High-brightness Beam Research Articles

Comparison of the coherent radiation-induced microbunching instability in a free-electron laser and a magnetic chicane

A self-amplified spontaneous emission free-electron laser (SASE FEL) is a device which is based on the creation of a very intense, relativistic electron beam which has very little temperature in all three phase planes. The beam in this system is described as having ``high brightness,'' and when it is bent repetitively in a magnetic undulator, undergoes a radiation-mediated microbunching instability. This instability can amplify the original radiation amplitude at a particular, resonant wavelength by many orders of magnitude. In order to obtain high brightness beams, it is necessary to compress them to obtain higher currents than available from the electron source. Compression is accomplished by the use of magnetic chicanes, which are quite similar to, if much longer than, a single period of the undulator. It should not be surprising that such chicanes also support a radiation-mediated microbunching interaction, which has recently been investigated, and has been termed coherent synchrotron radiation (CSR) instability. The purpose of this paper is to compare and contrast the characteristics of the closely related FEL and CSR microbunching instabilities. We show that a high-gain regime of the CSR instability exists which is formally similar to the FEL instability.

Simulations and experiments with space-charge-dominated beams

Beams in which space charge forces are stronger than the force from thermal pressure are nonneutral plasmas, since particles interact mostly via the long-range collective potential. An ever-increasing number of applications demand such high-brightness beams. The University of Maryland Electron Ring [P. G. O’Shea et al., Nucl. Instrum Methods Phys. Res. A 464, 646 (2001)], currently under construction, is designed for studying the physics of space-charge-dominated beams. Indirect ways of measuring beam emittance near the UMER source produced conflicting results, which were resolved only when a direct measurement of phase space indicated a hollow velocity distribution. Comparison to self-consistent simulation using the particle-in-cell code WARP [D. P. Grote et al., Fusion Eng. Design 32-33, 193 (1996)] revealed sensitivity to the initial velocity distribution. Since the beam is born with nonuniformities and granularity, dissipation mechanisms and rates are of interest. Simulations found that phase mixing by means of chaotic particle orbits is possible in certain situations, and proceeds much faster than Landau damping. The implications for using beams to model other N-body systems are discussed.

Improvement of brightness and output power of high-power laser diodes in the visible spectral region

This paper reports on a novel design for a high-brightness quantum-well AlGaInP laser-diode system operating at 635 nm. The spatial coherence of two broad area laser diodes is improved using spatial filtering and optical feedback from a narrow mirror stripe on each laser. To increase power, the outputs from the two systems are passively combined using a polarizing beam splitter. The result is a compact system providing a high-brightness beam. The beam-quality parameter is improved in one direction from M 2=12 and M 2=16 for the two lasers, to M 2=2.1 for the combined beam. The other direction of the beam is left unchanged due to its natural high quality with an M 2 value around 1.5. A procedure for efficient coupling of the two lasers into a small core-diameter optical fiber is given. This capability facilitates a wide range of applications including interstitial medical therapies, where delivery of the treatment light through thin optical fibers is essential.

Developing high brightness and high current beams for HIF injectors

The U.S. Heavy Ion Fusion Virtual National Laboratory is continuing research into ion sources and injectors that simultaneously provide high current (0.5–1.0 A) and high brightness (normalized emittance better than 1.0 π-mm-mr). The central issue of focus is whether to continue pursuing the traditional approach of large surface ionization sources or to adopt a multiaperture approach that transports many smaller “beamlets” separately at low energies before allowing them to merge. For the large surface source concept, the recent commissioning of the 2-MeV injector for the High Current eXperiment has increased our understanding of the beam quality limitations for these sources. We have also improved our techniques for fabricating large diameter aluminosilicate sources to improve lifetime and emission uniformity. For the multiaperture approach, we are continuing to study the feasibility of small surface sources and a RF-induced plasma source in preparation for beamlet merging experiments, while continuing to run computer simulations for better understanding of this alternate concept. Experiments into both architectures will be performed on a newly commissioned ion source test stand at Lawrence Livermore National Laboratory called STS-500. This stand test provides a platform for testing a variety of ion sources and accelerating structures with 500-kV, 17-μs pulses. Recent progress in these areas is discussed as well as plans for future experiments.

Noninvasive single-bunch matching and emittance monitor

On-line monitoring of beam quality for high brightness beams is possible only by using noninvasive instruments. For matching measurements, very few such instruments are available. One candidate is a quadrupole pickup. Therefore, a new type of quadrupole pickup has been developed for the 26 GeV Proton Synchrotron at CERN, and a measurement system consisting of two such pickups is now installed in this accelerator. Using the information from these pickups, it is possible to determine both injection matching and emittance in the horizontal and vertical planes, for each bunch separately. This paper presents the measurement method and some of the results from the first year of use, as well as comparisons with other measurement methods.

Self-consistent particle distribution of a bunched beam in RF field

An analytical solution for the self-consistent particle equilibrium distribution in an RF field with transverse focusing is found. The solution is attained in the approximation of a high brightness beam. The distribution function in phase space is determined as a stationary function of the energy integral. Equipartitioning of the beam distribution between degrees of freedom follows directly from the choice of the stationary distribution function. Analytical expressions for r– z equilibrium beam profile and maximum beam current in RF field are obtained.

A novel, very compact 1.0 MV singletron™ accelerator

Recently, a novel 1 MV terminal voltage, single ended accelerator has been put into operation at the Institute of Advanced Energy, Kyoto University, Japan. The coaxial Singletron™, in which three new technologies are made successfully for practical use, is characterised by a very small footprint of 1×2 m 2 and has been designed by High Voltage Engineering Europa B.V. It is part of an experimental set-up called DuET. The accelerator will be applied to simulate atom transmutation in neutron irradiated target materials in nuclear fusion material research. The accelerator is capable of delivering high brightness beams of e.g. hydrogen and helium. A typical brightness of >10 A m −2 rad −2 eV −1 was measured for a∼20 μA, 1 MeV H + beam. Initial test showed stable operation at 1 MV terminal voltage for three consecutive days with currents of 20 μA He + when the terminal was cooled directly.

Design of nondestructive emittance measurement device for H− beams

For the diagnostics of high-brightness particle beams as necessary for the European Spallation Source, spallation neutron source IFMIF (D1+), and HIDIF (Bi1+) nondestructive methods have to be developed. Conventional beam diagnostics are established, but are mostly destructive methods and suffer from the power density deposited on surfaces like slits or pinhole plates and beam loss. For negative ions a nondestructive method based on photoneutralization can be used to determine the transverse phase space distribution with high time resolution and a large number of phase space points. The interaction of laser light and H− ions produces a small number of neutral atoms as well as detached electrons. The beam of charged particles is separated magnetically from the neutral beam and the distribution of neutral hydrogen is measured. Under the assumption that the influence of the dipole is small enough, the laser induced emittance measurement can be used, e.g., near the entrance of radio-frequency quadrupole. The basic concept for the laser driven emittance measurement device will be presented together with design considerations and preliminary simulations on ion beam transport.

Challenges and applications of high brightness beams (plenary)

High intensity proton and H− beams are being used and proposed for a wide variety of applications such as spallation neutron sources, drivers for neutrino factories, and accelerator driven systems for waste transmutation and energy production. In cases involving storage rings or synchrotrons the designs are dependent on the production of intense, low emittance, chopped H− beams. Some of these applications are described and the effect of ion source performance on the designs is examined.

Thirty years of surface plasma sources for efficient negative ion production

Thirty years ago, July 1, 1971, significant enhancement of negative ion emission from a gas discharge following a small admixture of cesium was observed for the first time. This observation became the basis for the development of the Surface Plasma Source (SPS) for efficient production of negative ions. In the SPS, negative ions are produced from the interaction of plasma particles with electrodes on which adsorbed cesium reduces the surface work function. Following this discovery, the emission current density of negative ions increased rapidly from j∼0.01 A/cm2 to 3.7 A/cm2 with a flat cathode and up to 8 A/cm2 with optimized geometrical focusing in a long pulse SPS and up to 1 A/cm2 for a dc SPS. Charge-exchange cooling decreases a negative ion temperature below 1 eV and increases the negative ion brightness by many orders to a level compatible with the best proton sources, B=j/T>1 A/cm2 eV. The intensity of negative ion beams was increased from mA to tens of amperes. Features of different SPS’s, negative ion beam formation, transportation, space-charge neutralization over neutralization and instability damping are considered. Practical aspects of SPS operation and high brightness beam production will be discussed.

Developing high brightness beams for heavy ion driven inertial fusion

Heavy ion fusion (HIF) drivers require large currents and bright beams. In this article we review the two different approaches for building HIF injectors and the corresponding ion source requirements. The traditional approach uses large aperture, low current density ion sources, resulting in a very large injector system. A more recent approach merges high current density mini-beamlets into a large current beam and significantly reduces the injector size. Experiments are being prepared to demonstrate this approach.

Characterization of an 800 nm SASE FEL at saturation

Visible to Infrared SASE Amplifier is a free electron laser (FEL) designed to saturate at a radiation wavelength of 800 nm within a 4 m long, strong focusing undulator. Large gain is achieved by driving the FEL with 72 MeV, high brightness beam of BNL's accelerator test facility. We present measurements that demonstrate saturation in addition to the frequency spectrum of the FEL radiation. Energy, gain length and spectral characteristics are compared and shown to agree with simulation and theoretical predictions.

Space-charge effects in high brightness electron beam emittance measurements

The measurement of emittance in space-charge dominated, high brightness beam systems is investigated from conceptual, computational, and experimental viewpoints. As the self-field-induced collective motion in the low energy, high brightness beams emitted from photoinjector rf guns are more important in determining the macroscopic beam evolution than thermal spreads in transverse velocity; traditional methods for phase space diagnosis fail in these systems. We discuss the role of space charge forces in a traditional measurement of transverse emittance, the quadrupole scan. The mitigation of these effects by use of multislit- or pepper-pot-based techniques is explained. The results of a direct experimental comparison between quadrupole scanning and slit-based determination of the emittance of a 5 MeV high brightness electron beam are presented. These data are interpreted with the aid of both envelope and multiparticle simulation codes. It is shown that the ratio of the beam's $\ensuremath{\beta}$ function to its transverse plasma wavelength plays a central role in the quadrupole scan results. Methods of determining the presence of systematic errors in quadrupole scan data are discussed.

Longitudinal simulations and measurements of stretched bunches

Extending the method of Warnock and Ellison for the simulation of the Vlasov-Fokker-Planck equation for bunched beams [in Proceedings of the 2nd ICFA Advanced Accelerator Workshop on the Physics of High Brightness Beams, Los Angeles, CA, 1999 (World Scientific, Singapore, 2000)], we analyze arbitrary radio-frequency potentials and high- $Q$ impedances and study instabilities in stretched bunches. Results for the microwave instability driven by a broadband impedance and instability driven by high- $Q$ rf modes in stretched bunches are compared with measurements of the vacuum ultraviolet ring at the National Synchrotron Light Source. A method for the calculation of response functions is developed, and simulated response functions are compared to experimental results.

A novel ultra-short scanning nuclear microprobe: Design and preliminary results

The paper describes an optimized scanning nuclear microprobe (MP) with a new ultra-short (total length of 1.85 m) probe forming system based on a divided Russian quadruplet (DRQ) of magnetic quadrupole lenses. Modern electrostatic accelerators have a comparatively high beam brightness of about 10–25 pA/μm 2/mrad 2/MeV. This allows the MP proposed to provide a high lateral resolution even with large (1%) parasitic (sextupole and octupole) pole tip field components in all lenses. The features of the design permit the MP operation in the high current and low current modes with a short working distance and inexpensive quadrupole lenses. A new quadrupole doublet design has been developed for the MP. In the present work the calculated features of the new MP are compared with preliminary experimental results obtained with a similar system (total length of 2.3 m) at the INP in Cracow. The new MP is promising for studies of solids or biological samples with high resolutions (0.08–2 μm) in both modes under ambient conditions. A vertical version of the ultra-short MP can be very useful for single ion bombardments of living cells.

A high-brightness laser beam from a phase-locked multicore Yb-doped fiber laser array

We observed a high-brightness laser beam emitted from a closely packed array of seven Yb-doped single-mode fiber lasers embedded in a common cladding. The measured slope efficiency, greater than 65%, was achieved by clad-pumping from one end of the fiber. A theoretical model is established to provide a physical interpretation of the measured far field radiation patterns.

Self-consistent limited beam current in RF field

An analytical solution for a self-consistent particle equilibrium distribution in an RF field with uniform transverse focusing is applied to the problem of the high brightness beam current limit. The distribution function in phase space is determined as a stationary function of the energy integral. Particle distribution of the bright beam always tends to such a shape that the space charge beam potential is opposite to the external potential regardless of the applied field. Analytical expressions for r− z equilibrium beam profile and beam current limit in an RF field are obtained.

Using of slow positrons in various investigations – state of the art and perspectives

Slow positrons have become a unique probe in research of electronic structure of matter, in data acquisition on Fermi surface in metals and alloys, in studying of formation processes of structural and radiation defects in metals, semiconductors, polymers and nanocrystalline structures. First positron microscopes, PAES-spectrometers, positron ionization mass spectrometers and positron tomographs have been already created. In this paper results obtained by positron annihilation method in different research fields and possibilities of future progress in obtaining of high-intensity, high-brightness slow positron beams of variable energy are briefly discussed.

High-energy, high-power ytterbium-doped Q-switched fiber laser

We report on a Q -switched, cladding-pumped, ytterbium-doped large-mode-area fiber laser operating at 1090 nm that is capable of generating 2.3 mJ of output pulse energy at a 500-Hz repetition rate and more than 5 W of average output power at higher repetition rates in a high-brightness beam (M(2) = 3) . Using a similar fiber with a smaller core, we generated >0.5-mJ pulses in a diffraction-limited beam. Our results represent a threefold increase in pulse energy over previously published values for Q-switched fiber lasers and firmly establish fiber lasers as compact, multiwatt, multimillijoule pulse sources with large scope for both industrial and scientific applications.

Transverse phase space mapping of relativistic electron beams using optical transition radiation

Optical transition radiation (OTR) has proven to be a versatile and effective diagnostic for measuring the profile, divergence, and emittance of relativistic electron beams with a wide range of parameters. Diagnosis of the divergence of modern high brightness beams is especially well suited to OTR interference (OTRI) techniques, where multiple dielectric or metal foils are used to generate a spatially coherent interference pattern. Theoretical analysis of measured OTR and OTRI patterns allows precise measurement of electron beam emittance characteristics. Here we describe an extension of this technique to allow mapping of divergence characteristics as a function of transverse coordinates within a measured beam. We present the first experimental analysis of the transverse phase space of an electron beam using all optical techniques. Comparing an optically masked portion of the beam to the entire beam, we measure different angular spread and average direction of the particles. Direct measurement of the phase-space ellipse tilt angle has been demonstrated using this optical masking technique.