Search Results (11 total)

We have developed a theory to calculate both longitudinal and transverse impedances of a resistive short (typically shorter than the chamber radius) insert with cylindrical symmetry, sandwiched by perfectly conductive chambers on both sides. It is found that unless the insert becomes extremely thin (typically a few nm for a metallic insert) the entire image current runs on the thin insert, even in the frequency range where the skin depth exceeds the insert thickness, and therefore the impedance increases drastically from the conventional resistive-wall impedance. In other words, the wakefields do not leak out of the insert unless it is extremely thin. Formulas of the impedance valid for various cases of the insert are categorized in summary.

The 3 GeV Rapid Cycling Synchrotron (RCS) at Japan Proton Accelerator Research Complex is nearly at the operational stage with regard to the beam commissioning aspects. Recently, the design painting injection study has been commenced with the aim of high output beam power at the extraction. In order to observe the phase space footprint of the painting injection, a method was developed utilizing a beam position monitor (BPM) in the so-called single pass mode. The turn-by-turn phase space coordinates of the circulating beam directly measured using a pair of BPMs entirely positioned in drift space, and the calculated transfer matrices from the injection point to the pair of BPMs with several successive turns were used together in order to obtain the phase space footprint of the painting injection. There are two such pairs of BPMs placed in two different locations in the RCS, the results from which both agreed and were quite consistent with what was expected.

The 3-GeV rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC) was commissioned in October 2007, and successfully accomplished 3 GeV acceleration on October 31. Six run cycles through February 2008 were dedicated to commissioning the RCS, for which the initial machine parameter tuning and various underlying beam studies were completed. Then since May 2008 the RCS beam has been delivered to the downstream facilities for their beam commissioning. In this paper we describe beam tuning and study results following our beam commissioning scenario and a beam performance and operational experience obtained in the first commissioning phase through June 2008.

A ceramic chamber with Cu stripes is usually used as the vacuum chamber in a rapid cycling synchrotron. The Cu stripes terminate at either end as capacitors, and provide the low impedance for the circulating beam, and the high impedance for the induced current with the frequency components of the external time-dependent magnetic field (for example, injection bump magnets, bending magnets, etc.). It is important to be able to precisely estimate the field modulations inside the chamber when the field is excited, because any such field modulations can cause the beam characteristics to deteriorate. In this paper a theoretical approach to evaluate the field modulations in a quick and precise manner is developed.

The Napoly integral is the very useful method for calculations of wake potentials in structures where parts of the boundary extend below the beam pipe radius or the radii of the two beam pipes at both ends are unequal. It reduces CPU time a lot by deforming the integration path so that the integration contour is confined to the finite length over the gap of the structures. However, the original Napoly method cannot be applied to the transverse wake potentials in a structure where the two beam tubes on both sides have unequal radii . In this case, the integration path needed to be a straight line and the integration is stretched out to an infinite, in principle. We generalize the Napoly integrals so that integrals are always confined in a finite length even when the two beam tubes have unequal radii, for both longitudinal and transverse wake potential calculations. The extended method has been successfully implemented to the ABCI code.

A gap in the vacuum chamber stands between a beam and the outside world, and the theoretical elucidation of the interaction mechanism between the gap and the beam is of great importance to understand the interaction of any device with the beam. In this paper, we will present the formulas for the longitudinal and transverse impedances due to a gap in the beam chamber. In this process, we will derive the complete solutions of electromagnetic fields effective in the entire region, including the inside and the outside of the chamber, in a form that they can be easily numerically evaluated. The newly developed technique can provide new methods of solutions of electromagnetic fields also for a rather broad class of structures such as cavities. The numerical results of impedances are consistent with the ABCI results and their behavior in high frequency agrees well with the prediction of the diffraction theory. Our theory can also accurately reproduce the behavior of the impedance near and above the cutoff frequencies. In addition, our theory is applicable even to the impedances for nonrelativistic beams. We found that the broadband impedance of the small cavitylike structure can be estimated from the gap size and the chamber radius only, regardless of the exact shape of the structure. We also found that the transverse impedance of a gap has a large resonance peak at the frequency where the wavelength is equal to the chamber circumference. This resonance peak appears around 1–2 GHz in most of the cases, and we should be careful to design a ceramic break so that this transverse mode will not leak out to interact with nearby devices.

Since the resistive wall impedance for a beam pipe of a nonround cross section depends on the coordinates of a witness particle, the witness particle receives an incoherent tune shift. When the expression for the impedance of an infinitely thick chamber is applied to the calculation of this tune shift, it becomes infinite. We have derived the resistive wall impedance for a chamber with a finite thickness and calculated the tune shift. There is no ambiguity in this expression for the tune shift, because it is automatically finite.

A theory concerning the relation between the heating rate and temperature of hadron beams is formulated from a quantum point of view. This theory predicts that the heating rate can be reduced by increasing the lattice periodicity of the accelerator with its fixed tunes and circumference. This prediction is quite consistent with simulation results.

The existence and uniqueness of a solution for the Haissinski equation with a capacitive wake function has been analytically proved.

A purely inductive wake function was the only known case where a solution of the Haissinski equation did not exist beyond a certain threshold. This is due to the ill-defined treatment of the wake function. The solution is proved to exist beyond the threshold, if we define the wake function physically. The threshold of the stability for the solution exists but is lower than the former “threshold.”

We consider a scenario where the top-quark mass is generated dynamically, and study the implications of the present experimental values for m_t and the T parameter. We assume a technicolorlike scenario for inducing the W mass and an effective four-Fermi operator for inducing the top-quark mass. We also assume that only this four-Fermi operator is relevant at low energy. Then we estimate in detail the strength G and the intrinsic mass scale M of the four-Fermi operator. A unitarity bound is used to quantify the strength of G. We find that G / 4π∼1 and that M is of the order of Λ_TC≃1-2 TeV or less. Namely the four-Fermi operator cannot be treated as "pointlike" around the electroweak scale. Furthermore we estimate the contribution of the four-Fermi operator to the T parameter. We find that the QCD correction to the top-quark mass function reduces the contribution to the T parameter by about 40%. By comparing the results with the present experimental bound, we obtain another upper bound on M which is typically in the several to 10 TeV region.