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High-intensity, low-emittance, short electron bunches are needed to attain self-amplified spontaneous emission (SASE) in X-ray free electron lasers (X-FEL) and for injection in future laser-Wakefield accelerators (LWFA). At such a high density, conventional techniques using magnetic compression of long bunches suffer from collective effects such as coherent synchrotron radiation, limiting the brightness achievable. On the other hand, when creating very short electron bunches using a high intensity femtosecond laser the internal space charge forces rapidly decrease the brightness of the bunch. However, these space charge forces become negligibly small at relativistic energies due to the Lorentz force and contraction. Simulations have shown that acceleration to relativistic energies in a field of 1 GV/m would preserve enough of the phase-space density to meet the requirements for SASE and bunch length sufficiently small for injection into a LWFA. Conventional (RF) accelerators have maximum fields of approximately 100 MV/m and are usually operated in the 10-30 MV/m range. A novel approach was proposed by Srinivasan-Rao et al. [T. Srinivasan-Rao and J. Smedley, 1996] in which a short high-voltage pulse is applied to a simple diode configuration to generate a GV/m electric field.

The performance of a 1 MW, long pulse, 130–250 GHz free-electron maser being designed in the FOM-Institute of Plasmaphysics for fusion applications has been simulated by a fully 3-D, AC and DC space charge included, non-wiggle averaged, particle-pusher code. In comparison with previous designs a new shorter construction of a step-tapered undulator providing suppression of the double frequency amplification on the linear stage and a perfect matching of the power grow on the nonlinear stage is considered. The dependences of the linear gain and output power on the operating frequency, reflection coefficient, gap length between two undulator sections and electron beam focusing efficiency are examined. Particular attention has been devoted to the influence of space charge on the operation of the FEM oscillator. Longitudinal space charge reduces the linear gain but increases the level of power which can be reached inside the resonator. It has been shown that for a fixed undulator length there is an optimum value of the electron beam current density providing sufficiently high linear gain and output power.

Results of optimization of an excimer laser-induced plasma x-ray source for projection lithography in the range λ = 13–15 nm are reported. A conversion efficiency of >0.7% in 2% BW has been achieved with high-Z target materials. Two methods of reducing contamination of optical elements by target debris have been tested: usage of a thin target layer (for Ta 1 μm was found to be optimal) and of a heavy buffer gas. The effect of the use of Ta-tape target and Kr buffer gas has been measured by determining the reflectivity of a Mo-Si multilayer sample after 10 5 shots.

Soft x-ray emission spectra (250-875 eV) are presented for plasmas, produced by picosecond and nanosecond frequency-doubled Nd:YAG-glass laser pulses incident on 14 different target materials. The emitted spectra have been corrected for various apparatus functions which enables a direct comparison between plasmas produced by pico- and nanosecond laser pulses. The relative integrated emission intensity as a function of Z number, obtained from the corrected spectra, shows an oscillatory behavior, with distinct maxima for those elements exhibiting a dominant line emission in our photon energy window. We found for our two pulse lengths an approximately equal conversion efficiency from laser light into x-ray photons. General suggestions are given as to what target material should be used for different applications using the laser plasma as x-ray source in the energy range Studied.

Phase I of the FELIX project involved the demonstration of rapid tunability over a range of IR wavelengths covering at least 8–80 μm. This phase was completed successfully by the recent commissioning of FEL-2 for the 5–30 μm range; FEL-1 had already been covering the 16–110 μm range since summer 1991. In Phase II of the project, the emphasis will be on user applications. The potential user interest has been assessed at a User Workshop. At present, 30 proposals for experiments from five different disciplines have been received. Details are given on the user requirements for wavelength, bandwidth and pulse structure. User operation will commence in September and is expected to reach full capacity by spring 1993.

In this paper we present the status of a joint development programme on soft x-ray projection lithography (SXPL) integrating work on high brightness laser plasma sources, fabrication of multilayer x-ray mirrors, and patterning of reflection masks. We are in the process of optimization of a laser-plasma x-ray source and measured the conversion-efficiency of laser light into x-rays. Furthermore we present results of etching patterns in x-ray reflection masks and we discuss improved deposition techniques to produce multilayer coatings with enhanced reflectivity.

The influence of the impact of 300 as well as 800 eV Ar+ ions on the surface and interface roughness of a bilayer of Mo on top of Si is reported. The materials are deposited by electron beam evaporation. X-ray reflection and Auger electron spectroscopy are used as diagnostic tools. After initial smoothing of the Mo surface, roughnning is observed after a dose of 2 × 1019 Ar+ ions/cm2 for both ion energies. Etching with 800 eV ions improves the surface smoothness a factor of two more than using 300 eV ions. For 300 eV Ar+ ions interface mixing is observed at a Mo film thickness

The structure of Ni-C multilayer and single nickel layer samples has been analyzed before and after annealing, using two techniques: fluorescence EXAFS (F1EXAFS) at the Ni-K. edge and CuKα reflection. Annealing at a temperature of 450°C resulted in a change in the structure of the nickel layers from amorphous like to crystalline like. A reduction of the Bragg reflectivity by a factor of 7 was also found. Comparison between the EXAFS data of the annealed sample and of a nickel foil show a difference in the amplitude of the EXAFS. This is ascribed to a non-Gaussian atomic distribution of the backscattering atoms in the annealed sample around their average positions, whereas the atomic distribution in the (polycrystalline) Ni foil is a Gaussian one. From the annealing experiments we conclude that no irreversible changes take place in the structure of the nickel layers at temperatures below 200°C.

The effects of two forms of ion bombardment treatment on the reflectivity of multilayer X-ray coatings were compared: ion etching of the metal layers, taking place after deposition, and ion bombardment during deposition, the so-called ion assisted deposition. The ion beam was an Ar+ beam of 200 eV, and the multilayer coatings studied were W-C, Ni-C and W-Si. Cu K α reflection measurements (λ = 0.154 nm) of Ni-C and W-C multilayer coatings having ion etched metal layers showed considerably higher reflectivities than those of coatings having as-deposited layers; a factor 2.5 and 4, respectively. Ion etching of tungsten layers in W-Si coatings did not result in enhanced reflectivities. We applied ion assisted deposition to nickel in Ni-C multilayer coatings. We observed enhanced in-situ soft X-ray reflectivities of ion assisted deposited nickel layers with respect to as-deposited layers. However, the observed increases were less than those obtained by ion etching, which we ascribe to bombardment-induced intermixing at the underlying Ni-C interfaces. Finally we demonstrate that during ion assisted deposition the deposition and ion etch rates are not additive, but that the ion etch rate increases. This indicates that during deposition a number of atoms are temporarily loosely bound to the surface.

A laminar grating of 1200 1/mm was coated with an x-ray reflecting multilayer coating. The multilayer coating consisted of 41 alternating layers of ReW and C having a period of 2.3 nm. In this paper we report on diffraction measurements of the coated grating at the CuKα emission line. We describe its reflection behavior using a simple theoretical model and derive two diffraction conditions, corresponding to the grating relation and the Bragg law, for which peak intensities are to be observed. We find that grating order efficiencies are modulated by the multilayer reflection.

The reflectivity and the energy resolution of multilayer X-ray mirrors depend on the interface roughness created during deposition of the layers. We have investigated the influence of argon ion impact during and after deposition by evaporation of nickel on the interface roughness of an Ni/C multilayer. Roughness changes have been monitored by X-ray reflection. Etching of one and a half periods of nickel by argon ions of energy 200 eV has turned out to improve the reflectivity of one interface by a factor of 3.

High-intensity, low-emittance, short electron bunches are needed to attain Self-Amplified Spontaneous Emission (SASE) in X-ray Free Electron Lasers (X-FEL) and for injection in future Laser-Wakefield accelerators (LWFA). At such a high density, conventional techniques using magnetic compression of long bunches will suffer from collective effects such as coherent synchrotron radiation, limiting the brightness achievable. On the other hand, when creating very short electron bunches using a high intensity femtosecond laser the internal space charge forces will rapidly decrease the brightness of the bunch. However, these space charge forces become negligibly small at relativistic energies due to the Lorentz force and contraction. Simulations have shown that acceleration to relativistic energies in a field of 1 GV/m would preserve enough of the phase-space density to meet the requirements for SASE and bunch length sufficiently small for injection into a LWFA. Conventional (RF) accelerators have maximum fields of approximately 100 MV/m and are usually operated in the 10-30 MV/m range. A novel approach was proposed by Srinivasan-Rao et al. in which a short high-voltage pulse is applied to a simple diode configuration to generate a GV/m electric field.

We report a significant increase of the reflectivity of a soft x-ray Mo/Si multilayer mirror after low energy hydrogen ion beam bombardment of each of the Si layers after deposition. Cross section transmission electron microscopy pictures indicate no significant qualitative difference in interface roughness between the two samples. Elastic recoil detection and Rutherford backscattering spectrometry reveal a concentration of 22 at. % of H in the ion beam bombarded Si layers and a 12% reduction of the Si atomic density. Calculations using the measured atomic density and a very simple roughness model agree with the measured reflectivities. This is the first report of the modification of atomic density of Si in order to change the x-ray optical constants of the Si layer.

In the Fusion-FEM electrostatic Free Electron Maser, an electron beam loss current of less than 0.2% is essential for long-pulse operation. At reduced beam current, 3 A instead of the nominal 12 A, we have demonstrated electron beam acceleration and transport through the undulator at current losses less than 0.02%. This paper summarizes the results of beam transport experiments and outlines the simple yet effective beam diagnostics system. The status of the mm-wave cavity and low-power measurements are given. It is shown that cavity characteristics, such as losses, bandwidth, frequency-tuning and feedback coefficient, can be checked in situ via the output window, by injecting a low power test-signal into the mm-wave system and analyzing the reflected signal. (C) 1998 Elsevier Science B.V. All rights reserved.

At the FOM Institute for Plasma Physics ''Rijnhuizen'', The Netherlands, the commissioning of a high-power, electrostatic free-electron maser is in progress. The design target is the generation of 1 MW microwave power in the frequency range 130-260 GHz. The foreseen application of this kind of device is as a power source for electron cyclotron applications on magnetically confined plasmas. The device is driven by a high-power electron beam. For long-pulse operation a low loss current is essential. A 3-A electron beam has been accelerated to energies ranging from 1.35 to 1.7 MeV and transported through the undulator at current losses below 0.02%. Further, it was shown that the beam line accepts an electron energy variation of 5% with fixed beam optics. This is essential for rapid tuning of the microwave frequency, over 10%. Electron beam simulations have shown to be remarkably accurate both for the prediction of the lens settings and for the intercepted current. The operational settings of the beam line which give the highest current transmission are within a few percent of the simulated values.

We report on design work and testing of several main components of the FOM-Fusion-FEM project. Measurements on the electron gun are presented, showing a uniform beam current profile and a low halo current. A time-dependent simulation study of the interaction between the electron beam and the mm-waves shows that, for proper undulator tapering, longitudinal mode competition results in a quasi single-mode regime. Further, some high power and detailed low power measurements on mock-ups of the waveguide system are presented, showing that the reflection and outcoupling systems perform well.

We have investigated the details of the growth of electron-beam deposited molybdenum and silicon layers and the effect of ion-beam bombardment on the morphology and interface smoothness of those layers. Using in-situ X-ray reflectivity, θ-2θ reflectivity scans, Auger Electron Spectroscopy (AES) and Transmission Electron Microscopy (TEM) we find that a reduction in both Mo and Si surface roughness can occur as a result of the ion-beam bombardment. However, the overall smoothing effect is limited by interface mixing at the underlying interface and it is also dependent on the deposition morphology for Mo layers. When depositing multilayers with 10 periods of 6 nm we find a large (factor 4) improvement of the reflectivity as a result of the ion-beam bombardment. Our best results, obtained by polishing all Si layers after growth with 300 eV Kr+ ions, give a roughness of 0.3 nm for the Mo-on-Si interfaces and less than 0.5 nm for Si-on-Mo interfaces. Cross-section TEM pictures confirm the observations qualitatively.

The FOM Institute for Plasma Physics has obtained funding for the development of a 1 MW, long pulse, 140–250 GHz free-electron maser. The engineering design is presently being performed in an international collaboration. In this paper the main components of the free-electron maser, the electron beam line, the undulator and the millimetre-wave system, are described. The basic design parameters of these components, which are optimized with the help of a fully 3-D, particle-pusher, FEM simulation code, are reported.

The rapid development of high average power excimer lasers pushes the potential of a laser plasma x-ray source for SXPL. Now that the problem of debris production is solved at least at the prototype level [1] the source might be a candidate for operation in an industrial environment. The development of high reflectivity multilayer coatings for soft x-rays at normal incidence makes it possible to produce a demagnifying x-ray optical system. Together with an x-ray reflection mask all ingredients to develop a SXPL system are present. A cooperation of research institutes, semiconductor industry, resist and laser manufacturers has been set up to develop the process technology required.

Recently ion beam etching of electron beam deposited metal layers on C has resulted in higher x-ray reflection coefficients1. In spite of the widely recognized importance of Mo/Si multilayer coatings for projection lithography, little work has been done to use the etching technique on multilayer mirrors made of this combination of materials. As Mo/Si multilayer mirrors of d-values greater than 12 nm. have already been made with reflectivities very close to the theoretical maximum2, our main interest is in multilayers with smaller d-values.

We report on design and development activities of an integrated soft-X-ray projection system, consisting of all-reflective multilayer optics and a repetitive laser plasma soft-X-ray source.

The FELIX (free electron laser for infrared experiments) project involves the construction and operation of a rapidly tunable FEL in the spectral region between 3 and 300 μm. The spectral region between 8 and (at least) 80 μm is addressed in the first stage of the project. Nearly all components of FELIX are now available. In this article we review the design of the main components, discuss the status of the project, and present results of some performance tests.

Resultats de mesures effectuees sur des multicouches composees de 160 couches de Ni ou Ni x Si y (4 A) et C(18 A). Discussion des possibilites d'utiliser les couches multiples comme element dispersif dans un monochromateur a doubles cristaux

The Free Electron Laser (FEL) has reached a stage of development, in which serious consideration of its potential use in fusion research becomes obvious. The principles of operation of the FEL are discussed, together with the similarities and differences with classical devices like gyrotrons on the one hand, and ordinary lasers on the other hand. The FEL's potentially high efficiency and large unit size, combined with wide tunability, are among the outstanding characteristics. It is to be expected that operation in the ‘optical guiding’ regime, for which criteria will be given, is the route to enhanced performance of the devices. An overview is presented of current projects for FEL development, either in direct connection with, or possibly relevant for, fusion. Applications in this area include both heating, profile control and diagnostics in magnetic fusion, and driving implosions in inertial confinement.

The propagation of heat pulses originating from sawtooth activity in JET has been investigated in a series of limiter discharges with the following parameters: plasma current, Ip 3 MA, toroidal magnetic field, BT 3 T and elongation, κ = 1.45. The auxiliary power was varied such that the total power ranged from 2 to 13.5 MW. Electron temperature perturbations in a 20 cm region around a minor radius of r = (2/3)a were recorded with high time resolution, using a 12 channel electron cyclotron emission polychromator. From these measurements the electron heat diffusivity was derived. Over the whole range of powers considered, was found to be independent of power and to lie in the range of 2.5 ± 0.5 m2s−1. The quantity is compared to as derived from global power balance analysis. For Ohmic heating, the latter is lower than by a factor of 2.5. For increasing auxiliary power, approaches . A model for the dependence of the local χe n the temperature gradient is presented; it permits a unified description of the heat pulse behaviour, the deterioration of confinement and a certain degree of profile consistency. The model does not invoke non-local parameters such as the total power input. It is shown that the present heat pulse data, subject to this interpretation, contradict the τE scaling laws of the typical form τE ∝ P−0.5.

A review is given of the techniques to produce multilayer coatings with a d-spacing in the nanometer range, for application in X-ray optics. Methods to grow the films, to control thickness and to investigate layer boundaries are described. Investigations to obtain multilayer structures with a high energy resolution are discussed.

We review the status of the Dutch Free Electron Laser for Infrared eXperiments (FELIX), with which radiation in the range between 3 μm and 3 mm will be generated. Among our research objectives are (i) rapid tunability and (ii) mode reduction by means of an intracavity etalon. The first stage of the project deals with generation of radiation with a wavelength between 8 and 80 μm. The undulator of the former UK-FEL project will be used in this stage. The design of the accelerator, with which 70-A, 3-ps bunches are accelerated to a maximum energy of 45 MeV, is presented. It consists of a triode, a 4-MeV buncher, and two travelling-wave linac structures. Gain calculations leading to the best choice for the number of undulator periods are discussed.

We are currently engaged in a program on spectroscopy of electrons created in multi-photon ionization. Results on atomic Xe concerning additional absorption of photons in the N-photon continuum have been reported recently [1]. These data were obtained using a novel type of electron spectrometer described in a separate abstract.

An apparatus for time-resolved X-ray absorption measurements is discussed. The apparatus consists of a 15 ns, 20 J Nd:YAG/glass laser system, a polychromator for soft X-rays and a multichannel detector system. X-ray emission spectra of a laser produced plasma (LPP) for 15 different target materials are shown. Time resolved X-ray absorption measurements (90-190 eV) on clouds of Si particles, produced by laser irradiation of bulk Si, are presented. From these measurements the following results were obtained: Clusters are formed at an irradiance as low as 3.0 J/cm2; at an irradiance of 14 J/cm2 ionized species up to Si 4+ are formed; at an irradiance of J/cm2 at 15 ns pulse duration, a Si layer thickness of 80 A is removed.

In highly focussed nanosecond laser pulses the yield of electrons must be kept below about one hundred per lasershot, otherwise the space charge of the ions in the laser-focus will seriously influence the electron energy spectrum. Especially for the study of electrons produced in multiphoton ionization a spectrometer has been designed which accepts an opening angle of 2π sr. The laser beam is focussed in a region where a magnetic field of 1 T is present. The strong field diverges along the axis of a 50 cm. time-of-flight tube to a weak field of 10−3 T, in which an electron detector is positioned. Photoelectrons with any velocity in the direction of the detector spiral around the magnetic field lines and reach the detector. In the region of the field divergence the spread in forward velocities is reduced to 5×10−4 by an inversed magnetic-mirror principle. The total velocity is unaffected. Experiments show the resolution to be 20 meV for electrons up to 10 eV energy. By applying a retarding potential in the high magnetic field region it is possible to select an opening angle different from 2π sr, thus permitting measurements of angular distributions as well.

The roughness of a Ni-C interface, of interest for multilayer X-ray coatings, has been studied during growth using both X-ray reflection measurements and Auger electron spectroscopy. Preliminary measurements show that there is no diffusion of Ni into C or formation of nickel carbides at room temperature. The growth of Ni on C cannot be described by either smooth layer-by-layer growth or by island formation of Ni atoms on the C surface. An intermediate growth mechanism is proposed.

The thermal stability of the nickel-carbon system as well as the growth dynamics of nickel on carbon were investigated by AES. For temperatures higher than 120 degrees C, changes in the system, which can be ascribed to island formation are observed. Below 240 degrees C there is no indication of carbide formation. Changes in the Ni MNN peak are observed for temperatures higher than 240 degrees C but cannot be explained yet. For substrate temperatures of 20 and 120 degrees C a difference in the growth dynamics is observed when the film thickness exceeds 2.5 AA. The difference is explained by increasing island formation at high temperatures.

We have developed a pump-and-probe method to detect structural changes in surfaces under pulsed laser irradiation, by means of changes in the soft X-ray reflectivity. First results of reflectivity measurements of Si surfaces at X-ray energies around the Si L-edge (80–180 eV) are presented and compared to calculated reflectivities. The presence of the native silicon oxide layer is discussed. The measurements were performed with 20 ns X-ray pulses at an angle of incidence of 1.5° with respect to the surface, which was laser annealed at a power density of ~1 × 108 W/cm2. The reflectivity of the annealed surface, measured 30 ns after the anneal pulse, can be described by the product of the reflection by a solid surface and the absorption in the molten Si top layer. We have also performed preliminary measurements on the inter-diffusion process of a 5 A Ni overlayer into Si under pulsed laser irradiation. The reflection spectrum of this system shows, upon pulsed laser annealing, a clearly visible Si L-edge, indicating that the Ni has inter-diffused into the Si wafer. Apart from the Si edge, a new feature in the reflectivity becomes visible at 83 eV during annealing and remains visible afterwards. The feature was tentatively assigned as originating from Ni-oxide and/or Ni-silicide (Ni1+, M-edge). Measurements on the angle of incidence dependence of the reflectivity shows that the structures in the reflected signal are best visible at an angle of about 1.0°.