Search Results (16 total)

We describe a model of damage in rf cavities and show how this damage can limit cavity operation. We first present a review of mechanisms that may or may not affect the ultimate fields that can be obtained in rf cavities, assuming that mechanical stress explains the triggers of rf breakdown events. We present a method of quantifying the surface damage caused by breakdown events in terms of the spectrum of field enhancement factors, β, for asperities on the surface. We then model an equilibrium that can develop between damage and conditioning effects, and show how this equilibrium can determine cavity performance and show experimental evidence for this mechanism. We define three functions that quantify damage, and explain how the parameters that determine performance can be factored out and measured. We then show how this model can quantitatively explain the dependence of cavity performance on material, frequency, pulse length, gas, power supply, and other factors. The examples given in this paper are derived from a variety of incomplete data sets, so we outline an experimental program that should improve these predictions, provide mechanisms for comparing data from different facilities, and fill in many gaps in the existing data.

We have measured the effects of high (0–4.5 T) magnetic fields on the operating conditions of 805 MHz accelerating cavities, and discovered that the maximum accelerating gradient drops as a function of the axial magnetic field. While the maximum gradient of any cavity is governed by a number of factors including conditioning, surface topology and materials, we argue that J×B forces within the emitters are the mechanism for enhanced breakdown in magnetic fields. The pattern of emitters changes over time and we show an example of a bright emitter which disappears during a breakdown event. We also present unique measurements of the distribution of enhancement factors, β, of secondary emitters produced in breakdown events during conditioning. We believe these secondary emitters can also be breakdown triggers, and the secondary emitter spectrum helps to determine the maximum operating field.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

We present measurements of dark currents and x rays in a six cell 805 MHz cavity, taken as part of an rf development program for muon cooling, which requires high power, high stored energy, low frequency cavities operating in a strong magnetic field. We have done the first systematic study of the behavior of high power rf in a strong (2.5–4 T) magnetic field. Our measurements extend over a very large dynamic range in current and provide good fits to the Fowler-Nordheim field emission model assuming mechanical structures produce field enhancements at the surface. The locally enhanced field intensities we derive at the tips of these emitters are very large, (∼10  GV/m), and should produce tensile stresses comparable to the tensile strength of the copper cavity walls and should be capable of causing breakdown events. We also compare our data with estimates of tensile stresses from a variety of accelerating structures. Preliminary studies of the internal surface of the cavity and window are presented, which show splashes of copper with many sharp cone shaped protrusions and wires which can explain the experimentally measured field enhancements. We discuss a “cold copper” breakdown mechanism and briefly review alternatives. We also discuss a number of effects due to the 2.5 T solenoidal fields on the cavity such as altered field emission due to mechanical deformation of emitters, and dark current ring beams, which are produced from the irises by E×B drifts during the nonrelativistic part of the acceleration process.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

Signatures associated with electronic states confined in a parabolic quantum well in a GaAs/Al_xGa_1-xAs heterostructure have been observed in its piezomodulated reflectivity spectrum. The spectra also exhibit electronic transitions with states confined to the spin-orbit-split valence band as the initial states. A comparison of the relative intensity of 11H and 11L signatures in the piezomodulated and those in the photomodulated reflectivity spectrum emphasizes the contribution of the strain dependence of the energies of the confined states.

We report the results of a Brillouin-scattering study of corrugated Ag surfaces. The corrugation plays a dramatic role in the wave-vector–selection rules governing coupling to surface phonons, and this effect is substantially different when the effective wave vector of the surface corrugation is collinear or perpendicular to the scattering plane. In processes that involve the grating wave vector, we show that the coupling mechanism between light and phonons is governed by surface plasmons which introduce a new scattering interaction with unusual polarization features in the Brillouin-scattering process.

We report the investigation of Brillouin scattering from thermal surface acoustic waves on Ag-coated holographic gratings. The usual wave-vector conservation condition for Brillouin scattering is modified due to the added periodicity in the direction of phonon propagation, but this effect is observed only when surface plasmons act as an intermediate state in the scattering process. The involvement of surface plasmons results in an enhancement of the Brillouin scattering cross section which depends on the grating amplitude.

We report the observation of confined longitudinal-optical (LO) phonons in the first-order Raman spectrum of a GaAs/Al_xGa_1-xAs single quantum well. They are observed when the energy of the scattered radiation comes into the resonance with the electronic transitions of the quantum well, the resonance being tuned with temperature or with a tunable dye laser. The confined LO phonons observed in the first-order Raman spectrum have wave vectors extending to a significant fraction of the Brillouin zone and their frequencies agree well with those deduced from the bulk dispersion curve.

We report a continuation of our investigation of surface-plasmon-induced enhancement of Brillouin scattering of thermal surface acoustic waves of thin silver films. The experiments were performed in a Kretschmann attenuated-total-reflection configuration. In the backscattering mode, the surface plasmons are resonantly involved both in the incident and scattered radiation. The scattered radiation appears on the surface of a cone defined by the resonant interaction angle and contains Brillouin couplets, which are analyzed by a five-pass Fabry-Perot interferometer. The enhancement factor is determined from the ratio of the peak radiation on this cone to the Brillouin scattered radiation in an external reflection configuration not involving surface plasmons. The theory and experiment agree in that the double resonance gives a much larger enhancement than does the singly resonant process, which was observed in our earlier investigation for the forward-scattering configuration. The enhancement factor observed, 750 (±25%), is very dependent on the imaginary part of the dielectric constant of the Ag film, which is determined experimentally for our film as -9.0+i0.57. Finally, we contrast the results of our Brillouin scattering experiment with those of Raman scattering from chemical adsorbates.

We report the first observation of surface-enhanced Brillouin scattering involving the intimate interaction of the extended surface-plasmon (SP) polaritons of a silver film on a glass substrate with the thermal-equilibrium surface acoustic waves of the film. The role of the SP in the Brillouin scattering process is demonstrated and analyzed for a particular optical configuration, specifically, SP generation in the attenuated-total-reflection Kretschmann geometry, with Brillouin scattering observed on the air side of the silver film. Good agreement is obtained between theory and experiment, both in the form of the resonance in the Brillouin scattering signal and in the magnitude of the enhancement by the SP.

A new method has been designed for obtaining the hyperfine optical pumping of Na vapor. The pumping lamp is placed in a magnetic field and the light emitted in the field direction is used for excitation after crossing a circular polarizer selecting one of the Zeeman lines. This method has two interesting features: (i) The atoms can be pumped either in the lower or in the upper hyperfine sublevel by simply rotating a polarizer by an angle of 1 / 2π; (ii) the hyperfine pumping is obtained using both the D₁ and D₂ resonance lines without filtering losses. The pumping efficiency is good and it has been possible to measure the spin-exchange cross section between sodium atoms by studying the hyperfine relaxation rates as a function of the sodium density. The result is σ_ex=(1.109±0.005)×10^-14 cm².

We have measured the temperature dependence of the dynamic proton polarization in a 0.2-g sample of 95% 1-butanol and 5% water, containing 0.75% of the free radical porphyrexide, between 1.4 and 0.5 K. The largest polarization attained was 67% at 0.5 K. The feasibility of achieving comparable polarization in larger samples and the question of the origin of the observed temperature dependence are discussed.