High-brightness Beam Research Articles

Preparation of Single-Atom Tips and Their Field Emission Behaviors

We have developed a simple and reliable method for preparing single-atom tips. Electrochemical techniques are applied to deposit a noble metal film on the W tip. With the protection of the metal film, the tip can be stored and transferred in the ambient condition. After a gentle annealing of the plated tip in vacuum, a thermally and chemically stable nano-pyramid with single atom sharpness can be generated at the tip apex. The atomic structure of the tip is imaged by a field ion microscope (FIM) layer by layer through field evaporation. The corresponding field emission patterns can also be determined by the field emission microscopy (FEM). Most importantly, the single atom sharpness as well as the pyramidal structure can be regenerated for tens of times in vacuum simply by annealing if the apex is accidentally damaged. Field emission measurements indicate that the single-atom tips can emit stable electron beams of high brightness with a small extension angle. These desirable features make the single-atom tips very promising for future applications. [DOI: 10.1380/ejssnt.2006.233]

Ion beams and their applications in high-resolution probe formation

The most critical demand for high-brightness ion beams emerged from the realm of industrial applications of focused ion beams and it was met by using liquid metal ion source technology. Although recognized as a successful technology, liquid metal ion sources have major limitations in view of beam species. Noncontaminating ion beam species with beam brightness comparable to liquid metal ion source, i.e., /spl sim/10/sup 6/ A cm/sup -2/ sr/sup -1/ and energy spread of /spl sim/1 eV will be ideal for many cutting-edge applications including ion beam milling, mask repair, and imaging studies. This paper attempts to bring out the essential points in the development of high-resolution probe forming beams. With this specific objective in view, this study is based on an analysis of point sources and then a variety of other sources, under the general classification of "broad" source (primarily plasma sources), are discussed. Several sources in this category including a Penning-type source, a multicusp source, and new-generation high-charged state sources seem to have merits for circumventing the limitations of liquid metal ion sources, although further research is needed to improve the beam brightness and advance the state-of-the art.

A conceptual pre-injector design for the KEK-ERL test accelerator

A conceptual design for a test accelerator based on an energy recovery scheme is under way in developing a next generation light source at KEK. A high bright electron beam will be delivered from the test accelerator with a maximum energy of 200 MeV using 1.3 GHz superconducting accelerating cavities installed in two cryomodules. It is of great importance to design a pre-injector that can generate an electron beam with exceptionally high brightness and low emittances. A new simulation code is being developed to simulate the beam dynamics of the electron beam and to optimize the beam characteristics at the pre-injector. The simulation code describes semi-analytically the two-dimensional (transverse and longitudinal) beam dynamics of the electron beam based on a well-known envelope equation taking into account the space charge effect. The purpose of this code is not to rigorously simulate the particle dynamics, as is done by the particle tracking code “Parmela”, but rather to quickly optimize the design parameters of the accelerator components and beam characteristics themselves. This code has been validated by comparison with the results obtained from the “Parmela” code using the pre-injector design parameters. In this report, the development and some numerical results obtained from this new simulation code are presented in detail.

Emittance compensation with dynamically optimized photoelectron beam profiles

Much of the theory and experimentation concerning creation of a high-brightness electron beam from a photocathode, and then applying emittance compensation techniques, assumes that one must strive for a uniform density electron beam, having a cylindrical shape. On the other hand, this shape has large nonlinearities in the space-charge field profiles near the beam's longitudinal extrema. These nonlinearities are known to produce both transverse and longitudinal emittance growth. On the other hand, it has recently been shown by Luiten that by illuminating the cathode with an ultra-short laser pulse of appropriate transverse profile, a uniform density, ellipsoidally shaped bunch is dynamically formed, which then has linear space-charge fields in all dimensions inside of the bunch. We study here this process, and its marriage to the standard emittance compensation scenario that is implemented in most recent photoinjectors. It is seen that the two processes are compatible, with simulations indicating a very high brightness beam can be obtained. The robustness of this scheme to systematic errors is examined. Prospects for experimental tests of this scheme are discussed.

Development of focused ion beam systems with various ion species

Conventional focused ion beam systems employ a liquid-metal ion source (LMIS) to generate high-brightness beams, such as Ga+ beams. Recently there has been an increased need for focused ion beams in areas like biological studies, advanced magnetic-film manufacturing and secondary-ion mass spectroscopy (SIMS). In this article, status of development on focused ion beam systems with ion species such as O2+, P+, and B+ will be reviewed. Compact columns for forming focused ion beams from low energy (∼3keV), to intermediate energy (∼35keV) are discussed. By using focused ion beams, a SOI MOSFET is fabricated entirely without any masks or resist.

Upgrading a Duoplasmatron ion source to produce high brightness beam for nuclear microprobe applications with a tandem accelerator

Development of stable high brightness ion sources with accelerators will enhance the capabilities for micro-analysis as well as the micro-fabrication. Recently we have upgraded a Duoplasmatron ion source associated with a 1.7MV tandem (NEC, Pelletron) accelerator to produce high brightness proton beam for nuclear microprobe application at the Louisiana Accelerator Center. This has substantially increased the ion source life time as well as the accelerator beam brightness. The beam brightness was found to be highly heterogeneous, with paraxial rays about an order of magnitude brighter than surrounding rays.

Plans for the future of proton accelerators at CERN

The Large Hadron Collider, presently in construction at CERN, will be filled through a set of high performance proton accelerators providing the high brightness beam needed to reach the foreseen luminosity. Although this difficult project has top priority and uses most of the CERN resources, it is nevertheless time investigating improvements of the proton accelerator complex for physical cases beyond the LHC expectations. The needs of multiple physics communities have to be taken into account, as well as the necessity of consolidating the installations while keeping high reliability. This paper starts from the analysis and proposals made by the “High Intensity Proton” (HIP) working group [ http://ab-div.web.cern.ch/ab-div/Projects/hip/ , M. Benedikt, K. Cornelis, R. Garoby, E. Metral, F. Ruggiero, M. Vretenar, “Report of the High Intensity Protons Working Group”, CERN-AB-2004-022 OP/RF] to improve the performances of the PS and the SPS complex and better match the users requests in a staged scenario at short and medium term, and complement it, addressing the main possibilities beyond that horizon.

Recent optimization of the beam-optical characteristics of the 6 MV van de Graaff accelerator for high brightness beams at the iThemba LABS NMP facility

With the aim of improving the reliability and stability of the beams delivered to the nuclear microprobe at iThemba LABS, as well as optimization of the beam characteristics along the van de Graaff accelerator beamlines in general, relevant modifications were implemented since the beginning of 2003. The design and layout of the beamlines were revised. The beam-optical characteristics through the accelerator, from the ion source up to the analysing magnet directly after the accelerator, were calculated and the design optimised, using the computer codes TRANSPORT, IGUN and TOSCA. The ion source characteristics and optimal operating conditions were determined on an ion source test bench. The measured optimal emittance for 90% of the beam intensity was about 50πmmmrad for an extraction voltage of 6kV. These changes allow operation of the Nuclear Microprobe at proton energies in the range 1MeV–4MeV with beam intensities of tenths of a pA at the target surface. The capabilities of the nuclear microprobe facility were evaluated in the improved beamline, with particular emphasis to bio-medical samples.

Considerations on accelerator systems requirements and limitations for μ-probe applications

It is commonly accepted that the requirements for μ-probe applications on beam brightness and energy spread favour single-ended accelerators. With the Singletron™ systems HVE can offer high beam brightness (74Am−2rad−2eV−1) and low energy spread as well as high terminal voltage stability (∼10ppm). On the other hand, Tandetron™ systems have the advantage that no ion source maintenance is required in the tank as opposed to single-ended systems. Especially for the larger single-ended systems needed to obtain higher terminal voltages, source maintenance is time consuming. Tandem accelerators provide higher beam energies at the same terminal voltage. This offers the possibility of greater probe depths e.g. larger than 1mm for protons (10MeV) or over 300μm for He3 (15MeV) in biological samples, which can be desired in single-cell irradiation experiments.Single-ended and tandem systems exhibit several important differences. First, of course the difference in ion sources, as for a tandem system a negative ion source is needed. The development of these sources has led to the availability of the multicusp ion source that combines low beam emittance with high current. Furthermore, related to the stripping process needed with a tandem accelerator, energy straggling of the ions increases the energy spread and small angle scattering of the ions tends to lower the beam brightness.At HVE, we investigated the influence of the small angle scattering on the beam brightness. Various ion optical approaches were compared and it was concluded that a multicusp ion source provides the possibility to minimise the degradation in the beam brightness due to small angle scattering of the ions. It is expected that a beam brightness of 10–50Am−2rad−2eV−1 can be achieved with the Tandetron™ accelerator systems.

Electron effects in the Neutralized Transport Experiment (NTX)

The Neutralized Transport Experiment (NTX) at the Heavy Ion Fusion Virtual National Laboratory is exploring the performance of neutralized final focus systems for high-perveance heavy ion beams. To focus a high-intensity beam to a small spot requires a high-brightness beam. In the NTX experiment, a potassium ion beam of up to 400 keV and 80 mA is generated in a Pierce-type diode. At the diode exit, an aperture with variable opening provides the capability to vary the beam perveance. The beam is transported through four quadrupole magnets to a distance of 2.5 m. The beam can be neutralized and focused using a MEVVA plasma plug and a RF plasma source. We shall report on the measurement of the electron effects and the ways to mitigate the effects. Furthermore, we shall present the results of EGUN calculations consistent with the measurements effects of the electrons.

Production of a high brightness beam from a large surface source

An experiment was set up on a 500-kV test stand to study the beam optics of an extraction diode using a 10-cm diameter alumino-silicate source. The beam was also sent through an aperture to reduce aberrations, thus enhancing its brightness. In comparing the experimental results with WARP-3D computer simulations, we found excellent agreement in the emittance diagrams and in the beam current rise-time. By benchmarking the computer code, we now have the capability to perform detail optical designs and to create the proper voltage waveform for producing a current pulse with a fast rise-time. A fast rise-time pulse is desirable because many potential near-term HIF experiments will require short beam pulses which have a fast rise-time.

Cold atomic beams of high brightness

The possibility is studied for obtaining intense cold atomic beams by using the Renyi entropy to optimise the laser cooling process. It is shown in the case of a Gaussian velocity distribution of atoms, the Renyi entropy coincides with the density of particles in the phase space. The optimisation procedure for cooling atoms by resonance optical radiation is described, which is based on the thermodynamic law of increasing the Renyi entropy in time. Our method is compared with the known methods for increasing the laser cooling efficiency such as the tuning of a laser frequency in time and a change of the atomic transition frequency in an inhomogeneous transverse field of a magnetic solenoid.

Progress on plasma accelerators: from the energy frontier to tabletops

This paper reviews the concepts, recent progress and current challenges for realizing the tremendous electric fields in relativistic plasma waves for applications ranging from tabletop particle accelerators to high-energy physics. Experiments in the 1990s on laser-driven plasma wakefield accelerators at several laboratories around the world demonstrated the potential for plasma wakefields to accelerate intense bunches of self-trapped particles at rates as high as 100 GeV m−1 in millimetre-scale gas jets. These early experiments have been followed in the current decade by experiments that are advancing on several fronts—increasing the accelerated charge (to the several nanocoulomb level), producing higher transverse beam quality (to the mm mrad normalized emittance level) and accessing new physics regimes at higher laser power. Several groups are engaged in pursuing two key challenges for laser wakefield accelerators—producing beams with small energy spread and extending the interaction length from millimetres to centimetres and beyond. Major breakthroughs on both fronts have occurred in the past year. In parallel with the progress in laser-driven wakefields, particle-beam driven wakefield accelerators are making large strides. A series of experiments using the 30 GeV beam of the Stanford Linear Accelerator has demonstrated high-gradient acceleration of electrons and positrons in metre-scale plasmas as well as key scaling laws for a ‘plasma afterburner’, a concept for doubling the energy of a high-energy collider in a few tens of metres of plasma. In addition to wakefield acceleration, these and other experiments have demonstrated the rich physics bounty to be reaped from relativistic beam–plasma interactions. This includes plasma lenses capable of focusing particle beams to the highest energy density ever produced, collective radiation mechanisms capable of generating high-brightness x-ray beams, collective refraction of particles at a plasma interface and acceleration of intense proton beams from laser-irradiated foils.

Schottky-enabled photoemission in a rf accelerator photoinjector: possible generation of ultralow transverse thermal-emittance electron beam.

We present a clear signature of the Schottky effect in a rf photoinjector using photons with energy lower than the Mg cathode work function. This signature is manifested by the shift in the rf phase angle for the onset of the detection of photoelectrons via single-photon absorption and allows for a reasonable estimate of the field enhancement factor. This is a viable method to generate an electron beam with very low thermal emittance and thus, a high brightness beam.

A racetrack microtron with high brightness beams

Here we describe a racetrack microtron that provides electron beams at 12 energies from 4.85 to 34.2 MeV with ∼150 pC/bunch in ∼5 ps bunches having ∼10 mm mrad normalized transverse emittance. Our compact, inexpensive accelerator in addition to its external electron beams can generate electromagnetic radiation from ∼3 mm to ∼0.3 nm by a variety of mechanisms.

The Shanghai high-gain harmonic generation DUV free-electron laser

The Shanghai deep ultraviolet free-electron laser source (SDUV-FEL) is an HGHG FEL facility designed for generating coherent output with wavelength down to 88 nm. The design and the relevant R&D of this HGHG FEL source have been under way since 2000. Currently, a 150 MeV S-band electron injector is under construction as the first linac section to produce a high brightness beam. The design study and the present R&D status of the SDUV-FEL have been presented in this paper.

The Shanghai high-gain harmonic generation DUV free-electron laser

The Shanghai deep ultraviolet free-electron laser source (SDUV-FEL) is an HGHG FEL facility designed for generating coherent output with wavelength down to 88 nm. The design and the relevant R&D of this HGHG FEL source have been under way since 2000. Currently, a 150 MeV S-band electron injector is under construction as the first linac section to produce a high brightness beam. The design study and the present R&D status of the SDUV-FEL have been presented in this paper.

High current density beamlets from a rf argon source for heavy ion fusion applications

In a new approach to develop high current beams for heavy ion fusion, beam current at about 0.5 ampere per channel can be obtained by merging an array of high current density beamlets of 5 mA each. We have done computer simulations to study the transport of high current density beamlets and the emittance growth due to this merging process. In our radio frequency (rf) multicusp source experiment, we have produced a cluster of 61 beamlets using minimum gas flow. The current density from a 0.25 cm diameter aperture reached 100 mA/cm2. The normalized 4 rms emittance of 0.0186π mm mrad corresponds to an equivalent ion temperature of 2.08 eV. These results showed that the rf argon plasma source is suitable for producing high current density beamlets that can be merged to form a high current high brightness beam for heavy ion fusion application.

High-resolution primary ion beam probe for SIMS

High-brightness negative ion beams, especially O − beams, from a compact gaseous plasma source are studied to evaluate the suitability for high-resolution SIMS. Ion-optical calculations are made using the beam parameters from the new source. Results suggest that sub-100 nm spot with O − beam current of 10 pA is achievable. Beam focusing, the resulting beam current distributions at the target and the expected instrumental performance are discussed.

Optical properties of tapered laser cavities

The properties of a tapered semiconductor laser cavity, which can be used to generate a high-brightness optical beam, are studied. The cavity consists of a narrow straight waveguide section and a tapered power amplifier section. The influence of the input waveguide width and length, as well as of the length and taper angle of the power amplifier section, on the overall performance of the cavity are discussed. Finally, the application of beam spoilers is studied. In order to solve the wave equation, the wide-angle-beam propagation method incorporating a higher-order finite-difference scheme is applied.