Search Results (26 total)

The Fermilab photoinjector produces electron bunches of 1–12 nC charge with an energy of 16–18 MeV. Detailed measurements and optimization of the transverse emittance have been carried out for a number of beam line optics conditions, and at a number of beam line locations. The length of the bunches has also been measured, first for an uncompressed beam (as a function of the charge) and then for a compressed beam of 8 nC charge (as a function of the 9-cell cavity phase). These measurements are presented and compared with the simulation codes HOMDYN and ASTRA.

First principles density functional calculations for adsorption of Ti on Al₂O₃(0001) indicate that Ti:Al₂O₃(0001) interfaces become intermixed. Substitutional Ti replaces a surface Al atom rather than a subsurface Al, and the Al-terminated surface is unstable under Ti adsorption. Adsorbed Ti displaces the surface Al, resulting in a mixed Ti/Al interfacial layer instead of a sharp Ti:Al₂O₃ interface. Our results provide a coherent picture of the structural and electronic properties of this interface and are consistent with available experimental data.

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30–100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95–105 nm.

When a relativistic electron bunch traverses a structure, strong electromagnetic fields are induced in its wake. For a 12 nC bunch of duration 4.2 ps FWHM, the peak field is measured >0.5 MV/m. Time resolution of ∼5 ps is achieved using electro-optic sampling with a lithium tantalate (LiTaO₃) crystal and a short-pulse infrared laser synchronized to the beam. We present measurements for both the longitudinal and radial components of the field and relate them to the wall impedance.

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 (approximately 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width, and intensity fluctuations, are all consistent with the present models for SASE FELs.

The conceptional design of the proposed linear electron-positron collider TESLA is based on 9-cell 1.3 GHz superconducting niobium cavities with an accelerating gradient of E_acc≥25 MV/m at a quality factor Q₀≥5×10⁹. The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of E_acc≥15 MV/m. In a first series of 27 industrially produced TTF cavities the average gradient at Q₀  =  5×10⁹ was measured to be 20.1±6.2 MV/m, excluding a few cavities suffering from serious fabrication or material defects. In the second production of 24 TTF cavities, additional quality control measures were introduced, in particular, an eddy-current scan to eliminate niobium sheets with foreign material inclusions and stringent prescriptions for carrying out the electron-beam welds. The average gradient of these cavities at Q₀  =  5×10⁹ amounts to 25.0±3.2 MV/m with the exception of one cavity suffering from a weld defect. Hence only a moderate improvement in production and preparation techniques will be needed to meet the ambitious TESLA goal with an adequate safety margin. In this paper we present a detailed description of the design, fabrication, and preparation of the TESLA Test Facility cavities and their associated components and report on cavity performance in test cryostats and with electron beam in the TTF linac. The ongoing research and development towards higher gradients is briefly addressed.

Thermoelectric cooling, based upon the extraction of hot electrons and holes from a metallic electron gas, holds unrealized potential for refrigeration at cryogenic temperatures. We discuss the performance of two such electronic refrigerators: the quantum-dot refrigerator (QDR) and the normal-insulator-superconductor (NIS) refrigerator. We obtain the QDR base temperature using a numerical simulation and verify the validity of certain simplifying assumptions which allow refrigerating performance to be summarized on a diagram of ambient temperature versus electronic temperature. In this way, we find that the best refrigeration is obtained with the electronic distribution far from the equilibrium Fermi-Dirac function and the temperature reduction achieved is limited by the rate at which phonons are absorbed. We predict that, with sufficient thermal isolation, electronic devices could be cooled to a small fraction of the ambient temperature using these solid-state refrigerators. The NIS refrigerator should be capable of cooling thin-film devices from above 300 mK to below 100 mK; the QDR will cool macroscopic metallic samples in the μK or nK range. We also discuss topics related to thermoelectric refrigeration including other cryogenic thermoelectric cooling schemes, the validity of the linear-response theory of thermoelectric effects, the refrigerating efficiency of an optimized thermoelectric refrigerator, and the overall cooling power of thermoelectric refrigeration.

Current-imaging tunneling spectroscopy (CITS) was performed on cold-cleaved single crystals of YBa₂Cu₃O_7-x at 20 K. CITS data include I(V) curves taken simultaneously with a topographic scanning tunneling microscopic image. I(V) curves taken on CuO chains show an energy gap of about 20 meV which disappears near oxygen vacancies. We explain several features of large-junction I(V) measurements, photoemission spectroscopy, and single-point I(V) spectroscopy in terms of local effects detected by our CITS measurements. Finally, we consider the possibilities that this energy gap is due to either a charge-density wave or proximity-coupled superconductivity from the CuO₂ planes.

We have performed reversed-bias scanning tunneling microscopy at 20 K on the CuO chain layer of cold-cleaved single crystals of YBa₂Cu₃O_7-δ. We find 1.3-nm corrugations which change sign under bias polarity reversal. This behavior and the 1.3-nm wavelength are in agreement with the hypothesis that the one-dimensional CuO chains undergo a charge density wave transition. This is the first real-space evidence of such a transition in YBa₂Cu₃O_7-δ. We also explore the possibility that the corrugations are caused by Friedel oscillations or bipolaron scattering. Several-nm depressions along the CuO chains transform into broad swells and depressions as the bias voltage is increased.

We report the results of scanning tunneling microscopy (STM) and spectroscopy (STS) studies on high-quality single crystals of YBa₂Cu₃O_7-x (YBCO) which were cleaved in situ at 20 K prior to measurement. STS reveals an energy gap with 2Δ/kT_c=6–8 and conductance curves with small zero bias conductance. STM images show for the first time the complex atomic structure of the YBCO surface; atomic resolution disappears after heating to 70 K. Large features may be due to the electronic effects of oxygen disorder, and YBCO is shown to cleave between the BaO and CuO chain layers.

The energy dependence of the cross section for neutrino- and antineutrino-nucleon charged-current interactions has been determined from data taken in Fermilab's dichromatic neutrino beam. σ^ν / E=(0.669±0.003±0.024)×10^-38 cm²/GeV and σ^ν̅ / E=(0.340±0.003±0.02)×10^-38 cm²/GeV are found. These results are higher than some previous measurements.

We report on the observation of twelve like-sign (μ^-μ^-) neutrino-induced dimuon events with muon momenta greater than 9 GeV. The background from π and K decay is 1.3 events so that we conclude that this prompt signal is real with a significance greater than 1 in 10⁷. Although the overall rate is higher than present theoretical estimates, the kinematic distributions of these events are qualitatively consistent with a picture of charm-anticharm production. The ratio of μ^-μ^- / μ^- shows a strong energy dependence and rises to (2.5±1.0) × 10^-3 at E_ν=250 GeV.

The suggestion by Callan and Glashow that there is a new component of cosmic rays comprising stable massive charged particles is examined with respect to present experimental evidence. Direct analysis of particle masses at sea level, the angular distribution of low-energy muons, and the energies of energetic muons at great depths underground all point to the likely nonexistence of these particles, at least to the suggested extent of 10^-3 of the normal primary component.

From measurements on pure rotational transitions in the microwave region the moments of inertia, I_B in g cm²×10^-40, have been determined as follows: 32.8544 for C¹²H₃F, 33.7444 for C¹³H₃F, 81.0693 for C¹²HF₃, and 107.286 for P³¹F₃. The molecular dimensions determined are: for CH₃F, d_CH=1.109A, d_CF=1.385A, and ∠HCH = 110° 0′; for CHF₃, d_CF=1.326A, with d_CH=1.111A (assumed) and ∠FCF=110° (assumed); for PF₃, d_PF=1.546±0.008A with ∠FPF = 104±3° (assumed). The line breadth parameters, Δν, normalized to 1 mm of Hg pressure are for CH₃F 20 mc, for CHF₃ 18 mc, and for PF₃ 16 mc. Confirming evidences for the nuclear spin values, ½ for P³¹ and F¹⁹ have been obtained.

The phenomenon of the colored appearance of thin metallic films sputtered upon glass or upon other metals, when viewed in white light, is explained as being caused by interference. Some qualitative observations of cyclic changes of color are given which support this viewpoint. Mathematical expressions are derived from Maxwell's electromagnetic equations to express the change of phase as the wave train is reflected from a metal surface in air, from metal in metal, for refraction as the wave enters the metal from air and from air into metal. Use of the values of the indices of refraction and absorption for massive metals indicates fair substantiation of experimental results. Fritze's values of the optical constants for thin metals do not seem to give agreement between theory and experiment. The thickness of the film of copper necessary to produce a yellow color (interference for λ=0.48μ) when calculated from the index of refraction (1.13) is found to be three and one-half times larger than the value found by weighing the total deposit. The balance of evidence seems to support the theory of interference.

The intensity of a monochromatic beam of x-rays reflected from platinum, silver, and glass mirrors was measured for angles of incidence varying from 0.75 to 1.25 times the critical angle. Radiation having a wave-length of 0.69A was obtained by reflection from calcite. Values of the intensity of the reflected beam calculated by Thibaud's modification of Fresnel's equation were found to be in good agreement with experimental values obtained from platinum. Experimental results obtained from silver and glass mirrors do not agree quantitatively with the theoretical values. No explanation for the lack of concordance is offered.

Variation of critical angle with thickness of film for films deposited on platinum. —The total reflection of x-rays (λ=0.707A) is obtained from a series of thin nickel films of thickness varying from zero to 2.05×10^-5 cm. The films were sputtered upon thick platinum which was, in turn, sputtered upon a glass support. The critical angle for each film was measured. The values of the critical angle varied from 0.0040 radians (that of the bare platinum) to 0.0034 radians. With the exception of an increase in the critical angle (maximum value of 0.0043 radians) for the thinnest films, the critical angle decreased logarithmically with increasing thickness of the nickel film. The value obtained from the thickest film agrees with that calculated by the Lorentz dispersion formula. These results, together with those reported earlier seem to prove conclusively that the phenomenon of total reflection is not a purely surface phenomenon but is one which requires a layer of metal of probably definite thickness for the particular reflecting matter and wave-length of radiation used. An explanation for the variation of the critical angle with the thickness of the nickel films is given. This is based upon the assumption that the contribution to the intensity of the reflected ray made by the deepest electrons will be effective only when the total length of the path of the radiation in the metal is not too great. The maximum effective depth is that which reduces the intensity of the emerging radiation by absorption to a value less than that which may be detected in the reflected beam.

The total reflection of x-rays (λ=0.707A) is obtained from mirrors of sputtered nickel films having thicknesses from 0 to 3.3×10^-5 cm. The measured critical angles were found to vary from a minimum value of 0.0016 radians, that of blank glass, to a maximum value of 0.00339 radians, which was obtained from the thickest nickel film. A satisfactory agreement between the maximum experimental value of the critical angle and that calculated by the Lorentz dispersion formula in which the density of nickel was placed at 8.75 gm/cc, is used as evidence for concluding that the density of the nickel is entirely normal and also that the thickness of sputtered metal films which are to be used in x-ray reflection phenomena must be sufficiently large or misleading results may be obtained. The critical angle from a thick silver sputtered mirror was found to be identical with that obtained from a chemically prepared silver mirror which indicates that the density of silver is independent of the method of deposition.

Total reflection of a beam of x-rays (λ=0.7078A) is obtained from mirrors of glass, tin, silver, selenium, and zinc. The deviations of the reflected rays at the critical positions were measured on photographic films placed 312.5 cm from the mirrors. The index of refraction was calculated from the measured critical angle in each case, with the following results: crown glass δ×10⁶=1.711; tin 3.97; silver 5.78; selenium (vitreous) 2.67; and zinc 4.59. The experimental values of δ agree with those calculated by the Drude Lorentz dispersion formula to about 0.5%. It is shown in the case of the metals that there are undoubtedly in each atom 2 electrons which have the critical K absorption frequency.