Various projects under study require an angular-momentum-dominated electron beam generated by a photoinjector. Some of the proposals directly use the angular-momentum-dominated beams (e.g., electron cooling of heavy ions), while others require the beam to be transformed into a flat beam (e.g., possible electron injectors for light sources and linear colliders). In this paper we report our experimental study of an angular-momentum-dominated beam produced in a photoinjector, addressing the dependencies of angular momentum on initial conditions. We also briefly discuss the removal of angular momentum. The results of the experiment, carried out at the Fermilab/NICADD Photoinjector Laboratory, are found to be in good agreement with theoretical and numerical models.

Electrons generated and accumulated inside the beam-pipe form an “electron cloud” that interacts with a charged particle beam. If the number of electrons is sizable, this beam-cloud interaction can give rise to a two-stream instability, resulting in beam loss or emittance growth. The instability can occur within a single bunch, e.g., passing through the cloud on successive turns in a storage ring, or it can be a multibunch instability, where the motion of successive bunches is coupled via the electron cloud. In this paper, I review the experimental evidence for, simulation approaches to, and analytical treatments of single-bunch two-stream instabilities caused by an electron cloud. Depending on the parameter regime, this type of instability may resemble a coasting-beam instability, classical beam breakup, or transverse mode coupling. It can also cause long-term emittance growth. Despite the apparent similarities, a few fundamental differences distinguish the two-stream instability from a conventional impedance-driven instability, and limit the applicability of established accelerator-physics concepts, like “wakefield.” On the other hand, if, in addition to the electron cloud, space-charge forces, conventional impedance, or beam-beam interaction are also present, these can conspire so as to enhance the growth rate.

Accelerating polarized proton beams and storing them for many turns can lead to a loss of polarization when accelerating through energies where a spin rotation frequency is in resonance with orbit oscillation frequencies. First-order resonance effects can be avoided by installing Siberian snakes in the ring, devices which rotate the spin by $180°$ around the snake axis while not changing the beam’s orbit significantly. For large rings, several Siberian snakes are required. Here a criterion will be derived that allows one to find an optimal choice for the orientation of the snake axes. Rings with superperiod four are analyzed in detail, and the HERA proton ring is used as an example for approximate fourfold symmetry. The proposed arrangement of Siberian snakes matches their effects so that all spin-orbit coupling integrals vanish at all energies and therefore there is no first-order spin-orbit coupling at all for this choice, which I call snake matching. It will be shown that in general at least eight Siberian snakes are needed and that there are exactly four possibilities to arrange their axes. When the betatron phase advance between snakes is chosen suitably, four Siberian snakes can be sufficient. Since the spin motion depends on a particle’s trajectory, protons at different phase space positions generally have different spin directions. The time averaged polarization at each phase space point is parallel to the invariant spin field, and the spread of this spin field limits the polarization that can be stored. By the here presented choice of Siberian snakes this limit is completely eliminated up to first order in the transverse phase space coordinates. The invariant spin field and the amplitude-dependent spin tune are also computed without linearization to show the advantages of this choice of snakes. Ultimately, the goal of snake matching is to reduce the loss of polarization during the acceleration of the beam. To show that a favorable choice of snakes has been found, polarized protons are tracked for part of HERA-$p$’s acceleration cycle which shows that polarization is preserved best for the here proposed arrangement of Siberian snakes.

We present an in-depth analysis of the concept of spin precession frequency for integrable orbital motion in storage rings. Spin motion on the periodic closed orbit of a storage ring can be analyzed in terms of the Floquet theorem for equations of motion with periodic parameters, and a spin precession frequency emerges in a Floquet exponent as an additional frequency of the system. To define a spin precession frequency on nonperiodic synchrobetatron orbits we exploit the important concept of quasiperiodicity. This allows a generalization of the Floquet theorem so that a spin precession frequency can be defined in this case, too. This frequency appears in a Floquet-like exponent as an additional frequency in the system in analogy with the case of motion on the closed orbit. These circumstances lead naturally to the definition of the uniform precession rate and a definition of spin tune. A spin tune is a uniform precession rate obtained when certain conditions are fulfilled. Having defined spin tune we define spin-orbit resonance on synchrobetatron orbits and examine its consequences. We give conditions for the existence of uniform precession rates and spin tunes (e.g., where small divisors are controlled by applying a Diophantine condition) and illustrate the various aspects of our description with several examples. The formalism also suggests the use of spectral analysis to “measure” spin tune during computer simulations of spin motion on synchrobetatron orbits.

A possible method to realize a dispersion-free storage ring is described. The simultaneous use of a magnetic field $\mathbf{B}$ and an electric field $\mathbf{E}$ in bending regions, where the two fields are set perpendicular to each other, enables us to control the effect of momentum dispersion. When the relation $(1+1/{\gamma }_0^2)\mathbf{E}(\rho )=-{\mathbf{v}}_0\times \mathbf{B}$ is satisfied for a beam with the velocity $v_0$, the linear dispersion can be completely eliminated all around the ring. It is shown that the acceleration and deceleration induced by the electrostatic deflector counteracts the heating mechanism due to the shearing force from dipole magnets. The dispersion-free system is thus beneficial to producing ultracold beams. It looks probable that the technique will allow one to achieve three-dimensional crystalline beams. At ICR Kyoto University, an ion cooler storage ring S-LSR oriented for various beam physics purposes is now under construction. The application of the present idea to S-LSR is discussed and the actual design of the dispersionless bend is given.

Stray electrons can arise in positive-ion accelerators for heavy-ion fusion or other applications as a result of ionization of ambient gas or gas released from walls due to halo-ion impact, or as a result of secondary-electron emission. We summarize the distinguishing features of electron-cloud issues in heavy-ion-fusion accelerators and a plan for developing a self-consistent simulation capability for heavy-ion beams and electron clouds (also applicable to other accelerators). We also present results from several ingredients in this capability. (1) We calculate the electron cloud produced by electron desorption from computed beam-ion loss, which illustrates the importance of retaining ion reflection at the walls. (2) We simulate the effect of specified electron-cloud distributions on ion beam dynamics. We consider here electron distributions with axially varying density, centroid location, or radial shape, and examine both random and sinusoidally varying perturbations. We find that amplitude variations are most effective in spoiling ion beam quality, though for sinusoidal variations which match the natural ion beam centroid oscillation or breathing-mode frequencies, the centroid and shape perturbations can also have significant impact. We identify an instability associated with a resonance between the beam-envelope “breathing” mode and the electron perturbation. We estimate its growth rate, which is moderate (compared to the reciprocal of a typical pulse duration). One conclusion from this study is that heavy-ion beams are surprisingly robust to electron clouds, compared to a priori expectations. (3) We report first results from a long-time-step algorithm for electron dynamics, which holds promise for efficient simultaneous solution of electron and ion dynamics.

The linear coupling's parameterization in the uncoupled action-angle frame is introduced, where the two eigenmodes' phases at one point and the phase advances between two points in the ring are well defined. The general expressions of the coordinates $(x,x^{\prime },y,y^{\prime })$ are also given. The transfer matrix $\mathbf{P}$ from the action-angle frame to the laboratory frame can be obtained from the eigenvectors of the one-turn transfer map $\mathbf{T}$ or from Twiss and coupling parameters defined in Edwards-Teng parameterization. Matrix $\mathbf{P}$ can also be constructed from turn-by-turn beam position monitor (BPM) data. The phase ellipses and beam sizes in different projection planes are calculated through the action-angle parameterization. The two eigenmodes' Twiss and coupling parameters at point $s_2$ are described in terms of the section transfer map ${\mathbf{T}}_{1\to 2}$ and parameters at point $s_1$. The linear coupling's action-angle parameterization is useful for the analytical calculations and turn-by-turn BPM data interpretations.

Recently a new electron-bunch injection scheme for the laser wakefield accelerator has been proposed [JETP Lett. 74, 371 (2001); Phys. Rev. E 65, 046504 (2002)]. In this scheme, a low energy electron bunch, sent in a plasma channel just before a high-intensity laser pulse, is trapped in the laser wakefield, considerably compressed and accelerated to an ultrarelativistic energy. In this paper we show the possibility of the generation of an extremely short (on the order of $1\mu \mathrm{m}$ long or a few femtoseconds in duration) relativistic-electron-bunch by this mechanism. The initial electron bunch, which can be generated, for example, by a laser-driven photocathode rf gun, should have an energy of a few hundred keVs to a few MeVs, a duration in the picosecond range or less and a relatively low concentration. The trapping conditions and parameters of an accelerated bunch are investigated. The laser pulse dynamics as well as a possible experimental setup for the demonstration of the injection scheme are also considered.

The exact analytical expression for the longitudinal impedance of two-layer cylindrical tube with finite wall thickness is obtained. The numerical results for the copper-NEG, stainless steel-copper, and ceramic-copper laminated vacuum chamber impedances are given.

We show that, due to inaccurate calculations with a Volterra integral equation, the results obtained in the commented paper are not true. Such an awkwardness could be avoided if the authors would retain in their calculations initial conditions for the proton bunch centroid.

The authors disagree with Dr. D. V. Pestrikov’s assertion that “the results obtained in the commented paper are not true,” based on how Volterra’s integral equations are treated in the paper. The authors would agree, for clarity, that the equal sign ($=$) in Eqs. (34), (36), and (38) in the paper be replaced by some special symbol or the approximate sign ($\approx$) to indicate the omission of initial conditions. These and similar changes together with the revision to an error found by the authors have been published in a recent erratum.

The effect of “pressing out” of a short rf powerful pulse by an electron beam from a Bragg cavity pumped by a low-power input rf signal is studied. This effect occurs as a result of the transient process from the “cold” (without the electron beam) steady state of the system to the “hot” one. It is shown that the peak power of the output pulse can be significantly higher than both the pumping rf power and the electron beam power. As for the duration of the output pulse, it is fixed by the cold characteristics of the microwave system.

Model-independent analysis of beam dynamics in circular accelerators reveals spatial-temporal modes due to beam oscillations. Although such modes have been shown to be informative, their usefulness is limited by the lack of quantitative understanding in general. Here we present a simple approach to studying the modes analytically, which results in a quantitative understanding of the modes and an algorithm to extract the physical modes by untangling the mixed singular value decomposition modes. Particularly, we focus on the coupled betatron modes that are of great interest for high-energy colliders. A simple relationship between the coupling modes and the lattice functions is established, which not only provides a quantitative understanding but also lays the foundation for using these modes to measure machine properties and beam motion in phase space.

This paper presents a survey of the present theoretical understanding of collective processes and beam-plasma interactions affecting intense heavy ion beam propagation in heavy ion fusion systems. In the acceleration and beam transport regions, the topics covered include discussion of the conditions for quiescent beam propagation over long distances; the electrostatic Harris-type instability and the transverse electromagnetic Weibel-type instability in strongly anisotropic, one-component non-neutral ion beams; and the dipole-mode, electron-ion two-stream instability driven by an (unwanted) component of background electrons. In the plasma plug and target chamber regions, collective processes associated with the interaction of the intense ion beam with a charge-neutralizing background plasma are described, including the electrostatic electron-ion two-stream instability, the electromagnetic Weibel instability, and the resistive hose instability. Operating regimes are identified where the possible deleterious effects of collective processes on beam quality are minimized.

A concept for increasing the energy of a multibunch linear collider using plasma wakefields is examined. The realization of high beam quality and high efficiency (and high luminosity) requires more complexity than the original plasma afterburner concept proposed for doubling the energy of single bunch linear colliders. This paper discusses the possibilities of using alternate bunches in the train to drive the wake and accelerate upon it or alternately a few bunches to excite the wake and a single bunch to accelerate it. Simulation results indicate that an energy of collision/energy of linac ratio of 2.8 can be obtained with 4% energy spread and 0.29 relative luminosity by utilizing five drive bunches per accelerated bunch. The concept including transverse effects is modeled with 2D linear plasma wakefield theory.

We determine the impedance of a cylindrical metal tube (resistor) of radius $a$, length $g$, and conductivity $\sigma$ attached at each end to perfect conductors of semi-infinite length. Our main interest is in the asymptotic behavior of the impedance at high frequency $(k\gg 1/a)$. In the equilibrium regime, $k{a}^2\ll g$, the impedance per unit length is accurately described by the well-known result for an infinite length tube with conductivity $\sigma$. In the transient regime, $k{a}^2\gg g$, where the contribution of transition radiation arising from the discontinuity in conductivity is important, we derive an analytic expression for the impedance and compute the short-range wakefield. The analytic results are shown to agree with numerical evaluation of the impedance.

We investigate how collective modes and colored noise conspire to produce a beam halo with much larger amplitude than could be generated by either phenomenon separately. The collective modes are lowest-order radial eigenmodes calculated self-consistently for a configuration corresponding to a direct-current, cylindrically symmetric, warm-fluid Kapchinskij-Vladimirskij equilibrium. The colored noise arises from unavoidable machine errors and influences the internal space-charge force. Its presence quickly launches statistically rare particles to ever-growing amplitudes by continually kicking them back into phase with the collective-mode oscillations. The halo amplitude is essentially the same for purely radial orbits as for orbits that are initially purely azimuthal; orbital angular momentum has no statistically significant impact. Factors that do have an impact include the amplitudes of the collective modes and the strength and autocorrelation time of the colored noise. The underlying dynamics ensues because the noise breaks the Kolmogorov-Arnol’d-Moser tori that otherwise would confine the beam. These tori are fragile; even very weak noise will eventually break them, though the time scale for their disintegration depends on the noise strength. Both collective modes and noise are therefore centrally important to the dynamics of halo formation in real beams.

A 201.5 MHz, 50 mA, 2.0 MeV deuteron radio frequency quadrupole accelerator is proposed as the neutron generator for the neutron experiment facility project at Peking University, China. Based on better understanding of beam losses, some new optimization procedures concerning both longitudinal and transverse dynamics are adopted. Accordingly, the beam transmission efficiency is improved from 91.2% to 98.3% and the electrode length is shortened from 2.91 to 2.71 m. The fundamental physical analyses are performed to look inside the new design recipe and explain why it works.