Search Results (37 total)

The time-dependant current response of Pd∕AlN∕Si-based devices is investigated for different hydrogen concentrations. At a fixed applied voltage, the device current suddenly increases when hydrogen gas is turned on and the magnitude of this current shift varies with the hydrogen concentrations. Using first-principles simulations, the electronic structure of the Pd with different hydrogen concentrations in tetrahedric and octahedric positions is calculated. We find that when hydrogen loads the Pd metal, its Fermi energy changes, which affects the Fermi level of the Pd∕AlN∕Si device and thus its electrical response.

A barrier bucket with very small separation between the rf barriers (relative to the barrier widths) or even zero separation has its synchrotron tune decreasing rather slowly from a large value towards the boundary of the bucket. As a result, a large area at the bucket edges can become unstable under the modulation of rf voltage and/or rf phase. In addition, chaotic regions may form near the bucket center and extend outward under increasing modulation. Application is made to those barrier buckets once used in the process of momentum mining at the Fermilab Recycler Ring.

Turn-by-turn beam profile data measured at the Fermilab Booster are studied. Lattice models with experimental accelerator ramping parameters are used to obtain the lattice functions for data analysis. We studied the horizontal and vertical emittance growth behavior in different stages of a booster ramping cycle and its relation to the beam intensity. The transverse and longitudinal components in the horizontal beam width are separated by a fitting model which makes use of the different scaling rules of the beam momentum. We analyze the post-transition horizontal beam size oscillation based on a model where the longitudinal phase-space mismatch has resulted from rf voltage mismatch during the transition-energy crossing. We carried out systematic multiparticle simulation to show that the source of the vertical emittance growth is a combination of the random errors in skew-quadrupole and dipole fields, and the systematic Montague resonance. The effect of random quadrupole field is small for the Fermilab Booster because the betatron envelope tunes are reasonably far away from the half-integer stop band.

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.

Instead of slip stacking, an alternate method of doubling the linear intensity of the Fermilab Main Injector is discussed. This method makes use of rf barriers to transfer 12 booster batches from the Fermilab Booster to the Main Injector in 12 consecutive booster cycles, totaling 800 ms. After that, adiabatic capture of the beam into 53 MHz buckets can be accomplished in about 10 ms. Because the beam is debunched during the injection process and no rf voltage is required, the beam loading voltages in the rf cavities are small and can be eliminated by a combination of counterphasing and mechanical shorts.

Bunch length compression of a single proton bunch is studied by manipulations of the rf voltage and phase. Analytical expressions for the compression ratio, defined as the ratio of the initial to the final bunch lengths, are derived for a linear model. Results obtained from numerical simulations are compared with experimental results. The ultimate maximum compression ratios are found to be sqrt[2]/sqrt[3] σ_φ,i for the phase shift method and sqrt[8]/sqrt[3π] σ_φ,i for the voltage manipulation method, where σ_φ,i is the rms rf-phase spread of the initial beam bunch at the maximum rf voltage. Effects of space charge force are also studied.

An analytic theory of cumulative multibunch beam breakup in linear colliders is developed. Included is a linear variation of transverse focusing across the bunch train as might be applied, e.g., by chirping the radio frequency power sources or by using radio frequency quadrupole magnets. The focusing variation saturates the exponential growth of the beam breakup and establishes an algebraic decay of the transverse bunch displacement versus bunch number. A closed-form expression for the transverse bunch displacement is developed. It is used to quantify the total normalized emittance and thereby isolate the region of parameter space corresponding to high multibunch luminosity.

The evolution of the beam distribution in a double-rf system with a phase modulation on either the primary or secondary rf cavity was measured. We find that the particle diffusion process obeys the Einstein relation if the phase space becomes globally chaotic. When dominant parametric resonances still exist in the phase space, particles stream along the separatrices of the dominant resonance, and the beam width exhibits characteristic oscillatory structure. The particle-tracking simulations for the double-rf system are employed to reveal the essential diffusion mechanism. Coherent octupolar motion has been observed in the bunch beam excitation. The evolution of the longitudinal phase space in the octupole mode is displayed.

Single bunch instabilities for the 50GeV×50GeV muon collider are discussed. An impedance budget of the collider is estimated. A phase-slip factor of |η|=1×10^-6 is desired to avoid excessive rf systems. Potential-well distortion of a smooth bunch can be compensated by rf cavities. Accumulated growth in energy due to imperfections and noises in the muon bunch can be reduced by smoothing the bunch distribution before injection. The growth due longitudinal microwave instability is small because of the compensated rf cavities, the finite lifetime of the muons, and the choice of a small |η|. Beamloadings in the compensating rf cavities are large and suitable feed-forward cancellation is required. Transverse microwave instability can be damped by chromaticities and octupoles. Beam breakup can be cured by Balakin-Novokhatsky-Smirnov damping in principle, but is nontrivial in practice. When beam breakup is small, it can possibly be damped by a betatron tune spread in the beam.

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)].

The inductance of the vacuum chamber of the Proton Storage Ring at Los Alamos National Laboratory was intentionally increased by the introduction of ferrite rings to counteract the longitudinal space-charge effect of the intense beam. The magnetic permeability of the ferrite could be adjusted by introducing current into solenoids wound around the ferrite. Results show that the minimum rf voltage necessary to stabilize the beam against e-p instability may be reduced over that previously measured. The injected bunch length was observed to be longer when the ferrite was heavily biased so that its effect was reduced.

The proton driver for the muon collider must produce short pulses of protons in order to facilitate muon cooling and operation with polarized beams. In order to test methods of producing these bunches, we have operated the alternating-gradient synchrotron near transition and studied procedures which involved moving the transition energy γ_t to the beam energy. We were able to produce stable bunches of 3–5×10¹² protons with rms widths of σ=2.0–2.7ns for longitudinal bunch areas of 1.5eVs, in addition to making measurements of the lowest two orders of the momentum compaction factor.

Experimental observation of particle diffusion mechanism in the presence of overlapping parametric resonances generated by a time dependent rf phase modulation is analyzed. We find that the regime of fast emittance growth is associated with the rapid particle motion along the separatrix of a dominant parametric resonance, the slow growth regime is related to particle diffusion in the chaotic sea, and the emittance saturation occurs when beam particles fill the chaotic region bounded by an invariant torus. Experimental data observed at the Indiana University Cyclotron Facility (IUCF) Cooler Ring are shown to agree well with the theoretical analysis.

The stability of particle motion in a barrier rf system is studied. Parametric resonance strength functions for the barrier rf system with rf phase and voltage modulations are derived. We find that higher order parametric resonances of the barrier rf system are important. Tolerance of the rf phase modulational errors in the barrier rf system in the Fermilab Recycler, a cooling storage ring to recycle unused antiprotons from the Tevatron and to store newly produced cooled antiprotons, is analyzed. A constraint on the rate of bunch compression utilizing the barrier rf system is derived.

We investigate effects of quantum fluctuation, potential well distortion, quantum lifetime, and Touschek lifetime of the quasi-isochronous (QI) dynamical system. The Fokker-Planck equation is employed to study the equilibrium bunch distribution. The quantum lifetime in the moderate damping regime is compared with analytical formulae. The effects of harmonic radio-frequency phase modulation on equilibrium distribution function, quantum lifetime reduction, and the occurrence of stochastic resonance are studied. The formula for the Touschek lifetime for the QI dynamical system is derived and studied.

The problems of beam loss and emittance growth during transition crossing in a proton synchrotron have been major issues for many years. Recently we have developed a scheme that resolves some of these problems by eliminating rf focusing during transition crossing. The technique uses a flattened (nonsinusoidal) rf wave form which delivers the correct acceleration to all particles in the beam. This scheme has been tested in the Fermilab Main Ring accelerator by the addition of 13% of a third harmonic rf voltage to the fundamental accelerating rf voltage during the nonadiabatic period near the transition energy. Beam loss was completely eliminated, and longitudinal emittance dilution after transition remained below 15%. Simulations of longitudinal beam dynamics reproduce the data well.

We have observed longitudinal limit cycle oscillations of a proton beam when a critical threshold in the relative velocity between the proton beam and the cooling electrons has been exceeded. The threshold for the bifurcation of a fixed point into a limit cycle, also known as a Hopf bifurcation, was found to be asymmetric with respect to the relative velocity. Further experiments were performed to verify that the asymmetry was related to electron beam alignment with respect to the stored proton beam. The measured amplitudes of the ensuing limit cycle were used to determine the cooling drag force, which exhibits the essential characteristics of the magnetized cooling, where the limit cycle attractor can coexist with a damping-free region and/or a fixed point attractor. © 1996 The American Physical Society.

The longitudinal dynamics of a stored proton beam bunch, under the influence of a nonlinear damping force produced by electron cooling, was studied experimentally. The effect of the nonlinear damping force was explored by varying the relative velocity between the cooling electrons and the stored protons. Maintained longitudinal oscillations developed, which grew rapidly once a critical threshold in the relative velocity was exceeded. The bifurcation of a fixed point into a limit cycle is also known as a Hopf bifurcation. Comparisons of experimental data with numerical simulations and analytical calculations are made. Implications for cooled beam acceleration will be discussed.

The longitudinal dynamics of a stored proton beam bunch, acted upon by a nonlinear damping force, was studied experimentally at the Indiana University Cyclotron Facility Cooler Ring. The effect of the nonlinear damping force on synchrotron motion was explored by varying the relative velocity between the cooling electron and the stored proton beams. Maintained longitudinal oscillations were observed, whose amplitude grew rapidly once a critical threshold in the relative velocity between the proton and electron beams was exceeded. We attribute this phenomenon to a negative resistance instability occurring after a Hopf bifurcation.

The attractors of a double-rf system subject to rf phase modulation, in the presence of a weak damping force, were measured as a function of the modulation frequency. We found that the phase amplitudes of the attractors followed a simple predictable path related to the synchrotron tune of the double-rf system. These attractors were found to bifurcate at a modulation frequency near the maximum synchrotron frequency. We also found that the coherent synchrotron oscillations decohered rapidly at small synchrotron amplitudes but showed little decoherence at large synchrotron amplitudes. The experimental result has some implications for the Landau damping of coherent beam instabilities.

The betatron difference resonance, Q_x-2Q_z=-6, where the Q_x,z are the number of betatron oscillations per revolution, was studied at the Indiana University Cyclotron Facility cooler ring. Measurements of both vertical and horizontal coherent betatron oscillations were made, at a nonlinear resonance, after a pulsed dipole kick. We found that the Poincaré surface of section for the nonlinear resonance could be described by a simple Hamiltonian. The resonance strength and phase, as well as the tune shift, as a function of betatron amplitude, were deduced from the experimental data. Attempts to deduce the amplitude and phase of the time dependent fluctuations around the time averaged Poincaré surface of section will also be discussed.

When a double rf system is subjected to sinusoidal phase modulation, the Poincaré surfaces of the section display a rich spectrum of resonance islands. Stable and unstable fixed points of these resonance islands form a tree of bifurcation branches which can be explained as parametric resonances generated by external phase modulation. A semianalytic determination of the condition for the bifurcation of fixed points is presented for an autonomous Hamiltonian of one degree of freedom with sinusoidal time dependent perturbation.

The response of particles trapped in one-dimensional resonance islands to betatron tune modulation resembles, yet is not equivalent to, that of a parametric resonant system. Experimental data obtained at Indiana University Cyclotron Facility for the fourth-order resonance islands have confirmed this characteristic feature. The beam, driven by betatron tune modulation, was observed to travel from near the center of resonance islands toward the separatrix. The experimental data are characterized by the onset of a large response at a critical modulation amplitude and frequency, which are compared with theoretical models. Possible future experiments are suggested.

Experimentally obtained Poincaré maps in the resonant rotating frame for particle motion with linear coupling revealed invariant tori of the two-dimensional Hamiltonian. Using these tori, we obtained the linear coupling strength, the tune shift with betatron amplitude coefficients, and the proximity parameter to the resonance. The coupling strength obtained with this method agreed well with that obtained from measuring the betatron tune separation of the normal modes.