in the University of Maryland electron ring (UMER). We have studied combined photoelectric and thermionic emission from a dispenser cathode using a nitrogen laser operating at 337 nm. Results will be presented for the effect on emission of laser intensity, cathode temperature, and accelerating voltage.

Commissioning of the University of Maryland Electron Ring (UMER)

The University of Maryland electron ring (UMER) is a low-energy, high current recirculator for beam physics research. The ring is completed for multi-turn operation of beams over a broad range of intensities and initial conditions. UMER is addressing issues in beam physics with relevance to any applications that rely on intense beams of high quality. Examples are advanced accelerators, FEL’s, spallation neutron sources and future heavy-ion drivers for inertial fusion. We review the ring layout and operating conditions, and present a summary of beam physics areas that UMER is currently investigating and others that are part of the commissioning plan. We also emphasize the computer simulation work that is an integral part of the UMER project.

Longitudinal Dynamics in the University of Maryland Electron Ring

The University of Maryland Electron Ring (UMER) is a low energy electron recirculator for the study of space charge dominated beam transport. The system’s pulse length (100 ns) and large number of diagnostics make it ideal for investigating the longitudinal evolution of intense beams. Pulse shape flexibility is provided by the pulser system and the gridded gun, which has the ability to produce thermionic and photoemission beams simultaneously. In this paper, we report on the generation and evolution of novel line charge distributions in UMER. Professional Society.

Coherent terahertz radiation from optically pre-bunched electron beams

A bunched electron beam is an efficient source of coherent radiation at wavelengths longer than the bunch length. We describe an experiment demonstrating electron density modulation at 0.8 THz using an optically modulated drive laser to extract a prebunched beam from an RF photoinjector, which is preserved through acceleration to 75 MeV. Measurements of coherent THz transition radiation (CTTR) are presented at the prebunch frequency showing enhanced production over that produced from an unmodulated beam.

Intense Beam Experiments at the University of Maryland Electron Ring (UMER)

A detailed understanding of the physics of space‐charge dominated beams is vital for many modern accelerators. In that regard, low‐energy, high‐intensity electron beams provide an excellent model system. The University of Maryland Electron Ring (UMER) has been designed to study the physics of space‐charge dominated beams with extreme intensity in a strong focusing lattice with dispersion. At 10‐keV, 100 mA, the UMER beam has a generalized perveance of 0.0015. Though compact (11‐m in circumference), UMER is a very complex device. An update on construction and early experimental results is presented.