Solid deuterium (sD(2)) is used as an efficient converter to produce ultracold neutrons (UCN). It is known that the sD(2) must be sufficiently cold, of high purity and mostly in its ortho-state in order to guarantee long lifetimes of UCN in the solid from which they are extracted into vacuum. Also the UCN transparency of the bulk sD(2) material must be high because crystal inhomogeneities limit the mean free path for elastic scattering and reduce the extraction efficiency. Observations at the UCN sources at Paul Scherrer Institute and at Los Alamos National Laboratory consistently show a decrease of the UCN yield with time of operation after initial preparation or later treatment ("conditioning") of the sD(2). We show that, in addition to the quality of the bulk sD(2), the quality of its surface is essential. Our observations and simulations support the view that the surface is deteriorating due to a build-up of D-2 frost-layers under pulsed operation which leads to strong albedo reflections of UCN and subsequent loss. We report results of UCN yield measurements, temperature and pressure behavior of deuterium during source operation and conditioning, and UCN transport simulations. This, together with optical observations of sD(2) frost formation on initially transparent sD(2) in offline studies with pulsed heat input at the North Carolina State University UCN source, results in a consistent description of the UCN yield decrease.

For the SwissFEL at PSI beside the hard x-ray beamline to start with a soft x-ray line is planned to cover the wavelength between 0.7 and 7.0nm. For full control of the polarization of the FEL light, APPLE undulators are forseen. In this paper the design of these devices is introduced and the preliminary magnetic configuration together with the optimization strategy is presented in details.

A big challenge of the third generation synchrotron light sources is to increase the beam lifetime. In the medium energy machines like the Swiss Light Source (SLS) at the Paul Scherrer Institute, Switzerland and ELETTRA at Sincrotrone Trieste, Italy the beam life time is dominated by the large angle intra-beam scattering. A third harmonic superconducting rf system allows to cope efficiently with this effect. By increasing the beam induced harmonic voltage, the slope of the global accelerating voltage seen by the electrons during one revolution in the storage ring is flattened and the electron bunches are lengthened therefore reducing the intra-beam scattering. In summer 2002 an idle superconducting third harmonic cavity was installed at SLS followed shortly by another one at ELETTRA. At SLS the beam lifetime was increased by a factor of three and the beam current could be increased from 150 to 400mA [1]. This represented the first superconducting application of a high harmonic rf system in a storage ring. The superconducting cavity is cooled with liquid helium at 4.5K by a dedicated cryogenic system. During the past ten years of operation we had eliminated step-by-step all the cryogenic problems encountered with the operation of this facility such that now it operates unattended on a 12 month basis. Stable operation at 400mA is now well established as a standard for the SLS storage ring. The paper presents the cryogenic design of the superconducting harmonic cavities, the cryogenic system and the control system. The most important cryogenic operational problems encountered in the last ten years of operation are discussed and the solutions presented.

In order to find electron sources of low emittance and high quantum efficiency, single tip cathodes with a microstructured surface are investigated. Emission currents up to 310 A were obtained, by combining a 2 ns, 50 kV accelerating voltage pulse with a 266 nm wavelength, picosecond (sigma(t) = 6.2 ps) laser delivering a few mu J pulse energy. The multifilamentary cylindric Nb3Sn tip with a typical diameter of 0.8 mm provides quantum efficiencies up to 0.5%. The microstructured needle has also been tested in a combined diode-rf electron gun with 500 kV, 250 ns pulsed bias voltage as a first step towards reducing emittance-spoiling space-charge forces. DOI: 10.1103/PhysRevSTAB.16.043401

After 10 years of operation the wiggler-source Materials Science beamline at the Swiss Light Source was the first beamline to undergo a significant upgrade. The replacement of the W61 wiggler by the cryogenic undulator U14 makes the SLS the first wiggler free third generation light source. With the help of the cryogenic technology [1], the period length could be reduced from 19 mm to 14 mm. With a minimum gap of 3.8 mm and the x-ray energy range could be extended to nearly 40 keV. The undulator has been built in cooperation with SPring-8 and Hitachi. PSI designed the liquid-nitrogen-based cryogenic system and made the magnetic measurements under cryogenic conditions before installation. To be cost efficient, the undulator shares the cryogenic refrigeration system with the monochromator. Operational aspects like stability or temporal response to gap changes will be discussed as well as the spectral performance.

The 250MeV SwissFEL injector test facility is in operation since August 2010. Measurements with the “CTF2 Gun 5” photocathode S-band rf-gun show promising beam parameters and satisfy the requirements of the SwissFEL project. Since the performance of the electron source is fundamental for the stability and brightness of a free electron laser, further gun optimization studies are pursued. Under investigation is currently a 3.6 cell C-band gun. First ASTRA simulations indicate that with this gun the peakcurrent can be increased, thanks to a shorter laser pulse and a higher initial acceleration, by almost a factor of two, at slightly better emittance values than the S-band “PSI Gun 1”. Since the beam-quality depends also on the achieved performance of the cathode, several copper cathodes had been tested in the SwissFEL injector test facility to analyze the observed rapid degradation of quantum efficiency.

The current density limit for photoemission from metals was measured in an rf photogun to be below 10(9)  A/m2. We have achieved 1.6×10(11)  A/m2 by photofield emission from a new type of photocathode made from a metallic-composite, multifilamentary Nb3Sn wire driven by a 266 nm picosecond laser pulse and a 2 ns, 50 kV accelerating voltage. This cathode has a micrometer arrayed structure with tens of thousands of Nb/Nb3Sn filaments embedded in a bronze matrix. Our measurements revealed the existence of a new electron emission regime at high laser fluence (100  mJ/cm2). We have extracted stably, and without any surface ablation, up to 4800 pC of charge. This corresponds to 0.9% quantum efficiency, 100 times larger than what is measured from conventional metallic photocathodes. The unexpected large and stable charge extraction cannot be explained by the 3-step model. Thanks to the small emitting area, the measured emittance (0.6  mm·mrad) is low in spite of the high current density and space charge effects. This cathode will be of benefit for many applications based on short and bright electron bunches.

The cryogenic permanent magnet undulator (CPMU) is an insertion device in which the permanent magnets are cooled down to cryogenic temperatures to improve the magnetic performance in terms of the remanent field and coercivity. As it was found recently, the peak field and coercivity of permanent magnet materials like NdFeB are increasing as the temperature is decreased and reached a maximum at around 130 K. This temperature is not directly attainable by boiling of standard cryogenic fluids like liquid nitrogen (77 K) or liquid helium (4.2 K). We present a practical and reliable cooling method based on the thermal shunt principle with either liquid nitrogen or a cryocooler as a cold source. Design criteria, cryogenic analysis and the layout of a CPMU based on this principle are presented. A new CPMU with a magnetic period of 14 mm and a magnetic length of 1.7 m, has been manufactured and is presently installed and in operation at the Swiss Light Source (SLS) as part of a collaboration between PSI (Paul Scherrer Institute), SPring-8 and Hitachi Metals, Ltd.

A new type of ultra-cold neutron (UCN) source based on the spallation process is under construction at PSI. The essential elements are a pulsed proton beam with highest intensity (Ip≥2mA) and a low duty cycle (1%), a lead spallation target, a large D2O moderator and a solid deuterium (sD2) converter system. Spallation neutrons are thermalized in the D2O, further cooled and partially downscattered into the ultra-cold neutron range in the sD2. The expected UCN density is higher than 1000UCN/cm3 in typical experiments, an increase of almost two orders of magnitude over the best source currently available (at ILL). The new UCN source is expected to come into operation in 2009.

The ultra cold neutron source under construction at the Paul Scherrer Institute is a new facility dedicated to the production of UCN. An essential element of this source is a 5 K solid ortho-deuterium moderator. The cryogenic system of the UCN source is presented emphasizing on the thermal process, design criteria, and evaluation of heat loads in the D2 condenser, para-ortho D2 converter and moderator.

Within the scope of the low emittance gun project (LEG) at PSI, research is conducted into the development of a high brightness electron source suitable for compact, short wavelength free electron lasers. The gun, supposed to generate up to 5.5 Amperes of beam current, consists of a pulsed DC diode followed by a 1 1/2 cell RF gun. Using specialized codes, the performance offield emitter arrays is evaluated assuming realistic geometries. As an alternative, we examine the performance of conventional photo emission using copper cathodes, which we compare to that of field emitter arrays.

The Paul Scherrer Institute (PSI) in Switzerland is aiming to build a compact and cost-effective X-ray FEL facility for the wavelength range 0.1 – 10 nm. Based on the generation of very low emittance beams, it consists of a lowemittance electron source followed by high-gradient acceleration, and advanced accelerator technology for preserving the initial low emittance during further acceleration and bunch compression. In order to demonstrate the feasibility of the concept and the emittance preservation, a 250 MeV test facility will be built. This machine has been designed to be used as injector for the X-ray FEL at a later date. The accelerator design of the 250 MeV linac will be presented together with the status of the low emittance source and high gradient acceleration.

Today most of the X-rays Free-Electron Laser projects are based on state of the art RF guns, which aim at a nor- malized electron beam emittance close to 1 mm·mrad. In this paper we report on the progress made at PSI towards a hybrid DC + RF Low Emittance Gun (LEG) capable of producing a beam with an emittance below 0.1 mm·mrad. To reduce the intrinsic thermal emittance at the LEG cath- ode the electrons are extracted from nano-structured field- emitters. A gun test facility is under construction wherein after emission the beam is accelerated up to 500 keV in a diode before being injected and accelerated in a two- frequency 1.5-cell RF cavity. The fast acceleration in the diode configuration allows to minimize the emittance di- lution due to the strong space charge forces. The two- frequency RF structure is optimized to limit the emittance blow-up due to the non-linearity of the RF field.

The PSI FEL Project at the Paul Scherrer Institute in Switzerland incorporates the development of a low emittance gun as a driver for a cost-effective X-ray free- electron laser user facility (λs ≥ 0.1 nm, ħωs ≤ 12.4 keV). We investigate sources based on field-emitter technology and photoemission, followed by high gradient (Γ ≥ 0.25 GV/m) acceleration up to 1 MeV. We present a concept to preserve the emittance in the acceleration process where the first 250 MeV of acceleration is the most delicate. Experimentally we intend to verify the validity of ultra- high brightness acceleration over this energy range in the period 2008-2011.

The Japan Atomic Energy Research Institute has a collaboration with the European Union, the United States of America and the Russian Federation for the International Thermonuclear Experimental Reactor (ITER). In the engineering design activity for ITER, a test coil named QUench Experiments on Long Length (QUELL), using 91 m and 1/5-size ITER superconducting conductor, was fabricated by JAERI. The performance tests were carried out at the SULTAN facility in Switzerland where quench propagation, thermal and hydraulic characteristics were determined and development and test of new quench detection system were conducted. The thermal and hydraulic behavior was not known well. This conductor has a central channel to reduce the pressure drop. In order to investigate the thermal and hydraulic characteristic of the conductor, the pressure drop has been measured at 5-13 K and 2-11 g/s, and the friction factor of the central channel was calculated. In heat slug propagation, an inductive and resistive heater on the conductor has been used and the velocity of the heat front and input energy are estimated from the temperature change of conductor.

The Quench of Long Length (QUELL) experiments have been completed. The US contribution was to develop and implement both conventional and novel techniques in quench detection. The results of the quench detection experiments demonstrate that all US systems functioned as expected. The most important criteria for the comparison of the various quench detection systems were the time constant of the response and noise rejection by the system. The novel US internal (inside the CICC) sensors included; (1) a cowound voltage sensor, and (2) a fiberoptic temperature sensor. The internal sensors combined fast response with high noise rejection capacity, and proved to offer potentially high reliability for ITER. The conventional sensors, namely the absolute pressure transducer and Venturi flowmeters, confirmed inherent thermo-hydraulic time response limitations in measurements of pressure and flow that excludes them as quench detectors for ITER and other large scale CICC applications.

Today most of the X-rays Free-Electron Laser projects are based on state of the art RF guns, which aim at a normalized electron beam emittance close to 1 mm·mrad. In this paper we report on the progress made at PSI towards a hybrid DC + RF Low Emittance Gun (LEG) capable of producing a beam with an emittance below 0.1 mm·mrad. To reduce the intrinsic thermal emittance at the LEG cathode the electrons are extracted from nano-structured fieldemitters. A gun test facility is under construction wherein after emission the beam is accelerated up to 500 keV in a diode before being injected and accelerated in a twofrequency 1.5-cell RF cavity. The fast acceleration in the diode configuration allows to minimize the emittance dilution due to the strong space charge forces. The twofrequency RF structure is optimized to limit the emittance blow-up due to the non-linearity of the RF field.