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Beam instability caused by the electron cloud has been observed in positron and proton storage rings and it is expected to be a limiting factor in the performance of the positron damping ring (DR) of future linear colliders (LC) such as ILC and CLIC [1,2]. To test a series of promising possible electron cloud mitigation techniques as surface coatings and grooves, in the Positron low-energy ring (LER) of the PEP-II accelerator, we have installed several test vacuum chambers including (i) a special chamber to monitor the variation in the secondary electron yield of technical surface materials and coatings under the effect of ion, electron and photon conditioning in situ in the beam line (ii) chambers with grooves [3] in a straight magnetic-free section and (iii) coated chambers in a dedicated newly installed 4-magnet chicane [4] to study mitigations in a magnetic field region. In this paper, we describe the ongoing R&D effort to mitigate the electron cloud effect for the LC damping ring, focusing on the first experimental area and on results of the reduction in the secondary electron yield due to in situ conditioning.

The first experimental observation of magnetic resonances in electron clouds is reported. The resonance was observed as a modulation in cloud intensity for uncoated as well as TiN-coated aluminum surfaces in the positron storage ring of the PEP-II collider at SLAC. Electron clouds frequently arise in accelerators of positively charged particles, and severely impact the machines performance. The TiN coating was found to be an effective remedy, reducing the cloud intensity by three orders of magnitude.

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., Comment: 15 pages - 7 figures - 8 tables, submitted to NIM A

In many accelerators running positively charged beams, ionization of residual gas and secondary electron emission in the beam pipe will give rise to an electron cloud which can cause beam blow-up or the loss of the circulating beam. One solution to avoid the electron cloud is to ensure that the vacuum wall has low secondary emission yield (SEY). The SEY of thin films of TiN and sputter-deposited non-evaporable getter were measured for a variety of conditions, including the effect of recontamination in an ultra high vacuum environment.

Future colliders such as NLC and JLC will require a highly-polarized macropulse with charge that is more than an order of magnitude beyond that which could be produced for the SLC. The maximum charge from the SLC uniformly-doped GaAs photocathode was limited by the surface charge limit (SCL). The SCL effect can be overcome by using an extremely high (>1019 cm-3) surface dopant concentration. When combined with a medium dopant concentration in the majority of the active layer (to avoid depolarization), the surface concentration has been found to degrade during normal heat cleaning (1 hour at 600 C). The Be dopant as typically used in an MBE-grown superlattice cathode is especially susceptible to this effect compared to Zn or C dopant. Some relief can be found by lowering the cleaning temperature, but the long-term general solution appears to be atomic hydrogen cleaning., 11 pages, 3 figures, 1 table, contributed to 10th Workshop on Polarized Sources and Targets, Novosibirsk, Sept. 22-26, 2003, to be submitted to Nucl. Instrum. and Meth. A

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.

A high-gradient-doping technique is applied to strained polarized photocathodes. A 5.0– 7.5 nm p-type surface layer doped to 5×10 19 cm −3 is found sufficient to overcome the surface charge limit while maintaining high beam polarization. This technique can be employed to meet the charge requirements of the Next Linear Collider with a polarization approaching 80%.

The design of the Linac Coherent Light Source assumes that a low-emittance, 1-nC, 10-ps beam will be available for injection into the 15-GeV linac. The proposed rf photocathode injector that will provide a 150-MeV beam with rms normalized emittances of 1 mm in both the transverse and longitudinal dimensions is based on a 1.6-cell S-band rf gun that is equipped with an emittance compensating solenoid. The booster accelerator is positioned at the beam waist coinciding with the first emittance maximum and is provided with an accelerating gradient of ~25 MeV/m, i.e., the "new working point." The uv pulses required for cathode excitation will be generated by tripling the output of a Ti:sapphire laser system consisting of a highly stable cw mode-locked oscillator and two bow-tie amplifiers pumped by a pair of Q-switched Nd:YAG lasers. The large bandwidth of the Ti:sapphire system accommodates the desired temporal pulse shaping. Details of the design and the supporting simulations are presented., Comment: 13 pages (double spaced), 4 figures, contributed to The 23rd International Free Electron Laser Conference, Darmstadt, Germany, 20-24 August 2001

The PEP-II B-Factory at SLAC operates with aluminum alloy and copper vacuum chambers, having design positron and electron beam currents of 2 and 1 A, respectively. Titanium nitride coating of the aluminum vacuum chamber in the arcs of the positron ring is needed in order to reduce undesirable electron-cloud effects. The total secondary electron emission yield of TiN-coated aluminum alloy has been measured after samples of beam chamber material were exposed to air and again after electron-beam bombardment, as a function of incident electron beam angle and energy. The results may be used to simulate and better understand electron-cloud effects under actual operating conditions. We also present yield measurements for other accelerator materials because new surface effects are expected to arise as beam currents increase. Copper, in particular, is growing in popularity for its good thermal conductivity and self-radiation-shielding properties. The effect of electron bombardment, “conditioning”, on the yield of TiN and copper is shown.

A novel wire scanner has been developed with applications at linear accelerators where small beam spots or high intensities would otherwise destroy wires. A pattern of wires is supported on a thin membrane and remains in place even when punctured by the beam. Many scans can be made before the material needs to be replaced. Tests of the principle have been carried out at the Final Focus Test Beam at SLAC.

Spin-polarized electron photoemission has been studied for GaAs/GaAs$_{1-x}$P$_x$ strained superlattice cathodes grown by gas-source molecular beam epitaxy. The superlattice structural parameters are systematically varied to optimize the photoemission characteristics. The heavy-hole and light-hole transitions are reproducibly observed in quantum efficiency spectra, enabling direct measurement of the band energies and the energy splitting. Electron-spin polarization as high as 86% with over 1% quantum efficiency has been observed.

The Stanford 3-km linear accelerator at SLAC has operated exclusively since early 1992 using a polarized electron beam for its high-energy physics programs. The polarized electron source now consists of a diode-type gun with a strained-lattice GaAs photocathode DC biased at high voltage and excited with circularly polarized photons generated by a pulsed, Ti:sapphire laser system. The electron polarization at the source is > 80%. To date the source has met all the beam requirements of the SLC and fixed target programs with < 5% downtime.

The Stanford Linear Collider injector requires two 2 ns pulses of 4.5–5.5 × 10 10 electrons, separated by 61 ns at 120 Hz, from its source. Since 1992, these currents have been provided by a polarized electron source based on GaAs photocathodes. A beam polarization of 76 ± 4% has been measured at the end of the 50 GeV linac. At low photocathode quantum efficiencies, and for excitation near threshold, the maximum current delivered by the source is constrained, not by the space charge limit of the gun, but by a “charge limit” of the photocathode. The charge limited current is proportional to the photocathode quantum efficiency, but the proportionality varies for different photocathode types. Experience with high polarization strained GaAs photocathodes on a test beamline and on the SLC is presented.

Superconducting rf surface resistance measurements were made on oxidized Nb cavities that had been warmed for a few minutes to temperatures between 250 and 300° C. Warmed oxide layers were further studied using X-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary electron emission yield measurements. Warming usually reduced the temperature-dependent (BCS) part of the surface resistance by 10–20% while, for warming temperatures near 300° C, the low-temperature residual surface resistance increased by as much as 70 nΩ. Surface spectroscopy measurements showed that, at temperatures between 200 and 250° C, the oxide layer was chemically altered but remained surface-segregated; between 250 and 300° C, however, surface oxygen dissolved into the Nb bulk. Changes in the BCS resistance were consistent with a model based on a shortening of the electron mean free path by oxygen that had diffused into the Nb metal and based on the theoretical dependence of BCS resistance on electron mean free path. No specific cause for the increased residual resistance could be positively identified, although oxygen-induced surface roughening is a possibility.

The GaAsP/GaAs superlattice (SL) structure has been widely recognized as the most efficient spin polarized electron source with 90% maximum polarization and more than 1% quantum efficiency. The main spin depolarization mechanisms in these structures are: interband absorption smearing due to band-edge fluctuations, hole scattering between the heavy hole (HH) and light hole (LH) states that causes a broadening of the LH band, spin precession due to an effective magnetic field generated by the lack of crystal inversion symmetry and spin orbit coupling.

Possible remedies for the electron cloud in positron damping ring (DR) of the International Linear Collider (ILC) includes thin-film coatings, surface conditioning, photon antechamber, clearing electrodes and chamber with grooves or slots. We installed chambers in the PEP-II Low Energy Ring (LER) to monitor the secondary electron yield (SEY) of TiN, TiZrV (NEC) and technical accelerator materials under the effect of electron and photon conditioning in situ. We have also installed chambers with rectangular grooves in straight sections to test this possible mitigation technique. In this paper, we describe the ILC R&D ongoing effort at SLAC to reduce the electron cloud effect in the damping ring, the chambers installation in the PEP-II and latest results.

The ILC project will be the next large high energy physics tool that will use polarized electrons (and positrons). For this machine spin physics will play an important role. The polarized electron source design is based on electron injectors built for the Stanford Linear Collider (polarized) and Tesla Test Facility (un‐polarized). The ILC polarized electron source will provide a 5GeV spin polarized electron beam for injection into the ILC damping ring. Although most ILC machine parameters have been achieved by the SLC or TTF source, features of both must be integrated into one design. The bunch train structure presents unique challenges to the source laser drive system. A suitable laser system has not yet been demonstrated and is part of the ongoing R&D program for ILC at SLAC. Furthermore, ILC injector R&D incorporates photocathode development, increasing available polarization, and improving operational properties in gun vacuum systems. Another important area of research and development is advancing the design of DC and RF electron gun technology for polarized sources. This presentation presents the current status of the design and outlines aspects of the relevant R&D program carried out within the ILC community.

We investigated the effect of an accelerating field on the spin polarization of photo‐generated electrons in a 100nm thick GaAs based photocathode active region. By decreasing the transport time of the electrons and the number of scattering events that cause depolarization, we expected to increase the polarization as was indicated by Monte Carlo simulations of the scattering and transport time statistics of the electrons.A tungsten (W) grid was deposited on the cathode surface to provide a uniform voltage distribution across the cathode surface. The metal grid formed a Schottky contact with the semiconductor surface. The bias voltage was primarily dropped at the metal semiconductor interface region, which is the cathode active region. For positive surface bias, the accelerating voltage not only increased the polarization, but it also enhanced the quantum efficiency of the photocathode. Preliminary results verify the bias effect on both quantum efficiency and polarization by a factor of 1.8 and 1% respecti...

Metal photocathodes are commonly used in high-field rf guns because they are robust, straightforward to implement, and tolerate relatively poor vacuum compared to semiconductor cathodes. However, these cathodes have low quantum efficiency (QE) even at UV wavelengths, and still require some form of cleaning after installation in the gun. A commonly used process for improving the QE is laser cleaning. In this technique the UV-drive laser is focused to a small diameter close to the metal’s damage threshold and then moved across the surface to remove contaminants. This method does improve the QE, but can produce nonuniform emission and potentially damage the cathode. Ideally, an alternative process which produces an atomically clean, but unaltered, surface is needed. In this paper we explore using a hydrogen ion (H-ion) beam to clean a copper cathode. We describe QE measurements over the wavelength range of interest as a function of integrated exposure to an H-ion beam. We also describe the data analysis to obtain the work function and derive a formula of the QE for metal cathodes. Our measured work function for the cleaned sample is in good agreement with published values, and the theoretical QE as a function of photon wavelength is in excellent agreement with the cleaned copper experimental results. Finally, we propose an in situ installation of an H-ion gun compatible with existing s-band rf guns.

Improving and maintaining the quantum efficiency (QE) of a metal photocathode in an s-band RF gun requires a process for cleaning the surface. In this type of gun, the cathode is typically installed and the system is vacuum baked to {approx}200 degrees C. If the QE is too low, the cathode is usually cleaned with the UV-drive laser. While laser cleaning does increase the cathode QE, it requires fluences close to the damage threshold and rastering the small diameter beam, both of which can produce nonuniform electron emission and potentially damage the cathode. This paper investigates the efficacy of a low energy hydrogen ion beam to produce high-QE metal cathodes. Measurements of the QE vs. wavelength, surface contaminants using x-ray photoelectron spectroscopy and surface roughness were performed on a copper sample, and the results showed a significant increase in QE after cleaning with a 1keV hydrogen ion beam. The H-ion beam cleaned an area approximately 1cm in diameter and had no effect on the surface roughness while significantly increasing the QE. These results and a comparison with theory as well as a scheme for installing an H-ion cleaner on an s-band gun are presented.

Future electron-positron linear colliders require a highly polarized electron beam with a pulse structure that depends primarily on whether the acceleration utilizes warm or superconducting rf structures. The International Linear Collider (ILC) will use cold structures for the main linac. It is shown that a dc-biased polarized photoelectron source such as successfully used for the SLC can meet the charge requirements for the ILC micropulse with a polarization approaching 90%., 4 pages, 2 tables, contributed to SPIN04 Symposium (Trieste)

Any accelerator circulating positively charged beams can suffer from a build-up of an electron cloud (EC) in the beam pipe. The cloud develops through ionization of residual gases, synchrotron radiation and secondary electron emission and, when severe, can cause instability, emittance blow-up or loss of the circulating beam. The electron cloud is potentially a luminosity limiting effect for both the Large Hadron Collider (LHC) and the International Linear Collider (ILC). For the ILC positron damping ring, the development of the electron cloud must be suppressed. This paper discusses the state-of-the-art of the ongoing SLAC and international R&D program to study potential remedies.

Atomic hydrogen cleaning followed by heat cleaning at 450$^\circ$C was used to prepare negative-electron-affinity GaAs photocathodes. When hydrogen ions were eliminated, quantum efficiencies of 15% were obtained for bulk GaAs cathodes, higher than the results obtained using conventional 600$^\circ$C heat cleaning. The low-temperature cleaning technique was successfully applied to thin, strained GaAs cathodes used for producing highly polarized electrons. No depolarization was observed even when the optimum cleaning time of about 30 seconds was extended by a factor of 100.

Single strained, medium‐doped (5×1018 /cm3) GaAs photocathodes show the surface charge limit (SCL). The SCL poses a serious problem for operation of polarized electron sources at future linear colliders such as the NLC/JLC. A high‐gradient‐doping technique has been applied to address this problem. A 5 –7.5 nm p‐type surface layer doped to 5×1019/cm3 is found sufficient to overcome the SCL, while maintaining high beam polarization. This technique can be employed to meet the charge requirements of the NLC with a polarization approaching 80%.

The SLAC high-gradient-doped MOCVD-grown GaAs cathode presently in use consists of a strained GaAs low-doped layer (with a small admixture of P) capped by a few nanometers of highly Zn-doped GaAs, which is heat-cleaned at relatively high temperature and then activated by Cs/NF{sub 3} co-deposition. The high-gradient-doped structure solves the problem of the surface charge limit that the previously-used SLAC cathodes had, and this preparation procedure has produced satisfactory results. However, the preparation procedure has a few weaknesses that prevent cathodes from achieving the ultimate desired performance. The peak polarization is limited to 80% due to strain relaxation in the relatively thick strained layers. Also dopant loss causes the surface charge limit effect to reappear after multiple high-temperature heat-cleanings. In this paper, we will discuss recent progress made at SLAC that addresses these limitations, including using the MBE growth technique with Be doping and using the superlattice structure. In addition, to reduce the heat-cleaning temperature, an atomic hydrogen cleaning technique is explored.

For the 1992 operating cycle of the SLAC Linear Collider (SLC), the polarized electron source (PES) during its maiden run successfully met the pulse intensity and overall efficiency requirements of the SLC. However, the polarization of the bulk GaAs cathode was low (/spl sim/27%) and the pulse-to-pulse stability was marginal. We have shown that adequate charge for the SLC can be extracted from a strained layer cathode having P/sub espl sim/80% even though the quantum efficiency (QE) is >

An ultra-high vacuum loadlock system capable of operating at high voltage has been added to the SLC Polarized Electron Gun. The unit incorporates facilities for heat cleaning, activating and measuring the quantum efficiency of photocathodes. A tray of up to four photocathodes can be exchanged without bringing the activation unit or gun up to atmosphere. Low voltage quantum efficiencies of 20% have been obtained for bulk GaAs at 633 nm and 6% for a 0.3 micron GaAs layer at 755 nm. Results for other cathodes as well as operational characteristics are discussed. >

Our experience at SLAC with photoemission-based polarized electron sources has shown that the charge limit is an important phenomenon that may significantly limit the performance of a photocathode for applications requiring high intensity electron beams. In the process of developing high performance photocathodes for the ongoing and future SLC high energy physics programs, we have studied the various aspects of the charge limit phenomenon. We find that the charge limit effect arises as a result of the nonlinear response of a photocathode to high intensity light illumination. The size of the charge limit not only depends on the quantum efficiency of the cathode but also depends critically on the extraction electric field. In addition, we report the observation of charge oversaturation when the intensity of the incident light becomes too large. >

The Stanford Linear Accelerator Center, in collaboration with Argonne National Laboratory, Brookhaven National Laboratory, Los Alamos National Laboratory, Lawrence Livermore National Laboratory, and the University of California at Los Angeles, have collaborated to create a conceptual design for a Free-Electron-Laser (FEL) R&D facility operating in the wavelength range 1.5-15 {angstrom}. This FEL, called the ''Linac Coherent Light Source'' (LCLS), utilizes the SLAC linac and produces sub-picosecond pulses of short wavelength x-rays with very high peak brightness and full transverse coherence. The first two-thirds of the SLAC linac are used for injection into the PEP-II storage rings. The last one-third will be converted to a source of electrons for the LCLS. The electrons will be transported to the SLAC Final Focus Test Beam (FFTB) Facility, which will be extended to house a 122-m undulator system. In passing through the undulators, the electrons will be bunched by the force of their own synchrotron radiation to produce an intense, spatially coherent beam of x-rays, tunable in energy from 0.8 keV to 8 keV. The LCLS will include two experiment halls as well as x-ray optics and infrastructure necessary to make use of this x-ray beam for research in a variety of disciplines suchmore » as atomic physics, materials science, plasma physics and biosciences. This Conceptual Design Report, the authors believe, confirms the feasibility of constructing an x-ray FEL based on the SLAC linac.« less

Using laboratory scale and full size PEP-II vacuum chambers, chemical cleaning, glow discharge and thermal process effects were evaluated using surface analysis by X-ray photoelectron spectroscopy (XPS). These processes were optimized to reduce surface carbon and thereby minimize photodesorption gas loads. The relation of surface carbon to ion dose was investigated and compared for pure argon, 5% oxygen in argon, and pure hydrogen plasmas. Argon incorporation was noted only when the copper was oxidized in the mixed gas. Surfaces, stable in ambient atmosphere, were obtained having surface carbon values less than 10%. These optimized recipes will be used in processing copper vacuum chambers for the PEP-II B-Factory.

The SLAC Gun Test Laboratory apparatus, the first two meters of which is a replica of the SLAC injector, is used to study the production of intense, highly-polarized electron beams required for the Stanford Linear Collider and future linear colliders. The facility has been upgraded with a Mott polarimeter in order to characterize the electron polarization from photocathodes operating in a DC gun. In particular, SLAC utilizes p-type, biaxially-strained GaAs photocathodes which have produced longitudinal electron polarizations greater than 80% while yielding pulses of 5 A/cm/sup 2/ at an operating voltage of 120 kV. Among the experiments performed include studying the influences of the active layer thickness, temperature, quantum efficiency and cesiation on the polarization. The results might help to develop strained photocathodes with higher polarization.

A set of thin GaAs p-type negative electron affinity (NEA) photocathodes have been used to measure the yield of photoemitted electrons at high intensity excitation. The active layer thickness is 100 nm and the p-type doping ranges from 5\times10^{18} cm^{-3} to 5\times10^{19} cm^{-3} for a set of four samples. The results show that the surface escape probability is a linear function of the NEA energy. The surface photovoltage effect on photoemission is found to diminish to zero at a doping level of 5\times10^{19} cm^{-3}. The experimental results are shown to be in good agreement with calculations using a charge limit model based on surface photovoltage kinetics that assume a constant electron energy relaxation rate in the band bending region., LaTeX RevTeX 11 pages, 6 figures

Motivations gained from this paper are: (1) Suppress electron emission from high rf surface field components, e.g., SPEAR storage ring cavity tuners (1973), (2) Find a coating for superconducting Nb oxidation prevention (1980), (3) Develop a simple method for TiN-coating of LER Al alloy beam chambers (1998), and (4) Measure yields as a function of primary electron incidence angle, for simulating of the electron cloud effect (1999). Conclusions drawn from data: (1) Most electron-generated secondaries will have low energy ({approximately}4 eV) and

GaAs based photocathode research at SLAC will be described. Recent efforts have focused on both immediate applications and fundamental photocathode properties. This includes revisiting some old measurements with state-of-the-art instrumentation.

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.

The interactions of activated nitrogen (AN) with Cu(210) and Ni(210) and of O 2 with Ni(210) have been studied using LEED, RHEED and AES. Exposure to AN at 290 K produces (100) facets with a c(11 √2 × √2)R45°-N overlayer; on warming (520 K) in vacuum, (110) facets with a p(2 × 3)-N overlayer appear. Heating to 600 K causes desorption of N 2 and regeneration of the (210) surface. With O 2 on Ni(210) complicated facet patterns are observed. Facet orientations have been identified by a simple method involving rotation of the crystal. The results are discussed in relation to the stabilities of various Cu surfaces, particularly (410) and (530), in the presence of AN and O 2 .

Valence band x-ray photoelectron spectra from Fe(100) have been measured as a function of temperature to above the Curie temperature, Tc. The room temperature data can be reconciled with the theoretical one-particle density of states (DOS). At T = 1.034Tc, the data do not resemble the paramgnetic DOS of Fe as calculated in the disordered-local-moment limit.

Electron beam assisted adsorption and desorption of oxygen was studied by Auger electron spectroscopy (AES). Beam assisted adsorption was observed on clean as well as on oxidized surfaces. After an oxygen exposure of 1000 × 10−7 Torr min and continuous irradiation with beam voltage of 1.5 kV and beam current density 2 microA mm−2, the oxygen 510 eV signal amplitude from the point of beam impact was 2.5 times greater than the signal from the non-irradiated region. The Ge 89 eV signal showed a corresponding decrease. Enhanced adsorption occurred at beam energies as low as 16.5 eV. After irradiation, the oxidized surface was not carbon contaminated. Following an oxygen exposure of 30 min at 0.1 Torr and 550°C and subsequent additional beam assisted exposure of 1000 × 10−7 Torr min, the maximum oxide thickness was about 18 A. Beam assisted desorption did not occur from thin oxygen layers (0–510 eV signal strength less than 5 units, calculated oxide thickness about 6 A), but occurred from thick oxides and stopped after the signal amplitude had decreased to 5 units. Based on these results, a model for the structure of the oxygen layer covering the Ge(111) surface is proposed. Mechanisms for adsorption and desorption are discussed. The implications of beam assisted adsorption and desorption on electron beam operated surface measurements (LEED, AES, ELS, APS etc.) are stressed.

Summary Platelet aggregation is enhanced during the early development of adjuvant-induced arthritis in rats and returns to normal after the syndrome becomes established. Cross-over substitution studies indicate that ADP- and Collagen-induced aggregation are enhanced due to a factor present in the plasma of adjuvant-injected rats. The enhanced aggregation is effectively reduced by known inhibitors of platelet aggregation.

Various effects occur which can prevent attainment of the high Q’s and/or the high gradient fields necessary for the operation of radio‐frequency (rf) superconducting cavities. One of these effects, multipactor, both causes the cavity to detune during filling due to resonant secondary electron emission at the cavity walls, and lowers the quality factor (Q) by dissipative processes. TiN deposited onto the high‐field regions of room‐temperature Al cavities has been used at the Stanford Linear Accelerator Center to successfully reduce multipactor in the past. We have therefore studied TiN and its companion materials, NbN, NbC, and TiC, all on Nb substrates under several realistic conditions: (1) as‐deposited, (2) exposed to air, and (3) electron bombarded. The studied films (up to 14‐nm thickness) were sputter deposited onto sputter‐cleaned Nb substrates. Results indicate that all the materials tested gave substantially the same results. The maximum secondary electron yields for as‐deposited films were reduc...

Secondary electron yield measurements before and after Ar ion sputter cleaning were made on Nb and Nb compounds of interest for rf superconducting cavities. Total secondary electron yields (σ) for primary energies 20–1500 eV were measured for solid Nb (σmax=1.3 at 300 eV), anodized Nb2O5 (σmax=1.2 at 300 eV), and powders of Nb(σmax=1.0 at 400 eV), NbO (σmax=0.9 at 400 eV), NbO2 (σmax=1.0 at 400 eV), Nb2O5 (σmax=0.95 at 400 eV), NbC(σmax=0.8 at 400 eV), and NbN (σmax=0.8 at 500 eV). Determinations were made for Auger elemental sensitivities, and the relationship between Auger peak heights and oxide stoichiometry is discussed. The sputter etch rate of anodized Nb2O5 was measured by depth profiling anodic coatings of known thickness.

Mobility studies have been made on single crystal PbTe thin films grown on NaCl substrates. By subtracting the effects of lattice and surface scattering from the experimentally measured effective mobility, defect scattering is separated out. Temperature dependence of defect scattering in many films in the thickness range of 2000 to 10,000 A indicates dislocation scattering as opposed to the usual model of boundary limited scattering in lead chalcogenide films. Electron micrograph studies of these films support this result. Mobility increases with film thickness because the films get better structurally and the defect scattering decreases.

The effect of electron beam monitored gas adsorption on the clean Si surface is studied using Auger electron spectroscopy. It is shown that the beam affects the AES adsorption signal of CO and O2 on Si by dissociating the adsorbed molecules on the surface and subsequently promoting diffusion of atomic oxygen into the bulk. A qualitative explanation of the adsorption data is presented and the initial sticking probability of O2 on Si (111) surface is estimated to be S0 = 0.21.

The absorbed electron current for a clean Fe(100) surface as a function of energy rises steplike at the vacuum‐energy cutoff with an absorption close to 1. The smooth decrease of absorbed current at higher electron energy due to secondary electron emission is superimposed by a considerable amount of fine structure, the amplitude of which decreases with increasing energy. These features are found in good agreement with the results of a calculation of the elastic part of the electron reflection coefficient. Further, they are compared with the ferromagnetic electronic bulk band structure calculated above the vacuum energy. From the comparison with the experimental data, the energy dependence of the real and imaginary parts of the inner potential is determined.