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Title: Exact cancellation of emittance growth due to coupled transverse dynamics in solenoids and rf couplers

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Resource Type:
Journal Article: Published Article
Journal Name:
Physical Review Accelerators and Beams
Additional Journal Information:
Journal Volume: 21; Journal Issue: 1; Related Information: CHORUS Timestamp: 2018-01-17 10:04:21; Journal ID: ISSN 2469-9888
American Physical Society
Country of Publication:
United States

Citation Formats

Dowell, David H., Zhou, Feng, and Schmerge, John. Exact cancellation of emittance growth due to coupled transverse dynamics in solenoids and rf couplers. United States: N. p., 2018. Web. doi:10.1103/PhysRevAccelBeams.21.010101.
Dowell, David H., Zhou, Feng, & Schmerge, John. Exact cancellation of emittance growth due to coupled transverse dynamics in solenoids and rf couplers. United States. doi:10.1103/PhysRevAccelBeams.21.010101.
Dowell, David H., Zhou, Feng, and Schmerge, John. Wed . "Exact cancellation of emittance growth due to coupled transverse dynamics in solenoids and rf couplers". United States. doi:10.1103/PhysRevAccelBeams.21.010101.
title = {Exact cancellation of emittance growth due to coupled transverse dynamics in solenoids and rf couplers},
author = {Dowell, David H. and Zhou, Feng and Schmerge, John},
abstractNote = {},
doi = {10.1103/PhysRevAccelBeams.21.010101},
journal = {Physical Review Accelerators and Beams},
number = 1,
volume = 21,
place = {United States},
year = {Wed Jan 17 00:00:00 EST 2018},
month = {Wed Jan 17 00:00:00 EST 2018}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1103/PhysRevAccelBeams.21.010101

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  • Weak, rotated magnetic and radio frequency quadrupole fields in electron guns and injectors can couple the beam’s horizontal with vertical motion, introduce correlations between otherwise orthogonal transverse momenta, and reduce the beam brightness. This paper discusses two important sources of coupled transverse dynamics common to most electron injectors. The first is quadrupole focusing followed by beam rotation in a solenoid, and the second coupling comes from a skewed high-power rf coupler or cavity port which has a rotated rf quadrupole field. It is shown that a dc quadrupole field can correct for both types of couplings and exactly cancel theirmore » emittance growths. The degree of cancellation of the rf skew quadrupole emittance is limited by the electron bunch length. Analytic expressions are derived and compared with emittance simulations and measurements.« less
  • Bent solenoids can transmit charged particle beams while providing momentum dispersion. While less familiar than quadrupole and dipole systems, bent solenoids can produce superficially simple transport lines and large acceptance spectrometers for use at low energies. Design issues such as drift compensation and coupling sections between straight and bent solenoids are identified, and aberrations such as shears produced by perpendicular error fields are discussed. Examples are considered which provide the basis for the design of emittance exchange elements for the cooling system of a muon collider.
  • Self-assembled SiC quantum dots (QDs) are grown on Si substrates at a low substrate temperature of 400 degree sign C by means of low-frequency, inductively coupled plasma assisted rf magnetron sputtering from a sintered SiC target in a reactive Ar+H{sub 2} gas mixture. Effects of SiC target power and working gas pressure on the surface morphology and structural properties of SiC QDs are investigated. The growth dynamics of the QDs obeys cubic root-law behavior. With the increase of SiC target power, the growth rate increases greatly, resulting in nonuniform surface morphology and higher intensity of Si-C transmittance band. Scanning electronmore » microscopy shows that (i) at pressure below 1 Pa, SiC quantum dots are highly uniform and the average size of quantum dots increases with the increase of pressure; (ii) at pressure above 1 Pa, SiC quantum dots are nonuniform, and the size of quantum dots decreases with the increase of pressure. These behaviors are explained by the scattering effects and the surface mobility of the sputtered atoms. X-ray photoelectron and Fourier transform infrared spectroscopic results show that the predominant bonds are Si-C and the elemental composition of Si and C atoms is near stoichiometric.« less
  • It is well-known that the electron beam quality required for applications such as FEL’s and ultra-fast electron diffraction can be degraded by the asymmetric fields introduced by the RF couplers of superconducting linacs. This effect is especially troublesome in the injector where the low energy beam from the gun is captured into the first high gradient accelerator section. Unfortunately modifying the established cavity design is expensive and time consuming, especially considering that only one or two sections are needed for an injector. Instead, it is important to analyze the coupler fields to understand their characteristics and help find less costlymore » solutions for their cancellation and mitigation. This paper finds the RF coupler-induced emittance for short bunches is mostly due to the transverse spatial sloping or tilt of the field, rather than the field’s time-dependence. It is shown that the distorting effects of the coupler can be canceled with a static (DC) quadrupole lens rotated about the z-axis.« less
  • Simulations have identified charge-density variations as driving the dominant emittance growth mechanism for high-current, low-emittance induction linacs using solenoidal focusing, once the beam enters the emittance-dominated regime. In this paper, we use the radial equation of motion, including the nonlinearities resulting from radial density variations, to understand this effect. Nonlinearities in the beam{close_quote}s radial motion while in a solenoid arise from the noncancellation of the effects from the diamagnetic axial magnetic field and the potential depression of the beam, if the beam density is nonuniform. Any initial density variation drives a logarithmic increase in additional higher-order density variations (through themore » differential betatron motion), and an emittance growth that scales logarithmically, or greater (even potentially faster than linear), with the axial distance along the accelerator. The growth rate depends on the beam current, the focusing force, and the accelerating gradient, and for typical machine parameters, the growth rate can be faster than linear with distance. The magnitude of the emittance growth depends critically on the matching of the beam from the injector to the beamline. This formalism leads to a criterion of how uniform the beam density has to be and how well the beam needs to be matched in order not to have an unacceptable emittance growth. {copyright} {ital 1998 American Institute of Physics.}« less