R.E. Kirby

A Very High Charge, High Polarization Gradient-Doped Strained GaAs Photocathode

Abstract 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%.

Systematic study of polarized electron emission from strained GaAs/GaAsP superlattice photocathodes

Spin-polarized electron photoemission has been studied for GaAs∕GaAs1−xPx 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.

Surface properties of metal‐nitride and metal‐carbide films deposited on Nb for radio‐frequency superconductivity

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...

Energy-dependence of inner potential in Fe from low-energy electron absorption (target current)

Abstract For unpolarized low-energy electrons (0–60 eV) incident on a Fe(0 0 1)_ surface, the measured current I absorbed by the crystal (target current) displays a considerable amount of fine structure depending on the angle of incidence. Dynamical calculations of the elastic reflection coefficient R e show that - d 2 R e /d E 2 is in good agreement with d 2 I /d E 2 for suitably chosen real and imaginary parts of the inner potential. The energy dependence of the latter is thereby determined. Comparison with a bulk band structure calculated using the same real potential reveals a correpondence between target current minima and band gaps as well as current maxima and critical points.

Surface properties of Cr2O3

Thin (<5 nm) air‐oxidized Cr layers are deposited on the alumina output windows of radio‐frequency klystron tubes to prevent electron multipactor by reducing the secondary electron emission yield of the alumina surface. The top several nanometers of these layers appear to be Cr2O3. To compare the measured surface properties of these layers with those of clean stoichiometric Cr2O3, quasibulk Cr2O3 layers were produced by wet‐H2‐firing magnetron‐deposited Cr films on Cu substrates and characterized by x‐ray photoelectron‐, Auger electron‐, and electron energy loss spectra and secondary electron emission yield measurements. Other properties measured were x‐ray diffraction structure, sheet resistance, and optical reflectivity. In particular, the peak of the secondary electron yield was found to be ∼1.7, which is considerably higher than the <1 yield value reported earlier in the literature. The Cr2O3 Cr 2p x‐ray photoelectron core level spectrum was curve‐fit using Doniach–Sunjic line shapes and statistical f...

Photovoltage effects in photoemission from thin GaAs layers

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 x 10{sup 18} cm{sup 3} to 5 x 10{sup 19} cm{sup 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 x 10{sup 19} cm{sup 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.

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.

Oxide overlayers and the superconducting rf properties of yttrium-processed high purity Nb☆

Abstract 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 ferromagnetic to paramagnetic phase transition of Fe studied by x-ray photoelectron spectroscopy

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.

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.