RF Cavity Voltage Research Articles

Improved calibration of rf cavities for relativistic electron beams: Effects of secondary corrections and experimental verification

In the aspect of longitudinal beam bunching, the bunching strength can be controlled by the rf cavity phase and voltage. However, these machine parameters are different from those that interact with the beam itself. In order to gain control of the beam-cavity interaction, cavity calibration must be performed. Furthermore, it relies on fitting the beam energy gain versus cavity phase to a calibration function. Under the conventional assumption of relativistic beam conditions, the calibration function is a first harmonic sinusoidal function (a sinusoidal function with a period of $2\ensuremath{\pi}$). However, this expression is insufficient for a high-voltage bunching cavity. Due to beam acceleration inside the cavity, an energy bias and a second harmonic function should be included to modify the conventional calibration function, even for a relativistic electron beam. In this paper, we will derive this modification and provide a comparison to both the Coherent Electron Cooling Experiment and the impact-t simulation, respectively.

Simulation study of slow extraction for the Shanghai Advanced Proton Therapy facility

The Shanghai Advanced Proton Therapy facility employs third-integer slow extraction. In order to achieve accurate treatment, high-quality spill is needed. Therefore, parameters that may affect slow extraction should be investigated by simulation. A computer model of the synchrotron operation slow extraction was constructed with MATLAB®. By simulating the motion of the circulating protons, we could quantify the influence of machine and initial beam parameters on properties of the extracted beam, such as ripple, uniformity, stability, on- and off-time of the spill and spill width in the synchrotron. Suitable design parameters including the horizontal tunes, power supply ripple, longitudinal RF cavity voltage, RF-KO and the chromaticities were determined.

Measurement and compensation of horizontal crabbing at the Cornell Electron Storage Ring Test Accelerator

In storage rings, horizontal dispersion in the rf cavities introduces horizontal-longitudinal ($xz$) coupling, contributing to beam tilt in the $xz$ plane. This coupling can be characterized by a ``crabbing'' dispersion term ${\ensuremath{\zeta}}_{a}$ that appears in the normal mode decomposition of the 1-turn transfer matrix. ${\ensuremath{\zeta}}_{a}$ is proportional to the rf cavity voltage and the horizontal dispersion in the cavity. We report experiments at the Cornell Electron Storage Ring Test Accelerator where $xz$ coupling was explored using three lattices with distinct crabbing properties. We characterize the $xz$ coupling for each case by measuring the horizontal projection of the beam with a beam size monitor. The three lattice configurations correspond to (i) 16 mrad $xz$ tilt at the beam size monitor source point, (ii) compensation of the ${\ensuremath{\zeta}}_{a}$ introduced by one of two pairs of rf cavities with the second, and (iii) zero dispersion in rf cavities, eliminating ${\ensuremath{\zeta}}_{a}$ entirely. Additionally, intrabeam scattering is evident in our measurements of beam size vs rf voltage.

Interbunch extinction measurement at the BNL AGS for the KOPIO experiment

The KOPIO experiment at the BNL AGS required an extracted proton beam in which a debunched, coasting beam is forced between empty longitudinal RF buckets to form microbunches. The goal of the measurement described here was to obtain a low background determination of the fraction of protons coming from the AGS slowly extracted beam at times between microbunches (interbunch extinction). The effect on interbunch extinction of variations in the RF cavity voltage, the momentum dispersion of the beam and the main guide field voltage-ripple was studied. It was found that there exists a broad range of operating parameters that could allow the AGS to produce microbunches with the interbunch extinction better than the KOPIO experiment requirement. Results obtained in this study gave interbunch extinctions better than 10 −5 (1%) of KOPIO's upper limit.

Effect of rf cavity temperature variation on electron beams with rf voltage modulations

The correlation between the horizontal beam size versus the cavity temperature, observed at the Taiwan Light Source, is explained by the combined effects of (i) the cavity resonance frequency shift resulting from temperature change, (ii) the cavity voltage and synchronous phase angle change resulting from uncompensated beam loading in the low-level rf-feedback system, (iii) the synchrotron frequency shift due to the cavity voltage, and (iv) the rf cavity voltage modulation for alleviating the coupled-bunch instability. We find that the effect of higher-order parasitic-mode losses on the change of the horizontal beam size is small. This experimental method can be used to evaluate the intrinsic resolution of the low-level rf-feedback system.

Design of a compact synchrotron for medical applications

An optimal design of a low energy (300 MeV) proton synchrotron for medical applications is addressed. The machine has the following properties: (1) The transition energy is higher than the targets final proton beam energy of 300 MeV; (2) the betatron tunes are chosen such that the machine is free of systematic resonances; (3) the machine can accommodate both slow and fast extraction systems; and (4) the machine can provide rapid cycling operations depending on the rf cavity voltage. Applications of this low energy synchrotron are discussed.

RF voltage modulations and the coupled bunch instabilities

The effects of rf cavity voltage modulation on longitudinal beam dynamics were studied experimentally at Taiwan Light Source by using a streak camera system that measured the attractor amplitude of the beam bunch. Theoretical description of rf voltage modulation that includes the effect of nonzero synchronous phase has been developed. The characteristics of the quadrupole parametric resonance in the single bunch and multibunch modes were studied and compared with theory. The formation of beamlets in the bucket helps to damp the coherent coupled bunch oscillation driven by parasitic modes of the rf cavity. This beam manipulation technique can be employed to alleviate stability problems associated with high brightness electron storage rings.

Attainment of space-charge dominated beams in a synchrotron.

For a given rf cavity voltage, there is a maximum attainable peak current in an ion storage ring. This occurs when the electric field from the beam space charge is balanced by the field from the rf cavity. In this limit, the linear charge density distribution is parabolic and incoherent synchrotron motion is suppressed. The beam energy spread cannot be determined from the bunch length which depends only on the beam current and rf voltage. This has been observed using the high energy electron-cooling system at the Indiana University Cyclotron Facility.