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We present a design study of an innovative scheme to generate high rep rate (MHz-class) GeV electron beams by adopting a two-pass two-way acceleration in a superconducting (SC) linac operated in continuous wave (CW) mode. The electron beam is accelerated twice by being reinjected in opposite direction of propagation into the linac after the first passage. Acceleration in opposite directions is accomplished thanks to standing waves supported in rf cavities. The task of recirculating the electron beam when it leaves the linac after first pass is performed by a bubble-shaped arc compressor composed by a sequence of double bend achromat. In this paper we address the main issues inherent to the two-pass acceleration process and the preservation of the electron beam quality parameters (emittance, energy spread, peak current) required to operate x-ray free electron lasers (FEL) with low jitters in the amplitude, spectral and temporal domain, as achieved by operating in seeding and/or oscillator mode a CW FEL up to 1 MHz rep rate. Detailed start-to-end simulations are shown to assess the capability of this new scheme to double the electron beam energy as well as to compress the electron bunch length from picoseconds down to tens of femtoseconds. The advantage of such a scheme is to halve the requested linac length for the same final electron beam energy, which is typically in the few GeV range, as needed to drive an x-ray FEL. The AC power to supply the cryogenic plant is also significantly reduced with respect to a conventional single-pass SC linac for the same final energy. We are reporting also x-ray FEL simulations for typical values of wavelengths of interest (in the 200 eV--8 keV photon energy range) to better illustrate the potentiality of this new scheme.

The limits of performance of the European XFEL 3.9 GHz superconducting cavities were investigated. Most cavities exhibited high field Q slope, reaching the breakdown field at approximately $22\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. We hypothesize that this limit is a feature of high frequency cavities and can be explained by a thermal model incorporating field dependent surface resistance. The results obtained from simulations were in good agreement with experimental data obtained at 2 K.

We present a study of an innovative scheme to generate high repetition rate (MHz-class) GeV electron beams by adopting a two-pass two-way acceleration in a super-conducting Linac operated in Continuous Wave (CW) mode. The beam is accelerated twice in the Linac by being re-injected, after the first pass, in opposite direction of propagation. The task of recirculating the electron beam is performed by an arc compressor composed by 14 Double Bend Achromat (DBA). In this paper, we study the main issues of the two-fold acceleration scheme, the electron beam quality parameters preservation (emittance, energy spread), together with the bunch compression performance of the arc compressor, aiming to operate an X-ray Free Electron Laser. The requested power to supply the cryogenic plant and the RF sources is also significantly reduced w.r.t a conventional one-pass SC Linac for the same final energy.

There is a strong demand for small foot-print high-flux hard X-rays machines in order to enable a large variety of science activities and serve a multidisciplinary user community. For this purpose, two compact Inverse Compton Sources (ICSs) are currently being developed in Italy. The most recent one is the Bright and Compact X-ray Source (BriXS) which has recently been proposed to produce very energetic X-rays (up to 180 keV) and high photon flux (up to 1013 photons/s with expected bandwidth of 1-10%). BriXS will be installed in Milan and it will enable advanced large area radiological imaging applications to be conducted with mono-chromatic X-rays, as well as allowing basic fundamental science of matter and health sciences at both pre- and clinical levels. Based on an energy-recovery linac (ERL) scheme and superconducting technology, BriXS will operate in CW regime with an unprecedented electron beam repetition rate of 100 MHz. The second Italian ICS light source is the Southern Europe Thomson back-scattering source for Applied Research (STAR) which is currently installed at the University of Calabria (UniCal). STAR is a compact machine that has been designed to produce monochromatic and tunable, ps-long, polarized X-ray beams in the range 40-140 keV with a photon flux up to 1010 photons/s and energy bandwidth below 10%. The electron beam injector is based on normal-conducting technology in S-Band with a repetition rate up to 100 Hz.

The need of a fs-scale pulsed, high repetition rate, X-ray source for time-resolved fine analysis of matter (spectroscopy and photon scattering) in the linear response regime is addressed by the conceptual design of a facility called MariX (Multi-disciplinary Advanced Research Infrastructure for the generation and application of X-rays) outperforming current X-ray sources for the declared scope. MariX is based on the original design of a two-pass two-way superconducting linear electron accelerator , equipped with an arc compressor, to be operated in CW mode (1 MHz). MariX provides FEL emission in the range 0.2–8 keV with 1 0 8 photons per pulse ideally suited for photoelectric effect and inelastic X-ray scattering experiments. The accelerator complex includes an early stage that supports an advanced inverse Compton source of very high-flux hard X-rays of energies up to 180 keV that is well adapted for large area radiological imaging, realizing a broad science programme and serving a multidisciplinary user community, covering fundamental science of matter and application to life sciences, including health at preclinical and clinical level.

The pumping behaviour of sputter-ion pumps (SIPs), as a direct result of their underlying pumping mechanisms, is heavily influenced by several factors, including operation pressure and composition of the residual gas being pumped. Depending on their pumping history, SIPs can be thus characterised by transient phases in which some gases are released prior to the attainment of equilibrium between pumped and re-emitted species. In recent years, the feasibility of innovative vacuum pumps, based on the combination of SIPs with non-evaporable getter (NEG) technology, has been demonstrated; the optimisation of this coupling requires in-depth characterisations of SIPs, thus entailing the fine tuning of new specific testing procedures, which should flank and integrate the consolidated practice for SIP performance assessment. In this paper, some examples of the characterization of small SIPs are reported, in particular showing how the performances for Ar and CH 4 can be evaluated in view of the synergic coupling with NEGs.

The MariX project (Multi-disciplinary Advanced Infra-structure for Research with X-rays) is a free electron laser (FEL) light source proposed by the INFN-Milan. It will produce highly coherent X-rays, in the range 0.2-8 keV, with ultra-short pulses (10-50 fs) and a repetition rate up to 1MHz. At the same time, MariX will host a compact monochromatic X-ray source, called BriXS (Bright and compact X-ray Source), by using an inverse-Compton scattering scheme, with energies up to 180 keV and a repetition rate of 100 MHz (continuous-wave CW operation) that will generate fluxes up to 1013 photons per second. In this paper, the Radio-Frequency (RF) and beam dynamics designs of the electron injector for the MariX-FEL project are presented. The choice of the main devices, such as the electron gun and the accelerating linear accelerators, as well as the main parameters for CW operation are discussed in detail.

SRF cavities are complex electro-magnetic resonators operating at cryogenic temperature and achieving world-class quality factors and accelerating fields. LASA designed, fabricated and tested many cavities for several research projects.

Laser triggered photocathodes are key components of the electron sources for the XFELs. Research on semiconductor photocathodes started in the 90s at INFN LASA Milano by studying multialkali antimonied compounds (Cs3Sb, K3Sb, and K2CsSb), showing that the high QE was coupled with very high sensitivity to vacuum pollution. To avoid these deterioration issues, we focused our activity on Cs2Te which, while sensitive to UV light, has better resistance to gas contamination. Since then, we have a dedicated R&D program aimed at developing a always more reliable photocathode and its associated transport systems. Today our photocathodes are used at FLASH and at European XFEL at DESY Hamburg, PITZ at DESY Zeuthen, APEX in LBNL, FAST at FNAL and at LCLS-II at SLAC.

Physical review accelerators and beams 20(4), 042006 (2017). doi:10.1103/PhysRevAccelBeams.20.042006, We report the experience of the production, processing and qualification testing of the superconducting radio frequency cavities at 3.9 GHz for the third harmonic system at the European XFEL (EXFEL) injector. The rf structure concept, originally developed for the FLASH FEL facility, was adapted to the new interfaces provided by the EXFEL design and the cavities were procured from a qualified vendor, delivered ready for the testing at the INFN infrastructure. A total of 23 cavities, three prototypes and two batches of 10, have been realized and tested up to specifications, Published by American Physical Society, College Park, MD

Physical review accelerators and beams 19(9), 092001(2016). doi:10.1103/PhysRevAccelBeams.19.092001, The production of over 800 1.3-GHz superconducting (SC) cavities for the European X-ray Free Electron Laser (EXFEL), the largest in the history of cavity fabrication, has now been successfully completed. In the past, manufacturing of SC resonators was only partly industrialized; the main challenge for the EXFEL production was transferring the high-performance surface treatment to industry. The production was shared by the two companies RI Research Instruments GmbH (RI) and Ettore Zanon S.p.A. (EZ) on the principle of “build to print”. DESY provided the high-purity niobium and NbTi for the resonators. Conformity with the European Pressure Equipment Directive (PED) was developed together with the contracted notified body TUEV NORD. New or upgraded infrastructure has been established at both companies. Series production and delivery of fully-equipped cavities ready for cold rf testing was started in December 2012, and finished in December 2015. More than half the cavities delivered to DESY as specified (referred to “as received”) fulfilled the EXFEL specification. Further improvement of low-performing cavities was achieved by supplementary surface treatment at DESY or at the companies. The final achieved average gradient exceeded the EXFEL specification by approximately 25%. In the following paper, experience with the 1.3-GHz cavity production for EXFEL is reported and the main lessons learned are discussed., Published by American Physical Society, College Park, MD

In recent years the research on nuclear power generation focused on an innovative subcritical reactor concept, along with previous liquid-metal-cooled critical reactors. The accelerator-driven system reactor design matches higher and intrinsic safety requirements with the reduction of actinides and long-lived fission products, encumbrances on the nuclear waste final repository. The coupling of the accelerator technology with the reactor facility faces new challenges; the first is the design of the interface between accelerator and reactor. Currently two solutions are proposed and investigated: one with a solid beam-target window interface and the other one without a beam window. Our speculations focus on the windowless approach: No physical barrier is located in the interface region, so the ultrahigh vacuum environment of the accelerator is connected with the operative conditions of the reactor through an intermediate spallation target. In this work we describe our experimental activities and the numerical tool employed to give a basic characterization of the vacuum dynamics for a windowless interface, with particular regard given to proton beamline and target interface of the Ansaldo A80-XADS reference design.

The Photo Injector Test Facility at DESY Zeuthen (PITZ) was built to test and optimize electron sources for Free Electron Lasers and future linear colliders. The focus is on the production of intense electron beams with minimum transverse emittance and short bunch length as required for FEL operation. The experimental setup includes a 1.5 cell L-band gun cavity with coaxial RF coupler, a solenoid for space charge compensation, a laser capable to generate long pulse trains with variable temporal and spatial pulse shape, an UHV photo cathode exchange system, and an extensive diagnostics section. This contribution will give an overview on the facility and will mainly discuss the measurements of the electron beam transverse phase space. This will include measurements of the transverse and longitudinal laser profile, beam charge as a function of RF phase, and transverse emittance as a function of different parameters. The corresponding measurements of momentum and momentum spread as well as the RF commissioning results will be summarized. As a first application of the PITZ electron source it will be installed at the TESLA Test Facility Free Electron Laser at DESY Hamburg in autumn 2003.

TRASCO-ADS is a national funded program in which INFN, ENEA, and Italian industries work on the design of an accelerator driven subcritical system for nuclear waste transmutation. TRASCO is the Italian acronym for Transmutation (TRAsmutazione) of Waste (SCOrie). One of the most critical aspects in the design of an Accelerator Driven System is related to the interface region, which is the part of the beamline located between the accelerator, operating under UHV conditions, and the pressurized reactor vessel, consisting of a contained plenum of Pb-Bi eutectic (LBE). A so-called window could separate these two environments, but thermomechanical considerations and radioprotection issues point out that this component could be critical. In the windowless interface, no window is located between the linac and the spallation target. Only a suitable pumping and trapping system, for the gases and the vapors outcoming from LBE, divides the UHV accelerator and the spallation target vacuum. Vacuum gas dynamics theoretical considerations and calculations are presented in this article. The need for a validation of the theoretical models gave the motivation for an experimental work, whose results are also discussed. Scale-up of the experimental setup to the full system needs accurate analyses for a proper dimensioning of the system in the interface region.

The Photo Injector Test facility at DESY Zeuthen (PITZ) was built to develop electron sources for the TESLA Test Facility Free Electron Laser and future linear colliders. The main goal is to study the production of minimum transverse emittance beams with short bunch length at medium charge (∼1 nC). The facility includes a 1.5 cell L-band cavity with coaxial RF coupler, a solenoid for space charge compensation, a laser capable to generate long pulse trains, an UHV photo cathode exchange system, and different diagnostics tools. Besides an overview of the facility, its main components and their commissioning, this contribution will concentrate on the first measurements at PITZ with photoelectrons. This will include measurements of the transverse and longitudinal laser profile, charge and quantum efficiency, momentum and momentum spread, transverse electron beam profiles at different locations and first results on transverse emittance.

During the run 1998/1999 a new injector based on a laser-driven RF gun was brought in operation at the TESLA Test Facility (TTF) linac at DESY, in order to produce the beam structure and quality required either by TeV collider and SASE FEL experiments. High quantum efficiency cesium telluride photocathodes, prepared at Milano and transferred to DESY, have been successfully operated in the RF gun. A bunch charge of 50 nC, only limited by space charge effects, was achieved. The photocathodes have shown an operative lifetime of several months. A new cathode surface finishing has showed a promising decrease of the photocathode dark current. Measurements of dark current, quantum efficiency and lifetime are reported.

In the framework of the Tesla Test Facility (TTF) collaboration for the Injector II, we present a study on the formation and characterization of photoemissive films of Cs2Te grown on Mo substrata. The measured Quantum Efficiency (QE) has been analyzed in terms of the preparation procedures followed to obtain the photoemissive films. Using AES and XPS techniques we have monitored the evaporated materials (Te and Cs) growth into a film using a “step-by-step” analysis. We observed the formation of different compounds before the formation of a stoichiometric Cs2Te. The spectral response of the emissive layers has been measured in the wavelength range 254–514 nm, produced from a high-pressure mercury lamp and CW lasers. In view of the applications of this photoemitter, it is important to know the effect of gas pollution on a film prepared with a standard procedure. Since oxygen is one of the most reactive gases, oxygen contamination on Cs2Te films has been chosen as a criterion for the investigation of QE degradation. An exposure to 5 × 10−5 mbar s of oxygen has been found to be enough to reduce the QE by one order of magnitude. The XPS technique has been used to study the effects of the oxidation. We also investigated the “rejuvenation” effect (i.e., the recovery of QE after the degradation induced by the oxygen exposure) produced by the contemporary action of heating and illumination with 254 nm radiation from a mercury lamp. The results reported in this paper allowed us to produce reproducible and reliable photocathodes (QE = 13% at 254 nm) and to understand the cathode formation steps. Moreover, we reached an initial understanding of the pollution and rejuvenation mechanisms.

Different methods for the preparation of KCsSb photocathodes, suitable for electron sources in RF guns, are described in detail. The resulting material has been characterized by Auger depth profiling technique, and by its spectral response in the 250–600 nm range, as well. The results of lifetime experiments under operational and “laboratory” conditions are presented. The role of CO 2 in reducing the operational lifetime of the photocathodes and that of O 2 in improving the long-wavelength response are investigated.

Thin films of Cs2Te have been produced and analyzed by Auger depth profiling and x‐ray photoemission spectroscopy (XPS). The formation of the photoemissive material passes through different phases, each of them has been characterized by XPS and by its total yield in the spectral region 3.5–5 eV. Copper and molybdenum substrates have been considered. While Mo behaves to all practical purposes like an ideal support for Cs2Te, strong diffusion from the substrate material into the photoemissive film has been observed on photocathodes fabricated on Cu. The ruggedness of the photocathodes has been tested by exposing them to a few hundred Langmuirs of different gases, namely O2, CO2, CO, N2, and CH4. The last three have no effect on the photocathode lifetime, while a substantial reduction of the quantum efficiency has been observed after the exposure to oxygen. The main reason for this is the formation of a thick cesium oxide layer at the surface of the photocathode. However, the oxygen pollution can be partially recovered by the combined effect of heating the photocathode at 230 °C and illuminating the poisoned material with the 4.9 eV radiation. No rejuvenation has been observed under the effect of the temperature or the radiation alone.

High brightness electron sources for linac based short-wavelength free-electron lasers are developed and optimized for small transverse projected emittance at the photo-injector test facility at DESY, location Zeuthen (PITZ). A major part of the measurement program at PITZ is dedicated to transverse phase space optimization in order to fulfill the requirements of the European X-ray free-electron laser (European XFEL). A laser-driven RF-gun, treated with a dry-ice sublimation-impulse cleaning technique, a new photocathode laser system allowing short rise and fall times of the flat-top temporal distribution as well as several new diagnostic components have been installed at PITZ in 2008. The electrons generated via the photo-effect at a cesium telluride (Cs2Te) cathode are accelerated by a 1.6 cell L-band RF-gun cavity with a maximum accelerating gradient at the cathode of about 60 MV/m. The transverse projected emittance is measured using a single slit scan technique. In the 2008–2009 run period, a detailed characterization of the projected transverse emittance was performed at different operating conditions. Optimizations and measurement results as well as simulation predictions of the transverse projected emittance for bunch charges of 1, 0.5, 0.25 and 0.1 nC are presented and discussed in this paper. The geometric mean of the normalized projected rms emittance in both transverse directions for an electron bunch charge of 1 nC was measured to be

The photoinjector test facility at DESY, Zeuthen site (PITZ), was built to develop and optimize photoelectron sources for superconducting linacs for high-brilliance, short-wavelength free-electron laser (FEL) applications like the free-electron laser in Hamburg (FLASH) and the European x-ray free-electron laser (XFEL). In this paper, the detailed characterization of two laser-driven rf guns with different operating conditions is described. One experimental optimization of the beam parameters was performed at an accelerating gradient of about 43 MV/m at the photocathode and the other at about 60 MV/m. In both cases, electron beams with very high phase-space density have been demonstrated at a bunch charge of 1 nC and are compared with corresponding simulations. The rf gun optimized for the lower gradient has surpassed all the FLASH requirements on beam quality and rf parameters (gradient, rf pulse length, repetition rate) and serves as a spare gun for this facility. The rf gun studied with increased accelerating gradient at the cathode produced beams with even higher brightness, yielding the first demonstration of the beam quality required for driving the European XFEL: The geometric mean of the normalized projected rms emittance in the two transverse directions was measured to be 1.26±0.13 mm mrad for a 1-nC electron bunch. When a 10% charge cut is applied excluding electrons from those phase-space regions where the measured phase-space density is below a certain level and which are not expected to contribute to the lasing process, the normalized projected rms emittance is about 0.9 mm mrad.

Based on our experience on photocathode production, we present in this paper the results of the application of different optical diagnostic techniques on fresh and used photocathodes. These techniques allow studying effects like non uniformity, cathode aging, etc. In particular, photocathode optical parameters and QE characterization, both done at different wavelengths, give fundamentals information for the construction of a model of the photoemission process to be applied to Cs2Te photocathodes. These studies are useful for further improving key cathode features, such as its robustness and lifetime as well as to study and control the photocathodes thermal emittance.

For photocathode rf guns, the amount of dark current depends on the cavity surface as well as on the photocathodes. Smooth conditioning reduces the amount of dark current. Multipacting in the gun cavity changes the surface status of the cathodes and sometimes obstructs the gun operation because of vacuum interlocks. In this paper, dark current and multipacting features of an rf gun are presented including experimental and simulation studies.

The ability to produce high brightness electron beams as required for modern Free Electron Lasers (FELs) has been demonstrated during the first stage of the Photo Injector Test Facility at DESY Zeuthen (PITZ1). The electron source optimization at PITZ1 was successfully completed, resulting in the installation of the PITZ rf gun at the VUV-FEL (DESY, Hamburg). One of the main goals of the second stage of PITZ (PITZ2) is to apply higher gradients in the rf gun cavity in order to obtain smaller beam emittance by faster acceleration of the space charge dominated beams. In order to reach the required gradients a 10 MW klystron has to be installed and the gun cavity has to be conditioned for higher peak power. Another important goal of PITZ2 is a detailed study of the emittance conservation principle by using proper electron beam acceleration with a booster. Further photo injector optimization, including update of the photocathode laser and diagnostic tools, is foreseen as well. Recent progress on the PITZ developments will be reported.

Published online 16 November 2005 Groupe SERA; Many scientific disciplines ranging from physics, chemistry and biology to material sciences, geophysics and medical diagnostics need a powerful X-ray source with pulse lengths in the femtosecond range [1-4]. This would allow, for example, time-resolved observation of chemical reactions with atomic resolution. Such radiation of extreme intensity, and tunable over a wide range of wavelengths, can be accomplished using high-gain free-electron lasers (FEL) [5-10]. Here we present results of the first successful operation of an FEL at a wavelength of 32 nm, with ultra-short pulses (25 fs FWHM), a peak power at the Gigawatt level, and a high degree of transverse and longitudinal coherence. The experimental data are in full agreement with theory. This is the shortest wavelength achieved with an FEL to date and an important milestone towards a user facility designed for wavelengths down to 6 nm. With a peak brilliance exceeding the state-of-the-art of synchrotron radiation sources [4] by seven orders of magnitude, this device opens a new field of experiments, and it paves the way towards sources with even shorter wavelengths, such as the Linac Coherent Light Source [3] at Stanford, USA, and the European X-ray Free Electron Laser Facility [4] in Hamburg, Germany.

The SC RF group at INFN Milano-LASA is involved both in the TESLA/TTF collaboration and in the research and design activity on superconducting cavities for proton accelerators. Among these activities, some are supported by the European community within the CARE project. In the framework of the JRASRF collaboration we are developing a coaxial blade tuner for ILC (International Linear Collider) cavities, integrated with piezoelectric actuators for the compensation of the Lorenz force detuning and microphonics perturbation. Another activity, regarding the improved component design on SC technology, based on the information retrieving about the status of art on ancillaries and experience of various laboratories involved in SCRF, has started in our laboratory. Finally, in the framework of the HIPPI collaboration, we are testing two low beta superconducting cavities, built for the Italian TRASCO project, to verify the possibility to use them for pulsed operation. All these activities will be described here, together with the main results and the future perspectives.

Since the start-up of the laser driven rf gun based photoinjector at the TESLA Test Facility (TTF) late 1998, several cathodes have been used. We report on the properties of Cs/sub 2/Te and KCsTe cathodes under operating conditions, their quantum efficiency and lifetime. Darkcurrent emitted by the RF gun or the cathodes have been a major concern in the first year of operation. Meanwhile, new cathode production techniques and a conditioning effect of the RF gun has reduced the darkcurrent significantly.

2003 Particle Accelerator Conference, PAC2003, Portland, USA, 12 May 2003 - 16 May 2003; Hamburg : DESY 34-36 (2003)., The Photo Injector Test facility at DESY Zeuthen (PITZ) was built in order to study the production of high brightness electron beams, which are substantial for the successful operation of Free Electron Lasers (FEL) and linear colliders. The photoinjector at Zeuthen is based on a 1.5-cell L-band rf cavity with coaxial rf coupler equipped with emittance compensating solenoids, a laser capable to generate long pulse trains, an UHV photo cathode exchange system, and various diagnostics tools. The current goal of PITZ is a full characterization of the electron source, which will be installed at the TESLA Test Facility Free Electron Laser (TTF2-FEL) in autumn 2003. In the running periods before the gun is delivered toTTF2-FEL, the rf performance and the beam parameters will be measured in detail. The results presented in this contribution contain the measurements of dark current, driving laser parameters, beam charge, beam size along the facility, transverse emittance, momentum and momentum spread. The electron beam measurements will be presented in comparison with beam dynamics simulations., Published by DESY, Hamburg

R&D on high quantum efficiency (QE) alkali photocathode is in progress at Milano in the context of the ARES-TTF (Tesla Test Facility) program. High QE multialkali antimonide and cesium telluride cathodes have been produced in an advanced preparation system. Photoemitter preparation procedures have been set-up, in collaboration with the SESAMO lab. of the University of Modena, using Auger electron spectroscopy (AES): the AES technique was used, in conjunction with sputter erosion, to obtain the in-depth profile of the photoemissive film. In this way, photocathode preparation parameters (substratum cleaning procedure and temperature, alkali metal deposition rate, etc.) have been optimised. The spectral response of the photocathodes and the QE spatial distribution over the cathode surface has been measured using different light sources (254 nm

The facility at Fermilab was used to develop, fabricate, and commission an RF gun for the TeSLA Test Facility Linac. A time dependence is observed in the photo-emission and field emission from the photo-injector at Fermilab. The focussing solenoids can be adjusted to cause the QE and dark current to increase or decrease with time. The observed variation is in excess of a factor of 5 in the QE and a factor of 10 in the dark current, with time constants measured in hours. The evidence suggests that the source of the time-dependent behaviour is the Cs/sub 2/Te cathode.

We present experimental evidence that the free-electron laser at the TESLA Test Facility has reached the maximum power gain of 107 in the vacuum ultraviolet (VUV) region at wavelengths between 80 and 120 nm. At saturation the FEL emits short pulses with GW peak power and a high degree of transverse coherence. The radiation pulse length can be adjusted between 30 fs and 100 fs. Radiation spectra and fluctuation properties agree with the theory of high gain, single-pass free-electron lasers starting from shot noise.

High quantum efficiency photocathodes are routinely used as laser triggered emitters in the advanced high brightness electron sources based on radio frequency guns. The sensitivity of “semiconductor” type photocathodes to vacuum levels and gas composition requires special care during preparation and handling. This paper will discuss the results obtained using a novel pumping approach based on coupling a 20 l s−1 sputter ion getter pump with a CapaciTorr® D100 non evaporable getter (NEG) pump. A pressure of 8⋅10−8 Pa was achieved using only a sputter ion pump after a 6 day bake-out. With the addition of a NEG pump, a pressure of 2⋅10−9 Pa was achieved after a 2 day bake-out. These pressure values were maintained without power due to the ability of the NEG to pump gases by chemical reaction. Long term monitoring of cathodes quantum efficiencies was also carried out at different photon wavelengths for more than two years, showing no degradation of the photoemissive film properties.

We present the first observation of Self-Amplified Spontaneous Emission (SASE) in a free-electron laser (FEL) in the Vacuum Ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approx. 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width and intensity fluctuations all corroborate the existing models for SASE FELs., 6 pages including 6 figures; e-mail: joerg.rossbach@desy.de

A collaboration has been formed between FNAL, UCLA, LNFN Milano, the University of Rochester, and DESY to develop the technology of an RF photoinjector, followed by a superconducting cavity, to produce high bunch charge (8 nC) with low normalized emittance (

The present paper describes a two-module analog device for particle identification and spectra acquisition, using the power law technique with a counter telescope.

This paper discusses the simulation of analog signals from a nuclear radiation detector telescope. This goal is reached by means of a generator that reads 39 pairs of digital data punched on a paper tape and presents, as output, the corresponding analog pulses at a frequency variable up to 60 kHz.

This paper presents an updated status report on the Superconducting Cyclotron, under construction at the University of Milan during the past two years, with funding chiefly provided by the Italian National Institute for Nuclear Physics. The cyclotron is designed for a K of 800 and a K /SUB FOC/ of 200 and thus yields maximum energies of 100 MeV/n for fully stripped light ions and 20 MeV/n for uranium and other heavy ions. The pole radius is 90 cm with three sectors having an average spiral constant of 1/45.7 rad/cm. The average magnetic field will span between 22 and 48 K Gauss with a corresponding R.F. frequency range between 15 and 48 MHz. The anticipated peak dee voltage is 100 KV with harmonic operation from h = 1 to h = 4. The operating modes include the use of an internal PIG source, injection from a 16 MV tandem, and axial injection from an external ion source. This paper presents the design parameters and outlines the progress made so far.

In the framework of the ADS projects (Accelerator Driven System) developing worldwide, a reliability activity is on going to validate and qualify the linac accelerator design with focus on the general operational and design characteristics that shape the accelerator performance. Further quantitative results should be based on estimations mostly deriving from operational surveys at existing accelerator facilities. Currently, a validated accelerator component reliability data base has not yet been assembled, and because of the early stage of the ADS design in which several systems are not established at this time the topic can be addressed by the application of a preliminary FMEA (Failure Mode and Effect Analysis) methodology, helpful in the identification of reliability-critical areas, where modifications to the design can help to reduce the probability of system failures. In this paper, the preliminary results of this activity are presented together with possible solutions to improve the reliability of the reference linac design.

The quantitative description of low kinetic energy photoelectron emission in semiconductors is still an open question. In this article a model is developed to simulate the photoexcitation and transport of low kinetic energy electrons in Cs2Te. The statistical extension of the model, by Monte Carlo trajectory calculations, gives photon energy dependent quantum yields in agreement with experimental data. This is regarded as evidence that for near threshold photoemission in Cs2Te the dominant scattering mechanism is electron-phonon, while the mean energy scattering energy loss is 5 meV and the electron mean free path is 3 nm. The spatial distribution and the time response (0.4 ps) of the photoemitted electrons are estimated.

The total charge emitted from a polycrystalline Mo surface by 500 fs-264 nm laser pulses has been measured. Though a one-photon photoelectric effect is expected, a non-linear increase of the photoelectric yield was observed as a function of laser peak intensity, confirming earlier observations on Au, W and Zr. The threshold intensity for this non-linearity is 2 between 0.1 and 0.2 GW/ cm . The linear and non-linear regimes were clearly discerned in the experimental data. The non-equilibrium heating of the conduction electrons is considered as the cause of the observed non-linear behaviour. © 1999 Elsevier Science B.V. All rights reserved.

The total charge emitted from a polycrystalline Mo sample by 500 fs laser pulses at normal incidence is measured as a function of the laser peak intensity. Total yield data are taken at wavelengths of 527 and 264 nm. In both cases, a nonlinearity higher than expected is measured. A thermally enhanced regime is clearly observed when using 264 nm pulses for laser peak intensity larger than 0.1--0.2 ${\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$. This effect is interpreted on the basis of the nonequilibrium heating of the conduction electrons. Pump and probe photoemission data at 527 nm show a significant enhancement of the photoelectric sensitivity when the probe pulse is delayed by 1 ps from the pump. This enhancement is related to the growth of the available electron density induced by the nonequilibrium heating. Single pulse photoemission at this wavelength is not properly explained by a thermally assisted photoemission regime. This may indicate that other processes have a role in determining the photoemission yield.

Physical review letters 88, 104802 (2002). doi:10.1103/PhysRevLett.88.104802, Published by APS, College Park, Md.