F. Le Pimpec

Sharp reduction of the secondary electron emission yield from grooved surfaces

The effect of an artificially enhanced rough surface on the secondary electron yield (SEY) was investigated both theoretically and experimentally. Analytical studies on triangular and rectangular grooved surfaces show the connection between the characteristic parameters of a given geometry to the SEY reduction. The effect of a strong magnetic field is also discussed. SEY of grooved samples have been measured and the results agree with particle simulations using a Monte Carlo approach.

Electron Conditioning of Technical Aluminium Surfaces: Effect on the Secondary Electron Yield

The effect of electron conditioning on commercially available aluminium alloys 1100 and 6063 was investigated. Contrary to the assumption that electron conditioning, if performed long enough, can reduce and stabilize the secondary electron yield (SEY) to low values (⩽1.3, the value for many pure elements), the SEY of aluminium did not go lower than 1.8. In fact, it reincreased with continued electron exposure dose. The SEY was monitored as a function of electron dose and the surface chemistry was measured with x-ray photoelectron spectroscopy (XPS). The XPS carbon and aluminium core levels showed that the late increase in SEY is due to electron desorption of adsorbed gas, thereby exposing high-SEY Al2O3.

Quantum efficiency of technical metal photocathodes under laser irradiation of various wavelengths

Quantum efficiency studies for various laser wavelengths and various technical metal surfaces were carried out in a dedicated unbaked vacuum chamber in the absence of a significant electrical field. Copper, magnesium, aluminum, and aluminum–lithium photocathodes were irradiated by two different high power, high repetition rate, laser systems. We have observed an emission of electrons for photon energies below the work function of the material. This is explained by multiple photon absorption by the photocathode. We have not observed any degradation of the QE for these materials, but an improvement when irradiating them over a long period of time. This is contrary to observations made in RF photoguns.

Vacuum breakdown limit and quantum efficiency obtained for various technical metals using dc and pulsed voltage sources

For the SwissFEL project, an advanced high gradient low emittance gun is under development. Reliable operation with an electric field, preferably above 125 MV/m at a 4 mm gap, in the presence of an ultraviolet laser beam, has to be achieved in a diode configuration in order to minimize the emittance dilution due to space charge effects. In the first phase, a dc breakdown test stand was used to test different metals with different preparation methods at voltages up to 100 kV. The authors show that gradient achieved for rough machined (Ra<200 nm) metal electrodes followed by an argon glow plasma are similar to the one obtained using a mirrorlike electrode (Ra<40 nm). In addition, high gradient stability tests were also carried out over several days in order to prove reliable spark-free operation with a minimum dark current. In the second phase, electrodes with selected materials were installed in the 250 ns full width at half maximum, 500 kV electron gun and tested for high gradient breakdown and for quantu...For the SwissFEL project, an advanced high gradient low emittance gun is under development. Reliable operation with an electric field, preferably above 125 MV/m at a 4 mm gap, in the presence of an UV laser beam, has to be achieved in a diode configuration in order to minimize the emittance dilution due to space charge effects. In the first phase, a DC breakdown test stand was used to test different metals with different preparation methods at voltages up to 100 kV. In addition high gradient stability tests were also carried out over several days in order to prove reliable spark-free operation with a minimum dark current. In the second phase, electrodes with selected materials were installed in the 250 ns FWHM, 500 kV electron gun and tested for high gradient breakdown and for quantum efficiency using an ultra-violet laser. PACS numbers: 68.37.Vj, 29.25.Bx, 32.80.-t, 32.00.00 ∗Electronic address: frederic.le.pimpec@psi.ch

Nanoseconds field emitted current pulses from ZrC needles and field emitter arrays

The properties of the electron source define the ultimate limit of the beam quality in linear accelerators such as free electron lasers (FELs). The goal is to develop an electron gun delivering beam emittance lower than the current state of the art. Such a gun should reduce the cost and size of an x-ray FEL (XFEL). In this article we present two concepts of field emitter cathodes which could potentially produce low emittance beam. The first challenging parameter for such cathode is to emit peak current as high as 5 A. This is the minimum current requirement for the XFEL concept from Paul Scherrer Institut (http://leg.web.psi.ch). Maximum currents of 0.12 and 0.58 A have been reached, respectively, with field emitter arrays and single needle cathodes. Laser assisted field emission gave encouraging results to reach even higher peak current and to prebunch the beam.

The effect of gas ion bombardment on the secondary electron yield of TiN, TiCN and TiZrV coatings for suppressing collective electron effects in storage rings

In many accelerator storage rings running positively charged beams, ionization of residual gas and secondary electron emission (SEE) in the beam pipe will give rise to an electron cloud which can cause beam blow-up or loss of the circulating beam. A preventative measure that suppresses electron cloud formation is to ensure that the vacuum wall has a low secondary emission yield (SEY). The SEY of thin films of TiN, sputter deposited Non-Evaporable Getters and a novel TiCN alloy were measured under a variety of conditions, including the effect of re-contamination from residual gas.

Characterization of metallic field emitter array devices fabricated by molding for x-ray free electron laser applications

A low-emittance and high-brightness electron source is a prerequisite for the successful development of sub-nm wavelength x-ray free electron lasers (XFEL). For that application, a field emitter array (FEA) device equipped with a focusing gate is potentially advantageous over the state-of-the-art photocathode. In the low-emittance gun design of the PSI-XFEL project at the Paul Scherrer Institut, the cathode is assumed to emit above 0.2 nC within 10-40 ps, or -10 A, from an array of total area below 1 mm in diameter. So far, the current of the commercially available FEA device is limited to -0.1 A/mm2. To reach higher currents, we explore the field-emission properties of pyramidal-shaped molybdenum FEAs based on the molding-technique. For high-current applications, the pyramidal tip with low-aspect ratio is advantageous over a conical/cylindrical one because of the higher thermal conductance and thermal spread. Here we present fabrication and characterization of pyramidal-shaped molybdenum FEAs with relatively small numbers of tips (1) to measure the maximum current Imax per tip for single-gate devices, and (2) to explore the spatially- and energy-resolved photoemission in FEAs without gate electrodes.

Molecular desorption of a nonevaporable getter St 707 irradiated at room temperature with synchrotron radiation of 194 eV critical photon energy

Photon stimulated molecular desorption from a nonevaporable getter (NEG) St 707® (SAES Getters™) surface after conditioning and after saturation with isotopic carbon monoxide [cf. nomenclature in Handbook of Chemistry and Physics, 74th edition, edited by D. R. Lide (CRC Press, Boca Raton, 1994)] 13C18O, has been studied on a dedicated beamline at the EPA ring at CERN. The synchrotron radiation of 194 eV critical energy and with an average photon intensity of ∼1×1017 photons s−1 was impinging on the sample at perpendicular incidence. It is found that the desorption yields η (molecules/photon) of the characteristic gases in an UHV system (hydrogen, methane, carbon monoxide, and carbon dioxide) for a freshly activated NEG and for a NEG fully saturated with 13C18O are lower than that of 300 °C baked stainless steel.

Development of a low emittance electron gun based on field emission cathodes

Field emitters are investigated for use as a low emittance electron gun. Two available field emitter technologies are currently explored; the field emitter arrays (FEAs) with individual focusing and single tip cathode with robust and fairly blunt apex. The challenge is to achieve several amperes of peak current without tip destructions. Very good cathode and environmental conditioning procedures with extremely short emission duration (ns) at low repetition rate (10 Hz) gave encouraging results to reach high peak current emission. For a free electron laser application, very short emission durations are preferred. Such operation regime should prevent the tip from overheating so that higher current densities could be reached. Another possible low emittance electron sources are single needle tips made from etched wires and which can be coated and formed in order to carry high current emission. One way to achieve short emission duration is to use pulsed laser light illuminating the tip while high electric field is applied.

Surface analysis of oxygen free electrolytic-copper X-band accelerating structures and possible correlation to radio frequency breakdown events

X-band accelerator structures meeting the next linear collider design requirements have been found to suffer vacuum surface damage caused by rf breakdown, when processed to high electric-field gradients. Improved understanding of these breakdown events is desirable for the development of structure designs, fabrication procedures, and processing techniques that minimize structure damage. rf reflected wave analysis and acoustic sensor pickup have provided breakdowns localization in rf structures. Particle contaminations found following clean autopsy of four rf-processed traveling wave structures have been catalogued and analyzed. Their influence on rf breakdown, as well as that of several other material-based properties, are discussed.