Brightness Electron Beams Research Articles

X-ray free electron laser in a crystal as distributed feedback resonator

The theory of an X-ray free electron laser inside a crystal is developed. In the Bragg diffraction geometry, a crystal provides a distributed feedback. In the low-gain regime expressions for the generation threshold are found which take absorption into account. Cold- and warm-beam regimes both for magnetic and for laser wigglers are considered. A detailed explanation of the physics of the generation condition is given. The case of an electron beam propagating in a narrow slit, without multiple scattering is considered. The advantage of a two-dimensional diffraction geometry is shown to be that it provides lower generation threshold values than those in one-dimensional geometry. The warm-beam case is found to be more preferable due to less rigid requirements for the beam quality. Realization of the given scheme requires an approximately 30-fold increase of the brightness of electron beams in storage rings.

Status of the free electron laser experiment at UT/NERL

A free electron laser experiment utilizing an existing rf linear accelerator has been proposed at UT/NERL (Nuclear Engineering Research Laboratory, University of Tokyo). Preliminary parameter studies showed that the development of a high-brightness beam is necessary for the FEL experiments. Some modifications to the linac have been made to increase the electron beam brightness. As a result, the beam quality has been largely improved. The construction of a beamline for the FEL experiments has also been completed and a wiggler magnet is now under construction.

Extreme ultraviolet free-electron laser-based projection lithography systems

Free-electron laser (FEL) sources, driven by rf linear accelerators, have the potential to operate in the extreme ultraviolet (XUV) spectral range with more than sufficient average power for high-volume projection lithography. For XUV wavelengths from 100 to 4 nm, such sources will enable the resolution limit of optical projection lithography to be extended from 0.25 to 0.05 μm with an adequate total depth of focus (1 to 2 μpm). Recent developments of a photoinjector of very bright electron beams, high-precision magnetic undulators, and ring-resonator cavities raise our confidence that FEL operation below 100 nm is ready for prototype demonstration. We address the motivation for an XUV FEL source for commercial microcircuit production and its integration into a lithographic system, including reflecting reduction masks, reflecting XUV projection optics and alignment systems, and surface-imaging photoresists.

The Los Alamos FEL photoinjector drive laser

The drive laser system of the photoelectric injector for the Los Alamos free-electron laser (FEL) is described. This system consists of a 1.053 mu m mode-locked Nd:YLF oscillator, a grating-rhomb pulse compressor, a fast electrooptic shutter to reduce pulse repetition frequency, a pulsed Nd:YLF amplifier chain, and a KTP frequency doubler. The bright electron beams required for high performance FELs demand stringent specifications on drive laser output levels, beam quality, and stability. The design specifications, laser architecture, and operating performance following the most recent round of upgrades are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Compact rf-linac free-electron lasers

Design studies using the INEX simulation method are presented for an infrared compact rf-linac-driven FEL. The possibility of such a device relies upon two technological advances: a very bright electron beam from a linac with a photocathode injector, and a short-period, high-field wiggler.

First demonstration of a free-electron laser driven by electrons from a laser-irradiated photocathode

We report the results from the first operation of a free electron laser (FEL) driven by an electron beam from a laser-irradiated photocathode. The Rocketdyne/Stanford FEL achieved sustained oscillations, lasting in excess of three hours, driven by photoelectrons accelerated by the Stanford Mark III radiofrequency linac. A LaB6 cathode, irradiated by a tripled Nd: Yag mode-locked drive laser was the source of photoelectrons. The drive laser, operating at 95.2 MHz, was phase-locked to the 30th subharmonic of the S-band linac. Peak currents in excess of 125 A were observed and delivered to the Rocketdyne 2 m undulator which was operated as a stand-alone oscillator. Sustainable small-signal gain of 100% per pass was observed over a 2 h time period with periodic observation of small-signal gain as high as 150% per pass. Preliminary estimates of the electron-beam brightness deliverable to the undulator range from 3.5 × 1011 to 5.0 × 1011 A/(rad m)2.

Optimization of LaB6 Cathode Tip Shape for Maximum Brightness Over Lifetime

LaB6 cathodes are widely used as high brightness cathodes in electron microscopy and are capable of providing about five times the brightness of a tungsten hairpin filament. It is desirable to optimize the shape of the LaB6 tip to provide the highest possible brightness and to insure that this high level of brightness is maintained over the life of the cathode.It is well known that a high brightness electron beam is important in obtaining ultimate resolution in electron microscopy. Brightness is defined as the current density per unit solid angle, or amperes per square centimeter per steradian, in the electron beam. In electron microscopy, one would like to obtain the maximum possible brightness for the particular electron gun. Brightness is a conserved quantity, meaning that as the beam traverses the column, brightness can not be gained, only lost. Therefore, one must begin with the brightest possible cathode in order to obtain the best possible electron beam brightness.Much work has been done to determine the optimum LaB6 cathode tip shape and crystallographic orientation which will provide the highest brightness over cathode lifetime. The purpose of this is to review some of the previous results, present further data, and draw conclusions as to the optimum LaB6 cathode tip shape for high sustained brightness over cathode life. Nearly all commercially available LaB6 cathodes for electron microscopy employ an axially oriented LaB6 &lt;100&gt; crystal with a conical tip. Most are made with a full cone angle of 2α=90° (Figure 1). Some have a small radius, hemispherical point at the apex of the cone, while others simply have a small diameter truncation (flat) on top of the cone. The geometrical parameters affecting cathode brightness which will be considered here are hemispherical tip radius (R) and flat diameter (ϕ). Of primary interest is the dependence of brightness over lifetime with the variation of these parameters, and the comparison between the hemispherical and the flat tips.

Measured brightness of electron beams photoemitted from multicrystalline LaB6

Laser-driven semiconductor photoemitters can provide the very bright beams of electrons needed in advanced accelerators. However, these semiconductors are easily degraded in operation. Photoemissive testing of the compound LaB6, which is expected to be a more environmentally rugged material, has shown that under excimer laser irradiation normalized electron beam brightnesses of 6.7×106 , 2.6×106 , and 1.5×105 A/cm2 rad2 can be achieved at photon wavelengths, respectively, of 193, 248, and 308 nm.

Photoelectron sources: Selection and analysis

Laser-driven photoemissive cathodes have generated extremely bright pulsed electron beams with normalized brightness of 107 A/(cm2 rad2). Intense beams have been observed with pulse widths of 60 ps and 50 ns, indicating a wide temporal range over which this technology is applicable. Although only Cs3Sb and GaAs(Cs, O), both irradiated by a frequency-doubled Nd laser, have, so far, been studied, other semiconductor photoemitters are available and metal, or metal compound, photoemitters, driven by UV lasers, are possible contenders. Proper selection of a photocathode for a specific free electron laser application is outlined. The problem of plasma formation, due to the application of excessive electric fields on the electrodes, is also addressed. Such plasmas limit the maximum current densities that may be extracted at a given accelerating voltage and pulse width. A comparison between photocathodes, thermionic cathodes, and cold cathodes is made in terms of their respective brigthnesses and potential for operation at high current densities.

Quantum-mechanical constraints on electron-beam brightness

Bounds on the brightness and quality of fermion beams are derived from Pauli's exclusion principle and the finite density of quantum eigenstates for free fermions. These bounds are applied to electron-beam sources to derive limitations on the quality ( gamma J/ Delta gamma /sub z/) of the electron beams they generate. In the absence of electromagnetic fields, the quality of a relativistic electron beam of current density J and kinetic energy ( gamma -1)mc/sup 2/ cannot exceed (J gamma pi ec)/sup 1/2/ lambda /sub c//sup -3/2/, where lambda /sub c/ is the Compton wavelength. The normalized brightness cannot exceed 2 Delta gamma ecc lambda /sub c//sup -3/, where Delta gamma mc/sup 2/ is the electron energy spread, and will be 1/ pi times the beam quality for a beam of maximal quality. It is concluded that the qualities of electron beams produced by existing linear accelerators for free-electron lasers could be increased by six to eight orders of magnitude before reaching the quality limit. >

High gain free electron laser for heating and current drive in the ALCATOR-C tokamak

We have used the free electron laser particle simulation code FRED to examine the design of an FEL for amplifying radiation in the 1–2 mm wavelength range for use in electron heating and current drive in a tokamak device such as ALCATOR-C. We have used as a desired design goal a peak output power of 8 GW, with a minimum input power in the 1–100 W range. We have examined the effects of electron beam current, energy and brightness, laser frequency and input power as well as wiggler wavelength and overall wiggler length on performance of the FEL. We find that to obtain the desired power output, we require a 3 kA electron beam with a brightness of about 1 × 10 5 A/cm 2. For a 1 mm wavelength system, the beam energy must be at least 10 MeV, the laser input power must be at least 100 W, and the wiggler wavelength is 8 cm, with an overall length of at least 6 m. For a 2 mm wavelength system, the output power goals can be met with a 7 MeV electron beam, with 10 W of input laser power, and a wiggler with an 8 cm wavelength and total length of at least 6 m. The beam and laser requirements to meet the design criteria are less stringent for the 2 mm wavelength system. The magnet requirements to meet the design power level goal is more difficult to meet with a wiggler wavelength of 8 cm compared to a 9.8 cm wiggler wavelength. With the 10 MeV beam energy of the 1 mm system and the 7 MeV beam energy of the 2 mm system, the peak magnetic field changes from about 0.4 to 0.6 T when the wiggler wavelength changes from 9.8 to 8 cm.

Soft X-Ray free-electron laser with a laser undulator

We discuss the possibility of building a free-electron laser in the spectral region of tens of nanometers using a high-power laser pulse as the undulator. Requirements on the electron beam emittance, brightness, and energy spread are derived. The possibility of operation at the quantum limit is considered. The reduction of the gain due to diffraction of the undulator pulse (the variable mass-shift effect) is investigated, and several ways in which it could be overcome are suggested. Examples are presented showing that the device is feasible with moderate advances in current electron injector technology. Such a device would not require a large accelerator for high electron energies, nor a long wiggler made of permanent magnets.

Squeezing of the cavity vacuum by charged particles.

It is shown that the electromagnetic vacuum in a cavity is squeezed if one or several charges are present. For a bright electron beam typical of present high-intensity free-electron lasers, the effect is not necessarily small.

Pulse-laser-irradiated high-brightness photoelectron source

Linac-driven free-electron lasers, new powerful synchrotron radiation sources, and advanced high-power microwave devices will require very bright emitters of electrons to operate efficiently. By irradiating a Cs3Sb photocathode with a frequency-doubled, Nd:glass laser, we have obtained a very high normalized electron beam brightness of over 1011 A/m2/rad2. Peak currents emitted from the 1 cm2 surface in 50 ns pulses ranged up to 80 A. Measured normalized emittances were between 5 and 9 π mm mrad.

Some aspects concerning design for e-beam testability

Electron beam testing has become a powerful inspection technique for integrated circuits under operation, since an electron beam can act as a non-loading and non-damaging probe which can easily be adressed to any node of the circuit. A successful application of e-beam testing to integrated circuits of higher complexity, however, necessitates modifications of both test equipment and device layout. Essential developments of the testing set-up include use of bright electron beam guns, improvement of electron energy spectrometers and more sensitive signal processing. Furthermore a hybrid test system connecting a CAD design system equipment for VLSI and an e-beam tester has to be established indispensibly in order to obtain exact and automated probing of these nodes by the electron beam. However, there is also a demand for a modified circuit layout being appropriate to e-beam testing due to the following aspects: • - By theoretical treatment the influence of enlarged surfaces of test points on the achievable test duration has to be quantified. • - Numerical simulations quantitatively demonstrate the importance of circuit internal shielding electrodes in the direct vicinity of the probed node on the minimization of measurement errors caused by cross talk from neighbouring conductor tracks. Such calculations are treated for various parameters, such as device geometry, potential distributions and types of electron energy spectrometers. • - Extending the application of e-beam testing to the analysis of passivated devices by means of the capacitive coupling voltage contrast, this method may be strongly falsified by inhomogenous coupling of the local potential distributions at the metallization-passivation-interface with the passivations surface towards vacuum. Testing of multi-level-interconnection-devices is even more influenced by such falsifications and by this only possible for certain device geometries. From the results reported in this paper design rules can be deduced guaranteeing a sufficient e-beam testability for future integrated circuits.

Numerical determination of injector designs for high beam quality

The performance of a free electron laser strongly depends on the electron beam quality or brightness. The electron beam is transported into the free electron laser after it has been accelerated to the desired energy. Typically the maximum beam brightness produced by an accelerator is constrained by the beam brightness delivered by the accelerator injector. Thus it is important to design the accelerator injector to yield the required electron beam brightness. The DPC (Darwin Particle Code) computer code has been written to numerically model accelerator injectors. DPC solves for the transport of a beam from emission through acceleration up to the full energy of the injector. The relativistic force equation is solved to determine particle orbits. Field equations are solved for self-consistent electric and magnetic fields in the Darwin approximation. DPC has been used to investigate the beam quality consequences of AK (anode-cathode) gap, accelerating stress, electrode configuration and axial magnetic field profile.

Physics design for the ata tapered wiggler 10.6 μ FEL amplifier experiment

We are presently designing and constructing a high-gain, tapered wiggler 10.6 μ FEL amplifier to operate with the 50 MeV ATA e-beam. The initial experiments will be done with a constant period (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</sub> = 8 cm), 5 m-long linear wiggler. For an input laser power of 800 MW and electron beam brightness of 2.105 A/(rad-cm)2, we hope to achieve a trapped particle fraction ˜0.5 and an energy extraction efficiency of ˜2% with a ˜10% taper in the wiggler magnetic field. This taper corresponds to decelerating the trapped particle approximately two full ponderomotive well (i.e. bucket) heights. In this talk, we will discuss the physics motivations behind our tapered wiggler design and initial experimental diagnostics.

High-Brightness Photoemitier Injector for Electron Accelerators

A free-electron laser (FEL) oscillator, driven by an rf linac, requires a train of electron bunches delivered to an undulator. The electron-beam brightness requirement exceeds that available from a conventional buncher. The demonstrated high peak brightness of laser-illuminated photoemitters indicates that the conventional buncher system might be eliminated entirely without the usual large loss in beam brightness that occurs in bunchers. A photoemitter with a current density of about 200 A/cm2 is located on an end wall of an rf cavity to accelerate a 60-ps bunch of electrons to 1 MeV as rapidly as possible. Preliminary experimental work and simulation calculations are presented.

Promising cathode materials for high brightness electron beams

Materials for high brightness cathodes are evaluated by defining a figure of merit, which is based on the Richardson–Dushman equation. By calculating the figure of merit of borides, carbides, nitrides, and some refractory metals, it is concluded that carbides of transition metals such as TiC, ZrC, TaC, and HfC are promising for high brightness cathodes.

Improved sensitivity of backscattered electron detection using beam brightness modulation with phase sensitive detection in scanning Auger instruments

Commercial Auger instruments employ electron beam brightness modulation (BBM) to obtain N(E) data using a phase-sensitive detection system. Backscattered electron detection sensitivity can be improved by using this BBM system with phase-sensitive detection to detect the modulated signal from the video amplifiers. In the JEOL JAMP-10 instrument, a gain of ∼40 was realized extending the useable range of the detector to 3 keV beam potential or at 10 keV to 0.1 nA beam current.