Search Results (11 total)

The longitudinal wake is calculated for a bunch of ultrarelativistic electrons that is deflected by a magnet that is much shorter than the formation length of its coherent synchrotron radiation. Using this wake and the Coulomb field of an image bunch, we obtain formulas for the wake of coherent transition radiation. By modeling the coherent diffraction radiation of an iris as the coherent transition radiation from a hollow bunch, we calculate the on-axis longitudinal wake of coherent diffraction radiation for an arbitrary target shape. We also calculate the influence of an upstream magnet’s edge radiation upon the wake.

In a two-stage compression and acceleration system, where each stage compresses a chirped bunch in a magnetic chicane, wakefields affect high-current bunches. The longitudinal wakes affect the macroscopic energy and current profiles of the compressed bunch and cause microbunching at short wavelengths. For macroscopic wavelengths, impedance formulas and tracking simulations show that the wakefields can be dominated by the resistive impedance of coherent edge radiation. For this case, we calculate the minimum initial bunch length that can be compressed without producing an upright tail in phase space and associated current spike. Formulas are also obtained for the jitter in the bunch arrival time downstream of the compressors that results from the bunch-to-bunch variation of current, energy, and chirp. Microbunching may occur at short wavelengths where the longitudinal space-charge wakes dominate or at longer wavelengths dominated by edge radiation. We model this range of wavelengths with frequency-dependent impedance before and after each stage of compression. The growth of current and energy modulations is described by analytic gain formulas that agree with simulations.

The longitudinal wake is considered for a bunch of electrons that are suddenly accelerated to an ultrarelativistic velocity. This wake describes the wake of forward transition radiation, and it approximates the edge-radiation wake of a bunch exiting a bending magnet. The wake is large within the radiation formation zone, where it provides resistive impedance. A comparison with the computed wake downstream of a bending magnet yields good agreement, indicating that our wake expressions may be used to approximate the wake without numerical computation. For schemes in which a bunch produced by laser-plasma acceleration exits the plasma and then drives a free electron laser (FEL), the transition-radiation wake causes energy losses of many MeV that may affect the FEL process.

In an electron storage ring, coupling between dipole and quadrupole Robinson oscillations modifies the spectrum of longitudinal beam oscillations driven by radio-frequency (rf) generator phase noise. In addition to the main peak at the resonant frequency of the coupled dipole Robinson mode, another peak occurs at the resonant frequency of the coupled quadrupole mode. To describe these peaks analytically for a quadratic synchrotron potential, we include the dipole and quadrupole modes when calculating the beam response to generator noise. We thereby obtain the transfer function from generator-noise phase modulation to beam phase modulation with and without phase feedback. For Robinson-stable bunches confined in a synchrotron potential with a single minimum, the calculated transfer function agrees with measurements at the Aladdin 800-MeV electron storage ring. The transfer function is useful in evaluating phase feedback that suppresses Robinson oscillations in order to obtain quiet operation of an infrared beam line.

The effect of a higher-order mode upon longitudinal beam stability in an electron storage ring is modeled analytically and with simulations. Narrow band parasitic modes and broadband impedance are considered for the Aladdin and MAX-II electron storage rings. The simulations confirm that a passive harmonic cavity strongly suppresses parasitic coupled-bunch instabilities, in agreement with the analytic model. In the long-bunch regime where the bunch length exceeds the vacuum pipe radius, analytic modeling and simulations indicate that a harmonic cavity also suppresses the microwave instability. In the short-bunch regime where the bunch length is smaller than the vacuum pipe radius, analytic modeling and simulations indicate that tuning in a harmonic cavity may worsen the microwave instability.

Transverse hose instability may disrupt the propagation of a charged-particle beam in a channel of oppositely charged particles. A theoretical model predicts stabilization of this two-stream instability when the instability wavelength becomes smaller than the beam’s transverse size in a frame of reference where the instability’s phase velocity is nonrelativistic. Suppression of short-wavelength instability is also predicted when a proton beam propagates through a channel consisting of electrons and positive ions, consistent with previous experimental results.

Edge radiation may be extracted within a straight section of an electron storage ring by using a tilted mirror with a hole (or slot) for passage of the electron beam and its velocity field. At wavelengths much greater than the critical wavelength of the ring’s synchrotron radiation, the flux distribution upon the mirror is almost independent of wavelength. When the reflected radiation is extracted through a nearby window, diffraction reduces the extracted flux at wavelengths exceeding ∼2π times the hole radius. For the Aladdin 800 MeV electron storage ring, the extracted edge radiation may find application as a broadband source for wavelengths of 1 μm–60 mm or as a diagnostic of the bunch dimensions.

A radio frequency system with a fourth-harmonic “Landau” cavity suppresses coupled-bunch instabilities and increases the beam lifetime of the Aladdin electron storage ring. When the storage ring is operated with a small momentum compaction, instabilities limit the utility of the Landau cavity. Analytical modeling of instability frequencies and growth rates, simulations, and experiments suggest that the observed instabilities result from coupling between dipole and quadrupole Robinson modes.

When a gap in the electron bunch train prevents the trapping of ions, a transverse electron-ion instability may result from the ions created and lost during a single passage of the bunch train. A spread-frequency model is used to study this instability when the ions have a broad distribution of natural oscillation frequencies about the center of the electron beam. A growing disturbance saturates from Balakin-Novokhatsky-Smirnov damping at approximately the same time, and with the same total growth, as in the case without an ion frequency spread. At the tail of the bunch train, an unstable disturbance is amplified by a factor ∼exp(ω_iτ_b  ) before saturation occurs, where ω_i is a typical ion oscillation frequency and τ_b is the duration of the bunch train. Initially, the instability displays exponential growth in time, unlike the case where the ion-frequency spread is neglected. For a broad distribution of ion frequencies, instability may be prevented by a betatron damping rate that exceeds the incoherent betatron frequency shift induced by ions at the tail of the bunch train.

Gold disk targets were irradiated with and without induced spatial incoherence (ISI) at λ=0.526 μm and intensities of (1–5)×10¹⁴ W/cm². The distribution of laser energy on target and M-band x-ray (2–4 keV) pinhole images of the target were smoothed with ISI. Absorption of laser energy was increased by 5–10 % with ISI. Soft-x-ray conversion efficiency with and without ISI were equal within the data scatter and uncertainty of ∼10%.

We have observed plasma jets from laser-irradiated gold disk targets. The structures originate from low-intensity regions of the incident laser beam but propagate much faster than similar density surfaces from higher-intensity regions. The material is both cold and dense, as evidenced by its emission of soft x rays and absorption of hard x rays.