Search Results (14 total)

Computer simulations using the 2D code POSINST were used to study the formation of the electron cloud in the wiggler section of the positron damping ring of the International Linear Collider. In order to simulate an x-y slice of the wiggler (i.e., a slice perpendicular to the beam velocity), each simulation assumed a constant vertical magnetic field. At values of the magnetic field where the cyclotron frequency was an integral multiple of the bunch frequency, and where the field strength was less than approximately 0.6 T, equilibrium average electron densities were up to 3 times the density found at other neighboring field values. Effects of this resonance between the bunch and cyclotron frequency are expected to be non-negligible when the beam bunch length is much less than the product of the electron cyclotron period and the beam velocity, for a beam moving at v≈c. Details of the dynamics of the resonance are described.

By combining the method of images with calculus of complex variables, we provide a simple expression for the electric field of a two-dimensional (2D) static elliptical charge distribution inside a perfectly conducting circular cylinder. The charge distribution need not be concentric with the cylinder.

We revisit the estimation of the power deposited by the electron cloud (EC) in the arc dipoles of the Large Haydron Collider, by means of simulations. We adopt, as simulation input, a set of electron-related parameters closely resembling those used in recent simulations at CERN [F. Zimmermann, in LTC Meeting No. 40, CERN, 2005]. We explore values for the bunch population N_b in the range 0.4×10¹¹≤N_b≤1.6×10¹¹, peak secondary electron yield δ_max in the range 1.0≤δ_max≤2.0, and bunch spacing t_b either 25 or 75 ns. For t_b=25  ns we find that the EC average power deposition per unit length of beam pipe, dP̅ /dz, will exceed the available cooling capacity, which we take to be 1.7   W/m at nominal N_b [F. Zimmermann, in LHC MAC Meeting No. 17, 2005], if δ_max exceeds ∼1.3, but dP̅ /dz will be comfortably within the cooling capacity if δ_max≤1.2. For t_b=75  ns dP̅ /dz exceeds the cooling capacity only when δ_max>2 and N_b>1.5×10¹¹ taken in combination. The rediffused component of the secondary electron emission spectrum plays a significant role: if we artificially suppress this component while keeping δ_max fixed, dP̅ /dz is roughly cut in half for most values of N_b explored here, and in this case we find good agreement with earlier results [F. Zimmermann, in LTC Meeting No. 40, CERN, 2005], as expected. We provide a fairly detailed explanation of the mechanism responsible for such a relatively large effect. We assess the sensitivity of our results to numerical simulation parameters, and to physical parameters such as the photoelectric yield, bunch train length, etc. Owing to the lack of detailed knowledge of the electron emission spectrum, the sensitivity of dP̅ /dz to the rediffused component appears to be the most significant source of uncertainty in our results. Nevertheless, taking our results as a whole, the condition δ_max≤1.2 seems to be a conservative requirement for the cooling capacity not to be exceeded.

Present and future accelerators' performances may be limited by the electron cloud (EC) effect. The EC formation and evolution are determined by the wall-surface properties of the accelerator vacuum chamber. We present measurements of the total secondary electron yield (SEY) and the related energy distribution curves of the secondary electrons as a function of incident-electron energy. Particular attention has been paid to the emission process due to very low-energy primary electrons (<20  eV). It is shown that the SEY approaches unity and the reflected electron component is predominant in the limit of zero primary incident electron energy. Motivated by these measurements, we have used state-of-the-art EC simulation codes to predict how these results may impact the production of the electron cloud in the Large Hadron Collider, under construction at CERN, and the related surface heat load.

We have augmented the code POSINST to include solenoid fields and used it to simulate the buildup of electron cloud due in the PEP-II positron ring. We find that the distribution of electrons is strongly affected by the resonances associated with the cyclotron period and bunch spacing. In addition, we discover a threshold beyond which the electron density grows exponentially until it reaches the space charge limit. The threshold does not depend on the bunch spacing but does depend on the positron bunch population.

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.

Electron effects in the High Current Experiment (HCX) are studied via computer simulation. An approximate expression for the secondary electron yield for a potassium ion striking stainless steel is derived and compared with experimental results. This approximate expression has a peak of roughly 55 electrons at normal incidence at an ion energy of 60 MeV. Using an empirical angular dependence, the secondary electron yield is combined with a numerical simulation of the HCX ion beam dynamics to obtain an estimate for the number of secondary electrons expected per ion-wall collision in the HCX. This estimate is that approximately 150–200 electrons per ion collision may result in the HCX.

We have applied our simulation code POSINST to evaluate the contribution to the growth rate of the electron cloud instability in proton storage rings. In particular, we present here recent simulation results for the main features of the electron cloud in the storage ring of the Spallation Neutron Source at Oak Ridge, and updated results for the Proton Storage Ring at Los Alamos. A key ingredient in our model is a detailed description of the secondary electron emission process, including a refined model for the emitted energy spectrum, and for the three main components of the secondary yield, namely, the true secondary, rediffused and backscattered components.

Electron cloud instabilities in the Los Alamos Proton Storage Ring and those foreseen for the Oak Ridge Spallation Neutron Source are examined theoretically, numerically, and experimentally.

We provide a detailed description of a model and its computational algorithm for the secondary electron emission process. The model is based on a broad phenomenological fit to data for the secondary-emission yield and the emitted-energy spectrum. We provide two sets of values for the parameters by fitting our model to two particular data sets, one for copper and the other one for stainless steel.

In this paper we present a new approach, based on a shifted Green function, to evaluate the electromagnetic field in a simulation of colliding beams. Unlike a conventional particle-mesh code, we use a method in which the computational mesh covers only the largest of the two colliding beams. This allows us to study long-range parasitic collisions accurately and efficiently. We have implemented this algorithm in a new parallel strong-strong beam-beam simulation code. As an application, we present a study of a beam sweeping scheme for the LBNL luminosity monitor of the Large Hadron Collider.

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)].

We study the Hamiltonian form of Reggeon field theory on a lattice in the two-dimensional D = 2 transverse or impact-parameter space. The Hamiltonian formalism allows naturally for the privileged character of the longitudinal variable or rapidity, which is kept continuous. Based on recent results for the one-dimensional theory, we argue that we may truncate the single-site basis of the Hamiltonian by retaining the lowest two states only, and we arrive at a lattice spin model. In terms of Pauli spin matrices σ_k^n→ at each site n→ = (n₁,n₂), our Reggeon quantum spin model has the Hamiltonian H=Σn→(δ / 2)(1-σ_x^n→)+Σn→·i-^ A[1-2σ₊^{n→+i-^} )(1-2σ₊^n→)-σ_z^{n→+i-^} σ_z^n→], where i-^ represents the lattice unit vectors (1,0) and (0,1). The parameters δ and A are related to those of the original field theory. All the approximations are valid for small Regge slope α₀^′ in the region of the phase transition at output Regge intercept α(0)=1. The critical exponents of the original system can be determined by the properties of the low-energy states of H in strict analogy with the established relation between the φ⁴ theory in d = 2 and the ground state of the quantum Ising model with a transverse magnetic field in D = d-1 = 1 dimension. The general properties of the Reggeon quantum spin model are exhibited, and several new approximation schemes for Reggeon field-theoretic calculations are suggested. While the above Hamiltonian is adequate, by universality, to describe the critical Pomeron, systematic improvements can be made to study the theory away from criticality by retaining more states in the single-site basis of the Hamiltonian.