Search Results (21 total)

Since 2001, the Relativistic Heavy Ion Collider has experienced electron cloud effects, some of which have limited the beam intensity. These include dynamic pressure rises (including pressure instabilities), tune shifts, a reduction of the instability threshold for bunches crossing the transition energy, and possibly incoherent emittance growth. We summarize the main observations in operation and dedicated experiments as well as countermeasures including baking, nonevaporable getter coated warm beam pipes, solenoids, bunch patterns, antigrazing rings, prepumped cold beam pipes, scrubbing, and operation with long bunches.

The four electron stripping stages leading to fully stripped gold ions in the Relativistic Heavy Ion Collider (RHIC) are briefly described. The third stripper, which removes 46 electrons from the Au³¹⁺ ions leading to heliumlike Au⁷⁷⁺, offers the greatest challenges in terms of energy loss and induced energy spread. These problems are described in detail as well as recent advances in the design and performance of this stripper. Measurements performed with several carbon and aluminum strippers show general agreement with a semiempirical model but small systematic deviations suggest that some model adjustments may be in order. The best performance is predicted and obtained with a combined carbon-aluminum foil system. Measurements showing the enhanced performance in the alternating gradient synchrotron are described. The stripper that removes the last two electrons has also been improved and the results of relevant calculations and measurements are presented.

Nonscaling fixed field alternating gradient (FFAG) rings for cancer hadron therapy offer reduced physical aperture and large dynamic aperture as compared to scaling FFAGs. The variation of tune with energy implies the crossing of resonances during acceleration. Our design avoids intrinsic resonances, although imperfection resonances must be crossed. We consider a system of three nonscaling FFAG rings for cancer therapy with 250 MeV protons and 400   MeV/u carbon ions. Hadrons are accelerated in a common radio frequency quadrupole and linear accelerator, and injected into the FFAG rings at v/c=0.1294. H⁺/C⁶⁺ ions are accelerated in the two smaller/larger rings to 31 and 250  MeV/68.8 and 400   MeV/u kinetic energy, respectively. The lattices consist of doublet cells with a straight section for rf cavities. The gantry with triplet cells accepts the whole required momentum range at fixed field. This unique design uses either high-temperature superconductors or superconducting magnets reducing gantry magnet size and weight. Elements with a variable field at the beginning and at the end set the extracted beam at the correct position for a range of energies.

A major expense and design challenge in carbon/proton cancer therapy machines are the isocentric gantries. The transport elements of the carbon/proton gantry are presently made of standard conducting dipoles. Because of their large weight, of the order of ∼100   tons, the total weight of the gantry with support structure is ∼600   tons. The novel gantry design that is described here is made of fixed field superconducting magnets, thus dramatically reducing magnet size and weight compared to conventional magnets. In addition, the magnetic field is constant throughout the whole energy region required for tumor treatment. Particles make very small orbit offsets, passing through the beam line. The beam line is built of combined-function dipoles such as a nonscaling fixed field alternating gradient (NS-FFAG) structure. The very large momentum acceptance NS-FFAG comes from very strong focusing and very small dispersion. The NS-FFAG small magnets almost completely filled the beam line. They first make a quarter (or close to a quarter) of an arc bending upward and an additional half of a circle beam line finishing so that the beam is pointed towards the patient. At the end of the gantry, additional magnets with a fast response are required to allow radial scanning and to provide the required position and spot size. The fixed field combined-function magnets for the carbon gantry could be made of superconducting magnets by using low temperature superconducting cable or by using high temperature superconductors.

The Brookhaven Relativistic Heavy Ion Collider (RHIC) has been providing collisions of polarized protons at a beam energy of 100 GeV since 2001. Equipped with two full Siberian snakes in each ring, polarization is preserved during acceleration from injection to 100 GeV. However, the intrinsic spin resonances beyond 100 GeV are about a factor of 2 stronger than those below 100 GeV making it important to examine the impact of these strong intrinsic spin resonances on polarization survival and the tolerance for vertical orbit distortions. Polarized protons were first accelerated to the record energy of 205 GeV in RHIC with a significant polarization measured at top energy in 2005. This Letter presents the results and discusses the sensitivity of the polarization survival to orbit distortions.

Bent crystal channeling has been observed with protons and fully stripped gold ions in the Relativistic Heavy Ion Collider (RHIC). Prior to 2003, a bent crystal was installed in one ring of RHIC as the first stage of a two stage collimation system. The observed channeling efficiency was approximately 25%, less than half of original predictions. We show that this is due to a difference between the model and real Twiss parameters at the crystal location and our improved understanding of the beam halo. Collimation using the crystal was unsuccessful and raised background at the STAR detector by as much as a factor of 2 because of the low channeling efficiency. We give a report of our channeling studies in RHIC and describe our experience using the bent crystal as a collimator. The results are discussed and compared to simulations and theoretical predictions.

The antigrazing ridges installed in two Relativistic Heavy Ion Collider (RHIC) warm straight sections have been tested with beam. Significant reduction in electron cloud induced pressure rise has been observed. In addition, the pressure rise pattern at these locations has changed. The effect of antigrazing ridges on the RHIC warm section electron cloud are discussed, as well as possible mechanisms.

Linear coupling’s action-angle parametrization is convenient for interpretation of turn-by-turn beam position monitor (BPM) data. We demonstrate how to apply this parametrization to extract Twiss and coupling parameters in interaction regions (IRs), using BPMs on each side of a long IR drift region. Example data were acquired at the Relativistic Heavy Ion Collider, using an ac dipole to excite a single transverse eigenmode. We have measured the waist of the β function and its Twiss and coupling parameters.

We describe a fast and robust global decoupling scheme, coupling angle modulation. This novel technique introduces an extra rotating coupling coefficient into the coupled optics to determine the global decoupling strengths. The eigentune split is used as the observable during the modulation. The two eigentunes are tracked with a high-resolution phase locked loop tune measurement system. In the article, the principle of coupling angle modulation is presented, followed by its application to the Relativistic Heavy Ion Collider (RHIC). Coupling angle modulation coupling correction has been used for the global coupling correction on the nonstop RHIC ramp.

We present a design of nonscaling fixed field alternating gradient accelerators (FFAG) minimizing the dispersion action function H. The design is considered both analytically and via computer modeling. We present the basic principles of a nonscaling FFAG lattice and discuss optimization strategies so that one can accelerate over a broad range of momentum with reasonable apertures. Acceleration schemes for muons are discussed.

Measurement of the residual betatron coupling with skew quadrupole modulation is a new diagnostics technique. It was developed and tested at the Relativistic Heavy Ion Collider (RHIC) as a promising method for measuring coupling on the ramp. By modulating the strengths of skew quadrupole families, the two tunes' responses are precisely measured with the phase lock loop system. The projection ratio of the residual coupling coefficient onto the coupling modulation direction can be determined. In this article, the analytical solution to the skew quadrupole modulation is given. Dedicated beam studies were carried out in RHIC Run'04 and the results are presented. The ability to measure coupling on the ramp opens the possibility of continuously correcting coupling during acceleration.

Grazing collisions with the stainless steel beam pipes of gold ions, the so-called “halo scraping,” result in large secondary electron emission and surface molecular desorption yields in the Relativistic Heavy Ion Collider. Here we estimate electron emission yields as function of incidence angle, we show that desorption rates will follow a similar angular dependence at small angles, and we propose a simple approach to mitigate these effects.

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.

During the Relativistic Heavy Ion Collider commissioning in 2001 a fast transverse instability was observed on the ramp. In general this could be counteracted with increased chromaticity, resulting in Landau damping. However this method could not be applied around transition energy where chromaticities have to change sign. So octupoles were used near transition energy to create transverse Landau damping and avoid the transverse instability, emittance blowup, and beam loss. This paper describes the considerations that led to the present scheme, as well as experimental results.

A silicon crystal was used to channel and extract 70 GeV protons from the U-70 accelerator with an efficiency of 85.3±2.8%, as measured for a beam of ∼10¹² protons directed towards crystals of ∼2 mm length in spills of ∼2 s duration. The experimental data follow very well the prediction of Monte Carlo simulations. This demonstration is important in devising a more efficient use of the U-70 accelerator in Protvino and provides crucial support for implementing crystal-assisted slow extraction and collimation in other machines, such as the Tevatron, RHIC, the AGS, the SNS, COSY, and the LHC.

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 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.

Medium-energy accelerators are often confronted with problems during transition energy crossing, such as longitudinal microwave instability and nonlinear synchrotron motion. These problems can be avoided by an accelerator having a negative momentum-compaction factor. A modular method for designing a lattice with adjustable momentum-compaction factor is presented. The dispersion excursion of the basic flexible momentum compaction module can be minimized to less than the maximum dispersion of the focusing-defocusing (FODO) lattice containing the same FODO cells. The phase advance of the module can be adjusted to be an odd multiple of quarter betatron waves. We found that a lattice composed of such modules possesses excellent chromatic properties with excellent tunability, smaller systematic stop-band widths, and smaller sextupole distortion functions.